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Complex fault rupture during the 2003 Chengkung, Taiwan earthquake sequence from dense seismic array and GPS observations
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Tectonophysics 466 (2009) 184 – 204 www.elsevier.com/locate/tecto
Complex fault rupture during the 2003 Chengkung, Taiwan earthquake sequence from dense seismic array and GPS observations Bor-Shouh Huang a,⁎, Win-Gee Huang a , Yi-Ling Huang b , Lon-Chen Kuo a , Kou-Cheng Chen a , Jacques Angelier c a Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan Institute of Applied Geosciences, National Taiwan Ocean University, Keelung, Taiwan Universite Pierre et Marie Curie, Observatoire Oceanologique de Villefranche-sur-mer, France b
c
Available online 23 November 2007
Abstract The 2003 Chengkung earthquake has been debated because of inconsistent fault slips determined by seismic and geodetic observations. A pure thrust fault plane solution has been estimated by seismic waveform inversions form the mainshock and its major aftershocks; however, thrust slips with obvious northward horizontal motions are necessary to interpret GPS co-seismic deformation. Based on individual observation, its implication can be dramatically different. In this study, near source dense seismic array and continuous GPS observations were employed to examine the spatial-temporal variation of fault slip of this earthquake. And average rupture properties of major aftershocks were determined by stress inversion of earthquake mechanisms. Based upon strong motion observations, the near source two-dimensional ground motions were reconstructed from 54 free-field strong motion records with epicenter distances of less than 60 km. The reconstructed two-dimensional snapshots provide the visualization for seismic wave propagations near the source region. Based on the reconstructed seismic wave-fields, numerical modeling results confirmed that the dominant seismic energy of mainshock was released from a thrust fault slip and the same as its major aftershocks from stress inversions. No obvious seismic energy was released by some strike–slip type events. Observations from creepmeter and continuous GPS showed limited co-seismic deformations and obvious after-slips near the earthquake fault. The integrated analysis provided unique information to understand the quasi-static fault slips of an earthquake sequence. In this study, a model of sudden thrust slip with a horizontal creeping movement fault system has been proposed to interpret the rupture behavior of the Chengkung earthquake. During the process, the seismic energy was released from a thrust type fault rupture and following some strike–slip type slow fault movements. If the thrust slip is the nature of faulting in the Longitudinal Valley during major earthquakes, it should be significant to seismic hazards, source physics and its tectonic implications. However, the detailed horizontal creeping cannot be resolved by seismometers and low sampling rate GPS observations, presently. To completely understand the quasi-static faulting behavior of an earthquake, the installations of a high sampling rate strainmeter and continuous GPS monitoring near a fault are necessary. © 2007 Elsevier B.V. All rights reserved. Keywords: Chengkung earthquake; Co-seismic deformation; Creep rupture; Dense seismic array; GPS; Stress inversion
1. Introduction An earthquake rupture is considered to be a sudden slip on a fault resulting ground shaking and radiated seismic energy caused by the slip. However, it has been recognized that fault slips in a plate boundary take place within a source duration varying from seconds (an ordinary earthquake) to even years
⁎ Corresponding author. Address: P.O. Box 1-55, Nankang, Taipei, 115, Taiwan. E-mail address: [email protected] (B.-S. Huang). 0040-1951/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2007.11.025
(a silent earthquake) (Bürgmann et al., 2001; Yagi and Kikuchi, 2003). The slow movement of the Earth's surface at a fault is known as creep. Creep is considered as a more or less continuous movement occurring on faults due to ongoing tectonic deformation. Creeping fault segments do not tend to have large earthquakes and sudden slips should occur on a locked asperity. The observed creep can be gradual (months to years) or can occur in short episodes known as ‘creep events’ (lasting hours to days). Both steady creep and triggered slips along a fault have been well documented in Taiwan and other regions of the world (Lyons and Sandwell, 2003; Lee et al.,
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2001). Understanding the characteristics of creep and triggered slip, and their interaction along a fault is an important issue for both earthquake physics and earthquake hazards mitigation. However, limited by temporal and spatial samplings, the creep and its interaction with an earthquake rupture during or immediately after a large earthquake has not been well reported. The island of Taiwan is located on a section of the convergent boundary between the Eurasian plate and the Philippine Sea plate (Fig. 1). These two geological provinces are separated by the Longitudinal Valley, a clear suture zone between the two plates. The Coastal Range province is composed of rocks associated with a former island arc along its eastern side and the Central Range formed mainly of rocks belonging to the passive margin of South-East Asia, in west. The Longitudinal Valley, with a length of 150 km and average width of 4 km, is primarily
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an active left-lateral strike–slip fault. This plate boundary region is one of the highest seismicity areas of the world. In the past decades, abundant GPS data have been collected along the Longitudinal Valley (LV) area mainly by repeated campaigns surveys. The interseismic GPS-based velocity field shows a general convergent direction of 303°–324°, not exactly perpendicular to the orientation of the geological features (Yu and Kuo, 2001). The horizontal velocity field indicated a distinct discontinuity across the Longitudinal Valley fault (LVF) with a difference of velocity of about 30 mm/yr (Yu and Kuo, 2001). The creeping behavior of the LVF is prominent, especially in its southern part from Juisui to Taitung and extending for about 80 km. The horizontal shortening is principally revealed by shallow aseismic slips on the LVF (Lee et al., 2001).
Fig. 1. Map showing the topographic relief of southeastern Taiwan. Focal mechanisms of the 2003 Chengkung earthquake determined by USGS and BATS are shown in the right-top corner. Circle symbols show major earthquakes in the Chengkung area since 1951. The large circle represents events greater than magnitude 7, middle circle for events with magnitude between 6 and 7, and small circle for events with magnitude between 5 and 6. The inset shows the tectonic settings of Taiwan. Major geological provinces in eastern Taiwan, namely the Central Range (CR), Longitudinal Valley (LV) and Coastal Range (COR) are shown on the map. Three red color symbols are events proposed in this study (see detail in text). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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In the southern Longitudinal Valley, background seismicity is routinely reported by a regional seismic network, the Central Weather Bureau Seismic Network (CWBSN). A short-period seismic network covering the Taiwan area to monitor regional seismicity has validated a highly active and a large number of earthquakes clustered in this area where the strongest arccontinental collisions take place (Cheng, 1995; Lin, 2004). After the 1999 Chi-Chi earthquake, many new broadband, strong motion instruments and Global Position System (GPS) have been installed across the island of Taiwan to monitor large earthquake ground motions and its fault movement for seismic hazard reduction. The eastern Taiwan is a region to be densely monitored. Furthermore, in this same area, continuous strain monitoring of the LVF has been documented for more than 15 years and its behavior has been well reported (Lee et al., in press). Those records provide an opportunity to investigate the temporal and spatial behavior of the earthquake fault ruptures in eastern Taiwan. The aim of this study is to report observations and analyze results of fault behavior of the 10 December, 2003, Chengkung earthquake (magnitude Mw = 6.6) sequence. We present a join analysis of dense strong motion array data and GPS observations, and examine the spatial-temporal variation of the fault slip of this earthquake. Results of this study emerges an important issue of interaction of brittle fracture and slide movements during a complex collision boundary fault system.
