The Precursory Significance of Cumulative Slip of Repeating Earthquake Sequences Prior to

The Precursory Significance of Cumulative Slip of

Repeating Earthquake Sequences Prior to Moderately Strong Earthquakes — A Case Study of Four Remarkable Earthquake Sequences of

Haicheng-Xiuyan1

Li Yutong，Zhang Bo，Wang Liang，and Li Tongxia Earthquake Administration of Liaoning Province，Shenyang 110034，China 【期刊名称】中国地震研究（英文版） 【年(卷),期】2016(030)001 【总页数】11

【关键词】Key words:Earthquake sequence Repeating earthquake Waveform correlation Haicheng-Xiuyan Earthquake Research in China Volume 30，Number 1，2016

The Haicheng-Xiuyan region is an earthquake-prone area in Liaoning Province where earthquake sequences frequently occur and is regarded as the regional seismic window.In this area we found many earthquake events with the highest waveform similarity in the records of the same station from some remarkable seismic sequences，namely repeating earthquake sequences.In principle，rupture areas of the repeating events overlap with each other and are most closely located.Therefore these events may reflect the seismic process near the earthquake fault.In this paper，we identified four remarkable earthquake sequences

of Haicheng-Xiuyan by waveform cross-correlation.The result shows that the cumulative slip of repeating earthquakes is related to moderately strong earthquakes，among which the Xiuyan MS5.4 foreshock sequence has the strongest and most apparent pre-shock accelerating-like slip behavior.

INTRODUCTION

A repeating earthquake is a group of one or two of earthquake events that occur near a samelocation and have highly similar waveforms at the same station.Schaff et al.(2004a)defined a repeating earthquake from a recognizable angle as the waveform correlation coefficient≥0.8 recorded at least by one station.These repeating earthquakes usually appear in a large number in fault creep areas(Nadeau et al.，1995;Vidale et al.，1994;Schaff et al.，1998;Rubin，2002)，plate subduction belts(Wiens et al.，2001;Igarashi et al.，2003)and continental seismic active areas(Schaff et al.，2004a，2004b;Li et al.，2007，2009;Jiang Changsheng et al.，2008;Peng Hanshu et al.，2010). Recent research shows that there is a large amount of repeating earthquake information in the seismic signals from the seismic source of some large earthquakes.For example，there appeared a series of seismic ruptures from the source that lasted for some time before the MW7.6 Izmit earthquake in Turkey in 1999，and the seismic signals became

highly

similar

to

the

lasting

time

and

rupture

acceleration(Bouchon et al.，2011).Ni Sidao et al.(2010)also found that there were dozens of foreshocks with very similar waveforms before the Yushu MS7.1 main shock on April 14，2010.The occurrence sites of these earthquakes were very close to each other.Li et al.(2011)found in the same way that there was a group of repeating earthquakes near the foreshock sources of the Haicheng-Xiuyan MS5.4 earthquake in Liaoning in 1999.Their waveforms were very similar to each other and there was an increasing slip before the main shock.The high concentration of the spatial distribution of these repeating earthquakes，in a strict sense，demonstrates the characteristics of foreshocks which can be used to increase the effectiveness of the recognition of the main shock and measure the fine changes of the underground medium parameters.Therefore the study of the information conveyed by the similar waveforms before the main shocks is very significant for the determination of the nucleation period of main shocks and the precursors of unsteady slip and the slip accelerations before great earthquakes.

Schaff et al.(2004a)found in the study of repeating earthquakes in the Chinese mainland that repeating events may be generated by static or dynamic stress conveyance or the concentration of pore fluid effects on the asperities in the fault and the excitation of the barrier creeping.Templeton et al.(2008)believed that the larger earthquakes

around the asperities can cause increase of local stress，and accelerate the rupture of asperities.This mechanism partly explains the frequent occurrences of some repeating earthquakes.Beside the continuous slip of the fault for the generation of repeating earthquake，volcano eruptions also provide a repeating seismic source.Presently there are some attempts to use volcano eruptions for seismic predictions(Pandolfi et al.，2006;Johnson et al.，2010).

Theoretically，there can be a slow slip when the local stability is lost in the fault before moderately strong earthquakes.We can observe a series of repeating signals temporally from the seismic source.These signals are very slow in slip acceleration at first，but will speed up quickly before the earthquake.In fact，because of the complexity of the geological structure and stress，the temporal distribution of an earthquake sequence will not appear at just one location，but is usually distributed in a certain area.These small earthquakes interact with each other and it is difficult to decide when or whether the fault has lost stability.We can assume however that the internal small earthquake activities can be observed within a small body，and we can be closer to some extent to the actual fault activities.A feasible method currently is to use repeating earthquakes，i.e.by observing the amount and speed of the slip of earthquake sequences with highly similar waveforms generated by the natural repeating seismic sources，we can guess the

probability of a greater earthquake in or around the fault concerned. The Haicheng-Xiuyan region is one with the most active earthquake sequenced in Laoning Province，and also a concentrated area of moderately strong earthquakes.The MS7.3 Haicheng earthquake in 1975 and the MS5.4 Xiuyan earthquake in 1999 occurred in the region.These twoearthquakes are typical examples of successful prediction based on a foreshock sequence.Xue Ding et al.(2012)believed that the Haicheng region has a seismic window function，with some predictive effect for the region and adjunctive regions.Liaoning Digital Seismic Network had an early transformation，generating data as early as in 1999 and has collected abundant digital seismic wave data which serves a basic condition for the repeating earthquake study of the region.To assess the precursory nature of the earthquake sequence in this region，this paper uses waveform correlation to identify the repeating earthquakes of the significant sequences in the region，and the cumulative slip characteristics to determine the correlation with subsequent moderately strong earthquakes.

1 SIGNIFICANT EARTHQUAKE SEQUENCE IN THE

HAICHENG-XIUYAN REGION

The Haicheng-Xiuyan region is located in the north of the Liaodong peninsula in the Liaodong uplift area in the northeast of the China-Korea platform.The rupture structure of the region is well