Independence of Spatial and Temporal Static Stress-Drop Variability at The Geysers Geothermal Field

Abstract The variability of static stress drop of earthquakes has been described in several studies concerning injection-induced seismicity. In this work, we refer to temporal and spatial variability of static stress drop identified originally by Staszek et al. (2017) and Staszek et al. (2021) in the cluster of seismicity induced by cyclic fluid injection into Prati-9 and Prati-29 wells at The Geysers geothermal field (California, United States). Stress drops of events within this cluster fell in the range 0.7–58.3 MPa with a median value of 4.7 MPa. The temporal stress-drop variability described by Staszek et al. (2017) was identified among three time intervals and was found to be inversely related to the injection rate. The spatial variability of stress drop described by Staszek et al. (2021) resulted from the differences in stress drop between the events delineating identified fault and fractures. Here, we assess the significance of stress-drop variations in time in different groups of events and show that temporal stress-drop variability remains statistically significant even after excluding the fault-related events exhibiting significantly lower stress-drop values. Therefore, in this case the spatial and temporal variations in static stress drop are independent. Moreover, we confirm that there is an inverse relation between static stress drop and injection rate and conclude that pore pressure changes do influence the static stress drop of earthquakes in the area, regardless of the spatial heterogeneity of the reservoir. We deduce that static stress drop is a parameter influenced by both structure of the reservoir, that is, fracture and fault network, and the dominant physical process responsible for the generation of earthquakes. Therefore, we suggest that static stress drop should be interpreted after considering both spatial and temporal variability contributions, if possible, in the frame of specific structures.