Moderate-Magnitude Seismicity Remotely Triggered in the Taiwan Region by Large Earthquakes around the Philippine Sea Plate

Seismicity in the Taiwan region was studied before and after the 12 largest seismic events with body-wave magnitudes exceeding 6.5, occurring around the Philippine Sea plate in the 1973 to 1994 period. The total local seismicity rate involving events with magnitudes as small as 2.0 was not affected by remote large events. However , the number of local earthquakes with magnitude ML ------4.5 in the 15 days fol lowing a large event exceeded the number before it in 9 out of 12 cases studied. When earthquakes with ML >---4.0 were considered, then 10 cases showed an increase in seismic activity after the big event. This suggests a 75 to 83% probability of remote triggering. Introduction The triggering of earthquakes in populous areas by large, distant earthquakes has repeatedly been a subject of public speculation. In fact, the relationship between seismic activity in spatially isolated regions has been denied in professional seismological literature. However, the phenomenon was clearly demonstrated after the June 1992 Landers (California) earthquake that provoked a sudden increase in seismicity at distances of up to 1250 km from the epicenter (Hill et al., 1993; Anderson et al. , 1994; Bodin and Gomberg, 1994; Gomberg and Bodin, 1994). The mechanism of the triggering effect remains largely unclear. The most commonly proposed explanation is that some effect of the lowfrequency seismic waves from the mainshock destabilized the faults that were near failure (Anderson et al., 1994). In the meantime, one unexplained observation is that most of the triggered seismicity took place in the western Great Basin area to the north of the epicenter, whereas no discernible triggering was observed along the active San Andreas fault at much closer distances (Hill et al., 1993). One way to study remote triggering is to retroactively examine the existing seismicity catalogs in the world's most active regions. In this study, the area of Taiwan was selected because it is characterized by brisk seismicity rates. Taiwan's regional catalog provides a good opportunity to study the differences in seismic activity before and after major seismic events that have occurred in the western Pacific region throughout the past two decades. Seismotectonics and Database The island of Taiwan is situated on a boundary between the convergent Philippine Sea and Eurasian plates (Fig. 1). At present, the Philippine Sea plate moves toward the Eurasian plate at a rate of 7.1 cm/yr along the N307 ° direction (Seno et al., 1993). Due to the plate interaction, frequent seismicity and rapid crustal deformation are observed in Taiwan (e.g., Tsai, 1986; Yu and Chen, 1994). In 1972, the Taiwan Telemetered Seismic Network (TTSN) with approximately 25 stations was installed on the island to monitor earthquakes in the area of 21 to 26 ° N latitude and 119 to 123 ° E longitude, and it remained in operation until February 1991. The standard error of epicentral determination from this network was less than 5 km (Wang, 1989). Since 1991, the Central Weather Bureau, Taiwan (CWB) has upgraded the TTSN network into the advanced Central Weather Bureau Seismological Network (CWBSN). The number of stations increased to 75, and each station had three-component seismometers. The data management system of the CWBSN has been developed according to the guidelines of the Incorporated Research Institutions for Seismology (IRIS) consortium in the United States. After undergoing quality control, the time series are archived on the mass-storage devices. The TTSN catalog employs a duration magnitude scale. The magnitude cutoff is about 1.5, and the catalog completeness threshold over the region is about 2.0. On average, 380 events with duration magnitudes _-->2.0 are located monthly. The currently operating CWBSN network uses a local magnitude scale, which is calculated from the simulated WoodAnderson seismogram (Shin, 1993). Yeh et al. (1993) converted all the data available from the two networks to the same local magnitude scale. Analysis and Results To check the influence of the biggest earthquakes around the Philippine Sea plate on seismicity near Taiwan, the most significant events within the rectangular area be-