Coso Geothermal Field Research Articles

Seismicity zoning at Coso geothermal field and stress changes from fluid production and migration

The Coso geothermal field is a major geothermal power production site in the western United States. It has been observed that low-magnitude seismic events (M < 3.71) are unevenly distributed in three distinct zones, namely, nearfield (<3 km), midfield (3–6 km), and farfield (> 6 km) from the Coso geothermal plant. These zones exhibit distinct changes in earthquake location before and during geothermal production episodes that began in 1986. After 1986, the midfield region of the main flank experiences a significantly lower seismicity rate than the surrounding areas before production episodes. During 2014–2019, the farfield earthquakes cluster in the eastern and western parts of the greater Coso area, which is discernably different from how those pre-production earthquake events were distributed along the conjugate NW-SE and SW-NW trending structures across the main flank. Here, we analyze the stage of stress with finite-element-based poroelastic simulations to illustrate how the spatiotemporal evolution of the seismicity is associated with the pattern of stress perturbations caused by fluid migration amid the operations of geothermal power plants. Generally, ∼70% of co-production seismicity is found in zones of increased Coulomb stress between 2014 and 2019 at >99% confidence. Meanwhile, the midfield zone of seismic paucity overlaps with the zone of decreasing pore-fluid pressure. Overall, the results provide a physical explanation of how decadal geothermal operations at Coso have perturbed stress-field changes and contributed to the evolving characteristic seismic pattern, shedding insights into assessing the seismic hazard in other geothermal settings.

Mapping Geothermal Indicator Minerals Using Fusion of Target Detection Algorithms

Mineral mapping from satellite images provides valuable insights into subsurface mineral alteration for geothermal exploration. In previous studies, eight fundamental algorithms were used for mineral mapping utilizing USGS spectra, a collection of reflectance spectra containing samples of minerals, rocks, and soils created by the USGS. We used an ASD FieldSpec 4 Hi-RES NG portable spectrometer to collect spectra for analyzing ASTER images of the Coso Geothermal Field. Then, we established the ground-truth information and the spectral library by analyzing 97 samples. Samples collected from the field were analyzed using the CSIRO TSG (The Spectral Geologist of the Commonwealth Scientific and Industrial Research Organization). Based on the mineralogy study, multiple high-purity spectra of geothermal alteration minerals were selected from collected data, including alunite, chalcedony, hematite, kaolinite, and opal. Eight mineral spectral target detection algorithms were applied to the preprocessed satellite data with a proposed local spectral library. We measured the highest overall accuracy of 87% for alunite, 95% for opal, 83% for chalcedony, 60% for hematite, and 96% for kaolinite out of these eight algorithms. Three, four, five, and eight algorithms were fused to extract mineral alteration with the obtained target detection results. The results prove that the fusion of algorithms gives better results than using individual ones. In conclusion, this paper discusses the significance of evaluating different mapping algorithms. It proposes a robust fusion approach to extract mineral maps as an indicator for geothermal exploration.

Solid-earth tidal modulations of 2019 Ridgecrest earthquake sequence, California: any link with Coso geothermal field?

Solid-earth tidal modulations of 2019 Ridgecrest earthquake sequence, California: any link with Coso geothermal field?

