2005年宜蘭雙主震之震源機制解: 應用近場波形反演法探討區域地震機制
The Ilan Earthquake Pair on 5 March 2005: Discussion on Focal Mechanism Determination with Near-field Waveform Inversion
Date Issued
2007
Date
2007
Author(s)
Wang, Tien-Huei
DOI
en-US
Abstract
On 5 March 2005, an earthquake pair occurred at Ilan (121.84°E, 24.66°N), northeastern Taiwan. Both mainshocks have similar magnitude (ML = 5.9 and ML = 6.0). The first one was determined as a normal solution based on first-motion polarities of P wave from Central Weather Bureau Seismic Network (CWBSN) and Taiwan Strong Motion Instrumentation Program (TSMIP), while it was solved as a strike-slip solution by Broadband Array in Taiwan for Seismology (BATS) Centroid Moment Tensor (CMT) inversion. Normal faulting would be consistent with the opening and extension of Okinawa Trough on tectonic basis. On the other hand, a strike-slip solution would provide us one possible answer to the slip accommodation of compression. Thus, obtaining one reliable solution to this focal mechanism has become one important issue.
To resolve the focal mechanism mentioned above, we target the first mainshock for its solution inconsistency in two methods. Firstly we applied Moment Tensor Inversions using much denser TSMIP strong motion array in Ilan region. The kernel computer code was adopted from R. B. Herrmann (1985) and later modified by Huang (1994) and Chang (2005). Synthetic waveform is generated with wavenumber integration method to simulate the three-component time history from hypothesized point source. Results from 23 stations using single station approach suggest that lateral variance of velocity structures underneath Ilan plain are dramatic.
Multi-station inversion helped us to model this event at threshold magnitude(ML = 5.9) as point source. It serves to cancel out velocity variances from each station. We further applied multi-station approach to conceal velocity variation along different paths. The inversion result from fifteen stations provided a more convincing, averaged slip solution. This event was an oblique event in between normal component and strike-slip. The first motion solution simply presents initial rupture motion of an event, which could be irrelevant to subsequent slip motion of the whole cluster. By our result, we can conclude the initial rupture motion to be normal, and later strike-slip motion dominated the rest of this event. The multi-station approach is applicable for near-field TSMIP records to obtain reliable CMT solutions.
Subjects
宜蘭雙主震
波形反演
地震斷層機制解
P 波初動解
波數積分法
地震層析成像
多站反演
Ilan plain
earthquake pair
TSMIP
Moment Tensor Inversion
wavenumber integration
Multi-station inversion
first motion solution
CMT
Type
thesis
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