dc.description.abstract | The Yellowknife array (YKA), a small-aperture array, recorded many depth phase seismograms from earthquakes which occurred in Taiwan region. Such kinds of phases are unavailable to observe on Taiwan’s local seismic records. The depth phase(pP、pwP、sP、sS and so on) reflected from the surface then traveled to the recording station can provide unequivocal confirmation of focal depth. The pP-P interval depends strongly on focal depth and is independent of clock errors. It can give a good constrain for earthquakes locating that occurred out of seismic network and eliminate a trade-off relationship between the source depth and epicenter determination.
It is a simple and direct way to estimate focal depth using the pP-P interval on teleseismic records. However, the pP phase propagates a little bit longer path than P and dissipates more energy, so that the amplitude of pP phase is normally smaller than direct P. In this study, we proposed to use slant stack procedures to reduce the effects of random noise in the data and to enhance the depth phase signal. The surface reflections arrive behind the direct arrival and may be interfered by scattered P wave energy from both the source and receiver sides. The shape of the interference packet changes with depth and this information supplies a means of estimating the source depth. Comparing the observations with suitable synthetic seismograms will help us to identify the accurate source depth. For this reason, we adopt a forward modeling way to calculate synthetic waveforms in different focal depths to help us to recognize depth phase arrival. Finally, the source depth search can be quantitatively represented by a cross-correlation analysis to find the accuracy focal depth range.
First, we used two events which occurred in the seismic network to check our method is authentic. The result of this analysis showed our method indeed helped us easily control the range of depth and obtain a robust result. Moreover, using forward grid search to look for the best hypocenter location by S-P time difference of TSMIP records provided information about shallow 1-D S wave velocity stracture under the epicenter region at the same time.
Second, we applied our method and procedure to analyze other earthquakes which ML>4.5 and occurred offshore northeastern and southwestern Taiwan. All events are outside of Taiwan seismic network. Some earthquakes were far away from the seismic network and some did not have a good station distribution. The analyzed results of these events showed we can easily control the range of depth and estimate optimal results. The events in offshore southwestern Taiwan were selected from Pingtung Earthquake sequence. In this study, we chose 6 events for analyzing and the best depth solutions were accurately obtained. Analyzed results show that most depths of the offshore Pingtung earthquake sequence are slightly shallower than that reported by CWBSN except for the 2nd event that was the biggest event of the Pingtung earthquake sequence. Combining analyses from near source and teleseismic observations, we concluded that the rupture properties of the 2nd event began at a shallow depth, continued to grow to a depth of 56km where the largest rupture occurred and released the most energy.
Considering the arrival times of surface-reflected phases to locate an event depth, it can provide an excellent constrain in source depth. Moreover, accompanying with the forward waveform simulation in different source depth to help us to identify the accurate depth phase, we took the superiors of both methods to have a result with good stability and high depth resolution. Although this method is mainly applied for the events with a ML from 5.0 to 7.5, which features a complicate rupturing process and sometimes hardly to obtained an idea result from the comparing with theoretical simulation and observed waveforms. However, in overall speaking, it is a reliable method for the events outside of the array in source depth calculation. A simple but robust procedure to identify the depth of a seismic event has been developed and successfully demonstrated.
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