Ventura Basin Research Articles

Abstract: Map Restoration of a Quaternary Horizon across the Oak Ridge Fault, Offshore and Onshore Ventura Basin, California

Abstract: Map Restoration of a Quaternary Horizon across the Oak Ridge Fault, Offshore and Onshore Ventura Basin, California

Abstract: Comparison of the Seismicity and Associated Stress Systems to the GPS Strain Rates for the Ventura Basin and Northridge Regions

Abstract: Comparison of the Seismicity and Associated Stress Systems to the GPS Strain Rates for the Ventura Basin and Northridge Regions

Provenance of the upper Miocene Modelo Formation and subsidence analysis of the Los Angeles basin, southern California: Implications for paleotectonic and paleogeographic reconstructions

The Los Angeles basin has undergone three stages of development related to complex plate interactions within the evolving San Andreas transform system: transrotation (16–12 Ma), transtension (12–6 Ma), and transpression (6–0 Ma). Timing of these stages correlates with microplate-capture events along the continental margin, and is expressed in changes in subsidence rates and provenance within the Los Angeles basin. The Modelo Formation and related units were deposited in the northern part of the Los Angeles basin at bathyal depths during late Miocene time. The northern Los Angeles basin was segmented into three subbasins, in each of which coarse sediment was deposited as submarine fans (Puente, Tarzana, and Simi). A fourth fan system (Piru) formed in the Ventura basin, just north of the Los Angeles basin. The Puente, Tarzana, and Piru fans were derived from the San Gabriel block, which consists primarily of crystalline basement and lesser volcanic and sedimentary components. Sandstone within the Puente fan reflects unroofing of the central and eastern San Gabriel block. The Tarzana fan was derived primarily from the central San Gabriel block, and the Piru fan was derived primarily from the western San Gabriel block, which is distinctly characterized by Ca-rich plagioclase derived from Proterozoic anorthosite and related bodies. The lack of Ca-rich plagioclase in the other fans eliminates the western San Gabriel block as a possible source area, and confirms differentiation of the Ventura basin from the Los Angeles basin by late Miocene time. The Simi fan was derived from locally uplifted Cretaceous and Paleogene strata; sandstone composition reflects the recycling of these sediments. Subsidence and provenance analyses are consistent with the following paleogeographic and paleotectonic reconstruction. Beginning at approximately 16 Ma, transrotation of the Western Transverse Ranges induced extension and thermal subsidence of the Los Angeles basin area. A second pulse of extension and thermal subsidence occurred when motion began along the San Gabriel fault at 12 Ma. Right slip of 60–70 km occurred along the San Gabriel fault, which produced transtension in the Los Angeles basin area and deposition of the Puente, Tarzana, Simi, and Piru fan systems. At 6 Ma, transform motion was transferred to the San Andreas fault; transpression has dominated the Los Angeles basin since 6 Ma, including rapid uplift, flexural subsidence due to tectonic loading, and rapid sedimentary filling. The rapid subsidence and filling and the sudden switch between transtension and transpression in the Los Angeles basin are typical of strike-slip basins in general. However, initiation of the Los Angeles basin by transrotation reflects the uncommon process of microplate capture along the rapidly evolving California margin.

Compression directions in southern California (from Santa Barbara to Los Angeles Basin) obtained from borehole breakouts

