Agua Blanca Fault Research Articles

Behavior of Amagmatic Orogenic Geothermal Systems: Insights From the Agua Blanca Fault, Baja California, Mexico

AbstractAmagmatic geothermal systems within regional‐scale orogenic faults are promising renewable resources for heat and possibly electricity production. However, their behavior needs to be better understood to improve their exploration and assessment of energy potential. To provide more insight, we report geochemical, geological, and geophysical studies from seven hot spring sites strung along a 90 km segment of the Agua Blanca Fault, which traverses a mountainous region of northern Baja California, Mexico. Our results show that topographic heads drive infiltration of meteoric water deep into basement rocks, where it is heated according to the local geothermal gradients. Long paths lead to long water residence times and high 3He/Hetotal fractions. The hot water ascends along preferentially permeable zones within the ABF, discharging at temperatures from 37°C in inland springs to 102°C on the Pacific coast. Higher discharge temperatures correlate positively with the degree of extensional fault displacement (a proxy for fault permeability). Correlations between hydraulic head gradients, residence times, and 3He/Hetotal of the thermal waters show that the hydraulic head gradient controls the length and depth of the flow paths, whereas the magnitudes and locations of the discharge sites are controlled by fault permeability. Optimal conditions at the coast allow the 120°C temperature threshold for electricity production to be reached at relatively shallow depths (<2 km), demonstrating the potential of orogenic geothermal systems not only for exploitation of hot discharging water but also for EGS exploitation of the hot rocks that surround the water upflow zones.

Understanding hydrothermal behavior of the Maneadero geothermal system, Ensenada, Baja California, Mexico

A hydrogeological conceptual model has been developed that describes the geothermal system located at the Maneadero region in Ensenada, Baja California, Mexico. In addition, a two-dimensional steady state simulation of water flow and heat transfer of a model profile using COMSOL Multiphysics software is also presented. The proposed hydrogeological conceptual model considers that the old local anoxic subsurface seawater feeds the system, and a remnant magmatic intrusion in the surface from past volcanism at 2 km of depth works as heat source. The intersection between the south Agua Blanca Fault and the L3 alignment promote the fluids ascent to the submarine hydrothermal vent. The results of the simulation show a reservoir temperature of 164 °C and an affluent temperature of 105 °C, varying between 1 °C and 3 °C, respectively, with previous studies. Moreover, a geothermal potential of 0.48 MW was estimated for the simulated profile. The current policy of exploitation of geothermal resources in Mexico makes the development of these models particularly relevant, serving as a tool for their evaluation and management.

Time‐Invariant Late Quaternary Slip Rates Along the Agua Blanca Fault, Northern Baja California, Mexico

AbstractFault slip rates inform models of strain accumulation and release, which over geologic time may vary or remain constant depending on factors like structural complexity, fault strength, deformation rates, and proximity to other faults. In this study, we present a Late Pleistocene–Holocene slip history based on four new geologic slip rates for the Agua Blanca Fault (ABF), which transfers Pacific‐North American dextral plate boundary motion across the Peninsular Ranges of northern Baja California. Time‐averaged slip rates from three sites are 2.8 + 0.8/−0.6 mm/a since ~65.1 ka, 3.0 + 1.4/−0.8 mm/a since ~21.8 ka, 3.2 + 1.0/−0.6 mm/a since ~12.5 ka, and 3.5 + 5.1/−2.0 mm/a since ~1.4 ka; however, the actual slip rate may be closer to 4 mm/a when off‐fault slip and age interpretation uncertainties are considered. Significantly, although the ABF has more in common in terms of length, net offset, and slip rate with known variable slip rate faults, the most straightforward age and offset interpretations for the ABF suggest constant slip rates over ~10 kyr time scales. As with other constant slip rate faults, comparable neighboring faults that might modulate the ABF slip rate are absent, suggesting that fault interaction, or lack thereof, may be a more significant factor controlling fault behavior on this and potentially other faults. The new rates indicate that the ABF accommodates at least half of total slip across the Peninsular Ranges, clarifying strain partitioning for seismic forecasting models that previously lacked modern geologic slip rate constraints for this domain of the plate boundary.

