Investigation of the stress state on the fault planes and the magnitude of the seismic events occurred from geothermal reservoirs
Abstract
Occurrence of felt earthquakes is a critical environmental burden in geothermal development, and studies on control factors of the magnitude of the seismic events have been activated worldwide. We have identified fault planes of the large events occurred from engineered geothermal systems (EGS) sites, at Cooper Basin, Australia, and Basel, Switzerland, and Yanaizu-Nishiyama, a Japanese hydrothermal field. Shear/normal stress working on these fault planes was evaluated on the Mohr stress circles, comparing with the event magnitudes. It has been found that the large events at Basel and Yanaizu-Nishiyama occurred from fault planes where relatively large shear stress is working, although smaller events also occurred from fault planes with large shear stress. Identification of the fault planes of the larger events at Basel showed that large events mainly occurred from two types of sub-vertical fault planes with azimuth of WNW-ESE or N-S (see figure). FPSs of four felt earthquakes in Yanaizu-Nishiyama showed nearly common strike/dip. From these observations, it can be interpreted that the large events from Basel and Yanaizu-Nishiyama were likely to occur from particular fault planes with large shear stress within complex facture system. Similar relationship between shear stress and the magnitude has been also found by several seismologists (e.g. Terakawa et al., 2012). The selectivity in occurrence of the large events among fault planes under common shear stress suggests that there would be some additional factors to control scale of the failure. At Cooper Basin, where limited number of sub-horizontal fractures and vertical fractures connecting them compose the reservoir, the large events occurred from the sub-horizontal fault planes on which many smaller events also occurred. In this case, the moderate shear stress was working on the sub-horizontal fault planes, suggesting that the event magnitudes were mainly controlled by some unknown factors rather than the shear stress. Fig. 1: (a) Mohr stress circle at the depth of the main feed point at Basel. Circles show stress state on the fault planes of which orientation was estimated by SED. (b) A relation between the shear stress working on the fault planes and the moment magnitude. Orange squares: events of which FPSs were estimated by SED. Blue dots: smaller events of which FPSs were inferred by clustering analysis. (c) Rose diagram of the azimuth of identified fault planes for theh larger events. Stress state at the three fields
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.H51D1210M
- Keywords:
-
- 1822 HYDROLOGY Geomechanics;
- 7200 SEISMOLOGY