Cerro Prieto Fault Research Articles

High-resolution seismic imaging of the plate boundary in northern Baja California and southern California using double-pair double-difference tomography

We image seismic properties of the plate boundary through northern Baja California and southernmost California using a double-difference tomography and earthquake relocation algorithm that incorporates both event-pair and station-pair data. The data are derived from seismic networks in southern California (SCSN) and northern Baja California (CICESE). It comprises >700,000 travel times (P and S) associated with ∼200 seismic stations and >21,000 local earthquakes from 2003-2018, along with >2,000,000 event-pair and >1,500,000 station-pair double-differences derived from these travel times. In the upper crust, low VP values in the resultant models imply deep sediments and sedimentary rocks northwest of the Salton Sea and around the northern end of the Imperial fault. Low VS values indicate regions of rock damage at the northern ends of the Sierra Juarez fault zone and Laguna Salada fault, and the southern end of the San Jacinto fault zone (SJFZ). Shallow seismicity (d<956 km) highlights the Salton Buttes and Cerro Prieto volcanoes as high heat-flow thin crust regions. At mid to lower crustal depths, the sharpest contrast in crustal VP and most prominent low VS anomaly (V∼S3.4 km/s, VP/VS ∼ 1.9) occurs along a corridor extending northwestward from the Cerro Prieto fault (CPF), east of and subparallel to the Sierra El Mayor/Cucapah and into the southern SJFZ, indicating a continuous weak fluid-rich zone. At ∼25 km depth, we observe uppermost mantle velocities (V∼P7.8 km/s) in the eastern Salton Trough, crustal velocities (V<P7 km/s) around the Sierra El Mayor/Cucapah and Peninsular Ranges, and near mantle velocities (7.4 < VP < 7.8 km/s) beneath Laguna Salada. These results illuminate a thin crust in the Salton Trough, except for the Sierra El Mayor/Cucapah, and a thicker crust in the Peninsular Ranges. In all, high seismic activity, rapid changes in crustal structure and the prevalence of damage/deformation and fluids in a corridor extending from the CPF to the southern SJFZ emphasize this zone as a key component of the active plate boundary.

Subsurface structure revealed by seismic reflection images to the southwest of the Cerro Prieto pull-apart basin, Baja California

This work aims to characterize the crustal architecture and structure in an area of the Mexicali Valley between the southwestern part of the Cerro Prieto pull-apart basin, which contains the geothermal field of same name, and the southeastern edge of the Sierra Cucapah, Baja California, Mexico. Although the region surrounding the geothermal field has been extensively studied, the structural features in the vicinity are poorly known. Interpretations based on processed crooked-line 2D seismic reflection images made here reveal evidence of structural deformation and seismostratigraphic features in the study area. Two structural domains were identified adjacent to the Cerro Prieto fault: the northeastern domain, which belongs to the pull-apart basin, and the southwestern domain, which exhibits deformation associated with the Sierra Cucapah uplift. Subsurface images indicate the presence of the Cerro Prieto fault through lateral discontinuities in the reflectors; the fault is sub-vertically oriented and dips ∼80° to the east. Two seismic reflectors were identified to west of the Cerro Prieto fault, one of which corresponds to the acoustic basement. The other is associated with the top of an alluvial and deltaic facies and is described here for the first time. Three other seismic reflectors to the east of the Cerro Prieto fault correspond to gray shale sequences with interspersed sandstones in the Cerro Prieto geothermal field that were reported previously by other authors. The presence of two previously unknown structures are reported for the first time. The Sonora and Nayarit faults, with azimuths of ∼305° and ∼307° and lengths of ∼12 km and ∼9 km, respectively, are both located within the block bounded by the Cerro Prieto fault to the northeast and the Indiviso fault to the southwest. The La Puerta fault is also described for the first time, which exhibits reverse fault characteristics and lies transverse to the fault system of the Sierra Cucapah. The acoustic basement between the Sierra Cucapah and the Cerro Prieto fault is sub-horizontal and dipping slightly westward, and differs from the eastward-dipping acoustic basement observed to the south between the Sierra Las Pintas and the deltaic plain. Subsidence in the sequences within the Cerro Prieto basin can be more clearly observed after 500 m, with evidence in profile L-25 suggesting greater vertical displacement towards the southeast. Two areas of opportunity are reported for geothermal interest purposes. Despite the additional knowledge about regional tectonics and strucural background provided by this work, further studies are needed. The fault geometry remains poorly defined and due to its location, much of the fault is buried by sediments that prevent a full understanding of the potential danger in the region. Here, previously unknown structures are reported that represent geological and seismic risks. In addition to their scientific value, the findings have a potentially large societal impact due to the presence of several cities in the study area. The results should therefore be incorporated into future regional risk assessments.

