Seismicity and seismic stress in the Coso Range, Coso geothermal field, and Indian Wells Valley region, southeast-central California (original) (raw)
Related papers
Micro-Seismicity within the Coso Geothermal Field, California, from 1996-2012
We extend our previous catalog of seismicity within the Coso Geothermal field by adding over two and a half years of additional data to prior results. In total, we locate over 16 years of seismicity spanning from April 1996 to May of 2012 using a refined velocity model, apply it to all events and utilize differential travel times in relocations to improve the accuracy of event locations. The improved locations elucidate major structural features within the reservoir that we interpret to be faults that contribute to heat and fluid flow within the reservoir. Much of the relocated seismicity remains diffuse between these major structural features, suggesting that a large volume of accessible and distributed fracture porosity is maintained within the geothermal reservoir through ongoing brittle failure. We further track changes in b value and seismic moment release within the reservoir as a whole through time. We find that b values decrease significantly during 2009 and 2010, coincident with the occurrence of a greater number of moderate magnitude earthquakes (3.0 ≤ M L < 4.5). Analysis of spatial variations in seismic moment release between years reveals that localized seismicity tends to spread from regions of high moment release into regions with previously low moment release, akin to aftershock sequences. These results indicate that the Coso reservoir is comprised of a network of fractures at a variety of spatial scales that evolves dynamically over time, with progressive changes in characteristics of microseismicity and inferred fractures and faults that are only evident from a long period of seismic monitoring analyzed using self-consistent methods.
Micro-Seismicity and Seismic Moment Release Within the Coso Geothermal Field, California
We relocate 16 years of seismicity in the Coso Geothermal Field (CGF) using differential travel times and simultaneously invert for seismic velocities to improve our knowledge of the subsurface geologic and hydrologic structure. We expand on our previous results by doubling the number of relocated events from April 1996 through May 2012 using a new field-wide 3-D velocity model. Relocated microseismicity sharpens in many portions of the active geothermal reservoir, likely defining large-scale fault zones and fluid pressure compartment boundaries. However, a significant fraction of seismicity remains diffuse and does not cluster into sharply defined structures, suggesting that permeability is maintained within the reservoir through distributed brittle failure. The seismic velocity structure reveals heterogeneous distributions of compressional (Vp) and shear (Vs) wave speed, with Vs generally higher in the Main Field and East Flank and Vp remaining relatively uniform across the CGF, but with significant local variations. The Vp/Vs ratio appears to outline the two main producing compartments of the reservoir at depths below mean ground level of approximately 1 to 2.5 km, with a ridge of relatively high Vp/Vs separating the Main Field from the East Flank. Detailed analyses of spatial and temporal variations in earthquake relocations and cumulative seismic moment release in the East Flank reveal three regions with persistently high rates of seismic activity. Two of these regions exhibit sharp, stationary boundaries at the margins of the East Flank that likely represent barriers to fluid flow and advective heat transport. However, seismicity and moment release in a third region at the northern end of the East Flank spread over time to form an elongated NE to SW structure, roughly parallel both to an elongated cluster of seismicity at the southern end of the East Flank and to regional fault traces mapped at the surface. Our results indicate that high-precision relocations of micro-seismicity and simultaneous velocity inversions in conjunction with mapping of seismic moment release can provide useful insights into subsurface structural features and hydrologic compartmentalization within the Coso Geothermal Field.
Crustal Stress Heterogeneity in the Vicinity of Coso Geothermal Field, CA
Borehole induced structures in image logs of wells from the Coso Geothermal Field (CGF), CA record variation in the azimuth of principal stress. Image logs of these structures from five wells were analyzed to quantify the stress heterogeneity for three geologically distinct locations: two wells within the CGF (one in an actively produced volume), two on the margin of the CGF and outside the production area, and a control well several tens of kilometers south of the CGF. Average directions of S hmin and its standard deviation are similar along the eastern portion of the geothermal field at ~106±28°, but this is distinct from the western portion which has an azimuth of 081±18° and distinct from outside the geothermal field where the average azimuth is 092±47°. Spectral analysis was applied to the depth variation of stress direction and demonstrates that: (1) the data set contains distinct wavelengths of stress rotation, (2) that the relative power of these wavelengths in the total distribution of stress directions is fractally distributed and (3) in a manner consistent with earthquakes causing the stress rotations. The slope of the power spectrum quantifies the length-scale dependence of stress rotations for the volume of the brittle crust penetrated by each well. While the vertically averaged S hmin orientation for the three eastern wells inside the field varied by as little as 1˚, the spectral slopes inside the field varied by 0.4 log(deg 2 *m)(m), from the inside to the margin unproduced areas of the CGF.
