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Precursory seismicity changes associated with the Nemuro Peninsula earthquake, January 28, 2000
Journal of Asian Earth Sciences 21 (2002) 135–146 www.elsevier.com/locate/jseaes
Precursory seismicity changes associated with the Nemuro Peninsula earthquake, January 28, 2000 Qinghua Huanga,*, Guennadi A. Sobolevb a Department of Geophysics, Peking University, Beijing 100871, People’s Republic of China United Institute of Physics of the Earth, Russian Academy of Sciences, 10, B. Gruzinskaya, Moscow 123810, Russian Federation
b
Received 28 March 2001; revised 1 November 2001; accepted 8 March 2002
Abstract We investigated the characteristics of the precursory seismicity changes associated with the M ¼ 6:8 Nemuro Peninsula earthquake, January 28, 2000 by applying the RTL (Region – Time– Length) algorithm to the earthquake catalogue of the Japan Meteorological Agency (JMA). The RTL parameters at the epicentre indicated that a seismic quiescence started in 1995 and reached its minimum during October 1996. An activation stage with a duration of about eight months followed. Our detailed investigations indicated that the RTL anomaly around 1996 was not an artificial effect due to the changes of the model parameters, the upgrade of the JMA seismic network or the process of aftershock elimination. The spatial distribution of quiescence in 1996 revealed a significant anomaly in a broad region around the epicentre of the Nemuro Peninsula earthquake. Following the quiescence stage, an activation zone, which was on the order of the rupture length of this earthquake, was obtained around the epicentre during October 1996– July 1997. The consistency of the rupture region and the anomalous activation zone that appeared after the seismic quiescence stage may provide useful information for determining the future risk zones. This study may strengthen the understanding of the seismogenic process of strong earthquakes. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Seismicity; Quiescence; Activation; Precursor; Seismogenic process; Nemuro Peninsula earthquake
1. Introduction The laboratory rock experiments indicated that the acoustic emission may accelerate as the loading increases. However, the number of relatively weak signals tends to decrease after the loading reaches the maximum, because small cracks are no long generated due to the partial reduction of stress. During the final stage before the main rupture, acoustic activity increases again. In other words, acoustic emission can experience the stages of quiescence and activation prior to the main rupture (Sobolev, 1995). The field seismological observations also indicated the existence of quiescence and activation stages of seismicity in the rupture region of strong earthquakes. However, the primary attention was concentrated on seismic quiescence, which may be one of the most promising intermediate-term precursors (Wyss et al., 1984; Wyss and Habermann, 1988; Habermann, 1988; Kossobokov and Keilis-Borok, 1990; Taylor et al., 1991; Wiemer and Wyss, 1994; Takanami * Corresponding author. Tel.: þ86-10-6275-4057; fax: þ 86-10-62757860. E-mail address: [email protected] (Q. Huang).
et al., 1996; Katsumata and Kasahara, 1999). Many precursory seismic quiescences have been reported in and around focal areas a few years before earthquakes, e.g. earthquakes of Tonga – Kermadec (Wyss et al., 1984), Hokkaido (Taylor et al., 1991), Landers (Wiemer and Wyss, 1994), Kurile (Takanami et al., 1996; Katsumata and Kasahara, 1999), etc. There are several methods that can reveal the precursory seismic quiescence based on earthquake catalogues (Wyss and Habermann, 1988; Kossobokov and Keilis-Borok, 1990). However, few of these approaches deal with the seismic activation. A statistic method was developed recently to investigate the characteristics of seismicity changes, including both quiescence and activation patterns (Sobolev and Tyupkin, 1997, 1999; Huang and Sobolev, 2001; Huang et al., 2001). This method is called the RTL algorithm, where the name RTL comes from R: Region (epicentral distance), T: Time interval, and L: Length (rupture size). The RTL is a parameter reflecting the combination of three functions of distance, time and rupture length, respectively. All three parameters (time, place and magnitude) of earthquakes are taken into account with
1367-9120/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 7 - 9 1 2 0 ( 0 2 ) 0 0 0 3 2 - 9
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weighted coefficient in this algorithm. We will introduce briefly this algorithm in the Section 2. The RTL algorithm was initially tested using the earthquake catalogue of Kamchatka, Russia (Sobolev and Tyupkin, 1997, 1999). It was found that the strong earthquakes in Kamchatka with magnitude M . 7 had been preceded by seismic quiescence followed by activation. The seismic quiescence stage started 1.5– 3.5 years before the main shock and lasted 1 – 2.5 years. The subsequent stage of seismic activation had a duration varying from 6 months to 1.5 years. The linear dimension of the quiescence zone was several times larger than the rupture length of the main shock, while the activation zone was on the order of several tens of kilometres, comparable to the rupture length. Similar variation of seismicity changes of the 1995 M ¼ 7:2 Kobe earthquake was obtained recently by using the earthquake catalogue of the Japan Meteorological Agency (JMA) (Huang et al., 2001). We also investigated the seismic quiescence associated with the M ¼ 6:8 Nemuro Peninsula (Hokkaido) earthquake, which occurred at 146.71E longitude, 42.98N latitude, 23 h 21 min JST (Japan Standard Time) on January 28, 2000 with a focal depth of about 56 km, by applying the RTL algorithm to the JMA earthquake catalogue. The characteristics of seismic quiescence associated with this earthquake, which are revealed by both the RTL algorithm and the temporal variation of earthquake number in the epicentral zone, indicated that a significant quiescence appeared 3 years before the main shock and the anomalous quiescence zone was over 200 km long (Huang and Sobolev, 2001). Unfortunately, we investigated neither the influence of model parameters to the results nor the characteristics of seismic activation pattern in that short paper. In this study, after eliminating aftershocks from the original JMA earthquake catalogue and estimating the completeness of this catalogue, we investigate the temporal variation of seismicity changes and the spatial distribution of seismic quiescence associated with the Nemuro Peninsula earthquake using the RTL algorithm. We evaluate the influence of model parameters to the results, because seismicity changes may be caused either by the real changes associated with the development of earthquakes or by manmade changes (Habermann, 1987, 1991). We also discuss the characteristics of seismic activation and evaluate the spatial distribution of the seismic activation using another independent algorithm (Huang et al., 2001), which takes the increase of the effective rupture area into account and is to be introduced briefly in Section 2.
2. Methods In order to investigate the evolution characteristics of the seismic quiescence and activation patterns, we eliminate aftershocks from the JMA earthquake catalogue using the
algorithm developed by Molchan and Dmitrieva (1991). Because earthquakes may not be reported homogeneously due to the inhomogeneous distribution of seismic stations in Japan, an island country, we also estimate the completeness of this catalogue based on the power law of frequency– magnitude before applying the RTL algorithm. We will introduce briefly the principles of aftershock elimination and the completeness analysis in next section. 2.1. RTL algorithm The principle of the RTL algorithm has been introduced previously (e.g. Sobolev and Tyupkin, 1997; Huang et al., 2001). For the readers’ convenience and clear discussion, we repeat the main ideas of this algorithm in this section. In the RTL algorithm, we defined three functions of distance, time and rupture length as "
# ri Rðx; y; z; tÞ ¼ exp 2 2 Rtr ðx; y; z; tÞ; r0 i¼1 " # n X t 2 ti Tðx; y; z; tÞ ¼ exp 2 2 Ttr ðx; y; z; tÞ; t0 i¼1 " # n X li Lðx; y; z; tÞ ¼ 2 Ltr ðx; y; z; tÞ ri i¼1 n X
ð1Þ
where ðx; y; z; tÞ indicates the investigated position and time; li and ti are the rupture length and the occurrence time of the ith earthquake; ri is the distance from the position of ðx; y; zÞ to the epicentre of the ith event; n is the number of events with their three parameters satisfying some criteria, e.g. Mi $ Mmin (Mi is the magnitude of the ith earthquake and Mmin is the cut-off magnitude ensuring the completeness of the earthquake catalogue), ri # Rmax ¼ 2r0 and ðt 2 ti Þ # Tmax ¼ 2t0 (Rmax and Tmax are threshold distance and timespan, r0 and t0 are characteristic distance and time-span), dU # di # dL (di is the focal depth of the ith earthquake, dU and dL are the upper and lower cut-off depths); Rtr ðx; y; z; tÞ; Ttr ðx; y; z; tÞ and Ltr ðx; y; z; tÞ are the trends (background values) of Rðx; y; z; tÞ; Tðx; y; z; tÞ and Lðx; y; z; tÞ; respectively. One can see from Eq. (1) that Rðx; y; z; tÞ; Tðx; y; z; tÞ and Lðx; y; z; tÞ are three dimensionless functions. They are further normalised to their standard deviations sR, sT, and sL, respectively. The product of the above three functions is calculated as the RTL parameter, which describes the deviation from the background level of seismicity and is in units of standard deviation, s ¼ sR sT sL : A decrease of RTL means a decrease in seismicity compared to the background rate around the investigated location, i.e. a decrease of RTL represents a seismic quiescence. A recovery stage from the quiescence to the background level can be considered as an activation stage of seismicity. The rupture dimension li is given by an empirical relation
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with the magnitude Mi (Kasahara, 1981), log li ðkmÞ ¼ 0:5 Mi 2 1:8
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We defined the effective rupture area Seff as, ð2Þ
The rupture length of the M ¼ 6:8 Nemuro Peninsula earthquake estimated by Eq. (2) is on the order of several tens of kilometres. After taking into account the reports of seismic quiescence around the rupture zone and our previous experiences in Kamchatka and Kobe (Sobolev and Tyupkin, 1997, 1999; Huang et al., 2001), we assumed the characteristic distance r0 ¼ 50 km; i.e. the threshold distance Rmax ¼ 2r0 ¼ 100 km: We also assumed the characteristic time-span t0 ¼ 1 year; i.e. the threshold time-span Tmax ¼ 2t0 ¼ 2 years; based on our previous experiences (Sobolev and Tyupkin, 1997, 1999; Huang et al., 2001) and the fact that duration of seismic quiescence is generally on the order of one year. We make most of our calculations by taking r0 ¼ 50 km and t0 ¼ 1 year; although we also make the calculations by changing the characteristic distance r0 and the characteristic time-span t0 so that we can estimate the possible influence of these parameters to our results. Another criterion of the focal depth di satisfies di # dL ¼ 100 km (no upper cut-off focal depth dU), although we also made the calculations for the cases of (1) all di (i.e. no cut-off focal depths dU and dL), (2) dU ¼ 30 km # di # dL ¼ 100 km; and (3) di $ dU ¼ 30 km (no lower cut-off focal depth dL) in this study. Note that 30 km is a typical threshold depth for crust earthquakes. Therefore, the criterion of the upper cut-off focal depth dU ¼ 30 km may exclude or reduce the possible disturbance of crust seismicity to the Nemuro Peninsula earthquake, which occurred in the slab at a focal depth of about 56 km. Although the selection of the criterion of focal depth is somewhat arbitrary, to be discussed later, this criterion has little influence on our results. If we assume that the characteristics of the seismic quiescence at a certain location can be quantified by the minimum RTL value within the investigated period of time DT at this location, we can obtain the spatial distribution of the seismic quiescence by changing the calculated location. In this study, we change the location at a step of 0.258 along the direction of either longitude or latitude each time and repeat above calculations. It should be noted that the calculation of the RTL parameter at each location is made through the whole background period Tbk, although we take into account the minimum RTL value only within the study interval DT (Huang and Sobolev, 2001). 2.2. Increase of effective rupture area We developed another algorithm, which is independent of the RTL algorithm, to reveal the spatial distribution of the anomaly of seismic activation. We introduced this technique previously in Huang et al. (2001). We summarize the principle of this approach here for the readers’ convenience.
