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Research about San Andreas fault

Source-linked reports with geographic coverage including San Andreas fault.

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USGS1979

Changes in rate of fault creep

Aseismic slip or fault creep is occurring on many faults in California. Although the creep rates are generally less than 10 mm/yr in most regions, the maximum observed rate along the San Andreas fault between San Juan Bautista and Gold Hill in central California exceeds 30 mm/yr. Changes in slip rates along a 162 km segment of the San Andreas fault in this region have occurred at approximately the same time at up to nine alinement array sites. Rates of creep on the fault near the epicenters of moderate earthquakes ( M L 4–6) vary for periods of several years, decreasing before the main shocks and increasing thereafter, in agreement with prior observations based on creepmeter results. The change of surface slip rate is most pronounced within the epicentral region defined by aftershocks, but records from sites at distances up to 100 km show similar variations. Additionally, some variations in rate, also apparently consistent among many sites, have a less obvious relation with seismic activity and have usually taken place over shorter periods. Not all sites exhibit a significant variation in rate at the time of a regional change, and the amplitudes of the change at nearby sites are not consistently related. The time intervals between measurements at the nine array sites during a given period have not always been short with respect to the intervals between surveys at one site; hence, uneven sampling intervals may bias the results slightly. Anomalies in creep rates thus far observed, therefore, have not been demonstrably consistent precursors to moderate earthquakes; and in the cases when an earthquake has followed a long period change of rate, the anomaly has not specified time, place, or magnitude with a high degree of certainty. The consistency of rate changes may represent a large scale phenomenon that occurs along much of the San Andreas transform plate boundary.

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USGS1979

Preliminary analysis of clay gouge from a well in the San Andreas fault zone in central California

Drilling into the San Andreas fault zone has begun in central California as part of the U.S.G.S. program of fault zone studies. The purpose of drilling into the fault zone is threefold. First, it will allow recovery of material from depth in order to determine the composition of both solid and fluid phases. Second, it will allow determination of the physical state of the fault zone, that is, the state of stress, pore pressure, and temperature. Finally, it will allow emplacement of instruments at depth in the fault zone for monitoring experiments related to short-term earthquake prediction. In an attempt to drill to the depth of earthquake focii, the initial drill site was chosen at a locale known as Dry Lake Valley (Fig. 1). Although the entire central section of the San Andreas fault is characterized by moderate seismicity and aseismic creep, Dry Lake Valley has particularly shallow earthquakes as well as a fairly high creep rate (18-20 mm/year). Figure 2 is a longitudinal section along the San Andreas fault. The high precision earthquake locations shown are magnitude one or greater events that occurred in 1973-1975 (from W. Ellsworth, pers. comm.). The depth estimates are accurate to about + 0.5 km. It is obvious from the figure that earthquakes in the section of the fault near the well are quite shallow.

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USGS1978

Local magnetic field measurements, fault creep observations and local earthquakes on the San Andreas fault

Simultaneous records of local variations in magnetic field, fault creep and the occurrence times of local earthquakes have been obtained for the period early 1974 through mid-1977 along the San Andreas fault between the most southern extent of the 1906 earthquake fault break and the most northern extent of the 1857 break. The data utilized are primarily from stations located near the two ends of this section of the fault where strains accumulation is expected. The magnetic data show that local magnetic field variations up to 1.8 gammas with durations of a few minutes to several months have occurred. The creep data indicate both episodic events and changes in creep rate of up to 10 mm/yr lasting for six months or more. No clear magnetic transients or offsets are evident either simultaneous with or preceding the occurrence times of the episodic creep events by up to a day or so. Although some patterns of creep onset times at adjacent stations appear to correspond to some periods of longer term change in local magnetic field, these changes do not always occur and other groups of creep events have no corresponding changes in local magnetic field. Earthquakes with magnitudes less than 4.0 do not appear to correspond in time to local changes in magnetic field greater than 0.8 gamma or variations in the creep rate. In order to explain the observations presented in this study, it appears necessary to allow for a substantial amount of deep aseismic slip without any obvious attendant changes in the time distribution or size of the local earthquakes. Changes in stress related to the surface expression of aseismic slip on the San Andreas fault can be estimated from dislocation models fit to these data and to observations of simultaneous strains and tilts at points near the fault. These stress values are on average less than one bar near the surface but are probably more than 10 bars in localized regions at depths of a kilometer or so.