occurred near the small town of Chengkung in eastern Taiwan (Fig. 1). The main shock and its aftershocks were well recorded by the CWBSN, and the Broadband Array in Taiwan for Seismology (BATS), a broadband seismic network to immediately determine the CMT solution (Kao et al., 1998). The main shock focal mechanism was determined by BATS as a thrust fault striking in an N-S direction (Fig. 1). In the best fit double-couple mechanism had the following parameters: for plane 1, strike = 23°, dip = 42° and slip = 104°; for plane 2, strike = 184°, dip = 49° and slip = 77°. A similar solution was also reported by the USGS CMT solution. The focal depth of this event was located at the depth of 25 km by the BATS on the basis of the waveform inversion of the broadband seismograms. Fault normal cross-section of its aftershocks presented a distribution of eastward variable-dip angle seismic zone from near vertical at shallow depth and gradually decreasing its dip angle with depth (Lin, 2004). No obvious surface ruptures were found during this event. This event has been debated by the inconsistent fault slips determined by seismic and geodetic observations. A pure thrust fault plane solution has been estimated by seismic waveform inversion; however, thrust slips with obvious northward horizontal motions are necessary to interpret GPS co-seismic deformation. Complex geometries of a fault plane and fault creep are both considered as possible interpretations (Wu et al., 2006). 2.1. Observations
2. Major large earthquakes on the Longitudinal Valley of eastern Taiwan On November 25, 1951, a 7.3 magnitude earthquake occurred in eastern Taiwan. Hsu (1962) reported the observation of surface faulting after a series of large earthquakes in the LV in 1952. Several large earthquakes occurred in the eastern part of the LV near the city of Hualien and many aftershocks, a few with magnitude greater than 6, followed. Surface ruptures of this event took place in several place, Hsu (1962) associated it with Yuli fault. An oblique left-lateral surface rupture along the Longitudinal Valley with fault length 43 km was reported (Cheng, 1995). On its eastern side, a 10 km long, left-lateral fault trace which was parallel to the main fault was also found. However, it is difficult to assess the relative importance of dip–slip and strike– slip fault movement from surface faulting evidence alone. On May 28, 1992, a major earthquake (Origin time: 5/28/1992 23:19:35.6 GMT; ML = 5.4; 23.15 N, 121.35E; depth = 13.7) occurred again in the Chengkung area. No surface rupture was found. A permanent station of the Central Weather Bureau (CWB) has recorded this event with epicental distance about 5 km, not only the far-field but also the near-field displacements were clearly recorded. According to the full waveform modeling, the overall observed and synthetic seismograms are in good agreement for each component by the wave form inversion. In the best fit double-couple mechanism had the following parameters: for plane 1, strike = 280°, dip = 72° and slip = 170°; for plane 2, strike = 14°, dip = 81° and slip = 18°. It can be considered as near a pure thrust-faulting earthquake (Huang, 1994). On 10 December, 2003, a major earthquake [location = 23.058°N, 121.379°E; depth = 23 km; Mw = 6.6 after USGS]
The 10 December 2003 Chengkung earthquake was the first event, after the 1999 Chi-Chi earthquake, to occur in the Longitudinal Valley region with a magnitude greater than 6. It is also the first major eastern Taiwan inland event simultaneously recorded by both dense strong motion and GPS networks. More detail descriptions about observations within this event are following: 2.1.1. Strong motion array observations A strong motion network equipped with more than 400 freefield digital accelerometers and operated by the Central Weather Bureau has been constructed to monitor earthquake activities in Taiwan since 1990 (Shin, 1993). Each station in this network contains a force-balance accelerometer sensor with a flat response from DC to 50 Hz and signals are digitized at 200 samples per second or higher with a 16-bit or higher resolution (Liu et al.,1999). These accelerometer sensors are ± 2 g full scale and recorders have pre-event and post-event memory. The digital accelerometers are operated in the trigger mode. Most strong motion instruments are equipped with GPS to provide absolute timing. The 2003 Chengkung earthquake was well recorded by stations in the Longitudinal Valley region. This data set represents the densest near source strong ground motion observations for this area. The 54 free-field 3-component strong motion data were recorded with an epicentral distance less than 60 km during this event (Fig. 2). After correcting onset timing for those non-absolute timing records using the same procedure followed by Huang (2000), the network seismograms can be truly displayed according to its epicenter distance. Fig. 3
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Fig. 2. Map of southeastern Taiwan and strong motion array configuration. The blue triangles represent the strong motion stations of the CWB seismic network used in this study. Epicenter of the 2003 Chengkung earthquake is plotted as large red circle. Middle size red circles represent major aftershocks of the 2003 Chengkung earthquake with a magnitude greater than 5.0. Small blue dots represent one month's seismicity after the 2003 Chengkung earthquake. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
shows the corrected vertical component acceleration seismograms. It was found that the major seismic energy was suitably aligned. Although the focal mechanism of this earthquake can be determined by regional short-period and global seismic networks, however, in principle, first P-motion and CMT solutions do not provide further information about later rupture information of source rupture. For example, double events within an earthquake, the second event with different fault mechanism cannot be determined. However, the reconstructed seismic wavefield observed in near field has resolution to identify the possible second event with different focal mechanism. Herein, the observed dense array wave-fields provided an opportunity to independently examine the source parameters determined by regional short-period or global
seismic network, and to discuss the source rupture behavior of this earthquake. 2.1.2. GPS observations Before the 2003 Chengkung earthquake, many continuous GPS and campaign-surveyed stations have been installed and well maintained in the southeastern Taiwan area. Immediately after the earthquake, the GPS field survey around the epicentral area was repeated to measure the co-seismic and postseismic surface displacements (Chen et al., 2006). Finally, processed data from eighteen continuously recording GPS stations and more than 86 campaign-surveyed stations were used to detect the co-seismic crustal deformation in the region around epicenter of the Chengkung earthquake (Fig. 4).
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Fig. 3. Vertical component acceleration record section for the 2003 Chengkung earthquake with a distance from 0 to 70 km. The vertical bar in each seismogram represents the picking for its first P-arrival time.
Significant displacements have been observed in particular for the stations near the epicenter. The co-seismic horizontal displacements in the hanging-wall showed a fan-shape distribution with vectors towards the west and all of the GPS stations were raised by the Chengkung earthquake. The elevation changes generally decreased away from the epicenter area (Chen et al., 2006). According to the data
recorded by the near fault continuously recording GPS stations, processed results provide detailed information not only for the co-seismic deformation but also for the temporal variations of post-seismic deformation. Several nearby station pairs, which showed reverse crustal motions, were examined to study the temporal variations of fault movements. Among them, only two continuous station pairs (Fig. 5) showed
Fig. 4. (a) Horizontal co-seismic displacements relative to Paisha, Penghu. The 95% confidence error ellipse is shown at the tip of each co-seismic vector. The epicenter is marked by a solid star. LV and LVF denote the Longitudinal Valley and Longitudinal Valley Fault, respectively. (b) The elevation changes in GPS height for the coseismic displacement (modified from Chen et al., 2006).
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Fig. 5. Position variations in two near fault station pairs, SHAN-TAPO and S105-ERPN (station positions can be found in Fig. 4). For each subplot, the abscissa denotes the time and year, and the ordinates are the north, east and up components of each baseline, respectively. The central vertical line in each subplot indicates the time of the earthquake, the number on the left hand side of the line expresses the relative co-seismic displacement and the number on the right hand side of the line means the relative post-seismic displacement of the earthquake (modified from Chen et al., 2006).
significant post-seismic movements. Results from both pairs indicated that post-seismic movements occurred at Chihshang and its southern area (Chen et al., 2006). 3. Analysis and results During the 10 December 2003 Chengkung earthquake, strong ground motions were well recorded by the densely deployed stations in the source area. Spatial wave-fields (snapshots) could, therefore, be reconstructed using the recorded data. In this study, to enhance the coherence seismic wave propagation characteristics, the recorded accelerograms were previously converted to velocity seismograms and utilized a band-pass filter with corner frequencies at 0.1 and 0.5 Hz, respectively for the further analysis. The spatial wave-fields were interpolated from the common-time amplitudes of records (Huang, 2000). One example is shown in Fig. 6 and represents instantaneous, vertical velocity distribution at time 24.8 s after the onset of the source rupture. To reconstruct this snapshot, the griding algorithm developed by Smith and Wessel (1990) was employed. The same procedure used to construct Fig. 6 could be easily applied to other selected times and other horizontal components. Waveform analysis can be subsequently based on
serial discrete snapshots at equal time intervals. An alternative form of presentation of surface ground motion is illustrated through animation using successive displays of dense timesampled snapshots. Such animation can be easily constructed and displayed using a variety of available visualization software on personal computers. Using animation, the ground motion, the seismic energy radiated from the rupture plane and the propagation of disturbances and wave fronts over the surface can be observed in an uninterrupted fashion. This form of presentation is most helpful in the interpretation of source behavior and wave propagation effects. Based upon the successive presentation of the snapshots in dense time steps, the near source coherent waveforms of the 2003 Chengkung earthquake can be thoroughly traced during propagation. Some complex waveforms may be induced by the multiple source asperities in the fault plane, seismic waves trapped inside the fault zone and soft soil amplifications. In Fig. 7, snapshots at several time steps were selected. Each panel in Fig. 7 represents the vertical velocity of ground surface at a given time. These snapshots provided a time history of the spatially-dependent wave-fields. Based upon those snapshots, wave propagation features can be clearly followed and discussed. Analysis of this event shows that the
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Fig. 6. Spatial distribution of vertical velocities (original time 24.8 s) near the source area of the 2003 Chengkung earthquake. To avoid spatial aliasing, the seismograms were processed with a band-pass filter at a period of 2–10 s and are the same as figures in Fig. 7. The amplitudes are normalized following the color bar.
filtered ground motions have major amplitudes in its horizontal directions and the dominant radiated seismic energy was recognized as a direct S-wave emerged at the surface 5 s after the earthquake occurrence. Most radiated seismic energy surrounded the epicenter (Fig. 7). The major characteristics of the 2-D ground motion were its symmetric circular wave fronts spreading over the source area. From successive
snapshots, it was found that seismic energy radiation from the source rupture was completed within 25 s after the initiation of the rupture (Fig. 7) and the duration of the source rupture was estimated to be 20 s. From the 34.8 second snapshot of Fig. 7, seismic amplifications were found to be trapped along the Longitudinal Valley where thick sediments were locally distributed.
