- © 2014 by the Seismological Society of America
Online Material: Figure, table with double‐difference event locations.
Hydraulic fracturing has been widely applied in development of tight sand and shale gas reservoirs, in which high‐pressure fluids are injected into target zones to enhance the reservoir permeability so that gas can be more efficiently recovered. The opening and growing of tensile fractures, as well as shearing slip along fractures during stimulation treatment are thought to be the major mechanisms inducing microseismic events around the treatment well (Shemeta and Anderson, 2010). Therefore, microseismic monitoring is a valuable approach to assess the fracturing process. For example, the locations of microseismic events are used to determine fracture network geometry, and their focal mechanisms are helpful for understanding how the fractures are stimulated. The information derived from microseismic monitoring is helpful for reservoir simulation and assessment (e.g., Rutledge and Phillips, 2003; Warpinski, 2009; Maxwell, 2010).
However, injecting fluids into underground formations, especially wastewater disposal into deep wells, has caused felt or damaging earthquakes with magnitudes larger than 4 in some cases (Ellsworth, 2013). The well‐documented cases include Rocky Mountain Arsenal in the 1960s (Healy et al., 1968), wastewater disposal in Texas (Frohlich et al., 2011), Oklahoma (Holland, 2013), and Arkansas (Horton, 2012). In fact, an anomalous increase in earthquake activity has occurred in the central and eastern United States over the past few years, which is mainly due to deep injection of low‐pressure wastewater into deep strata or basement formations (Ellsworth, 2013). Hydraulic fracturing using high‐pressure fluids can induce a lot of earthquakes, …