Hybrid focal mechanism determination of induced seismicity using tectonic stress data

TitleHybrid focal mechanism determination of induced seismicity using tectonic stress data
Publication TypeConference Proceedings
Year of Conference2016
AuthorsFuse, K, Mukuhira, Y, Naoi, M, Moriya, H, Ito, T, Asanuma, H, Haring, M
Conference NameAGU Fall Meeting
Conference LocationSan Francisco

Faultplane solutions (FPSs) of earthquake are generally estimated using Pwave first motions observed at seismic stations. FPSs can provide information on fracture geometry in EGS or shale gas reservoir, but accurate estimation of FPSs of microseismic (MS) events induced in those fields is usually difficult owing to limited number of MS stations. This study aims to constrain FPSs estimated from insufficient number of stations, using other available geophysical data. In this analysis, we used regional stress data and wellhead pressure during stimulation. First, we estimate all possible FPSs from Pwave first motion observed at MS network (Fig. (a)). Next, regional stress data is used to calculate critical pore pressure (CPP) for arbitrary FPSs based on coulomb failure criterion (Fig. (b)). Then, ‘slip index’ is evaluated for each FPSs by combining the FPSs with the CPP (Fig. (c)). Note that slip index indicates relative potential for fractures to slipÍž the maximum/minimum CPP corresponds to 0/1 for slip index. Finally, we eliminated FPSs whose CPP was higher than the wellhead pressure at the event occurrence time (Fig. (d)). We applied this method to one of the induced seismic events recorded during hydraulic stimulation in Basel, Switzerland. The reliable FPSs of this event, the two squares in Fig. (a), were estimated by Swiss Seismological service (SED) with about 80 stations. We also estimated FPSs of this event by using only 6 MS stations with following the process, to check the validity of the method. Red dots in Fig. (a) show the pole distribution of all FPSs that can explain all Pwave first motion polarities (black/white circles indicates push/pull) of the 6 MS stations. Fig. (c) shows the contour of slip index evaluated from the CPP, and the modified one with the wellhead pressure (=30MPa) are shown in Fig. (d). In Fig. (d), the high slip index area agrees with one of the FPSs determined by SED. This result indicates that we successfully obtained well constrained FPSs.