Deformation In Southern California Research Articles

Large-Scale Land Deformation Monitoring over Southern California with Multi-Path SAR Data

Southern California, USA, has been suffering severe surface deformation due to its active crustal movement under the north–south compression of the Pacific Plate and North American Plate. Meanwhile, affected by groundwater extraction and recharge, oil exploitation, surface subsidence, uplift, and seasonal deformation occur commonly in this region. In this paper, multi-path SAR datasets were collected to investigate and monitor surface deformation in Southern California. The unified simultaneous least squares (USLS) approach is applied to remove the deformation discontinuity between adjacent SAR image paths. Multiple deformation patterns of structural faults, groundwater withdrawal, and oil exploitation are observed with the interferometric synthetic aperture radar (InSAR) technique. The InSAR-derived results were validated with GPS monitoring data. The correlations between land deformation and groundwater withdrawal, faults, and precipitation were intensively analyzed, finding out and mastering the magnitude and characteristics of ground deformation in Southern California.

Crustal Deformation in Southern California Constrained by Radial Anisotropy From Ambient Noise Adjoint Tomography

AbstractWe build a new radially anisotropic shear wave velocity model of Southern California based on ambient noise adjoint tomography to investigate crustal deformation associated with Cenozoic evolution of the Pacific‐North American plate boundary. Pervasive positive radial anisotropy (4%) is observed in the crust east of the San Andreas Fault (SAF), attributed to subhorizontal alignment of mica/amphibole foliation planes resulting from significant crustal extension. Substantial negative anisotropy (6%) is revealed in the middle/lower crust west of the SAF, where high shear wave speeds are also observed. The negative anisotropy could result from steeply dipping amphibole schists in a shear zone developed during Laramide flat slab subduction. Alternatively, it could be caused by the crystal preferred orientation (CPO) of plagioclase, whose fast axis aligns orthogonally to a presumed subhorizontal foliation. The latter new mechanism highlights potentially complex CPO patterns resulting from different lithospheric mineralogy, as suggested by laboratory experiments on xenoliths from the region.

Joint Correction of Ionosphere Noise and Orbital Error in L-Band SAR Interferometry of Interseismic Deformation in Southern California

The accuracy of L-band synthetic aperture radar (SAR) differential interferometry (InSAR) on crustal deformation studies is largely compromised by ionosphere path delays on the radar signals. The ionosphere effects cause severe ionospheric distortion such as azimuth streaking and long wavelength phase distortion similar to orbital ramp error. Effective detection and correction of ionospheric phase distortion from L-band InSAR images are necessary to measure and accurately interpret surface displacement. In this paper, we investigate the performance improvement of L-band InSAR interseismic deformation measurements in southern California through the joint correction of both ionosphere noise and orbital error. Our results show that this method can effectively remove orbit and ionosphere phase distortions. In comparison with in situ GPS measurements, the achieved InSAR measurement accuracy is improved from ~ 30 mm to ~ 10 mm by the proposed joint correction method. We show that, after the joint correction, the remaining atmosphere noise can be further mitigated through stacking, leading to an RMS error of ~ 4.7 mm/year in resultant line-of-sight velocity, as compared with ~ 11.3 mm/year before the correction. Our results demonstrate that the proposed joint correction technique provides a promising way to jointly correct orbital and ionospheric artifacts in L-band InSAR studies of crustal deformation.

Slip rates and off‐fault deformation in Southern California inferred from GPS data and models

