A note on the depth recurrence and strain release of large Vrancea earthquakes

TECTONOPHYSICS ELSEVIER

Tectonophysics 272 (1997) 291-302

A note on the depth recurrence and strain release of large Vrancea earthquakes Mihnea-Corneliu Oncescu *, Klaus-Peter Bonjer Geophysical Institute of Karlsruhe University, Hertz Str. 16, D-76187, Karlsruhe, Germany

Received 13 July 1995; accepted 6 June 1996

Abstract The large 1940 Vrancea event with body wave magnitude m~ = 7.3 was investigated in terms of the depth of rupture initiation and extent. It was found that the rupture started at 150 km depth and propagated downwards. The ruptured zones of the 1977, 1986 and 1990 Vrancea events were estimated from the distribution of aftershocks located with the joint hypocentre determination method. Their depth ranges are 90-110 km, 130-150 km and 70-90 km, respectively. Because the body wave magnitudes mb were saturated for the last three events and the surface wave magnitudes Ms were underestimated (not being corrected for depth) we chose the moment magnitudes to quantify the four events: 7.7 (1940), 7.4 (1977), 7.1 (1986) and 6.9 (1990), respectively. The depth interval 110-130 km is found to be unbroken since at least 1838 or 1802 and therefore is expected to accommodate the next large moment release (Mw = 7.0 to 7.4). The strain in the vertical and in the SE-NW directions is about 0.0003, giving a moderate strain rate of about 1 0 - 1 3 s - 1 . A correlation between the depth interval of the large events and the year of occurrence in a century is suggested, offering a physical basis to the observed quasi-cyclicity. Keywords: seismic moment; strain; Vrancea

1. Introduction The Vrancea region in the Eastern Carpathians is characterized by a very clustered and persistent intermediate depth seismic activity. The understanding of the tectonic causes of those characteristics experienced a complex evolution. A review of the older models can be found in Sandulescu (1984) and of the latest ones in Horvath (1993) or Morley (1993). Intermediate depth earthquakes from the Vrancea region with magnitudes greater than about 6.5 occur * Corresponding author. Fax: +49 lani @gpiwapI .physik.uni-karlsruhe.de

721-71173. E-mail:

with a frequency of about two per century (e.g., Purcaru, 1979; Radu, 1979). It was therefore surprising that during this century the frequency of occurrence was doubled through the events in 1940, 1977, 1986 and 1990 (see Table 1 and Fig. 1). It was even more astonishing that the large event from 1986 apparently ruptured the same zone as the large 1940 event. Being unlikely that such an amount of strain energy could be accumulated in a relatively 'slow moving' region (about 1 crrdyear after Enescu (1993)) during only 46 years, we tried to answer the following questions: (1) Did the two large events indeed rupture the same zone? ; (2) What is the amount of strain released during that period of time? ; and (3) Is there

0040-1951/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0040- 195 1 (96)00263-6

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M.-C Oncescu, K.-P Bonjer/Tectonophysics 272 (1997) 291-302

! IJ.I

w.

I

[

I

I

I,

t

!

answer the first question, we showed that the zones ruptured by the 1940 and 1986 events are different. We redetermined the focal depth of the 1940 event from p P - P time differences and located the main shock relative to the first one in order to get a rough idea of the rupture direction and extent. We then delimited the ruptured zones of the other three large events (1977, 1986 and 1990) by looking at their aftershock distribution. We had good aftershock locations for the 1986 and 1990 aftershocks. For the 1977 event, we relocated the aftershocks listed by Fuchs et al. (1979) using the joint hypocentre determination (JHD) method. To answer the second question we determined the strain tensor and the strain rate from the fault plane solutions and the seismic moments of these four large events. To answer the third question we suggest a correlation between the depth intervals of the ruptured zones and the occurrence year in a century, attributing to each depth interval a separate quasi-cyclicity.

2. The November 10, 1940 earthquake

LONGITUDE Fig. 1. Position map of the analysedVrancea events. Topography of the Carpathianregion is illuminatedfrom the west. any correlation between the depth recurrence of large events and the quasi-cyclicity observed by Purcaru (1974, 1979) and later by Marza et al. (1994)? To

This earthquake was the largest Vrancea event recorded instrumentally. Due to the lack of epicentral stations and to the lack of precision in arrival time readings, the depth of this event appears with different values in different catalogues. For example, a depth of 133 km is reported by Radu (1979), 135 km by Constantinescu and Marza (1980), but 150

Table 1 List of earthquakes analysed in this study, No.