2011
We have synthesized the characteristics of the seismogenic zone in the east Los Angeles basin by analyzing earthquake data recorded during the past 30 years (1981-2010). The seismicity is distributed along the Whittier fault, with the majority of earthquakes located adjacent to the south side, in the depth range from 0 to 9 km, with b value of 1:1 0:05 and mostly normal and strike-slip faulting. Within the depth range of 9-12 km, the seismicity is scattered uniformly across the region, the b value is 1:0 0:05, and all three faulting styles are present. At the deepest depths (12-18 km), seismicity is sparse and primarily limited to a few clusters striking north; these deeper earthquakes primarily have reverse fault motion, and the b value is 0:78 0:04. Inversion of high-quality focal-mechanism data for the orientation of the regional stress field showed that the direction of maximum compressional stress rotates from N12°W at shallow depth to due north at the bottom of the seismogenic zone. Similarly, a depth dependence is observed in stress drops calculated from P-wave source spectra, which indicate stress drop generally increases from ∼7 MPa at shallow depth (3 km) to ∼53 MPa at the base of the seismogenic zone (17 km). Overall, our results provide new evidence for the vertical partitioning of styles of deformation and state of stress within this complex fault system in the east Los Angeles basin.
Journal of Geophysical Research, 1995
The southern San Joaquin Valley (SSJV) stress state is characterized by systematic variations in the regional principal stress directions and relative magnitudes, as well as local and possibly temporal variations that appear to be the result of the 1952 Ms 7.8 Kern County earthquake. The regional maximum horizontal principal stress (SHmax) orientation in the San Joaquin Valley systematically rotates from ~NE-SW compression along the western margins of the valley to ~N-S compression in the SSJV. This ~N-S SHmax stress direction in the SSJV is consistent with active development of a ~E-W trending structural fabric of fold axes and shallow thrust, and the reverse dip-slip motion observed during the Kern County earthquake on the south-southeast dipping White Wolf fault. Contemporary seismicity (30-40 years after the mainshock) clusters in two separate areas along the White Wolf fault: the southwest region where the 1952 earthquake nucleated, and the northeast region near where the earthquake rupture terminated. Earthquakes in the southwest and northeast regions show a diversity of focal mechanisms that include strike-slip, reverse, oblique, and, to a lesser extent, normal slip. Inversion of earthquake focal mechanisms for in situ stress in the southwest region indicates a strike-slip/reverse stress regime with S 1 oriented approximately perpendicular to the ruptured fault plane implying low frictional strength in the nucleation zone of the 1952 earthquake. Inversion of focal plane mechanisms in the northeast region indicates a strike-slip stress regime with S 2 nearly perpendicular to secondary 1952 rupture planes also implying low frictional strength. These results indicate near-complete stress drops for fault planes associated with the 1952 earthquake (and some of the contemporary earthquakes), implying fault surfaces which are frictionally weak (i.e., slip planes subparallel to principal stress planes). Based on the observed stress state in the southern San Joaquin Valley, much of the seismicity may be the result of elevated fluid pressures within these active fault zones. 6249 Zoback, 1994]. In brief, observations of breakouts 10-20 km north of the White Wolf fault indicate a -N-S SHmax stress direction. However, near the epicentral region of the 1952 earthquake, breakouts show -NE-SW compression which is in contrast to the long-term regional -N-S SHmax stress direction that is responsible for the north-south convergence seen in the geologic fabric and the breakout data farther north. Motivated by these observations, this study examines the contemporary seismicity to characterize the style of faulting within the White Wolf fault zone where earthquakes focal depths approach 25 km. All earthquakes for the 1983-1993 period were relocated using a crustal velocity structure and station corrections estimated from earthquake travel time inversions. Focal mechanisms of nearly 250 earthquakes (ML > 1.8) were then inverted for information about the orientation and relative magnitudes of the in situ stresses using a grid search inversion method
Journal of Geophysical Research, 1986