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USGS1978

Continuous tilt, strain, and magnetic field measurements near four earthquakes (ML = 3.6 to 3.8) on the San Andreas fault, California

Four moderate earthquakes (M = 3.6 to 3.8) have occurred on the San Andreas fault in central California since October, 1977. These earthquakes are the first since 1974 to occur at this magnitude level within the array of tilt, strain, and magnetic instruments between Chalome (35.726N, 12.249 W) and San Francisco (37.79N, 122.23W) shown in Figure 1. They offer, therefore, one of the few opportunities to search for indications of precursive ground deformation. Furthermore, several other geophysical parameters such as resistivity, creek, radon, seismicity, geodetic strain, etc, are also monitored and allow comparisons for some of these events. This note reports comparative data and some possible implications in continuous strain, tilt, magnetic field and other measurements obtained from instruments within 10 km of the epicenters.

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USGS1978

How often will earthquakes recur on the San Andreas Fault?

The relationship between magnitude and abundance of earthquakes, called a recurrence curve, has been derived for many regions of the world from seismographic records. AS an example, Clarence Allen and his associates at the California Institute of Technology have obtained recurrence rates for the southern California region by incorporating data from over 10,000 earthquakes recorded between 1934 and 1963. My own approach to estimating average recurrence intervals has been somewhat different. I have used the history of slip rates along the San Andreas fault that are preserved in the geologic record. The main advantage in this method is that is samples a very long period of time, which gives a better estimate of the recurrence of small earthquakes.

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USGS1976

On simultaneous tilt and creep observations on the San Andreas Fault

THE installation of an array of tiltmeters along the San Andreas Fault 1 has provided an excellent opportunity to study the amplitude and spatial scale of the tilt fields associated with fault creep. We report here preliminary results from, and some implications of, a search for interrelated surface tilts and creep event observations at four pairs of tiltmeters and creepmeters along an active 20-km stretch of the San Andreas Fault. We have observed clear creep-related tilts above the instrument resolution (10 −8 rad) only on a tiltmeter less than 0.5 km from the fault. The tilt events always preceded surface creep observations by 2–12 min, and were not purely transient in character.

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USGS1976

Catalog of earthquakes along the San Andreas fault system in central California, July-September 1973

Numerous small earthquakes occur each day in the Coast Ranges of central California. The detailed study of these earthquakes provides a tool for gaining insight into the tectonic and physical processes responsible for the generation of damaging earthquakes. This catalog contains the fundamental parameters for earthquakes located within and adjacent to the seismograph network operated by the National Center for Earthquake Research (NCER), U. S. Geological Survey, during the period July-Sptember, 1973. The motivation for these detailed studies has been described by Pakiser and others (1969) and by Eaton and others (1970). Similar catalogs of earthquakes for the years 1969, 1970, and 1971 have been prepared by Lee and others (1972b, c, and d). Catalogs for the first, second, third, and fourth quarters of 1972 and the first and second quarters of 1973 have been prepared by Wesson and others (1972a, b, 1973b, and 1974a and b), and by Bufe and others (1975). The basic data contained in these catalogs provide a foundation for further studies. This catalog contains data on 949 earthquakes in central California. Arrival times at 132 seismograph stations were used to locate the earthquakes listed in this catalog. Of these 117 were telemetered stations operated by NCER. Readings from the remaining 15 stations were obtained through the courtesy of the Seismographic Stations, University of California, Berkeley (UCB), and the California Department of Water Resources, Sacramento. The Seismographic Stations of the University of California, Berkeley, have for many years published a bulletin describing earthquakes in northern California and the surrounding area and listing readings at UCB Stations from more distant events. The purpose of the present catalog-is not to replace the UCB Bulletin, but rather to supplement it, by describing the seismicity of a portion of central California in much greater detail.