Ubiquitous Earthquake Dynamic Triggering in Southern California

AbstractEarthquakes can be dynamically triggered by the passing waves of other distant events. The frequent occurrence of dynamic triggering offers tangible hope in revealing earthquake nucleation processes. However, the physical mechanisms behind earthquake dynamic triggering have remained unclear, and contributions of competing hypotheses are challenging to isolate with individual case studies. To gain a systematic understanding of the spatiotemporal patterns of dynamic triggering, we investigate the phenomenon in southern California from 2008 to 2017. We use the Quake Template Matching catalog and an approach that does not assume an earthquake occurrence distribution. We develop a new set of statistics to examine the significance of seismicity‐rate changes as well as moment‐release changes. Our results show that up to 70% of 1,388 global M ≥ 6 events may have triggered earthquakes in southern California. The triggered seismicity often occurred several hours after the passing seismic waves. The Salton Sea Geothermal Field, San Jacinto fault, and Coso Geothermal Field are particularly prone to triggering. Although adjacent fault segments can be triggered by the same earthquakes, the majority of triggered earthquakes seem to be uncorrelated, suggesting that the process is primarily governed by local conditions. Further, the occurrence of dynamic triggering does not seem to correlate with ground motion (e.g., peak ground velocity) at the triggered sites. These observations indicate that nonlinear processes may have primarily regulated the dynamic triggering cases.

Tidal modulation of seismicity at the Coso geothermal field

Studying the seismicity triggering response of fault systems to periodic stress fluctuations can improve our understanding of earthquake nucleation, rupture failure processes, and local stress states. Geothermal fields are well known to be susceptible to triggering, as the injection and extraction activities change the local stress and fluid flow conditions. Here, we examine the modulation of earthquakes by Earth tides within California's Coso geothermal field (CGF) and its vicinity. To maximize our resolution to detect modulation of small earthquakes, we take advantage of the new Quake Template Matching catalog in southern California, which has nearly twice as many events in the Coso region as the standard catalog and is complete down to about magnitude 0.3. We observe strong tidal triggering of earthquakes within the CGF, even though the fluctuations of tidal stresses are small (∼2 kPa). The tidally-triggered earthquakes tend to occur near the time of maximum tensile tidal stress. The signal is strongest near the edges of the zone of new production wells, suggesting fluid pressure gradients encourages triggering at tidal periods.

Ridgecrest aftershocks at Coso suppressed by thermal destressing.

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.

Optimally Oriented Remote Triggering in the Coso Geothermal Region

AbstractDefining the nature of faulting, the stress needed to trigger earthquakes, and the stress state of faults remains fundamental to understanding the earthquake cycle. Studying remote dynamic triggering allows us to probe faults to systemically address the fundamental physical mechanisms of faulting. Using 13 years of data (2004–2016) from the EarthScope USArray Transportable Array and the Southern California Seismic Network, we search for remotely triggered seismicity in an extended region encompassing the Coso Geothermal Field (CGF+), California. We first apply a short term to long‐term average ratio detector to high‐pass (5 Hz) filtered waveforms spanning ±5 hr encompassing 211 M ≥ 7 global earthquakes. We visually inspect these waveforms to identify uncatalogued local earthquakes. We use the augmented local earthquake catalog to investigate remote earthquake triggering in the CGF+ region. Of the 211 remote mainshocks, we find 32 (15%) produce a statistically significant increase in seismicity following the P wave arrival in CGF+. An additional nine mainshocks had local earthquakes coincident with the surface waves passage but lacked a significant rate increase. Of the 41 (19%) triggering mainshocks, 28 and 13 exhibit instantaneous and delayed triggering, respectively. We find no correlation between triggering and mainshock depth, peak dynamic stress, nor mainshock focal mechanism type. Instead, results suggest the CGF+ may be optimally oriented for remote triggering from mainshocks in the West Pacific and South America and propose the reason for this is that the transient dynamic stresses align favorably with the local stress field (most compressive horizontal stress, SHmax) to promote triggering.