In Southern California, fault slip rate estimates along the San Andreas fault (SAF) and Garlock fault from geodetically constrained kinematic models are systematically at the low end or lower than geologic slip rate estimates. The sum of geodetic model slip rates across the eastern California shear zone is higher than the geologic sum. However, the ranges of reported model and geologic slip rate estimates in the literature are sufficiently large that it remains unclear whether these apparent discrepancies are real or attributable to epistemic uncertainties in the two types of estimates. We further examine uncertainties in geodetically derived slip rate estimates on major faults in Southern California by conducting a suite of inversions with four kinematic models. Long‐term rigid elastic block models constrained by the geologic slip rates cannot fit the present‐day GPS‐derived velocity field. Deforming (permanent off‐fault strain) elastic block models and viscoelastic earthquake cycle block models constrained by geologic slip rates can fit the present‐day GPS‐derived velocity field with 28–33% of the total geodetic moment rate occurring as distributed deformation off of the major faults. Models incorporating viscoelastic mantle flow predict systematically higher slip rates than purely elastic models on many of the major Southern California faults with ranges of (elastic/viscoelastic) 29–34/30–37 mm/yr for the Carrizo SAF segment, 20–24/20–32 mm/yr for the Mojave SAF segment, 14–17/18–22 mm/yr for the Coachella SAF segment, 13–19/14–22 mm/yr for the San Jacinto fault, and 5–11/5–11 mm/yr for the western Garlock fault.

Investigating Ductile Lithosphere Deformation in Southern California

A workshop to investigate properties and processes of ductile lithosphere deformation was held in Menlo Park, Calif. This workshop provided overviews of the newest results from lab, field, and geodetic modeling disciplines relevant to better understanding earthquake processes in Southern California. A principal goal of the workshop was to bring together niche experts in tectonic geodesy, laboratory rock mechanics, and field observations of ductile rocks to gain better interdisciplinary understanding of the stress transfer process between brittle‐elastic upper crust and ductile lithosphere in Southern California.

Small‐scale upper mantle convection and crustal dynamics in southern California

We present numerical modeling of the forces acting on the base of the crust caused by small‐scale convection of the upper mantle in southern California. Three‐dimensional upper mantle shear wave velocity structure is mapped to three‐dimensional density structure that is used to load a finite element model of instantaneous upper mantle flow with respect to a rigid crust, providing an estimate of the tractions acting on the base of the crust. Upwelling beneath the southern Walker Lane Belt and Salton Trough region and downwelling beneath the southern Great Valley and eastern and western Transverse Ranges dominate the upper mantle flow and resulting crustal tractions. Divergent horizontal and upward directed vertical tractions create a tensional to transtensional crustal stress state in the Walker Lane Belt and Salton Trough, consistent with transtensional tectonics in these areas. Convergent horizontal and downward directed vertical tractions in the Transverse Ranges cause approximately N–S crustal compression, consistent with active shortening and transpressional deformation near the “Big Bend” of the San Andreas fault. Model predictions of crustal dilatation and the forces acting on the Mojave block compare favorably with observations suggesting that small‐scale upper mantle convection provides an important contribution to the sum of forces driving transpressional crustal deformation in southern California. Accordingly, the obliquity of the San Andreas fault with respect to plate motions may be considered a consequence, rather than a cause, of contractional deformation in the Transverse Ranges, itself driven by downwelling in the upper mantle superimposed on shear deformation caused by relative Pacific–North American plate motion.

Southern California Earthquake Center Geologic Vertical Motion Database

The Southern California Earthquake Center Geologic Vertical Motion Database (VMDB) integrates disparate sources of geologic uplift and subsidence data at 104‐ to 106‐year time scales into a single resource for investigations of crustal deformation in southern California. Over 1800 vertical deformation rate data points in southern California and northern Baja California populate the database. Four mature data sets are now represented: marine terraces, incised river terraces, thermochronologic ages, and stratigraphic surfaces. An innovative architecture and interface of the VMDB exposes distinct data sets and reference frames, permitting user exploration of this complex data set and allowing user control over the assumptions applied to convert geologic and geochronologic information into absolute uplift rates. Online exploration and download tools are available through all common web browsers, allowing the distribution of vertical motion results as HTML tables, tab‐delimited GIS‐compatible text files, or via a map interface through the Google Maps™ web service. The VMDB represents a mature product for research of fault activity and elastic deformation of southern California.