Mechanical Models Suggest Fault Linkage through the Imperial Valley, California, U.S.A.

AbstractThe Imperial Valley hosts a network of active strike‐slip faults that comprise the southern San Andreas fault (SAF) and San Jacinto fault systems and together accommodate the majority of relative Pacific–North American plate motion in southern California. To understand how these faults partition slip, we model the long‐term mechanics of four alternative fault networks with different degrees of connectivity through the Imperial Valley using faults from the Southern California Earthquake Center Community Fault Model version 5.0 (v.5.0). We evaluate model results against average fault‐slip rates from the Uniform California Earthquake Rupture Model v.3 (UCERF3) and geologic slip‐rate estimates from specific locations. The model results support continuous linkage from the SAF through the Brawley seismic zone to the Imperial and to the Cerro Prieto faults. Connected faults decrease surface strain rates throughout the region and match more slip‐rate data. Only one model reproduces the UCERF3 rate on the Imperial fault, reaching the lower bound of 15 mm/yr. None of the tested models reproduces the UCERF3 preferred rate of 35 mm/yr. In addition, high‐strain energy density rates around the Cerro Prieto fault in all models suggest that the UCERF3 preferred rate of 35 mm/yr may require revision. The Elmore Ranch fault‐slip rate matches the UCERF3 rate only in models with continuous linkage. No long‐term slip‐rate data are available for the El Centro and Dixieland faults, but all models return less than 2 mm/yr on the El Centro fault and 3.5–9.6 mm/yr on the Dixieland fault. This suggests that the Dixieland fault may accommodate a significant portion of plate‐boundary motion.

Systematic heat flow measurements across the Wagner Basin, northern Gulf of California

A primary control on the geodynamics of rifting is the thermal regime. To better understand the geodynamics of rifting in the northern Gulf of California we systematically measured heat-flow across the Wagner Basin, a tectonically active basin that lies near the southern terminus of the Cerro Prieto fault. The heat flow profile is 40 km long, has a nominal measurement spacing of ∼1 km, and is collocated with a seismic reflection profile. Heat flow measurements were made with a 6.5-m violin-bow probe. Although heat flow data were collected in shallow water, where there are significant temporal variations in bottom water temperature, we use CTD data collected over many years to correct our measurements to yield accurate values of heat flow. After correction for bottom water temperature, the mean and standard deviation of heat flow across the western, central, and eastern parts of the basin are 220 ± 60, 99 ± 14, 889 ± 419 mW m−2, respectively. Corrections for sedimentation would increase measured heat flow across the central part of basin by 40 to 60%. We interpret the relatively high heat flow and large variability on the western and eastern flanks in terms of upward fluid flow at depth below the seafloor, whereas the lower and more consistent values across the central part of the basin are suggestive of conductive heat transfer. Moreover, heat flow across the central basin is consistent with gabbroic underplating at a depth of 15 km and suggests that continental rupture here has not gone to completion.