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Fig. 7. Ground motion snapshots of vertical velocity in the near-source area. The first snapshot, on the left at the top, was taken 5 s after the onset of the source rupture. The succeeding panels were snapshots with original times as noted in each panel. The last snapshot depicts the surface motion 44 s after the initiation of the rupture process. For amplitudes refer to color bar of Fig. 6.
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According to BATS CMT solution as focal mechanism, finite fault modeling for co-seismic crustal deformation of the 10 December 2003 Chengkung earthquake was computed using a numerical code for modeling co-seismic response of the Earth's crust to earthquakes (Wang et al., 2006) and shown in Fig. 8. The rupture is considered as similar fault geometry as BATS CMT solution to simulate the Chengkung earthquake. It is found that the horizontal slips are symmetric on both sides of fault end. The vertical deformation shows a simple upward movement pattern in its hanging-wall. A series fault models with various dip angles, fault widths and lengths are tested to compare with the GPS determined co-seismic crustal deformation of this event (Fig. 4). No model shows similar pattern of observed results. However, the GPS observed co-seismic deformation pattern is similar to the computation of a two fault segment model (Fig. 9). This model includes one square pure thrust fault plane with a major strike–slip oblique thrust fault superposed on its northern end. To verify the GPS recorded strike–slip faulting related to aftershocks occurred within this earthquake sequence, it is helpful to examine the rupture properties of major aftershocks. Fortunately, the focal mechanisms of the Chengkung earthquake sequence have been well determined by the regional Taiwan broadband seismic network (thus, BATS). In this study, BATS was employed to determine the aftershocks from December 10, 2003 (mainshock) to January 25, 2004 and has been employed to estimate seismic energy release. The determined focal mechanisms are shown in Fig. 10. As shown in Fig. 10(a), each aftershock is plotted at its epicenter as a double-couple focal mechanism of ‘beachball’ stereoplots (an equal-area projection of the lower hemisphere, with compression dihedra in black and extension dihedra in white). It was found that most of aftershocks show pure thrust focal mechanisms. The cross-section projected all events in the direction of azimuth N122°E is shown in Fig. 10(b) and revealed a clear east-dipping fault plane. No observable strike– slip fault mechanism events were found within this earthquake sequence. In this study, the stress inversion method based on the slip shear stress component (SSSC) criterion (Angelier, 2002) was applied to determine the regional stress direction which is related to the earthquake sequences. During this earthquake sequences, all the BATS determined focal mechanisms within a 1° × 1° area (as shown in Fig. 10) are selected for stress inversion. Stress directions with confidence ellipses from inversion of focal mechanisms of earthquakes for the Chengkung earthquake sequence is shown in Fig. 10(c) and based upon a steroplot (Schmidt's projection, lower hemisphere). Numerical information about stress tensors and inversion parameters can be found in Table 1. The averaged stress inversion can be considered as the maximum principle direction of a pure thrust fault rupture as the same as those major aftershocks. Furthermore, it also implicated that major seismic energy were released by thrust type aftershocks. Combined with information from dense seismic array analysis, the aftershock and GPS observations, analyzed results of this study offer some constraints for the study of the entire earthquake sequence source rupture behavior.
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4. Discussion and conclusions From 2-D ground motions animation, successive snapshots and snapshot panels of Fig. 7 reveal that the long period seismic wave-fields show a clearly symmetric pattern with respect to the earthquake epicenter. Those near-field strong motion wave-field characteristics were confirmed by numerical simulations caused by a pure thrust fault rupture (Huang, 1989; Huang et al., 2004). Based on the method developed by Huang et al., (2004), the computed 2-D near-source wave field, using a single pure thrust fault mode which is the same source model to compute coseismic crustal deformation of Fig. 8, is shown in Fig. 11. The major subsidence is found in the footwall side of fault and this pattern is consistent to the snapshots as shown in Fig. 7. Implication of this study indicated that the 2-D ground motion simulation presents independent evidence for the focal mechanism of the 2003 Chungkung earthquake as a thrust event with limited strike direction slip. The thrust fault characteristic estimated in this study is consistent to the determined CMT solutions according to the regional broadband seismic network (BATS) and global seismic network (reported by USGS) and indicates that the radiated seismic energy recorded on near source, regional and teleseismic distances show a pure thrust fault behavior. No obvious second event of strike–slip faulting was found during the rupture process of this event. However, numerical co-seismic crustal deformation modeling from a simple pure thrust model (Fig. 8) do not show consistent results as observations based on GPS data (Fig. 5). The co-seismic observations showed that, near the Chengkung area, the block east of Longitudinal Valley moved to the west with a direction similar to the plate motion direction. However, crustal motion north of the epicenter showed a large left-lateral horizontal slip component. Based upon co-seismic deformation determined by GPS observations, a pure thrust fault plane with an oblique left-lateral thrust faulting is required to fit the source mechanism of the 2003 Chengkung earthquake (Fig. 9). According to the two fault segments model, 2-D ground motion were simulated as shown in Fig. 12. It is found that the pattern is very different to the observed 2-D ground motions (Fig. 7). Thus, the fault model fit the co-seismic crustal deformation cannot fit the seismic energy release recorded by the strong motion array. Other source ruptures models determined by coseismic crustal deformation modeling are both a single fault plane model with significant strike–slip movements and a double fault plane model equipped with one pure deep thrust fault and a shallow strike–slip fault, respectively (Cheng et al., 2004; Wu et al., 2006). Similar numerical modeling indicated that 2-D ground motion simulation based on both models do not fit the observed 2-D ground motions. The inconsistent results from seismic moment tensor inversion and GPS co-seismic crustal deformation inversion have been debated not only for its source rupture types but also its tectonic implications. Several models may explain this conflict. In this study, a model of sudden thrust slip with horizontal creeping movement fault system has been proposed to interpret the rupture behavior of this event because co-seismic deformation determined from
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Fig. 8. Numerical simulation for co-seismic crustal deformation of a pure thrust faulting with uniform slips within a source area of 15 × 15 km2 and 15 km beneath surface. Fault parameters are strike = 0°, dip = 60° and slip = 90°. The rectangular within each panel is the projection of the modeled fault-plan.
the campaign-surveyed GPS observations should include crustal movement within several days after the earthquake. The horizontal creeping along existing faults cannot be separated from co-seismic deformation. Actually, monitoring active fault creep in southeast Taiwan has a long history. Clearly horizontal aseismic slips near the source region of this event have been well documented (Yu and Kuo, 2001; Lee et al., 2001). Furthermore, creepmeter observations across the Chihshang fault which was ruptured by this event showed limited horizontal co-seismic shortening, but significant post-seismic creep (Lee et al., 2005). The creeping behavior is also found on the near source continuous GPS observations as significant
post-seismic horizontal crustal deformation near Chengkung and its southern area (Fig. 5). It shows limited co-seismic deformations but obvious after-slips between both sides of fault trace. Integrating all available information, the source process of the 2003 Chengkung earthquake can be considered to be induced by the action of oblique plate collision on the LVF suture zone. Thus, in the southeastern Taiwan region, the plate convergent direction is not perpendicular to the orientation of the plate boundary and the major faults should be thrusts with strike–slip components. During the interseismic period, the elastic rebound seismic energy is usually accumulated along the
Fig. 9. Numerical simulation for co-seismic crustal deformation of a two segment faulting model. Each model is assumed as uniform slips within a source area of 15 × 15 km2 and 15 km beneath surface. Left segment with fault parameters of strike = 0°, dip = 60° and slip = 90° and right segment with fault parameters of strike = 0°, dip = 80° and slip = 40°.