Borehole elongation in 71 drill holes was used to infer breakout orientation and directions of maximum horizontal principal stress SH for six areas west of the San Andreas fault in southern California: Santa Barbara, Ojai, Central Ventura Basin, East Ventura Basin, West Los Angeles Basin, and East Los Angeles Basin. Breakouts were determined from analysis of oriented four‐arm caliper data. The breakouts form at the position of the maximum compressive stress on the borehole wall; if the borehole is vertical and parallel to one of the principal stress directions, the breakouts will form parallel to the minimum horizontal principal stress Sh orthogonal to the maximum horizontal principal stress SH. Observations from deviated boreholes permit some constraints on the relative magnitudes of the principal stresses. In most cases the data permit either a thrust faulting (Sv<Sh<SH) or strike‐slip faulting (Sh<Sv<SH) stress regime with NE to NW directions of SH. These results are broadly consistent with results from focal mechanism studies [Hauksson, 1990; Li, 1996] and with breakout and focal mechanism data present in the world stress map database [Zoback, 1992]. However, we find systematic variations in SH directions suggestive of strong heterogeneity in the stress field at shallow depths, similar to that present in the Cajon Pass borehole [Shamir and Zoback, 1992]. Anomalous NW directions of SH in the San Fernando Valley region and near the Whittier fault may be related to structural complexities and/or lateral ramps in nearby fault systems.

Three‐dimensional VP and VP/VS Velocity Models of the Los Angeles basin and central Transverse Ranges, California

We use P and S arrival times from 5225 earthquakes and 53 explosions, recorded by the Southern California Seismographic Network, to invert for the three‐dimensional P‐ velocity (VP) and the P and S‐velocity ratio (VP/ VS) in the central Transverse Ranges and the Los Angeles basin. To model long‐wavelength features of the velocity structure, we invert for the models by interpolating models determined successively from a sparse, medium, to a dense grid, with 40, 20, and 10 km spacing of horizontal grid nodes. Layers of grid nodes are placed at depths of 1, 4, 6, 8, 12, 16, and 20 km. The data variance decreased about 80% in the gradational inversion. Ample data from the 1994 Northridge and other earthquake sequences, the rich background seismicity, and the dense station distribution along with controlled sources made the model well resolved, except along the edges, to the southwest in the offshore region, and at depths greater than 20 km. The VP model images the shape of the Los Angeles and east Ventura basins down to depths of 8 and 12 km. Three low‐velocity areas at 1 km depth in the Los Angeles basin that coincide with recent sediment depositional areas are also imaged. The north edge of the Peninsular Ranges, the Santa Monica, and the San Gabriel Mountains, form discontinuous high‐velocity ridges extending to depths of 20 km. The high VP/VS ratios in the near surface are consistent with high pore fluid pressures in the basin sediments. At depth beneath the east Ventura basin the high VP/VS and high VP suggest the presence of ophiolitic assemblages or mid‐Miocene volcanics. In contrast, a body with normal VP and low VP/VS that is identified in the upper crust beneath Santa Monica Bay may be a fragment of quartz‐rich continental crust. The VP model of both the Ventura and the Los Angeles basins shows features that suggest deformation of the hanging wall or basin closure. At seismogenic depths of 16 km the hypocenters of moderate‐sized and large earthquakes are located within or adjacent to high‐VP‐velocity bodies. In most cases these high‐velocity bodies form the upper block, consistent with shortening of the lower crust as described in thick‐skinned tectonic interpretations.

Deformation rates across the Placerita (Northridge Mw = 6.7 aftershock zone) and Hopper Canyon segments of the western transverse ranges deformation belt