Tectonic and Anthropogenic Deformation at the Cerro Prieto Geothermal Step-Over Revealed by Sentinel-1A InSAR

The Cerro Prieto geothermal field (CPGF) lies at the step-over between the imperial and the Cerro Prieto faults in northern Baja California, Mexico. While tectonically this is the most active section of the southern San Andreas Fault system, the spatial and temporal deformation in the area is poorly resolved by the sparse global positioning system (GPS) network coverage. Moreover, interferograms from satellite observations spanning more than a few months are decorrelated due to the extensive agricultural activity in this region. Here we investigate the use of frequent, short temporal baseline interferograms offered by the new Sentinel-1A satellite to recover two components of deformation time series across these faults. Following previous studies, we developed a purely geometric approach for image alignment that achieves better than 1/200 pixel alignment needed for accurate phase recovery. We construct interferometric synthetic aperture radar time series using a coherence-based small baseline subset method with atmospheric corrections by means of common-point stacking. We did not apply enhanced spectral diversity because the burst discontinuities are generally small (<1.4 mm) and can be effectively captured during the atmospheric corrections. With these algorithms, the subsidence at CPGF is clearly resolved. The maximum subsidence rate of 160 mm/yr, due to extraction of geothermal fluids and heat, dominates the ~40 mm/yr deformation across the proximal ends of the imperial, the Cerro Prieto, and the indiviso faults.

Initiation, evolution and extinction of pull-apart basins: Implications for opening of the Gulf of California

We present a model for the origin, crustal architecture, and evolution of pull-apart basins. The model is based on results of three-dimensional upper crustal elastic models of deformation, field observations, and fault theory, and is generally applicable to basin-scale features, but predicts some intra-basin structural features. Geometric differences between pull-apart basins are inherited from the initial geometry of the strike-slip fault step-over, which results from the forming phase of the strike-slip fault system. As strike-slip motion accumulates, pull-apart basins are stationary with respect to underlying basement, and the fault tips propagate beyond the rift basin, increasing the distance between the fault tips and pull-apart basin center. Because uplift is concentrated near the fault tips, the sediment source areas may rejuvenate and migrate over time. Rift flank uplift results from compression along the flank of the basin. With increasing strike-slip movement the basins deepen and lengthen. Field studies predict that pull-apart basins become extinct when an active basin-crossing fault forms; this is the most likely fate of pull-apart basins, because basin-bounding strike-slip systems tend to straighten and connect as they evolve. The models show that larger length-to-width ratios with overlapping faults are least likely to form basin-crossing faults, and pull-apart basins with this geometry are thus most likely to progress to continental rupture. In the Gulf of California, larger length-to-width ratios are found in the southern Gulf, which is the region where continental breakup occurred rapidly. The initial geometry in the northern Gulf of California and Salton Trough at ~6Ma may have been one of widely-spaced master strike-slip faults (lower length-to-width ratios), which our models suggest inhibits continental breakup and favors straightening of the strike-slip system by formation of basin-crossing faults within the step-over, as began ~1.2Ma when the San Jacinto and Elsinore - Cerro Prieto fault systems formed.

Fault geometry inversion and slip distribution of the 2010 Mw 7.2 El Mayor‐Cucapah earthquake from geodetic data

AbstractThe 4 April 2010 Mw 7.2 El Mayor‐Cucapah (EMC) earthquake in Baja, California, and Sonora, Mexico, had primarily right‐lateral strike‐slip motion and a minor normal‐slip component. The surface rupture extended about 120 km in a NW‐SE direction, west of the Cerro Prieto fault. Here we use geodetic measurements including near‐ to far‐field GPS, interferometric synthetic aperture radar (InSAR), and subpixel offset measurements of radar and optical images to characterize the fault slip during the EMC event. We use dislocation inversion methods and determine an optimal nine‐segment fault geometry, as well as a subfault slip distribution from the geodetic measurements. With systematic perturbation of the fault dip angles, randomly removing one geodetic data constraint, or different data combinations, we are able to explore the robustness of the inferred slip distribution along fault strike and depth. The model fitting residuals imply contributions of early postseismic deformation to the InSAR measurements as well as lateral heterogeneity in the crustal elastic structure between the Peninsular Ranges and the Salton Trough. We also find that with incorporation of near‐field geodetic data and finer fault patch size, the shallow slip deficit is reduced in the EMC event by reductions in the level of smoothing. These results show that the outcomes of coseismic inversions can vary greatly depending on model parameterization and methodology.

