1) Fault behaviors and seismotectonics along a mega-thrust in the plate suture
During the last couples of years, I spent a great of efforts, together with some collaborators, on studying fault behaviors and seismotectonics along on-land mega-thrust. We were focusing particularly in the plate suture of the Longitudinal Valley Fault between the Philippine Sea plate and Eurasia in eastern Taiwan as a study case. The Longitudinal Valley fault (LVF), revealing a rather rapid slip rate of 2-3 cm/yr, indeed provides a good opportunity for better understanding behaviors of active fault in the plate suture. We have paid particular attention on three areas, from north to south: the Hualien area, the Chihshang fault area, and the Taitung area. Each segment illustrates different behaviors of slip, which reflect mainly on the tectonic context of the area. Hereafter indicates recent progresses of our research in different but relevant topics.
• Fault behaviors vs. tectonic context:
By detailed repeated geodetic measurements, including GPS and leveling, together with the regional geological structural analysis we found that (1) the Hualien area is characterized by a lateral and upward tectonic extrusion, (2) the Chihshang area is characterized by a single major oblique reverse faulting, although minor deformation occurred on the hanging wall as well as the footwall, (3) the Taitung area is characterized by a complex three-fault system, with two strands of east-dipping LVF and a west-dipping blind thrust under the eastern edge of the Central Range.
• Behaviors of creeping fault
Using our self-developed creep meters, combined with the dense near-fault and far-fault campaigned geodetic measurements, we characterized the slip behaviors of the Chihshang fault, a rapid creeping fault at a slip rate of 3 cm/yr, in space and time, from surface to depth. We also highlighted a strong seasonal effect on the shallow creep/locked segment on the Chihshang fault.
• Postseismic slip
We have highlighted the behaviors of shallow post-seismic slip of the 2003 Mw6.8 Chengkung earthquake, which was interpreted as the effect of dynamic frictional instability along the fault plane and has a geologically strong relation with the near-surface thick unconsolidated deposits.
• Near-fault Surface Deformation
We have highlighted the near fault surface deformation along several active segments of the LVF, including the Milun fault in Hualien, the Chihshang fault in Chihshang and the Luyeh fault and the Lichi fault in Taitung.
• Monitoring of active fault and seismic mitigation
We deployed a multi-disciplinary system, which includes GPS, leveling, creep meter, tri-lateration survey, seismometers, groundwater observation, and tiltmeter, to monitor closely the activity of the active faults on the LVF.
2) Characterization of Co-seismic Surface Rupture of the Chi-Chi earthquake
• Regional structure of the 1999 fault planes
We have highlighted the influence of the regional inherited fold structure. The south-plunging regional synclinal fold in the north of the 1999 fault acted as a strain guide and led to a nearly 90-degree rotation of the trend of the surface ruptures in the northern termination. Furthermore, the northernmost segment of the 1999 fault overprinted on pre-existing folds indicate the segment is a growing young active fault segment although it showed a relatively obscure morphological expression compared to the main segment of the Chelungpu fault to the south.
• Structural and kinematics analysis along the scarp of the 1999 surface rupture
We found different types of deformation occurred strongly related to the five fault segments distinguished. We also have made some detailed quantitative study on the surface fault deformation and the fault slip amounts. These studies gave some rare excellent examples to characterize the deformation structures on the reverse fault earthquake scarps.
• Earthquake geology and paleoseismology study
We have conducted a trench study across the 1999 surface ruptures in order to evaluate the paleoseimicity along this reactivated Chelungpu fault. We found a penultimate event at the Wufeng excavation site occurred no longer than 200 years ago. We attributed this paleoearthquake event to be the 1845 big earthquake according to the historical literature.