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At least 865 records · Page 48Linked to original sources
USGS1994

Routine estimation of earthquake source complexity: The 18 October 1992 Colombian earthquake

We describe two methods, suitable for routine application to teleseismic recordings, that characterize the time history of seismic events. Stacking short-period signals from large regional arrays provides stable estimates of high-frequency radiation from the source, and an empirical Green's function deconvolution procedure extracts reliable, broadband time functions suitable for analysis of faulting complexity and the spatio-temporal extent of rupture. Combined, these procedures characterize the source radiation of large events ( M s > 7) between 200- and 0.5-sec periods.

Bulletin of the Seismological Society of America↗
USGS1995

Strain accumulation in Owens Valley, California, 1974 to 1988

Strain accumulation observed over the 1974 to 1988 interval in a 25 by 100 km aperture trilateration network spanning Owens Valley is adequately described by a strain rate that is uniform in space and time. The tensor strain-rate components referred to a coordinate system with the 2 axis directed N18°W (parallel to the trend of the valley) and the 1 axis N72°E are ∈˙11′ = 0.042 ± 0.014 μstrain/yr, ∈˙12′ = -0.058 ± 0.007 μstrain/yr, and ∈˙22′ = 0.002 ± 0.014 μstrain/yr; quoted uncertainties are standard deviations and extension is reckoned positive. Across the 25-km breadth of the network, this amounts to 1.0 ± 0.3 mm/yr extension normal to the axis of the valley, 2.9 ± 0.4 mm/yr right-lateral shear across the axis, and no extension parallel to the axis. If the measured strain accumulation is attributed to slip on the deeper section of the Owens Valley fault with the uppermost 10 km of the fault locked, the observed right-lateral deformation would imply about 7 mm/yr right-lateral slip on the buried fault, much greater than the geologic estimate of 2 ± 0.5 mm/yr right-lateral secular slip (Beanland and Clark, 1994). Nor is the observed uplift profile across the valley consistent with continuing normal slip on just the deep segment of the Owens Valley fault; normal slip at depth on the Sierra frontal fault also seems to be required. The observed deformation across Owens Valley apparently implies processes more complicated than those represented by the conventional model of strain accumulation along a throughgoing fault.

California↗
USGS1995

Near real-time monitoring of seismic events and status of portable digital recorders using satellite telemetry

Near real-time monitoring of seismic events and status of portable 16-bit digital recorders has been established for arrays near Parkfield, Mammoth Lakes, and San Francisco, California. This monitoring system provides near real-time seismic event identification (rough location and magnitude) and a cost-effective means to maintain arrays at near 100% operational level. Principal objectives in the design of this system have been portability and low-cost telemetry. The system has been developed to use portable digital seismic recorders ( GEOS —General Earthquake Observation System) and portable data collection platforms ( DCP 's) for the Geostationary Operational Environmental Satellite (GEOS) telemetry system. Data are transferred asynchronously from the GEOS seismic system through a microprocessor-controlled interface every 10 min. The interface stores, determines priority, converts, and synchronously transfers these data to a Sutron Corp. model 8004 DCP for transmission through the GEOS satellite telemetry system. Event parameters include trigger time, peak amplitude, time of peak amplitude, and event duration. Instrument configuration parameters, transmitted at system start-up time and every 24 hr, include recording parameters, trigger parameters, GEOS software version, clock reference, and location parameter. Instrument status includes battery voltage, number of events, and percentage of tape usage. These data are transmitted as appropriate to the U.S. Geological Survey satellite downlink and computers located in Menlo Park, California, where they are processed and displayed.

Bulletin of the Seismological Society of America↗
USGS1995

Ribbon Cliff landslide Washington, and the earthquake of 14 December 1872

Estimates of the epicentral location and maximum intensity of the earthquake of 14 December 1872, the largest and oldest historic earthquake documented in the Pacific Northwest, are controversial largely because the estimates are based on ground effects. The Ribbon Cliff landslide is one of the more critical ground effects used to argue that the epicenter was in the vicinity of Lake Chelan in central Washington. Sketchy historical accounts link the Ribbon Cliff landslide to the 1872 earthquake, but a subsequent study disputed the historical accounts and, on the basis of dendrochronology, concluded that the landslide occurred more than 100 yr prior to the earthquake. However, Quaternary stratigraphic relations and the results of multiple dating techniques reported here indicate that the main Ribbon Cliff landslide probably occurred within a 14-yr period that includes the time of the 1872 earthquake. Although our study supports the historical accounts that link the landslide to the December 1872 earthquake, it does not prove that seismic shaking triggered the landslide.