Slip history and the role of the Agua Blanca fault in the tectonics of the North American–Pacific plate boundary of southern California, USA and Baja California, Mexico

Slip history and the role of the Agua Blanca fault in the tectonics of the North American–Pacific plate boundary of southern California, USA and Baja California, Mexico

A multidisciplinary approach to estimate slip rate-magnitude and recurrence time for a segment of a major active fault. Case study: The Agua Blanca Fault, Valle de Agua Blanca, B.C., México

The segmented major Agua Blanca Fault is one of the largest geologically active faults in northern Baja California, México. It extends 130 km inland, from the Pacific Ocean to the San Matías Pass (near the Gulf of California). Geologic evidence shows that the Agua Blanca Fault has been active during most of the Quaternary. To estimate the seismic hazard that the Agua Blanca Fault represents for the region, it is necessary to recognize its seismic activity. However, seismic data and historical registries of major seismic events associated to this fault are limited. In addition, structural evidence required to calculate the slip rates is scarce. Thus, in order to determine fault movements and hence its seismic risk, it was necessary to use other geological techniques. For that reason, a different multidisciplinary approach to estimate slip rates, magnitude and recurrence times for a geologically active major fault was tested in a selected segment of the Major Agua Blanca Fault (study area: Valle de Agua Blanca, locality: Rancho La Cocina-Rancho Agua Blanca). In this case, correlation of the major alluvial fans displaced by the fault by the use of the clast counting technique and other geological data was used in order to estimate slip rates, magnitude and recurrence times for the Valle de Agua Blanca segment of the fault. By using this method, two major distinctive ancient fans and four small recent fans dissected and offset by the fault within the Valle de Agua Blanca were identified. Fault displacement was established, and the slip rate was calculated to be ∼6 mm/yr consistent with the amount obtained in other studies within the area by the use of other techniques. The average length of each segment of the Agua Blanca Fault is between 20 and 30 km, a surface rupture of 20–30 km accompanies earthquakes of Ms 6.7–7.0. This is the case in the Valle Agua Blanca, which is the boundary between two major eastern fault segments 20–35 km long. These lengths, and the slip rate obtained, and using the Bonilla et al., (1984) regression relationship developed for worldwide strike-slip faults, we can estimate the recurrence interval for earthquakes with magnitudes between 6 and 7. The recurrence interval for earthquakes with Ms = 6 to 7 generated by the Agua Blanca Fault in the Valle Agua Blanca is between 75 and 200 yr, with average ground displacements of 0.5–1.25 m.

Geothermal prospects in the Baja California Peninsula

Geothermal resource evaluation was accomplished for the Baja California Peninsula hydrothermal systems. Numerous geothermal areas were identified along the eastern coast of the Peninsula, which are related with the Gulf of California opening process. The western coast presents hydrothermal activity in the northern part related with regional faults like the Agua Blanca Fault nearby Ensenada. The southern part of the Peninsula also has abundant geothermal resources related with local grabens. The heat in place evaluation was accomplished for five geothermal areas where geophysical data were available to estimate the reservoir volume. The minimum calculated potential is more than 400MWe. There are still more than six geothermal prospects with calculated temperatures above 200°C that lack reliable information on the reservoir volume and would increase the geothermal potential of the Peninsula. Surface heat loss from the submarine systems reaches more than 6000MWt.

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) …

View of an intact oceanic arc, from surficial to mesozonal levels: Cretaceous Alisitos arc, Baja California