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Fig. 10. Analyzed results of stress inversion of focal mechanisms of earthquakes for the Chengkung earthquake sequence, from December 10, 2003 (mainshock) to January 25, 2004. (a) A map of earthquakes considered in the inversion. (b) A cross-section showing the same earthquakes. (c) The results of the inversion. Axes σ1 (maximum compressive stress), σ2 (intermediate stress) and σ3 (minimum stress) shown as five-branched star, four-branched star and three-branched star, respectively. Confidence ellipses are also shown (60%, 75% and 90%). Pairs of large arrows indicate the resulting directions compression (convergent arrows). Grey patterns around arrows indicate confidence intervals (increasingly darker for 90%, 75%, and 60%). The value of the ratio of principal stress differences, Φ = (σ2 − σ3)/ (σ1 − σ3), is indicated within a bar from 0 (base) to 1 (top). Numerical information about stress tensors and inversion parameters are listed in Table 1.
plate boundary perpendicular direction and the stress which parallels to the plate boundary is generally released in creep type deformation. When an earthquake occurs, seismic waves are released from thrust component rebound, and the horizontal slips of fault represent an adjustment of total plate motions and limited seismic energy are released from horizontal plate motions. The complex fault ruptures are possibly induced by fault interactions from a sudden regional stress release and/or seismic wave triggering during the earthquake. The implication of this study indicates that partition of seismogenic and aseismic
slip may have occurred simultaneously in eastern Taiwan and seismic observations based on dense seismic array, GPS and creepmeters, will help to resolve the time-dependent processes of earthquake faulting. Thus, the fault rupture can be analyzed as a quasi-static rupture process. However, as reported by Chen et al., (2006), the detailed horizontal creep cannot be resolved by low sampling rate GPS observations and the same as it cannot be resolved by strong motion array analysis reported by this study. Furthermore, the source processes of slip separation can happen in a single fault plane, but it is also possible to occur
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Table 1 Results of stress inversion of focal mechanisms of earthquakes for the Chengkung earthquake sequence Reference
BATS
ωacc (%)
N
60
14
acc
N
rej
5
σ1 axis
σ2 axis
σ3 axis
Trend
Plunge
Trend
Plunge
Trend
Plunge
296
3
27
13
194
77
Φ
ωm (%)
tm (%)
αm (deg)
0.50
88 ± 10
91 ± 10
12 ± 8
ωacc, minimum ratio omega requested for retaining a datum. ω may range from −100 (total misfit) to 100 (perfect fit). Nacc, number of accepted data (noted in terms of nodal planes). Nrej represents the number of rejected data. The orientations of principal stress axes σ1 (maximum compressive stress), σ2 (intermediate stress) and σ3 (minimum stress) given in terms of trends and plunges, in degrees. Φ, ratio of principal stress differences, Φ = (σ2 − σ3)/(σ1 − σ3). ωm, average ratio omega of accepted data. τm, average dimensionless shear stress from 0 to 100% (that is, average shear stress with respect to maximum shear stress). αm, represents the average angle between shear stress and slip, from 0 to 180°. Note that the main estimator, ω, and the two related subsidiary estimators, τ and α, are obtained through consideration of both the nodal planes (a choice between nodal planes would result in better values, but has not been done). For complete explanation of the method, the reader is referred to the description by Angelier (2002).
on different fault planes. The identification of fault types is important to rupture process details and seismic hazard reduction. However, until recently, not enough data were available to identify this difference. To completely understand the quasi-static rupture process of an earthquake, the installation of a high sampling rate strainmeter and continuous GPS near a fault are necessary. Indeed, the source process of the 2003 Chengkung earthquake is not a unique case in eastern Taiwan. Wu et al. (1989) reported three large earthquakes in the vicinity of the Longitudinal Valley having predominantly thrust focal mechanisms. From a geological point of view, the LVF is an active fault with significant left-
lateral strike–slip component. A large earthquake in eastern Taiwan with left-lateral strike–slip mechanism becomes a highly possible source candidate. However, if the thrust slip is the nature of faulting in the Longitudinal Valley during major earthquakes, the design for seismic hazards, the rupture behavior of earthquake source and its possible implications for tectonic setting will be dramatically different as our presently understanding. For example, reexamination for the fault rupture of the 1951 magnitude 7.3 earthquake, although no seismic waveform data was available to determine its source mechanism and left-lateral surface breaks were found during the earthquake (Cheng, 1995), this event has the possibility of being a pure thrust event and
Fig. 11. Numerical simulation for 2-D ground displacements using the same fault model as Fig. 8. Color tape of each panel shows relative amplitude of snapshot. Arrival time of each snapshot is shown in the top. A simple pure thrust faulting was employed in this modeling. The rectangular square within each panel is the projection of the modeled fault-plan.
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Fig. 12. Numerical simulation for 2-D ground displacements using the same fault model as Fig. 9. Definitions are the same as Fig. 11. A two fault segment model was employed in this modeling. Two rectangular squares within each panel are the projection of the modeled fault-plans.
should be considered for future large earthquakes for building design and seismic hazard reduction. Acknowledgements The authors wish to express their appreciation to the Central Weather Bureau and the Institute of Earth Sciences, Academia Sinica for providing the data used in this study. This study was supported by Academia Sinica and the National Science Council, R.O.C., under grants NSC93-2119-M-001-014 and NSC92-2116-M-001-009. References Angelier, J., 2002. Inversion of earthquake focal mechanisms to obtain the seismotectonic stress IV—a new method free of choice among nodal planes. Geophys. J. Int. 150 (3), 588–609. Bürgmann, R., Kogan, M.G., Levin, V.E., Scholz, C.H., King, R.W., Steblov, G.M., 2001. Rapid aseismic moment release following the 5 December, 1997. Kronotsky, Kamchatka, earthquake. Geophys. Res. Lett. 28, 1331–1334. Cheng, L.W., Lee, J.C., Wu, Y.M., Hu, J.C., 2004. Inversion of Co-seismic Deformation of Chengkung Earthquake in Eastern Taiwan revealed by Strong Motion and Continuous GPS, in AGU Fall Meeting, San Francisco. Chen, H.Y., Yu, S.B., Kuo, L.C., Liu, C.C., 2006. Co-seismic and postseismic displacement of the 10 December 2003 (Mw 6. 8) Chengkung earthquake, eastern Taiwan. Earth Planet Space, 58, 1–17. Cheng, S.N., 1995. The study of stress distribution in and around Taiwan, Ph.D. thesis, 225 pp., Natl. Cent. Univ., Chungli, Taiwan. Hsu, T.L., 1962. Recent faulting in the Longitudinal Valley of eastern Taiwan. Mem. Geol. Soc. China 1, 95–102.