Delayed Dynamic Triggered Seismicity in Northern Baja California, México Caused by Large and Remote Earthquakes

We analyze the seismicity in northern Baja California, Mexico, that occurred one month before and one month after the 11 March 2011 ( M w 9.1) Tohoku‐Oki, Japan, earthquake and for two other large and remote earthquakes, the 27 February 2010 central Chile ( M w 8.8) earthquake and the 11 April 2012 northern Sumatra earthquake ( M w 8.6). The northern region of Baja California exhibits high microseismic activity and moderate‐size earthquakes. The seismicity in this region is monitored by the seismic network Red Sismica del Noroeste de Mexico (RESNOM) operated by the Centro de Investigacion Cientifica y de Educacion Superior de Ensenada (CICESE). We use the RESNOM seismic catalog to search for changes in local seismic rates that occurred after the passing of surface waves generated by the three large earthquakes studied. When we compare one month of seismicity before and after the M w 9.1 Japan earthquake, in the biggest of the three events analyzed, we observe the absence of triggered seismicity in the northern Peninsular Ranges and an increase of seismicity south of the Mexicali Valley, where the Imperial fault jumps southwest and the Cerro Prieto fault continues south. We also observe an increase of seismicity rate in the Cucapah fault and in the San Pedro Martir fault regions. The Cucapah fault region, the most active of the three regions during the studied period, shows an increase of seismicity several days after the 2010 Chile ( M w 8.8) and the 2011 Japan ( M w 9.1) earthquakes. This observation can be interpreted as evidence of delayed seismicity and confirms the observation of Rubinstein et al. (2009) that earthquakes tend to be triggered in regions with high ambient seismicity rates. We observed delayed triggering of earthquakes generated by two of the three remote earthquakes analyzed. These observations can be explained by dynamic triggering of prolonged fault creep as proposed by Shelly et al. (2011). Online Material: Maps of seismicity and M c.

Structural Geometry of a Sector of the Colorado River Delta, Baja California, Mexico, Based on Seismic Reflections

A structural study in the SW section of the Colorado River delta using seismic reflection data is presented. The study area is located along the Cerro Prieto transform fault, which extends from the northern Gulf of California through the Mexicali Valley and is an active fault within the Pacific-North American plate boundary zone. The research was supported by a database of five seismic profiles with a total length of 215 km, collected in the early 80’s by Petroleos Mexicanos. The results show a high density of faults, most of which are buried by sediments. Within the Cerro Prieto fault zone, several faults were identified, such as: Palmas, Mesa, and Pangas Viejas, until now unknown. In addition, even though the Indiviso fault was investigated and superficially identify prior to this work, herein mapped at depth. West of the Cerro Prieto fault zone lies the Las Tinajas basin, bound by the Dunas and Saldana faults and by the Montague basin to the southeast. The deformation zone along the plate boundary is 18-km-wide, stretching from the Cerro Prieto fault in the east to the Pangas Viejas fault in the west. The orientations of the faults are NW–SE, and if projected from the southern side of the Sierra Cucapah southward, the faults tend to join the Cerro Prieto fault. In the Las Tinajas basin, the acoustic basement is deeper than 5,000 m. Some of the largest vertical displacements generated by the 2010 7.2-Mw El Mayor-Cucapah earthquake occurred southeast of the epicenter and coincided with the location of the Pangas Viejas Fault, which is buried by sediments. Before this event, seismic activity was very low, and no structures were known in the area. In this paper, we demonstrate that there are at least seven major faults that may now pose a high seismic hazard.

