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At least 865 records · Page 48Linked to original sources
USGS1988

Failure of self-similarity for large (Mw > 81/4) earthquakes

Compares teleseismic P-wave records for earthquakes in the magnitude range from 6.0-9.5 with synthetics for a self-similar, omega 2 source model and conclude that the energy radiated by very large earthquakes (Mw > 81/4) is not self-similar to that radiated from smaller earthquakes (Mw < 81/4). Furthermore, in the period band from 2 sec to several tens of seconds, it is concluded that large subduction earthquakes have an average spectral decay rate of omega -1.5. This spectral decay rate is consistent with a previously noted tendency of the omega 2 model to overestimate Ms for large earthquakes.

Bulletin of the Seismological Society of America↗
USGS1988

The seismic radiation from composite models of faulting

The failure of an asperity, i.e., the dynamic rupture of a small fault area with finite stress drop surrounded by a broken or weak fault area which has no stress drop but which slips after the asperity fails, is proposed as a model for the rupture process of a subevent in a composite earthquake. The rupture area of the composite earthquake surrounding the subevent is modeled by the weak fault area surrounding the asperity in the subevent model. The resulting seismic moment of the subevent is proportional to the stress drop and the rupture area of the subevent, as well as the radius of the composite earthaquake. By setting the stress drops of the asperity models equal to the dynamic stress drops of the subevents, the composite earthquake can be modeled as the sum of a set of subevents which cover the rupture area of the composite earthquake. The scaling of the high- and low-frequency radiation from composite earthquakes composed of asperities is commensurate with generally observed spectral scaling laws, in contrast to composite earthquakes composed of cracks, or smaller earthquakes. A simple filtering strategy is proposed for filtering the waveforms radiated by cracks to approximate the waveforms radiated by asperities. The P and S waves radiated by an ML = 5.2 earthquake which occurred on 9 May 1983, at Coalinga, California, are simulated using the P and S waves radiated by an ML = 3.6 aftershock. The aftershock waveforms are first filtered to approximate the radiation from asperities with the appropriate rupture areas, and then the waveforms of 12 asperity subevents are summed together to simulate the waveforms and spectra of the composite earthquake.

Bulletin of the Seismological Society of America↗
USGS1988

The Argos seismic data message system

A reliable, inexpensive method for sending limited daily seismic data messages from remote observatories to the National Earthquake Information Center has been developed for use with the Argos satellite system. Data messages are compressed on a microcomputer and passed automatically to a simple transmitter. About 4 hr later, the data are available at the National Earthquake Information Center, where they are decompressed and reformatted into standard telegrams for use in quick epicenter determinations. Epicenter data are available daily to the international scientific community.

Bulletin of the Seismological Society of America↗
USGS1988

Inversion for slip distribution using teleseismic P waveforms: North Palm Springs, Borah Peak, and Michoacan earthquakes

We have inverted the teleseismic P waveforms recorded by stations of the Global Digital Seismograph Network for the 8 July 1986 North Palm Springs, California, the 28 October 1983 Borah Peak, Idaho, and the 19 September 1985 Michoacan, Mexico, earthquakes to recover the distribution of slip on each of the faults using a point-by-point inversion method with smoothing and positivity constraints. In the inversion procedure, a fault plane with fixed strike and dip is placed in the region of the earthquake hypocenter and divided into a large number of subfaults. Rupture is assumed to propagate at a constant velocity away from the hypocenter, and synthetic ground motions for pure strike-slip and dip-slip dislocations are calculated at the teleseismic stations for each subfault. The observed seismograms are then inverted to obtain the distribution of strike-slip and dip-slip displacement for the earthquake.

