San Andreas Fault Research Articles

Internal structure and weakening mechanisms of the San Andreas Fault

New observations of the internal structure of the San Gabriel fault (SGF) are combined with previous characterizations of the Punchbowl fault (PF) to evaluate possible explanations for the low frictional strength and seismic characteristics of the San Andreas fault (SAF). The SGF and PF are ancient, large‐displacement faults of the SAF system exhumed to depths of 2 to 5 km. These fault zones are internally zoned; the majority of slip was confined to the cores of principal faults, which typically consist of a narrow layer (less than tens of centimeters) of ultracataclasite within a zone of foliated cataclasite several meters thick. Each fault core is bounded by a zone of damaged host rock of the order of 100 m thick. Orientations of subsidiary faults and other fabric elements imply that (1) the maximum principal stress was oriented at large angles to principal fault planes, (2) strain was partitioned between simple shear in the fault cores and nearly fault‐normal contraction in the damaged zones and surrounding host rock, and (3) the principal faults were weak. Microstructures and particle size distributions in the damaged zone of the SGF imply deformation was almost entirely cataclastic and can be modeled as constrained comminution. In contrast, cataclastic and fluid‐assisted processes were significant in the cores of the faults as shown by pervasive syntectonic alteration of the host rock minerals to zeolites and clays and by folded, sheared, and attenuated cross‐cutting veins of laumontite, albite, quartz, and calcite. Total volume of veins and neocrystallized material reaches 50% in the fault core, and vein structure implies episodic fracture and sealing with time‐varying and anisotropic permeability in the fault zone. The structure of the ultracataclasite layer reflects extreme slip localization and probably repeated reworking by particulate flow at low effective stresses. The extreme slip localization reflects a mature internal fault structure resulting from a positive feedback between comminution and transformation weakening. The structural, mechanical, and hydrologic characteristics of the Punchbowl and San Gabriel faults support the model for a weak San Andreas based on inhomogeneous stress and elevated pore fluid pressures contained within the core of a seismogenic fault. Elevated fluid pressures could be repeatedly generated in the core of the fault by a combination of processes including coseismic dilatancy and creation of fracture permeability, fault‐valve behavior to recharge the fault with fluid, post‐seismic self‐sealing of fracture networks to reduce permeability and trap fluids, and time‐dependent compaction of the core to generate high pore pressure. The localized slip and fluid‐saturated conditions are wholly compatible with additional dynamic weakening by thermal pressurization of fluids during large seismic slip events, which can help explain both the low average strength of the San Andreas and seismogenic characteristics such as large stress relief. In addition, such a dynamic weakening mechanism is expected only in mature fault zones and thus could help explain the apparent difference in strength of large‐displacement faults from smaller‐displacement, subsidiary seismogenic faults.

Variations in Strength and Slip Rate Along the San Andreas Fault System

Convergence across the San Andreas fault (SAF) system is partitioned between strike-slip motion on the vertical SAF and oblique-slip motion on parallel dip-slip faults, as illustrated by the recent magnitude M(s) = 6.0 Palm Springs, M(s) = 6.7 Coalinga, and M(s) = 7.1 Loma Prieta earthquakes. If the partitioning of slip minimizes the work done against friction, the direction of slip during these recent earthquakes depends primarily on fault dip and indicates that the normal stress coefficient and frictional coefficient (micro) vary among the faults. Additionally, accounting for the active dip-slip faults reduces estimates of fault slip rates along the vertical trace of the SAF by about 50 percent in the Loma Prieta and 100 percent in the North Palm Springs segments.

Crustal strain near the Big Bend of the San Andreas Fault: Analysis of the Los Padres‐Tehachapi Trilateration Networks, California

