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USGS1985

Crustal refraction profile of the Long Valley caldera, California, from the January 1983 Mammoth Lakes earthquake swarm

Seismic-refraction profiles recorded north of Mammoth Lakes, California, using earthquake sources from the January 1983 swarm complement earlier explosion refraction profiles and provide velocity information from deeper in the crust in the area of the Long Valley caldera. Eight earthquakes from a depth range of 4.9 to 8.0 km confirm the observation of basement rocks with seismic velocities ranging from 5.8 to 6.4 km/sec extending at least to depths of 20 km. The data provide further evidence for the existence of a partial melt zone beneath Long Valley caldera and constrain its geometry. This evidence is in the form of pronounced secondary arrivals, which we interpret as waves that have propagated through a volume with low Q (the magma chamber) and reflected from the lower boundary of the southern edge of this volume at depths of 18 to 20 km.

California↗
USGS1985

Coupling of ocean bottom seismometers to sediment: Results of tests with the U.S. Geological Survey ocean bottom seismometer

The response of an ocean bottom seismometer (OBS) to a transient pull that excites the natural OBS-sediment coupling resonance can be modeled as a mass-spring-dashpot system in which the resonant frequency and damping are functions of instrument mass and bearing radius and of the physical properties of the sediment (primarily the shear modulus). For the very soft sediments sometimes found on the sea floor, this resonance may be within the main frequency band of interest (2 to 15 Hz) for many common instrument configurations. To test the model and to find an anchor that would shift the coupling resonance to a higher frequency and decrease its amplitude, we conducted a series of tests which measured the response of the vertical and horizontal components of the U.S. Geological Survey OBS to transient pulls as a function of anchor configuration and sediment properties. The tested anchors included a concrete “flowerpot,” a tripod, a plate, and a perforated plate. Sites were on soft, organic-rich ooze and on firm sand. Several small shots were also fired at the ooze site in order to compare the response of the plate and “flowerpot” anchors to seismic signals. For a given anchor at a given site, the observed response was very repeatable. We found that the model predicts the vertical coupling response quite well and that good vertical coupling can be achieved with the plate or perforated-plate anchors. The response to the horizontal pulls, however, was similar and resonant for all anchors.

Bulletin of the Seismological Society of America↗
USGS1985

Effects of the 1983 Coalinga, California, earthquake on creep along the San Andreas fault

The M//L approximately equals 6. 5 earthquake that occurred near Coalinga, California, on May 2, 1983 induced changes in near-surface fault slip along the San Andreas fault. Coseismic steps were observed by creepmeters along a 200-km section of the San Andreas. some of the larger aftershocks induced additional steps, both right-lateral and left-lateral, and in general the sequence disrupted observed creep at several sites from preseismic long-term patterns. Static dislocation models can approximately explain the magnitudes and distribution of the larger coseismic steps on May 2. The smaller, more distant steps appear to be the abrupt release of accumulated slip, triggered by the coseismic strain changes, but independent of the strain change amplitudes.

California↗
USGS1985

Quaternary tectonic setting of the 1983 Borah Peak earthquake, central Idaho

The 1983 Borah Peak earthquake was accompanied by extensive surface faulting along a part of the Lost River fault that has abundant evidence of latest Quaternary (last 15,000 yr) offset. This fault and two similar range-front normal faults along the Lemhi Range and Beaverhead Mountains lie in an area of basin-and-range structure in central Idaho that is part of a roughly V-shaped belt of latest Quaternary surface faulting that extends from the Wasatch fault, through the Yellowstone area, to the Lost River fault. The position of this belt may be related to the outward migration of a thermal front associated with the northeastward progression of late Cenozoic silicic volcanism along the Yellowstone-Snake River Plain axis. The central segments of the Lost River, Lemhi, and Beaverhead faults have been active more recently, and probably more active throughout Quaternary time, than the southern and northern segments. The main 1983 surface faulting occurred in an area of high structural relief along a central segment of the Lost River fault that has ruptured in latest Quaternary time, which suggests that comparable areas along other range fronts in the area should be regarded as likely sites of future surface faulting. Other perspective of fault behavior suggest additional possible sites, and all segments of the range-front faults are regarded as capable of surface faulting.