Hypocentral locations and first-motion data from the southern California seismic network are used to infer the present pattern of deformation along the southern San Andreas fault. The study area lies between the San Jacinto fault and Desert Hot Springs and includes the San Bernardino Mountains and San Gorgonio Pass. This region has some of the deepest earthquakes observed anywhere along the entire San Andreas system, exhibits a complex surface geology characterized by both right-and left-lateral faults, has high topographic relief as a result of recent uplift, and is a potential site for the nucleation of a great earthquake. Although this area is unusually seismogenic, little activity can be directly associated with major throughgoing faults. Seismicity is also generally absent in the upper 5 km. The predominant style of faulting above 10-12 km is oblique slip with a large reverse component. The spatial distribution of relocated hypocenters and first-motion data suggests the presence of a system of left-slip faults striking northeast. This pattern of faulting, in conjunction with an unusual set of normal and reverse focal mechanisms, is interpreted as the clockwise rotation of a small set of crustal blocks subject to regional right-lateral shear. At depths greater than about 10 km, seismicity defines a wedge-shaped volume undergoing pervasive internal deformation on a combination of strike-slip and low-angle thrust faults. Velocity structures determined from earthquake arrival times suggest a low-velocity zone at about 10 km below the San Bernardino Mountains but not below the San Jacinto Mountains. This is nearly the same depth as the transition between the two layers of different kinematic behavior seen south of the Mill Creek-Mission Creek branch of the San Andreas fault and the maximum depth of seismicity seen north of that fault. The low-velocity zone and the transition between block rotations and the deeper deformation may thus correspond to a detachment under much of this region and would imply that the overthrust San Bernardino Mountains are allochthonous. The present pattern of seismic deformation in shocks of small to moderate size may only characterize the interval between large earthquakes and may change systematically as the region prepares to accommodate large right-lateral displacements. INTRODUCTION The seismic deformation of southern California has been largely controlled by the evolution and slip history of the San Andreas fault system [e.g., Allen, 1981 ]. Large, damaging earthquakes of magnitude near 8 occurred along this plate boundary in 1857 and 1906 (Figure 1), yet the segment in southern California south of Cajon Pass has not experienced a large or great earthquake in historic time. Geologic excavations suggest that the last major slip event for this southern segment appears to have occurred in either the late 1600s or early 1700s, and a repeat time of the order of 300 years is not unreasonable [Sieh, 1984b; Weldon and $ieh, 1985]. The fault segment northwest of Cajon Pass formed the southern part of the 1857 rupture zone and has an estimated repeat time of 150 years [Sieh, 1984a]. Should these sections fail together, the resulting great earthquake could involve rupture from as far north as Palmdale to as far south as the Salton Sea (C-D, Figure 1) and could cause damage in excess of several billion dollars [Federal Emergency Management Agency, 1980]. An
Regional tectonic stress near the San Andreas fault in central and southern California
Geophysical Research Letters, 2004
Throughout central and southern California, a uniform NNE-SSW direction of maximum horizontal compressive stress is observed that is remarkably consistent with the superposition of stresses arising from lateral variations in lithospheric buoyancy in the western United States, and farfield Pacific-North America plate interaction. In central California, the axis of maximum horizontal compressive stress lies at a high angle to the San Andreas fault (SAF). Despite relatively few observations near (±10 km) the fault, observations in the greater San Francisco Bay area indicate an angle of as much as 85°, implying extremely low fault strength. In southern California, observations of stress orientations near the SAF are rotated slightly counterclockwise with respect to the regional field. Nevertheless, we observe an approximately constant angle between the SAF and the maximum horizontal stress direction of 68 ± 7°along $400 km of the fault, indicating that the SAF has moderately low frictional strength in southern California.