Power‐law distribution of fault slip‐rates in southern California

The spatial partitioning of deformation in the continental crust and, in particular, at plate boundary zones is determined by the distribution of fault slip‐rates. Analytic and numerical models of strain accumulation in the elastic upper crust have been divided into those that parameterize faulting as localized on a finite length fault system comprised of relatively few fast slip‐rate faults, or as distributed throughout a continuum of relatively slow slip‐rate faults. We demonstrate that in the southern California fault system, between the Pacific and North American plates, both geologically and geodetically constrained fault slip‐rate catalogs obey a power‐law frequency distribution. Using this empirically constrained scaling relationship we derive an analytic expression for the partitioning of potency accumulation rate, which determines the distribution and magnitude of slip localization. This model describes the kinematics of both micro‐plate and continuum deformation models, and predicts that ∼97% of the deformation in southern California is accommodated on faults slipping at >1 mm/yr which is consistent with models of continental deformation which explicitly represent a large though finite number of deforming structures.

Characterization of the time‐dependent strain field at seismogenic depths using first‐motion focal mechanisms: Observations of large‐scale decadal variations in stress along the San Andreas Fault system

We present a method for summing moment tensors derived from first‐motion focal mechanisms to study temporal dependence in features of the subsurface regional strain field. Time‐dependent processes are inferred by comparing mechanisms summed over differing time periods. We apply this methodology to seismogenic zones in central and southern California using focal mechanisms produced by the Northern and Southern California Seismograph Networks for events during 1980–1999. We find a consistent pattern in both the style of deformation (strike‐slip versus compressional) and seismicity rate across the entire region. If these temporal variations are causally related, it suggests a temporal change in the regional‐scale stress field. One change consistent with the observations is a rotation in the regional maximum horizontal compressive stress direction, followed by a reversal to the original direction. Depending upon the dominant style of deformation locally, this change in orientation of the regional stress will tend to either enhance or hinder deformation. The mode of enhanced deformation can range from increased microseismicity and creep to major earthquakes. We hypothesize that these temporal changes in the regional stress field are the result of subtle changes in apparent relative plate motion between the Pacific and North American plates, perhaps due to long‐range postseismic stress diffusion. Others have hypothesized that small changes in plate motion over thousands of years, and/or over decades, are responsible for changes in the style of deformation in southern California. We propose that such changes, over the course of just a few years, also affect the style of deformation.

Recent crustal deformation in southern California deduced from the restoration of folded and faulted strata

A rigid element method of restoration (UNFOLD) is used to restore competent folded and faulted layers of the Ventura and Los Angeles basins to their initial horizontal state. Comparison of initial (undeformed) state with present (deformed state) allows one to estimate both the finite crustal deformation and its associated horizontal displacement field (relative to an arbitrary fixed line). Including data from the Santa Barbara Channel basin, the total finite displacement field for the western Transverse Ranges and vicinity (within the Pacific plate) is inferred from this map restoration and is modeled as a double fan closure. This model implies a 10° clockwise rotation of the northern boundary of the western Transverse Ranges and a 5° counterclockwise rotation of its northeast boundary. Lateral variation of the deformation reveals the heterogeneity of the subsurface deformation. Most of the major thrusts appear to initiate as en echelon structures along the left‐lateral northern margin and the right‐lateral northeastern margin of the studied area. The resulting deformation and displacement values closely match those derived by other geological methods (section balancing techniques or fault slip measurements) and by geophysical methods (geodetic, paleomagnetic, and focal mechanism data). Map restoration thus is a method that can independently quantify both local and regional deformation including folds and faults. This method also reveals the zones where problems of compatibility appear with the available geological and geophysical data and thus where the next studies might be focused.

Principal component analysis of interseismic deformation in southern California

Eight trilateration networks along the San Andreas fault in southern California have been surveyed repeatedly (8 to 19 times) within an approximately 15‐year interval between 1971 and 1992. The data for each network were analyzed by principal component analysis to represent the observed changes in line length as a superposition of individual modes: Lij ‐ Li = ∑k Aik Ck (tj), where Lij is the length of the ith line measured at the time tj, Li is the average length of the ith line, and AikCk(tj) is the kth mode. Each mode is described by a common time function Ck(tj) that is evaluated only at the times tj of the surveys, and the participation of each line in that mode is described by a constant amplitude Aik for that line. For all networks the deformation is largely described by the first mode, and the first‐mode time function is essentially linear in time. That is, the deformation in each of the networks appears to be steady. Two anomalies in the accumulation of deformation were observed: In the network near Palmdale a transient strain event occurred in the second mode in early 1982, and in the network within Cajon Pass, fluctuations in the accumulation of deformation somewhat greater than would be attributed to measurement error were found. Although both anomalies are formally significant given the estimated measurement error, I am not confident that either anomaly is real. The principal conclusion is that the accumulation of interseismic deformation in the southern California trilateration networks is steady within the precision of measurement.

