Banning Fault Research Articles

Insights into the geometry and evolution of the southern San Andreas fault from geophysical data, southern California

AbstractTwo new joint gravity-magnetic models in northern Coachella Valley provide additional evidence for a steep northeast dip of the Mission Creek strand of the southern San Andreas fault (southern California, USA). Gravity modeling indicates a steep northeast dip of the Banning fault in the upper 1–2 km in northern Coachella Valley. The Mission Creek strand and its continuation to the southeast (Coachella segment) coincide with the northeastern margin of a Cenozoic basin and are marked by prominent gravity and magnetic gradients that are consistent with these strands of the San Andreas fault having accommodated >160 km of right-lateral and 1–5 km of vertical displacement. These anomalies are best fit by a moderate to steep northeast dip. Such a geometry is further supported by seismicity, reflectivity, geodesy, and boundary-element modeling. We explore the possibility that these fault strands forming the margin of Coachella Valley were originally near vertical and have rotated into their present orientation by underplating of a localized high-velocity, lower-crustal prong within the Peninsular Ranges batholith. Reconstructions of San Andreas fault offset suggest that this crystalline body was translated into the San Gorgonio Pass area at the time of major fault reorganization at 1.1–1.3 Ma.

Dynamic models of earthquake rupture along branch faults of the eastern San Gorgonio Pass region in California using complex fault structure

AbstractGeologic data suggest that the Coachella Valley segment of the southern San Andreas fault (southern California, USA) is past its average recurrence time period. At its northern edge, this right-lateral fault segment branches into the Mission Creek and Banning strands of the San Andreas fault. Depending on how rupture propagates through this region, there is the possibility of a throughgoing rupture that could lead to the channeling of damaging seismic energy into the Los Angeles Basin. The fault structures and potential rupture scenarios on these two strands differ significantly, which highlights the need to determine which strand provides a more likely rupture path and the circumstances that control this rupture path. In this study, we examine the effect of different assumptions about fault geometry and initial stress pattern on the dynamic rupture process to test multiple rupture scenarios and thus investigate the most likely path(s) of a rupture that starts on the Coachella Valley segment. We consider three types of fault geometry based on the Southern California Earthquake Center Community Fault Model, and we create a three-dimensional finite-element mesh for each of them. These three meshes are then incorporated into the finite-element method code FaultMod to compute a physical model for the rupture dynamics. We use a slip-weakening friction law, and consider different assumptions of background stress, such as constant tractions and regional stress regimes with different orientations. Both the constant and regional stress distributions show that rupture from the Coachella Valley segment is more likely to branch to the Mission Creek than to the Banning fault strand. The fault connectivity at this branch system seems to have a significant impact on the likelihood of a throughgoing rupture, with potentially significant impacts for ground motion and seismic hazard both locally and in the greater Los Angeles metropolitan area.

Holocene geologic slip rate for the Banning strand of the southern San Andreas Fault, southern California

AbstractNorthwest directed slip from the southern San Andreas Fault is transferred to the Mission Creek, Banning, and Garnet Hill fault strands in the northwestern Coachella Valley. How slip is partitioned between these three faults is critical to southern California seismic hazard estimates but is poorly understood. In this paper, we report the first slip rate measured for the Banning fault strand. We constrain the depositional age of an alluvial fan offset 25 ± 5 m from its source by the Banning strand to between 5.1 ± 0.4 ka (95% confidence interval (CI)) and 6.4 + 3.7/−2.1 ka (95% CI) using U‐series dating of pedogenic carbonate clast coatings and 10Be cosmogenic nuclide exposure dating of surface clasts. We calculate a Holocene geologic slip rate for the Banning strand of 3.9 + 2.3/−1.6 mm/yr (median, 95% CI) to 4.9 + 1.0/−0.9 mm/yr (median, 95% CI). This rate represents only 25–35% of the total slip accommodated by this section of the southern San Andreas Fault, suggesting a model in which slip is less concentrated on the Banning strand than previously thought. In rejecting the possibility that the Banning strand is the dominant structure, our results highlight an even greater need for slip rate and paleoseismic measurements along faults in the northwestern Coachella Valley in order to test the validity of current earthquake hazard models. In addition, our comparison of ages measured with U‐series and 10Be exposure dating demonstrates the importance of using multiple geochronometers when estimating the depositional age of alluvial landforms.