Seff ¼
1 X Si 1 X ðMi 2Mref Þ ¼ 10 ; DT i Sref DT i
ð3Þ
where DT is any time interval of interest and Sref is the rupture area corresponding to a reference magnitude Mref. In this study, DSeff ¼ Seff 2 Sbk is chosen as a parameter of the seismic activation, where Seff is the effective rupture area in an investigated activation period DT and Sbk is the averaged background one, which can be calculated by Eq. (3) after replacing DT by the period of background Tbk.
3. Data analysis We analyse the JMA earthquake catalogue before applying the RTL algorithm. Because the manual operation of data in the JMA earthquake catalogue has been replaced by the computer operation since 1961, we choose the earthquake catalogue from January 1, 1961 to March 31, 2000 in this study. For the first step, we eliminate aftershocks from the above JMA catalogue using the program written by V.B. Smirnov on the basis of the algorithm of Molchan and Dmitrieva (1991). The principle of separating aftershocks from the rest of events, which are called background, is based on the comparison of their functions and their distribution in time and space. Background events are assumed to be distributed evenly. Aftershocks are assumed to have a bell-shaped (Gaussian) distribution on the plane (only earthquakes epicenters are taken into account) and are distributed in time according to the Omori law. The algorithm efficiency was tested by Smirnov who compared aftershock sequences obtained from regional catalogues with the use of this algorithm and aftershocks catalogues compiled independently by other authors. The discrepancy of the number of aftershocks obtained is approximately 5%. After eliminating aftershocks from the JMA earthquake catalogue, we evaluate the completeness of this catalogue using the algorithm developed by Smirnov (1998). The evaluation of the completeness of the earthquake catalogue is based on the histograms of earthquake distribution by magnitude. The problem is to find the minimal value Mmin with which the recurrence plot is linear in the range of M $ Mmin. One can find the detailed introduction of this technique in Smirnov (1998), Huang et al. (2001). This algorithm was realised by Smirnov as a set of programs, which allow making assessments of the representative character of earthquake catalogues in varying time intervals and in desired spatial cells. The application of the algorithm of the completeness assessment to a number of regional catalogues testified to its efficiency (Smirnov, 1998).
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Fig. 1. The completeness analysis using the JMA earthquake catalogue from January 1, 1961 to March 31, 2000. (a) Temporal variation of the minimum magnitude, Mmin around the epicentre of the M ¼ 6:8 Nemuro Peninsula (Hokkaido) earthquake, January 28, 2000. (b) Spatial distribution of Mmin during January 1, 1984–March 31, 2000. The shadowed zone indicates the investigated region in this study.
Fig. 1(a) plots the temporal variation of the cut-off magnitude Mmin in a circular zone with a radius of Rmax ¼ 100 km; and a center at (146.71E, 42.98N), the epicenter of the Nemuro Peninsula earthquake. It showed that Mmin decreased with time. This is due to the improvements/ changes of the observing instruments or the processing systems of the JMA seismic stations. We found that the threshold magnitude Mmin after 1995 is less than 3.0, which is consistent with the detectability of the JMA seismic network in the Hokkaido area (Kuwayama, 1999). However, the remaining period up to January 28, 2000 is too short for calculating the reliable background variations using the RTL algorithm. We obtained a magnitude of 3.6 as the threshold magnitude of the earthquakes in the study zone after January 1, 1984. The spatial distribution of the threshold magnitude Mmin was also estimated as shown in Fig. 1(b) by using the JMA earthquake catalogue for January 1, 1984 –March 31, 2000. We found that Mmin ¼ 3:6 can cover most parts of the Japanese islands including the epicentral zone. Therefore, we use all events with the magnitude M $ 3.6 in the JMA earthquake catalogue for January 1, 1984– March 31, 2000 in this study. It should be mentioned that we introduced another
Fig. 2. (a) The RTL parameter at the epicentre of the 2000 Nemuro Peninsula earthquake varies with time. A significant quiescence appeared in 1996, followed by an activation pattern. The arrow indicates the occurrence time of the main shock. (b) The temporal variations of three functions R(146.71E, 42.98N, 0, t ), T(146.71E, 42.98N, 0, t ), and L(146.71E, 42.98N, 0, t ) at the epicentre. They decreased simultaneously during 1995– 1996.
criterion of maximum threshold magnitude Mmax ¼ 5:5 in our previous study on the seismic quiescence associated with the Nemuro Peninsula earthquake (Huang and Sobolev, 2001), because we want to avoid the possible disturbance to background seismicity of large events due to the possible deviation of the power law of frequencymagnitude at large magnitudes (Main, 1992; Hamilton and McCloskey, 1997; Ikeya and Huang, 1997). However, there is a potential danger that artificial changes can be generated by introducing two cut-off magnitudes Mmin and Mmax (Habermann, 1987, 1991). In order to investigate whether or not our previous results of seismic quiescence pattern prior to the Nemuro Peninsula earthquake is an artificial effect, we excluded the maximum threshold magnitude Mmax in our current study.
4. Results We calculated the temporal variations of the RTL parameter (in units of standard deviation, s ) at the epicenter of the M ¼ 6:8 Nemuro Peninsula earthquake based on the earthquakes in the circular zone with a radius of Rmax in the JMA earthquake catalogue. The spatial distributions of seismic quiescence and activation were also investigated using the methods introduced before. Fig. 2(a) shows the temporal variation of the RTL
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Fig. 3. The spatial distribution of seismic quiescence in 1996. A significant seismic quiescence appeared three years before the Nemuro Peninsula earthquake. The anomalous zone reached 200 km. The scale on the right corresponds to the RTL value in units of standard deviation. The black star represents the epicentre of the Nemuro Peninsula earthquake (146.71E, 42.98N). There occurred another 4 moderate earthquakes with M $ 6.0 during January 1, 1997–March 31, 2000. The grey stars show the epicentres of these 4 events.
parameter at longitude 146.71E, latitude 42.98N, the epicenter of the Nemuro Peninsula earthquake. This RTL curve is obtained on the basis of the RTL algorithm, which was introduced in the previous section. All events used in the above calculations satisfy the following criteria: magnitude Mi $ 3.6, focal depth di # 100 km, epicentral distance ri # Rmax ¼ 100 km; and time interval ðt 2 ti Þ # Tmax ¼ 2 years: An obvious seismic quiescence was obtained from this RTL curve, followed by a significant activation stage. The quiescence started in 1995 and reached its minimum in October, 1996. The largest deviation from the background was about 26s. This result is consistent with the one that we obtained for the case of introducing a maximum threshold magnitude Mmax ¼ 5:5 in the previous study (Huang and Sobolev, 2001). We will discuss the correlation between them in the next section. The temporal variations of three functions, R(146.71E, 42.98N, 0, t ), T(146.71E, 42.98N, 0, t ), and L(146.71E, 42.98N, 0, t ) are shown in Fig. 2(b). These three functions deviated consistently from their background levels during 1995 – 1996. The spatial distribution of the seismic quiescence is plotted in Fig. 3. We calculated the quiescence distribution during January 1 –December 31, 1996, because a significant quiescence stage was revealed during this period by the RTL parameter (Fig. 2(a)). A significant quiescence anomaly appeared around the epicenter of the Nemuro Peninsula earthquake. The linear dimension of this anomalous zone was over 200 km. Fig. 4 shows the spatial distribution of the seismic activation (parameter DSeff) during an interval of 0.7 year until mid-1997 (i.e. from mid-October 1996 to early July
1997), because an obvious activation pattern was detected during this period by the RTL parameter (Fig. 2(a)). Compared to the anomalous quiescence zone, a relative smaller zone of activation anomaly on the order of several tens of kilometers up to 100 km was obtained around the epicentre of the main shock.
5. Discussion Obviously, the completeness of an earthquake catalogue is important for an investigation of seismicity (Wyss and Habermann, 1988). That’s why we have estimated the threshold magnitude of the JMA catalogue before applying our algorithm. In this study, we focused our discussions on the land and its coast regions (i.e. the shadowed parts as shown in Fig. 1(b)). There may be a contradiction between the completeness of a catalogue and the reliability of statistics, because the larger the threshold magnitude Mmin, the fewer the events within a certain interval. One can see clearly from Fig. 1(b) that the Mmin is spatially inhomogeneous. The threshold magnitude Mmin ¼ 3; which was used in our previous analysis on the Kobe earthquake, is sufficient for most parts of Japan, including the land area of Hokkaido (Fig. 1(b), also see Kuwayama (1999)). However, Mmin around the epicentral zone of the Nemuro Peninsula earthquake is 3.6. That’s why we used Mmin ¼ 3:6 in this study. As mentioned before, the selections of some model parameters are somewhat empirical and arbitrary. Therefore, we investigate the possible influence of these parameters to our previous result so that we can judge
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Fig. 4. The spatial distribution of seismic activation during mid-October, 1996–early July, 1997. The epicentre of the Nemuro Peninsula earthquake (black star) located almost in the centre of this anomalous zone, which is on the order of several tens of kilometres. Another anomalous activation zone in Tokai area may have some relation with the earthquake swarms in the Izu Island region.
