Focal-Mechanism Determination in Taiwan by Genetic Algorithm

Abstract

We determined the focal-mechanism solutions for earthquakes with magnitude M L ≥ 4:0 that occurred in the Taiwan region between 1991 and 2005. Firstmotion polarities of P waves recorded at over 700 seismic stations in Taiwan were used. Because of the large number of events and stations involved, we implemented the genetic algorithm in a nonlinear global search for the focal-mechanism solutions. The algorithm was tuned and validated through synthetic tests. We finally determined the focal mechanisms of 1635 events with good qualities among 4188 earthquakes. Focal-mechanism solutions for a majority of the earthquakes display a dominant pattern of thrust-fault type reflecting the compressive stress field due to the plate collision. Normal-fault events occurred at intermediate depths in subduction zones, which is likely the result of the bending of the subducting slabs. Strike-slip faults are also found within the Eurasia plate around the Peikang Basement High and in collision zones near Ilan where the geometry of the colliding plates is complex. Our study provides a database of focal mechanisms for studying seismogenic structures and plate tectonics. This database can also be used by structural seismologists to compute synthetics for waveform tomography studies.

The Preliminary Study of the 4 March 2010 Mw 6.3 Jiasian, Taiwan Earthquake Sequence

“…A three-dimensional velocity model (Wu et al 2009b) was used instead of the onedimensional velocity model used in CWBSN location algorithm (Chen 1995). After earthquake relocation, we applied the genetic search algorithm, FPsearch (Wu et al 2008a), to determine the first motion focal mechanisms using. This method provides a quality index, Q fp , to measure if the solution is good.…”

Tectonic Implication of the 5th March 2005, Doublet Earthquake in Ilan, Taiwan

“…Based on seismological observations, Kao et al (1998) suggests that this region is located where an interaction exists between the extension of the opening of OT and the compression of the PSP oblique collision. Generally, normal faulting in this region would be consistent with the opening and extension of the OT (e.g., Wang et al 2000;Shyu et al 2005b;Wu et al 2008a). A strike-slip focal mechanism may imply the lateral extrusion at the transition zone between the Taiwan mountain range and the OT (Liang et al 2005).…”

“…The discrepancies of different focal mechanism solutions are expected given that the solutions obtained from first-motion polarities reflect the high-frequency behavior of the initial earthquake ruptures, whereas the Harvard and BATS solutions are obtained from long-period waveforms and more representative of the average behavior of the entire sources. However, in Taiwan area, only earthquakes occurred in this region have such phenomenon (Wu et al 2008a). Thus, this phenomenon may reflect a tectonic-related characteristics.…”

“…These variations in moment tensor focal mechanisms might result from different input parameters, such as velocity structure, amplitude-ratio, and period, under different algorithms (e.g., Helffrich 1997). On the other hand, a pure normal faulting is determined for the first shock of 2005 March (Wu et al 2008a) based on first P-wave polarities from CWB Seismic Network (CWBSN) and Taiwan Strong Motion Instrumentation Program (TSMIP). Unfortunately, since the first motion polarities of the second shock were strongly influenced by the first shock in the southwest quadrant, no clear first-motion polarity can be identified for the second shock in the third quadrant.…”

“…However, based on first motion polarities, the first shock has a normal focal mechanism (Wu et al 2008a). This discrepancy has caused a debate over the focal mechanism solution because different focal mechanisms have different tectonic implications.…”