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Aftershock Observations of the 1999 Chi-Chi, Taiwan Earthquake
Abstract
We conducted aftershock observations of 20 September, 1999 Chi-Chi, Taiwan Earthquake beginning 15 days after the main shock. We deployed 20 seismographs twice in a 100km by 100 km area around the focal area of the main shock. Each observation period lasted one month. Two months of observations yielded about 80GB of continuously recorded aftershock data, from which a large number of aftershocks are identified. Among these we located 736 after-shocks from a four-day record. Taking lateral heterogeneity in the crustal structure into account, we have a clear distribution of the aftershocks. There are three particular trends in the east-west cross-section: an east-dipping plane, a nearly flat plane distribution, and a deeper cluster. These trends correspond to the fault plane of the main shock, the hypothesized decollement between the accretionary wedge, and the upper boundary of the Eurasian Plate, and intra-plate activity respectively. 1999年9月21日(現地時間)に台湾で起きた集集地震の臨時余震観測を行なった.震源断層面を取り囲むように, 100km四方の領域に20点の観測点を配置した.観測は, DATに連続記録を収録する方式で,乾電池で約1ヶ月間稼働することができる.本震発生2週間後の10月7日から12月22日まで観測を行ない,数多くの余震を観測した,このうち初期の4日間のデータを再生し, 736個の余震の震源決定を行なった.地殻の速度構造の不均質を考慮した観測点補正値を導入し,詳細な震源分布を得た.東下がりの余震分布は本震の震源断層面を表し,ほぼ水平な分布は付加帯とユーラシアプレートとの境界面であるデコルマ,そしてより深部の分布はプレート内で発生した地震を表している.
Supplementary resource (1)
Data
February 2013
... The dominant rupture propagated from south to the north, and the seismic energy was concentrated in the northern part in 20 s; although the fault rupture form was still dominated by reverse, it also had a strong strike-slip component. The seismic energy radiation was highly non-uniform, and the major seismic energy radiation was identified from several asperity ruptures [28][29][30][31]. Most of the multiple inherent velocity pulses extracted in our work are consistent with this rupture process. ...
Multi-pulse characteristics of near-fault ground motions, such as the number of inherent pulses, pulse periods, and amplitudes, have notable influences on the response of structures. To investigate these important parameters, an automatic detection procedure, which is conducted on the rough pulse signal that is extracted by the HHT method, is proposed in this work. This procedure can localize all inherent pulses in the time domain independently and discontinuously. Important parameters can be automatically obtained at the same time. Then, statistical relationships between these multi-pulse parameters and earthquake parameters, including moment magnitudes, site conditions (Vs30), rupture distances and types of faults, are investigated comprehensively. With an increasing number of pulses, the multi-pulse ground motions are more likely to be recorded in relatively limited areas. All pulse periods in a velocity record are similar to each other and can be represented by the period of the pulse with the largest energy (TPE). TPEs are almost identical to periods of the first pulse in the time domain (TP1). They are related not only to magnitudes but also to fault-types and site conditions. New empirical models are proposed in this work according to fault-types that can predict most TPEs across all magnitudes (from 5.7 to 7.6). For amplitudes, all PGVs of inherent pulses can be expressed by the PGV of the pulse with the largest energy (PGVE) in a linear attenuation relationship. PGVEs are related to rupture distances, site conditions, and fault-types. New empirical models are also developed in this work.
... From the constraints provided by the excellent coverage of our data in the Chi-Chi earthquake source rupture region, one thing is clear, during the first 15 minutes after the Chi-Chi mainshock most of the aftershocks occurred in an east- dipping narrow zone, which may be associated with the Chi-Chi earthquake rupture zone. Some deeper events were located with focal depths Ͼ20 km and they most likely happened on a high-angle, west-dipping fault (e.g., Hirata et al., 2000;Chen et al., 2002;Lee et al., 2002;Lin and Ando, 2004;Wu et al., 2004). Also Figure 6c shows that the larger aftershocks tend to occur near the junction of the two conjugate faults (dashed lines in Fig. 6c) and along the westdipping fault. ...