Seismic velocity structures in the southern California plate-boundary environment from double-difference tomography

SUMMARY We present tomographic images of crustal structures in the southern California plate-boundary area, with a focus on the San Jacinto fault zone (SJFZ), based on double-difference inversions of earthquake arrival times. Absolute arrival times of 247 472 P and 105 448 S wave phase picks for 5493 earthquakes recorded at 139 stations in southern California are used. Starting with a layered 1-D model, and continuing in later iterations with various updated initial models, we invert the data for Vp and Vs in a 270 km long, 105 km wide and 35 km deep volume around the SJFZ using a spatially variable grid with higher density around the SJFZ. The examined volume stretches from Cajon Pass to the northernmost Imperial Fault Zone and includes portions of the southern San Andreas Fault (SAF), the Elsinore Fault and the Brawley Seismic Zone in the Salton Trough. Because differential traveltimes used in the double-difference inversions are most sensitive to near-source structures, we obtain high resolution around the earthquake sources. After 30 iterations we improve the average traveltime misfit by a factor of 16. Though ray coverage is limited at shallow depths, we obtain detailed images of seismic velocities from 3 to 20 km throughout much of the study area. Our final velocity results show zones of low-velocity and anomalous Vp/Vs ratios associated with various fault strands and sedimentary basins, along with clear velocity contrasts across the SJFZ and the southern SAF. The velocity reductions in fault zone regions are generally highest in geometrically complex areas (up to 30–50 per cent in the top few kilometres), are higher for Vs than for Vp, and follow a flower-type pattern with depth. In the central section of the SJFZ, from the San Jacinto valley to the trifurcation area, the northeast side of the fault has generally higher seismic velocities than the southwest block. The obtained contrasts of Vp are more persistent and higher (up to 20 per cent) than the contrasts of Vs (up to 15 per cent), although the differences may stem (at least partially) from the higher resolution of Vp images. In the SJFZ sections to the northwest and southeast, there are patches with reversed velocities contrasts especially in the shallow crust near the San Jacinto Valley and other basins. Along the Banning fault there is no clear velocity contrast. For the southern SAF, the northeast side has generally lower seismic velocities in the seismogenic zone with patches of contrast reversals in the shallow crust. In the Brawley Seismic Zone, the northeast side has somewhat lower velocities in the ∼20 km section near the southern SAF and higher velocities farther to the southwest. The imaged features have important implications for various aspects of earthquake and crustal dynamics in the region.

Geophysical evidence for wedging in the San Gorgonio Pass structural knot, southern San Andreas fault zone, southern California

Geophysical data and surface geology define intertonguing thrust wedges that form the upper crust in the San Gorgonio Pass region. This picture serves as the basis for inferring past fault movements within the San Andreas system, which are fundamental to understanding the tectonic evolution of the San Gorgonio Pass region. Interpretation of gravity data indicates that sedimentary rocks have been thrust at least 5 km in the central part of San Gorgonio Pass beneath basement rocks of the southeast San Bernardino Mountains. Subtle, long-wavelength magnetic anomalies indicate that a magnetic body extends in the subsurface north of San Gorgonio Pass and south under Peninsular Ranges basement, and has a southern edge that is roughly parallel to, but 5–6 km south of, the surface trace of the Banning fault. This deep magnetic body is composed either of upper-plate rocks of San Gabriel Mountains basement or rocks of San Bernardino Mountains basement or both. We suggest that transpression across the San Gorgonio Pass region drove a wedge of Peninsular Ranges basement and its overlying sedimentary cover northward into the San Bernardino Mountains during the Neogene, offsetting the Banning fault at shallow depth. Average rates of convergence implied by this offset are broadly consistent with estimates of convergence from other geologic and geodetic data. Seismicity suggests a deeper detachment surface beneath the deep magnetic body. This interpretation suggests that the fault mapped at the surface evolved not only in map but also in cross-sectional view. Given the multilayered nature of deformation, it is unlikely that the San Andreas fault will rupture cleanly through the complex structures in San Gorgonio Pass.