Structural characteristics of the Altar Basin, Northwest Sonora, Mexico

Eight two-dimensional, multichannel seismic reflection lines were acquired, processed, and interpreted to study the structure of the Altar Basin, which is part of the Salton Trough tectonic province. We identified two basin-bounding zones characterized by different degrees of strain: the Cerro Prieto–Altar deformation zone (CPADZ) and the Altar–Caborca deformation zone (ACDZ). The CPADZ is bounded on the west by the Cerro Prieto fault and on the east by the Altar fault. To the north, the strike of both faults changes slightly from a NW to more NNW direction. In the CPADZ, the thickness of the crust decreases southward towards the Gulf of California, and is associated with a deformation-developing fault. The CPADZ has a rotation component orientating these faults in an oblique direction to the Cerro Prieto fault, whereas within the ACDZ, a geometric coherence of synthetic and antithetic faults exists, creating horsts and graben striking N37° W. The Altar fault is recognized by basement interruption, with a vertical component of ∼1 km, striking at N37° W and dipping 83° SW. On the northeastern side of the Altar Basin, the basement configuration shows that the minimum time of basement record (∼0.4 s of two-way travel time) and the time curve gradient decrease in the NE–SW direction. The depocentre is ∼6 km deep in the central-west portion of the basin. We identified a graben between the Rosario and Tinajas Altas mountains (Rosario Basin). The extension–connection of the Altar and Rosario basins to the south is not well defined; nevertheless, these basins could represent the link between the Colorado River and the Gulf of California during the late Miocene, whereas this link was abandoned in the Pliocene as subsidence migrated towards the northwest into the Cerro Prieto and Laguna Salada basins.

The Role of Fluids in Promoting Seismic Activity in Active Spreading Centers of the Salton Trough, California, USA

We interpret seismic activity in the active spreading centers of the Salton Trough to indicate 1) a magmatic intrusion in the lower crust beneath the active Brawly, Cerro Prieto, Imperial, Elsinore, and San Jacinto fault systems; and 2) fluids in the upper crust that have been released from that magmatic body. The absence of a magmatic body and fluids at the location of fossil spreading centers along the Sand Hill and Algodones faults ndicated by little or no seismic activity in those areas. We show several lines of evidence to point out that both melt and fluids related to the seismic activity. In particular, receiver function analysis, Vp/Vs ratios, and tomographic data reveal low velocity zones coincide with the location of the active spreading centers. High Vp/Vs ratios and low velocity zones in the lower crust and upper mantle attributed to melt inclusion, while low Vp/Vs ratios in the upper crust are attributed water inclusions. Frequency-mag- nitude distributions characterized by high b-values in southern California; high b-values have also been associated with crustal fluids. A crustal scale model developed from the receiver functions, gravity, and magnetic data supports the existence of a magmatic intrusion within about 20 km of the surface southwest of the Salton Sea, that intrusion extends for 70 km in a SW-NE direction.

Geometry of the Wagner basin, upper Gulf of California based on seismic reflections

The Wagner basin occupies the northernmost spreading centre in the Gulf of California, located along the Pacific‐North America plate boundary. It is filled with sediments from the Colorado River that obscure its bathymetric expression; therefore it is not as well defined as other basins in the central and southern Gulf of California. To define the geometry and extension of the Wagner basin, a 2D multi‐channel seismic reflection database was used. Data were collected by Petroleos Mexicanos (PEMEX) in 1979–1980. The most important regional structural features identified are the Consag and Wagner normal faults and the Cerro Prieto strike‐slip fault. These structures play an important role in the development of the basin. The Consag fault, described for the first time in this paper, marks the western side of the basin. The eastern and northwest limits are bound by the Cerro Prieto and Wagner faults respectively. The Wagner fault intersects the Cerro Prieto fault at an angle of 130°, bending the depocentre in a NW direction, adjacent to the Cerro Prieto fault zone. The northernmost segment of the Consag fault bends 25° in a NE direction and joins the Cerro Prieto fault at an angle of 110°. Greater subsidence (up to 300 m) takes place along the northern trace of the Cerro Prieto fault, with a downthrown displacement of 400 m. The Consag and Wagner breaks obliquely intersect the Cerro Prieto fault, and, inasmuch as both are normal faults, they have small horizontal slip components which generated oblique displacement. This structural pattern is different relative to the pattern of basins located south of Wagner basin, such as the Upper and Lower Delfin basins. The orientations of the normal faults are perpendicular to the master fault (Ballenas transform fault). The relationship between normal and transform faults in the Wagner basin and the observed ‘S’ shape are typical of a basin that has not yet reached maturity. As a result of this study, the previously uncertain area (∼1330 km2) and perimeter (158 km) of the Wagner basin were defined.

