SCEC Award Number 14097 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Effect of geothermal operations on earthquake source spectra
Investigator(s)
Name Organization
Xiaowei Chen Woods Hole Oceanographic Institution Jeffrey McGuire Woods Hole Oceanographic Institution
Other Participants
SCEC Priorities 2f, 2b, 2c SCEC Groups Seismology, EFP, FARM
Report Due Date 03/15/2015 Date Report Submitted N/A
Project Abstract
 The borehole network operated by CalEnergy provides high quality dataset for the microearthquakes in the Salton Sea geothermal region. The network was open to public from 2008 to early 2014, which recorded over 7000 earthquakes in the geothermal field; the majority of earthquakes are concentrated in the middle cluster during this time period, and the feature of three sub-clusters remains prominent. With high-resolution earthquake location based on 3D velocity model, we identify clear depth separation between larger (M≥2.5) and smaller (M<2.5) events in the middle cluster. Our preliminary spectral analysis has revealed reduced fault strength near the main fault that hosted the M5 2005 earthquake swarm (low stress drop), where two main earthquake bursts in late 2009 and 2010 occurred. In the middle cluster, stress drop increases from 1.5 MPa closest to injection wells to 5 MPa at 300 m from injection wells. Beyond 0.3 km, the stress drop is nearly constant, suggesting that the spatial window of directly induced seismicity maybe limited in 300m from injection wells. The results suggest complex interaction between fluid and faults in the geothermal field. We are continuing to integrate the stress drop, the velocity model, and the high-resolution location to get better understanding of the geomechanical process of the seismicity in the geothermal field.
Intellectual Merit The research contributes to integrated understanding of the geomechanical process within the active geothermal field. The new findings of reduced stress drop near the main fault zone, and stress drop distance dependence within 300m is important for understanding the spatial influence window of injection activity, and interaction between fluid circulation and pre-existing weak zones.
Broader Impacts The project results are beneficial for learning earthquake hazards, risks and earthquake physics. Due to the recent increase in earthquakes in central US, the students at the University of Oklahoma are interested in learning more about induced seismicity, and enrolled in my seminar on “induced seismicity”. A master student at OU is currently working on this project with Xiaowei Chen. The research results are relevant to a few ongoing research projects at other institutions, for example, the shear-wave velocity changes in the geothermal field by Jeff McGuire at WHOI, the earthquake detection by Zhigang Peng at Gatech.
Exemplary Figure Figure 4. (a) Map view of stress drop in the middle cluster after applying spatial-median filter (average stress drop from nearest 50 events). Black circles are injection wells. X1-X2 and Y1-Y2 mark two profiles. (b) Left: Cross-section view along profile X1-X2; Right: Cross-section view along profile Y1-Y2. (c) Stress drop versus distance (from nearest active injection wells). The low stress drop near the main fault zone (southeastern portion) is clear. The vertical errorbars are 25% and 75% of stress drop in each distance bin.
Figure credits: Xiaowei Chen