California↗
USGS1988

Effects of three-dimensional velocity structure on the seismicity of the 1984 Morgan Hill, California, aftershock sequence

A three-dimensional velocity model for the area surrounding the 24 April 1984 Morgan Hill earthquake has been developed by simultaneously inverting local earthquake and refraction arrival-time data. This velocity model corresponds well to the surface geology of the region, predominantly showing a low-velocity region associated with the sedimentary sequence to the southwest of the Madrone Springs fault. This low-velocity sequence extends to a depth of approximately 5 km. Previously constructed geologic cross-sections for this region had suggested that these sediments extended to a depth of less than 2 km. Relocation of 2,179 hypocenters with the three-dimensional velocity model did not show great differences when compared to locations done with a one-dimensional velocity model with station corrections. In both cases, the Calaveras fault is steep but has a noticeable dip to the northeast and the seismicity does not intersect the surface along the trace of the Calaveras fault in the southern half of the rupture zone. The greatest differences between the two sets of locations was that the locations using the three-dimensional velocity model were deeper, and fewer earthquakes were located within the low-velocity region. The focal mechanisms were also determined for 946 earthquakes using both the one-dimensional and three-dimensional earth models. Both earth models yield similar focal mechanisms for these earthquakes.

California↗
USGS1988

Aftershock patterns and main shock faulting

We have compared aftershock patterns following several moderate to large earthquakes with the corresponding distributions of coseismic slip obtained from previous analyses of the recorded strong ground motion and teleseismic waveforms. Well-located aftershock hypocenters are projected onto the main shock fault plane, and their positions are examined relative to the zones of coseismic displacement indicated by the estimated distributions of main shock slip. We also examine the aftershock focal mechanisms, when these data are available, in an attempt to identify possible patterns of secondary faulting within the aftershock zone.

California↗
USGS1988

Crustal velocities near Coalinga, California, modeled from a combined earthquake/explosion refraction profile

Crustal velocity structure for the region near Coalinga, California, has been derived from both earthquake and explosion seismic phase data recorded along a NW-SE seismic-refraction profile on the western flank of the Great Valley east of the Diablo Range. Comparison of the two data sets reveals P-wave phases in common which can be correlated with changes in the velocity structure below the earthquake hypocenters. In addition, the earthquake records reveal secondary phases at station ranges of less than 20 km that could be the result of S- to P-wave conversions at velocity interfaces above the earthquake hypocenters. Two-dimensional ray-trace modeling of the P-wave travel times resulted in a P-wave velocity model for the western flank of the Great Valley comprised of: (a) a 7- to 9-km-thick section of sedimentary strata with velocities similar to those found elsewhere in the Great Valley (1.6 to 5.2 km/sec); (b) a middle crust extending to about 14 km depth with velocities comparable to those reported for the Franciscan assemblage in the Diablo Range (5.6 to 5.9 km/sec); and (c) a 13- to 14-km-thick lower crust with velocities similar to those reported beneath the Diablo Range and the Great Valley (6.5 to 7.30 km/sec). This lower crust may have been derived from subducted oceanic crust that was thickened by accretionary underplating or crustal shortening.

California↗
USGS1988

Teleseismic and near-field analysis of the Nahanni earthquakes in the Northwest Territories, Canada

The analysis of the Nahanni earthquakes of October 5, 1985 (MS 6.6), and December 23, 1985 (MS 6.9), will have important implications for the assessment of seismic hazards in intraplate environments. To maximize the information available to seismic engineers, broadband data recorded teleseismically are analyzed jointly with strong-motion data recorded in the near field. The time-domain analysis of teleseismic data yields the source mechanisms, depths, and complexities of rupture of each earthquake. Both earthquakes occurred as shallow thrusts with centroid depths (6 to 7 km) and shallowly dipping fault planes that correspond well with the aftershock distributions obtained from a local survey run by the Canadian Geological Survey. The shallow nodal plane for the October 5 earthquake dips 30° to the WSW, while the shallow nodal planes of the subevents for the December 23 earthquake dip an average of 23° to the WSW. The October 5 earthquake has an impulsive initial rupture, followed by a weak subevent of longer duration but smaller moment release. The December 23 earthquake exhibits more complexity, being comprised of three subevents of similar size. The subevent delays derived from the teleseismic analysis are used to help interpret arrivals in records of ground velocity recorded in the near field of the December 23 earthquake. The rupture geometries inferred from the joint near- and far-field analysis suggest that the rupture processes were unusually complicated and that the 2g peak that occurs late in one of the near-field records could be a localized phenomenon. Spectral analyses of the teleseismic P waves yield the following source parameters for the October 5 and December 23 earthquakes, respectively: the seismic moments are 1.2 and 1.8 × 1026 dyne-cm, the radiated energies are 1.8 and 2.8 × 1021 dyne-cm, and the dynamic stress drops are 65 and 50 bar. The acceleration source spectra of both earthquakes exhibit an intermediate slope (| üα(ω) | ∝ω) from 0.03 to 0.3 Hz, suggesting that the earthquakes represent the failure of asperities. Extrapolating the teleseismic P-wave spectra to estimate the near-field S-wave spectra yields good fits to the acceleration spectra from two strong motion records, but underestimates the spectra from a third strong motion record with the strongest, but possibly localized, accelerations.