The Alisitos arc is an approximately 300 × 30 km oceanic arc terrane that lies in the western wall of the Peninsular Ranges batholith south of the modern Agua Blanca fault zone in Baja California. We have completed detailed mapping and dating of a 50 × 30 km segment of this terrane in the El Rosario to Mission San Fernando areas, as well as reconnaissance mapping and dating in the next 50 × 30 km segment to the north, in the San Quintin area. We recognize two evolutionary phases in this part of the arc terrane: (I) extensional oceanic arc, characterized by intermediate to silicic explosive and effusive volcanism, culminating in caldera-forming silicic ignimbrite eruptions at the onset of arc rifting, and (II) rifted oceanic arc, characterized by mafic effusive and hydroclastic rocks and abundant dike swarms. Two types of units are widespread enough to permit tentative stratigraphic correlation across much of this 100-km-long segment of the arc: a welded dacite ignimbrite (tuff of Aguajito), and a deepwater debris-avalanche deposit. New U–Pb zircon data from the volcanic and plutonic rocks of both phases indicate that the entire 4000-m-thick section accumulated in about 1.5 MY, at 111–110 MY. Southwestern North American sources for two zircon grains with Proterozoic 206Pb / 207Pb ages support the interpretation that the oceanic arc fringed North America rather than representing an exotic terrane. The excellent preservation and exposure of the Alistos arc terrane makes it ideal for three-dimensional study of the structural, stratigraphic and intrusive history of an oceanic arc terrane. The segment mapped and dated in detail has a central major subaerial edifice, flanked by a down-faulted deepwater marine basin to the north, and a volcano-bounded shallow-water marine basin to the south. The rugged down-faulted flank of the edifice produced mass wasting, plumbed large-volume eruptions to the surface, and caused pyroclastic flows to disintegrate into turbulent suspensions that mixed completely with water. In contrast, gentler slopes on the opposite flank allowed pyroclastic flows to enter the sea with integrity, and supported extensive buildups of bioherms. Caldera collapse on the major subaerial edifice ponded the tuff of Aguajito to a thickness of at least 3 km. The outflow ignimbrite forms a marker in nonmarine to shallow marine sections, and in deepwater sections it occurs as blocks up to 150 m long in a debris-avalanche deposit. These welded ignimbrite blocks were deposited hot enough to deform plastically and form peperite with the debris-avalanche matrix. The debris avalanche was likely triggered by injection of feeder dikes along the basin-bounding fault zone during the caldera-forming eruption. Intra-arc extension controlled very high subsidence rates, followed shortly thereafter by accretion through back-arc basin closure by 105 Ma. Accretion of the oceanic arc may have been accomplished by detachment of the upper crust along a still hot, thick middle crustal tonalitic layer, during subduction of mafic–ultramafic substrate.

Seismic cycle and rheological effects on estimation of present‐day slip rates for the Agua Blanca and San Miguel‐Vallecitos faults, northern Baja California, Mexico

Geodesy can be used to infer long‐term fault slip rates, assuming a model for crust and upper mantle rheology. We examine the sensitivity of fault slip rate estimates to assumed rheology for the Agua Blanca and San Miguel‐Vallecitos faults in northern Baja California, Mexico, part of the Pacific–North America plate boundary zone. The Agua Blanca fault is seismically quiet, but offset alluvial fans indicate young activity. Current seismicity is confined to the nearby San Miguel‐Vallecitos fault, a small offset fault better aligned with plate motion. GPS measurements between 1993 and 1998 suggest that both faults are active, with a combined slip rate of 4–8 mm yr−1 regardless of rheological model. However, slip rate estimates for the individual faults are sensitive to assumed rheology. Elastic half‐space models yield 2–3 mm yr−1 for the Agua Blanca fault, and somewhat faster rates for the San Miguel‐Vallecitos fault, 2–4 mm yr−1, with uncertainties of about 1 mm yr−1. Models incorporating viscoelastic rheology and seismic cycle effects suggest a faster slip rate for the Agua Blanca fault, 6 ± 1 mm yr−1, and a slower rate for the San Miguel‐Vallecitos fault, 1 ± 1 mm yr−1, in better agreement with geological data, but these rates are sensitive to assumed rheology. Numerical simulations with a finite element model suggest that for similar rheological and friction conditions, slip on the San Miguel‐Vallecitos fault should be favored due to better alignment with plate motion. Long‐term faulting processes in the larger offset Agua Blanca fault may have lowered slip resistance, allowing accommodation of motion despite misalignment with plate motion.

A Northward-propagating Earthquake Sequence in Coastal Southern California?