Within the hour immediately after the 1999 Chi-Chi mainshock, only 40 aftershocks were located by the Taiwan Central Weather Bureau Seismic Network (CWBSN) because of the power outage in more than half of the island and the limited dynamic range of the CWBSN high-gain instruments. Here, by analyzing 20 nearfield, on-scale records from the Taiwan Strong-Motion Instrumentation Program (TSMIP), we determined a catalog of 296 aftershocks with M-L >= 3.4 within the first hour after the Chi-Chi mainshock. Focal mechanisms were also determined for 24 of these aftershocks. The frequency-magnitude relation obtained from the 296 aftershocks indicates that the catalog is complete above magnitude M-L 4.3. Most of the aftershocks occurred in a small-slip region of the mainshock immediately to the north of the mainshock epicenter. Spatially, the aftershocks appear to migrate downward from the mainshock hypocenter. Later in the hour, the aftershocks began to concentrate in the fringe area of the main rupture. During the first hour, the b-value as defined by events in the 4.3 < M-L < 6 magnitude range is about 1, the value of the background seismicity, and the frequency of larger events is higher.
On April 25th, 2015, the magnitude Mw 7.8 Gorkha earthquake ruptured the Main Himalaya Thrust (MHT) in central Nepal as a result of a thrust type event. Comprehensive evaluations of this event and its aftershocks have been undertaken to understand the locations and mechanisms of the aftershocks. Here we model co-seismic displacement and strain caused by the Gorkha earthquake using different slip models, and then compare those results with the focal information. We have the following conclusions: 1) previous work suggests that there are two aftershock belts after the Gorkha earthquake, and we further suggest that the negative ΔCFS lobe(s) beneath the maximum slip led to the seismic gap between the two seismic belts; 2) aftershocks associated with the Gorkha earthquake occurred not only along the MHT fault plane, but also 10–20 km deeper than the depth of the maximum slip zone, triggered by the ΔCFS increase, this stress change pattern and distribution of aftershocks coincide with the recently proposed ‘top-down’ effect, that the stress pulses produced by earthquake in upper crust can drive aftershocks in the lower crust; 3) we suggest that the normal-fault aftershock 40 km north of the maximum slip zone was triggered by the Gorkha earthquake, the ΔCFS on the nodal plane (strike 178, dip 52, rake −78) driven from different slip models all increase more than 1 bar; 4) The aftershocks' mechanisms are strongly related to the deformation field caused by the mainshock, the displacement directions always nearly parallel to the maximum principal stress directions or the shear directions of the aftershocks' focal mechanism solutions. Our studies give suitable explanations to the distribution of the aftershocks accompanied with the Gorkha earthquake, which is also of great significance for the analysis of aftershocks under the similar seismogenic environments.
https://authors.elsevier.com/c/1ZPJz98wdpiV
We investigated distance measuring indices to express peek ground accelerations in the near source region of reverse faults. Two indices, the equivalent hypocentral distance, Xeq, and the shortest distance to the fault, Xsh, are often applied to ground motion prediction equations (GMPE). Strong motions in the near fault region are calculated based on the Stochastic Green's function method for the hypothetical fault model. Advantage of Xeq in comparison with Xsh is shown to explain the distribution of peak ground accelerations in the near source region, including the hanging wall effect. Near source strong motion records from two events, the 2004 Mid Niigata prefecture earthquake and the 1999 Chi-Chi earthquake in Taiwan, are also investigated. We confirmed that the strong motion generation areas play an important role when we apply the Xeq to GMPE as distance measuring indices.
Microearthquakes with magnitude down to 0.3 were detected by the Taiwan
Chelungpu-fault Drilling Project Borehole Seismometers (TCDPBHS).
Despite the large coseismic slip of 12 m at the drill site during the
1999 Chi-Chi earthquake, our studies show very little seismicity near
the TCDPBHS drill site 6 yr after the Chi-Chi main shock. The
microearthquakes clustered at a depth of 9-12 km, where the Chelungpu
thrust fault turns from a 30° dipping into the horizontal
decollement of the Taiwan fold-and-thrust tectonic structure. Continuous
GPS surveys did not observe post-slip deformation at the larger slip
region and no seismicity was observed near the drill site. Therefore we
suggest that the thrust belt above the decollement is locked during this
interseismic period. We further investigated source parameters of 242
microearthquakes by fitting ω-2-shaped Brune source
spectra to our observation data using a frequency-independent Q model.