whether the RTL anomaly in 1996 (Fig. 2(a)) is an artificial effect of the influence of model parameters or a real precursor of the Nemuro Peninsula earthquake. In addition, we discuss the evolutionary characteristics of seismic quiescence and activation patterns in this section. 5.1. Influence of model parameters on the RTL anomaly The seismic quiescence of the Nemuro Peninsula earthquake started in 1995 and lasted until October 1996, about 3 years before the main shock (Fig. 2(a)). The time duration was about 1.5 years and the largest deviation from the background level was 2 25.94s. The distance function R(146.71E, 42.98N, 0, t ), time function T(146.71E, 42.98N, 0, t ), and rupture length L(146.71E, 42.98N, 0, t ) decreased consistently from their background levels during 1995– 1996 (Fig. 2(b)). As mentioned in the previous section, these three functions are normalised by their standard deviations sR, sT, and sL, respectively. It would be possible to estimate roughly the probability of these variations. The values of these three functions at the time when the RTL parameter has the largest deviation in October 1996 are 2 2.71sR, 2 2.46sT, and 2 3.88sL, respectively. After assuming a normal distribution, the occurrence probabilities of the above anomalies are 0.00672, 0.01390, and 0.0001,
respectively. If these three functions are completely independent, the occurrence probability of the above RTL anomaly in October 1996 would be as low as 0.93 £ 1028. Therefore, the RTL anomaly in October 1996 is a significant anomaly compared to the background level of seismicity. As mentioned in the previous section, we introduced in this study some ‘free’ parameters such as cut-off focal depths dU and dL, characteristic distance r0, and characteristic time-span t0. We also introduced another criterion of maximum cut-off magnitude Mmax in our previous study (Huang and Sobolev, 2001). We chose these parameters as dL ¼ 100 km; r0 ¼ 50 km ðRmax ¼ 2r0 ¼ 100 kmÞ and t0 ¼ 1 year ðTmax ¼ 2t0 ¼ 2 yearsÞ in our current calculations. Although the criteria of dU and dL are somewhat arbitrary, the criteria of r0 and t0 are chosen not only on the basis of our previous experiences, but also on the basis of the reports of rupture region and duration time of seismic quiescence. In order to investigate the possible influence on the results, we repeat our calculations by changing these parameters. For simplification, we calculate the RTL curves after changing only one parameter in each case. We also evaluate the correlation coefficients. First, we make a comparison between the RTL parameter in Fig. 2(a) and the one in our previous study as a criterion of maximum cut-off magnitude Mmax ¼ 5:5 (Huang and
Table 1 Correlation of RTL values between different model parameters of cut-off magnitude and cut-off focal depths dU and dL (threshold distance Rmax ¼ 2r0 and threshold time interval Tmax ¼ 2t0 ) Cases
Correlation coefficient
A B
dL ¼ 100 km M # 5.5
r0 ¼ 50 km No dU, dL
0.7654
0.9998
t0 ¼ 1 year dU ¼ 30 km dL ¼ 100 km 0.8326
M $ 3.6 dU ¼ 30 km No dL 0.8287
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Fig. 5. The RTL parameter at the epicentre for the case of dU ¼ 30 km and dL ¼ 100 km:
Sobolev, 2001). In both cases, we obtained a significant quiescence anomaly during 1995 – 1996 with a minimum in October 1996. The correlation coefficient between the above two cases is 0.7654 (see Table 1). The statistic test based on a model of linear correlation analysis indicated that significant correlation exists for the above two cases at a significant level of 0.05 (Bendat and Piersol, 2000). Next, we discuss the influence of the cut-off focal depths dU and dL, which are somewhat arbitrary in this study. We re-calculated the RTL parameter for the following three cases: (1) without cut-off focal depths dU and dL; (2) introducing the upper cut-off depth dU ¼ 30 km (this is a typical threshold depth of earthquakes in crust) and keeping the lower cut-off focal depth dL ¼ 100 km; (3) introducing the upper cut-off depth dU ¼ 30 km and excluding the lower cut-off depth dL. In all of the above three cases, we keep other parameters unchanged (i.e. M $ 3.6, r0 ¼ 50 km and t0 ¼ 1 year). We found that there is little influence on the RTL parameter from the changes of the cut-off focal depths dU and dL (e.g. the RTL curve of case (2) is plotted in Fig. 5). We also estimated the correlation coefficient between the result in Fig. 2(a) and one of these three cases. We obtained the correlation coefficients of 0.9998, 0.8326 and 0.8287, respectively (see Table 1). The statistic test indicated that significant correlation exists at a significant level of 0.05. The changes in the cut-off focal depths dU and dL do not significantly influence our results. We also investigate the influence of the characteristic distance r0 and the characteristic time-span t0 after taking the cut-off focal depths dU ¼ 30 km and dL ¼ 100 km: The temporal variations of the RTL parameter in the case of r0 ¼ 25 km ðRmax ¼ 2r0 ¼ 50 kmÞ and r0 ¼ 75 km ðRmax ¼ 2r0 ¼ 150 kmÞ are quite similar to that in Fig. 5 (we keep other parameters unchanged, i.e. M $ 3.6, dU ¼ 30 km; dL ¼ 100 km and t0 ¼ 1 year). The correlation coefficients are 0.9189 and 0.9779, respectively for the cases between r0 ¼ 25 km and r0 ¼ 50 km and between r0 ¼ 75 km and r0 ¼ 50 km (see Table 2). The statistic test indicated that significant correlation exists at a significance level of 0.05 for the above two cases. We repeat the calculations by changing t0 from 1 year to 0.5 year ðTmax ¼ 2t0 ¼ 1 yearÞ and 1.5 years ðTmax ¼ 2t0 ¼
141
3 yearsÞ and keeping other parameters unchanged (i.e. M $ 3.6, dU ¼ 30 km; dL ¼ 100 km and r0 ¼ 50 km). We obtained in both cases the similar characteristics of seismicity changes to that in Fig. 5. The correlation coefficients are 0.5612 and 0.3916, respectively for the above two cases (see Table 2). Both cases show significant correlation with our previous result (Fig. 5) at a significance level of 0.05. Above analyses indicate that the RTL result we obtained previously in Fig. 2(a) is unlikely an artificial effect due to the selection of some model parameters, dU, dL, r0 and t0. Thus, we can conclude reasonably that the RTL anomaly, which appeared in 1996, would correlate with the preparation stage of the M ¼ 6:8 Nemuro Peninsula earthquake. 5.2. Evolution of seismic quiescence and activation patterns The seismic quiescence of the Nemuro Peninsula earthquake started in 1995 and lasted until the end of 1996, three years before the main shock (Fig. 2(a)). The duration time was about 1.5 years. The activation pattern appeared at the end of 1996 with a duration time of about 0.7 year. In the case of the Kobe earthquake, the quiescence started two years before the main shock with a duration of about 1.5 years. The followed activation lasted 0.7 year. In contrast to the Kobe case, the Nemuro Peninsula earthquake did not occur immediately after the activation stage, but after a delay of 2.5 years. A similar time delay after the activation stage was also reported in Kamchatka (Sobolev and Tyupkin, 1999). A M ¼ 7:7 earthquake occurred in the northern Gulf of Kamchatka on December 5, 1997, about 1.5 years after the end of the activation stage. Generally, a future earthquake most probably occurs after the establishment of the successive quiescence and activation patterns. Therefore, seismicity changes may provide useful information for earthquake prediction. However, the existence of the time delay, which was reported in this study and the previous studies in Kamchatka and Izmit (Sobolev and Tyupkin, 1999; Huang et al., 2002), makes it difficult to determine the occurrence time of a future event within the accuracy required in short-term prediction. It may be that the investigated candidate zone has already been in a critical state. Therefore, macrofractures around this zone may be triggered by the changes of any local, regional or global factors such as meteorological or cosmic phenomena, the occurrence of neighbour or remote earthquakes, etc. The spatial distribution of seismic quiescence of the Nemuro Peninsula earthquake indicated that the length of this anomalous zone was over 200 km (Fig. 3), several times larger than the rupture length, which is usually on the order of several tens of kilometres. In the case of the M ¼ 7:2 Kobe earthquake, the anomalous quiescence zone was over 300 km (Huang et al., 2001), larger than that of the M ¼ 6:8 Nemuro Peninsula earthquake. Unfortunately, the
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Table 2 Correlation of RTL values between different model parameters of threshold distance Rmax and time-span Tmax (Rmax ¼ 2r0 and Tmax ¼ 2t0 ) Cases Correlation coefficient
A B
dU ¼ 30 km r0 ¼ 25 km 0.9189
dL ¼ 100 km r0 ¼ 75 km 0.9779
correlation between the size of the anomalous quiescence zone and the magnitude of the following event is still unclear, because the geological structure or seismo-tectonic background in the Kobe and Hokkaido regions would be somewhat different. We obtained some small anomalous quiescence zones during the same period (Fig. 3). There occurred another 4 earthquakes with M $ 6.0 and d # 100 km in the displayed zone (see Fig. 1(b)) during January 1, 1997 –March 31, 2000. We marked these earthquakes in Fig. 3. Two moderate earthquakes with M $ 6.0 occurred around the small anomalous zones in the northeastern part of Honshu and southern Kyushu, respectively. However, we are not sure whether these small zones have reasonable correlations with the nearby earthquakes, because we do not have experience in revealing the seismogenic processes of such moderate earthquakes using the RTL algorithm. It should be mentioned that no quiescence anomalies occurred around the other two events. Although we cannot understand this phenomenon very well at the current stage, we can make a reasonable deduction that the occurrence probability and the significance of a quiescence anomaly for a moderate earthquake should be much lower than for a large event. Of course, above phenomenon might have other causes, e.g. the relatively high threshold magnitude of Mmin ¼ 3:6 used in this study (we have to fix the calculation condition when we evaluate the seismic quiescence map) may reduce some
r0 ¼ 50 km t0 ¼ 0:5 year 0.5612
t0 ¼ 1 year t0 ¼ 1:5 years 0.3916
M $ 3.6
seismogenic information in other areas with lower threshold magnitudes (Fig. 1(b)). Thus, additional investigation of these moderate events would be interesting, but beyond the scope of this paper. We also investigated the spatial variations of the seismic quiescence pattern for different time windows. We found that the anomaly around the epicentre of the Nemuro Peninsula earthquake disappeared before the main shock. Fig. 6 shows the quiescence map from January 29, 1999– January 28, 2000. The previous quiescence anomaly around the epicentre as shown in Fig. 3 disappeared. Instead, another significant quiescence anomaly was observed around the Tokai area, where many earthquake swarms and volcanic eruptions occurred during June –September 2000. There might be some correlation between this quiescence anomaly and the above earthquake swarms in the Izu Island region. A significant anomalous activation zone on the order of several tens of kilometres up to 100 km appeared before the Nemuro Peninsula earthquake (Fig. 4). The epicenter was located almost in the centre of this anomalous zone. This result is consistent with those obtained for the Kobe earthquake and the earthquakes in Kamchatka, although their appearance time before the main shocks is somewhat different (Sobolev and Tyupkin, 1997, 1999; Sobolev et al., 2002; Huang et al., 2001). Therefore, the anomalous activation zone obtained by the algorithm in this study
Fig. 6. The seismic quiescence map for January 29, 1999–January 28, 2000. The quiescence anomaly around the epicentre of the Nemuro Peninsula earthquake disappeared during this period. The significant anomaly around the Tokai area may have some correlation with the earthquake swarms and volcanic eruptions in the Izu Island region during June–September, 2000.