In the region of the Los Padres‐Tehachapi geodetic network, the San Andreas fault (SAF) changes its orientation by over 30° from N40°W, close to that predicted by plate motion for a transform boundary, to N73°W. The strain orientation near the SAF is consistent with right‐lateral shear along the fault, with maximum shear rate of 0.38±0.01 μrad/yr at N63°W. In contrast, away from the SAF the strain orientations on both sides of the fault are consistent with the plate motion direction, with maximum shear rate of 0.19±0.01 μrad/yr at N44°W. The strain rate does not drop off rapidly away from the fault, and thus the area is fit by either a broad shear zone below the SAF or a single fault with a relatively deep locking depth. The fit to the line length data is poor for locking depth d less than 25 km. For d of 25 km a buried slip rate of 30 ± 6 mm/yr is estimated. We also estimated buried slip for models that included the Garlock and Big Pine faults, in addition to the SAF. Slip rates on other faults are poorly constrained by the Los Padres‐Tehachapi network. The best fitting Garlock fault model had computed left‐lateral slip rate of 11±2 mm/yr below 10 km. Buried left‐lateral slip of 15±6 mm/yr on the Big Pine fault, within the Western Transverse Ranges, provides significant reduction in line length residuals; however, deformation there may be more complicated than a single vertical fault. A subhorizontal detachment on the southern side of the SAF cannot be well constrained by these data. We investigated the location of the SAF and found that a vertical fault below the surface trace fits the data much better than either a dipping fault or a fault zone located south of the surface trace.

Seismotectonics of the easternmost transverse ranges, California: Relevance for seismic potential of the southern San Andreas Fault

Earthquake locations, depths and focal mechanisms from the Southern California Regional Network (1977–1985) are used to identify the orientation and sense of slip of active subsurface faults in the Easternmost Transverse Ranges (ETR). The ETR are separated from the Salton Trough province by the southernmost strands of the San Andreas fault (SAF). Much of the seismicity in the ETR is concentrated well northeast of the SAF at relatively shallow depths under the Little San Bernardino Mountains. Many of these earthquakes reflect slip on steeply dipping, left‐lateral faults striking northeast to east, at relatively high angles to the adjacent SAF. Focal mechanisms in the ETR show predominantly strike‐slip, normal, or oblique‐normal faulting, and share common near‐horizontal T axes striking WNW. P axes range from near vertical to near horizontal and strike mostly NNE. In contrast, reverse and strike‐slip focal mechanisms that exhibit persistent north trending, near‐horizontal P axes characterize the San Gorgonio Pass area immediately to the west. These different patterns of strain geometries are inferred to represent changes in local stress regime and clearly establish a boundary between contrasting tectonic styles of contemporary secondary deformation along the SAF. This boundary, which in the Coachella Valley may be the Mission Creek fault, is also distinguished by abrupt changes in (1) rate and depth of seismic activity; (2) topography; (3) Quaternary vertical deformation; (4) strikes and dips of major branches of the SAF; and (5) seismic velocities in the crust and upper mantle. The preponderance of secondary normal faulting in the ETR versus secondary reverse faulting in the San Gorgonio Pass region suggests that fault‐normal stress is much less across the SAF adjacent to the ETR. If a friction law where strength is proportional to normal stress applies to the SAF, then a smaller tectonic shear stress would be required tor slip in large earthquakes along the Salton Trough segment. In this case, the southernmost SAF may have shorter repeat times, smaller average displacements, and lower moment release rates than earthquakes that rupture through the major restraining bend in San Gorgonio Pass.

Morphology and growth history of delgada fan: Implications for the Neogene evolution of Point Arena Basin and the Mendocino Triple Junction