Lithospheric imaging via teleseismic scattering tomography

The coda of the teleseismic P phase consists largely of energy scattered by small inhomogeneities in the receiver-side lithosphere. Given large collections of teleseismic data from dense permanent networks, previous workers have successfully back-propagated coda energy back to scattering source points using various kinematic migration schemes, as well as by inverting using an inverse scattering/radon transform approach. Under the Born approximation, seismic scattering is a linear process; therefore it is possible to approach coda scattering as a linear waveform inversion problem, mathematically similar to transmission-based tomography. Assuming ray-theoretical propagation and Rayleigh scattering, we pose the inverse scattering problem in tomographic form, and recover perturbations in density and P and S velocities from Pp and Ps scattered data. The method is applied to data from the Southern California Seismic Network (SCSN) covering the San Jacinto–Anza region. The results show a considerable correlation between seismicity and velocity perturbation structure, particularly in the region between the Mission Creek and Banning fault branches. Features connecting the Coyote Creek and Elsinore faults at right angles are correlated with seismicity lineations and may represent conjugate faulting with no surface expression.

Delineation of Faulting and Basin Geometry along a Seismic Reflection Transect in Urbanized San Bernardino Valley, California

Fourteen kilometers of continuous, shallow seismic reflection data acquired through the urbanized San Bernardino Valley, California, have revealed numerous faults between the San Jacinto and San Andreas faults as well as a complex pattern of downdropped and uplifted blocks. These data also indicate that the Loma Linda fault continues northeastward at least 4.5 km beyond its last mapped location on the southern edge of the valley and to within at least 2 km of downtown San Bernardino. Previously undetected faults within the valley northeast of the San Jacinto fault are also imaged, including the inferred western extension of the Banning fault and several unnamed faults. The Rialto-Colton fault is interpreted southwest of the San Jacinto fault. The seismic data image the top of the crystalline basement complex across 70% of the profile length and show that the basement has an overall dip of roughly 10° southwest between Perris Hill and the San Jacinto fault. Gravity and aeromagnetic data corroborate the interpreted location of the San Jacinto fault and better constrain the basin depth along the seismic profile to be as deep as 1.7 km. These data also corroborate other fault locations and the general dip of the basement surface. At least 1.2 km of apparent vertical displacement on the basement is observed across the San Jacinto fault at the profile location. The basin geometry delineated by these data was used to generate modeled ground motions that show peak horizontal amplifications of 2–3.5 above bedrock response in the 0.05- to 1.0-Hz frequency band, which is consistent with recorded earthquake data in the valley.

Seismic behavior of the southern San Andreas fault zone in the northern Coachella Valley, California: Comparison of the 1948 and 1986 earthquake sequences

Abstract Moderate earthquakes in 1948 and 1986 that occurred along the southern San Andreas fault zone are examined to help understand the subsurface geometry and seismic behavior of active fault segments in the northern Coachella Valley. The 1986 North Palm Springs earthquake occurred between the Banning and Mission Creek traces of the San Andreas fault zone. Based on data from both portable and permanent stations, aftershocks of the 1986 event define a nearly planar surface that strikes about N60° to 70°W, is about 15-km long, and dips northeast from near the Banning surface trace at about 45° to 50°. Relocation of the mainshock using revised station corrections and a local velocity model inverted from the aftershock arrival times indicates a location at 34°N00.26′, 116°W36.34′, and a focal depth of 10.4 km. The first-motion focal mechanism for the mainshock indicates pure right-slip on a plane dipping northeast at 40° to 45° with a strike of N60°W. Previous moment-tensor solutions (e.g., Hartzell, 1989) exhibit significant oblique-reverse slip on a plane with a more westerly strike and a seismic moment of 1.4 to 1.7 × 1018 N-m (Mw = 6.1). Actual and synthetic Wood-Anderson recordings indicate a local magnitude of ML 6.0. The 1948 Desert Hot Springs earthquake produced waveforms similar to 1986, although amplitudes for 1948 are typically 20 to 30% larger. The 1948 mainshock is relocated at 33°N55.2′, 116°W28.9′, with a focal depth of 12 km, a revised magnitude of ML 6.3, and an estimated seismic moment of 2 to 3 × 1018 N-m. Based on arrival-time information from close (<20 km) portable stations deployed in 1948 to 1949, the 1948 aftershock distribution is about 15-km long, 8- to 10-km wide, and abuts the 1986 aftershock sequence to the southeast along the Banning fault. The 1948 aftershock zone is consistent with a focal mechanism that exhibits predominantly right-slip motion (rake 169°) on a plane that strikes N55°W and dips more steeply northeast at 60° to 70°. In 1986, the horizontal extent of the rupture may have been controlled by secondary cross faults and possible changes in fault geometry, while the down-dip extent may have been controlled by the presence at depth of a regional detachment. In 1948, the rupture likely had a steeper dip than in 1986 and extended southeast as far as the northern Indio Hills, where the surface trace of the Banning fault undergoes an approximate 7° change in strike. These results indicate that the Banning fault is nonvertical, is likely segmented according to fault dip, as well as fault strike, and has been the primary locus of recent moderate-sized earthquake activity in the northern Coachella Valley.