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anomaly, which was detected around the epicentre during late 1996– mid-1997 (Fig. 4), disappeared. 5.3. Possible influence of aftershocks of the 1994 M ¼ 8:1 Kurile earthquake
Fig. 7. The temporal variation of the aftershocks of the 1994 M ¼ 8:1 Kurile earthquake. (a) all Aftershocks. (b) Aftershocks with M $ 3.6.
may be used to locate the future risk regions, although it is still difficult to estimate exactly the occurrence time of a future earthquake. It should be mentioned that another intense activation anomaly was detected around the Izu Islands of the Tokai area (Fig. 4). No single event with M . 6 occurred around this region after 1997. However, there are a lot of earthquake swarms during 1994 – 1998 (Japan Meteorological Agency, 1999). The time window of our calculation on the activation distribution is from mid-October 1996 to early July 1997. There were five swarms during this period: (1) October 15, 1996 – November 10, 1996, (2) December 17, 1996 – December 24, 1996, (3) March 3, 1997 – March 26, 1997, (4) June 20, 1997 – June 26, 1997, and (5) June 27, 1997 – July 15, 1997 (Japan Meteorological Agency, 1999). Note that we have not excluded clustered earthquakes from the catalogue in this study. This may be why a significant activation anomaly appeared in this region. However, there is another swarm from April 20, 1998 to June 2, 1998. Therefore, we cannot deny the possibility that the above anomalous activation zone in the Izu Island region has some correlation with this swarm. Further studies on this topic would be interesting, but beyond the scope of this paper. We also evaluated the spatial variations of the seismic activation pattern for other time windows. We found, from the spatial distribution of the seismic activation from January 29, 1999 –January 28, 2000, that the activation
Our above discussions indicated that the quiescence anomaly in 1996 is unlikely an artificial effect due to the selection of model parameters. The temporal and spatial evolution of seismicity changes also suggested that these changes would have reasonable correlations with the buildup leading to the M ¼ 6:8 Nemuro Peninsula earthquake. Unfortunately, a strong earthquake with M ¼ 8:1 occurred at Kurile on October 4, 1994. The epicentral distance between the Kurile and Nemuro Peninsula earthquakes is about 92 km. Because we took Rmax ¼ 2r0 ¼ 100 km in most of our calculations, the 1994 Kurile earthquake and its aftershocks would have some influence on our results. Therefore, it would be important to investigate whether or not the quiescence anomaly in 1996 is an artificial effect due to the elimination process of the aftershocks of the Kurile earthquake. First, we investigated the characteristics of the aftershocks in time and space. The temporal variation of the aftershocks showed an exponential decay (Fig. 7). Fig. 7(a) includes all aftershocks of the Kurile earthquake, while Fig. 7(b) shows the aftershocks with M $ 3.6. The aftershocks occurred from October 4, 1994 to October 13, 1996. These aftershocks were distributed within an ellipse zone as shown in Fig. 8. Because most of these aftershocks were located within our investigated time window and spatial zone, we should expect a detectable influence to the RTL results after taking these aftershocks into account. However, as mentioned before, aftershocks should not be included in the RTL calculations. Therefore, the above problem about the influence of aftershocks would be equivalent to the problem about the influence of aftershock elimination. For the above purpose, we evaluated the temporal evolution of earthquakes in a circular zone within 100 km of the epicentre in the Nemuro Peninsula earthquake. Fig. 9(a) and (b) plot the temporal variations of the cumulative number of all earthquakes and those with M $ 3.6 in the original JMA earthquake catalogue. One can see that the aftershocks of the Kurile earthquake significantly disturb the linear relationship of the cumulative number in the study zone. After applying the aftershock elimination technique to the JMA earthquake catalogue, we found that the aftershocks of the Kurile earthquakes were eliminated effectively as shown in Fig. 10. There is a clear linear relationship in the cumulative number in the study zone until the end of 1996 (Fig. 10(a)). The cumulative number after 1996 satisfies another linear relationship. The sudden change around the end of 1996 is due to the introduction of the data share project of the JMA seismic network and the local network in the Hokkaido region (Y. Ishikawa, private communication). If the process of aftershock elimination
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Fig. 8. The spatial distribution of the aftershocks of the 1994 M ¼ 8:1 Kurile earthquake. (a) All aftershocks. (b) Aftershocks with M $ 3.6.
leads to an artificial quiescence, one should detect such quiescence from the variation of the cumulative number, too. Besides, the artificial quiescence would appear during late 1994– late 1996. However, we did not detect the anomalous quiescence for the case of all main shocks (Fig. 10(a)). Even though we detected a significant quiescence for the case of main shocks with M $ 3.6 as shown in Fig. 10(b), the quiescence started at the end of 1993, before
Fig. 9. The temporal variation of the cumulative number of earthquakes in the study zone (within 100 km from the epicentre of the 2000 M ¼ 6:8 Nemuro Peninsula earthquake). The earthquake data are from the original JMA earthquake catalogue. (a) All events. (b) Events with M $ 3.6.
rather than after the Kurile event. Therefore, it is unlikely that the 1996 quiescence anomaly is an artificial effect due to the process of aftershock elimination. It should be mentioned that, although the JMA seismic network has been changed many times (Ohtake and Ishikawa, 1995; Y. Ishikawa, private communication), the effect due to the upgrade of the JMA network at the end of
Fig. 10. The temporal variation of the cumulative number of main shocks in the study zone (within 100 km from the epicentre of the 2000 M ¼ 6:8 Nemuro Peninsula earthquake). The earthquake data are from the earthquake catalogue after eliminating aftershocks from the original JMA catalogue. (a) All events. (b) Events with M $ 3.6.
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1996 was reduced significantly for the case of a complete catalogue of main shocks with M $ 3.6 (Fig. 10(b)). Because we applied the RTL algorithm to the above complete earthquake catalogue in this study, the 1996 anomaly would not be an artificial effect due to the upgrade of the JMA seismic network, either. The evolution of the seismicity changes of the 2000 Nemuro Peninsula earthquake revealed by the RTL algorithm showed similar characteristics to those obtained for some strong earthquakes in Russia (Sobolev and Tyupkin, 1997, 1999) and Japan (Huang and Nagao, 2002; Huang et al., 2001). This similarity of seismicity changes prior to strong earthquakes in different geological or tectonic regions may reflect the natural evolution of seismogenic process. Tests in Russia and Japan indicated that the RTL algorithm is an effective tool for revealing the seismic quiescence and activation patterns before strong earthquakes.
6. Conclusions We investigated seismicity changes of quiescence and activation patterns prior to the M ¼ 6:8 Nemuro Peninsula (Hokkaido) earthquake, January 28, 2000 by applying the RTL (Region– Time –Length) algorithm to the earthquake catalogue of the JMA. The main shock occurred 2.5 years after the completion of the quiescence and activation stages. After evaluating the influence of different model parameters, we conclude that the quiescence anomaly in 1996, which was revealed by the RTL algorithm, would have a reliable correlation with the buildup leading to the Nemuro Peninsula earthquake. The linear size of the anomalous quiescence zone is several times larger than the rupture length of this earthquake. An anomalous activation zone on the order of several tens of kilometres appeared around the epicentre. Most of the results in this study are consistent with those obtained for the Kobe earthquake and the strong earthquakes in Kamchatka. The regularity of the seismicity changes, which has been revealed by the RTL algorithm at different geological and/or seismo-tectonic regions, may provide the best opportunity for the application of this algorithm to seismic risk analysis.
Acknowledgments The authors thank the Japan Meteorological Agency (JMA) for providing the earthquake catalogue and V.B. Smirnov for offering the programs of aftershock elimination and completeness estimation of earthquake catalogue. The authors also thank Dr Y. Ishikawa and Prof. H.K. Gupta for useful comments. This study was supported by the Frontier Research System of RIKEN (The Institute of Physical and Chemical Research), Japan.
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References Bendat, J.S., Piersol, A.G., 2000. Random Data: Analysis and Measurement Procedures, Wiley, New York. Habermann, R.E., 1987. Man-made changes in seismicity rates. Bulletin of the Seismological Society of America 77, 141–159. Habermann, R.E., 1988. Precursory seismic quiescence: past, present, and future. Pure Applied Geophysics 126, 279– 318. Habermann, R.E., 1991. Seismicity rate variations and systematic changes in magnitudes in teleseismic catalogs. Tectonophysics 193, 277 – 289. Hamilton, T., McCloskey, J., 1997. Breakdown in power-law scaling in an analogue model of earthquake rupture and stick-slip. Geophysical Research Letters 24, 465–468. Huang, Q., Sobolev, G.A., 2001. Seismic quiescence prior to the 2000 M ¼ 6.8 Nemuro Peninsula earthquake. Proceedings of the Japan Academy 77, 1–6. Huang, Q., Sobolev, G.A., Nagao, T., 2001. Characteristics of the seismic quiescence and activation patterns before the M ¼ 7.2 Kobe earthquake, January 17, 1995. Tectonophysics 337, 99–116. Huang, Q., Nagao, T., 2002. Seismic quiescence before the 2000 M ¼ 7.3 Tottori earthquake. Geophysical Research Letters 29 in press. Huang, Q., Oncel, A.O., Sobolev, G.A., 2002. Precursory seismicity changes associated with the Mw ¼ 7:4 Izmit (Turkey) earthquake, August 17, 1999. Geophysical Journal International (in press). Ikeya, M., Huang, Q., 1997. Earthquake frequency and moment magnitude relations for mainshocks, foreshocks and aftershocks: Theoretical b values. Episodes 20, 181–184. Japan Meteorological Agency, 1999. Seismicity in Izu Peninsula and its surrounding area. Report of the Coordinating Committee for Earthquake Prediction, Japan 61, 1–4 (in Japanese). Katsumata, K., Kasahara, M., 1999. Precursory seismic quiescence before the 1994 Kurile earthquake (Mw ¼ 8.3) revealed by three independent seismic catalogs. Pure Applied Geophysics 155, 443–470. Kasahara, K., 1981. Earthquake Mechanics, Cambridge University Press, Cambridge. Kossobokov, V.G., Keilis-Borok, V.I., 1990. Localization of intermediateterm earthquake prediction. Journal of Geophysical Research 95, 763– 772. Kuwayama, T., 1999. Earthquake detectivity after establishing the seismic network for Tsunami forecast. Technical Report of Sapporo Local Center 133, 1–8 (in Japanese). Main, I.G., 1992. Earthquake scaling. Nature 357, 27–28. Molchan, G.M., Dmitrieva, O.E., 1991. Identification of aftershocks: review and new approaches. Computative Seismology 24, 19 –50 (in Russian). Ohtake, M., Ishikawa, Y., 1995. Seismic observation networks in Japan. Journal of Physics of the Earth 43, 563–584. Smirnov, V.B., 1998. Earthquake catalogs: evaluation of data completeness. Volcanology and Seismology 19, 433–446. Sobolev, G.A., 1995. Fundamental of Earthquake Prediction, Electromagnetic Research Centre, Moscow. Sobolev, G.A., Tyupkin, Y.S., 1997. Low-seismicity precursors of large earthquakes in Kamchatka. Volcanology and Seismology 18, 433–446. Sobolev, G.A., Tyupkin, Y.S., 1999. Precursory phases, seismicity precursors, and earthquake prediction in Kamchatka. Volcanology and Seismology 20, 615–627. Sobolev, G.A., Huang, Q., Nagao, T., 2002. Phases of earthquake’s preparation and by chance test of seismic quiescence anomaly. Journal of Geodynamics 33, 413– 424. Takanami, T., Sacks, I.S., Snoke, J.A., Motoya, Y., Ichiyanagi, M., 1996. Seismic quiescence before the Hokkaido–Toho–Oki earthquake of October 4, 1994. Journal of Physics of the Earth 44, 193–203. Taylor, D.W.A., Snoke, J.A., Sacks, I.S., Takanami, T., 1991. Seismic quiescence before the Urakawa – Oki earthquake. Bulletin of the Seismological Society of America 81, 1255–1271.