Abstract Aftershocks of the 1994 Northridge (Mw = 6.7) earthquake provide insights into the geometry of the seismic source faults in the San Fernando Valley and the east Ventura Basin and allow the calculation of deformation rates for the region. The Northridge thrust and Santa Susana faults dip in opposite directions, with the Northridge thrust entirely beneath the Santa Susana fault. These opposing reverse faults interact, resulting in a folded active Santa Susana fault and uplift in the footwall block of that reverse fault. Two balanced cross sections suggest thick-skinned deformation of the western Transverse Ranges. The western section, across the Modelo lobe segment of the north-dipping San Cayetano fault and the easternmost surface trace of the south-dipping Oak Ridge fault, is west of any aftershocks of the Northridge earthquake and has been termed the Hopper Canyon segment of the deformation belt. Structural modeling predicts a dip of 46° S on the Oak Ridge fault at seismogenic depths. Horizontal shortening rates are calculated by adding the products of the dip-slip displacements and the cosines of the dips of both faults. The eastern cross section shows the Northridge mainshock, with a 45° south-dipping nodal plane at a depth of 18 km. Aftershocks reach a depth of 20 km. In a thin-skinned paradigm, a hinge should occur at the surface near the Santa Monica Mountains due to rocks moving from a decollement at the brittle-plastic transition and changing dip as they move up the ramp. No hinge of that magnitude occurs there. Calculation of horizontal short-ening rates across this part of the western Transverse Ranges must take into account the displacement on both the Northridge thrust (eastern extension of the Oak Ridge fault) and the Santa Susana fault (Placerita segment). Horizontal shortening rates are 8.2 ± 2.4 mm/yr across the Modelo lobe segment of the San Cayetano fault and the Oak Ridge fault and 5.7 ± 2.5 mm/yr across the Northridge thrust and the Santa Susana fault. These rates are consistent with those based on tectonic geodesy using GPS. Dip-slip displacement rates on the faults are 1.7 mm/yr for the Northridge thrust since 2.3 Ma, 4.1 ± 0.4 mm/yr for the Oak Ridge fault since 500 ka, 5.9+3.9/−3.8 mm/yr for the Santa Susana fault since 600 to 2300 ka, and 7.4 ± 3.0 mm/yr for the Modelo lobe segment of the San Cayetano fault since 500 ka. This indicates that the slip rates on the north-dipping, the eastern San Cayetano, and the Santa Susana faults are comparable, but the slip rate on the south-dipping faults decreases eastward; the slip rate on the Oak Ridge fault in the Ventura basin is more than double that of the Northridge thrust.

Simultaneous inversion of local and teleseismic data for the crust and mantle structure of southern California

We determined a detailed three-dimensional seismic velocity structure of the crust and mantle to a depth of 800 km beneath southern California using local and teleseismic data simultaneously. We used 131 372 P-wave arrival times from 6437 local earthquakes and 12 134 travel time residuals from 193 teleseismic events recorded by 251 seismic stations of the Caltech-US Geological Survey Southern California Seismic Network. Compared with previous local and teleseismic studies, the station coverage is considerably improved, and the number of data used is greatly increased in the present study. This, together with the local and teleseismic joint inversion approach, results in a unified three-dimensional (3-D) P-wave velocity model of the crust and mantle in southern California with a higher resolution than the previous models. The result obtained shows very heterogeneous structures in the crust and upper mantle in southern California. Shallow crustal structures correlate well with surface geological features. Sedimentary basins such as the Los Angeles Basin, Ventura Basin and Santa Maria Basin are imaged well as low velocities, and batholiths such as the Peninsular Ranges and San Gabriel Mountains are imaged as high velocities. The velocity in the crust is generally low in the Mojave Desert. The crustal velocities change abruptly across major faults such as the San Andreas, San Jacinto, Elsinore and Garlock faults. In the upper mantle, three major anomalies are found, beneath the Transverse Ranges, Salton Trough and Southern Sierra Nevada. The Transverse Ranges feature appears as a curtain-like, east-trending and north-dipping high-velocity anomaly with a thickness of approximately 50 km, and extends most deeply at its eastern end to about 250 km. The Salton Trough feature is composed of low velocities which trend southeast and extend to approximately 200 km depth. The Southern Sierra Nevada feature is a slab-like high-velocity anomaly which extends to about 250 km. The mantle velocity is generally low in the upper 100–200 km beneath the volcanic areas in southern California. The seismic velocity anomalies in the mantle beneath southern California are considered to result from small-scale mantle convection.

