Induced microseismic event with strong rupture directivity and superimposed attenuation effects

Rupture directivity is a fundamental effect well known mainly for large natural earthquakes. Its observation for microseismic events is difficult due to small rupture size and short duration, usually insufficient coverage of monitoring array and attenuation along wave propagation paths. Here, we detect the rupture directivity for an induced microseismic event (M (sub w) approximately 1.2) recorded by a dense surface starlike array during hydraulic fracturing of a shale reservoir in China. We use durations of initial P-wave arrivals as a proxy to peak frequency content. The observed directional and offset dependence of the peak frequencies can be explained by superimposed effects of the rupture directivity of fast, possibly supershear rupture propagation and attenuation, permitting the determination of the event's fault plane orientation. Furthermore, we implement a simple statistical correction to the amplitudes, proving the inverted source mechanism to be stable, only with a slightly lower, yet unreliable nonshear component.