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Wiemer, S., Wyss, M., 1994. Seismic quiescence before the Landers (M ¼ 7.5) and Big Bear (M ¼ 6.5) 1992 earthquakes. Bulletin of the Seismological Society of America 84, 900– 916. Wyss, M., Habermann, R.E., 1988. Precursory seismic quiescence. Pure Applied Geophysics 126, 319–332.
Wyss, M., Habermann, R.E., Griesser, J.C., 1984. Seismic quiescence and asperities in the Tonga – Kermadec Arc. Journal of Geophysical Research 89, 9293–9304.
Precursory seismicity changes associated with the Nemuro Peninsula earthquake, January 28, 2000 Qinghua Huanga,*, Guennadi A. Sobolevb a Department of Geophysics, Peking University, Beijing 100871, People’s Republic of China United Institute of Physics of the Earth, Russian Academy of Sciences, 10, B. Gruzinskaya, Moscow 123810, Russian Federation
b
Received 28 March 2001; revised 1 November 2001; accepted 8 March 2002
Abstract We investigated the characteristics of the precursory seismicity changes associated with the M ¼ 6:8 Nemuro Peninsula earthquake, January 28, 2000 by applying the RTL (Region – Time– Length) algorithm to the earthquake catalogue of the Japan Meteorological Agency (JMA). The RTL parameters at the epicentre indicated that a seismic quiescence started in 1995 and reached its minimum during October 1996. An activation stage with a duration of about eight months followed. Our detailed investigations indicated that the RTL anomaly around 1996 was not an artificial effect due to the changes of the model parameters, the upgrade of the JMA seismic network or the process of aftershock elimination. The spatial distribution of quiescence in 1996 revealed a significant anomaly in a broad region around the epicentre of the Nemuro Peninsula earthquake. Following the quiescence stage, an activation zone, which was on the order of the rupture length of this earthquake, was obtained around the epicentre during October 1996– July 1997. The consistency of the rupture region and the anomalous activation zone that appeared after the seismic quiescence stage may provide useful information for determining the future risk zones. This study may strengthen the understanding of the seismogenic process of strong earthquakes. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Seismicity; Quiescence; Activation; Precursor; Seismogenic process; Nemuro Peninsula earthquake
1. Introduction The laboratory rock experiments indicated that the acoustic emission may accelerate as the loading increases. However, the number of relatively weak signals tends to decrease after the loading reaches the maximum, because small cracks are no long generated due to the partial reduction of stress. During the final stage before the main rupture, acoustic activity increases again. In other words, acoustic emission can experience the stages of quiescence and activation prior to the main rupture (Sobolev, 1995). The field seismological observations also indicated the existence of quiescence and activation stages of seismicity in the rupture region of strong earthquakes. However, the primary attention was concentrated on seismic quiescence, which may be one of the most promising intermediate-term precursors (Wyss et al., 1984; Wyss and Habermann, 1988; Habermann, 1988; Kossobokov and Keilis-Borok, 1990; Taylor et al., 1991; Wiemer and Wyss, 1994; Takanami * Corresponding author. Tel.: þ86-10-6275-4057; fax: þ 86-10-62757860. E-mail address: [email protected] (Q. Huang).
et al., 1996; Katsumata and Kasahara, 1999). Many precursory seismic quiescences have been reported in and around focal areas a few years before earthquakes, e.g. earthquakes of Tonga – Kermadec (Wyss et al., 1984), Hokkaido (Taylor et al., 1991), Landers (Wiemer and Wyss, 1994), Kurile (Takanami et al., 1996; Katsumata and Kasahara, 1999), etc. There are several methods that can reveal the precursory seismic quiescence based on earthquake catalogues (Wyss and Habermann, 1988; Kossobokov and Keilis-Borok, 1990). However, few of these approaches deal with the seismic activation. A statistic method was developed recently to investigate the characteristics of seismicity changes, including both quiescence and activation patterns (Sobolev and Tyupkin, 1997, 1999; Huang and Sobolev, 2001; Huang et al., 2001). This method is called the RTL algorithm, where the name RTL comes from R: Region (epicentral distance), T: Time interval, and L: Length (rupture size). The RTL is a parameter reflecting the combination of three functions of distance, time and rupture length, respectively. All three parameters (time, place and magnitude) of earthquakes are taken into account with
1367-9120/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 7 - 9 1 2 0 ( 0 2 ) 0 0 0 3 2 - 9
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weighted coefficient in this algorithm. We will introduce briefly this algorithm in the Section 2. The RTL algorithm was initially tested using the earthquake catalogue of Kamchatka, Russia (Sobolev and Tyupkin, 1997, 1999). It was found that the strong earthquakes in Kamchatka with magnitude M . 7 had been preceded by seismic quiescence followed by activation. The seismic quiescence stage started 1.5– 3.5 years before the main shock and lasted 1 – 2.5 years. The subsequent stage of seismic activation had a duration varying from 6 months to 1.5 years. The linear dimension of the quiescence zone was several times larger than the rupture length of the main shock, while the activation zone was on the order of several tens of kilometres, comparable to the rupture length. Similar variation of seismicity changes of the 1995 M ¼ 7:2 Kobe earthquake was obtained recently by using the earthquake catalogue of the Japan Meteorological Agency (JMA) (Huang et al., 2001). We also investigated the seismic quiescence associated with the M ¼ 6:8 Nemuro Peninsula (Hokkaido) earthquake, which occurred at 146.71E longitude, 42.98N latitude, 23 h 21 min JST (Japan Standard Time) on January 28, 2000 with a focal depth of about 56 km, by applying the RTL algorithm to the JMA earthquake catalogue. The characteristics of seismic quiescence associated with this earthquake, which are revealed by both the RTL algorithm and the temporal variation of earthquake number in the epicentral zone, indicated that a significant quiescence appeared 3 years before the main shock and the anomalous quiescence zone was over 200 km long (Huang and Sobolev, 2001). Unfortunately, we investigated neither the influence of model parameters to the results nor the characteristics of seismic activation pattern in that short paper. In this study, after eliminating aftershocks from the original JMA earthquake catalogue and estimating the completeness of this catalogue, we investigate the temporal variation of seismicity changes and the spatial distribution of seismic quiescence associated with the Nemuro Peninsula earthquake using the RTL algorithm. We evaluate the influence of model parameters to the results, because seismicity changes may be caused either by the real changes associated with the development of earthquakes or by manmade changes (Habermann, 1987, 1991). We also discuss the characteristics of seismic activation and evaluate the spatial distribution of the seismic activation using another independent algorithm (Huang et al., 2001), which takes the increase of the effective rupture area into account and is to be introduced briefly in Section 2.