An integrated 3D velocity inversion—joint hypocentral determination relocation analysis of events in the Northridge area

Abstract A subset of 3371 events recorded in the Northridge area by the Southern California Seismic Network during January to April 1994 was relocated with the joint hypocentral determination (JHD) technique. This analysis showed two unexpected results: (a) the JHD locations are shifted about 3.9 km on average in a northwest direction with respect to the locations determined using a single-event location (SEL) program, and (b) the station corrections vary between −0.55 and 1.26 sec, a rather large range. In addition, the JHD locations are less scattered than the SEL locations. For each station, the weighted average of the arrival time residuals obtained when the events are located with the SEL program (which does not apply distance or error weighting) are generally smaller than the corresponding JHD corrections. The locations determined with SEL and using the weighted average residuals as station corrections do not differ much from the SEL locations, but on average the RMS residuals become as small as those corresponding to the JHD locations. As the magnitude of the station corrections indicates the presence of large lateral velocity variations, a 3D velocity model for the area was determined using the arrival times of 1012 events recorded by at least 17 stations. The initial velocity model was that used routinely by the Southern California Earthquake Center. The first two layers (5.5- and 10.5-km thick) were subdivided into 100 blocks each (12 × 12 km). These layers show a pronounced low-velocity anomaly (24% and 16%, respectively) immediately to the northwest of the epicentral area. This low-velocity zone coincides with the west Ventura Basin. Another pronounced low-velocity zone to the southeast of the epicentral area reflects the presence of the Los Angeles Basin. The locations obtained with the 3D velocity model are consistently to the southeast of the JHD locations, 2.4 km on average. To establish the effect of these pronounced lateral velocity variations on the SEL and JHD locations, synthetic travel times were analyzed. The synthetic times were generated for event locations determined by JHD (shifted by various amounts) and the 3D velocity model and were subsequently treated as the actual data. The most important result of this analysis is that the JHD locations are affected by a quasi-systematic shift in a northwest direction (up to about 2.7 km on average, depending on the initial shift) but that the relative locations are well preserved. Therefore, both the velocity inversion of the actual data and the analysis of the synthetic data indicate that the JHD locations determined for the actual data are quasi-systematically mislocated. To account for this mislocation, an overall shift of 2.5 km to the southeast was applied to all the JHD locations. One of the most important implications of the shifted locations is the possibility that the northeasterly dipping Santa Susana fault, to the northwest of the epicentral area, was seismically active during the aftershock sequence. This feature is more diffuse in other published locations.

Paleocene to Holocene paleodeltas of ancestral Colorado River offset by the San Andreas fault system, southern California

Research Article| September 01, 1996 Paleocene to Holocene paleodeltas of ancestral Colorado River offset by the San Andreas fault system, southern California Jeffrey L. Howard Jeffrey L. Howard 1Department of Geology, Wayne State University, Detroit, Michigan 48202 Search for other works by this author on: GSW Google Scholar Author and Article Information Jeffrey L. Howard 1Department of Geology, Wayne State University, Detroit, Michigan 48202 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1996) 24 (9): 783–786. https://doi.org/10.1130/0091-7613(1996)024<0783:PTHPOA>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jeffrey L. Howard; Paleocene to Holocene paleodeltas of ancestral Colorado River offset by the San Andreas fault system, southern California. Geology 1996;; 24 (9): 783–786. doi: https://doi.org/10.1130/0091-7613(1996)024<0783:PTHPOA>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Upper Paleocene to middle Miocene fluvial-deltaic rocks in the Los Angeles and Ventura basins were deposited by a Colorado paleoriver prior to 300 km of dextral displacement on the San Andreas fault. During the late Miocene, movement on the fault and associated rifting in the Salton trough rerouted the paleoriver into the proto–Gulf of California. Since the Pliocene, translation of a moving Colorado deltaic depocenter along the fault has filled the Salton trough progressively southeastward. The postulated Colorado paleoriver extended north approximately along the lower course of the present river where it joined with Gila and Amargosa paleotributaries. Late Cenozoic tectonism disrupted the Amargosa paleoriver, and the modern course of the Colorado River through the lower Grand Canyon was developed. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Convergence rates across a displacement transfer zone in the western Transverse Ranges, Ventura basin, California