High precision earthquake locations and subsurface velocity structure provide potential insights into fracture system geometry, fluid conduits and fluid compartmentalization critical to geothermal reservoir management. We analyze 16 years of seismicity to improve hypocentral locations and simultaneously invert for the seismic velocity structure within the Coso Geothermal Field (CGF). The CGF has been continuously operated since the 1980's and is separated into two main compartments: the main field and the east flank. These compartments are at higher temperatures than the immediate surroundings. We find that relocated seismicity in the main field is shallower than in the east flank and occurs at the same depths as the injection and production wells, while the east flank seismicity extends about 1 km below the injection and production wells and is occurring almost exclusively in regions of high temperature. In the east flank, many of the earthquakes appear to align along planar features, suggesting through-going, pre-existing faults that may act as conduits for fluid and heat transport. The seismic velocity structure is heterogeneous, with compressional wave speed (Vp) generally lower in the main field when compared to the east flank and shear wave speed (Vs) varying more significantly in the shallow portions of the reservoir. The Vp/Vs ratio appears to outline the two main compartments of the reservoir, with a narrow zone of relatively high Vp/Vs separating the main field from the east flank. In the deeper portion of the reservoir this zone becomes less prominent. Several factors influence Vp/Vs ratios in geothermal systems including temperature, fracture geometry/density, and fluid saturation or pore pressure. Comparison of the distribution of Vp/Vs ratios with a temperature model generated from well logs reveals a first-order correlation between regions of low Vp/Vs ratio and high temperature. However, there is a better correlation between the distribution of productioninduced microseismicity and Vp/Vs ratio, especially where high seismicity density occurs within the regions of high temperature, suggesting that these low Vp/Vs ratios most likely result from changes in fluid saturation or pore pressure.
Earthquake Source Parameters and Fault Kinematics In the Eastern California Shear Zone
Bulletin of the Seismological Society of …, 1998
Based on waveform data from a profile of aftershocks following the north-south trace of the June 28, 1992 Landers rupture across the Mojave desert, we construct a new velocity model for the Mojave region which features a thin, slow crust. Using this model, we obtain source parameters, including depth and duration, for each of the aftershocks in the profile, and in addition, any significant (M > 3.7) Joshua Tree-Landers aftershock between October, 1994 for which coherent TERRAscope data were available. In all, we determine source parameters and stress-drops for 45 significant (M w > 4) earthquakes associated with the Joshua Tree and Landers sequences, using a waveform grid-search algorithm. Stress drops for these earthquakes appear to vary systematically with location, with respect to previous seismic activity, proximity to previous rupture (i.e., with respect to the Landers rupture), and with tectonic province. In general, for areas north of the Pinto Mountain fault, stress-drops of aftershocks located off the faults involved with the Landers rupture are higher than those located on the fault, with the exception of aftershocks on the newly recognized Kickapoo (Landers) fault. Stress drops are moderate south of the Pinto Mountain fault, where there is a history of seismic swarms but no single through-going fault. In contrast to aftershocks in the eastern Transverse ranges, and related to the 1992 Big Bear, California, sequence, Landers events show no clear relationship between stress-drop and depth. Instead, higher stress-drop aftershocks appear to correlate with activity on nascent faults, or those which experienced relatively small slip during mainshock rupture.
Three-dimensional anatomy of a geothermal field, Coso, southeast-central California
Memoir 195: Geologic Evolution of the Mojave Desert and Southwestern Basin and Range, 2002
This paper reviews geophysical and seismological imaging in the Coso geothermal field, located in southeast-central California. The Coso geothermal production area covers ϳ6 10 km 2 . Although regional seismicity is addressed, as it sheds light on the magma, or heat, sources in the field, the primary focus of this paper is on the main production area. Three-dimensional inversions for P-and S-wave velocity variations, distribution of attenuation, and anisotropy are presented side-by-side so that anomalies can be compared spatially in a direct manner. Velocity inversions for P and S waves are combined for direct determination of Poisson's ratio and indirect estimation of variations of porosity in the field. Anomalies southeast of Sugarloaf Mountain are prominent on nearly all analyses. The anomalies coincide with high levels of seismicity and with stress anomalies as determined from earthquake focal mechanism analysis and seismic anisotropy distribution. The anomalies also correlate with high heat flow in the field and the termination of geothermal production to the south. I speculate that an intrusion is present in this region that causes significant perturbation of stress in the field.