Washington↗
USGS1996

Low-velocity fault-zone guided waves: Numerical investigations of trapping efficiency

Recent observations have shown that shear waves trapped within low-velocity fault zones may be the most sensitive measure of fault-zone structure (Li et al. , 1994a, 1994b). Finite-difference simulations demonstrate the effects of several types of complexity on observations of fault-zone trapped waves. Overlying sediments with a thickness more than one or two fault-zone widths and fault-zone step-overs more than one or two fault widths disrupt the wave guide. Fault kinks and changes in fault-zone width with depth leave readily observable trapped waves. We also demonstrate the effects of decreased trapped wave excitation with increasing hypocentral offset from the fault and the effects of varying the contrast between the velocity in the fault zone and surrounding hard rock. Careful field studies may provide dramatic improvements in our knowledge of fault-zone structure.

Bulletin of the Seismological Society of America↗
USGS1996

Site amplification at five locations in San Francisco, California: A comparison of S waves, codas, and microtremors

We compare microtremor data to weak-motion S -wave and coda recordings at sites in San Francisco in order to clarify the range of applicability of microtremor data to ground-motion prediction. We also compare S -wave results to coda results. For each type of data, we compute spectral ratios of motions from two soil/rock station pairs and from an uphole/downhole pair in the Marina district. We compute horizontal/vertical ratios (Nakamura's method) at a soil site, a rock site, and the surface and borehole instruments. In the station-pair analyses, microtremor data show amplifications at the same fundamental frequency as S waves, but the frequencies of other peaks do not agree. The amplification at frequencies higher than 2 Hz is greater in the microtremor data. Station-pair ratios of coda data generally show spectral peaks occurring at the same frequencies, but with levels varying from one to four times the amplification from S -wave ratios. Nakamura's method of analyzing microtremors agrees better with S -wave station-pair results than the microtremor station-pair method over a limited frequency band that varies from station to station.

California↗
USGS1996

The 25 March 1993 Scotts Mills, Oregon, earthquake and aftershock sequence: Spatial distribution, focal mechanisms, and the mount angel fault

The 25 March 1993 M L = 5.7 crustal earthquake near Scotts Mills, Oregon, was the largest earthquake to occur in the Pacific Northwest in over a decade. The mainshock was located at 45.033°N, 122.586°W and at a depth of about 15.1 km, based on arrival time data from the short-period Pacific Northwest Seismograph Network. Beginning about 12 h after the mainshock, investigators from the U.S. Geological Survey deployed 22 digital seismographs to record aftershocks. Using data from the temporary and permanent stations, we analyzed a subset of 50 aftershocks with quality locations. Hypocenters of these aftershocks lie on a northwest-trending steeply dipping plane (strike 290 ± 10°, dipping 60 ± 5° to the north-northeast), in agreement with the preferred slip plane of the mainshock focal mechanism solution (strike 294°, dipping 58° to the north-northeast). The planar structure defined by the aftershock locations may be a southeast continuation of the Mount Angel Fault, a reverse fault identified from both surface and subsurface evidence. The mapped southeast extent of the Mount Angel Fault is located less than 10 km west of the Scotts Mills epicentral region. In addition, the mainshock focal mechanism solution, with a combination of reverse motion and right-lateral strike slip, has a geometry and sense of motion consistent with the Mount Angel Fault. While aftershock focal mechanisms are varied, P axes are consistently oriented in a subhorizontal north-south direction. This earthquake sequence, together with the geological and geophysical evidence for the Mount Angel Fault, suggests a significant crustal earthquake hazard for this region of northwest Oregon.

Oregon↗
USGS1996

Earthquake mechanisms from linear-programming inversion of seismic-wave amplitude ratios

The amplitudes of radiated seismic waves contain far more information about earthquake source mechanisms than do first-motion polarities, but amplitudes are severely distorted by the effects of heterogeneity in the Earth. This distortion can be reduced greatly by using the ratios of amplitudes of appropriately chosen seismic phases, rather than simple amplitudes, but existing methods for inverting amplitude ratios are severely nonlinear and require computationally intensive searching methods to ensure that solutions are globally optimal. Searching methods are particularly costly if general (moment tensor) mechanisms are allowed. Efficient linear-programming methods, which do not suffer from these problems, have previously been applied to inverting polarities and wave amplitudes. We extend these methods to amplitude ratios, in which formulation on inequality constraint for an amplitude ratio takes the same mathematical form as a polarity observation. Three-component digital data for an earthquake at the Hengill-Grensdalur geothermal area in southwestern Iceland illustrate the power of the method. Polarities of P , SH , and SV waves, unusually well distributed on the focal sphere, cannot distinguish between diverse mechanisms, including a double couple. Amplitude ratios, on the other hand, clearly rule out the double-couple solution and require a large explosive isotropic component.