Date

Origin time Latitude Longitude Depth Mo (UT) (°N) C°E) (km) (Nml

M,,

1 2

Nov. 10, 1940 01:39 Mar. 04, 1977 19:21

45.8 45.77

26.7 26.76

150 a 94 h

5.1.102(j 7.7 1.5.1020 7.4

3 4

Aug. 30, 1986 21:28 May 30, 1990 10:40

45.52 45.87

26.49 26.87

131" 89 d

0.6.102o 7.1 0.3.1020 6.9

mB Ms 7,3

-

mb

7.2

6.3

7.0 6.7

6.4 6.7

Strike Dip (o) (o)

S l i p Source (o) of FPS

224 194 220 227 232

76 87 116 104 89

62 41 76 65 58

e t g g

The seismic moments Mo are averages over values obtained by different methods. The moment magnitudesMw are those determined in this study. For the othermagnitudescales see text. Sources of adopted fault plane solutions(FPS) are given in the last column. aThis study. hlSC. CTrifuand Oncescu (1987). dTrifu et al. (1990). eRadu and Oncescu (1980). fTavera (1991) for shocks 1 and 2 or R~ikersand Mtiller (1982) for shocks 1 and 3. gCMT solutions.

293

M.-C Oncescu, K.-P. Bonjer / Tectonophysics 272 (1997) 291-302

km by Abe and Kanamori (1979) or Purcaru and Berckhemer (1982). Furthermore, the multiple shock character of this event (Iosif and Iosif, 1982) made all depth determinations difficult.

KEW-

.1..I

NS

t,! . . . .

A~.t ~.....

2.1. Depth redetermination

To determine the depth of this event we chose to use the pP-P time differences. This time difference is a strong function of depth and a very weak one of epicentral distance. Moreover, the procedure is based on arrival time differences and does not depend on the absolute timing. We used the whole set of pP-P intervals listed in the ISS bulletin for the range in epicentral distances of 25 ° to 95 °. We did not have stations in the SE quadrant, for which Perrot et al. (1996) noticed biased amplitudes and arrival times for the teleseismic pP and sP phases due to strong lateral anomalies above and southeastwards of the Vrancea zone. Using a set of 33 data we came up with an average focal depth of 133 4- 27 km. We considered the scatter of 27 km as too large to be accepted and deleted all data corresponding to depths outside the range between 133 - 27 = 106 km and 133 + 27 = 160 kin. With the remainder of 21 data the scatter was considerably smaller and the corresponding depth was 150 4- 8 km. First of all, we are able to explain now from which data sets the previous depth determinations (133-135 km and 150 km, respectively) came from. The unusual high scatter in the p P - P intervals is generated by the existence of two shocks (see Fig. 2). Many stations, probably due to low magnification and/or large distances, reported preponderantly the interval PP1-P2, which is smaller than the interval P P r P l . The first (smaller) shock Pl was in many cases not seen. This means that the ISS data set is contaminated by too many too small pP-P intervals, which makes the depth of this event too small. We adopted for the depth of the first shock the value of 150 km, which is the depth where the rupture started. 2.2. 'Master event' analysis

There were very few seismograms available for the aftershocks of this large event. Therefore, in or-

t

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~1 ~Z:~r.-.~.~=~.<,,-;:~.a:.,,--..:, ... . . . . . ~ ,.- .~.- . . . . . . . . . .

(To=10s)

STU

NS

• Pl~2 (To=30s)

Fig. 2. Examples of seismograms for the 1940 event displaying the double shock character on different types of seismographs. The segments drawn under each seismogram represent a 10-s interval. The distance/azimuth for the stations KEW and STU are 270/344° and 18°/289°, respectively.The E-W component at station STU has reversed sign. der to determine how the rupture propagated, we did not look at the aftershock distribution. We determined instead the relative position of shock No. 2 with respect to shock No. 1. To do this, we used the time difference between the two shocks Atzl = P2 - P1 made on 27 seismograms that we collected. Due to the slow drum rotation (15 mrn/min or 30 mm/min), the errors of these readings are estimated to be about 0.5 s. The 'master event' analysis permits the determination of the differences in origin times r and in position dN, dE and dz along the northward, eastward and downward axes, by solving the linear system of equations: At21 i = z -- (1/Vp)(dNeNi -F dEeEi -k dzezi)

