Geothermal and volcanic areas are prone to earthquake triggering1,2. The Coso geothermal field in California lies just north of the surface ruptures driven by the 2019 Ridgecrest earthquake (moment magnitude Mw = 7.1), in an area where changes in coseismic stress should have triggered aftershocks3,4. However, no aftershocks were observed there4. Here we show that 30 years of geothermal heat production at Coso depleted shear stresses within the geothermal reservoir. Thermal contraction of the reservoir initially induced substantial seismicity, as observed in the Coso geothermal reservoir, but subsequently depleted the stress available to drive the aftershocks during the Ridgecrest sequence. This destressing changed the faulting style of the reservoir and impeded aftershock triggering. Although unlikely to have been the case for the Ridgecrest earthquake, such a destressed zone could, in principle, impede the propagation of a large earthquake.
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The seismic catalogue10 is publicly available from the Southern California Earthquake Data Center (https://scedc.caltech.edu/data/alt-2011-dd-hauksson-yang-shearer.html). The Coso field well location and flow rate data are available from the California Department of Conservation (https://maps.conservation.ca.gov/doggr/wellfinder, https://www.conservation.ca.gov/calgem/geothermal/manual/Pages/production.aspx). Simulation data are available in the Caltech data repository (https://doi.org/10.22002/D1.1455). Source data are provided with this paper.
The TOUGH–FLAC coupled simulator and all input files are available in the Caltech data repository (https://doi.org/10.22002/D1.1455).
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This study was supported by the National Science Foundation via the IUCR center Geomechanics and Mitigation of Geohazards (award number 1822214) and via the Southern California Earthquake Center (SCEC). The SCEC is funded by NSF Cooperative Agreement EAR-1600087 and USGS Cooperative Agreement G17AC00047.
The authors declare no competing interests.
Peer review information Nature thanks Roland Burgmann, J. Ole Kaven and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Extended data figures and tables
Extended Data Fig. 1 Seismicity before and after the Ridgecrest mainshock (5 July 2019) in and around the Coso area.
We divided the area into different domains. a–d, Relocated seismicity10 of all magnitude, before (2010 to the Mw = 7.1 mainshock; grey circles) and after (Mw = 7.1 mainshock to the end of 2019; blue circles) the Ridgecrest earthquake. a–h, We compare the spatial distribution (a–d) and cumulative magnitude–frequency distribution (e–h) of earthquakes before (black) and after (blue) the mainshock for the Coso volcanic area (a, e), Cactus flat (b, f), Coso geothermal field (c, g) and the northwest edge of the Mw = 7.1 event (d, h). Red rectangles in a define the areas for each plot. Black triangles in c indicate the locations of geothermal wells. The density of aftershocks above the detection threshold (Mw > 1) is about two orders of magnitude lower in the Coso geothermal field (c) than in the surrounding areas (b, d). A similar result has been reported previously4.
a, Distribution of seismicity (Mw > 1)10. b, Seismicity history over the entire Coso area. c–f, Seismicity history of event magnitude (circles; left axis) and annual rate (black line; right axis) for each fault zone (c–e, Coso main field; f, east flank area), as indicated in a. The fault zones are selected on the basis of the expression of the seismic cloud.
Extended Data Fig. 3 Change of focal mechanism in the Coso main field area and effective stress changes predicted by the simulation.
a, The rake angle shows that the proportion of normal faulting (−120° to about −60°) increases with time. b, c, Ternary plots53 show that the formal mechanism is more diverse, with increased normal faulting in the later operation period (2001–2019; c) than in the earlier period (1981–2000; b). d–f, Time-dependent maximum horizontal (red), minimum horizontal (blue) and vertical (black) stress at different depths, calculated as an average along a 1-km baseline at the centre of the reservoir at each depth (d, 1,000 m; e, 1,500 m; f, 1,750 m). The stresses within the reservoir (e, f) decline with time, but the rate of decline in vertical stress is lower than that in maximum horizontal stress. The simulation predicts an increase in the proportion of normal faulting and diversity of focal mechanism, as observed in a–c.
Blue, dark green and light green blocks represent the reservoir, upper host and lower host elements, respectively. The right-hand side shows the repeating five-spot pattern of injectors (triangles) and producers (circles). Inset, initial horizontal stresses (σ1, maximum; σ3, minimum) calculated from Coso field data (Fig. 1a, inset). Vertical stress (σv) is calculated as gravitational stress for an effective density of 1,400 kg m−3 at every time step. The x axis is chosen to be parallel to the dominant fault orientation in the main field (Fig. 1a, inset), which is parallel to the main fault ruptured in the Mw = 7.1 Ridgecrest earthquake. Roller and shear stress boundaries are applied corresponding to the initial stress, as shown in the inset. The ground surface is stress-free.