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USGS1975

The nature of surface tilt along 85 km of the San Andreas fault-preliminary results form a 14-instrument array

The continuous monitoring of surface deformation near active faults is clearly necessary for an understanding of elastic strain accumulation and elastic and anelastic strain release associated with earthquakes. Fourteen 2-component tiltmeters have been installed in shallow boreholes along 85 km of the currently most active section of the San Andreas fault in the western United States. These instruments operate at a sensitivity of 10 −8 radians. Five of these tiltmeters, extending along one 35 km section of the fault, have been in operation since June 1973. The results indicate that regional tectonic tilting has occurred before more than ten individual earthquakes or groups of earthquakes with epicenters within ten earthquake source dimensions of one or more instruments. This tilting has a time scale of up to a month depending on earthquake magnitude. The amplitude of these tilts exceeds by almost an order of magnitude that expected from a dislocation model of the source using seismically determined parameters. No indication of rapid or accelerated tilt just prior to these earthquakes has been seen.

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USGS1974

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 2 L 2 ) 12 where a = (a 2 + b 2 L 2 ) 12 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.

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USGS1973

Recurrence of seismic migrations along the central California segment of the San Andreas fault system

VERIFICATIONS of tectonic concepts 1 concerning seafloor spreading are emerging in a manner that has direct bearing on earthquake prediction. Although the gross pattern of worldwide seismicity contributed to the formulation of the plate tectonic hypothesis, it is the space-time characteristics of this seismicity that may contribute more toward understanding the kinematics and dynamics of the driving mechanism long speculated to originate in the mantle. If the lithosphere is composed of plates that move essentially as rigid bodies, then there should be seismic edge effects associated with this movement. It is these interplate effects, especially seismic migration patterns, that we discuss here. The unidirectional propagation at constant velocity (80 km yr −1 east to west) for earthquakes ( M ≥7.2) on the Antblian fault for the period 1939 to 1956 (ref. 2) is one of the earliest observations of such a phenomenon. Similar studies 3,4 of the Alaska Aleutian seismic zone and certain regions of the west coast of South America suggest unidirectional and recurring migrations of earthquakes ( M ≥7.7) occur in these areas. Between these two regions along the great transform faults of the west coast of North America, there is some evidence 5 for unidirectional, constant velocity and recurrent migration of great earthquakes. The small population of earthquakes (M>7.2) in Savage's investigation 5 indicates a large spatial gap along the San Andreas system in central California from 1830 to 1970. Previous work on the seismicity of this gap in central California indicates that the recurrence curves remain relatively constant, independent of large earthquakes, for periods up to a century 6 . Recurrence intervals for earthquakes along the San Andreas Fault have been calculated empirically by Wallace 7 on the basis of geological evidence, surface measurements and assumptions restricted to the surficial seismic layer. Here we examine the evidence for recurrence of seismic migrations along the San Andreas fault system of central California for earthquakes of magnitude M ≥5.

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USGS1970

Earthquake recurrence intervals on the San Andreas fault

Possible recurrence intervals between earth- quakes of different magnitude that may be generated along the San Andreas fault are derived by relating long-term offset rates since mid-Tertiary time, displacements, and lengths of breaks recorded for historic earthquakes, and tectonic creep rates. The recurrence interval for earthquakes of different magnitude at a given point on the fault is believed to follow the relation: (1) R x = D / (S - C) where: R x = recurrence interval at a point on the fault, D = displacement accompanying an earthquake of given magnitude (related empirically to Richter magnitude), S = long-term strain rate (from offset of geo- logic units), C = tectonic creep rate. The recurrence interval for earthquakes of different magnitudes for the total length of the fault is then derived by weighting equation (1) according to the number of break lengths in the total length as follows: (2) R t = DL / (S - C)L t where: R t = recurrence interval for entire fault, L = length of break (related empirically to Richter magnitude), L t = total length of fault. Tectonic creep is believed to be related to Richter magnitude, for example, small for segments of the fault characterized by earthquakes of large magnitude, and large for segments characterized by small earthquakes; and equations (1) and (2) can be weighted according to this relationship

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