Long‐range side scan (GLORIA) sonographs and seismic reflection data acquired during a survey of the western U.S. Exclusive Economic Zone in 1984, coupled with information from Deep Sea Drilling Project sites, provide new insights into the growth and evolution of the Delgada Fan. Construction of the fan commenced in the latest Miocene (∼6 Ma) following the filling of the Neogene Point Arena Basin. The fan presently covers more than 50×103 km2 of the Pacific plate and contains approximately 15×103 km3 of predominantly terrigenous detritus. The large size of the fan is incompatible with the small present‐day supply of sediment to the canyon system. The GLORIA data show the Delgada Fan to be a hybrid‐type fan, exhibiting characteristics of both elongate and radial fans. The morphology and volume of the fan, along with evidence for a decline in accumulation rates on the lower fan during the Quaternary period, suggest that the fan experienced an early growth phase (latest Miocene and Pliocene) characterized by relatively rapid progradation of elongate fan lobes followed by a period (Quaternary) of slower growth that has featured a shift of depocenters to sites closer to the canyons and a transition to distributary channels bordered by less prominent levees and overbank deposits. We examine the growth of Delgada Fan in relation to the Neogene evolution of the North American‐Pacific plate boundary using a series of paleogeographic reconstructions based on recently published time displacement histories of the Mendocino triple junction (MTJ), the San Andreas fault (SAF), and the Pacific plate, upon which the fan rests. The time displacement curves for the SAF and the MTJ suggest that the MTJ and Mendocino Fracture Zone overtook and passed Point Arena Basin at about 10 Ma when the basin lay immediately southwest of the present San Francisco Bay area. We suggest that the MTJ joined the SAF at approximately that time and location, thus making the SAF the master fault in the transform system. This interpretation is compatible with evidence from seismic reflection profiles over the fan, which demonstrate that the fan and the canyon system and therefore Point Arena Basin have moved as a unit since the inception of fan growth (∼6–7 Ma). Point Arena Basin lay southwest of the San Francisco area at 10–12 Ma, and the passage of the MTJ caused the disruption of the forearc shelf and slope and the development of local uplifted and subsiding blocks. In particular, uplift of the “bay block” immediately east of the SAF may have provided the source area for the late Miocene sediments that filled Point Arena Basin and set the stage for the growth of Delgada Canyon and Fan system. The growth rate of the fan has decreased, and the style of deposition has changed as the system was tectonically transported to its present location adjacent to the small youthful drainages of the King Range.

Slip budget and potential for a M7 earthquake in central California

The slip rate budget of the San Andreas fault (SAF) in central California, which is approximately 33 mm/yr, is accounted for by a change in the slip release mechanism along the fault. In the NW section of the fault, between Bear Valley and Monarch Peak, creep apparently accounts for the slip budget with the seismicity contributing negligibly. The section at Parkfield marks the transition from a creeping to a locked fault trace. Since the M8 1857 earthquake five M6 earthquakes have occurred but have not completely accounted for the slip budget. Southeast of Parkfield, the SAF has been locked since 1857. From Cholame to Bitterwater Valley this section now lags the deep slip by the amount of slip released in 1857; consequently faulting in this section could occur at the time of the next Parkfield earthquake. If this earthquake releases the slip deficit accumulated in the transition zone and in the locked zone, the earthquake will have a moment‐magnitude M7.2.

Geodetic evidence of the seismic potential at Parkfield, California

We use observed line length change rates and shallow fault slip rates at Parkfield, California to estimate the slip rate at depth on the San Andreas fault. One model featuring a nearly monotonic transition from creeping behavior on the northwest to locked behavior on the southeast fits the data acceptably well. An isolated asperity fits the data better, but not significantly. Evidence for the asperity comes from two lines connecting stations where mildly inconsistent data have been collected. Thus evidence for the asperity is marginal. Stored elastic energy southeast of Parkfield could produce an earthquake of magnitude up to 8.0 that would have displacement and rupture length similar to historic earthquakes of the same size. Smaller events are of course more likely, but the geodetic data do not favor a "characteristic" M=6 event.

Magnetic anomalies around the North Anatolian Fault

The nature of the observed magnetic anomalies surrounding the San Andreas fault (SAF) is of interest to researchers studying fault kinematics and earthquake dynamics. In this work we will present and interpret data collected around the North Anatolian fault (NAF) by Honkura et al. (1985) which exhibit anomalies similar to those observed along the SAF. Magnetic and surface geologic data from five profiles straddling the North Anatolian Fault were used in conjunction with a two dimensional magnetic inversion in an attempt to identify the source of the magnetic anomalies. As a result of this study, we conclude that all long wavelength anomalies (λ ≥ 500m) could be attributed to the changes in the relative magnetic susceptibility across geologic units. Thus, they are due to lateral variations in the near surface geology and are not due to the concentration of a particular material in the fault zone. However, there was one exception, a short wavelength anomaly (λ ≃ 150m) was observed on profile IJ. Based on the results of the inversion, it appears that the anomaly may be due to a small sliver of material located in the upper part of the fault zone. If that interpretation is correct, the short wavelength anomaly is clearly of different nature than the other anomalies.

