International Geologiical Congress - Oslo 2008


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GHZ-04 Earthquake hazard assessment and geotechnics


Fault behaviors of a rapid creeping reverse fault at the plate suture: The Chihshang fault in eastern Taiwan


Jian-Cheng Lee, Academia Sinica (Taiwan)
Jacques Angelier, Geosciences Azur (France)
Hao-Tsu Chu, Central Geological Survey (Taiwan)
Jyr-Ching Hu, National Taiwan University (Taiwan)


The 35-km-long Chihshang fault is one of the most active segments of the Longitudinal Valley Fault, the plate suture between the converging Philippine and Eurasian plates in eastern Taiwan. Two moderate to big earthquakes of M 6.2 and M 6.5 resulted from rupturing of the Chihshang fault with observable surface ruptures, occurred in 1951 and 2003, respectively. In between the 50-year inter-seismic period, the Chihshang fault reveals a seasonal creeping behavior at a rather rapid rate of about 20-30 mm/yr, at least during the last 30 years with instrumental observation. Based on in-site measurements, including creep meters (once per day), campaign near-fault dense geodetic networks (twice per year for leveling and GPS) across the Chihshang fault zone since 1998, together with earlier measurements of markers on civil features since 1989, we characterize the interseismic fault motion at the surface level. During the last 15 years, the Chihshang fault showed a seasonal creep, however, with a significant slow down a few years before the 2003 M=6.5 earthquake. As for the 2003 earthquake, which ruptured at about 20 km in depth, produced only a few cm of surface offset on the fault. By contrast, significant post-seismic slip occurred at the near surface level along the fault, that produced surface rupturing as well as anticline folding for about 10-15 cm horizontal shortening during the 6 months following the main shock. We interpret the large post-seismic near-fault deformation as a result of velocity strengthening of frictional instability at the shallow level, mainly due to thick unconsolidated surface deposits and significant fault gauge. Together strong relation between rainfall (groundwater) and fault movement inferred from the seasonal creep, we anticipate that hydro-mechanical coupling with the fault friction plays an important role on triggering of fault creep, at least at the uppermost km, and might affect the stress/strain in the more deeper part of the fault in the seismogenic zone. A drilling project together with a variety of monitoring of fault movement as well as on-site measurements is now undergoing. We aim at better understanding fault behaviors at the shallow level in a great detail.


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