Present‐day crustal deformation in southern California

The effects of laterally homogeneous mantle electrical conductivity have been included in steady. Using an extensive set of precise geodetic measurements, we have developed a detailed picture of present‐day deformation rates in southern California. This large set of measurements, amounting to nearly 2000 repeated distance measurements over the period 1973 to 1991, comes from the U.S. Geological Survey's Geodolite trilateration program, involving their combined Anza, Joshua Tree, and Salton networks. Building on previous results from these data, we are able to present the deformation field as estimates of the rate of horizontal strain accumulation in small four‐station subnetworks of the overall 89‐station network. Using this technique, the spatial details of the 18‐year average strain rate field can be determined. By correlating these spatial details with the tectonics of the region we are able to understand better how deformation is partitioned across this highly complex margin between the Pacific and North American tectonic plates. Some of the more interesting findings of this study are that (1) the vast majority of strain rate estimates show a pattern of nearly pure shear as would be expected in a transcurrent environment, (2) the fastest accumulation of surface strain in southern California is along the San Jacinto Fault west of the Salton Sea, not along the San Andreas Fault, (3) strain accumulation rate along the length of the San Jacinto Fault increases toward the southeast as the fault enters the Imperial Valley, (4) a large area near the southern end of the Salton Sea, where the San Andreas Fault meets the Brawley Seismic Zone, is undergoing areal dilatation, which is in part consistent with the formation of crust at a spreading center, and (5) deformation at the transition zone between the San Andreas Fault and the Eastern California Shear Zone also appears to be the result of crustal spreading.

GPS measurements of regional deformation in Southern California: Some constraints on performance

Geodetic measurements are an important tool for understanding plate boundary zone deformation. They provide data on fault motion rates and indirectly provide constraints on crustal rheology, earthquake processes, and evolution of geologic structures. In southern California the pattern of strain resulting from interaction of the Pacific and North American plates is very complex, and a robust geodetic experiment requires dense spatial sampling. In addition, the signals of interest may have strain rates smaller than 10−7/yr. Taken together, these conditions imply the need for geodetic techniques that are both economical, enabling large numbers of measurements, and highly accurate. Conventional terrestrial surveying techniques can satisfy the requirements for sampling density and accuracy for some applications, but are limited to line‐of‐sight, or distances less than about 50 km [e.g., Snay et al, 1983; Savage et al., 1986]. The precision a of these techniques may be described by σ = (a2 + b2L2)frac12; (1) where L is baseline length (station separation) and a and b are constants representing, respectively, length‐independent and length‐dependent sources of error [Savage and Prescott, 1973]. For high‐precision Geodolite surveys, a = 3 mm, and b = 2 × 10−7 [Savage et al., 1987].

Review of magnetic and electric field effects near active faults and volcanoes in the U.S.A.