The devastating 1999 M 7.4 Izmit, Turkey earthquake was “no surprise” (Reilinger et al. , 2000) because it occurred as part of a sequence of earthquakes that have ruptured nearly the entire length of the strike-slip North Anatolian Fault zone (> 1,000 km) since 1912. The 1999 rupture occurred on a segment previously identified as a seismic gap (Toksoz et al. , 1979) and recently studied to understand mechanisms of stress transfer and earthquake triggering along strike-slip faults (Stein et al. , 1997). We suggest that an analogous rupture sequence spanning the last few centuries may be in its later stages along southern California coastal faults. As we will discuss, recently published fault investigations in the northern Baja California peninsula (Mexico) and coastal southern California (USA) reveal evidence for geologically contemporaneous or sequential earthquakes along a > 300-km-length, predominantly strike-slip seismic zone. This coastal fault zone includes structures previously mapped as the Agua Blanca, Rose Canyon, San Joaquin Hills, and southern Newport-Inglewood Fault zones (Figures 1 and 2). Radiocarbon dating and the historic record indicate that moderate to large earthquakes occurred after A.D. 1640 ± 160, 1523 to 1769, and 1635 to 1855 on the Agua Blanca Fault (Rockwell et al. , 1993), on the Rose Canyon Fault (Rockwell and Murbach, 1999), and in the San Joaquin Hills (Grant et al. , 2002), respectively. Additionally, a moderate to large ( M ≥ 6.5) earthquake is interpreted for the near offshore region in A.D. 1800 based on historical accounts (Toppozada et al. , 1981). Finally, a Mw 6.4 earthquake on the southern Newport-Inglewood Fault zone (NIFZ) followed in 1933 (Barrows, 1974; Hauksson and Gross, 1991) and increased the Coulomb stress on the northern NIFZ in Los Angeles (Stein et al. , 1994). The date of last surface rupture of the …

Tectonic implications for the along-strike variation of the Peninsular Ranges batholith, southern and Baja California

The Jurassic-Cretaceous Peninsular Ranges batholith of southern and Baja California exhibits several along-strike variations that are most pronounced across the Agua Blanca fault. These variations include differences in the age of magmatism, degree of inheritance in zircons, depositional environment of volcanic strata, extra-arc basin geography, lateral extent of continentally derived flysch strata, and the structures attending juxtaposition of the two segments with the North American continental margin and with each other. We propose that these variations, when taken together, imply that the two segments of the western zone currently juxtaposed by the active Agua Blanca fault were accreted to North America diachronously and did not share a common history prior to the late Early Cretaceous. The Santiago Peak arc segment north of the fault developed on oceanic lithosphere juxtaposed with the continental margin prior to and during arc magmatism. Conversely, the Alisitos arc segment developed on oceanic lithosphere exotic to North America prior to accretion at 115‐108 Ma. If this model is correct, it implies that the Agua Blanca fault initiated as a transpressional continuation of the suture joining the latter arc segment to the continent and that a large portion of the forearc that existed between them was subducted.

New kinematic models for Pacific‐North America Motion from 3 Ma to Present, II: Evidence for a “Baja California Shear Zone”

We use new models for present‐day Pacific‐North America motion to evaluate the tectonics of offshore regions west of the Californias. Vandenburg in coastal Alta California moves at the Pacific plate velocity within uncertainties (∼1 mm/yr) after correcting for strain accumulation on the San Andreas and San Gregorio‐Hosgri faults with a model that includes a viscoelastic lower crust. Modeled and measured velocities at coastal sites in Baja California south of the Agua Blanca fault, a region that most previous models consider Pacific plate, differ by 3–8 mm/yr, with coastal sites moving slower that the Pacific plate. We interpret these discrepancies in terms of strain accumulation on known on‐shore faults, combined with right lateral slip at a rate of 3–4 mm/yr on additional faults offshore peninsular Baja California in the Pacific. Offshore seismicity, offset Quaternary features along the west coast of Baja California, and a discrepancy between the magnetically determined spreading rate in the Gulf Rise and the total plate rate from a geological model provide independent evidence for a “Baja California shear zone.”