Huang, B.S., 1989. Modeling seismic source geometry and rupture process by finite element method (in Chinese), Ph.D. Thesis, National Central University, Chung-Li, Taiwan, ROC, 136 pp. Huang, B.S., 1994. Estimation of source parameters by the inversion of near source strong motion wave forms. TAO 5, 11–26. Huang, B.S., 2000. Two-dimensional reconstruction of the surface ground motions of an earthquake: the September 21, 1999, Chi-Chi, Taiwan earthquake. Geophys. Res. Lett., 27, 3025–3028. Huang, Y.L., Huang, B.S., Wen, K.L., 2004. Simulation for near source twodimensional wave field and its application to the study of ground motion characteristics of the 1999 Chi-Chi, Taiwan earthquake. TAO 15, 33–46. Kao, H., Jian, P.R., Ma, K.F., Huang, B.S., Liu, C.C., 1998. Moment-tensor inversion for offshore earthquakes east of Taiwan and their implications to regional collision. Geophys. Res. Lett 25, 3619–3622. Lee, J.C., Angelier, J., Chu, H.T., Hu, J.C., Jeng, F.S., 2001. Continuous monitoring of an active fault ina plate-suture zone: a creepmeter study of the Chihshang active fault, eastern Taiwan. Tectonophysics 333, 219–240. Lee, J.C., Angelier, J., Chu, H.T., Hu, J.C., Jeng, F.S., 2005. Monitoring active fault creep as a tool in seismic hazard mitigation. Insights from creepmeter study at Chihshang, Taiwan, C.R. Geosciences 337 (13), 1200–1207. Lee, J.C., Chu, H.T., Angelier, J., Chu, H.T., Chen, H.Y., Yu, S.B., in press. Quantitative analysis of surface co-seismic faulting and post-seismic creep accompanying the 2003, Mw = 6.5 Chengkung earthquake in eastern Taiwan, J. Geophys. Res. Lin, C.-H., 2004. Repeated foreshock sequences in the thrust faulting environment of eastern Taiwan. Geophys. Res. Lett. 31, L13601. doi:10.1029/ 2004GL019833. Liu, K.S., Shin, T.C., Tsai, Y.B., 1999. A free-field strong motion network in Taiwan: TSMIP. TAO 10, 377–396. Lyons, S., Sandwell, D., 2003. Fault creep along the southern San Andreas from interferometric synthetic aperture radar, permanent scatterers, and stacking. J. Geophys. Res. 108 (B1), 2047. doi:10.1029/2002JB001831.
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Shin, T.C., 1993. Progress summary of the Taiwan strong motion instrumentation program, Symp. on the Taiwan strong motion instrumentation program, 1–10. Smith, W.H.F., Wessel, P., 1990. Gridding with continuous curvature splines in tension. Geophysics 55, 293–305. Wang, R., Lorenzo-Martín, Francisco, Roth, Frank, 2006. PSGRN/PSCMP—a new code for calculating co- and post-seismic deformation, geoid and gravity changes based on the viscoelastic-gravitational dislocation theory. Comput Geosci 32, 527–541. Wu, F.T., Chen, K.C., Wang, J.H., McCaffrey, R., Salzberg, D., 1989. Focal mechanisms of recent large earthquakes and the nature of faulting in the Longitudinal Valley of eastern Taiwan. Proc. Geol. Soc. China 32, 157–177.
Wu, Y.M., Chen, Y.G., Shin, T.C., Kuochen, H., Hou, C.S., Hu, J.C., Chang, C.H., Wu, C.F., Teng, T.L., 2006. Co-seismic versus interseismic ground deformations, fault rupture inversion and segmentation revealed by 2003 Mw 6.8 Chengkung earthquake in eastern Taiwan. Geophys. Res. Lett. 33, L02312. doi:10.1029/2005GL024711. Yagi, Y., Kikuchi, M., 2003. Partitioning between seismogenic and aseismic slip as highlighted from slow slip events in Hyuga-nada, Japan. Geophys. Res. Lett. 30 (2), 1087. doi:10.1029/2002GL015664. Yu, S.B., Kuo, L.C., 2001. Present-day crustal motion along the Longitudinal Valley fault, eastern Taiwan. Tectonophysics 333, 199–217.
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Complex fault rupture during the 2003 Chengkung, Taiwan earthquake sequence from dense seismic array and GPS observations Bor-Shouh Huang a,⁎, Win-Gee Huang a , Yi-Ling Huang b , Lon-Chen Kuo a , Kou-Cheng Chen a , Jacques Angelier c a Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan Institute of Applied Geosciences, National Taiwan Ocean University, Keelung, Taiwan Universite Pierre et Marie Curie, Observatoire Oceanologique de Villefranche-sur-mer, France b
c
Available online 23 November 2007
Abstract The 2003 Chengkung earthquake has been debated because of inconsistent fault slips determined by seismic and geodetic observations. A pure thrust fault plane solution has been estimated by seismic waveform inversions form the mainshock and its major aftershocks; however, thrust slips with obvious northward horizontal motions are necessary to interpret GPS co-seismic deformation. Based on individual observation, its implication can be dramatically different. In this study, near source dense seismic array and continuous GPS observations were employed to examine the spatial-temporal variation of fault slip of this earthquake. And average rupture properties of major aftershocks were determined by stress inversion of earthquake mechanisms. Based upon strong motion observations, the near source two-dimensional ground motions were reconstructed from 54 free-field strong motion records with epicenter distances of less than 60 km. The reconstructed two-dimensional snapshots provide the visualization for seismic wave propagations near the source region. Based on the reconstructed seismic wave-fields, numerical modeling results confirmed that the dominant seismic energy of mainshock was released from a thrust fault slip and the same as its major aftershocks from stress inversions. No obvious seismic energy was released by some strike–slip type events. Observations from creepmeter and continuous GPS showed limited co-seismic deformations and obvious after-slips near the earthquake fault. The integrated analysis provided unique information to understand the quasi-static fault slips of an earthquake sequence. In this study, a model of sudden thrust slip with a horizontal creeping movement fault system has been proposed to interpret the rupture behavior of the Chengkung earthquake. During the process, the seismic energy was released from a thrust type fault rupture and following some strike–slip type slow fault movements. If the thrust slip is the nature of faulting in the Longitudinal Valley during major earthquakes, it should be significant to seismic hazards, source physics and its tectonic implications. However, the detailed horizontal creeping cannot be resolved by seismometers and low sampling rate GPS observations, presently. To completely understand the quasi-static faulting behavior of an earthquake, the installations of a high sampling rate strainmeter and continuous GPS monitoring near a fault are necessary. © 2007 Elsevier B.V. All rights reserved. Keywords: Chengkung earthquake; Co-seismic deformation; Creep rupture; Dense seismic array; GPS; Stress inversion
1. Introduction An earthquake rupture is considered to be a sudden slip on a fault resulting ground shaking and radiated seismic energy caused by the slip. However, it has been recognized that fault slips in a plate boundary take place within a source duration varying from seconds (an ordinary earthquake) to even years
⁎ Corresponding author. Address: P.O. Box 1-55, Nankang, Taipei, 115, Taiwan. E-mail address: [email protected] (B.-S. Huang). 0040-1951/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2007.11.025
(a silent earthquake) (Bürgmann et al., 2001; Yagi and Kikuchi, 2003). The slow movement of the Earth's surface at a fault is known as creep. Creep is considered as a more or less continuous movement occurring on faults due to ongoing tectonic deformation. Creeping fault segments do not tend to have large earthquakes and sudden slips should occur on a locked asperity. The observed creep can be gradual (months to years) or can occur in short episodes known as ‘creep events’ (lasting hours to days). Both steady creep and triggered slips along a fault have been well documented in Taiwan and other regions of the world (Lyons and Sandwell, 2003; Lee et al.,
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2001). Understanding the characteristics of creep and triggered slip, and their interaction along a fault is an important issue for both earthquake physics and earthquake hazards mitigation. However, limited by temporal and spatial samplings, the creep and its interaction with an earthquake rupture during or immediately after a large earthquake has not been well reported. The island of Taiwan is located on a section of the convergent boundary between the Eurasian plate and the Philippine Sea plate (Fig. 1). These two geological provinces are separated by the Longitudinal Valley, a clear suture zone between the two plates. The Coastal Range province is composed of rocks associated with a former island arc along its eastern side and the Central Range formed mainly of rocks belonging to the passive margin of South-East Asia, in west. The Longitudinal Valley, with a length of 150 km and average width of 4 km, is primarily
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an active left-lateral strike–slip fault. This plate boundary region is one of the highest seismicity areas of the world. In the past decades, abundant GPS data have been collected along the Longitudinal Valley (LV) area mainly by repeated campaigns surveys. The interseismic GPS-based velocity field shows a general convergent direction of 303°–324°, not exactly perpendicular to the orientation of the geological features (Yu and Kuo, 2001). The horizontal velocity field indicated a distinct discontinuity across the Longitudinal Valley fault (LVF) with a difference of velocity of about 30 mm/yr (Yu and Kuo, 2001). The creeping behavior of the LVF is prominent, especially in its southern part from Juisui to Taitung and extending for about 80 km. The horizontal shortening is principally revealed by shallow aseismic slips on the LVF (Lee et al., 2001).