Seismicity Rate Change at the Coso Geothermal Field Following the July 2019 Ridgecrest Earthquakes

ABSTRACT Many geothermal and volcanic regions experience remote and regional triggering following large earthquakes. The transient or permanent changes in stresses acting on faults and fractures can lead to changes in seismicity rates following either the passage of teleseismic waves or the permanent change in stresses following regional events. One such region of prevalent triggering is the Coso Geothermal Field (CGF) in eastern California, which is located roughly 30 km to the north of the 2019 Mw 7.1 Ridgecrest epicenter. Previous regional earthquakes have not only seemingly caused increase in seismicity rate surrounding the CGF, but also showed an absence of such rate increases in the CGF itself. To test whether seismicity rates in the CGF were dissimilar to the surrounding area following the Mw 7.1 Ridgecrest earthquake, I carry out seismicity rate change calculations using a catalog of seismicity compiled using a local seismic network and find that the behavior at CGF is identical to the surrounding area. Comparisons of seismicity rate changes calculated using a regional-network-derived catalog, and the local-network-derived catalog show that for a moderate, regional earthquake (2009 Mw 5.2 Olancha, California), the local network catalog reveals a change in seismicity rate whereas the regionally network catalog shows no significant changes. The differences are possibly related to incomplete sampling of seismicity using the regional network due to the existence of a shallow brittle–ductile transition centered on the CGF. The CGF, thus, is prone to triggering from both teleseismic and regional earthquakes.

A Stress-Similarity Triggering Model for Aftershocks of the Mw 6.4 and 7.1 Ridgecrest Earthquakes

ABSTRACTThe July 2019 Mw 6.4 and 7.1 Ridgecrest earthquakes triggered numerous aftershocks, including clusters of off-fault aftershocks in an extensional stepover of the Garlock fault, near the town of Olancha, and near Panamint Valley. The locations of the off-fault aftershocks are consistent with the stress-similarity model of triggering, which hypothesizes that aftershocks preferentially occur in areas where the mainshock static stress change tensor is similar in orientation to the background stress tensor. The background stress field is determined from the inversion of earthquake focal mechanisms, with the spatial resolution adapted to the local density of earthquakes. The mainshock static stress change is computed using finite-source models for the Mw 6.4 foreshock and Mw 7.1 mainshock. I quantify the similarity between these two stress fields using the tensor dot product of the normalized deviatoric stress tensors. The off-fault aftershocks in the Garlock stepover and the Olancha area fall within lobes of positive stress similarity, whereas the aftershocks near Panamint Valley are partially within a lobe. The cluster in the Garlock fault stepover and the smaller of two clusters near Olancha occur in regions of locally anomalous background stress that results in higher stress similarity. I compute the spatial density of M≥2.0 aftershocks and find that the aftershock density increases as a function of stress similarity, with a factor of ∼15 difference between high stress-similarity and low stress-similarity areas. This result is robust with respect to the choice of mainshock model and the uncertainty of the background stress field. The aftershock density varies substantially inside the high stress-similarity lobes, however, indicating that other variable background conditions, such as material properties, temperature, and fluid pressure, may also be playing a role. Specifically, temperature and fluid pressure conditions might help explain the low rate of aftershocks in the Coso geothermal field.

Geodetic Measurements and Numerical Models of Deformation at Coso Geothermal Field, California, USA, 2004–2016

We measure transient deformation at Coso geothermal field using interferometric synthetic aperture radar (InSAR) data acquired between 2004 and 2016 and relative positions estimated from global positioning system (GPS) to quantify relationships between deformation and pumping. We parameterize the reservoir as a cuboidal sink and solve for best-fitting reservoir dimensions and locations before and after 2010 in accordance with sustainability efforts implemented in late 2009 at the site. Time-series analysis is performed on volume changes estimated from pairs of synthetic aperture radar (SAR) and daily GPS data. We identify decreasing pore-fluid pressure as the dominant mechanism driving the subsidence observed at Coso geothermal field. We also find a significant positive correlation between deformation and production rate.

