Imaging global seismic arrivals by stacking BATS array broadband seismograms

Abstracteismologist uses body wave travel times to research earth structure. They average global travel time curves with a lot of seismic arrival times collected by Global Seismic Network（GSN）to locate earthquakes and make reference earth velocity model. However the aperture of GSN is more than hundred kilometers, GSN data is hard to study the detailed earthquake source and earth structure. Building regional seismic networks is necessary. Because of the non-uniform of global seismic zone distribution, the seismic phases recorded by regional array are different from GSN. Broadband Array in Taiwan for Seismology（BATS） has operated more than ten years and recorded large seismic data for further application. Stacking seismograms from BATS will understand what phases can well record by network.n this study, I selected earthquakes recorded by BATS from 1994 to 2005. Shallow events（depth < 30Km） and deep events（depth 30~100Km） with magnitude（Mw） larger than 6.0 are selected, respectively. The stacking procedure starts by calculating a ratio function, from a short-term-average（STA） to long-term-average（LTA） window for each point along the trace. This is a form of automatic gain control（AGC） that is similar to that used by Shearer（1991） and Earle and Shearer（1994）. Ratio functions are stacked in bins corresponding to the epicentral distance and origin time of each event. The final stack is normalized by the number of traces per bin. Since the visibility of different body-wave phases varies as a function of frequency, the Butterworth band-pass filter is used before stacking. n the result, due to the location of major earthquake belts relative to BATS, the dataset is distributed unevenly in epicenter distance. There is a large spike in the distribution of the shallow events(depth<30km) between 20°~40°,75°~85°,135°~145° due to the seismically active subduction zones in the southwestern Pacific, Italy, and the Andes. The phases like P, Pdif, S, PP, SS, SKP, PKiKP, PKPdif could be observed in BATS. The travel time image provides empirical travel times and is also a useful education tool.