Bulletin of the Seismological Society of America↗
USGS1996

Self-healing slip pulses in dynamic rupture models due to velocity-dependent strength

Seismological observations of short slip duration on faults (short rise time on seismograms) during earthquakes are not consistent with conventional crack models of dynamic rupture and fault slip. In these models, the leading edge of rupture stops only when a strong region is encountered, and slip at an interior point ceases only when waves from the stopped edge of slip propagate back to that point. In contrast, some seismological evidence suggests that the duration of slip is too short for waves to propagate from the nearest edge of the ruptured surface, perhaps even if the distance used is an asperity size instead of the entire rupture dimension. What controls slip duration, if not dimensions of the fault or of asperities?

Bulletin of the Seismological Society of America↗
USGS1996

Urban seismology - Northridge aftershocks recorded by multi-scale arrays of portable digital seismographs

We deployed portable digital seismographs in the San Fernando Valley (SFV), the Los Angeles basin (LAB), and surrounding hills to record aftershocks of the 17 January 1994 Northridge California earthquake. The purpose of the deployment was to investigate factors relevant to seismic zonation in urban areas, such as site amplification, sedimentary basin effects, and the variability of ground motion over short baselines. We placed seismographs at 47 sites (not all concurrently) and recorded about 290 earthquakes with magnitudes up to 5.1 at five stations or more. We deployed widely spaced stations for profiles across the San Fernando Valley, as well as five dense arrays (apertures of 200 to 500 m) in areas of high damage, such as the collapsed Interstate 10 overpass, Sherman Oaks, and the collapsed parking garage at CalState Northridge. Aftershock data analysis indicates a correlation of site amplification with mainshock damage. We found several cases where the site amplification depended on the azimuth of the aftershock, possibly indicating focusing from basin structures. For the parking garage array, we found large ground-motion variabilities (a factor of 2) over 200-m distances for sites on the same mapped soil unit. Array analysis of the aftershock seismograms demonstrates that sizable arrivals after the direct S waves consist of surface waves traveling from the same azimuth as that of the epicenter. These surface waves increase the duration of motions and can have frequencies as high as about 4 Hz. For the events studied here, we do not observe large arrivals reflected from the southern edge of the San Fernando Valley.

California↗
USGS1996

Reflected seismic waves and their effect on strong shaking during the 1989 Loma Prieta, California, earthquake

Our data indicate that critical and postcritical reflections from crustal layers and the Moho produced increased shaking at discrete distances along the San Francisco Peninsula during the 1989 Loma Prieta earthquake. These reflections may have produced an increase in amplitude that is as much as 10 times greater than that of the direct arrival. Peak amplitude-distance patterns measured from explosive sources, synthetic seismograms, aftershocks, and the mainshock of the 1989 Loma Prieta earthquake indicate that (1) point sources, such as explosions, produce similar peak amplitude-distance relations as distributed, double-couple sources when the crustal structure is approximately a Poisson solid; (2) peak amplitudes from smaller point sources (explosions) may be scaled to those of larger sources (earthquakes); and (3) reflections caused a pattern of high amplitudes at specific distances along the San Francisco Peninsula that geographically correlates with areas of reported damage following from the Loma Prieta mainshock. Our study indicates that critical and postcritical reflections were stronger influences on the locations of strong shaking than local geology because a number of sites around the San Francisco Bay that are underlain by unconsolidated sediments experienced much less shaking than sites that were underlain by hard rock. Furthermore, some severely shaken hard-rock sites were farther from the epicenter than many of the less severely shaken “soft-sediment” sites. Models of the regional crustal structure and seismic-wave propagation paths may be useful in identifying sites around the San Francisco Bay region that are at risk due to reflected seismic energy, and this type of analysis may be useful in other seismically active regions.