Fig. 1. Map showing the topographic relief of southeastern Taiwan. Focal mechanisms of the 2003 Chengkung earthquake determined by USGS and BATS are shown in the right-top corner. Circle symbols show major earthquakes in the Chengkung area since 1951. The large circle represents events greater than magnitude 7, middle circle for events with magnitude between 6 and 7, and small circle for events with magnitude between 5 and 6. The inset shows the tectonic settings of Taiwan. Major geological provinces in eastern Taiwan, namely the Central Range (CR), Longitudinal Valley (LV) and Coastal Range (COR) are shown on the map. Three red color symbols are events proposed in this study (see detail in text). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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In the southern Longitudinal Valley, background seismicity is routinely reported by a regional seismic network, the Central Weather Bureau Seismic Network (CWBSN). A short-period seismic network covering the Taiwan area to monitor regional seismicity has validated a highly active and a large number of earthquakes clustered in this area where the strongest arccontinental collisions take place (Cheng, 1995; Lin, 2004). After the 1999 Chi-Chi earthquake, many new broadband, strong motion instruments and Global Position System (GPS) have been installed across the island of Taiwan to monitor large earthquake ground motions and its fault movement for seismic hazard reduction. The eastern Taiwan is a region to be densely monitored. Furthermore, in this same area, continuous strain monitoring of the LVF has been documented for more than 15 years and its behavior has been well reported (Lee et al., in press). Those records provide an opportunity to investigate the temporal and spatial behavior of the earthquake fault ruptures in eastern Taiwan. The aim of this study is to report observations and analyze results of fault behavior of the 10 December, 2003, Chengkung earthquake (magnitude Mw = 6.6) sequence. We present a join analysis of dense strong motion array data and GPS observations, and examine the spatial-temporal variation of the fault slip of this earthquake. Results of this study emerges an important issue of interaction of brittle fracture and slide movements during a complex collision boundary fault system.
occurred near the small town of Chengkung in eastern Taiwan (Fig. 1). The main shock and its aftershocks were well recorded by the CWBSN, and the Broadband Array in Taiwan for Seismology (BATS), a broadband seismic network to immediately determine the CMT solution (Kao et al., 1998). The main shock focal mechanism was determined by BATS as a thrust fault striking in an N-S direction (Fig. 1). In the best fit double-couple mechanism had the following parameters: for plane 1, strike = 23°, dip = 42° and slip = 104°; for plane 2, strike = 184°, dip = 49° and slip = 77°. A similar solution was also reported by the USGS CMT solution. The focal depth of this event was located at the depth of 25 km by the BATS on the basis of the waveform inversion of the broadband seismograms. Fault normal cross-section of its aftershocks presented a distribution of eastward variable-dip angle seismic zone from near vertical at shallow depth and gradually decreasing its dip angle with depth (Lin, 2004). No obvious surface ruptures were found during this event. This event has been debated by the inconsistent fault slips determined by seismic and geodetic observations. A pure thrust fault plane solution has been estimated by seismic waveform inversion; however, thrust slips with obvious northward horizontal motions are necessary to interpret GPS co-seismic deformation. Complex geometries of a fault plane and fault creep are both considered as possible interpretations (Wu et al., 2006). 2.1. Observations
2. Major large earthquakes on the Longitudinal Valley of eastern Taiwan On November 25, 1951, a 7.3 magnitude earthquake occurred in eastern Taiwan. Hsu (1962) reported the observation of surface faulting after a series of large earthquakes in the LV in 1952. Several large earthquakes occurred in the eastern part of the LV near the city of Hualien and many aftershocks, a few with magnitude greater than 6, followed. Surface ruptures of this event took place in several place, Hsu (1962) associated it with Yuli fault. An oblique left-lateral surface rupture along the Longitudinal Valley with fault length 43 km was reported (Cheng, 1995). On its eastern side, a 10 km long, left-lateral fault trace which was parallel to the main fault was also found. However, it is difficult to assess the relative importance of dip–slip and strike– slip fault movement from surface faulting evidence alone. On May 28, 1992, a major earthquake (Origin time: 5/28/1992 23:19:35.6 GMT; ML = 5.4; 23.15 N, 121.35E; depth = 13.7) occurred again in the Chengkung area. No surface rupture was found. A permanent station of the Central Weather Bureau (CWB) has recorded this event with epicental distance about 5 km, not only the far-field but also the near-field displacements were clearly recorded. According to the full waveform modeling, the overall observed and synthetic seismograms are in good agreement for each component by the wave form inversion. In the best fit double-couple mechanism had the following parameters: for plane 1, strike = 280°, dip = 72° and slip = 170°; for plane 2, strike = 14°, dip = 81° and slip = 18°. It can be considered as near a pure thrust-faulting earthquake (Huang, 1994). On 10 December, 2003, a major earthquake [location = 23.058°N, 121.379°E; depth = 23 km; Mw = 6.6 after USGS]
The 10 December 2003 Chengkung earthquake was the first event, after the 1999 Chi-Chi earthquake, to occur in the Longitudinal Valley region with a magnitude greater than 6. It is also the first major eastern Taiwan inland event simultaneously recorded by both dense strong motion and GPS networks. More detail descriptions about observations within this event are following: 2.1.1. Strong motion array observations A strong motion network equipped with more than 400 freefield digital accelerometers and operated by the Central Weather Bureau has been constructed to monitor earthquake activities in Taiwan since 1990 (Shin, 1993). Each station in this network contains a force-balance accelerometer sensor with a flat response from DC to 50 Hz and signals are digitized at 200 samples per second or higher with a 16-bit or higher resolution (Liu et al.,1999). These accelerometer sensors are ± 2 g full scale and recorders have pre-event and post-event memory. The digital accelerometers are operated in the trigger mode. Most strong motion instruments are equipped with GPS to provide absolute timing. The 2003 Chengkung earthquake was well recorded by stations in the Longitudinal Valley region. This data set represents the densest near source strong ground motion observations for this area. The 54 free-field 3-component strong motion data were recorded with an epicentral distance less than 60 km during this event (Fig. 2). After correcting onset timing for those non-absolute timing records using the same procedure followed by Huang (2000), the network seismograms can be truly displayed according to its epicenter distance. Fig. 3
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Fig. 2. Map of southeastern Taiwan and strong motion array configuration. The blue triangles represent the strong motion stations of the CWB seismic network used in this study. Epicenter of the 2003 Chengkung earthquake is plotted as large red circle. Middle size red circles represent major aftershocks of the 2003 Chengkung earthquake with a magnitude greater than 5.0. Small blue dots represent one month's seismicity after the 2003 Chengkung earthquake. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
shows the corrected vertical component acceleration seismograms. It was found that the major seismic energy was suitably aligned. Although the focal mechanism of this earthquake can be determined by regional short-period and global seismic networks, however, in principle, first P-motion and CMT solutions do not provide further information about later rupture information of source rupture. For example, double events within an earthquake, the second event with different fault mechanism cannot be determined. However, the reconstructed seismic wavefield observed in near field has resolution to identify the possible second event with different focal mechanism. Herein, the observed dense array wave-fields provided an opportunity to independently examine the source parameters determined by regional short-period or global
seismic network, and to discuss the source rupture behavior of this earthquake. 2.1.2. GPS observations Before the 2003 Chengkung earthquake, many continuous GPS and campaign-surveyed stations have been installed and well maintained in the southeastern Taiwan area. Immediately after the earthquake, the GPS field survey around the epicentral area was repeated to measure the co-seismic and postseismic surface displacements (Chen et al., 2006). Finally, processed data from eighteen continuously recording GPS stations and more than 86 campaign-surveyed stations were used to detect the co-seismic crustal deformation in the region around epicenter of the Chengkung earthquake (Fig. 4).
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Fig. 3. Vertical component acceleration record section for the 2003 Chengkung earthquake with a distance from 0 to 70 km. The vertical bar in each seismogram represents the picking for its first P-arrival time.
