Kuo-Fong Ma

The Chi‐Chi, Taiwan earthquake: Large surface displacements on an inland thrust fault

In the early morning (01:47 local time) of September 21, 1999, the largest earthquake of the century in Taiwan (Mw=7.6, ML=7.3) struck the central island near the small town of Chi-Chi. The hypocenter was located by the Central Weather Bureau Seismological Center at 23.87°N, 120.75°E, with a depth of about 7 km. There were extensive surface ruptures for about 85 km along the Chelungpu fault with vertical thrust and left lateral strike-slip offsets. The maximum displacement of about 9.8 meters is among the largest fault movements ever measured for modern earthquakes. There was severe destruction in the towns of Chungliao, Nantou,Taichung, FengYuan, and Tungshi, with over 2300 fatalities and 8700 injuries.

Slip zone and energetics of a large earthquake from the Taiwan Chelungpu-fault Drilling Project

Determining the seismic fracture energy during an earthquake and understanding the associated creation and development of a fault zone requires a combination of both seismological and geological field data. The actual thickness of the zone that slips during the rupture of a large earthquake is not known and is a key seismological parameter in understanding energy dissipation, rupture processes and seismic efficiency. The 1999 magnitude-7.7 earthquake in Chi-Chi, Taiwan, produced large slip (8 to 10 metres) at or near the surface, which is accessible to borehole drilling and provides a rare opportunity to sample a fault that had large slip in a recent earthquake. Here we present the retrieved cores from the Taiwan Chelungpu-fault Drilling Project and identify the main slip zone associated with the Chi-Chi earthquake. The surface fracture energy estimated from grain sizes in the gouge zone of the fault sample was directly compared to the seismic fracture energy determined from near-field seismic data. From the comparison, the contribution of gouge surface energy to the earthquake breakdown work is quantified to be 6 per cent.

Spatial and Temporal Distribution of Slip for the 1999 Chi-Chi, Taiwan, Earthquake

We investigated the rupture process of the 1999 Chi-Chi, Taiwan, earth- quake, using high-quality near-source strong-motion records, broadband teleseismic displacement waveforms, and well-distributed Global Positioning System (GPS) data. The near-source strong-motion displacement waveforms recorded significant static offsets of up to 8 m. The teleseismic displacement records show a significant pulse with duration of about 18 to 20 sec. Taking into account the surface displacements observed along the Chelungpu fault, we considered two fault geometries: a single planar fault and a two-segment fault with a northeast-striking section near the north- ern end. Using the finite-fault model with variable slip vectors, we derived two models of the temporal and spatial slip distribution of the earthquake. The GPS data provided good surface displacement constraints for the slip-distribution determina- tion. The spatial slip distribution is generally consistent with field observations. The results for the simple fault model show a large asperity located in the region about 25 to 55 km north of the hypocenter with maximum slip of about 15 m. When we use the two-segment model, the asperity further extends to the region where the fault bends toward the northeast with a maximum slip of up to 20 m. A large amount of right-lateral slip beneath station TCU068 is necessary to explain its observed large west movement. It implies a local converging slip at the corner where the fault bends to the northeast. The slip amplitude near the hypocenter is about 3 to 6 m. The seismic moments determined from the various data sets are within the range of 2 to 4 10 27 dyne cm. Most of the slip concentrated at shallow depths (less than 10 km). The total rupture duration is about 28 sec, and the rupture velocity is 75% to 80% of the shear-wave velocity. The slip vector shows a clockwise rotation during the fault rupture. The static stress drop of the large asperity region is comparable with the dynamic stress drop, as observed directly from the slip velocity at the station near the large slip region.

Slip history and dynamic implications of the 1999 Chi‐Chi, Taiwan, earthquake

We investigate the rupture process of the 1999 Chi-Chi, Taiwan, earthquake using extensive near-source observations, including three-component velocity waveforms at 36 strong motion stations and 119 GPS measurements. A three-plane fault geometry derived from our previous inversion using only static data [ Ji et al., 2001 ] is applied. The slip amplitude, rake angle, rupture initiation time, and risetime function are inverted simultaneously with a recently developed finite fault inverse method that combines a wavelet transform approach with a simulated annealing algorithm [ Ji et al., 2002b ]. The inversion results are validated by the forward prediction of an independent data set, the teleseismic P and SH ground velocities, with notable agreement. The results show that the total seismic moment release of this earthquake is 2.7 × 10^20 N m and that most of the slip occurred in a triangular-shaped asperity involving two fault segments, which is consistent with our previous static inversion. The rupture front propagates with an average rupture velocity of ∼2.0 km s^(−1), and the average slip duration (risetime) is 7.2 s. Several interesting observations related to the temporal evolution of the Chi-Chi earthquake are also investigated, including (1) the strong effect of the sinuous fault plane of the Chelungpu fault on spatial and temporal variations in slip history, (2) the intersection of fault 1 and fault 2 not being a strong impediment to the rupture propagation, and (3) the observation that the peak slip velocity near the surface is, in general, higher than on the deeper portion of the fault plane, as predicted by dynamic modeling.

