Rupture Propagation Imaging Across Scales: from Large Earthquakes to Microseismic Events

We have developed a method for tracing the rupture propagation of microseismic events. We referred to it as microseismic rupture propagation imaging (MRPI), which is an adaptation of the back projection technique from global seismology. Hence, we shifted back recorded waveforms to a grid of possible source locations and obtained a coherent phase stack that migrated according to the migration of the rupture front. Using synthetic ruptures and the corresponding waveforms obtained by finitedifference modeling, we tested the viability of the approach for a reservoir model with the properties and geometry of the monitoring system of the Basel-1 geothermal reservoir. First, we found that an estimation of the rupture location, orientation, direction, and length was feasible in this environment. The method was then applied to the four largest events (ML 1⁄4 3.1–3.4) recorded at the Basel-1 reservoir. We found that the obtained rupture lengths and orientations were reasonably consistent with independent estimates from seismic moments, stress drops, and fault-plane solutions. MRPI allowed us to solve the ambiguity between the actual fault plane and the auxiliary plane. The derived fault planes and rupture directions for the three best-determined events indicated that the failure process was directed preferentially from the periphery toward the injection source. This agreed with the observation that hypocenters of large-magnitude-induced events tend to occur on the edges of the stimulated volume. The results also corroborated the recently proposed idea that induced events were more probable to occur on preexisting faults if the potential rupture surface lay within the stimulated volume. INTRODUCTION In global seismology, tracking the rupture fronts of earthquakes with magnitudes from large (M > 6) to megathrust (M > 8) has recently become rather common. The most prominent applications are, e.g., the 2004 Sumatra-Andaman earthquake, the 2010 Maule, Chile, earthquake, or the 2011 Tohoku, Japan, earthquake (e.g., Krüger and Ohrnberger, 2005; Kiser et al., 2011, 2012; Bayer et al., 2012). A frequently used technique is to backproject the teleseismic P-wave seismograms recorded at an array or at a seismic network to a grid of possible source locations. This method is referred to as the back-projection technique (e.g., Ishii et al., 2007; Walker and Shearer, 2009) or the source-scanning algorithm (Kao and Shan, 2007) and is built on the constructive and destructive stack of seismic signals. It provides information on the energy release and yields estimates of rupture properties, such as direction, speed, and duration. To improve the precision, usually, coherent phases recorded by dense and large arrays are used so that different array-based timecorrection techniques can be applied. A slightly different approach is followed by Allmann and Shearer (2007), who stack local S-wave records during the M6.0 Parkfield earthquake, 2004, to identify a high-frequency subevent. Using back projection is a direct and elegant way to obtain independent information on the characteristics of an earthquake. One main advantage of the method is to produce an estimate of the rupture geometry without the assumption of a rupture model. To date, there is, to our knowledge, no method of imaging the rupture geometry of microseismic events. Usually, the characteristic rupture dimensions are inferred indirectly from the measured body-wave spectra, and the results depend on a specific theoretical model (e.g., Brune, 1970; Madariaga, 1976). Few studies have used the empirical Green’s function method to estimate the rupture direction and rupture velocity of clustered microseismic events (e.g., Jost et al., 1998). Here, we modify the global back-projection technique and introduce the microseismic rupture propagation imaging (MRPI). Although the key idea of MRPI is similar to back projection, it difManuscript received by the Editor 3 December 2014; revised manuscript received 1 June 2015. Freie Universität Berlin, Institute of Geophysics, Berlin, Germany. E-mail: jonas.folesky@geophysik.fu-berlin.de; joern@geophysik.fu-berlin.de; shapiro@ geophysik.fu-berlin.de. © 2015 Society of Exploration Geophysicists. All rights reserved. 1 GEOPHYSICS, VOL. 80, NO. 6 (NOVEMBER-DECEMBER 2015); P. 1–9, 9 FIGS. 10.1190/GEO2014-0572.1