This simulation result is identical to that shown in Fig. 2, but with a lower flow rate, set to match the injection rate via permeability reduction. Other parameters are identical to our reference simulation (Fig. 2). a, Reported injection and production flow rates from the Coso field (thin lines) and simulation (bold lines). b, Ground deformation recovered from InSAR measurements between May 1996 and June 199814. Image adapted with permission from ref. 14, American Geophysical Union. c, Cumulative line-of-sight (LOS) surface displacement at the end of the simulation (year 30). The white arrow shows the line-of-sight unit vector (0.38, −0.09, 0.92)14. d, Time evolution of maximum line-of-sight displacement (black line) and observations (blue solid lines)14,33,34, along with their extrapolations (blue dashed lines). e, Shear stress at the end of the simulation (year 30) in the orientation parallel to the Mw = 7.1 rupture in the reservoir area (inset).
a, Pressure change at well bores (black straight line) and well blocks (the block where the imaginary well bore is embedded; coloured lines). The pressure gap between the well bore and well block is larger at injection than at production, owing to the low temperature and consequent low fluid viscosity. b, Pressure drop distribution at the end of the simulation. The white rectangle indicates the 4 km × 4 km × 3 km reservoir area. The pressure drops by around 5.5 MPa in the reservoir, and the halo of pressure drop extends beyond the reservoir area.
a, Line-of-sight surface displacement recovered from InSAR measurements between May 1996 and June 1998 (identical to Fig. 2d). Image adapted with permission from ref. 14, American Geophysical Union. b, Predicted line-of-sight surface displacement at the end of the isothermal simulation. All parameters are identical to those for the reference simulation (Fig. 2), which accounts for thermal strain. c, Time evolution of maximum line-of-sight displacement of the isothermal (red) and non-isothermal (black; Fig. 2f) simulations, together with observations (blue solid lines)14,33,34 and their interpolations (blue dashed lines). The ‘no thermal stress case’ represents the subsidence from pressure depletion alone. d, Observed and predicted ground displacements projected along the line of sight (arrow in b) of the InSAR images14,33. The black solid line are from our reference simulation (Fig. 2); the red solid line is from the isothermal simulation (no thermal strain). The curves are normalized by the maximum displacement of about 65 cm and about 35 cm for the reference and no-thermal-stress cases, respectively (c). The case from ref. 14 is measured between September 1993 and May 1996, with a maximum displacement of about 8 cm (west–east) and about 5 cm (south–north); the case from ref. 33 is measured between February 2008 and October 2009, with a maximum displacement of around 2 cm.
The simulation geometry and parameters are identical to those for the reference simulation (Fig. 3), except for a lower internal friction coefficient of 0.3. a, Change in Coulomb stress at the end of the simulation (year 30), calculated for faults parallel to the main rupture (inset). b, Shear stress at the end of the simulation (year 30). Shear stress in the reservoir area (white rectangle) is strongly depleted owing to rock failure. c, Mohr circle representation of stress changes during the simulation. Maximum and minimum effective normal stress are calculated at the centre of the reservoir (stresses averaged along the yellow line in b). The Mohr circle at year 0 is smaller than for the higher friction cases (Fig. 3c) owing to initial failure. The grey dashed line indicates the input failure criteria in this simulation. d, Change in shear stress at the centre of the reservoir (averaged along the yellow line in b).
a, As in Fig. 3c. b, As in a, except that the reservoir is fully elastic (no failure). When the reservoir is fully elastic (that is, when failure and the resulting drop in stress are neglected), normal stresses become impossibly large in tension. c, d Evolution of normal and shear stress relative to the orientation of the Ridgecrest fault (Extended Data Fig. 4, inset) at a depth of 1,500 m, for the Mohr–Coulomb failure model (c) the fully elastic model (d). With the failure model (c), the stresses naturally approach zero over time, as a result of shear and tensile failure; in the fully elastic case (d), normal stresses transit through zero and become highly tensile when the shear stress drops, as a result of failure being ignored. The wiggles in the well pattern area of the reservoir are due to the non-uniform distribution of temperature driving differential thermal stresses.
Extended Data Fig. 11 Cumulative shear strain at the conclusion of the reference simulation (after 30 years of production).
The largest change in strain occurs in the well pattern area, where the change in temperature is largest. The maximum shear strain is about 1.1 × 10−3, which is approximately two orders of magnitude larger than the strain released by seismicity, as estimated from the sum of all seismic moments (see text).
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Im, K., Avouac, JP., Heimisson, E.R. et al. Ridgecrest aftershocks at Coso suppressed by thermal destressing. Nature 595, 70–74 (2021). https://doi.org/10.1038/s41586-021-03601-4
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