dc.description.abstract | In the past decade, many studies had proved the influences of static stress changes on spatial or temporal distribution of the aftershocks and further large earthquakes. By contrast, other studies have deepened the argument by resolving stress changes on aftershock focal mechanisms, which removes the assumption that the aftershocks are optimally oriented for failure.
The 21st September 1999 Chi-Chi, Taiwan, earthquake (Mw=7.6) produced a remarkable set of data and clearly exhibits stress transfer. Large amount of GPS static measurements and strong motion acceleration records recorded this event. Such a data set offers a unique opportunity to understand the earthquake process and the generation of ground motions. Furthermore, 7 of continuous monitoring GPS stations were set up near the Chelungpu fault, mostly within three weeks after the occurrence of Chi-Chi mainshock and about 80 campaign-surveyed stations were resurveyed up to 7 times from September 1999 to December 2000.
We explore how Coulomb stress transfer might control aftershock distribution, long-term seismicity, and postseismic slip in a ramp-flat thrust fault system. The Mw=7.6 Chi-Chi shock, with a surface-cutting 30°-dipping ramp fault merging into a near-horizontal d?collement, is representative of continental thrust systems throughout the world, and so inferences drawn from this uniquely well-recorded event may be widely applicable elsewhere. The 3D distribution of aftershocks and their focal mechanisms are consistent with the calculated spatial distribution of Coulomb stress changes. Here one compares the percentage of planes on which failure is promoted after the main shock relative to the percentage beforehand. For Chi-Chi we find a 28% increase for thrust and an 18% increase for strike-slip mechanisms, commensurate with increases reported for other large main shocks. However, perhaps the chief criticism of static stress triggering is the difficulty in observing predicted seismicity rate decreases in the stress shadows, or sites of Coulomb stress decrease. Detection of sustained drops in seismicity rate demands a long catalog with a low magnitude of completeness and a high seismicity rate, conditions that are met at Chi-Chi. We find four lobes with statistically significant seismicity rate declines of 40–90% for 50 months, and they coincide with the stress shadows calculated for strikeslip faults, the dominant faulting mechanism. The rate drops are evident in uniform cell calculations, 100-month time series, and by visual inspection of the M≧3 seismicity. An additional reason why detection of such declines has proven so rare emerges from this study: there is a widespread increase in seismicity rate during the first 3 months after Chi-Chi, and perhaps many other main shocks, that might be associated with a different mechanism. And nearly all the M≧6 aftershocks are found to be promoted by several bars as a result of the mainshock.
We further consider whether the stresses imparted by the coseismic slip could have triggered postseismic slip on the fault. We find a fair correlation between the inferred postseismic slip and regions of calculated stress increase on the ramp and d?collement. The correlation of stress with slip is best if the fault friction is very high (μ?=0.8) along the uppermost 5 km of the ramp, and if friction is exceedingly low (μ?=0.0) along the d?collement. Finally, we search for a change in aftershock distribution and rate caused by the postseismic d?collement slip. We find a marked decrease in aftershocks with respect to Omori decay where the postseismic slip is calculated to further depress the Coulomb stress, and an increase of seismicity and the rate of M ≧ 5.0 earthquakes in the corresponding positively stressed zones.
The GPS observations suggest significant slip on the hanging wall of the Chelungpu fault, while little surface deformation is observed on the footwall. Repeated precise leveling survey across central Taiwan also shows significant uplift on the hanging wall of the Chelungpu faults during Aug. 2002 to Mar. 2004. we consider both of the afterslip and viscoelastic rebound behaviors during postseismic period to compare with geodesy data. They show good correlation between GPS observation on the hanging wall with the calculation deformation based on afterslip. On the footwall, by contrast, the calculation deformation based on viscoelastic rebound seems fit the observation better.
We also estimate the stress evolution along the faults near the Chelungpu fault. During coseismic period, the shear stress along Changhua fault seems promoted by mainshock, while the normal stress is dropped. For the Shiaomao and Hsuilikeng faults, the normal stress is promoted, while the shear stress is dropped. During 15 mo. after Chi-Chi, the shear stress is dropped at shallow part of the Changhua fault, while it is enhanced at deep part. It also shows the normal stress along all of the faults is promoted them to failure. 50 years after Chi-Chi, the shear stress at deeper part of the Changhua, Shiaomao and most part of Hsuilikeng is promoted to failure. For the normal stress, except at shallow part of the Changhua fault, it shows stress dropped at rest of the faults. | en_US |