Bulletin of the Seismological Society of America↗
USGS1988

Elastic-wave propagation and site amplification in the Salt Lake Valley, Utah, from simulated normal faulting earthquakes

The two-dimensional seismic response of the Salt Lake valley to near- and far-field earthquakes has been investigated from simulations of vertically incident plane waves and from normal-faulting earthquakes generated on the basin-bounding Wasatch fault. The response to normal faulting earthquakes was simulated using a two-dimensional finite-element method and the plane-wave response was calculated from two-dimensional finite-difference simulations. The plane-wave simulations were then compared with observed site amplifications in the Salt Lake valley, based on seismic recordings from nuclear explosions in southern Nevada, that show 10 times greater amplification within the basin than measured values on hard-rock sites. While previous studies attribute this increased site amplification to the near-surface unconsolidated/consolidated alluvial fill contact, our synthetic seismograms suggest that in the frequency band 0.3 to 1.5 Hz at least one-half the site amplification can be attributed to the impedance contrast between the basin sediments and higher velocity basement rocks. Synthetic seismograms from vertically incident plane-wave sources and buried double-couple sources predict large amplitude Rayleigh-wave propagation from the edges of the basin and, in general, uniform site amplification. In contrast, near-field simulations of basin-bounding, normal-faulting earthquakes predict large-amplitude Rayleigh waves propagating westward from the fault across the basin. Spectra of synthetic accelerograms computed from the normal-faulting earthquakes shows that spectral amplification within the basin is primarily due to source directivity with a maxima near the surface projection of the fault that decays rapidly away from the fault. Importantly, the synthetic modeling of near-field earthquake sources show that near-field directivity effects are important and should be considered in an earthquake hazard assessment of the Salt Lake valley and similar geologic settings along the Wasatch Front.

Utah↗
USGS1989

Surface faulting along the Superstition Hills fault zone and nearby faults associated with the earthquakes of 24 November 1987

The M 6.2 Elmore Desert Ranch earthquake of 24 November 1987 was associated spatially and probably temporally with left-lateral surface rupture on many northeast-trending faults in and near the Superstition Hills in western Imperial Valley. Three curving discontinuous principal zones of rupture among these breaks extended northeastward from near the Superstition Hills fault zone as far as 9 km; the maximum observed surface slip, 12.5 cm, was on the northern of the three, the Elmore Ranch fault, at a point near the epicenter. Twelve hours after the Elmore Ranch earthquake, the M 6.6 Superstition Hills earthquake occurred near the northwest end of the right-lateral Superstition Hills fault zone. Surface rupture associated with the second event occurred along three strands of the zone, here named North and South strands of the Superstition Hills fault and the Wienert fault, for 27 km southeastward from the epicenter. In contrast to the left-lateral faulting, which remained unchanged throughout the period of investigation, the right-lateral movement on the Superstition hills fault zone continued to increase with time, a behavior that was similar to other recent historical surface ruptures on northwest-trending faults in the Imperial Valley region.

California↗
USGS1989

Three-dimensional records of surface displacement on the Superstition Hills fault zone associated with the earthquakes of 24 November 1987

Seven quadrilaterals, constructed at broadly distributed points on surface breaks within the Superstition Hills fault zone, were repeatedly remeasured after the pair of 24 November 1987 earthquakes to monitor the growing surface displacement. Changes in the dimensions of the quadrilaterals are recalculated to right-lateral and extensional components at millimeter resolution, and vertical components of change are resolved at 0.2 mm precision. The displacement component data for four of the seven quadrilaterals record the complete fault movement with respect to an October 1986 base. These data fit with remarkable agreement the power law