Idaho↗
USGS1985

A note on the effect of bottom currents on an ocean bottom seismometer

Two three-component ocean bottom seismometers and a current meter were deployed a few hundred meters apart on the southern Blake Plateau off the United States eastern coast to study the effect of near-bottom currents on the background noise level of seismometers. Although analysis of the data is limited somewhat by instrumental problems, the increase in current speed, which ranged from 2 to 25 cm/sec, is correlated with a broadband increase in the noise level at frequencies below 10 Hz. Intermittent periods of narrow-band 8-Hz noise, which were also observed, are not correlative with bottom currents and were tentatively attributed to passing ships. Details of the mechanism of generation of the background noise cannot be determined from the present data set.

Blake Plateau↗
USGS1985

Characteristics of the aftershock sequence of the Borah Peak, Idaho, earthquake determined from digital recordings of the events

The U.S. Geological Survey, Menlo Park, deployed and maintained a network of twelve digital instruments over the 2 weeks following the 28 October 1983 Borah Peak, Idaho, earthquake. The network recorded 45 events with M ≧ 3.0, and 6 events with M ≦ 4.0. The epicenters are located in a narrow band which parallels the trace of the surface fauiting up to the Willow Creek summit; the depths of the events range from 5 to 16 km. In the south, the distribution of hypocenters delineate a plane which dips to the southwest at 50°; to the north, the hypocenters dip steeply to the east. Composite focal mechanisms for three groups of events show normal faulting mechanisms; the mechanism of the aftershocks in the north appear rotated in both strike and dip from the aftershocks in the south. The seismic moments of the aftershocks increase with increasing hypocentral depth below 12 km. The dynamic stress drops of the events do not show any systematic variation with depth, however. Most of the events with large stress drops are clustered in the northwest limb of the aftershock distribution; the average stress drop of the southern events is 31 ± 16 bars, while the average stress drop of the events in the northwest limb is 77 ± 52 bars. This clustering of events with large stress drops marks an apparent stress concentration, possibly associated with the arrest of the main shock rupture propagation by a fracture barrier at depth.

Idaho↗
USGS1985

Deformation near the epicenter of the 1984 Round Valley, California, earthquake

A trilateration network extending from near Mammoth Lakes to Bishop, California, was resurveyed following the 23 November 1984, Round Valley earthquake (ML = 5.8). The network had previously been surveyed in 1982. Deformation apparently associated with the Round Valley earthquake was detected as well as deformation due to the expansion of a magma chamber 8 km beneath the resurgent dome in the Long Valley caldera and right-lateral slip on the uppermost 2 km of the 1983 rupture surface in the south moat of the caldera. The deformation associated with Round Valley earthquake suggests left-lateral slip on the north-northeasterly striking vertical plane defined by the aftershock hypocenters located by A. S. Ryall. The earthquake moment implied by the deformation is about 3.8·1017 N-m, a value equivalent to an earthquake magnitude ML = 5.7 in good agreement with the observed magnitude of 5.8. A 0.053 km3 expansion of the magma chamber and 0.32 m slip on the 1983 rupture surface in the 1982-1985 interval was also required to account for the observed deformation.

California↗
USGS1985

High-frequency observations and source parameters of microearthquakes recorded at hard-rock sites

We have estimated the source parameters of 53 microearthquakes recorded in July 1983 which were aftershocks of the Miramichi, New Brunswick, earthquake that occurred on 9 January 1982. These events were recorded by local three-component digital seismographs at 400 sps/component from 2-Hz velocity transducers sited directly on glacially scoured crystalline basement outcrop. Hypocentral distances are typically less than 5 km, and the hypocenters and the seven digital seismograph stations established all lie essentially within the boundaries of a granitic pluton that encompasses the faults that ruptured during the main shock and major aftershocks. The P-wave velocity is typically 5 km/sec at the surface and at least 6 km/sec at depths greater than about 1 km.