2. Methods In order to investigate the evolution characteristics of the seismic quiescence and activation patterns, we eliminate aftershocks from the JMA earthquake catalogue using the
algorithm developed by Molchan and Dmitrieva (1991). Because earthquakes may not be reported homogeneously due to the inhomogeneous distribution of seismic stations in Japan, an island country, we also estimate the completeness of this catalogue based on the power law of frequency– magnitude before applying the RTL algorithm. We will introduce briefly the principles of aftershock elimination and the completeness analysis in next section. 2.1. RTL algorithm The principle of the RTL algorithm has been introduced previously (e.g. Sobolev and Tyupkin, 1997; Huang et al., 2001). For the readers’ convenience and clear discussion, we repeat the main ideas of this algorithm in this section. In the RTL algorithm, we defined three functions of distance, time and rupture length as "
# ri Rðx; y; z; tÞ ¼ exp 2 2 Rtr ðx; y; z; tÞ; r0 i¼1 " # n X t 2 ti Tðx; y; z; tÞ ¼ exp 2 2 Ttr ðx; y; z; tÞ; t0 i¼1 " # n X li Lðx; y; z; tÞ ¼ 2 Ltr ðx; y; z; tÞ ri i¼1 n X
ð1Þ
where ðx; y; z; tÞ indicates the investigated position and time; li and ti are the rupture length and the occurrence time of the ith earthquake; ri is the distance from the position of ðx; y; zÞ to the epicentre of the ith event; n is the number of events with their three parameters satisfying some criteria, e.g. Mi $ Mmin (Mi is the magnitude of the ith earthquake and Mmin is the cut-off magnitude ensuring the completeness of the earthquake catalogue), ri # Rmax ¼ 2r0 and ðt 2 ti Þ # Tmax ¼ 2t0 (Rmax and Tmax are threshold distance and timespan, r0 and t0 are characteristic distance and time-span), dU # di # dL (di is the focal depth of the ith earthquake, dU and dL are the upper and lower cut-off depths); Rtr ðx; y; z; tÞ; Ttr ðx; y; z; tÞ and Ltr ðx; y; z; tÞ are the trends (background values) of Rðx; y; z; tÞ; Tðx; y; z; tÞ and Lðx; y; z; tÞ; respectively. One can see from Eq. (1) that Rðx; y; z; tÞ; Tðx; y; z; tÞ and Lðx; y; z; tÞ are three dimensionless functions. They are further normalised to their standard deviations sR, sT, and sL, respectively. The product of the above three functions is calculated as the RTL parameter, which describes the deviation from the background level of seismicity and is in units of standard deviation, s ¼ sR sT sL : A decrease of RTL means a decrease in seismicity compared to the background rate around the investigated location, i.e. a decrease of RTL represents a seismic quiescence. A recovery stage from the quiescence to the background level can be considered as an activation stage of seismicity. The rupture dimension li is given by an empirical relation
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with the magnitude Mi (Kasahara, 1981), log li ðkmÞ ¼ 0:5 Mi 2 1:8
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We defined the effective rupture area Seff as, ð2Þ
The rupture length of the M ¼ 6:8 Nemuro Peninsula earthquake estimated by Eq. (2) is on the order of several tens of kilometres. After taking into account the reports of seismic quiescence around the rupture zone and our previous experiences in Kamchatka and Kobe (Sobolev and Tyupkin, 1997, 1999; Huang et al., 2001), we assumed the characteristic distance r0 ¼ 50 km; i.e. the threshold distance Rmax ¼ 2r0 ¼ 100 km: We also assumed the characteristic time-span t0 ¼ 1 year; i.e. the threshold time-span Tmax ¼ 2t0 ¼ 2 years; based on our previous experiences (Sobolev and Tyupkin, 1997, 1999; Huang et al., 2001) and the fact that duration of seismic quiescence is generally on the order of one year. We make most of our calculations by taking r0 ¼ 50 km and t0 ¼ 1 year; although we also make the calculations by changing the characteristic distance r0 and the characteristic time-span t0 so that we can estimate the possible influence of these parameters to our results. Another criterion of the focal depth di satisfies di # dL ¼ 100 km (no upper cut-off focal depth dU), although we also made the calculations for the cases of (1) all di (i.e. no cut-off focal depths dU and dL), (2) dU ¼ 30 km # di # dL ¼ 100 km; and (3) di $ dU ¼ 30 km (no lower cut-off focal depth dL) in this study. Note that 30 km is a typical threshold depth for crust earthquakes. Therefore, the criterion of the upper cut-off focal depth dU ¼ 30 km may exclude or reduce the possible disturbance of crust seismicity to the Nemuro Peninsula earthquake, which occurred in the slab at a focal depth of about 56 km. Although the selection of the criterion of focal depth is somewhat arbitrary, to be discussed later, this criterion has little influence on our results. If we assume that the characteristics of the seismic quiescence at a certain location can be quantified by the minimum RTL value within the investigated period of time DT at this location, we can obtain the spatial distribution of the seismic quiescence by changing the calculated location. In this study, we change the location at a step of 0.258 along the direction of either longitude or latitude each time and repeat above calculations. It should be noted that the calculation of the RTL parameter at each location is made through the whole background period Tbk, although we take into account the minimum RTL value only within the study interval DT (Huang and Sobolev, 2001). 2.2. Increase of effective rupture area We developed another algorithm, which is independent of the RTL algorithm, to reveal the spatial distribution of the anomaly of seismic activation. We introduced this technique previously in Huang et al. (2001). We summarize the principle of this approach here for the readers’ convenience.
Seff ¼
1 X Si 1 X ðMi 2Mref Þ ¼ 10 ; DT i Sref DT i
ð3Þ
where DT is any time interval of interest and Sref is the rupture area corresponding to a reference magnitude Mref. In this study, DSeff ¼ Seff 2 Sbk is chosen as a parameter of the seismic activation, where Seff is the effective rupture area in an investigated activation period DT and Sbk is the averaged background one, which can be calculated by Eq. (3) after replacing DT by the period of background Tbk.
3. Data analysis We analyse the JMA earthquake catalogue before applying the RTL algorithm. Because the manual operation of data in the JMA earthquake catalogue has been replaced by the computer operation since 1961, we choose the earthquake catalogue from January 1, 1961 to March 31, 2000 in this study. For the first step, we eliminate aftershocks from the above JMA catalogue using the program written by V.B. Smirnov on the basis of the algorithm of Molchan and Dmitrieva (1991). The principle of separating aftershocks from the rest of events, which are called background, is based on the comparison of their functions and their distribution in time and space. Background events are assumed to be distributed evenly. Aftershocks are assumed to have a bell-shaped (Gaussian) distribution on the plane (only earthquakes epicenters are taken into account) and are distributed in time according to the Omori law. The algorithm efficiency was tested by Smirnov who compared aftershock sequences obtained from regional catalogues with the use of this algorithm and aftershocks catalogues compiled independently by other authors. The discrepancy of the number of aftershocks obtained is approximately 5%. After eliminating aftershocks from the JMA earthquake catalogue, we evaluate the completeness of this catalogue using the algorithm developed by Smirnov (1998). The evaluation of the completeness of the earthquake catalogue is based on the histograms of earthquake distribution by magnitude. The problem is to find the minimal value Mmin with which the recurrence plot is linear in the range of M $ Mmin. One can find the detailed introduction of this technique in Smirnov (1998), Huang et al. (2001). This algorithm was realised by Smirnov as a set of programs, which allow making assessments of the representative character of earthquake catalogues in varying time intervals and in desired spatial cells. The application of the algorithm of the completeness assessment to a number of regional catalogues testified to its efficiency (Smirnov, 1998).
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Fig. 1. The completeness analysis using the JMA earthquake catalogue from January 1, 1961 to March 31, 2000. (a) Temporal variation of the minimum magnitude, Mmin around the epicentre of the M ¼ 6:8 Nemuro Peninsula (Hokkaido) earthquake, January 28, 2000. (b) Spatial distribution of Mmin during January 1, 1984–March 31, 2000. The shadowed zone indicates the investigated region in this study.
Fig. 1(a) plots the temporal variation of the cut-off magnitude Mmin in a circular zone with a radius of Rmax ¼ 100 km; and a center at (146.71E, 42.98N), the epicenter of the Nemuro Peninsula earthquake. It showed that Mmin decreased with time. This is due to the improvements/ changes of the observing instruments or the processing systems of the JMA seismic stations. We found that the threshold magnitude Mmin after 1995 is less than 3.0, which is consistent with the detectability of the JMA seismic network in the Hokkaido area (Kuwayama, 1999). However, the remaining period up to January 28, 2000 is too short for calculating the reliable background variations using the RTL algorithm. We obtained a magnitude of 3.6 as the threshold magnitude of the earthquakes in the study zone after January 1, 1984. The spatial distribution of the threshold magnitude Mmin was also estimated as shown in Fig. 1(b) by using the JMA earthquake catalogue for January 1, 1984 –March 31, 2000. We found that Mmin ¼ 3:6 can cover most parts of the Japanese islands including the epicentral zone. Therefore, we use all events with the magnitude M $ 3.6 in the JMA earthquake catalogue for January 1, 1984– March 31, 2000 in this study. It should be mentioned that we introduced another
Fig. 2. (a) The RTL parameter at the epicentre of the 2000 Nemuro Peninsula earthquake varies with time. A significant quiescence appeared in 1996, followed by an activation pattern. The arrow indicates the occurrence time of the main shock. (b) The temporal variations of three functions R(146.71E, 42.98N, 0, t ), T(146.71E, 42.98N, 0, t ), and L(146.71E, 42.98N, 0, t ) at the epicentre. They decreased simultaneously during 1995– 1996.
criterion of maximum threshold magnitude Mmax ¼ 5:5 in our previous study on the seismic quiescence associated with the Nemuro Peninsula earthquake (Huang and Sobolev, 2001), because we want to avoid the possible disturbance to background seismicity of large events due to the possible deviation of the power law of frequencymagnitude at large magnitudes (Main, 1992; Hamilton and McCloskey, 1997; Ikeya and Huang, 1997). However, there is a potential danger that artificial changes can be generated by introducing two cut-off magnitudes Mmin and Mmax (Habermann, 1987, 1991). In order to investigate whether or not our previous results of seismic quiescence pattern prior to the Nemuro Peninsula earthquake is an artificial effect, we excluded the maximum threshold magnitude Mmax in our current study.
4. Results We calculated the temporal variations of the RTL parameter (in units of standard deviation, s ) at the epicenter of the M ¼ 6:8 Nemuro Peninsula earthquake based on the earthquakes in the circular zone with a radius of Rmax in the JMA earthquake catalogue. The spatial distributions of seismic quiescence and activation were also investigated using the methods introduced before. Fig. 2(a) shows the temporal variation of the RTL
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Fig. 3. The spatial distribution of seismic quiescence in 1996. A significant seismic quiescence appeared three years before the Nemuro Peninsula earthquake. The anomalous zone reached 200 km. The scale on the right corresponds to the RTL value in units of standard deviation. The black star represents the epicentre of the Nemuro Peninsula earthquake (146.71E, 42.98N). There occurred another 4 moderate earthquakes with M $ 6.0 during January 1, 1997–March 31, 2000. The grey stars show the epicentres of these 4 events.
parameter at longitude 146.71E, latitude 42.98N, the epicenter of the Nemuro Peninsula earthquake. This RTL curve is obtained on the basis of the RTL algorithm, which was introduced in the previous section. All events used in the above calculations satisfy the following criteria: magnitude Mi $ 3.6, focal depth di # 100 km, epicentral distance ri # Rmax ¼ 100 km; and time interval ðt 2 ti Þ # Tmax ¼ 2 years: An obvious seismic quiescence was obtained from this RTL curve, followed by a significant activation stage. The quiescence started in 1995 and reached its minimum in October, 1996. The largest deviation from the background was about 26s. This result is consistent with the one that we obtained for the case of introducing a maximum threshold magnitude Mmax ¼ 5:5 in the previous study (Huang and Sobolev, 2001). We will discuss the correlation between them in the next section. The temporal variations of three functions, R(146.71E, 42.98N, 0, t ), T(146.71E, 42.98N, 0, t ), and L(146.71E, 42.98N, 0, t ) are shown in Fig. 2(b). These three functions deviated consistently from their background levels during 1995 – 1996. The spatial distribution of the seismic quiescence is plotted in Fig. 3. We calculated the quiescence distribution during January 1 –December 31, 1996, because a significant quiescence stage was revealed during this period by the RTL parameter (Fig. 2(a)). A significant quiescence anomaly appeared around the epicenter of the Nemuro Peninsula earthquake. The linear dimension of this anomalous zone was over 200 km. Fig. 4 shows the spatial distribution of the seismic activation (parameter DSeff) during an interval of 0.7 year until mid-1997 (i.e. from mid-October 1996 to early July
1997), because an obvious activation pattern was detected during this period by the RTL parameter (Fig. 2(a)). Compared to the anomalous quiescence zone, a relative smaller zone of activation anomaly on the order of several tens of kilometers up to 100 km was obtained around the epicentre of the main shock.