Three cross sections are balanced and retrodeformed to the top of the Pleistocene Saugus Formation and to a horizon close to the base of the Jaramillo subchron to yield crustal shortening and shortening rates for the western Transverse Ranges of California. The cross sections compare the shortening that occurs along a transfer zone in which displacement is transferred eastward from a surface reverse fault (Red Mountain fault) to a blind thrust and from that to a combination of both a surface reverse fault (San Cayetano fault) and the blind thrust. Using an age of 250±50 ka for the top of the Saugus Formation based on amino‐acid racemization of fossil mollusks, crustal shortening rates appear to have abruptly accelerated through time from 3+4/−3 to 5±4 mm/yr between 250 and 975 ka to 20±6 to 28±1 mm/yr since 250±50 ka. Using an age of 500 ka for the top of the Saugus Formation based on paleomagnetic stratigraphy, there is little increase in deformation rates through time, ranging from 5±5 to 8±6 mm/yr between 975–500 ka and from 10±3 to 14±1 mm/yr since 500 ka. Crustal convergence rates determined by Global Positioning System surveys, taken over 4.6 years, indicate a shortening rate of 7–10 mm/yr across the basin. This is consistent with the slower rates of deformation calculated using an age of 500 ka for the top of the Saugus Formation.

The Oak Ridge fault system and the 1994 Northridge earthquake

THE 17 January 1994 Northridge earthquake (moment magnitude M w = 6.7) in the San Fernando Valley in southern California illuminated a hitherto unrecognized blind reverse fault with a moderate southward dip 1. This fault lies beneath the active north-dipping Santa Susana fault system and uplifted both the footwall and the hanging wall of the Santa Susana fault during the earthquake1. Here we argue that footwall uplift on the Santa Susana fault before the earthquake could have been used to identify the Northridge blind thrust as an active fault. Moreover, we propose that the Northridge earthquake occurred on a continuation of the Oak Ridge fault system, which reaches the surface in the Ventura basin to the west. Slip rates on the western part of this fault system 2, 3 are nearly three times larger than on the Northridge blind thrust 4, increasing the probability of a N or t bridge-sized earthquake in the heavily populated Ventura basin.

Fault Seal Mechanisms in Contractional and Transpressional Settings: Examples From the Ventura Basin, Southern California: ABSTRACT

Fault Seal Mechanisms in Contractional and Transpressional Settings: Examples From the Ventura Basin, Southern California: ABSTRACT

A Comparison of Undiscovered Oil and Gas Resource Estimates, Los Padres National Forest in the Ventura Basin Province, California: ABSTRACT

Two recent assessments of the undiscovered oil and gas resources of Los Padres National Forest lands in the Ventura Basin Province using different methodologies and personnel show remarkable coincidence of estimated resources. The 1989 U.S. Geological Survey assessment was part of a National appraisal. In the Ventura Basin Province, two separate plays were assessed and a percentage of resources from these plays was allocated to Federal lands. By this allocation, the undiscovered oil and gas resources of this part of the Los Padres National Forest are estimated to range from <10-140 MMBO (means probability 60 MMBO, million barrels of oil) and 10-250 BCFG (mean probability 110 BCFG, billion cubic feet of gas). In 1993, the U.S. Forest Service completed an oil and gas assessment of the entire 1.8 million-acre Los Padres National Forest as part of a Reasonably Foreseeable Oil and Gas Development Scenario. In those areas of the forest considered to have high potential for the occurrence of oil and gas deposits, a deposit simulation model was used. This method is based on a fundamental reservoir engineering formula in the USGS computer program, FASPU (Fast Appraisal System for Petroleum-Universal). By this method, the undiscovered oil and gas resource ofmore » this part of the Los Padres National Forest are estimated to range from 0-182 MMBO (mean probability 56 MMBO) and 9-233 BCFG (mean probability 103 BCFG). An additional 6 MMBO (mean probability) is allocated to forest lands with medium potential within this province but not to any specific prospects. The remarkable coincidence of estimate resources resulting from such different assessment methods and personnel is noteworthy and appears to provide an increased measure of confidence in the estimates.« less