(1)

where vp is the P-wave velocity at focus depth, ei is the unit vector leaving the focus toward station i = 1. . . . . N. We solved the overdetermined system (Eq. 1) of N = 27 equations and 4 unknowns using the least-squares method. The results are: r

=

5.7 4- 0.3 s

dN

=

-9.1+2.5km

dE

=

--12.6 4- 1.6km

dz

=

14.2 4- 3.7 km

(2)

294

M.-C. Oncescu, K.-P. Bonjer/ Tectonophysics 272 (1997) 291-302 -0.5 8

0.0 I

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cos(theta) Fig. 3. 'Master event' analysis of the arrival time difference At versus cos(0) for the 1940 event.

We displayed the fit of the data in Fig. 3 as the linear dependence between At21i and cos(0)i = (dNeNi q- dEeEi + dzezi)/d, where d = (d~ + d~ + d27)I/2 is the slant distance between shock No. 1 and shock No. 2. The results of Eq. 2 give d = 21 km, shock No. 2 being 14.2 km deeper than shock No. 1. This is the most important result of this analysis, indicating that the rupture propagated obliquely downwards. A by-product of this analysis is the information that shock No. 2 has a position making an azimuth of 234 ° and dipping 42 ° relative to the position of shock No. 1. This position (full diamond in Fig. 4) permits the identification of the actual rupture plane as being the one dipping northwestward, in agreement with independent determinations by Purcaru (1979) or by Oncescu (1987a). If we use the fault length of 52 km estimated by Enescu et al. (1979) and the rupture dip of 42 °, we come up with the rupture ending at about 185 km depth, which practically coincides with the lower limit of the seismic activity in the Vrancea region (180 km). The value o f d / r : 3.7 km/s can be interpreted as a mean rupture velocity and represents a reasonable fraction, namely 0.75, of the shear wave velocity at that depth (Oncescu, 1984). Similar mean rupture velocities were determined for the other three large

events: 3.7 km/s for the 1977 event (R~ikers and Mailer, 1982); 3.5 km/s for the 1986 event (Oncescu, 1989); and 3.9 km/s for the 1990 event (Trifu et al., 1990).

3. The depth distribution of the ruptured zones and the associated event magnitudes The next step was to analyse which depth intervals were broken by each one of the large events (Table l) and to discuss the different magnitudes reported for those earthquakes. We will show that those magnitudes are either incorrect or inappropriate. The body wave magnitudes mb are incorrect because they are saturated and the surface wave magnitudes Ms are inappropriate because they are applied to intermediate depth events without correction for depth. Both factors tend to make those magnitudes too small. We will disregard the surface wave magnitude values because we do not know the depth correction factor and we will show in the following why the body wave magnitudes are saturated. We therefore use the moment magnitude Mw, defined by Kanamori (1977) as: Mw = [log(M•) - 9.1 ]/1.5 (for M• in N.m)to correctly compare the size of those events based on their rupture area. We recall that the seismic moment M• = # D S , where # is the shear modulus at focus depth, D is the final dislocation on the earthquake fault (of the order of metres for these events) and S is the rupture area.

3.1. The November 10, 1940 event According to the depth redetermination and to the 'master event' results, the ruptured zone of this event is situated approximately between 150 and 180 km depth. The body wave magnitude for this event, given by Abe and Kanamori (1979) as well as by Purcaru and Berckhemer (1982), is mB = 7.3. It was determined from body waves with a predominant period of about 8 s, which corresponds to a wavelength of about 65 km, larger than the length of the fault (52 km after Enescu et al. (1979)). This means that the magnitude mB is probably not saturated. Because the instructions to determine mB were to read the maximum amplitude in the first 10 s after the first P

295

M.-C. Oncescu, K.-P. Bonjer / Tectonophysics 272 (1997) 291-302

Nov. 10, 1940 N

Mar. 04, 1977

N

I

P w ,x'

T1

s

8

Aug. 30, 1986

May 30, 1990

N

W~

P1

N

E

S

W

S

Fig. 4. Fault plane solutions of the events in Table 1 (equal area projection on the lower hemisphere). Full diamonds for the first two events represent the position of the main shock relative to the first shock (1940, this study; 1977, Riikers and Miiller (1982)). Open squares denote compression (P) axes and full squares denote tension (T) axes.

arrival and because the intervals P2-P] are less than 10 s, it means that this magnitude indeed corresponds to the second (larger) shock. There was no surface wave magnitude Ms given for this event because Ms is defined only for focal depths smaller than 60 km. For deeper events, Ms will be underestimated. The average seismic moment of this event is 5.1.1020 N m (see Table 1), corresponding to a moment magnitude Mw -----7.7. The magnitude M = 7.4 currently attributed to this event in many catalogues is incorrectly used as Ms or as MCR as defined by Gutenberg and Richter (1954).