Quantifying the heterogeneity of the tectonic stress field using borehole data

AbstractThe heterogeneity of the tectonic stress field is a fundamental property which influences earthquake dynamics and subsurface engineering. Self‐similar scaling of stress heterogeneities is frequently assumed to explain characteristics of earthquakes such as the magnitude‐frequency relation. However, observational evidence for such scaling of the stress field heterogeneity is scarce. We analyze the local stress orientations using image logs of two closely spaced boreholes in the Coso Geothermal Field with subvertical and deviated trajectories, respectively, each spanning about 2 km in depth. Both the mean and the standard deviation of stress orientation indicators (borehole breakouts, drilling‐induced fractures, and petal‐centerline fractures) determined from each borehole agree to the limit of the resolution of our method although measurements at specific depths may not. We find that the standard deviation in these boreholes strongly depends on the interval length analyzed, generally increasing up to a wellbore log length of about 600 m and constant for longer intervals. We find the same behavior in global data from the World Stress Map. This suggests that the standard deviation of stress indicators characterizes the heterogeneity of the tectonic stress field rather than the quality of the stress measurement. A large standard deviation of a stress measurement might be an expression of strong crustal heterogeneity rather than of an unreliable stress determination. Robust characterization of stress heterogeneity requires logs that sample stress indicators along a representative sample volume of at least 1 km.

Absence of remote earthquake triggering within the Coso and Salton Sea geothermal production fields

AbstractGeothermal areas are long recognized to be susceptible to remote earthquake triggering, probably due to the high seismicity rates and presence of geothermal fluids. However, anthropogenic injection and extraction activity may alter the stress state and fluid flow within the geothermal fields. Here we examine the remote triggering phenomena in the Coso geothermal field and its surrounding areas to assess possible anthropogenic effects. We find that triggered earthquakes are absent within the geothermal field but occur in the surrounding areas. Similar observation is also found in the Salton Sea geothermal field. We hypothesize that continuous geothermal operation has eliminated any significant differential pore pressure between fractures inside the geothermal field through flushing geothermal precipitations and sediments out of clogged fractures. To test this hypothesis, we analyze the pore‐pressure‐driven earthquake swarms, and they are found to occur outside or on the periphery of the geothermal production field. Therefore, our results suggest that the geothermal operation has changed the subsurface fracture network, and differential pore pressure is the primary controlling factor of remote triggering in geothermal fields.

Discriminating Characteristics of Tectonic and Human‐Induced Seismicity

We analyze statistical features of background and clustered subpopulations of earthquakes in different regions in an effort to distinguish between human‐induced and natural seismicity. Analysis of end‐member areas known to be dominated by human‐induced earthquakes (The Geyser geothermal field in northern California and TauTona gold mine in South Africa) and regular tectonic activity (the San Jacinto fault zone in southern California and the Coso region, excluding the Coso geothermal field in eastern central California) reveals several distinguishing characteristics. Induced seismicity is shown to have (1) higher rate of background events (both absolute and relative to the total rate), (2) faster temporal offspring decay, (3) higher rate of repeating events, (4) larger proportion of small clusters, and (5) larger spatial separation between parent and offspring, compared to regular tectonic activity. These differences also successfully discriminate seismicity within the Coso and Salton Sea geothermal fields in California before and after the expansion of geothermal production during the 1980s. Online Material: Figures of examined regions, earthquake epicenters, and joint distributions of the time and space components of the nearest‐neighbor distance.

Differentiating induced and natural seismicity using space‐time‐magnitude statistics applied to the Coso Geothermal field

AbstractA remarkable characteristic of earthquakes is their clustering in time and space, displaying their self‐similarity. It remains to be tested if natural and induced earthquakes share the same behavior. We study natural and induced earthquakes comparatively in the same tectonic setting at the Coso Geothermal Field. Covering the preproduction and coproduction periods from 1981 to 2013, we analyze interevent times, spatial dimension, and frequency‐size distributions for natural and induced earthquakes. Individually, these distributions are statistically indistinguishable. Determining the distribution of nearest neighbor distances in a combined space‐time‐magnitude metric, lets us identify clear differences between both kinds of seismicity. Compared to natural earthquakes, induced earthquakes feature a larger population of background seismicity and nearest neighbors at large magnitude rescaled times and small magnitude rescaled distances. Local stress perturbations induced by field operations appear to be strong enough to drive local faults through several seismic cycles and reactivate them after time periods on the order of a year.