Sedimentation Response to Earthquake-Related Events, Middle Eocene Ventura Basin, California: ABSTRACT

Middle Eocene strata in the Topatopa Mountains, northeastern Ventura Basin, California, were deposited as five unconformity-bounded depositional sequences in a seismically-active basin characterized by rapid, episodic subsidence. Analysis of stratigraphic geometries, detailed facies analysis, and backstripping-derived subsidence rates indicate that tectonically-induced differential subsidence caused basin-wide migrations of the Topatopa depocenter at least every 2 m.y. Unusually abundant convolute laminations and a slumped interval may record the influence of earthquakes. The convulate laminations do not grade downward into ripple laminations and are not associated with dewatering dikes and pipes as would be expected if the laminations were formed by shear from an overlying current or by dewatering during rapid burial. Thus, formation during shock-induced liquefaction is more likely. Also a 20-m thick slumped interval in the uppermost Cozy Dell Formation underlies a sequence boundary interpreted as a surface across which depocenter migration took place. Association of this slumped interval with a tectonically-formed surface is consistent with deposition in a seismically-active environment. Rapid, differential subsidence in the Topatopa depocenter was probably episodic and associated with seismic events. Rapid, episodic subsidence with attending earthquakes is recorded in Holocene strata of the Humboldt basin, an analog to the Topatopa depocenter. Also, A Middle Tertiarymore » slumped interval in the southernmost San Joaquin basin is attributed to a seismic event. Similar earthquake-related events are recorded by sedimentation patterns in the Middle Eocene Ventura Basin and may be evident in the strata of other active-margin basins as well.« less

Late Cenozoic Tectonics of the East Ventura Basin, Transverse Ranges, California

The east Ventura basin originated in the middle Miocene as a rift system bounded on one side by the Oak Ridge-Simi Hills structural shelf and on the other side by a granitic ridge parallel to the San Gabriel fault. This fault began accumulating right slip 10-12 m.y. ago at a rate of 4.5-9 mm/yr (depending on whether total slip is 45 or 60 km), slowing to about 1 mm/yr in the Quaternary. North of the Santa Clara River, rifting ended prior to deposition of the uppermost Miocene-lower Pliocene Towsley Formation. South of the Santa Clara River, the rift axis shifted southwest toward the Oak Ridge-Simi Hills shelf as the Towsley Formation accumulated against a normal-fault ancestor of the Santa Susana fault. A change to contractile tectonics occurred in the Pliocene with depos tion of the Fernando Formation, when the Newhall-Potrero anticline developed as a monocline above a blind reverse fault; the Pico anticline to the southeast and the Temescal and Hopper Ranch-Modelo anticlines to the northwest may have a similar origin. Tectonic inversion and displacement on the southwest-verging Santa Susana fault began about 0.5 Ma based on appearance of locally-derived clasts in the upper Saugus Formation and its equivalents, and continues today, along with the southwest-verging San Cayetano fault farther west. Also active are northeast-verging backthrusts occurring in the east Ventura basin fold belt, and a segment of the San Gabriel fault which now acts as a northeast-dipping oblique-slip reverse fault. Northeast-trending discontinuities and structures divide the present deformation zone into four segments. In the Hopper Canyon segment at the west end of the area in the west Ventura basin, the San Cayetano fault places Miocene Modelo Formation over Pliocene-Pleistocene strata more than 5 km thick, largely at maximum burial. To the southeast, the Newhall-Potrero segment is characterized by north-vergent backthrusts within the east Ventura fold belt and by southward thrusting of the basin sequence (tectonic inversion) over the structural shelf on the Santa Susana fault. Farther southeast, the Placerita segment is marked by reverse faulting on both the Santa Susana fault and the San Gabriel fault. Southeast of the basin in the San Fernando Valley, the Sylmar segment contains a thick Pli cene-Pleistocene sequence overridden by basement rocks of the San Gabriel Mountains as well as the south-verging Mission Hills-Granada Hills and Northridge Hills fault zones.