5. Discussion Obviously, the completeness of an earthquake catalogue is important for an investigation of seismicity (Wyss and Habermann, 1988). That’s why we have estimated the threshold magnitude of the JMA catalogue before applying our algorithm. In this study, we focused our discussions on the land and its coast regions (i.e. the shadowed parts as shown in Fig. 1(b)). There may be a contradiction between the completeness of a catalogue and the reliability of statistics, because the larger the threshold magnitude Mmin, the fewer the events within a certain interval. One can see clearly from Fig. 1(b) that the Mmin is spatially inhomogeneous. The threshold magnitude Mmin ¼ 3; which was used in our previous analysis on the Kobe earthquake, is sufficient for most parts of Japan, including the land area of Hokkaido (Fig. 1(b), also see Kuwayama (1999)). However, Mmin around the epicentral zone of the Nemuro Peninsula earthquake is 3.6. That’s why we used Mmin ¼ 3:6 in this study. As mentioned before, the selections of some model parameters are somewhat empirical and arbitrary. Therefore, we investigate the possible influence of these parameters to our previous result so that we can judge
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Fig. 4. The spatial distribution of seismic activation during mid-October, 1996–early July, 1997. The epicentre of the Nemuro Peninsula earthquake (black star) located almost in the centre of this anomalous zone, which is on the order of several tens of kilometres. Another anomalous activation zone in Tokai area may have some relation with the earthquake swarms in the Izu Island region.
whether the RTL anomaly in 1996 (Fig. 2(a)) is an artificial effect of the influence of model parameters or a real precursor of the Nemuro Peninsula earthquake. In addition, we discuss the evolutionary characteristics of seismic quiescence and activation patterns in this section. 5.1. Influence of model parameters on the RTL anomaly The seismic quiescence of the Nemuro Peninsula earthquake started in 1995 and lasted until October 1996, about 3 years before the main shock (Fig. 2(a)). The time duration was about 1.5 years and the largest deviation from the background level was 2 25.94s. The distance function R(146.71E, 42.98N, 0, t ), time function T(146.71E, 42.98N, 0, t ), and rupture length L(146.71E, 42.98N, 0, t ) decreased consistently from their background levels during 1995– 1996 (Fig. 2(b)). As mentioned in the previous section, these three functions are normalised by their standard deviations sR, sT, and sL, respectively. It would be possible to estimate roughly the probability of these variations. The values of these three functions at the time when the RTL parameter has the largest deviation in October 1996 are 2 2.71sR, 2 2.46sT, and 2 3.88sL, respectively. After assuming a normal distribution, the occurrence probabilities of the above anomalies are 0.00672, 0.01390, and 0.0001,
respectively. If these three functions are completely independent, the occurrence probability of the above RTL anomaly in October 1996 would be as low as 0.93 £ 1028. Therefore, the RTL anomaly in October 1996 is a significant anomaly compared to the background level of seismicity. As mentioned in the previous section, we introduced in this study some ‘free’ parameters such as cut-off focal depths dU and dL, characteristic distance r0, and characteristic time-span t0. We also introduced another criterion of maximum cut-off magnitude Mmax in our previous study (Huang and Sobolev, 2001). We chose these parameters as dL ¼ 100 km; r0 ¼ 50 km ðRmax ¼ 2r0 ¼ 100 kmÞ and t0 ¼ 1 year ðTmax ¼ 2t0 ¼ 2 yearsÞ in our current calculations. Although the criteria of dU and dL are somewhat arbitrary, the criteria of r0 and t0 are chosen not only on the basis of our previous experiences, but also on the basis of the reports of rupture region and duration time of seismic quiescence. In order to investigate the possible influence on the results, we repeat our calculations by changing these parameters. For simplification, we calculate the RTL curves after changing only one parameter in each case. We also evaluate the correlation coefficients. First, we make a comparison between the RTL parameter in Fig. 2(a) and the one in our previous study as a criterion of maximum cut-off magnitude Mmax ¼ 5:5 (Huang and
Table 1 Correlation of RTL values between different model parameters of cut-off magnitude and cut-off focal depths dU and dL (threshold distance Rmax ¼ 2r0 and threshold time interval Tmax ¼ 2t0 ) Cases
Correlation coefficient
A B
dL ¼ 100 km M # 5.5
r0 ¼ 50 km No dU, dL
0.7654
0.9998
t0 ¼ 1 year dU ¼ 30 km dL ¼ 100 km 0.8326
M $ 3.6 dU ¼ 30 km No dL 0.8287
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Fig. 5. The RTL parameter at the epicentre for the case of dU ¼ 30 km and dL ¼ 100 km:
Sobolev, 2001). In both cases, we obtained a significant quiescence anomaly during 1995 – 1996 with a minimum in October 1996. The correlation coefficient between the above two cases is 0.7654 (see Table 1). The statistic test based on a model of linear correlation analysis indicated that significant correlation exists for the above two cases at a significant level of 0.05 (Bendat and Piersol, 2000). Next, we discuss the influence of the cut-off focal depths dU and dL, which are somewhat arbitrary in this study. We re-calculated the RTL parameter for the following three cases: (1) without cut-off focal depths dU and dL; (2) introducing the upper cut-off depth dU ¼ 30 km (this is a typical threshold depth of earthquakes in crust) and keeping the lower cut-off focal depth dL ¼ 100 km; (3) introducing the upper cut-off depth dU ¼ 30 km and excluding the lower cut-off depth dL. In all of the above three cases, we keep other parameters unchanged (i.e. M $ 3.6, r0 ¼ 50 km and t0 ¼ 1 year). We found that there is little influence on the RTL parameter from the changes of the cut-off focal depths dU and dL (e.g. the RTL curve of case (2) is plotted in Fig. 5). We also estimated the correlation coefficient between the result in Fig. 2(a) and one of these three cases. We obtained the correlation coefficients of 0.9998, 0.8326 and 0.8287, respectively (see Table 1). The statistic test indicated that significant correlation exists at a significant level of 0.05. The changes in the cut-off focal depths dU and dL do not significantly influence our results. We also investigate the influence of the characteristic distance r0 and the characteristic time-span t0 after taking the cut-off focal depths dU ¼ 30 km and dL ¼ 100 km: The temporal variations of the RTL parameter in the case of r0 ¼ 25 km ðRmax ¼ 2r0 ¼ 50 kmÞ and r0 ¼ 75 km ðRmax ¼ 2r0 ¼ 150 kmÞ are quite similar to that in Fig. 5 (we keep other parameters unchanged, i.e. M $ 3.6, dU ¼ 30 km; dL ¼ 100 km and t0 ¼ 1 year). The correlation coefficients are 0.9189 and 0.9779, respectively for the cases between r0 ¼ 25 km and r0 ¼ 50 km and between r0 ¼ 75 km and r0 ¼ 50 km (see Table 2). The statistic test indicated that significant correlation exists at a significance level of 0.05 for the above two cases. We repeat the calculations by changing t0 from 1 year to 0.5 year ðTmax ¼ 2t0 ¼ 1 yearÞ and 1.5 years ðTmax ¼ 2t0 ¼
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3 yearsÞ and keeping other parameters unchanged (i.e. M $ 3.6, dU ¼ 30 km; dL ¼ 100 km and r0 ¼ 50 km). We obtained in both cases the similar characteristics of seismicity changes to that in Fig. 5. The correlation coefficients are 0.5612 and 0.3916, respectively for the above two cases (see Table 2). Both cases show significant correlation with our previous result (Fig. 5) at a significance level of 0.05. Above analyses indicate that the RTL result we obtained previously in Fig. 2(a) is unlikely an artificial effect due to the selection of some model parameters, dU, dL, r0 and t0. Thus, we can conclude reasonably that the RTL anomaly, which appeared in 1996, would correlate with the preparation stage of the M ¼ 6:8 Nemuro Peninsula earthquake. 5.2. Evolution of seismic quiescence and activation patterns The seismic quiescence of the Nemuro Peninsula earthquake started in 1995 and lasted until the end of 1996, three years before the main shock (Fig. 2(a)). The duration time was about 1.5 years. The activation pattern appeared at the end of 1996 with a duration time of about 0.7 year. In the case of the Kobe earthquake, the quiescence started two years before the main shock with a duration of about 1.5 years. The followed activation lasted 0.7 year. In contrast to the Kobe case, the Nemuro Peninsula earthquake did not occur immediately after the activation stage, but after a delay of 2.5 years. A similar time delay after the activation stage was also reported in Kamchatka (Sobolev and Tyupkin, 1999). A M ¼ 7:7 earthquake occurred in the northern Gulf of Kamchatka on December 5, 1997, about 1.5 years after the end of the activation stage. Generally, a future earthquake most probably occurs after the establishment of the successive quiescence and activation patterns. Therefore, seismicity changes may provide useful information for earthquake prediction. However, the existence of the time delay, which was reported in this study and the previous studies in Kamchatka and Izmit (Sobolev and Tyupkin, 1999; Huang et al., 2002), makes it difficult to determine the occurrence time of a future event within the accuracy required in short-term prediction. It may be that the investigated candidate zone has already been in a critical state. Therefore, macrofractures around this zone may be triggered by the changes of any local, regional or global factors such as meteorological or cosmic phenomena, the occurrence of neighbour or remote earthquakes, etc. The spatial distribution of seismic quiescence of the Nemuro Peninsula earthquake indicated that the length of this anomalous zone was over 200 km (Fig. 3), several times larger than the rupture length, which is usually on the order of several tens of kilometres. In the case of the M ¼ 7:2 Kobe earthquake, the anomalous quiescence zone was over 300 km (Huang et al., 2001), larger than that of the M ¼ 6:8 Nemuro Peninsula earthquake. Unfortunately, the
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Table 2 Correlation of RTL values between different model parameters of threshold distance Rmax and time-span Tmax (Rmax ¼ 2r0 and Tmax ¼ 2t0 ) Cases Correlation coefficient
A B
dU ¼ 30 km r0 ¼ 25 km 0.9189
dL ¼ 100 km r0 ¼ 75 km 0.9779
correlation between the size of the anomalous quiescence zone and the magnitude of the following event is still unclear, because the geological structure or seismo-tectonic background in the Kobe and Hokkaido regions would be somewhat different. We obtained some small anomalous quiescence zones during the same period (Fig. 3). There occurred another 4 earthquakes with M $ 6.0 and d # 100 km in the displayed zone (see Fig. 1(b)) during January 1, 1997 –March 31, 2000. We marked these earthquakes in Fig. 3. Two moderate earthquakes with M $ 6.0 occurred around the small anomalous zones in the northeastern part of Honshu and southern Kyushu, respectively. However, we are not sure whether these small zones have reasonable correlations with the nearby earthquakes, because we do not have experience in revealing the seismogenic processes of such moderate earthquakes using the RTL algorithm. It should be mentioned that no quiescence anomalies occurred around the other two events. Although we cannot understand this phenomenon very well at the current stage, we can make a reasonable deduction that the occurrence probability and the significance of a quiescence anomaly for a moderate earthquake should be much lower than for a large event. Of course, above phenomenon might have other causes, e.g. the relatively high threshold magnitude of Mmin ¼ 3:6 used in this study (we have to fix the calculation condition when we evaluate the seismic quiescence map) may reduce some
r0 ¼ 50 km t0 ¼ 0:5 year 0.5612
t0 ¼ 1 year t0 ¼ 1:5 years 0.3916
M $ 3.6
seismogenic information in other areas with lower threshold magnitudes (Fig. 1(b)). Thus, additional investigation of these moderate events would be interesting, but beyond the scope of this paper. We also investigated the spatial variations of the seismic quiescence pattern for different time windows. We found that the anomaly around the epicentre of the Nemuro Peninsula earthquake disappeared before the main shock. Fig. 6 shows the quiescence map from January 29, 1999– January 28, 2000. The previous quiescence anomaly around the epicentre as shown in Fig. 3 disappeared. Instead, another significant quiescence anomaly was observed around the Tokai area, where many earthquake swarms and volcanic eruptions occurred during June –September 2000. There might be some correlation between this quiescence anomaly and the above earthquake swarms in the Izu Island region. A significant anomalous activation zone on the order of several tens of kilometres up to 100 km appeared before the Nemuro Peninsula earthquake (Fig. 4). The epicenter was located almost in the centre of this anomalous zone. This result is consistent with those obtained for the Kobe earthquake and the earthquakes in Kamchatka, although their appearance time before the main shocks is somewhat different (Sobolev and Tyupkin, 1997, 1999; Sobolev et al., 2002; Huang et al., 2001). Therefore, the anomalous activation zone obtained by the algorithm in this study
Fig. 6. The seismic quiescence map for January 29, 1999–January 28, 2000. The quiescence anomaly around the epicentre of the Nemuro Peninsula earthquake disappeared during this period. The significant anomaly around the Tokai area may have some correlation with the earthquake swarms and volcanic eruptions in the Izu Island region during June–September, 2000.