3.2. The March 4, 1977 event

According to the 'master event' analysis of Mailer et al, (1978), R~ikers and Miiller (1982) and Iosif et al. (1983), and to the aftershock locations by Fuchs et al. (1979), the depth interval ruptured by this multiple event is between approximately 85 and 115 km. To precise the limits, we relocated those aftershocks using the JHD method. The histogram of the depth distribution of our relocations is presented in Fig. 5. The rupture zone is interpreted to lie approximately between 90 and 110 km, confirming the 'master event' analysis.

296

M.-C. Oncescu, K.-£ Bot~/er/'£ectonophysics 272 (1997) 291-302 0

20

40

60

60

3.3. The August 30, 1986 event 60

990

80

8O

100

~ 120

977

:

100

9

120

a 140

1986

160

180

140

160

1940 i 0

380 20

40

60

Number of aftershocks

Fig. 5. Histograms with the depth distribution of aftershocks for the 1977, 1986 and 1990 events. The available aftershocks of the 1977 event (in light grey) were multiplied by ten. Stars denote rupture initiation, diamonds main shock for multishock events and arrows rupture propagation. For the rupture propagation of the 1977 event see also text.

The problem with the magnitude of the 1977 event is somewhat complicated. Firstly, because it was a multiple shock and secondly, because all reported body wave magnitudes on the mb scale (between 6.1 and 6.4) are saturated. The multiple shock character, with a duration of about 14 s between the first and the last shock according to the above-cited studies, plays an important role because the instructions to determine mb are to read the maximum amplitude in the first 5 s. By this, the main shock was missed. Then, the saturation arises from the fact that m b is determined at a period of about 1 s, which corresponds to a wavelength of about 8 kin. However, the length of the fault estimated by R~ikers and MOller (1982) is about 50 kin. To determine the size we use again the seismic moment, which has an average value of 1.5-102o N m (see Table 1). This corresponds to a moment magnitude Mw = 7.4. A magnitude value of M = 7.2 is incorrectly listed as Ms or as MCR by Radu (1979) or Constantinescu and Marza (1980). The determinations of Ms = 7.2 done routinely for this event by international seismological agencies are certainly underestimated because the event depth is again outside the definition range.

Precise aftershock relocations (JHD) were performed by Trifu and Oncescu (1987). The histogram of the depth distribution of those locations is presented in Fig. 5. Again, we associate the ruptured zone with the aftershock distribution that, according to the above authors, ranges in depth approximately between 130 and 150 1,an. The magnitude mb = 6.4 is probably again saturated. The fault length of this event is about 30 km (after Trifu and Oncescu, 1987) and m b cannot "see' faults longer than about 10 kin. The average seismic moment is 0.6.102o N m (see Table 1), which corresponds to a moment magnitude Mw = 7.1. A magnitude value M = 7.0 is incorrectly listed as Ms or as M~R by Radu (1991). Again, routine Ms determinations are not appropriate. 3.4. The May 30, 1990 event Good JHD aftershock relocations are also available for this event (Trifu et al., 1990). The histogram of the depth distribution of those locations is presented in Fig. 5 too. Again, we associate the ruptured zone with the aftershock distribution: approximately between 70 and 90 km depth. A large aftershock occurred 13 h after the main shock at a depth of about 80 kin. This would suggest, together with the aftershock distribution, that the rupture propagated upwards. For the other large events, there is ample evidence that the rupture propagated downwards (1940 and 1986) or laterally with a small downward component (1977). The latter was also related to the multiple shock character of this event (MOiler et al., 1978; R/ikers and Mtiller, 1982; Iosif et al., 1983). The body wave magnitude mb = 6.4 is probably again saturated for this event having a fault length of about 17 km (after Trifu et al., 1990). The average seismic moment is 0.3.102o N m (see Table 1), which corresponds to a moment magnitude Mw = 6.9. A magnitude value M = 6.7 is incorrectly listed as Ms or as MOR by Radu (1991). Again, routine Ms determinations are not appropriate. 3.5. Conversion formulas Because Mw should be equal to Ms in this magnitude range, we interpret the difference of about 0.2