Dynamic triggering of microearthquakes in three geothermal/volcanic regions of California

AbstractGeothermal/volcanic regions are most susceptible to local earthquake triggering by regional and remote earthquakes. Transient stresses caused by surface waves of these earthquakes can activate critically stressed faults. Though earthquakes can be triggered in geothermal/volcanic regions, it is less understood how these regions differ in their triggering responses to distant earthquakes. We conduct a systematic survey of local earthquakes triggered by distant earthquakes in three geothermal/volcanic regions of California: Long Valley Caldera, Coso Geothermal Field, and Geysers Geothermal Field. We examine waveforms of distant earthquakes with magnitudes ≥ 5.5 occurring between 2000 and 2012 and compute β statistics to confirm the significance of our findings. We find that Long Valley, Coso, and Geysers vary in triggering frequency—2.0%, 3.8%, and 6.8% in the 12 year period, respectively—and when compared to the triggering of deep tectonic tremors along the Parkfield‐Cholame section of San Andreas Fault (9.2% in the 12 year period). Stress triggering thresholds vary among the regions with Long Valley having the highest of ~5 kPa and ~1 kPa for the other regions. Because dynamic stresses from distant earthquakes are similar in these three regions, the varying triggering behavior likely reflects faults having a tendency to be at or near failure. This is compatible with Geysers having a higher a value in the Gutenberg‐Richter relationship and higher geothermal productivity than the other two regions. The observation of more frequent triggering of tremor than microearthquakes is consistent with recent laboratory studies on increasing triggerability with lower effective stress.

Three‐dimensional Vp and Vp/Vs models in the Coso geothermal area, California: Seismic characterization of the magmatic system

AbstractWe combine classic and state‐of‐the‐art techniques to characterize the seismic and volcanic features in the Coso area in southern California. Seismic tomography inversions are carried out to map the variations of Vp, Vs, and Vp/Vs beneath Coso. The velocities in the top layers of our model are correlated with the surface geological features. The Indian Wells Valley, with high silica content sediment strata, shows low‐velocity anomalies up to 3 km depth, whereas the major mountain ranges, such as the south Sierra Nevada and the Argus Range, show higher velocities. The resulting three‐dimensional velocity model is used to improve absolute locations for all local events between January 1981 and August 2011 in our study area. We then apply similar‐event cluster analysis, waveform cross correlation, and differential time relocation methods to improve relative event location accuracy. A dramatic sharpening of seismicity patterns is obtained after using these methods. We also estimate high‐resolution near‐source Vp/Vs ratio within each event cluster using the differential times from waveform cross correlation. The in situ Vp/Vs method confirms the trend of the velocity variations from the tomographic results. An anomalous low‐velocity body with low Vp, Vs, and Vp/Vs ratios, corresponding to the ductile behavior underlying the Coso geothermal field from 6 to 12 km depth, can be explained by the existence of frozen felsic magmatic materials with the inclusion of water. The material is not likely to include pervasive partial melt due to a lack of high Vp/Vs ratios.

A joint geophysical analysis of the Coso geothermal field, south-eastern California

Three-dimensional density models derived from gravity data and two-dimensional resistivity models derived from magnetotelluric data collected in the vicinity of the Coso geothermal field are analyzed in order to determine the source region of the geothermal field. The derived models show zones of both low resistivity and low density at and below 6km depth in the Devils Kitchen and the Coso Hot Springs areas. These zones agree with seismic reflection and tomography results which found a high amplitude reflector at 5km and low velocities zones below 5km. We interpret the density and resistivity zones to indicate the presence of cooling magmatic material that provides the heat for the shallower geothermal system in these regions. A zone marked by high resistivity and low density was found to lie directly above the interpreted partially melted region extending to within 1km depth below the surface in the reservoir region where it is capped by a low resistivity clay zone. In addition, the density models indicate that the high density bodies occurring under volcanic outcrops may be mafic intrusions.