Significant displacements have been observed in particular for the stations near the epicenter. The co-seismic horizontal displacements in the hanging-wall showed a fan-shape distribution with vectors towards the west and all of the GPS stations were raised by the Chengkung earthquake. The elevation changes generally decreased away from the epicenter area (Chen et al., 2006). According to the data
recorded by the near fault continuously recording GPS stations, processed results provide detailed information not only for the co-seismic deformation but also for the temporal variations of post-seismic deformation. Several nearby station pairs, which showed reverse crustal motions, were examined to study the temporal variations of fault movements. Among them, only two continuous station pairs (Fig. 5) showed
Fig. 4. (a) Horizontal co-seismic displacements relative to Paisha, Penghu. The 95% confidence error ellipse is shown at the tip of each co-seismic vector. The epicenter is marked by a solid star. LV and LVF denote the Longitudinal Valley and Longitudinal Valley Fault, respectively. (b) The elevation changes in GPS height for the coseismic displacement (modified from Chen et al., 2006).
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Fig. 5. Position variations in two near fault station pairs, SHAN-TAPO and S105-ERPN (station positions can be found in Fig. 4). For each subplot, the abscissa denotes the time and year, and the ordinates are the north, east and up components of each baseline, respectively. The central vertical line in each subplot indicates the time of the earthquake, the number on the left hand side of the line expresses the relative co-seismic displacement and the number on the right hand side of the line means the relative post-seismic displacement of the earthquake (modified from Chen et al., 2006).
significant post-seismic movements. Results from both pairs indicated that post-seismic movements occurred at Chihshang and its southern area (Chen et al., 2006). 3. Analysis and results During the 10 December 2003 Chengkung earthquake, strong ground motions were well recorded by the densely deployed stations in the source area. Spatial wave-fields (snapshots) could, therefore, be reconstructed using the recorded data. In this study, to enhance the coherence seismic wave propagation characteristics, the recorded accelerograms were previously converted to velocity seismograms and utilized a band-pass filter with corner frequencies at 0.1 and 0.5 Hz, respectively for the further analysis. The spatial wave-fields were interpolated from the common-time amplitudes of records (Huang, 2000). One example is shown in Fig. 6 and represents instantaneous, vertical velocity distribution at time 24.8 s after the onset of the source rupture. To reconstruct this snapshot, the griding algorithm developed by Smith and Wessel (1990) was employed. The same procedure used to construct Fig. 6 could be easily applied to other selected times and other horizontal components. Waveform analysis can be subsequently based on
serial discrete snapshots at equal time intervals. An alternative form of presentation of surface ground motion is illustrated through animation using successive displays of dense timesampled snapshots. Such animation can be easily constructed and displayed using a variety of available visualization software on personal computers. Using animation, the ground motion, the seismic energy radiated from the rupture plane and the propagation of disturbances and wave fronts over the surface can be observed in an uninterrupted fashion. This form of presentation is most helpful in the interpretation of source behavior and wave propagation effects. Based upon the successive presentation of the snapshots in dense time steps, the near source coherent waveforms of the 2003 Chengkung earthquake can be thoroughly traced during propagation. Some complex waveforms may be induced by the multiple source asperities in the fault plane, seismic waves trapped inside the fault zone and soft soil amplifications. In Fig. 7, snapshots at several time steps were selected. Each panel in Fig. 7 represents the vertical velocity of ground surface at a given time. These snapshots provided a time history of the spatially-dependent wave-fields. Based upon those snapshots, wave propagation features can be clearly followed and discussed. Analysis of this event shows that the
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Fig. 6. Spatial distribution of vertical velocities (original time 24.8 s) near the source area of the 2003 Chengkung earthquake. To avoid spatial aliasing, the seismograms were processed with a band-pass filter at a period of 2–10 s and are the same as figures in Fig. 7. The amplitudes are normalized following the color bar.
filtered ground motions have major amplitudes in its horizontal directions and the dominant radiated seismic energy was recognized as a direct S-wave emerged at the surface 5 s after the earthquake occurrence. Most radiated seismic energy surrounded the epicenter (Fig. 7). The major characteristics of the 2-D ground motion were its symmetric circular wave fronts spreading over the source area. From successive
snapshots, it was found that seismic energy radiation from the source rupture was completed within 25 s after the initiation of the rupture (Fig. 7) and the duration of the source rupture was estimated to be 20 s. From the 34.8 second snapshot of Fig. 7, seismic amplifications were found to be trapped along the Longitudinal Valley where thick sediments were locally distributed.
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Fig. 7. Ground motion snapshots of vertical velocity in the near-source area. The first snapshot, on the left at the top, was taken 5 s after the onset of the source rupture. The succeeding panels were snapshots with original times as noted in each panel. The last snapshot depicts the surface motion 44 s after the initiation of the rupture process. For amplitudes refer to color bar of Fig. 6.
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According to BATS CMT solution as focal mechanism, finite fault modeling for co-seismic crustal deformation of the 10 December 2003 Chengkung earthquake was computed using a numerical code for modeling co-seismic response of the Earth's crust to earthquakes (Wang et al., 2006) and shown in Fig. 8. The rupture is considered as similar fault geometry as BATS CMT solution to simulate the Chengkung earthquake. It is found that the horizontal slips are symmetric on both sides of fault end. The vertical deformation shows a simple upward movement pattern in its hanging-wall. A series fault models with various dip angles, fault widths and lengths are tested to compare with the GPS determined co-seismic crustal deformation of this event (Fig. 4). No model shows similar pattern of observed results. However, the GPS observed co-seismic deformation pattern is similar to the computation of a two fault segment model (Fig. 9). This model includes one square pure thrust fault plane with a major strike–slip oblique thrust fault superposed on its northern end. To verify the GPS recorded strike–slip faulting related to aftershocks occurred within this earthquake sequence, it is helpful to examine the rupture properties of major aftershocks. Fortunately, the focal mechanisms of the Chengkung earthquake sequence have been well determined by the regional Taiwan broadband seismic network (thus, BATS). In this study, BATS was employed to determine the aftershocks from December 10, 2003 (mainshock) to January 25, 2004 and has been employed to estimate seismic energy release. The determined focal mechanisms are shown in Fig. 10. As shown in Fig. 10(a), each aftershock is plotted at its epicenter as a double-couple focal mechanism of ‘beachball’ stereoplots (an equal-area projection of the lower hemisphere, with compression dihedra in black and extension dihedra in white). It was found that most of aftershocks show pure thrust focal mechanisms. The cross-section projected all events in the direction of azimuth N122°E is shown in Fig. 10(b) and revealed a clear east-dipping fault plane. No observable strike– slip fault mechanism events were found within this earthquake sequence. In this study, the stress inversion method based on the slip shear stress component (SSSC) criterion (Angelier, 2002) was applied to determine the regional stress direction which is related to the earthquake sequences. During this earthquake sequences, all the BATS determined focal mechanisms within a 1° × 1° area (as shown in Fig. 10) are selected for stress inversion. Stress directions with confidence ellipses from inversion of focal mechanisms of earthquakes for the Chengkung earthquake sequence is shown in Fig. 10(c) and based upon a steroplot (Schmidt's projection, lower hemisphere). Numerical information about stress tensors and inversion parameters can be found in Table 1. The averaged stress inversion can be considered as the maximum principle direction of a pure thrust fault rupture as the same as those major aftershocks. Furthermore, it also implicated that major seismic energy were released by thrust type aftershocks. Combined with information from dense seismic array analysis, the aftershock and GPS observations, analyzed results of this study offer some constraints for the study of the entire earthquake sequence source rupture behavior.