M.-C. Oncescu, K.-P Bonjer / Tectonophysics 272 (1997) 291-302 magnitude units between Mw and Ms for the last three events as the depth correction factor for the Msz surface magnitude scale. However, we will not advocate the further use of Ms, even corrected for depth, but definitely prefer Mw to quantify the intermediate depth Vrancea earthquakes. This scale has the great advantage that it can be used for large and small events, as well as for crustal and deep events. Empirical conversion relationships between different magnitude scales and the moment magnitude for various types of Carpathian earthquakes were given by Oncescu (1987b): for example, if ME is a local magnitude determined from the signal duration, then for small intermediate depth earthquakes holds: Mw = 0.74Mc + 0.80 (for2.6 < ME < 5.0)

N

W

E

(3a) S

and for small crustal earthquakes holds Mw = 0.52Mc + 1.10 (for 2.9 < ME < 4.7)

297

(3b)

One of the most useful relationship is the one converting the (old) MGR magnitude into the Mw magnitude: Mw = 0.92MGR + 0.81 (for5.3 < MGR < 7.4) (3C) 4. The strain tensor

The determination of the strain tensor ~ij was based on the fault plane solutions (Fig. 4) for the four large events listed in Table 1. The formula used (Kostrov, 1974) was modified in the sense that each event k was allowed to have a different shear modulus/zh in the rupture zone and also a different volume V involved in the strain release: Nev 1 6iJ = 2 Z [M~; ) / ( V(v) Iz~k)) ] (4) k=l

M~jk) is the seismic moment of event k and Nev is the total number of events (in this application, four). The shear moduli at four different focus depths were taken as 8.2, 7.1, 7.5 and 6.7 times l01° N/m: (see Table 1), respectively, according to a standard Earth model. To estimate the volumes involved in the strain release, we multiplied the fault surface by 10 km. This value is about the thickness of the aftershock zones, as measured perpendicular to the fault plane and it comes from the precise locations of the 1986 and 1990 aftershocks. For the four events

Fig. 6. Eigenvectors of the strain tensor in the Vrancea region plotted in an equal area projection on the lower hemisphere (el < e2 < e3).

in Table 1, the fault surfaces were respectively taken from: Enescu et al. (1979), 2500 km2; Fuchs et al. (1979), 2000 km2; Trifu and Oncescu (1987), 725 km2; and Trifu et al. (1990), 300 km 2. The seismic moments were computed with the relation:

M(k)ij = Mo(t (k) l~ i tj(k) --Pi (k)_(k)., Pj ) / z

(5)

Mo is the scalar seismic moment (see Table 1), ti and Pi are the unit vectors along the tension axis T and the pressure axis P. We also computed the eigenvalues el < e2 < e3 and the associated eigenvectors (see Fig. 6). The three eigenvalues correspond to the maximum compression, intermediate and maximum extension axis, respectively. The results are listed in Table 2. The most striking elements are the large vertical, downgoing, extension, and the important SE-NW compression. The directions and signs of the e~- and eE-axes are the same as those determined from geodetic measurements in the epicentral region. These measurements have been performed in the Siriu polygon just before and after the 1986 event (Schmitt et al., 1990). The results (see Table 2) are similar to those obtained by Radu and Oncescu (1985) using a different data set but a similar method. This is easily

298

M.-C. Oncescu, K.~P.Bonjer / Tectonophysic.~ 272 (1997) 291-302

Table 2 Strain components, eigenvalues and eigenvectors Component

Strain (10-3 )

8NN SNE ENZ SEE EEZ Szz sl

--0.09 ± 0.05 +0.12 i 0.02 --0.12 + 0.03 --0.17 ± 0.04 +0. I0 ± 0.04 +0.26 ± 0.03 -0.30 0.01 +0.31 0.47

s2

s3 R

Azimuth (%

Dip (%

307±4 39±3 146 ± 8

16±3 5±3 74 ± 3

N stands for positive towards the north, E for positive towards the east and Z for positive downwards. R is the 'shape factor" of the strain tensor defined as R = (el - s_~)/(El s~).