Evidence from precise earthquake hypocenters for segmentation of the San Andreas Fault in San Gorgonio Pass

We use precise hypocenter patterns and focal mechanisms to investigate the presence or absence of a continuous strike‐slip fault at depth connecting the San Bernardino strand of the San Andreas fault with the Coachella Valley segment of the Banning fault. We inverted 560,000 arrival times from 23,000 earthquakes (1981–1993) for high‐quality hypocenters and three‐dimensional P wave velocity structure in a 1° by 2° area centered on the San Gorgonio Pass. Cross‐sectional plots of relocated earthquakes reveal an abrupt 5 to 7 km high step in the maximum depth of hypocenters. The step riser defines a near‐vertical, locally curved surface that extends westerly more than 60 km from the Coachella Valley segment of the San Andreas fault to the San Jacinto fault. A hypothetical continuous vertical San Andreas fault through San Gorgonio Pass would cross the step at an oblique angle. We suggest that the step is the expression of the contact between different basement rock types juxtaposed by large‐scale right‐slip motion on the ancestral San Andreas fault. South of the step in Peninsular Ranges type basement (intrusives), brittle failure occurs down to about 20‐km depth, while north of the step in San Bernardino type basement (Pelona schist), brittle failure occurs to only about 13‐km depth. The step provides a piercing plane that should be offset about 3 km right laterally by an active, continuous, vertical San Andreas fault. Within the resolution of our mapping the step is not offset in this manner, implying either that there has not been a throughgoing vertical fault at depth, that a throughgoing fault has not experienced enough slip to offset the step, or that a throughgoing fault is not vertical and dips north over the top of the step. Hypocentral patterns and focal mechanisms indicate distributed deformation (thrust, normal, and strike‐slip faulting) over a large volume in the San Gorgonio Pass region; there is no evidence of hypocenter or slip vector alignments that would indicate a throughgoing, continuous, near‐vertical San Andreas fault. In summary, we find no evidence indicating a continuous fault at seismogenic depth connecting the San Bernardino strand and Coachella Valley segment of the San Andreas fault zone. We speculate that this is because the 3 km of right slip on the San Bernardino stand of the San Andreas fault and Coachella Valley segment of the Banning fault has not been sufficient to form a single new structure through the 15‐ to 20‐km gap between the two previously unconnected segments. This implies that large earthquake rupture on the San Andreas fault may be inhibited from propagating through San Gorgonio Pass, thus limiting the maximum magnitudes on the southern San Andreas fault.