A neotectonic study of the San Miguel-Vallecitos fault, Baja California, Mexico

Abstract The San Miguel strand of the San Miguel-Vallecitos fault zone displaces an alluvial ridge 23+5/−10 m in the vicinity of Dolores Mountain. The same section of fault produced surface rupture during the M6.8 earthquake of 1956. We estimate a minimum age of the ridge at between 110+150/−60 ka by comparing soil development on the offset ridge to two soil chronosequences in southern California. Dividing the measured offset by the age of the ridge places a maximum range on the fault slip rate of 0.2+0.35/−0.15 mm/yr. Excavations along and across the fault at the same site revealed at least 80 cm and probably 115 cm of right-lateral offset of a now-buried stream channel deposit. We attribute the 115-cm offset to the 1956 M6.8 earthquake, a displacement that is slightly larger than previously reported for this earthquake. Dividing the 80 to 115 cm of displacement by the calculated fault slip rate yields an estimated return time for similar-sized events of about 5.8+17/−4.3 ka. Similarly, dividing the slip expected for rupture of the entire 160-km length of the San Miguel-Vallecitos fault zone by the fault slip rate yields an estimate of return time of Mw7.8 earthquakes of about 80 ka. The slip rate determined from this study is at least an order of magnitude less than that contributed by the Agua Blanca fault and indicates that the San Miguel fault zone transfers less than about 1% of the plate motions. The San Miguel fault shows a complex fault trace and registers a small value of cumulative geologic offset (maximum of 0.6 km). When combining the geological estimates of magnitude and return time of the largest earthquakes with magnitude-frequency data recorded along the San Miguel fault by the RESNOR seismic network during the period 1976 to 1991, we observe that the shape of the magnitude-frequency distribution along the fault may be described by the Gutenberg-Richter relationship Log n = a − bM. In contrast, along-strike-slip faults in southern California that are characterized by orders of magnitude more cumulative offset and less complex fault traces, we observe that extrapolation of earthquake frequency statistics from the instrumental record of seismicity severely underestimates the rate of occurrence of maximum expected events. We speculate that the relatively high productivity of small earthquakes along the San Miguel fault reflects a more heterogeneous stress field associated with the incipient and complex nature of the fault trace.

The July 1986 Oceanside (ML = 5.3) earthquake sequence in the Continental Borderland, Southern California

An earthquake of M_L = 5.3 occurred at 32°58.7′N, 117°51.5′W southwest of Oceanside in San Diego County at 13:47 13 July 1986 (UT). This main shock was followed by an extensive aftershock sequence, with 55 events of M_L ≧ 3.0 during July 1986. The epicenters of the main shock and aftershocks are located at the northern end of the San Diego Trough-Bahia Soledad fault zone (SDT-BS) where it changes strike from northwest to a more westerly direction through a left offset or a bend in the fault. The northwest-striking SDT-BS is one of three strike-slip fault systems that constitute the offshore Agua Blanca fault system. The spatial distribution of the aftershocks indicates a unilateral 7- to 9-km long rupture to the east-southeast away from the epicenter of the main shock. The focal mechanism of the main shock also has an east-southeast striking and south-dipping plane with mostly reverse movement on it. Focal mechanisms of the M_L ≧ 3.0 aftershocks show both reverse and strike-slip movement. The reverse focal mechanisms indicate that this sequence may have occurred on a thrust fault that provides for a left stepping offset or a bend in the San Diego Trough fault as movement is transferred to the west along the Santa Cruz-Catalina Island escarpment. Some of the aftershocks that are located to the southeast of the main shock and have strike-slip focal mechanisms suggest activation of the northwest-trending San Diego Trough fault. A stress inversion of the focal mechanism data shows that the maximum principal stress determined from the focal mechanisms of the main shock and 22 aftershocks that occurred within 36 hours of the main shock has an azimuth of S30°W plunging 18°. The maximum principal stress determined from 30 aftershocks that occurred from 15 July to 2 October 1986 has an azimuth of S20°W, plunging 18°. The φ-values (the measure of the relative sizes of the principal stresses) are approximately 0.07 and 0.1, respectively, indicating that the intermediate and minimum principal stress are of similar magnitude. The results of the stress inversion, and the focal mechanisms that showed reverse faulting, suggest that the Inner Continental Borderland offshore from Oceanside is not currently a pure-strike-slip tectonic regime but rather a strike-slip mixed with reverse faulting regime. When the 52 aftershock focal mechanisms are divided into four groups and the stress inversion is repeated, the change in stress can be described as a progressive counter-clockwise rotation of 14° of the orientation of the maximum principal stress to a more southerly direction, with the greatest change in stress orientation observed shortly after the main shock. The abundance of aftershocks may be related to the large temporal variation in stress orientation that, in turn, may have resulted from the small stress drop of the main shock.

