Temporal changes in shear-wave splitting at an isolated swarm of small earthquakes in 1992 near Dongfang, Hainan Island, southern China

Abstract

An isolated swarm of small earthquakes occurred in 1992, near Dongfang on Hainan Island, southern China. The Institute of Geophysics, State Seismological Bureau of China, monitored the swarm with five DCS‐302 digital accelerometers for three months from 1992 June 1. 18 earthquakes, with magnitudes ML ranging from 1.8 to 3.6, were well located by five stations, and shear‐wave splitting varying azimuthally was analysed on 27 seismic records from these events. The mean polarization azimuth of the faster shear wave was WNW. Time delays between the split shear waves at two stations varied with time and space. The time delays at one station fell abruptly after earthquakes of magnitudes 3.1 and 3.6, but did not change significantly at the second station. This behaviour is consistent with the delay‐time changes being caused by changes in the aspect ratio of vertical liquid‐filled (EDA) cracks. Thus, the variation in shear‐wave‐splitting time delay could be due to changes in crustal stress related to nearby small‐magnitude earthquake activity. The connection between earthquake activity and crustal stress variation measured by shear‐wave splitting leaves the door open for possible observations of crustal stress transients related to the onset of an earthquake; however, our data cannot be considered as definite evidence for such precursors.

Shear-Wave Splitting in a Critical Crust: the Next Step

“…The approach of fracture criticality, and the consequent earthquake, could be recognized by monitoring shear-wave splitting as above small earthquakes. Such changes have been observed (with hindsight) by Crampin et al (1990Crampin et al ( , 1991, Booth et al (1990), Liu et al (1997), and Gao et al (1998). More controlled investigations could be made by cross-hole observations between deviated boreholes (Crampin and Zatsepin, 1997b).…”

“…[3] Heffer and Bevan (1990) (1997), Gao et al (1998) [4] [9] Crampin et al (1998) Crampin (1997a [10] King et al (1994) [5] [11] Zatsepin and Crampin (1996) [6] Crampin and Booth (1989) [12] Crampin et al (1997).…”

Anisotropy Study of the February 4th 2008 Swarm in Nw Peloponnesus (Greece)

“…Temporal variations in shear-wave time-delays have been observed before several earthquakes with magnitudes M 1.7 to M 7.7 (Crampin and Peacock, 2008). These observations have been performed in different regions, worldwide, such as Iceland (Crampin et al, 1999), China (Gao et al, 1998), U.S.A. (Liu et al, 1997) and Taiwan (Crampin and Gao, 2005). Whenever there is adequate data, these characteristic patterns of temporal variation are seen before all larger earthquakes and earthquakes can be stress-forecasted (Crampin, 1999).…”

Anisotropy in the Shallow Crust Observed around the San Andreas Fault Before and After the 2004 M 6.0 Parkfield Earthquake

“…Several shear-wave splitting studies near recently ruptured fault zones have claimed to see temporal evolution of the crack field following a large magnitude earthquake or prior to earthquake swarms (e.g., Gao et al, 1998;Tadokoro et al, 1999;Teanby et al, 2004). However, a variety surveys have seen no postseismic change in anisotropic parameters in the months following large magnitude earthquakes (e.g., Cochran et al, 2003;Peng and Ben-Zion, 2004).…”