California↗
USGS1989

Inverting measurements of surface slip on the Superstition Hills fault

We derive and test a set of inversions of surface-slip measurements based on the empirical relation u(t) = uf/(1 + T/t)c proposed by Sharp and Saxton (1989) to estimate the final slip uf, the power-law exponent c, and the power-law duration T. At short times, Sharp's relation behaves like the simple power law, u(t) ∼ u1tc, where u1 is the initial slip, that is, the slip at 1 day after the earthquake. At long times, the slip approaches the final slip asymptotically. The inversions are designed in part to exploit the accuracy of measurements of differential slip; that is, measurements of surface slip which are made relative to a set of nails or stakes emplaced after the earthquake. We apply the inversions to slip measurements made at 53 sites along the Superstition Hills fault for the 11 months following the M = 6.2 and 6.6 earthquakes of 24 November 1987. In general, estimates of the initial slip and the power law exponent are well resolved, while estimates of the power-law durations and the final slip are less well resolved because the durations of the surface slip measurements are often less than the derived power-law durations. The slip on the three fault strands is a relatively smooth function of position; the initial slip and final slip are well correlated. The time dependence of surface slip at the 53 sites is roughly similar along the entire fault, where the power-law exponents are distributed as c = 0.14 ± 0.04 and the power-law durations range from 100 < T < 1000 days.

California↗
USGS1989

Pre-earthquake displacement and triggered displacement on the Imperial fault associated with the Superstition Hills earthquake of 24 November 1987

Two right-lateral slip events, about 3 weeks apart in November 1987, broke the surface discontinuously along probably similar, nearly 20 km lengths of the northern Imperial fault. The first displacement, at about the beginning of November, was accompanied by a surface tilt representing deep vertical motion or distributed strain. This movement may have been part of a more regional event that also involved the southern San Andreas fault, although the evidence there is questionable. The later surface offset was triggered, probably by the second main shock of the 24 November earthquakes located in the Superstition Hills, about 37 km northwest of the Imperial fault. The maximum observed displacement was less than 4 cm on both occasions; for the triggered movement the maximum slip occurred on a branch strand near the northern extremity of the fault.

California↗
USGS1989

Moment-tensor solutions for the 24 November 1987 Superstition Hills, California, earthquakes

The teleseismic long-period waveforms recorded by the Global Digital Seismograph Network from the two largest Superstition Hills earthquakes are inverted using an algorithm based on optimal filter theory. These solutions differ slightly from those published in the Preliminary Determination of Epicenters Monthly Listing because a somewhat different, improved data set was used in the inversions and a time-dependent moment-tensor algorithm was used to investigate the complexity of the main shock. The foreshock (origin time 01:54:14.5, mb 5.7, Ms 6.2) had a scalar moment of 2.3 × 1025 dyne-cm, a depth of 8 km, and a mechanism of strike 217°, dip 79°, rake 4°. The main shock (origin time 13:15:56.4, mb 6.0, Ms 6.6) was a complex event, consisting of at least two subevents, with a combined scalar moment of 1.0 × 1026 dyne-cm, a depth of 10 km, and a mechanism of strike 303°, dip 89°, rake −180°.

California↗
USGS1989

Rupture process of the Ms 6.6 Superstition Hills, California, earthquake determined from strong-motion recordings: application of tomographic source inversion

We analyze strong-motion recordings of the M s 6.6 Superstition Hills earthquake to determine the timing, location, spatial extent, and rupture velocity of the subevents that produced the bulk of the high-frequency (0.5 to 4 Hz) seismic energy radiated by this shock. The earthquake can be characterized by three principal subevents, the largest ones occurring about 3 and 10 sec after initiation of rupture. Timing relationships between pulses on the seismograms indicate that the three subevents are located within 8 km of each other along the northern portion of the Superstition Hills fault. The two largest subevents display different directivity effects. We apply a tomographic source inversion to the integrated accelerograms to determine the slip acceleration on the fault as a function of time and distance, based on a one-dimensional fault model. The azimuthal distribution of amplitudes for the second subevent can be largely explained by a rupture that propagated about 2 km to the southeast along the Superstition Hills fault at a velocity about equal to the P -wave velocity. An alternative model with rupture propagating to the northeast along a conjugate fault plane can also account for the observed directivity of this subevent, but it is not supported by the aftershock distribution. The third subevent ruptured to the southeast along an 8-km long portion of the Superstition Hills fault at about the shear-wave velocity. This rupture propagation caused the relatively large accelerations and velocities observed in strong-motion records for stations southeast of the hypocenter. The long time intervals between the subevents and their relative proximity to each other indicate a very slow component to the rupture development. The southern half of the Superstition Hills fault did not generate significant high-frequency strong ground motion, although it showed substantial co-seismic surface displacement. The subevents are situated along the same northern portion of the fault where most of the aftershocks are located. The locations of the subevents appear to be controlled by bends in the fault mapped at the surface and by changes in basement structure at depth.