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anomaly, which was detected around the epicentre during late 1996– mid-1997 (Fig. 4), disappeared. 5.3. Possible influence of aftershocks of the 1994 M ¼ 8:1 Kurile earthquake
Fig. 7. The temporal variation of the aftershocks of the 1994 M ¼ 8:1 Kurile earthquake. (a) all Aftershocks. (b) Aftershocks with M $ 3.6.
may be used to locate the future risk regions, although it is still difficult to estimate exactly the occurrence time of a future earthquake. It should be mentioned that another intense activation anomaly was detected around the Izu Islands of the Tokai area (Fig. 4). No single event with M . 6 occurred around this region after 1997. However, there are a lot of earthquake swarms during 1994 – 1998 (Japan Meteorological Agency, 1999). The time window of our calculation on the activation distribution is from mid-October 1996 to early July 1997. There were five swarms during this period: (1) October 15, 1996 – November 10, 1996, (2) December 17, 1996 – December 24, 1996, (3) March 3, 1997 – March 26, 1997, (4) June 20, 1997 – June 26, 1997, and (5) June 27, 1997 – July 15, 1997 (Japan Meteorological Agency, 1999). Note that we have not excluded clustered earthquakes from the catalogue in this study. This may be why a significant activation anomaly appeared in this region. However, there is another swarm from April 20, 1998 to June 2, 1998. Therefore, we cannot deny the possibility that the above anomalous activation zone in the Izu Island region has some correlation with this swarm. Further studies on this topic would be interesting, but beyond the scope of this paper. We also evaluated the spatial variations of the seismic activation pattern for other time windows. We found, from the spatial distribution of the seismic activation from January 29, 1999 –January 28, 2000, that the activation
Our above discussions indicated that the quiescence anomaly in 1996 is unlikely an artificial effect due to the selection of model parameters. The temporal and spatial evolution of seismicity changes also suggested that these changes would have reasonable correlations with the buildup leading to the M ¼ 6:8 Nemuro Peninsula earthquake. Unfortunately, a strong earthquake with M ¼ 8:1 occurred at Kurile on October 4, 1994. The epicentral distance between the Kurile and Nemuro Peninsula earthquakes is about 92 km. Because we took Rmax ¼ 2r0 ¼ 100 km in most of our calculations, the 1994 Kurile earthquake and its aftershocks would have some influence on our results. Therefore, it would be important to investigate whether or not the quiescence anomaly in 1996 is an artificial effect due to the elimination process of the aftershocks of the Kurile earthquake. First, we investigated the characteristics of the aftershocks in time and space. The temporal variation of the aftershocks showed an exponential decay (Fig. 7). Fig. 7(a) includes all aftershocks of the Kurile earthquake, while Fig. 7(b) shows the aftershocks with M $ 3.6. The aftershocks occurred from October 4, 1994 to October 13, 1996. These aftershocks were distributed within an ellipse zone as shown in Fig. 8. Because most of these aftershocks were located within our investigated time window and spatial zone, we should expect a detectable influence to the RTL results after taking these aftershocks into account. However, as mentioned before, aftershocks should not be included in the RTL calculations. Therefore, the above problem about the influence of aftershocks would be equivalent to the problem about the influence of aftershock elimination. For the above purpose, we evaluated the temporal evolution of earthquakes in a circular zone within 100 km of the epicentre in the Nemuro Peninsula earthquake. Fig. 9(a) and (b) plot the temporal variations of the cumulative number of all earthquakes and those with M $ 3.6 in the original JMA earthquake catalogue. One can see that the aftershocks of the Kurile earthquake significantly disturb the linear relationship of the cumulative number in the study zone. After applying the aftershock elimination technique to the JMA earthquake catalogue, we found that the aftershocks of the Kurile earthquakes were eliminated effectively as shown in Fig. 10. There is a clear linear relationship in the cumulative number in the study zone until the end of 1996 (Fig. 10(a)). The cumulative number after 1996 satisfies another linear relationship. The sudden change around the end of 1996 is due to the introduction of the data share project of the JMA seismic network and the local network in the Hokkaido region (Y. Ishikawa, private communication). If the process of aftershock elimination
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Fig. 8. The spatial distribution of the aftershocks of the 1994 M ¼ 8:1 Kurile earthquake. (a) All aftershocks. (b) Aftershocks with M $ 3.6.
leads to an artificial quiescence, one should detect such quiescence from the variation of the cumulative number, too. Besides, the artificial quiescence would appear during late 1994– late 1996. However, we did not detect the anomalous quiescence for the case of all main shocks (Fig. 10(a)). Even though we detected a significant quiescence for the case of main shocks with M $ 3.6 as shown in Fig. 10(b), the quiescence started at the end of 1993, before
Fig. 9. The temporal variation of the cumulative number of earthquakes in the study zone (within 100 km from the epicentre of the 2000 M ¼ 6:8 Nemuro Peninsula earthquake). The earthquake data are from the original JMA earthquake catalogue. (a) All events. (b) Events with M $ 3.6.
rather than after the Kurile event. Therefore, it is unlikely that the 1996 quiescence anomaly is an artificial effect due to the process of aftershock elimination. It should be mentioned that, although the JMA seismic network has been changed many times (Ohtake and Ishikawa, 1995; Y. Ishikawa, private communication), the effect due to the upgrade of the JMA network at the end of
Fig. 10. The temporal variation of the cumulative number of main shocks in the study zone (within 100 km from the epicentre of the 2000 M ¼ 6:8 Nemuro Peninsula earthquake). The earthquake data are from the earthquake catalogue after eliminating aftershocks from the original JMA catalogue. (a) All events. (b) Events with M $ 3.6.
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1996 was reduced significantly for the case of a complete catalogue of main shocks with M $ 3.6 (Fig. 10(b)). Because we applied the RTL algorithm to the above complete earthquake catalogue in this study, the 1996 anomaly would not be an artificial effect due to the upgrade of the JMA seismic network, either. The evolution of the seismicity changes of the 2000 Nemuro Peninsula earthquake revealed by the RTL algorithm showed similar characteristics to those obtained for some strong earthquakes in Russia (Sobolev and Tyupkin, 1997, 1999) and Japan (Huang and Nagao, 2002; Huang et al., 2001). This similarity of seismicity changes prior to strong earthquakes in different geological or tectonic regions may reflect the natural evolution of seismogenic process. Tests in Russia and Japan indicated that the RTL algorithm is an effective tool for revealing the seismic quiescence and activation patterns before strong earthquakes.
6. Conclusions We investigated seismicity changes of quiescence and activation patterns prior to the M ¼ 6:8 Nemuro Peninsula (Hokkaido) earthquake, January 28, 2000 by applying the RTL (Region– Time –Length) algorithm to the earthquake catalogue of the JMA. The main shock occurred 2.5 years after the completion of the quiescence and activation stages. After evaluating the influence of different model parameters, we conclude that the quiescence anomaly in 1996, which was revealed by the RTL algorithm, would have a reliable correlation with the buildup leading to the Nemuro Peninsula earthquake. The linear size of the anomalous quiescence zone is several times larger than the rupture length of this earthquake. An anomalous activation zone on the order of several tens of kilometres appeared around the epicentre. Most of the results in this study are consistent with those obtained for the Kobe earthquake and the strong earthquakes in Kamchatka. The regularity of the seismicity changes, which has been revealed by the RTL algorithm at different geological and/or seismo-tectonic regions, may provide the best opportunity for the application of this algorithm to seismic risk analysis.
Acknowledgments The authors thank the Japan Meteorological Agency (JMA) for providing the earthquake catalogue and V.B. Smirnov for offering the programs of aftershock elimination and completeness estimation of earthquake catalogue. The authors also thank Dr Y. Ishikawa and Prof. H.K. Gupta for useful comments. This study was supported by the Frontier Research System of RIKEN (The Institute of Physical and Chemical Research), Japan.
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