Geologic Structure and Tectonics of Inner Continental Borderland of Northern Baja California: ABSTRACT

Detailed marine geophysical surveys of the inner California continental borderland west of northern Baja California show that the region is underlain by two major, northwest-trending, Quaternary, dextral wrench fault systems. The San Clemente fault system lies along the western part of the inner borderland and is delineated by the San Clemente and San Isidro fault zones. Together, these fault zones connect to form a long (300 km), narrow (5-10 km), continuous zone of faulting that is very similar to the larger San Andreas fault system onshore. The Agua Blanca fault system is a complex zone of shear delineated by three or more subparallel wrench fault zones in the eastern part of the inner borderland. The westernmost San Diego Trough-Bahia Soledad fault zone consists of relatively long (50 km), continuous, main fault traces which cut the Quaternary sediments of the nearshore basin trough. The Coronado Bank-Agua Blanca fault zone is more complicated, with numerous discontinuous, subparallel, right- and left-stepping, anastomosing fault traces which are associated with significant structural relief. A nearshore zone of faults, marked by the Newport-Inglewood-Rose Canyon fault zone in the north and the Estero-Descanso fault zone in the south, parallels the coast and defines the eastern boundary ofmore » the California continental borderland structural province. All of these eastern fault zones merge into the transpeninsular Agua Blanca fault, and their N30/sup 0/W trend differs substantially from the trend of the major peninsular ranges fault zones.« less

Seismic Expression of Late Quaternary Banda Submarine Canyon and Fan Offshore Northern Baja California: ABSTRACT

High-resolution seismic reflection profiles obtained throughout the inner California continental borderland offshore northwestern Baja California, Mexico, show the presence of numerous modern submarine canyons and associated fans. One set of these, the Banda submarine canyon/fan, is of relatively recent origin, as demonstrated by onlap of the basal fan sediments against an acoustically transparent, presumably hemipelagic deposit. Late Quaternary sedimentation rates inferred from isotopically dated piston core samples place the age of the postulated hemipelagic unit at approximately 650,000 years ago. The Banda submarine canyon heads within the Bahia Todos Santo and passes through a narrow gorge between Punta Banda and Islas Todos Santos. It is proposed that this submarine canyon and fan system formed entirely during late Quaternary time, following the breach of the Punta Banda ridge during a late Pleistocene high sea level stand. The presence of an ancient, buried channel exiting to the north out of Bahia Todos Santos probably marks the head of an earlier submarine canyon which acted as the conduit of clastic sediments from Valle Maneadero to the deep borderland basins. The now active Banda submarine canyon pirated the supply of terrigenous clastics from this older canyon. The active Agua Blanca fault zone cuts acrossmore » the head of Banda submarine canyon, suggesting that tectonic movements may have played a role in the development of the Banda submarine canyon and fan system.« less

Source parameters of the Ensenada Bay earthquake swarm, Baja California, Mexico

Seismic data collected from the Ensenada Bay earthquake swarm of late 1981 were used to calculate the spectra of ground displacement. Data from the stations of Ensenada (ENX) and Cerro Bola (CBX), at epicentral distances of 14 and 57 km, respectively, were used to evaluate source parameters. The focal depths determined for these events were less than 10 km. The focal mechanism was a strike-slip fault type, with the plane of motion striking N52°W, parallel to the Agua Blanca Fault. Seismic moments ranging from 3.44 × 1019 to 5.99 × 1020 dyn∙cm (3.44 × 1014 to 5.99 × 1015 N∙cm) were estimated for events with local magnitudes in the range 1.7–2.3. The source dimensions were found to be 186 ± 36 m and the stress drops between 3 and 66 bar (0.3 and 6.6 MPa), comparable to results obtained in previous studies of shallow events (depths <10 km). The Ensenada swarm could be attributed to a localized zone of high-strain energy at the intersection of two faults. Ratios of P to S corner frequencies were evident for only five events; they were 1.39 ± 0.38. Magnitude and seismic moment from other studies were compared with the Ensenada data in the range of magnitudes 0–3. All the data can be accommodated by log M0 = 1.5 ML + (16.9 ± 1.1). The Ensenada earthquake swarm and the Victoria earthquake swarm, which occurred in the Mexicali valley in 1978, have similar source radii and corner frequencies for the same range of seismic moments.