explained by the fact that the main difference in the data sets is that Radu and Oncescu (1985) included also smaller events, down to a magnitude of about 5, and that they could not include, of course, the large events from 1986 and 1990. On one hand, the influence of smaller events is small, because each contribution to the strain tensor is weighted by the scalar seismic moment Mo. On the other hand, the small events mirror the strain release of larger events, as was pointed out by Oncescu (1986a) or Oncescu and Trifu (1987). The standard errors in Table 2 were computed starting with a magnitude error of 0.1 units and with an angle error of 5 ° , very reasonable assumptions for our data set. The resulting standard errors are relatively small. This could mean that the large events in the Vrancea region are governed by the same geodynamical process. Whether or not this is a vertical, gravitational sinking of a subducted and partially delaminated slab (Fuchs et al., 1979), a horizontal S E - N W push 'escaped' in a downward motion of a continuous, but 90 ° bended, slab (Roman, 1970) or combination of the two processes (Oncescu, 1984) still remains to be discriminated (see also Figs. 8 and 9). With the values from Table 2 and a time interval of 50 years (1940-1990), we get an average strain rate of the order of 10 -13 S-1, both in the vertical and in the S E - N W direction. This value is one to two orders of magnitude smaller than the values

determined by Kiratzi and Papazachos (1995) for the Aegean area and the North Anatolian Fault zone. However, the direction of the largest compressive deformation in the deeper part of the Vrancea region ( S E - N W ) agrees with the one in the crustal domain of the North Anatolian Fault and of the Calabrian Arc. This direction coincides also with the direction of Shma× in most of the European crust (Miiller et al., 1992).

5. Discussion We have shown that the 1940 event occurred below 150 km depth and that the 1977, 1986 and 1990 events occurred approximately between 90ol 10 km, 130-150 km and 70090 km depth, respectively. We have also shown that the magnitudes of these events were systematically underestimated, so that their energy release was underestimated by a factor of two, which is significant in the energy budget of a seismic region. This means, first of all, that there is a region between approximately 110 and 130 km depth that did not accommodate a large event since a very long time: either 1838 (Mw = 7.3) or 1802 (Mw = 7.8). These magnitudes were converted from those given by Purcaru (1979) using Eq. 3c. The 1802 event

REMARKS:

M w

o Moderate crustal seismic activity

<5.5

40

Low seismic activity, low velocities . . . . .

80

7.4

71 160

~ 70

6.9

May 30, 1990 9O

130 150

7.7

Not ruptured since 1802 or 1838 August 30, 1986 November 10, 1940

180

200 3.7

March 4, 1977

~220

No present seismic activity Deepest event ever recorded (May 16, 1982)

h(km)

Fig. 7. Depth distribution of ruptured (white) and not ruptured (grey) zones in the Vrancea region.

M.-C. Oncescu, K.-P.. Bonjer/Tectonophysics 272 (1997) 291-302

is the largest event ever reported for the Vrancea region. A depth of 130 km was macroseismically estimated by Radu and Utale (1992). So, it might be that this last depth interval (110-130 km) was not ruptured since the beginning of the last century. However, because the unruptured region is comparable in size with the other regions ruptured during this century, the associated moment release would probably not exceed a moment magnitude of Mw = 7.0 to 7.4. Secondly, one should not forget the depth interval

299

between 40 and 70 km. However, due to the very reduced seismic activity (Fuchs et al., 1979) and low velocities (Oncescu, 1984; Oncescu et al., 1984), there are good reasons to believe that this interval cannot accommodate stress build-up. The nature of this 'seismic gap' is difficult to assert. The thickness of the crust in the epicentral region is estimated at 43--44 km from deep seismic sounding data (Radulescu and Pompilian, 1991). Fuchs et al. (1979) proposed that the intermediate depth seismogenic volume is decoupled from the crust and that the gap

W

0

i-

5 r,,

LONGITUDE Fig. 8. Epicentral distribution of best recorded microearthquakeshaving occurred between 1982-1989, relocated with the JHD method. Topography is illuminated from the west. Black crosses stand for events with the focal depth less then 50 km, the open white diamonds for the deeper events. For each of the two groups, the size of the symbolsincreases with depth.

