Memory effect in M ≥ 7 earthquakes of Taiwan

Abstract

The M ≥ 7 earthquakes that occurred in the Taiwan region during 1906–2006 are taken to study the possibility of memory effect existing in the sequence of those large earthquakes. Those events are all mainshocks. The fluctuation analysis technique is applied to analyze two sequences in terms of earthquake magnitude and inter-event time represented in the natural time domain. For both magnitude and inter-event time, the calculations are made for three data sets, i.e., the original order data, the reverse-order data, and that of the mean values. Calculated results show that the exponents of scaling law of fluctuation versus window length are less than 0.5 for the sequences of both magnitude and inter-event time data. In addition, the phase portraits of two sequent magnitudes and two sequent inter-event times are also applied to explore if large (or small) earthquakes are followed by large (or small) events. Results lead to a negative answer. Together with all types of information in study, we make a conclusion that the earthquake sequence in study is short-term corrected and thus the short-term memory effect would be operative.

Appendix: a brief history of seismic observations in Taiwan

From 1897 to 1901, the seismographs were installed at Keelung, Taipei, Taichung, Tainan, Penghu, and Hengchun. Later, the Wiechert seismographs were installed at Taipei in 1928, at Tainan, Hualien, and Alishan in 1933, at Taitung and Hengchung in 1934, and at Taichung in 1935. Totally, 17 stations, each being equipped with three-component, low-gain displacement seismometers, were constructed. In addition, the old-fashioned accelerometers were also installed at some stations. For all stations, the seismograms were recorded in the analog form. The clocks of all stations were not synchronous, because the technicians at the local stations individually timed the clocks. This is the main shortcoming of the network. There were remarkable errors for the arrival times, thus resulting in high uncertainty in earthquake location. From the study of time residuals at the 17 stations of the network, Chan and Wang (1990) found that the largest residual could be up to 10 s. In the end of the Second World War, this seismic network was transferred to the Taiwan Weather Bureau (now the Central Weather Bureau, CWB). Although instrumentally recorded seismograms are available since 1897, numerous early seismograms cannot be found. Since 1953, a bulletin (with four volumes annually) of earthquake data including the phases and arrival times has been published by the CWB.

From 1972, a network, named as the Taiwan Telemetered Seismographic Network (TTSN) was constructed under the support by the National Science Council. During 1972 to 1991, the TTSN was operated by the Institute of Earth Sciences (IES), Academia Sinica for monitoring earthquakes in Taiwan. This network consisted of 24 stations, each equipped with a vertical high-gain and analog velocity seismometer. The earthquake magnitude used by the TTSN was the duration magnitude. Wang (1989) described this network in details. Since 1991, the old seismic network of CWB has been upgraded and numerous new stations have been added. In 1991, the TTSN was merged into CWB’s network to form a new CWB Seismic Network (CWBSN). The earthquake magnitude of the earthquake catalog has been unified to be the local magnitude. A detailed description about the CWBSN can be found in Shin (1992) and Shin and Chang (2005). At present, the CWBSN is consisted of 72 stations, each equipped with three-component velocity seismometers. The seismograms are recorded in both high- and low-gain forms. This network provides high-quality digital earthquake data to the seismological community. Hence, large earthquakes, at least for those with M ≥ 5, from 1900 were detected completely from regional seismic networks.