Late Quaternary Structure and Convergence Rates in the Coastal Ventura Basin: ABSTRACT

Late Quaternary Structure and Convergence Rates in the Coastal Ventura Basin: ABSTRACT

Late Cenozoic Tectonics of the East Ventura Basin, Transverse Ranges, California: ABSTRACT

Late Cenozoic Tectonics of the East Ventura Basin, Transverse Ranges, California: ABSTRACT

Seismicity and Tectonics of the Western Transverse Ranges: Ventura Basin Region to the San Fernando Valley: ABSTRACT

Seismicity and Tectonics of the Western Transverse Ranges: Ventura Basin Region to the San Fernando Valley: ABSTRACT

Space geodetic measurement of crustal deformation in central and southern California, 1984–1992

We estimate the velocity field in central and southern California using Global Positioning System (GPS) observations from 1986 to 1992 and very long baseline interferometry (VLBI) observations from 1984 to 1991. Our core network includes 12 GPS sites spaced approximately 50 km apart, mostly in the western Transverse Ranges and the coastal Borderlands. The precision and accuracy of the relative horizontal velocities estimated for these core stations are adequately described by a 95% confidence ellipse with a semiminor axis of approximately 2 mm/yr oriented roughly north‐south, and a semimajor axis of approximately 3 mm/yr oriented east‐west. For other stations, occupied fewer than 5 times, or occupied during experiments with poor tracking geometries, the uncertainty is larger. These uncertainties are calibrated by analyzing the scatter in three types of comparisons: (1) multiple measurements of relative position (“repeatability”), (2) independent velocity estimates from separate analyses of the GPS and VLBI data, and (3) rates of change in baseline length estimated from the joint GPS+VLBI solution and from a comparison of GPS with trilateration. The dominant tectonic signature in the velocity field is shear deformation associated with the San Andreas and Garlock faults, which we model as resulting from slip below a given locking depth. Removing the effects of this simple model from the observed velocity field reveals residual deformation that is not attributable to the San Andreas fault. Baselines spanning the eastern Santa Barbara Channel, the Ventura basin, the Los Angeles basin, and the Santa Maria Fold and Thrust Belt are shortening at rates of up to 5 ± 1, 5 ± 1, 5 ± 1, and 2 ± 1 mm/yr, respectively. North of the Big Bend, some compression normal to the trace of the San Andreas fault can be resolved on both sides of the fault. The rates of rotation about vertical axes in the residual geodetic velocity field differ by up to a factor of 2 from those inferred from paleomagnetic declinations. Our estimates indicate that the “San Andreas discrepancy” can be resolved to within the 3 mm/yr uncertainties by accounting for deformation in California between Vandenberg (near Point Conception) and the westernmost Basin and Range. Strain accumulation of 1–2 mm/yr on structures offshore of Vandenberg is also allowed by the uncertainties. South of the Transverse Ranges, the deformation budget must include 5 mm/yr between the offshore islands and the mainland.

Geodetic measurement of deformation in the Ventura Basin Region, southern California

We have measured the deformation in the Ventura basin region, southern California, with Global Positioning System (GPS) measurements carried out over 4.6 years between 1987 and 1992. The deformation within our network is spatially variable on scales of tens of kilometers, with strain rates reaching 0.6 ± 1 μrad/yr in the east‐central basin. Blocklike rotations are observed south and northwest of the basin where the maximum shear strain rates are an order of magnitude lower (0.06 ± 1 μrad/yr to the south). We also observed clockwise rotations of 1°–7°/m.y. Shear strain rates determined by comparing angle changes from historical triangulation spanning several decades and GPS measurements give consistent, though less precise, results. The geodetic rates of shortening across the basin and Western Transverse Ranges are lower than those estimated from geological observations, but the patterns of deformation from the two methods agree qualitatively.