Fractal analysis applied to characteristic segments of the San Andreas Fault

Fractal theory is applied in a quantitative analysis of the San Andreas fault (SAF) geometry. The method, which directly measures the increase in total fault length with a decrease in ruler size, gives the fractal dimension D and scaling properties for the chosen length band 0.5–1000 km. A physical interpretation of D is that it measures the irregularity of the fault trace in the selected band. The fault is subdivided into six segments of distinctive seismic behavior. A “main” fault trace which shows either maximum coseismic slip or creeping was selected for analysis, with three alternative branches examined for the SAF system south of San Bernardino. Branches of the fault trace are not considered in this work. Fractal dimensions calculated for the different segments range from 1.0008 to 1.0191, and are different from 1.0 to the 95% confidence interval. These small changes reflect the overall smoothness of the main fault trace. Significant variations in D among segments indicate heterogeneities in the fault smoothness along strike. D also changes significantly from the short‐length band to the long‐length band where the demarcation point ranges from 1 to 2 km. The short‐length band has larger D values. A slight correlation is obtained between the fractal dimension of the main trace and the extent of subparallel faulting. This indicates some correspondence between the main fault trace irregularity and the complexity of subsidiary fault traces in plan view.

Gravity anomaly and density structure of the San Andreas fault zone

A densely spaced gravity survey across the San andreas fault zone was conducted near Bear Valley, about 180 km south of San Francisco, along a cross-section where a detailed seismic reflection profile was previously made byMcEvilly (1981). WithFeng andMcEvilly's velocity structure (1983) of the fault zone at this cross-section as a constraint, the density structure of the fault zone is obtained through inversion of the gravity data by a method used byParker (1973) andOldenburg (1974). Although the resulting density picture cannot be unique, it is better constrained and contains more detailed information about the structure of the fault than was previously possible. The most striking feature of the resulting density structure is a deeply seated tongue of low-density material within the fault zone, probably representing a wedge of fault gouge between the two moving plates, which projects from the surface to the base of the seismogenic zone. From reasonable assumptions concerning the density of the solid grains and the state of saturation of the fault zone the average porosity of this low-density fault gouge is estimated as about 12%. Stress-induced cracks are not expected to create so much porosity under the pressures in the deep fault zone. Large-scaled removal of fault-zone material by hydrothermal alteration, dissolution, and subsequent fluid transport may have occurred to produce this pronounced density deficiency. In addition, a broad, funnel-shaped belt of low density appears about the upper part of the fault zone, which probably represents a belt of extensively shattered wall rocks.

Tectonic Evolution of the Transverse Ranges of Southern California: ABSTRACT

The Transverse Ranges of southern California trend anomalously east-west in a tectonic regime otherwise dominated by the northwest-southeast trending San Andreas fault system. Plate tectonics theory offers an explanation for the origin of the Transverse Ranges. Convergence of the North American Plate (NAP) with the East Pacific Rise (EPR) and the overriding of the EPR by the NAP south of the Mendocino Fracture Zone led to development of northwest-trending, right-lateral faults on the leading edge of the NAP in southern California. Subsequent deflection of the NAP to the southwest by the still active Gorda-Juan de Fuca Ridge segment of the EPR resulted in southwesterly deflection of the San Andreas fault (SAF) forming the big bend in that fault. These plate movements are responsible for the east-west trend and juxtaposition of major components of the Transverse Ranges. The east-west-trending Santa Ynez Range represents northward-moving rocks on the south side of the SAF, which were deflected westerly and did not negotiate the big bend in that fault. The elevated central part of the Transverse Ranges from the big bend southeasterly to Cajon Pass is under compression as northwesterly moving blocks on the south side of the SAF converge on southwesterly moving blocks on the north side of that fault. The easternmost range in the Transverse Ranges, the San Bernardino Mountains, is under similar compression on the north side of the bent portion of the SAF. End_of_Article - Last_Page 678------------