Bathymetry and active geological structures in the Upper Gulf of California

Bathymetric surveys made between 1994 and 1998 in the Upper Gulf of California revealed that the bottom relief is dominated by narrow, up to 50 km long, tidal ridges and intervening troughs. These sedimentary linear features are oriented NW-SE, and run across the shallow shelf to the edge of Wagner Basin. Shallow tidal ridges near the Colorado River mouth are proposed to be active, while segments in deeper water are considered as either moribund or in burial stage. Superposition of seismic swarm epicenters and a seismic reflection section on bathymetric features indicate that two major ridge-troughs structures may be related to tectonic activity in the region. Off the Sonora coast the alignment and gradient of the isobaths matches the extension of the Cerro Prieto Fault into the Gulf. A similar gradient can be seen over the west margin of the Wagner Basin, where in 1970 a seismic swarm took place (Thatcher and Brune, 1971) overlapping with a prominent ridge-trough structure in the middle of the Upper Gulf. It is believed that these major elongated features can be linked to active geological structures reflecting the geometry of the deep basement in the Wagner Basin.

Shape and Dimensions of the Cerro Prieto Pull-Apart Basin, Mexicali, Baja California, Mexico, Based on the Regional Seismic Record and Surface Structures

The Salton Trough is a wide, actively subsiding basin where a system of an en-echelon dextral transform faults and pull-apart basins is operating. This tectonic province represents the connection between the Gulf of California fault system and the San Andreas transform fault. Within the depression, several sub-basins, such as the Cerro Prieto geothermal field in Mexicali, Baja California, Mexico, represent an incipient spreading center along the southern limits of the Salton Trough. The Cerro Prieto Basin has developed between two major, subparallel, right-stepping active transforms, the Imperial and Cerro Prieto faults. General agreement exists regarding the regional tectonic interpretation of the Cerro Prieto Basin; however, published documentation is lacking regarding its shape and dimensions. In this paper, we address this problem. We have integrated seismic data available for the whole study area and evidence from surface breaks. Our study shows that the most important factor controlling formation of the Cerro Prieto pull-apart basin is the slip on the Cerro Prieto and Imperial faults. This tectonic activity has triggered subsidence in the intervening area, creating several subsurface normal faults, oriented oblique and parallel to the major strands, that control the subsidence. All of this has resulted in the development of a NE-SW trending, immature, pull-apart basin with an areal extent of 183 km2. Thus the Cerro Prieto Basin is an important structure in terms of geology and geothermal resources.

Surface Rupture of the Morelia Fault Near the Cerro Prieto Geothermal Field, Mexicali, Baja California, Mexico, during the Mw 5.4 Earthquake of 24 May 2006

The Cerro Prieto geothermal field, considered an active developing rift basin, is located in the southern limits of the Salton Trough tectonic province. The tectonic activity along the Gulf of California–Salton Trough has created spreading centers linked through a set of transform faults. Such faults are characterized by right lateral motion obliquely oriented to the gulf axis, which produces a transtensional stress regime along the Pacific–North American plate boundary. In particular, the Cerro Prieto basin has developed between the major Cerro Prieto and Imperial right-stepping transform faults, which are part of the San Andreas–Gulf of California fault system (figure 1). Both faults are seismically active and have produced several magnitude 7 earthquakes. However, in the past decades only the Imperial fault has produced superficial ground breakage during earthquakes. This is in contrast to earthquake ruptures on the Cerro Prieto fault, which have not extended up to the ground surface, so this fault trace is not yet well-constrained. The epicenter of the 24 May 2006 Mw 5.4 earthquake was located along the northern end of the Cerro Prieto fault. In a field survey performed after the earthquake, superficial fractures were clearly observed at short distances from the epicenter. Based on our field observations, we address the question of whether such fractures are evidence of rupture along the Cerro Prieto fault or are associated with a conjugate fault. ▴ Figure 1. Tectonic map of the Gulf of California region, including the peninsula of Baja California and Sonora, Mexico; California; Arizona; and the main tectonic elements: North American plate, Pacific Plate, East Pacific Rise, Gulf extensional province (GEP), Cerro Prieto fault (CPf), Imperial fault (If), Agua Blanca fault (ABf), Laguna Salada fault (LSf), Elsinore fault (Ef), San Jacinto fault (SJf), San Andreas fault (SAf), and Garlock fault (Gf). On 24 May 2006 at 04:20 (UTC) …

