Source Characterization of Microearthquakes Induced by Hydraulic Fracturing with Empirical Green's Function

In this paper, we retrieved relative source time functions (RSTF) and estimated the source parameters for microearthquakes (M= -1.9 to -2.6) induced by hydraulic injection at Fenton Hill, New Mexico, using an empirical Green's function (EGF) method. Seismic waveform of a small event in seismic doublets or multiplets (Gelle and Meuller, 1980), defined as co-located events with similar focal mechanisms, within a hydraulic fracture zone, is treated as the EGF and is deconvolved from that of a larger event in the doublets or multiplets to retrieve the relative source time function. Time domain analysis of the RSTFs reveals the source complexity of the induced microearthquakes. The azimuthal variation of the RSTF indicates that the rupture propagates to the northwest, which is consistent with the growth direction of the hydraulic fracture zone determined by Li and Cheng (1995) with a seismicity temporal-spatial distribution pattern. The source duration of the induced events ranges from 2 to 8 ms and the source radii are estimated to be 4 to 12 meters. Values of stress drops are from 1 to 19 bars. Significant variation of the stress drops may reflect the heterogeneity of the stress field in the hydraulic fracture zone and its vicinity and indicate that the stress field heterogeneity extends down to a few meters.