Abstract
Three earthquake sequences, each with two main earthquakes of almost the same magnitudes (ML from 5.9 to 7.0 with differences less than 0.1), have recently been observed in Taiwan. The two largest earthquakes in each sequence occurred with a short delay time between them, were strikingly similar in terms of magnitude, location and/or focal mechanism and are referred to as doublets. They were markedly distinct from typical single mainshock sequences. Our estimated static stress field generated by the first shock in the doublet shows that the second shock and most of their aftershocks were located within a region where static stress increased substantially. Thus, a possible explanation for earthquake doublet is that seismic energy for each shock had accumulated independently within adjacent crustal volumes, separated by an asperity, and that the second shock is triggered by the increased static stress after the first one. An important implication of doublet sequence is that works by emergency response teams after the first earthquake could be made more hazardous by the second.
Highlights
On 26 December 2006, an earthquake sequence with two identical major earthquakes (ML = 7.0) occurred off the southwest coast of Taiwan, approximately 20 ~ 30 km west southwest of Hengchun, a small town at the southern tip of Taiwan (Fig. 1)
Two residents in Hengchun were killed in a house that collapsed after the second major earthquake
The earthquake data used in this study were recorded by two seismic networks in the Taiwan area, the Central Weather Bureau Seismic Networks (CWBSN), with 79 stations on Taiwan (Shin et al 2000) and offshore islands, and Broadband Array in Taiwan for Seismology (BATS), with a total of 22 stations
Summary
On 26 December 2006, an earthquake sequence with two identical major earthquakes (ML = 7.0) occurred off the southwest coast of Taiwan, approximately 20 ~ 30 km west southwest of Hengchun, a small town at the southern tip of Taiwan (Fig. 1) Both the Japan Meteorological Agency and the Pacific Tsunami Warning Center immediately issued a tsunami warning for the South China Sea region, the northern Luzon islands. We have carefully examined all of 214 earthquakes (ML > 5.5) listed on the Earthquake Catalogue from 1974 to 2006 and found another two earthquake sequences with two identical moderate or strong earthquakes whose magnitude differences are less than 0.1 They were the Suao doublet on 6 March 2005 and the South China Sea doublet on 8 Oct. 2006. These two as well as the Hengchun doublets share distinct characteristics and deserve further study
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