Source mechanisms of three moderate California earthquakes of July 1986

Abstract We studied the source processes of three moderate size earthquakes in California: the M s = 6.0 event of July 8, 1986, near Palm Springs, the M s = 5.8 shock of July 13, 1986, offshore of Oceanside, and the M s = 6.2 event of July 21, 1986, in the Chalfant Valley, on the east side of the Sierra Nevada. The centroidal source parameters are estimated by inversion of long-period P and SH waves recorded at teleseismic distances by seismographs of worldwide networks. A more detailed description of the source is then obtained by analyzing broadband seismograms. The North Palm Springs earthquake had a simple rupture process. Most of the seismic moment was released in the first 3 sec in a single step. The strike of the source fault plane follows the average trend of the Banning fault and its dip agrees with the dip of the fault plane delineated by the aftershocks. The faulting mechanism of the main shock is right-lateral strike-slip with a sizable reverse component that is consistent with uplift north of the Banning fault. The earthquake offshore of Oceanside had a slightly more complex rupture process. The far-field source-time function consists of two pulses that represent the release of most of the moment in the first 3 sec. The source mechanism indicates reverse faulting with the compressional axis trending northeast, almost perpendicular to the trend of mapped faults offshore. The event does not appear to be associated with major mapped faults in the area, but the strike of one of the nodal planes agrees with the trend of the major mapped faults offshore and its mechanism is consistent with the missing component of plate convergence not taken up by the onshore strike-slip fault systems, and the idea of a weak San Andreas fault system. The rupture process of the Chalfant Valley earthquake was yet more complicated. The source-time function indicates that in a 4 sec time window three asperities were broken. The source mechanism indicates right-lateral strike-slip faulting, with a small normal-faulting component on a fault that dips approximately 50° southwest. Earthquake activity suggests that presently the deformation in the northern part of the Owens Valley (which includes the Chalfant Valley) is dominated by conjugate strike-slip faults which strike obliquely to the main range bounding normal faults. The waveforms indicate the presence of a low-velocity zone at a depth of about 20 km in the crust in the Chalfant Valley. A synthesis of information from waveform modeling, first-motion fault-plane solutions, and aftershock distributions indicates that the nucleation and/or termination of rupture for the North Palm Springs and the Chalfant Valley events were controlled by preexisting fault geometry.

Seismomagnetic Observation During the 8 July 1986 Magnitude 5.9 North Palm Springs Earthquake

A differentially connected array of 24 proton magnetometers has operated along the San Andreas fault since 1976. Seismomagnetic offsets of 1.2 and 0.3 nanotesla were observed at epicentral distances of 3 and 9 kilometers, respectively, after the 8 July 1986 magnitude 5.9 North Palm Springs earthquake. These seismomagnetic observation are the first obtained of this elusive but long-anticipated effect. The data are consistent with a seismomagnetic model of the earthquake for which right-lateral rupture of 20 centimeters is assumed on a 16-kilometer segment of the Banning fault between the depths of 3 and 10 kilometers in a region with average magnetization of 1 ampere per meter. Alternative explanations in terms of electrokinetic effects and earthquake-generated electrostatic charge redistribution seem unlikely because the changes are permanent and complete within a 20-minute period.

Trace-fractures on the banning fault created in association with the 1986 North Palm Springs earthquake

Trace-fractures on the banning fault created in association with the 1986 North Palm Springs earthquake

Transverse Ranges Province: A Unique Structural-Petrochemical Belt across the San Andreas Fault System

The San Gabriel and San Bernardino mountain ranges of southwestern California, and associated mountains and basins westward to the Pacific Ocean, make up a unique east-trending geomorphic, stratigraphic, petrologic, and structural belt 400 km long that is offset only a few tens of kilometers right laterally by northwest-striking faults of the San Andreas type. Spot correlations across these faults, suggesting displacements of hundreds of kilometers, perhaps have other explanations. Within the transverse ranges and basins, the east trend is shown by the general petrology, the crystalline rock patterns, pre- and post-batholithic structural features, and batholithic chemical patterns. The east-west unity is especially striking west of the San Andreas fault, but it is also evident in the central and southern San Bernardino Mountains, east of that fault. The Salton Trough and the southern boundary of the Transverse Ranges Province appear to be the most important provincial boundaries in southwestern California. West of the San Jacinto fault, which is just southwest of the San Andreas fault and nearly parallel to it, the Transverse Ranges Province is bounded on the south by the Malibu Coast–Cucamonga fault zone. At and just east of the San Jacinto fault, the southern boundary fault zone is displaced by faulting 30 to 40 km to the southeast, and is continued as the Banning fault. East of the eastern (Mission Creek) branch of the San Andreas fault, east-trending faults are present, but the San Bernardino Mountains rocks and structures merge into the southeast-trending Little San Bernardino Mountains.