We find that the static stress drop increases significantly with
increasing seismic moment. However, due to the intense debate on this
topic of scaling-relations and the related self-similarity of
earthquakes, we further improve the data analysis and correct for path
and site effects using the Projected Landweber Deconvolution (PLD)
method for events within some clusters. The PLD method analyses the
source time functions of the larger and the smaller event by an
iterative technique. As a result we received source dimensions and
stress drops of larger events including path and site effect
corrections. The results from the PLD method are less scattered and also
show a positive relation between static stress drop and seismic moment.
We find a similar positive trend for the apparent stress scaling with
seismic moment.
High-resolution 3-D VP and VS velocity models around the source region of the Mw7.6 Chi-Chi, Taiwan, earthquake show significant lateral and vertical variations. The mainshock occurred within a narrow low velocity zone along the Chelungpu fault. A sudden increase of velocity and seismicity took place across the Shuilikeng fault to the east. Most aftershocks were located in areas of high VP and VS. A sharp east-dipping zone extending from the Shuilikeng fault on the surface to a depth of about 15km separates the low velocity region in the west from the high velocity region in the east. An apparent high-angle west-dipping seismic zone lies at depths of 15–30km beneath the western Central Range. Numerous aftershocks with normal faulting were located west of the middle segment of the Chelungpu fault.
We document the structural context of the 1999 Chi-Chi earthquake (Mw=7.6) in western Taiwan, which is one of the best-instrumented thrust-belt earthquakes. The main surface break and large slip (3–10 m) is on two segments of the shallow otherwise aseismic bedding-parallel Chelungpu–Sanyi thrust system, which shows nearly classic ramp-flat geometry with shallow detachments (1–6 km) in the Pliocene Chinshui Shale and Mio-Pliocene Kueichulin/Tungkeng Formations. However, rupture is complex, involving at least six faults, including a previously unknown deeper thrust (8–10 km) on which the rupture began. We compare the coseismic displacements with a new 3D map of the Chelungpu–Sanyi system. The displacements are spatially and temporally heterogeneous and well correlated with discrete geometric segments of the 3D shape of the fault system. Geodetic displacement vectors are statistically parallel to the nearest adjacent fault segment and are parallel to large-scale oblique fault corrugations. The displacement magnitudes are heterogeneous at several scales, which requires in the long term other non-Chi-Chi events or significant aseismic deformation. The Chelungpu thrust has a total displacement of ∼14 km but the area of largest Chi-Chi slip (∼10 m) is on a newly propagated North Chelungpu Chinshui detachment (∼0.3 km total slip) which shows abnormally smooth rupture dynamics.
We use a Global Positioning System (GPS)-derived surface velocity field of Taiwan for the time period between 1993 and 1999 to infer interseismic slip rates on subsurface faults. We adopt a composite elastic half-space dislocation model constrained by the observed horizontal velocities projected into the direction of plate motion (306°). The GPS data are divided into northern and southern regions and the velocities in each region are projected into single profiles. The model fault geometry includes a shallowly dipping décollement, based on the balanced geological cross-sections in the Coastal Plain and Western Foothills, and a two-segment fault representing the Longitudinal Valley Fault (LVF) in eastern Taiwan. The décollement is composed of two fault segments, one extending west under the Central Range (CR) and one extending east of the LVF, with estimated slip rates of about 35 and 80 mm/yr, respectively. The optimal geometry of décollement is subhorizontal (2°∼11°) at a depth of 8∼9 km. The inferred surface location of the western end point of dislocation in the northern profile is located 15 km east of the Chelungpu Fault, while in the southern section, it is located beneath the Chukou Fault. The elastic dislocation model successfully matches the horizontal velocity data, and predicts elastic strain accumulation in the Western Foothills that will presumably be released in future earthquakes. However, considered over multiple earthquake cycles, our model cannot explain the topography of the CR and thus fails to predict the active mountain building process in Taiwan. This failure indicates that both horizontal and vertical velocity fields require a more complex rheological model that incorporates inelastic behavior.
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