300

M.-C. Oncescu, K.-P. Bon/er / Tectonophysic.~ 272 (1997) 291-302

is due to an infusion of mantle material characterized by low viscosities and non-brittle properties. Thirdly, we interpret the region below 180 km (see Fig. 7), although characterized by a positive velocity anomaly (Oncescu et al., 1984), as being too heated to accumulate strain and to present brittle properties. According to the model of Demetrescu and Andreescu (1994), the seismogenic zone above 180 km depth lies within the 800°C isotherm, offeting still brittle properties for ultramafic materials; below that depth, the temperature becomes too high. So, to summarize, the next depth interval prone to a large energy release is estimated to be between 110 and 130 km depth (see also Fig. 7). Purcaru (1974, 1979) and later Marza et al. (1994) noticed that there is a certain regularity in the year of occurrence of the large Vrancea earthquakes during this millennium. The earthquakes with magnitudes greater than about 6.5 occurred almost always during the years 0-10, 30-40 and 70-90 of each century. Because of the relatively moderate strain rate that we found, it is unlikely that more than one large event per century could occur in the same depth interval (rupture the same region). This means that the occurrence of two (or more) events per century is due to a superposition of events from different NW

SE

'

depth intervals. We therefore interpret the 'quasicycles' of about 100 years found by Purcaru (1974, 1979) as being the recurrence period of the same depth interval. Moreover, we speculate that the occurrence of large Vrancea earthquakes in certain time bands does reflect the energy release within certain depth intervals. The broadness of the third time band tyears 70-90) could be due to the superposition of three depth intervals: 90-110 km (where the 1977 event occurred), 130-150 km (where the 1986 event occurred) and 70-90 km (where the 1990 event occurred). The second time band (years 30-40) would characterize the 150-180 krn depth interval (where the 1940 event occurred) and the first time band (years 0-10) would characterize the still unruptured interval between 110-130 km depth. So, if there will be a large event at the beginning of the next century (years 0-10), it will probably rupture the 110-130 krn depth interval and will have a maximum magnitude in the range Mw = 7.0 - 7.4. We also relocated the best recorded microearthquakes in the Vrancea region during 19821989 with the JHD method. The data set consisted of 262 events having minimum seven clear P arrivals and at least three S arrivals. The epicentres are displayed in Fig. 8. Fig. 9 presents two verti-

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~

i

200

i

r

~

!

~

1

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240

Fig. 9. Vertical NW-SE and SW-NE cross-sections displaying the depth distribution of the microearthquakes in Fig. 8.

M.-C. Oncescu, K.-P. Bonjer / Tectonophysics 272 (1997) 291-302

cal and perpendicular cross-sections. Firstly, we note the nearly vertical to subvertical distribution of the seismic activity. Secondly, we note that a very narrow zone of hypocentres, first obtained by Oncescu (1984) and only about 10 km wide in the NW-SE direction, begins just in the 110-130 km depth interval and is connected to a change of angle in the distribution of the seismic activity. This could be associated with a geometric barrier, a region harder to break, but in the same time a region that can accumulate large stresses. Oncescu (1986b) determined for the microearthquakes within this depth interval larger stress drops in comparison with those from above and below. Our simplified model takes into account only the depth parameter. The horizontal extent of the rupture zones varied between 11 km for 1990 event (Trifu et al., 1990) and about 50 km for the 1977 event (R~ikers and MUller, 1982). This increases the variability of the moment release. The maximum possible horizontal extension, as seen from the SW-NE section in Fig. 9, is about 40 km in the upper part and about 60 km in the lower part of the intermediate depth seismogenic zone. In principle, the rupture of each depth interval is associated with the breaking of a single asperity. These asperities break independently from one another, but by chance they could break simultaneously (they are synchronized). In this situation, a very large event, as the one from 1802 (in the first time band and according to our interpretation in the 110-130 km depth interval), with Mw > 7.8, is a very rare event. The other possibility is that the mechanical coupling between the asperities is large, so that when an asperity breaks, it might trigger an adjacent one. In this situation, the great 1802 event would be also a maximal event, but not necessarily a rare one (Kanamori and McNally, 1982). As the earthquake catalogues for the last millennium show only one very large event in the Vrancea region (in 1802), it is likely that the mechanical coupling between asperities in different depth intervals is low and that very large events are very rare.

Acknowledgements We thank T. Iosif and S. Iosif for making available to us unpublished data on the 1940 earthquake,

301

as well as the staff of seismic stations who made available to us copies of original seismograms. We also thank A. Deschamps and J. Perrot for providing preliminary results before publication. This study was supported by the SFB108 and IDNDR Projects of Karlsruhe University. Contribution No. 687 of the Geophysical Institute of Karlsruhe University.

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