Earthquake seasonality before the 1906 San Francisco earthquake

Since 1855, 40 earthquakes of magnitude ⩾5.5 have occurred on the San Andreas Fault (SAF) system in coastal central California, between latitudes 36°30′ N and 39°30′ N (Table 1)1. Of the included 27 ‘independent’ earthquakes, which occurred more than 1yr after or 1 geocentric degree distant from a previous ⩾5.5 mag earthquake, 3 occurred in summer, 7 in autumn, 4 in winter and 13 in spring. All of the spring earthquakes occurred during the 25-yr interval that culminated with the great San Francisco earthquake (8.25 mag) in the spring of 1906 (Fig. 1a, b). I report here that the observed frequency of the spring earthquakes before 1906 significantly exceeds (binomial P = 0.002) seasonal frequencies expected from random variation in the earthquake rate, and thus invites serious inquiry into the subject of earthquake seasonality. More importantly, the time-clustering of spring earthquakes before 1906 raises the question of whether accelerated seasonal seismicity may herald the end of the ‘seismic cycle’ suspected1 to modulate the long-term pattern of seismicity on the northern segment of the SAF system.

Implications of an elastic analysis of in situ stress measurements near the san andreas fault : McGarr, A; Zoback, M D; Hanks, T C J Geophys Res, V87, NB9, 10 Sept 1982, P7797–7806

Implications of an elastic analysis of in situ stress measurements near the san andreas fault : McGarr, A; Zoback, M D; Hanks, T C J Geophys Res, V87, NB9, 10 Sept 1982, P7797–7806

Large creep events on the Imperial fault, California

During September 1975, three creepmeters were installed across the Imperial fault and one across the southern end of the Brawley fault, two members of the San Andreas fault system in the Imperial Valley of southeastern California. These supplement earlier installations in the Imperial Valley of one dial-gauge creepmeter and six alignment arrays (Figure 1 and Table 1). Each creepmeter uses either an invar or a stainless steel wire suspended in a horizontally buried pipe, and crosses the fault at an angle of about 45°. Those at Heber Road and Tuttle Ranch are of the design described by Yamashita and Burford (1973), using a linear variable differential transformer as the displacement transducer, and are 20 m long. The three more northerly creepmeters are similar in design to that described by Smith and Wyss (1968), and are between 12 and 15 m in length. Those at Harris Road and Ross Road use a potentiometer in a bridge circuit as the displacement transducer for continuous recording. The Superstition Hills creepmeter is equipped with a dial-gauge micrometer, which is read periodically.

Static deformation of a laterally inhomogeneous half-space by a two-dimensional strike-slip fault.

On the basis of the elastic dislocation theory, formulas are derived for the displacement, strain and stress fields of an arbitrarily situated, two-dimensional strike-slip fault in a laterally inhomogeneous medium. It is assumed that the elastic half-space is made up of n+1 isotropic and homogeneous media being in welded contact and separated by n vertical interfaces. The explicit expressions of coefficients which appear in the formulas are listed for the case n=2. They are checked with the previous results obtained with the help of the method of images. As an application of the formulas derived, the surface displacement field of a vertical strike-slip fault is determined. Three different models are analyzed on the basis of the data concerning medium structure along a section of the San-Andreas fault near Bear Valley. In the presence of lateral inhomogeneities of the medium, it is shown that the elastic field of an earthquake should exhibit distinct asymmetry. This conclusion is confirmed by the existing geodetic data. In particular, the Tango earthquake of 1927 is discussed in greater detail.

Geodimeter measurements of slip and strain accumulation along the san andreas fault

The U.S. Geological Survey conducts repeated geodimeter surveys of trilateration networks in central California in order to study the processes of slip and strain accumulation along the San Andreas fault. The precision of distance measurement is described by a standard deviation σ = (a 2 + b 2L 2) 1 2 where a = 3mm, b = 2 · 10 −7, and L is the line length. Within the precision of measurement, no anomalous strain episodes preceding earthquakes or even strain discontinuities at the time of earthquakes were detected from repeated measurements of lines near the epicenters of small (magnitude 4.5–5.1) earthquakes. Annual measurements of small (5-km aperture) strain polygons near the San Andreas fault have not proved strain accumulation in a 3-year period. Repeated measurements of longer lines over periods of 8 to 14 years indicate changes that cannot be attributed to fault slip and must represent strain accumulation at the level of a few parts in 10 7 per year.

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.

The California Earthquake of April 18, 1906. State Earthquake Investigation Committee A. C. Lawson

<i>The California Earthquake of April 18, 1906</i>. State Earthquake Investigation Committee A. C. Lawson