Geological and seismic evidence of a new branch of the Agua Blanca Fault

The Agua Blanca fault is a major, trans‐peninsular, right‐lateral fault located in northern Baja California. Its WNW‐ESE orientation is markedly different from the general trend of the San Andreas‐Gulf of California fault system. From a geological point of view, the Agua Blanca fault is considered active, but there has been little significant seismic activity directly associated with it. On October 9, 1981, the onset of an earthquake swarm was detected at the Ensenada seismic station (ENX), with a S‐P time average of 2 sec. Shortly after this, an array of up to seven portable seismic stations was installed by CICESE around Todos Santos Bay. More than 180 events were recorded during the following 10 days, after which the local seismicity decreased. A second swarm consisting of 100 microearthquakes was detected between November 28 and December 5, 1981. The epicentral locations of those events recorded at four or more local stations all lie inside Todos Santos Bay. The composite fault plane solution for both swarms indicates a right‐lateral strike‐slip vertical fault with a strike of N52°W. Geological observations allow us to conclude that the seismic activity reported here is associated with the Agua Blanca fault zone.

A study of microseismicity in Northern Baja California, Mexico

abstract Five microearthquake instruments were operated for 2 months in 1974 in a small mobile array deployed at various sites near the Agua Blanca and San Miguel faults. An 80-km-long dection of the San Miguel fault zone is presently active seismically, producing the vast majority of recorded earthquakes. Very low activity was recorded on the Agua Blanca fault. Events were also located near normal faults forming the eastern edge of the Sierra Juarez suggesting that these faults are active. Hypocenters on the San Miguel fault range in depth from 0 to 20 km although two-thirds are in the upper 10 km. A composite focal mechanism showing a mixture of right-lateral and dip slip, east side up, is similar to a solution obtained for the 1956 San Miguel earthquake which proved consistent with observed surface deformation.

Garlock Fault: An Intracontinental Transform Structure, Southern California

The northeast- to east-striking Garlock fault of southern California is a major strike-slip fault with a left-lateral displacement of at least 48 to 64 km. It is also an important physiographic boundary since it separates along its length the Tehachapi–Sierra Nevada and Basin and Range provinces of pronounced topography to the north from the Mojave Desert block of more subdued topography to the south. Previous authors have considered the 260-km-long fault to be terminated at its western and eastern ends by the northwest-striking San Andreas and Death Valley fault zones, respectively. We interpret the Garlock fault as an intracontinental transform structure which separates a northern crustal block distended by late Cenozoic basin and range faulting from a southern, Mojave block much less affected by dilational tectonics. Earlier ideas that the Garlock fault terminates eastward at the Death Valley fault zone appear to us to be in error, although right-lateral offsetting of the Garlock along that zone by about 8 km is necessary. Displacement along the Garlock fault must increase westward from its eastern terminus, a point of zero offset now buried beneath alluvial deposits in Kingston Wash to the east of the Death Valley fault zone. Much of the displacement on the Garlock fault due to east-west components of basin and range faulting appears to have been derived from block faulting in the area between Death Valley and the Nopah Range. Westward displacement of the crustal block north of the Garlock by extensional tectonics within it totals 48 to 60 km in the Spangler Hills–Slate Range area and probably continues to increase westward at least as far as the eastern frontal fault of the Sierra Nevada. Westward shifting of the northern block of the Garlock has probably contributed to the westward bending or deflection of the San Andreas fault where the two faults meet. Many earlier workers have considered that the left-lateral Garlock fault is conjugate to the right-lateral San Andreas fault in a regional strain pattern of north-south shortening and east-west extension, the latter expressed in part as an eastward displacement of the Mojave block away from the junction of the San Andreas and Garlock faults. In contrast, we regard the origin of the Garlock fault as being directly related to the extensional origin of the Basin and Range province in areas north of the Garlock. Recent models for development of that province related to intracontinental spreading east of an east-dipping subduction zone along the Cenozoic margin of western North America may best account for the differential east-west extension which has occurred in the crustal blocks to the north and south of the Garlock fault. Other possible examples of intracontinental transform faults in the southwestern Cordillera with geometries similar to that of the Garlock fault include the left-lateral Santa Cruz–Sierra Madre fault zone along the southern margin of the western Transverse Ranges, and the right-lateral Las Vegas shear zone and Agua Blanca fault of Baja California.