Abstract Synchronized measurements of geomagnetic field have been recorded along 800 km of the San Andreas fault and in the Long Valley caldera since 1974, and during eruptions on Mount St. Helens since 1980. For shorter periods of time, continuous measurements of geoelectric field measurements have been made on Mount St. Helens and near the San Andreas fault where moderate seismicity and fault slip frequently occurs. Significant tectonic and volcanic events for which nearby magnetic and electric field data have been obtained include: (1) two moderate earthquakes ( M L > 5.8) for which magnetometers were close enough to expect observable signals (about three source lengths), (2) one moderate earthquake ( M S 7.3) for which magnetometers were installed as massive fluid outflow occurred during the post-seismic phase, (3) numerous fault creep events and moderate seismicity, (4) a major explosive volcanic eruption and numerous minor extrusive eruptions, and (5) an episode of aseismic uplift. For one of the two earthquakes with M L > 5.8, seismomagnetic effects of −1.3 and −0.3 nT were observed. For this event, magnetometers were optimally located near the epicenter and the observations obtained are consistent with simple seismomagnetic models of the event. Similar models for the other event indicate that the expected seismomagnetic effects are below the signal resolution of the nearest magnetometer. Precursive tectonomagnetic effects were recorded on two independent instruments at distances of 30 and 50 km from a M L 5.2 earthquake. Longer-term changes were recorded in one region in southern California where a moderate M L 5.9 earthquake has since occurred. Surface observations of fault creep events have no associated magnetic or electrical signature above the present measurement precision (0.25 nT and 0.01%, respectively) and are consistent with near-surface fault failure models of these events. Longer-term creep is sometimes associated with corresponding longer-term magnetic field perturbations. Correlated changes in gravity, magnetic field, areal strain, and uplift occurred during episodes of aseismic deformation in southern California primarily between 1979 and 1983. Because the relationships between these parameters agrees with those calculated from simple deformation and tectonomagnetic models, the preferred explanation appeals to short-term strain episodes independently detected in each data set. An unknown source of meteorologically generated noise in the strain, gravity, and uplift data and an unknown, but correlated, disturbance in the absolute magnetic data might also explain the data. No clear observations of seismoelectric or tectonoelectric effects have yet been reported. The eruption of Mount St. Helens generated large oscillatory fields and 9 ± 2 nT offset on the only surviving magnetometer. A large-scale traveling magnetic disturbance passed through the San Andreas array from 1 to 2 h after the eruption. Subsequent extrusive eruptions generated small precursory magnetic changes in some cases. These data are consistent with a simple volcanomagnetic model, magneto-gas dynamic effects, and a blast excited traveling ionospheric disturbance. Traveling ionospheric disturbances (TIDs), also generated by earthquake-related atmospheric pressure waves, may explain many electromagnetic disturbances apparently associated with earthquakes. Local near-fault magnetic field transients rarely exceed a few nT at periods of a few minutes and longer.

Measurements of coseismic deformation in southern California: 1972–1982

Since 1972, a number of static strain changes have been measured at Piñon Flat Observatory, southern California, as a result of regional faulting. As recorded by a collection of long‐baselength strainmeters and tiltmeters, these signals are in good quantitative agreement with the deformation calculated for a dislocation in an elastic half‐space. In contrast, colocated short‐baselength sensors are found to produce spurious results whose size is much larger than expected theoretically and well correlated with peak site acceleration. Calculations show that the root‐mean‐square (rms) of the coseismic signals from several different instruments at one site, or from several sites at known hypocentral distances, provide a good estimate of the seismic moment, a result confirmed by the observations. Static moment estimates for the four largest events in this period are Imperial Valley 1979, M0 = 9.0 × 1018Nm; Mexicali Valley 1980, M0 = 6.1 × 1018Nm; Westmorland 1981, M0 = 5.3 × 1017Nm; and Brawley 1979, M0 = 2.3 × 1017 Nm. These estimated moments generally exceed those determined seismically by about 25%, which is as expected since they include some postseismic deformation. Results from the Mexicali Valley earthquake are particularly interesting, indicating an average fault strike approximately 9° different from the historic fault trace. Several small to moderate earthquakes (ML 3.7–5.2) were also well enough recorded to estimate source parameters. For the only other detectable events in the period 1972–1982 (a total of three others for which the strain change Δεrms was greater than 0.4 nε), high accelerations caused most of the instruments at the site to malfunction. No precursory signals were evident for any of the earthquakes recorded at this site.