Stress Heterogeneity in the Cerro Prieto Geothermal Field, Baja California, Mexico

We studied the background seismic activity in the Cerro Prieto geothermal field (CPGF). Earthquakes were located below the exploitation area of the CPGF, between 3 and 12 km deep. This activity is located on a horstlike structure below the geothermal field and coincides with the zone of maximum subsidence in the CPGF. Two earthquake swarms occurred, one in November 2000 and the second in February 2001, along the southeast–northwest strike of the Cerro Prieto fault and in the neighborhood of the Cerro Prieto volcano. Magnitudes of the located microevents range from –0.3 to 3.5. A V p/ V s ratio of 1.91 for the region below the volcano suggests a water-saturated medium and/or a partial-melt medium. Seventy-six focal mechanisms of individual microearthquakes reveal significant variation in the orientation of the P and T axes. Focal mechanism inversions using the Gephart (1990) code also reveal significant lateral stress variation of the best-fit stresses. Below the CPGF, the orientation of the best-fit maximum, intermediate, and minimum stresses are (plunge, azimuth): σ 1 = (66°, 72°), σ 2 = (6°, 328°), and σ 3 = (24°, 235°). For the November 2000 earthquake swarm, the best-fit principal stress orientations are σ 1 = (12°, 117°), σ 2 = (11°, 25°), and σ 3 = (74°, 253°), whereas for the February 2001 earthquake swarm they are σ 1 = (36°, 235°), σ 2 = (24°, 127°), and σ 3 = (45°, 11°). The σ 1 orientations are considerably different from the orientation of the maximum compressive regional stress that lies approximately north–south. On 1 June and 10 September 1999, two earthquakes of M w 5.2 and 5.3 occurred in the basement at depths of 7.4 and 3.8 km below the CPGF. Waveform modeling resulted in fault-plane geometries given by strike = 236°, dip = 60°, slip = –58° (normal) and strike = 10°, dip = 90°, slip = 159° (right-lateral strike slip), respectively. An observed triangular source time function of 0.7 sec and a double source with a total duration of 1.9 sec were used to calculate the synthetics seismograms for these two events. Static stress drops and seismic moments for the June and September events are Δ σ = 82.5 MPa, M = 7.65 × 1016 N m and Δ σ = 31.3 MPa and M = 1.27 × 1017 N m. These stress drops are typical of continental events rather than of events originating in spreading centers. We concluded from the focal mechanism inversions of the background seismicity and the June and September 1999 events that complex stress heterogeneity exits in the CPGF as a result of the continual thinning of the crust in the Cerro Prieto basin.

Seismic activity at the Cerro Prieto Geothermal Area (Mexico) from August 1994 to December 1995, and its relationship with tectonics and fluid exploitation

A continuous monitoring of the Cerro Prieto geothermal field was carried out from August 1994 to December 1995 to investigate the seismicity of tectonic origin and the seismicity triggered by exploitation activities. Hypocenters for 148 events were located mainly around the northern end of the Cerro Prieto fault and within the geothermal zone. The estimated focal depths range mostly from 1 to 6 km, for earthquakes with magnitudes from 0.5 to 4.6. It was difficult to distinguish between natural and triggered seismicity. We observed a single case of an earthquake for which a correlation in time and space seems plausible with a sharp increase of fluid injection in a well located at less than 1 km from the epicenter. Fault plane solutions of 26 events with magnitudes between 2.8 and 4.6 indicate right‐lateral strike‐slip motion for earthquakes that occur near the Cerro Prieto and Imperial faults. Inside the geothermal zone, events with normal components of motion were recorded, as expected from the extensional stress regime of the pull‐apart basin laying between both faults.

