Study of potential injection-induced earthquakes in Rongchang area with sparse network, Sichuan basin, China

Abstract:

It has been demonstrated that injections of high-pressure fluids into underground formations are capable of inducing significant earthquakes. The seismic hazards caused by fluid injection have become important topics of political and scientific discussion in recent years (Ellsworth,2013). Earthquake source parameters are crucial in studies of injection-induced earthquakes. However the accuracy of source parameters based on traditional travel-time method is limited by the available data recorded in sparse seismic network. The location error is often sizable in the area with sparse network; sometimes it can reach 10 km or more (Bondár et al., 2004). Fortunately, if there are one or two close seismic stations, we can improve the precision of source parameters by analyzing three-component seismic waveforms in details.In this study, we first obtained a shallow crust velocity model using the joint inversion of short-period Rayleigh wave group velocity and ellipticity for Rongchang (ROC) region in Chongqing, China. With the better-constrained velocity model, we relocated earthquakes in the vicinity of the seismic station ROC by analyzing three-component seismic waveforms and then determined the focal depths with the aid of local sPL depth phases. About 160 M~2.0 earthquakes during 2002 to 2014 were relocated with an uncertainty of 1~2 km. Results show that earthquakes in Rongchang area are along a geologic fault in the vicinity of fluid injection wells at depths of 2~5 km. Focal mechanism of the September 10, 2010 M4.7 Rongchang earthquake was inverted using the gCAP method and its b-value was estimated based on enhanced catalog via sliding-window cross-correlation. Focal mechanism of the earthquake is consistent with the fault’s strike; its b-value of 1.7 is significantly different from typical tectonic earthquakes. Therefore we propose that seismicity in Rongchang and adjacent areas tend to be induced by fluid injections.