Curie Point Depth Estimates from Aeromagnetic Data from Death Valley and Surrounding Regions, California

Aeromagnetic data were analyzed to determine the Curie point depth (CPD) by power density spectral and three-dimensional inversion methods within and surrounding Death Valley in southern California. We calculated the CPD for 0.5° regions using 2D power density spectral methods and found that the CPDs varied between 8 and 17 km. However, the 0.5° region may average areas that include shallow and deep CPDs, and because of this limitation, we used the 3D inversion method to determine if this method may provide better resolution of the CPDs. The final 3D model indicates that the depth to the bottom of the magnetic susceptible bodies varies between 5 and 23 km. Even though both methods produced roughly similar results, the 3D inversion method produced a higher lateral resolution of the CPDs. The shallowest CPDs occur within the central and southern Death Valley, Panamint Valley, Coso geothermal field and the Tecopa hot springs region. Deeper (>15 km) CPDs occur over outcropping granitic and Precambrian lithologies in the Panamint Range, Grapevine Mountains, Black Mountains and the Argus Range. The shallowest CPD occurs within the central Death Valley over a possible seismically imaged magma body and slightly deeper values occur within the Panamint Valley, southern Death Valley and Tecopa Hot Springs. The shallow CPD values suggest that partially molten material may also be found in these latter regions. The CPD computed heat flow values for the region suggest that the entire area has high heat flow values (>100 mW m−2), on the other hand, locally extremely high values (>200 mW m−2) occur within the Panamint Valley, the southern and central Death Valley and Tecopa Hot Springs region. These locally high heat flow values may be related to midcrustal magma bodies; but additional geophysical experiments are needed to determine if the magma bodies exist.

Research Spotlight: Spooky action at a distance, for earthquakes

On 27 February 2010 a magnitude 8.8 earthquake rocked millions of Chileans from their slumber, but the South American people weren't the only ones so affected. New research by Peng et al. suggests that the trembling off the Chilean coast could have triggered a swarm of shallow earthquakes nearly 10,000 kilometers away in central California. The team identified four earthquakes with magnitudes of 2 or higher, with the largest of the group being a M 3.5 quake that rattled the Coso geothermal field. The affected region is plagued by small earthquakes, but when the authors calculated the chance of a swarm so neatly following the Chilean earthquake, they inferred that the two seismic events were probably connected. The authors also saw a cluster of deep, low‐frequency earthquakes along the Parkfield‐Cholame section of the San Andreas Fault that also appeared to be influenced by the Chilean event. The researchers suggest that the timing of the California earthquakes was affected by the arrival of Love waves—horizontal surface movement that would have traveled out from the epicenter of the Chilean earthquake, pushing already stressed faults over the edge. (Geophysical Research Letters, doi:10.1029/2010GL045462, 2010)

Delayed triggering of microearthquakes by multiple surface waves circling the Earth

It is well known that direct surface waves of large earthquakes are capable of triggering shallow earthquakes and deep tremor at long-range distances. However, it is not clear whether multiple surface waves circling the Earth could also trigger/modulate seismic activities. Here we conduct a systematic search of remotely triggered microearthquakes near the Coso Geothermal Field in central California following the 2010 Mw 8.8 Chile earthquake. We find a statistically significant increase of microearthquakes in the first few hours after the Chile mainshock. These observations of apparently delayed earthquake triggering do not follow the Omori-law decay with time since the largest ML 3.5 event occurred during the large-amplitude Love waves. Instead, they are better correlated with the first three groups of multiple surface waves (G1 − R1, G2 − R2, and G3). Our observation provides an alternative explanation of delayed triggering of microearthquakes at long-range distances, at least in the first few hours after large earthquakes.