SAN ANDREAS FAULT ZONE IN SAN GORGONIO PASS, SOUTHERN CALIFORNIA

Research Article| March 01, 1957 SAN ANDREAS FAULT ZONE IN SAN GORGONIO PASS, SOUTHERN CALIFORNIA CLARENCE R ALLEN CLARENCE R ALLEN DIVISION OF THE GEOLOGICAL SCIENCES, CALIFORNIA INSTITUTE OF TECHNOLOGY, PASADENA, CALIFORNIA Search for other works by this author on: GSW Google Scholar GSA Bulletin (1957) 68 (3): 315–350. https://doi.org/10.1130/0016-7606(1957)68[315:SAFZIS]2.0.CO;2 Article history received: 09 Aug 1956 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation CLARENCE R ALLEN; SAN ANDREAS FAULT ZONE IN SAN GORGONIO PASS, SOUTHERN CALIFORNIA. GSA Bulletin 1957;; 68 (3): 315–350. doi: https://doi.org/10.1130/0016-7606(1957)68[315:SAFZIS]2.0.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 SocietyGSA Bulletin Search Advanced Search Abstract Unusual features of the San Andreas fault in the San Gorgonio Pass area of Southern California are the absence of rift topography, absence of lateral stream offsets, an abrupt change in trend of the fault trace, seismic evidence for the predominance of thrusting over strike-slip faulting, and a lack of great earthquakes in historic time.Rocks of the San Gorgonio igneous-metamorphic complex crop out over most of the map area and constitute a metamorphic terrane of intermediate to basic composition that has been intimately intruded and partially reconstituted by Mesozoic(?) quartz monzonite. Flaser gneiss, greenschist, and piedmontite-bearing gneiss are distinctive rock types in the otherwise rather uniform migmatitic gneisses. Most of the younger rocks of the pass area are sedimentary and reflect a history of recurrent deformation and deposition; they range in age from upper Miocene(?) to Quaternary, and nearly all are of alluvial-fan or flood-plain origin.Quaternary alluvial fans that once buried a former rugged topography are now being dissected along the north side of the pass, leaving numerous surfaces of low relief and associated stream terraces. Warping of these surfaces suggests Quaternary arching of the mountain range along a north-south axis.Within San Gorgonio Pass, the San Andreas fault curves abruptly southward from its normal southeast trend and butts into the eastward-trending Banning fault at 45°. The Banning fault is a major tectonic feature that delineates the north side of the pass and forms the southern limit of the Transverse Ranges in this region. Thrust and reverse movements of at least 5000 feet have taken place on this fault in Quaternary time, and, although there is little evidence of Recent lateral displacements, late Pliocene and Pleistocene right-hand strike-slip movements totaling at least 5 miles are suggested. The Mill Creek and Mission Creek faults are major branches of the San Andreas fault that diverge northward; both evince considerable late Cenozoic vertical displacement, but possible lateral movements are unknown.This study neither proves nor disproves the existence of lateral displacements amounting to perhaps hundreds of miles along the San Andreas fault zone as a whole. But if large lateral displacements have taken place, they must have been followed by deformation and disruption of the fault traces then existent, because lateral movements of even 1 mile are difficult to reconcile with the complex surface geometry of faults within the pass area. Faults previously considered branches of the San Andreas, particularly the Mission Creek and San Jacinto faults, may have absorbed much of the lateral strain, and the Banning fault may represent an ancestral San Andreas fault—now deformed into a Transverse Range fault. The deformation and disruption of former breaks appear to represent a pattern that is typical of the entire eastern half of the Transverse Ranges, where elements of San Andreas and Transverse Range structure have been vying for control; evidently one set has alternated with the other in attaining temporary dominance. Southeast of San Gorgonio Pass it is not clear which, if any, fault trace deserves the name of San Andreas, and it is suggested that the entire area between the Elsinore fault on the west and the east side of the Salton depression on the east be termed the San Andreas fault zone. 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.