Miramichi↗
USGS1985

Foreshocks and time-dependent earthquake hazard assessment in southern California

The probability that an earthquake in southern California (M ≧ 3.0) will be followed by an earthquake of larger magnitude within 5 days and 10 km (i.e., will be a foreshock) is 6 ± 0.5 per cent (1 S.D.), and is not significantly dependent on the magnitude of the possible foreshock between M = 3 and M = 5. The probability that an earthquake will be followed by an M ≧ 5.0 main shock, however, increases with magnitude of the foreshock from less than 1 per cent at M ≧ 3 to 6.5 ± 2.5 per cent (1 S.D.) at M ≧ 5. The main shock will most likely occur in the first hour after the foreshock, and the probability that a main shock will occur decreases with elapsed time from the occurrence of the possible foreshock by approximately the inverse of time. Thus, the occurrence of an earthquake of M ≧ 3.0 in southern California increases the earthquake hazard within a small space-time window several orders of magnitude above the normal background level.

California↗
USGS1985

General P, type-I S, and type-II S waves in anelastic solids; inhomogeneous wave fields in low-loss solids

The physical characteristics for general plane-wave radiation fields in an arbitrary linear viscoelastic solid are derived. Expressions for the characteristics of inhomogeneous wave fields, derived in terms of those for homogeneous fields, are utilized to specify the characteristics and a set of reference curves for general P and S wave fields in arbitrary viscoelastic solids as a function of wave inhomogeneity and intrinsic material absorption. The expressions show that an increase in inhomogeneity of the wave fields causes the velocity to decrease, the fractional-energy loss ( Q −1 ) to increase, the deviation of maximum energy flow with respect to phase propagation to increase, and the elliptical particle motions for P and type-I S waves to approach circularity. Q −1 for inhomogeneous type-I S waves is shown to be greater than that for type-II S waves, with the deviation first increasing then decreasing with inhomogeneity. The mean energy densities (kinetic, potential, and total), the mean rate of energy dissipation, the mean energy flux, and Q −1 for inhomogeneous waves are shown to be greater than corresponding characteristics for homogeneous waves, with the deviations increasing as the inhomogeneity is increased for waves of fixed maximum displacement amplitude. For inhomogeneous wave fields in low-loss solids, only the tilt of the particle motion ellipse for P and type-I S waves is independent to first order of the degree of inhomogeneity. Quantitative estimates for the characteristics of inhomogeneous plane body waves in layered low-loss solids are derived and guidelines established for estimating the effect of inhomogeneity on seismic body waves and a Rayleigh-type surface wave in low-loss media.

Bulletin of the Seismological Society of America↗
USGS1985

A general earthquake-observation system (GEOS)

Microprocessor technology has permitted the development of a General Earthquake-Observation System (GEOS) useful for most seismic applications. Central-processing-unit control via robust software of system functions that are isolated on hardware modules permits field adaptability of the system to a wide variety of active and passive seismic experiments and straightforward modification for incorporation of improvements in technology. Various laboratory tests and numerous deployments of a set of the systems in the field have confirmed design goals, including: wide linear dynamic range (16 bit/96 dB); broad bandwidth (36 hr to 600 Hz; greater than 36 hr available); selectable sensor-type (accelerometer, seismometer, dilatometer); selectable channels (1 to 6); selectable record mode (continuous, preset, trigger); large data capacity (1. 4 to 60 Mbytes); selectable time standard (WWVB, master, manual); automatic self-calibration; simple field operation; full capability to adapt system in the field to a wide variety of experiments; low power; portability; and modest costs. System design goals for a microcomputer-controlled system with modular software and hardware components as implemented on the GEOS are presented. The systems have been deployed for 15 experiments, including: studies of near-source strong motion; high-frequency microearthquakes; crustal structure; down-hole wave propagation; teleseismicity; and earth-tidal strains.

Bulletin of the Seismological Society of America↗
USGS1988

The northeastern Ohio earthquake of 31 January 1986: Was it induced?

On 31 January 1986, at 11:46 EST, an earthquake of mb = 5.0 occurred about 40 km east of Cleveland, Ohio, and about 17 km south of the Perry Nuclear Power Plant. The earthquake was felt over a broad area, including 11 states, the District of Columbia, and parts of Ontario, Canada, caused intensity VI-VII at distances of 15 km, and generated relatively high accelerations (0.18 g) of short duration at the Perry plant. Thirteen aftershocks were detected as of 15 April, with six occurring within the first 8 days. Two of the aftershocks were felt. Magnitudes for the aftershocks ranged from about 0.5 to 2.5. Focal depths for all of the earthquakes ranged from 2 to 6 km. Except for one small earthquake, all of the aftershocks occurred in a very tight cluster with a north-northeast orientation. Focal mechanisms of the aftershocks exhibit predominantly oblique right-slip motion on nearly vertical nodal planes oriented N15° to 45°E, with a nearly horizontal P axis north of east.

