Seismic Velocity Anomalies beneath the Tatun Volcano Group, Northern Taiwan
Date Issued
2015
Date
2015
Author(s)
Lin, Tzu-Yu
Abstract
The Tatun Volcano Group (TVG) is situated in northern Taiwan, where hydrothermal activity is prevailing. According to the empirical definition of active volcano, TVG can be categorized as composed of active volcanoes. Therefore, scientists are eager to know whether there is a magma chamber beneath the TVG. Since earthquakes occurred in this area are too shallow to get a good local seismic tomography, it’s difficult to estimate possible velocity anomaly of magma chamber and/or hydrothermal system. Instead, we use natural seismic waves generated by large earthquakes worldwide to image the possible velocity anomaly beneath TVG. We systematically compare the differences of first arrival times from tele-seismic earthquakes in 2014 for finding any low-velocity zone within the crust. The waveform data was obtained from Taiwan Volcano Observatory – Tatun (TVO). If the seismic waves pass the low-velocity zone, the arrival-time will be delayed. We used cross-correlation function to get first-arrival times from those earthquake records, and calculated theoretical arrival times through least squares method. Then, we compared “observed” arrival times and “calculated” arrival times for examining possible delay as seismic waves passing through the low-velocity zones beneath TVG if there is any. The results show that arrival times of some of the tele-earthquakes occurred on the other side of the Earth appear to have significant delay, particularly for those recorded at Station Ba-Yan (BY) and near by. In addition, we drew the ray-paths of those earthquake waves with arrival-time delay, and tried to find the intersection of ray-paths. The obtained “crossing area” is under BY and Da-You-Keng (DYK) area, about 10 km deep. This may imply the existence of possible low-velocity zones, which is likely an indication of partial melting materials, existing beneath Stations BY and DYK.
Subjects
Tatun Volcano Group
First-arrival time
Low-velocity zone
Tele-earthquake
Seismic wave
Type
thesis
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