California↗
USGS1996

Application of ground-penetrating radar to investigation of near-surface fault properties in the San Francisco Bay region

In many geologic environments, ground-penetrating radar (GPR) provides high-resolution images of near-surface Earth structure. GPR data collection is nondestructive and very economical. The scale of features detected by GPR lies between those imaged by high-resolution seismic reflection surveys and those exposed in trenches and is therefore potentially complementary to traditional techniques for fault location and mapping. Sixty-two GPR profiles were collected at 12 sites in the San Francisco Bay region. Results show that GPR data correlate with large-scale features in existing trench observations, can be used to locate faults where they are buried or where their positions are not well known, and can identify previously unknown fault segments. The best data acquired were on a profile across the San Andreas fault, traversing Pleistocene terrace deposits south of Olema in Marin County; this profile shows a complicated multi-branched fault system from the ground surface down to about 40 m, the maximum depth for which data were recorded.

California↗
USGS1996

Shallow velocity structure and Poisson's ratio at the Tarzana, California, strong-motion accelerometer site

The 17 January 1994, Northridge, California, earthquake produced strong ground shaking at the Cedar Hills Nursery (referred to here as the Tarzana site) within the city of Tarzana, California, approximately 6 km from the epicenter of the mainshock. Although the Tarzana site is on a hill and is a rock site, accelerations of approximately 1.78 g horizontally and 1.2 g vertically at the Tarzana site are among the highest ever instrumentally recorded for an earthquake. To investigate possible site effects at the Tarzana site, we used explosive-source seismic refraction data to determine the shallow (<70 m) P -and S -wave velocity structure. Our seismic velocity models for the Tarzana site indicate that the local velocity structure may have contributed significantly to the observed shaking. P -wave velocities range from 0.9 to 1.65 km/sec, and S -wave velocities range from 0.20 and 0.6 km/sec for the upper 70 m. We also found evidence for a local S -wave low-velocity zone ( LVZ ) beneath the top of the hill. The LVZ underlies a CDMG strong-motion recording site at depths between 25 and 60 m below ground surface ( BGS ). Our velocity model is consistent with the near-surface (<30 m) P - and S -wave velocities and Poisson's ratios measured in a nearby (<30 m) borehole. High Poisson's ratios (0.477 to 0.494) and S -wave attenuation within the LVZ suggest that the LVZ may be composed of highly saturated shales of the Modelo Formation. Because the lateral dimensions of the LVZ approximately correspond to the areas of strongest shaking, we suggest that the highly saturated zone may have contributed to localized strong shaking. Rock sites are generally considered to be ideal locations for site response in urban areas; however, localized, highly saturated rock sites may be a hazard in urban areas that requires further investigation.

California↗
USGS1996

On the coherence of ground motion in the San Fernando valley

We present an analysis of the coherence of seismic ground motion recorded on alluvial sediments in the San Fernando Valley, California. Using aftershocks of the 17 January 1994 M w 6.7 earthquake recorded at a quasi-dense array of portable stations, we analyze the coherence of three well-recorded magnitude 3.7 to 4.0 events over the frequency range 0.5 to 15 Hz and a distance range of 0.5 to 5.3 km. All stations are located at sites with broadly similar near-site geology, characterized by medium to fine-grain Quaternary alluvial sediments. On average, relatively high values of coherence are observed for distances up to 3 to 4 km and frequencies up to 2 to 3 Hz; coherence drops sharply at frequencies near and above 3 Hz. Although average coherence functions are described reasonably well by a log-linear relationship with frequency, the curves at all distances exhibit a flattening at low frequencies that is not consistent with previous observations of coherence at hardrock sites. The distance decay of coherence is also markedly less strong, with high coherence values observed over station separations corresponding to multiple wavelengths. This may reflect fundamental differences in shallow-wave propagation in the two environments, with high-frequency scattering relatively more dominant in regions of hard-rock near-surface geology. Within a sedimentary basin or valley, the site response itself generally reflects a resonance phenomenon that may tend to give rise to more uniform ground motions. However, previous studies have demonstrated the existence of pathological focusing and amplification effects within complex sedimentary basin environments such as the greater Los Angeles region; our results undoubtedly do not quantify the full range of ground-motion variability at all sites, but rather represent the level of that variability that can be expected, and quantified, for typical source/receiver paths.

