A Seismological Study of Sources: Earthquakes and Landquakes
Date Issued
2012
Date
2012
Author(s)
Chao, Wei-An
Abstract
Due to the density of seismograph stations in Taiwan is among the highest of the world and high seismicity, provides a lot of seismic data of small-to large-sized earthquakes. Frequent earthquakes of various magnitudes provide valuable information for understanding the regional seismotectonic environment. The first part of the dissertation focuses on the coseismic deformation which can be determined from strong-motion records of large earthquakes. The baseline corrections are often required to obtain reliable coseismic deformation because baseline offsets lead to unrealistic permanent displacement. We use strong-motion records from three large earthquakes in Taiwan (1999 Mw7.6 Chi-Chi, 2003 Mw6.8 Chengkung and 2006 Mw6.1 Taitung earthquakes) to illustrate our automatic baseline correction procedure. Based on the comparisons for coseismic deformation results from previous studies, our new procedure is suitable for quick and reliable determination of coseismic deformation from strong-motion records.
According to recent 3D structural models in Taiwan are already available, and efficient numerical methods have been developed for calculating Green’s function, a database is established for strain Green’s tensors (SGTs) obtained by the finite-difference method in this study. With this SGTs database, 3D synthetic seismograms can be computed by simple retrieving the appropriate SGTs from database and applying the reciprocity theorem. Here, we develop an approach to determining the focal mechanisms of small and moderate earthquakes by combining three effective ingredients: (1) an optimization criterion including both first-motion polarity and P-waveform fitting, (2) Green’s functions in 3D velocity models, and (3) a grid search for source depth and focal mechanism based on the genetic algorithm (GA). We apply this approach to determine the focal mechanisms and centroid depths of the 1999 Chia-yi earthquake sequence. Results show dominant strike-slip and thrust mechanisms that are in good agreement with previous results based upon P-wave first-motion polarities and moment-tensor inversions.
To further understand the characteristic of source rupture process, we propose an approach of imaging source slip distribution by direct back-projection of local absolute P-wave displacement in strong-motion records. This approach is applied to estimate the slip distribution of an earthquake occurred in March 4, 2010, in Jiasian in southern Taiwan. Our resulting slip images are consistent with the distribution of aftershocks and slip distributions obtained from dislocation model and finite-fault inversion. Aforementioned highly automatic and efficient approaches allow for rapid determinations of coseismic deformations, fault-plane solutions and slip distributions after earthquakes using strong-motion records, providing important, useful and timely information for seismic hazard mitigation.
Landquakes (e.g. rock collapse, rock slide, debris flow and rock avalanche) induced by the excessive rainfall during typhoon season can be catastrophic, both in loss of human life and to the economy. Thus, landquake risks have become an important issue in global natural mitigation effort. Conventional seismic data analysis offers a unique approach to studying landquake that is independent of and complementary to other types data. In this study, we apply time-frequency analysis to detect 12 landquakes in Taiwan during the passage of Typhoon Morakot in 2009, which resulted in 675 death, 24 missing, and an estimated economic loss of $3.3 billion. These events were recorded by seismic stations of the Broadband Array in Taiwan for Seismology (BATS). We develop an automatic and efficient approach for rapid determination of landquake centroid location and collapse area, and for identifying dam-formation event using records from existing real-time broadband seismic network, thus providing an important alternative for landquake hazard mitigation.
Subjects
strong-motion records
coseismic deformation
genetic algorithm
strain Green’s tensors
fault-plane solution
landquake
Type
thesis
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