Ohio↗
USGS1988

Geometry of the Juan de Fuca plate beneath Washington and northern Oregon from seismicity

Earthquake hypocenters within the subducting Juan de Fuca plate beneath Washington and northern Oregon are interpreted as showing that the direction of plate dip changes from northeast beneath the Puget Sound region to east-southeast beneath southwestern Washington. The shallowest hypocenters within the Juan de Fuca plate are between 30- to 40-km depth, and the distribution of these events strikes north-northeast from near the mouth of the Columbia River to the northern Olympic Mountains. The distribution of hypocenters between 40 to 50 km generally strikes parallel with the shallowest events, but shows a significant broadening beneath the eastern Olympic Mountains and Puget Sound. Events with depths greater than 50 km south of the 1965 Seattle earthquake (mb = 6.5) strike north-northeast, approximately parallel with the shallower distributions; however, north of this event, the distribution of these deeper hypocenters strikes northwest. This change in the distribution of earthquake hypocenters reflects an upward arching of the Juan de Fuca plate plate beneath Puget Sound compared with the depth of the plate beneath southwestern Washington. The T axis calculated for the 1949 South Puget Sound earthquake (MS = 7.1) is oriented to the southeast, and the 20° plunge of the T axis is in good agreement with the plate dip angle determined from the earthquake hypocenters. We conclude that the 1949 earthquake resulted at least in part from down-dip tensional forces within the subducting Juan de Fuca plate. One consequence of the change in the direction of plate dip is that volcanic front in Washington is everywhere perpendicular to the dip of the Juan de Fuca plate.

Oregon, Washington↗
USGS1988

Parkfield, California, liquefaction prediction (USA)

The primary purpose of this short note is to formally record the liquefaction prediction (Holzer et al., 1986) made in connection with this predicted earthquake. In addition, this note serves to alert the seismic engineering community to special instrumentation being installed at the prediction site. The instrumentation will consist of 4 downhole accelerometers at depths ranging from 3-30 m, a surface accelerometer, 7 dynamic piezometers distributed in the sand strata between depths of 5 and 15 m, and a network of bench marks for measuring permanent ground deformation.

Califonia↗
USGS1988

Vertical seismic profiling of Oroville microearthquakes: Velocity spectra and particle motion as a function of depth

Direct evidence of site distortion of P - and S -wave microearthquake source spectra at Oroville, California, is presented. The data were gathered by placing vertical and three-component seismometers at 90 m intervals in a 500 m borehole through the Cleveland Hill normal fault, on which the 1975, M L = 5.7 Oroville earthquake took place. High-pressure, hydraulic locking mechanisms were used to firmly lock the seismometer packages against the borehole wall. Digital, event-triggering GEOS recorders were used to receive the data. Some 30 seismic events, including 12 microearthquakes, were recorded during the 4 months the seismometers were deployed. By comparing the velocity spectra of microearthquake waves at the different depths, it can be seen that scattering and attenuation in the shallow crust around the borehole dominantly affect high-frequency S waves. Above 15 Hz, the value of the apparent S -wave quality factor, Q as , for the upper 500 m of crust at this site is 9. One feature of this low Q as is a gross difference between S -wave “corner” frequencies observed at ground level and 500 m downhole. For example, the uphole and downhole S -wave corner frequencies for the M coda = 0.4 microearthquake of Julian Day 259 differ by a factor of 2 or more. Low quality factors and depth-dependent corner frequencies were also observed for P waves, but these data are less definitive due to lower signal-to-noise ratios. Based on their three-component particle motions and polarizations, the direct S waves of the microearthquakes appear to be composed of two phases, which have similar amplitudes but different apparent velocities. It is possible that the velocity differences are a result of anisotropy in the underlying rock.

California↗