California↗
USGS1996

Rare, large earthquakes at the laramide deformation front - Colorado (1882) and Wyoming (1984)

The largest historical earthquake known in Colorado occurred on 7 November 1882. Knowledge of its size, location, and specific tectonic environment is important for the design of critical structures in the rapidly growing region of the Southern Rocky Mountains. More than one century later, on 18 October 1984, an m b 5.3 earthquake occurred in the Laramie Mountains, Wyoming. By studying the 1984 earthquake, we are able to provide constraints on the location and size of the 1882 earthquake. Analysis of broadband seismic data shows the 1984 mainshock to have nucleated at a depth of 27.5 ± 1.0 km and to have ruptured ‖ 2.7 km updip, with a corresponding average displacement of about 48 cm and average stress drop of about 180 bars. This high stress drop may explain why the earthquake was felt over an area about 3.5 times that expected for a shallow earthquake of the same magnitude in this region. A microearthquake survey shows aftershocks to be just above the mainshock's rupture, mostly in a volume measuring 3 to 4 km across. Focal mechanisms for the mainshock and aftershocks have NE-SW-trending T axes, a feature shared by most earthquakes in western Colorado and by the induced Denver earthquakes of 1967.

Colorado, Wyoming↗
USGS1996

Large-amplitude moho reflections (SmS) from Landers aftershocks, Southern California

Closely spaced aftershocks of the 28 June 1992 Landers earthquake ( M w 7.3) were used to make event record sections that show the transverse components of S and SmS arrivals at a distance of 70 to 170 km. For the data recorded toward the north in the Mojave desert, large SmS phases are observed with amplitudes 2 to 5 times greater than the direct S . For similar distances to the south, the SmS arrival is comparable to or smaller than the S . Comparisons to synthetic seismograms indicate that the large-amplitude SmS phases are produced by the simple crustal structure of the Mojave desert that allows a large Moho reflection. In contrast, the more complex geologic structure to the south partitions the seismic energy into a more complicated set of seismic phases, so that the Moho reflection is diminished in amplitude. The large SmS phases observed in the Mojave enhance the overall ground motions by a factor of 2 to 3. This suggests that when damaging earthquakes occur in other regions of simple crustal structures, Moho reflections will produce amplified strong motions at distance ranges around 100 km depending on the local structure.

California↗
USGS1996

Tectonics and seismicity of the southern Washington Cascade range

Geophysical, geological, and seismicity data are combined to develop a transpressional strain model for the southern Washington Cascades region. We use this model to explain oblique fold and fault systems, transverse faults, and a linear seismic zone just west of Mt. Rainier known as the western Rainier zone. We also attempt to explain a concentration of earthquakes that connects the northwest-trending Mount St. Helens seismic zone to the north-trending western Rainier zone. Our tectonic model illustrates the pervasive effects of accretionary processes, combined with subsequent transpressive forces generated by oblique subduction, on Eocene to present crustal processes, such as seismicity and volcanism.

Washington↗
USGS1996

Earthquake site response in Santa Cruz, California

Aftershocks of the 1989 Loma Prieta, California, earthquake are used to estimate site response in a 12-km 2 area centered on downtown Santa Cruz. A total of 258 S -wave records from 36 aftershocks recorded at 33 sites are used in a linear inversion for site-response spectra. The inversion scheme takes advantage of the redundancy of the large data set for which several aftershocks are recorded at each site. The scheme decomposes the observed spectra into source, path, and site terms. The path term is specified before the inversion. The undetermined degree of freedom in the decomposition into source and site spectra is removed by specifying the site-response factor to be approximately 1.0 at two sites on crystalline bedrock. The S -wave site responses correlate well with the surficial geology and observed damage pattern of the mainshock. The site-response spectra of the floodplain sites, which include the heavily damaged downtown area, exhibit significant peaks. The largest peaks are between 1 and 4 Hz. Five floodplain sites have amplification factors of 10 or greater. Most of the floodplain site-response spectra also have a smaller secondary peak between 6 and 8 Hz. Residential areas built on marine terraces above the floodplain experienced much less severe damage. Site-response spectra for these areas also have their largest peaks between 1 and 4 Hz, but the amplification is generally below 6. Several of these sites also have a secondary peak between 6 and 8 Hz. The response peaks seen at nearly all sites between 1 and 4 Hz are probably caused by the natural resonance of the sedimentary rock column. The higher amplifications at floodplain sites may be caused by surface waves generated at the basin margins. The secondary peak between 6 and 8 Hz at many sites may be a harmonic of the 1- to 4-Hz peaks.

California↗