Contemporary plate motion and crustal deformation

The measurement of relative plate velocities during the past few years is a signal accomplishment in earth science, leading to refinement of the precepts of steady motion of plate interiors and cyclic deformation along plate margins. Regrettably, deformation premonitory to an earthquake has yet to be detected with confidence. Delineation of the spatial and temporal buildup of strain between earthquakes, however, has put limits on models of the earthquake cycle. A diverse set of fault structures and crustal rheologic conditions can explain the pattern of surface strain accumulation and release along strike‐slip faults. In contrast, the geometry of thrusts and normal faults, revealed by earthquake deformation, has been found to differ markedly from expectations. Geodetic observations have proved vital to monitor the ascent of magma through the Earth's crust and to predict volcanic eruptions at the Earth's surface. Episodic vertical and horizontal deformation in southern California remains a subject of dispute; if anything, it is less certain than it once seemed.

Correction to “An approach to modeling present‐day deformation in southern California” by J. B. Rundle

Correction to “An approach to modeling present‐day deformation in southern California” by J. B. Rundle

An approach to modeling present‐day deformation in southern California

This paper presents an approach to modeling the complete, time‐dependent deformation in southern California. We make use of a variety of techniques to include the far‐field plate motion, stress relaxation in the asthenosphere, aseismic fault slip within the elastic lithosphere, and the complex, three‐dimensional nature of the faults within the southern California region. Among the conclusions of the work here are the following: (1) the data exhibit a marginal preference for a relatively thin lithosphere in southern California, (2) the most sensitive determinant for lithospheric thickness and viscosity of the asthenosphere is the rotation rate of the motion vectors toward or away from major faults, (3) the major faults in southern California are slipping at depth, at or near their average Holocene rates, (4) a shallow, active decollement beneath the Transverse Ranges is apparently needed to realistically model the strain changes observed there, and (5) this modeling technique offers the best approach for understanding the complex deformation field in southern California.

A Kinematic model of southern California

We propose a kinematic model for southern California based on late Quaternary slip rates and orientations of major faults in the region. Internally consistent motions are determined assuming that these faults bound rigid blocks. Relative to North America, most of California west of the San Andreas fault is moving parallel to the San Andreas fault through the Transverse Ranges and not parallel to the motion of the Pacific plate. This is accomplished by counterclockwise rotation of California south of the San Andreas fault and by the westward movement of central California north of the Gar lock fault. The velocities of the blocks are calculated along several paths in southern California that begin in the Mojave Desert and end off the California coast. A path that crosses the western Transverse Ranges accumulates the accepted relative North America‐Pacific plate velocity, whereas paths to the north and south result in a significant missing component of motion. This implies the existence of a zone of active deformation in southern California that is interpreted to include the western Transverse Ranges and northwest trending, predominately strike‐slip faults close to the coast both north and south of the Transverse Ranges. Strain on this system accounts for about a third of the total North America‐Pacific plate motion.

Geophysical interpretation of satellite laser ranging measurements of crustal movement in California

As determined by satellite laser ranging the rate of contraction of a 900 km baseline between sites located near Quincy in northern California and San Diego in southern California is about 61–65 mm/yr with a formal uncertainty of about 10 mm/yr (Christodoulidis et al., 1985). The measured changes in baseline length are a manifestation of the relative motion between the North America and Pacific tectonic plates. This long baseline result is compared to measurements made by more conventional means on shorter baselines. Additional information based on seismiscity, geology, and theoretical modelling is also analyzed. Deformation lying within a few tens of kilometers about the major faults in southern California accounts for most, but not all, of the observed motion. Further motion is attributable to a broader-scale deformation in southern California. Data suggesting crustal movements north of the Garlock fault, in and near the southern Sierra Nevada and local motion at an observatory are also critically reviewed. The best estimates of overall motion indicated by ground observations lie between 40 and 60 mm/yr. This lies within one or two standard deviations of that deduced from satellite ranging but the possibility of some unresolved deficit cannot be entirely dismissed. The long time scale RM2 plate tectonic model of Minster and Jordan (1978) predicts a contraction between 47 and 53 mm/yr depending on the extension rate of the Basin and Range. Thus the ground based observations, SLR results, and RM2 rates differ at about the 10 mm/yr level but are not inconsistent with one another within the data and model uncertainties.