Global Positioning System constraints on fault slip rates in southern California and northern Baja, Mexico

We use Global Positioning System (GPS) estimates of horizontal site velocity to constrain slip rates on faults comprising the Pacific‐North America plate boundary in southern California and northern Mexico. We enlist a simple elastic block model to parameterize the distribution and sum of deformation within and across the plate boundary. We estimate a Pacific‐North America relative plate motion rate of 49 ± 3 mm/yr (one standard deviation), consistent with NUVEL‐1A estimates. We are able to resolve robust slip rate estimates for the southernmost San Andreas, San Jacinto, and Elsinore faults (26 ± 2, 9 ± 2, and 6 ± 2 mm/yr, respectively) and for the Imperial and Cerro Prieto faults (35 ± 2 and 42 ± 1 mm/yr, respectively), accounting for about 86% of the total plate motion. The remaining 14% appears to be accommodated to the west of these fault systems, probably via slip along the San Clemente fault and/or the San Miguel, Vallecitos, Rose Canyon, and Newport‐Inglewood fault systems. These results are highly consistent with paleoseismic estimates for slip rates implying that off‐fault strain accumulation within the deforming zone of the plate boundary is largely elastic. We estimate that the seismically quiescent, southernmost San Andreas fault has incurred about 8.2 m of slip deficit over the last few hundred years, presumably to be recovered during a future large earthquake.

Stress orientation and anisotropy based on shear-wave splitting observations in the Cerro Prieto fault area, Baja California, Mexico

Digital seismograms continuously recorded from 1988 to 1992 by two stations of the RESNOM seismic network in northern Baja California, Mexico, were used to search for probable shear-wave anisotropic characteristics in the region of the Cerro Prieto fault. Shear-wave splitting was identified in many of the three-component records analyzed. We measured the polarization direction of the leadingS wave inside theS-wave window as well as the delay times between fast and slow phases on those records displaying shear-wave splitting. For station CPX, which is nearest the Imperial Valley region to the north, the preferred polarization direction found in this study (azimuth 180°±10°) coincides with the direction of the regional maximum compressive stress determined for the region. This polarization direction can be interpreted in terms of the “Extensive Dilatancy Anisotropy” model as the effect of vertical parallel aligned cracks. The preferred polarization direction measured at LMX, however, gives an azimuth of 45°±5°. Thus, it appears that faults and fractures aligned oblique to the main tectonic trend have a greater influence on the anisotropic characteristics of the crust south of Cerro Prieto volcano than that of the regional stress field. Time delays between slow and fastS waves observed at CPX appear constant from 1988 to 1992 while delays measured at LMX for the same interval indicate a small increase with time which cannot be attributed to azimuthal variations of paths.

Earthquake source mechanisms and transform fault tectonics in the Gulf of California

We have determined the source parameters of 19 earthquakes in the Gulf of California from an inversion of long‐period P and SH waveforms. Fifteen of the earthquakes, on transforms between the Cerro Prieto fault and the Tamayo Fracture Zone, are characterized by right‐lateral strike‐slip faulting, and all but one of these likely reflect the relative motion between the North American and Pacific plates. Slip vectors from these events constitute an improved data set for determination of North American‐Pacific motion. Most centroid depths are poorly resolved because of trade‐offs between depth and source time function. Both the ranges in acceptable centroid depths and the average fault width estimated from cumulative moment release during this century are compatible with the hypothesis that seismic slip does not extend below about 10‐km depth, or the approximate depth of the nominal 800°C isotherm, along the principal oceanic transforms of the gulf. Two normal‐faulting earthquakes during a swarm in the northern gulf have centroid depths of 6 ± 3 and 3 ± 2 km, consistent with earthquake centroid depths in other settings (Arctic, northern Red Sea) where plate separation has carried continental rifting nearly to completion. A thrust‐faulting earthquake near the Tres Marias escarpment has a centroid depth of 11 ± 2 km and probably reflects relative motion between the Rivera and North American plates. The transition from oceanic transform faulting to continental transform faulting in the northern gulf is marked by a broadening of the zone of deformation and a 10° change in the strike of the principal fault accommodating plate motion. Fault kinematics in the transition zone may be understood in terms of an unstable FFF triple junction, with the Agua Bianca and San Miguel faults acting to transfer a portion of the plate motion to faults west of the San Andreas system and with the Delfin and Wagner basins forming as a direct result of slip on the three fault systems meeting in the junction region.