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4. Discussion and conclusions From 2-D ground motions animation, successive snapshots and snapshot panels of Fig. 7 reveal that the long period seismic wave-fields show a clearly symmetric pattern with respect to the earthquake epicenter. Those near-field strong motion wave-field characteristics were confirmed by numerical simulations caused by a pure thrust fault rupture (Huang, 1989; Huang et al., 2004). Based on the method developed by Huang et al., (2004), the computed 2-D near-source wave field, using a single pure thrust fault mode which is the same source model to compute coseismic crustal deformation of Fig. 8, is shown in Fig. 11. The major subsidence is found in the footwall side of fault and this pattern is consistent to the snapshots as shown in Fig. 7. Implication of this study indicated that the 2-D ground motion simulation presents independent evidence for the focal mechanism of the 2003 Chungkung earthquake as a thrust event with limited strike direction slip. The thrust fault characteristic estimated in this study is consistent to the determined CMT solutions according to the regional broadband seismic network (BATS) and global seismic network (reported by USGS) and indicates that the radiated seismic energy recorded on near source, regional and teleseismic distances show a pure thrust fault behavior. No obvious second event of strike–slip faulting was found during the rupture process of this event. However, numerical co-seismic crustal deformation modeling from a simple pure thrust model (Fig. 8) do not show consistent results as observations based on GPS data (Fig. 5). The co-seismic observations showed that, near the Chengkung area, the block east of Longitudinal Valley moved to the west with a direction similar to the plate motion direction. However, crustal motion north of the epicenter showed a large left-lateral horizontal slip component. Based upon co-seismic deformation determined by GPS observations, a pure thrust fault plane with an oblique left-lateral thrust faulting is required to fit the source mechanism of the 2003 Chengkung earthquake (Fig. 9). According to the two fault segments model, 2-D ground motion were simulated as shown in Fig. 12. It is found that the pattern is very different to the observed 2-D ground motions (Fig. 7). Thus, the fault model fit the co-seismic crustal deformation cannot fit the seismic energy release recorded by the strong motion array. Other source ruptures models determined by coseismic crustal deformation modeling are both a single fault plane model with significant strike–slip movements and a double fault plane model equipped with one pure deep thrust fault and a shallow strike–slip fault, respectively (Cheng et al., 2004; Wu et al., 2006). Similar numerical modeling indicated that 2-D ground motion simulation based on both models do not fit the observed 2-D ground motions. The inconsistent results from seismic moment tensor inversion and GPS co-seismic crustal deformation inversion have been debated not only for its source rupture types but also its tectonic implications. Several models may explain this conflict. In this study, a model of sudden thrust slip with horizontal creeping movement fault system has been proposed to interpret the rupture behavior of this event because co-seismic deformation determined from
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Fig. 8. Numerical simulation for co-seismic crustal deformation of a pure thrust faulting with uniform slips within a source area of 15 × 15 km2 and 15 km beneath surface. Fault parameters are strike = 0°, dip = 60° and slip = 90°. The rectangular within each panel is the projection of the modeled fault-plan.
the campaign-surveyed GPS observations should include crustal movement within several days after the earthquake. The horizontal creeping along existing faults cannot be separated from co-seismic deformation. Actually, monitoring active fault creep in southeast Taiwan has a long history. Clearly horizontal aseismic slips near the source region of this event have been well documented (Yu and Kuo, 2001; Lee et al., 2001). Furthermore, creepmeter observations across the Chihshang fault which was ruptured by this event showed limited horizontal co-seismic shortening, but significant post-seismic creep (Lee et al., 2005). The creeping behavior is also found on the near source continuous GPS observations as significant
post-seismic horizontal crustal deformation near Chengkung and its southern area (Fig. 5). It shows limited co-seismic deformations but obvious after-slips between both sides of fault trace. Integrating all available information, the source process of the 2003 Chengkung earthquake can be considered to be induced by the action of oblique plate collision on the LVF suture zone. Thus, in the southeastern Taiwan region, the plate convergent direction is not perpendicular to the orientation of the plate boundary and the major faults should be thrusts with strike–slip components. During the interseismic period, the elastic rebound seismic energy is usually accumulated along the
Fig. 9. Numerical simulation for co-seismic crustal deformation of a two segment faulting model. Each model is assumed as uniform slips within a source area of 15 × 15 km2 and 15 km beneath surface. Left segment with fault parameters of strike = 0°, dip = 60° and slip = 90° and right segment with fault parameters of strike = 0°, dip = 80° and slip = 40°.
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Fig. 10. Analyzed results of stress inversion of focal mechanisms of earthquakes for the Chengkung earthquake sequence, from December 10, 2003 (mainshock) to January 25, 2004. (a) A map of earthquakes considered in the inversion. (b) A cross-section showing the same earthquakes. (c) The results of the inversion. Axes σ1 (maximum compressive stress), σ2 (intermediate stress) and σ3 (minimum stress) shown as five-branched star, four-branched star and three-branched star, respectively. Confidence ellipses are also shown (60%, 75% and 90%). Pairs of large arrows indicate the resulting directions compression (convergent arrows). Grey patterns around arrows indicate confidence intervals (increasingly darker for 90%, 75%, and 60%). The value of the ratio of principal stress differences, Φ = (σ2 − σ3)/ (σ1 − σ3), is indicated within a bar from 0 (base) to 1 (top). Numerical information about stress tensors and inversion parameters are listed in Table 1.
plate boundary perpendicular direction and the stress which parallels to the plate boundary is generally released in creep type deformation. When an earthquake occurs, seismic waves are released from thrust component rebound, and the horizontal slips of fault represent an adjustment of total plate motions and limited seismic energy are released from horizontal plate motions. The complex fault ruptures are possibly induced by fault interactions from a sudden regional stress release and/or seismic wave triggering during the earthquake. The implication of this study indicates that partition of seismogenic and aseismic
slip may have occurred simultaneously in eastern Taiwan and seismic observations based on dense seismic array, GPS and creepmeters, will help to resolve the time-dependent processes of earthquake faulting. Thus, the fault rupture can be analyzed as a quasi-static rupture process. However, as reported by Chen et al., (2006), the detailed horizontal creep cannot be resolved by low sampling rate GPS observations and the same as it cannot be resolved by strong motion array analysis reported by this study. Furthermore, the source processes of slip separation can happen in a single fault plane, but it is also possible to occur
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Table 1 Results of stress inversion of focal mechanisms of earthquakes for the Chengkung earthquake sequence Reference
BATS
ωacc (%)
N
60
14
acc
N
rej
5
σ1 axis
σ2 axis
σ3 axis
Trend
Plunge
Trend
Plunge
Trend
Plunge
296
3
27
13
194
77
Φ
ωm (%)
tm (%)
αm (deg)
0.50
88 ± 10
91 ± 10
12 ± 8
ωacc, minimum ratio omega requested for retaining a datum. ω may range from −100 (total misfit) to 100 (perfect fit). Nacc, number of accepted data (noted in terms of nodal planes). Nrej represents the number of rejected data. The orientations of principal stress axes σ1 (maximum compressive stress), σ2 (intermediate stress) and σ3 (minimum stress) given in terms of trends and plunges, in degrees. Φ, ratio of principal stress differences, Φ = (σ2 − σ3)/(σ1 − σ3). ωm, average ratio omega of accepted data. τm, average dimensionless shear stress from 0 to 100% (that is, average shear stress with respect to maximum shear stress). αm, represents the average angle between shear stress and slip, from 0 to 180°. Note that the main estimator, ω, and the two related subsidiary estimators, τ and α, are obtained through consideration of both the nodal planes (a choice between nodal planes would result in better values, but has not been done). For complete explanation of the method, the reader is referred to the description by Angelier (2002).
on different fault planes. The identification of fault types is important to rupture process details and seismic hazard reduction. However, until recently, not enough data were available to identify this difference. To completely understand the quasi-static rupture process of an earthquake, the installation of a high sampling rate strainmeter and continuous GPS near a fault are necessary. Indeed, the source process of the 2003 Chengkung earthquake is not a unique case in eastern Taiwan. Wu et al. (1989) reported three large earthquakes in the vicinity of the Longitudinal Valley having predominantly thrust focal mechanisms. From a geological point of view, the LVF is an active fault with significant left-
lateral strike–slip component. A large earthquake in eastern Taiwan with left-lateral strike–slip mechanism becomes a highly possible source candidate. However, if the thrust slip is the nature of faulting in the Longitudinal Valley during major earthquakes, the design for seismic hazards, the rupture behavior of earthquake source and its possible implications for tectonic setting will be dramatically different as our presently understanding. For example, reexamination for the fault rupture of the 1951 magnitude 7.3 earthquake, although no seismic waveform data was available to determine its source mechanism and left-lateral surface breaks were found during the earthquake (Cheng, 1995), this event has the possibility of being a pure thrust event and
Fig. 11. Numerical simulation for 2-D ground displacements using the same fault model as Fig. 8. Color tape of each panel shows relative amplitude of snapshot. Arrival time of each snapshot is shown in the top. A simple pure thrust faulting was employed in this modeling. The rectangular square within each panel is the projection of the modeled fault-plan.
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Fig. 12. Numerical simulation for 2-D ground displacements using the same fault model as Fig. 9. Definitions are the same as Fig. 11. A two fault segment model was employed in this modeling. Two rectangular squares within each panel are the projection of the modeled fault-plans.
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