‘Negative repeating doublets’ in an aftershock sequence
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2013
Received: 28 June 2012
Accepted: 28 January 2013
Published: 17 September 2013
We observed some ‘negative repeating doublets’, with nearly opposite three-component waveforms, in the aftershock sequence of the 2008 Wenchuan earthquake. The ‘negative repeaters’ are identified by using the broadband seismic record (with frequency range from 1 to 10 Hz) of the Wenchuan station (WCH) with a near epicenter distance from 19.7 to 26.6 km. These opposite three-component waveforms are not due to the changing of polarities of the seismic station.
In recent years, ‘repeating earthquakes’ in the sense of waveform cross-correlation have been observed over a wide range of magnitudes and diverse tectonic environments such as creeping zones of major faults (Vidale et al., 1994; Rubin, 2002), interplate subduction zones (Igarashi, 2003; Uchida et al., 2010), and inland regions (Schaff and Richards, 2004b, 2011). The properties of ‘repeating earthquakes’, such as the magnitude-recurrence interval scaling, have been utilized to study the physics of earthquakes and faulting; for example, fault healing (Vidale et al., 1994), earthquake source properties (Nadeau and McEvilly, 1997; Rubin et al., 1999; Uchida et al., 2007), slip rate at depth (Nadeau and McEvilly, 1999; Li et al., 2011a, b), temporal variation of deep structure (Poupinet et al., 1984; Zhang et al., 2008), and strength of asperities (Sammis and Rice, 2001). ‘Repeating events’ have also been used to evaluate and improve earthquake location practice (Jiang and Wu, 2006; Schaff and Richards, 2004a; Jiang et al., 2008, 2012) and enhance the capability of detecting foreshocks and aftershocks (Schaff, 2010).
In the sense of waveform cross-correlation, ‘repeating earthquakes’ display nearly identical seismograms at a common station, share the same fault patch, have nearly the same centroid of moment release, and probably have similar focal mechanisms (Rubin, 2002; Schaff and Richards, 2004b). It is somewhat interesting, therefore, that in the ‘repeating aftershocks’ of the 2008 Wenchuan earthquake, there are ‘unusual’ repeating doublets, which have nearly opposite three-component waveforms. This kind of ‘negative doublets’ have been mentioned by Hauksson and Shearer (2005) when using the HypoDD algorithm to relocate events in Southern California. Schaff (2010) discussed similar cases of the 1999 Xiuyan, China, earthquake sequence, that some portions of the seismograms are similar while others are nearly opposite. Nevertheless, this kind of ‘negative repeating events’ are not common in observations and worthy of special attention.
2. Data Used and Method for Analysis
Unlike traditional processing, which usually uses vertical-component seismograms, we use the three-component waveforms. A threshold of 0.8 was prescribed for the cross-correlation coefficient (cc) to identify the ‘repeaters’: the case that abs(cc) of each component of two events is not less than 0.8 is used to select the event pair as a ‘repeating doublet’.
3. ‘Repeating Events’ Identified: Regular ‘Repeating Doublets’ versus ‘Negative Repeating Doublets’
Earthquake doublets recorded at the WCH station. Blue circles indicate the doublets with positive correlation coefficients, while red dots indicate the ‘negative doublets’. Red circles show the 2 events which have both ‘positive repeater/s’ and ‘negative repeater/s’. Parameters (as registered in the seismological network bulletins) of these 2 events and their ‘positive repeaters’ and ‘negative repeaters’ are: 138 2009-09-25 04:02:25.90 31.33N 103.69E M 2.7 Event 1 145 2009-10-29 15:18:06.08 31.33N 103.71E M 1.5 positive partner 151 2009-11-26 16:12:25.20 31.33N 103.71E M 1.8 negative partner 202 2011-02-03 06:59:37.74 31.30N 103.70E M 1.7 Event 2 167 2010-02-05 16:15:00.05 31.29N 103.67E M 2.5 positive partner 205 2011-02-23 21:42:04.62 31.30N 103.66E M 2.2 negative partner
4. Conclusions and Discussion
In the aftershock sequence of the 2008 Wenchuan earthquake, from 2008/10/01 to 2011/09/15 around the WFSD-1 drilling site, and identified by the seismic waveforms (within the frequency range 1–10 Hz) of the WCH station, we observed 5 among 155 ‘repeating doublets’ with almost opposite waveforms for all the three components. Such ‘negative repeating doublets’ are not likely the result of a polarity change at the seismic station. Clear and simple P-waveforms and S-waveforms at a near-source station are a useful aid in the study of polarity changes. And this is especially clear when considering three-component, rather than single-component, seismograms.
In the identification of ‘repeating doublets’, a commonly used criterion is that for at least one station, the vertical component seismograms from an ‘earthquake pair’ have a cross-correlation coefficient (cc) no less than 0.8 (for example, Schaff and Richards, 2004b). Compared with other cases, the WCH station in this study is characterized by its near source distance being less than 30 km. Limited near source stations prevent us from undertaking a detailed investigation of the physical cause of these ‘negative doublets’ based on the single-station recordings, although the three-component recordings provide more reliable results of cross-correlation. In the single station case, apparently it is not possible to discriminate whether the ‘negative doublets’ are caused by a ‘reversed focal mechanism’, that is, the slips of the two events are nearly opposite to each other, or they are just caused by a composition of the focal mechanism and the special station-event configuration (referring to figures 4.5 and 4.6 of Aki and Richards, 1980). It is worth noting that the event pair shown in Fig. 3, as well as other pairs which are not shown, are characterized by a large S-phase and small P-phase, with the S-phase being the predominant contributor to the negative cross-correlation coefficient, and the P-phase unstable as per positive or negative cross correlation. Classical seismology shows that the case of takeoff seismic rays near to the normal direction of the seismic rupture, or along its perpendicular direction, may cause this feature of seismograms (Aki and Richards, 1980). This implies that the ‘completely reversed focal mechanism’ case cannot be excluded. Whether or not near source recordings are more suitable to identify the ‘negative repeaters’ is a question to be answered in the future, and whether the ‘negative doublets’ are a special feature of aftershocks, or are common for all types of earthquake sequences, is another open question at the present time.
Throughout the paper, following previous studies, we have used the wording ‘repeating events’. Following convention, we use the word ‘repeating’ in the sense of waveform cross-correlation. Note that the frequency range, 1–10 Hz, determines the resolution of the ‘repeater’ identification. That is, ‘repeating’ events identified by waveform cross-correlation can only constrain the two events within the size of a wavelength (in this case, about 0.5 km), rather than to make sure that the two events are physically ‘repeating’. Also, note that the earthquake itself has a limited size of fracture. In this case, it is still too early to discuss whether the ‘repeating events’, including the ‘negative repeating pairs’, are located in the same fault patch. As shown in our previous studies (Li et al., 2011b), at the present time what can be done practically, rather than to challenge this limit of resolution, is to reduce the spatial range of the traditional analysis of seismicity to the size of a ‘repeating event cluster’. Nevertheless, from the perspective of either the cause of ‘repeating earthquakes’ or the practical need for identifying ‘repeating earthquakes’, such ‘negative repeating doublets’ are worthy of further investigation.
Thanks to L. B. Han, C. S. Jiang, H. S. Peng, and W. T. Wang for help and discussion concerning software and data processing, to X. F. Zheng for providing the waveform data, to P. G. Richards for stimulating discussion, and to the anonymous reviewers for constructive comments and valuable suggestions. This work is supported by the WFSD project.
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