California↗
USGS1989

Microearthquake spectra from the Anza, California, seismic network: Site response and source scaling

We analyzed spectra of local microearthquakes recorded by the Anza, California, seismic network to isolate the effects of site response and to investigate the scaling of source parameters for small earthquakes. Spectra of microearthquakes (M < 2; Mo < 1019 dyne-cm) at Anza have shapes characteristic of the receiver sites and are generally independent of the source region. Thus, the site response is a major conditioner of the observed spectral shape. To remove the effects of site response from the spectra of a M ∼ 3 event and isolate its source spectrum, we divided by the spectra of an adjacent aftershock used as an empirical Green's function event. The spectral ratios indicate that the apparent corner frequencies of small earthquakes (Mo < 1019dyne-cm) observed at even the high-fmax stations on hard rock are much lower than the source corner frequencies. The spectral ratios are consistent with stress drop remaining constant with decreasing seismic moment, for events with moments as small as 1018 dyne-cm. The spectral ratios display remarkable agreement between sites which showed vast differences in their original spectra, indicating that the spectral division effectively removed the site response. The source spectrum of the M ∼ 3 event has a high-frequency spectral fall-off of about ω−2. An apparent dependence of high-frequency fall-off with seismic moment in the original spectra can also be explained by the effects of site response. The difference between the P- and S-wave corner frequencies and high-frequency roll-offs in the observed spectra for these events is the result of the site response and is not a source property. The shapes of the spectra of microearthquakes at Anza can largely be explained by attenuation at shallow depth with a frequency-independent Q. For some sites, near-surface resonances are also apparent in the spectra of microearthquakes. It is indicated by t* values determined for each site that Qp ∼ Qs for the shallow low-Q layer. Further evidence of low near-surface Q is observed in the anomalous spectra of an unusually shallow earthquake (source depth ∼ 1 km) in the network. The spectra from this shallow event are depleted in high-frequency energy at most stations, relative to those of deeper events. This observation can be explained by a low-Q surficial zone. For stations of the network situated on alluvium, this low-Q layer has a maximum thickness of about 3 km and maximum P- and S-wave Q values of 30 to 50.

California↗
USGS1989

An exact anelastic model for the free-surface reflection of P and S-I waves

Exact anelastic solutions incorporating inhomogeneous waves are used to model numerically S -I and P waves incident on the free surface of a low-loss anelastic half-space. Anelastic free-surface reflection coefficients are computed for the volumetric strain and displacement components of inhomogeneous wave fields. For the problem of an incident homogeneous S -I wave in Pierre shale, the largest strain and displacement amplitudes for the reflected P wave occur at angles of incidence for which the particle motion for the reflected inhomogeneous P wave is elliptical (minor/major axis = 0.6), the specific absorption ( Q P −1 ) is greater (300 per cent) and the velocity is less (25 per cent) than those for a corresponding homogeneous P wave, the direction of phase propagation is not parallel to the free surface, and the amplitude of the wave shows a significant increase with depth (6 per cent in one wavelength). Energy reflection coefficients computed for this low-loss anelastic model show that energy flow due to interaction of the incident and reflected waves reach maxima (30 per cent of the incident energy) near large but nongrazing angles of incidence. For the problem of an incident homogeneous P wave in Pierre shale, the inhomogeneity of the reflected S wave is shown not to contribute to significant variations in wave field characteristics over those that would be expected for a homogeneous wave.

Bulletin of the Seismological Society of America↗