Determination of Stress State from Focal Mechanisms of Microseismic Events Induced During Hydraulic Injection at the Hijiori Hot Dry Rock Site

Abstract

Ð The stress state at the Hijiori hot dry rock site was estimated based on the inversion from focal mechanisms of microseismic events induced during hydraulic injection experiments. The best ®t stress model obtained by inverting 58 focal mechanisms of seismic events simultaneously indicates that the maximum principal stress r 1 is vertical, while the minimum principal stress r 3 is horizontal and trends north-south. The average mis®t between the stress model and all the data is 6.8°. The inversion results show that the average mis®t is small enough to satisfy the assumption of homogeneity in the focal mechanism data and that the 95% con®dence regions of r 1 and r 3 are well constrained, i.e., they do not overlap, suggesting that the inversion results are acceptable. The stress estimates obtained by the focal mechanism inversion essentially agree with other stress estimates previously obtained. It is therefore concluded that the focal mechanism inversion method provides a useful tool for estimating the stress state. The hypocentral distributions of microseismic events associated with the hydraulic fracturing experiments are distributed around the plane that spreads to almost east±west from the injection wells and declines to the north at a high angle. The vertical orientation and east±west strike of the seismic events are essentially coplanar with the caldera ring-fault structure in the southern portion of the Hijiori Caldera. This indicates that tensile fractures of intact rock were not being created, but pre-existing fractures were being re-opened and developed in the direction of the maximum horizontal principal stress, although microseismic events were caused by shear failures.

Stress tensor changes related to fluid injection at The Geysers geothermal field, California

“…Stress inversion techniques have also been applied to induced seismicity related to fluid injection into different types of reservoirs [ Oppenheimer , ; Feng and Lees , ; Sasaki and Kaieda , ; Bohnhoff et al ., ]. These studies have mainly focused on spatial analyses around the geothermal area and particularly on the variations in stress field orientation with depth.…”

“…[3] Stress inversion techniques have also been applied to induced seismicity related to fluid injection into different types of reservoirs [Oppenheimer, 1986;Feng and Lees, 1998;Sasaki and Kaieda, 2002;Bohnhoff et al, 2004].…”

Fault mechanisms of induced seismicity at the superdeep German Continental Deep Drilling Program (KTB) borehole and their relation to fault structure and stress field

“…This is a widely used approach for analyzing induced seismicity in geothermal and hydrocarbon reservoirs [e.g., Fehler et al , 1987; Phillips et al , 2002] based on the assumption that the most effective coupling of hydraulic overpressures (injection pressure) occurs along planar fracture planes. In the current study we combine the geometrical structures derived from the hypocenter distribution with the source mechanisms obtained in the previous section (similar comparisons have previously been done by Sasaki and Kaieda [2002]); that is, we search for planar structures which exhibit a similar source mechanism. We find that a total of 66 events including 40 with fault plane solutions are grouped into four planar structures to which we refer as structure 1–4 in the following.…”

Basement Fault Reactivation in the Zakho– Silopi Region, Iraqi Kurdistan – Turkey Border

“…These focal mechanisms were created by the FOCMEC program using the Pwave first motion data (Snoke et al, 1984). The FOCMEC program provides more realistic solutions even if there is a poor data or limited number of stations (Sasaki and Kaieda, 2002;Snoke 2003;Oros, 2013). The QuickBird images with 0.6 m resolution, dated 22 June 2005, were used for mapping of lineament patterns in the Zakho -Silopi region using the ArcGIS software.…”