Transition from oblique subduction to collision: Earthquakes in the southernmost Ryukyu arc‐Taiwan region

Tectonic characteristics of the region between Taiwan and the southernmost Ryukyu arc are inferred from a detailed analysis of local seismicity and source parameters of 62 recent earthquakes of 5.5≤mb≤6.6. Five major seismogenic structures can be delineated: the Collision Seismic Zone (CSZ), the Interface Seismic Zone (ISZ), the Wadati-Benioff Seismic Zone (WBSZ), the Lateral Compression Seismic Zone (LCSZ), and the Okinawa Seismic Zone (OSZ). In the CSZ, located along the east coast of Taiwan and offshore, earthquake focal mechanisms show horizontal P axes distributed in two directions, 287°±10° and 333°±16°, possibly reflecting a strain partition associated with the relative plate convergence between the Eurasia plate and the Philippine Sea plate. The corresponding seismic strain tensor indicates a maximum compressive strain rate of 1.2×10−7 yr−1 along 293° and a comparable extension in vertical direction, presumably resulted from plate collision in the region. The geometry of the ISZ, which is distorted significantly at its westernmost end, can be approximated by a north dipping plane that is gradually pushed northward with increasing dip. The seismogenic portion of the interface spans a short depth range from ∼10 km to ∼35 km. A clear pattern of earthquake slip partition is observed; the average slip vector residual is as large as 35°. Seismic strain patterns within the subducted Philippine Sea slab show predominantly downdip extension between 80 and 120 km and downdip compression at ∼270 km, different from the pattern of strain segmentation observed for the rest of the Ryukyu arc where the northern and southern portions are dominated by downdip extension and compression, respectively. Owing to the large convergence obliquity, the slab is descending at a rate significantly slower near Taiwan than in the southern Ryukyu. Thus we interpret the appearance of downdip extension within the subducted lithosphere as a combined result of oblique subduction and the slabs negative buoyancy. A number of thrust or oblique strike-slip earthquakes between ISZ and WBSZ show a consistent pattern of lateral compression with P axes oriented roughly parallel to the local strike of the trench-arc system. They are probably due to the compressive strain originated from the collision and transmitted laterally within the lithosphere. Shallow normal-faulting earthquakes show successive rotation of T axes from approximately N-S in the Okinawa trough to approximately E-W in northeast Taiwan, possibly as a result of interaction between the extension from the opening of the Okinawa trough and the compression from collision. One normal event (January 18, 1991, mb=5.9) occurred in the Central Range with T axis roughly parallel to the structural trend of Taiwan, implying that the nature of the orogeny in Taiwan has changed from “thin-skinned” deformation to lithospheric collision involving the whole crust and uppermost mantle.

Evidence for fault lubrication during the 1999 Chi-Chi, Taiwan, earthquake (Mw7.6)

The ground motion data of the 1999 Chi-Chi, Taiwan, earthquake exhibit a striking difference in frequency content between the north and south portions of the rupture zone. In the north, the ground motion is dominated by large low-frequency displacements with relatively small high-frequency accelerations. The pattern is opposite in the south, with smaller displacements and larger accelerations. We analyze the fault dynamics in light of a fault lubrication mechanism using near-field seismograms and a detailed rupture model. The fault zone contains viscous material (e.g., gouge), in which pressure increases following the Reynolds lubrication equation. When the displacement exceeds a threshold, lubrication pressure becomes high enough to widen the gap, thereby reducing the area of asperity contact. With less asperity contact, the fault slips more smoothly, suppressing high-frequency radiation.

Spatial slip distribution of the September 20, 1999, Chi‐Chi, Taiwan, Earthquake (MW7.6) —Inverted from teleseismic data

The teleseismic waveforms of the MW7.6 September 20, 1999 Chi-Chi earthquake were examined to obtain the quick information on the fault rupture process. The deconvolution results show the fault ruptured from south to the north, and revealed the west movement of the hanging wall to the footwall on the eastern dipping plane. The spatial slip distribution shows that the earthquake was mainly composed by a large asperity with a dimension of about 45km × 15km. The maximum slip was about 8 m located at about 45 km to the north of the epicenter. The slip distributions obtained in this study have a good agreement with the observed surface breaks. The comparable static and the dynamic stress drops of about 11 Mpa in the large slip region indicate that the melting/fluid pressurization might have taken place in this earthquake. It reduces the dynamic friction and results in the large slip observed.

Moment-tensor inversion for offshore earthquakes east of Taiwan and their implications to regional collision

Reliable determination of source parameters for offshore earthquakes east of Taiwan with mb<5.5 was a difficult task because of the poor azimuthal coverage by local network and the lack of signals at teleseismic distances. We take advantage of the recently established “Broadband Array in Taiwan for Seismology” (BATS) to invert seismic moment tensors for 7 such events occurred in 1996. To cope with different patterns of background noise and unknown structural details, we utilize variable frequency bands in the inversion and adapt a two-step procedure to select best velocity models for individual epicenter-station paths. Our results are consistent with the overall patterns of regional collision and indicate that the resulting compressive stress has caused significant intraplate deformation within the Philippine Sea plate. Simulation of the regions geological evolution and orogenic processes should take this factor into account and allow the Philippine Sea plate to deform internally.

Stress orientations of Taiwan Chelungpu‐Fault Drilling Project (TCDP) hole‐A as observed from geophysical logs

[1] The Taiwan Chelungpu-fault Drilling Project (TCDP) drilled a 2-km-deep research borehole to investigate the structure and mechanics of the Chelungpu Fault that ruptured in the 1999 Mw 7.6 Chi-Chi earthquake. Geophysical logs of the TCDP were carried out over depths of 500–1900 m, including Dipole Sonic Imager (DSI) logs and Formation Micro Imager (FMI) logs in order to identify bedding planes, fractures and shear zones. From the continuous core obtained from the borehole, a shear zone at a depth of 1110 meters is interpreted to be the Chelungpu fault, located within the Chinshui Shale, which extends from 1013 to 1300 meters depth. Stress-induced borehole breakouts were observed over nearly the entire length of the wellbore. These data show an overall stress direction (N115E) that is essentially parallel to the regional stress field and parallel to the convergence direction of the Philippine Sea plate with respect to the Eurasian plate. Variability in the average stress direction is seen at various depths. In particular there is a major stress orientation anomaly in the vicinity of the Chelungpu fault. Abrupt stress rotations at depths of 1000 m and 1310 m are close to the Chinshui Shale’s upper and lower boundaries, suggesting the possibility that bedding plane slip occurred during the Chi-Chi earthquake. Citation: Wu, H.-Y., K.-F. Ma, M. Zoback, N. Boness, H. Ito, J.-H. Hung, and S. Hickman (2007), Stress orientations of Taiwan Chelungpu-Fault Drilling Project (TCDP) hole-A as observed from geophysical logs, Geophys. Res. Lett., 34, L01303, doi:10.1029/2006GL028050.

In situ measurement of the hydraulic diffusivity of the active Chelungpu Fault, Taiwan

[1] Hydraulic diffusivity controls fluid pressure and hence affects effective normal stress during rupture. Models suggest a particularly spectacular example of fluid pressurization during the Mw = 7.6 1999 Chichi earthquake when pressurization may have reduced highfrequency shaking in the regions of large slip if the fault was sufficiently sealed. We investigate in situ hydraulic diffusivity which is the key parameter in such models through a cross-hole experiment. We find a diffusivity of D =( 7 ±1 )� 10 � 5 m 2 /s, which is a low value compatible with pressurization of the Chelungpu fault during the earthquake. In most poroelastic media, the hydraulic storativity S lies between 10 � 7 and 10 � 5 ,s o that the transmissivity T along the fault zone is comprised between 10 � 11 m 2 /s and 10 � 9 m 2 /s. The corresponding permeability (10 � 18 –10 � 16 m 2 ) is at most one hundred times larger than the value obtained on core samples from the host rock. The fault zone is overpressurized by 0.06 to 6 MPa, which is between 0.2% and 20% of the lithostatic pressure. Citation: Doan, M. L., E. E. Brodsky, Y. Kano, and K. F. Ma (2006), In situ measurement of the hydraulic diffusivity of the active Chelungpu Fault, Taiwan, Geophys. Res. Lett., 33, L16317, doi:10.1029/2006GL026889.