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At least 883 records · Page 49Linked to original sources
USGS1996

Monte Carlo simulation of peak-acceleration attenuation using a finite-fault uniform-patch model including isochrone and extremal characteristics

A finite-fault statistical model of the earthquake source is used to confirm observed magnitude and distance saturation scaling in a large peak-acceleration data set. This model allows us to determine the form of peak-acceleration attenuation curves without a priori assumptions about their shape or scaling properties. The source is composed of patches having uniform size and statistical properties. The primary source parameters are the patch peak-acceleration distribution mean, the distribution standard deviation, the patch size, and patch-rupture duration. Although our model assumes no scaling of peak acceleration with magnitude at the patch, the peak-acceleration attenuation curves, nevertheless, strongly scale with magnitude ( da p /dM ) ≠ 0, and the scaling is distance dependent ( da p /dM ) ∝ f ( r ).

Bulletin of the Seismological Society of America↗
USGS1996

The accuracy of seismic estimates of dynamic strains: an evaluation using strainmeter and seismometer data from Piñon Flat Observatory, California

The dynamic strains associated with seismic waves may play a significant role in earthquake triggering, hydrological and magmatic changes, earthquake damage, and ground failure. We determine how accurately dynamic strains may be estimated from seismometer data and elastic-wave theory by comparing such estimated strains with strains measured on a three-component long-base strainmeter system at Piñon Flat, California. We quantify the uncertainties and errors through cross-spectral analysis of data from three regional earthquakes (the M 0 = 4 × 10 17 N-m St. George, Utah; M 0 = 4 × 10 17 N-m Little Skull Mountain, Nevada; and M 0 = 1 × 10 19 N-m Northridge, California, events at distances of 470, 345, and 206 km, respectively). Our analysis indicates that in most cases the phase of the estimated strain matches that of the observed strain quite well (to within the uncertainties, which are about ±0.1 to ±0.2 cycles). However, the amplitudes are often systematically off, at levels exceeding the uncertainties (about 20%); in one case, the predicted strain amplitudes are nearly twice those observed. We also observe significant ɛ φφ strains ( φ = tangential direction), which should be zero theoretically; in the worst case, the rms ɛ φφ strain exceeds the other nonzero components. These nonzero ɛ φφ strains cannot be caused by deviations of the surface-wave propagation paths from the expected azimuth or by departures from the plane-wave approximation. We believe that distortion of the strain field by topography or material heterogeneities give rise to these complexities.

California↗
USGS1996

Site response for urban Los Angeles using aftershocks of the Northridge earthquake

Ground-motion records from aftershocks of the 1994 Northridge earthquake are used to estimate site response in the urban Los Angeles area. Over 1300 shear-wave records from 61 sources and 90 sites are used in a linear inversion for source and site-response spectra. The methodology makes no assumptions about the shape of the source spectrum. To obtain a stable unique inverse, a Q model and geometrical spreading factor are assumed. In addition, the site response at a hard-rock site is constrained to be approximately 1.0 with a kappa of 0.02. The site-response spectra compare favorably with the results of previous and on-going investigations in Los Angeles. A couple of first-order effects are lower site response in the surrounding mountains, dominated by Mesozoic and Tertiary rocks, and higher values in the San Fernando and Los Angeles Basins, containing surficial Pleistocene and Holocene alluvial deposits. Results show good correlation of high site-response spectral values with localized areas of severe damage (Interstate 10 collapse, Sherman Oaks, Northridge, Interstate 5/14 collapse). However, widespread trends in site response across the sedimentary basins are not obvious. The data suggest that site responses are lower near the southern margin of the San Fernando Valley for sources to the north, due to north-dipping sedimentary structures. But the general pattern of site response is characterized by high variability on length scales less than a kilometer. Variations of a factor of 2 in site response are observed over the length scale of 200 m and for the same surficial geologic unit. For some of the alluvial basin sites, surface-wave generation is a significant contributor to elevated site response at lower frequencies, below 2 Hz. The total damage pattern for the Northridge earthquake is influenced by strong source directivity to the north and strong local site effects. The correlation of weak-motion site-response estimates with areas of significant damage demonstrates the value of these field measurements in future urban planning and in the reduction of seismic risk in urban areas.

California↗
USGS1996

Directional topographic site response at Tarzana observed in aftershocks of the 1994 Northridge, California, earthquake: Implications for mainshock motions

The Northridge earthquake caused 1.78 g acceleration in the east-west direction at a site in Tarzana, California, located about 6 km south of the mainshock epicenter. The accelerograph was located atop a hill about 15-m high, 500-m long, and 130-m wide, striking about N78°E. During the aftershock sequence, a temporary array of 21 three-component geophones was deployed in six radial lines centered on the accelerograph, with an average sensor spacing of 35 m. Station C00 was located about 2 m from the accelerograph. We inverted aftershock spectra to obtain average relative site response at each station as a function of direction of ground motion. We identified a 3.2-Hz resonance that is a transverse oscillation of the hill (a directional topographic effect). The top/base amplification ratio at 3.2 Hz is about 4.5 for horizontal ground motions oriented approximately perpendicular to the long axis of the hill and about 2 for motions parallel to the hill. This resonance is seen most strongly within 50 m of C00. Other resonant frequencies were also observed. A strong lateral variation in attenuation, probably associated with a fault, caused substantially lower motion at frequencies above 6 Hz at the east end of the hill. There may be some additional scattered waves associated with the fault zone and seen at both the base and top of the hill, causing particle motions (not spectral ratios) at the top of the hill to be rotated about 20° away from the direction transverse to the hill. The resonant frequency, but not the amplitude, of our observed topographic resonance agrees well with theory, even for such a low hill. Comparisons of our observations with theoretical results indicate that the 3D shape of the hill and its internal structure are important factors affecting its response. The strong transverse resonance of the hill does not account for the large east-west mainshock motions. Assuming linear soil response, mainshock east-west motions at the Tarzana accelerograph were amplified by a factor of about 2 or less compared with sites at the base of the hill. Probable variations in surficial shear-wave velocity do not account for the observed differences among mainshock acceleration observed at Tarzana and at two different sites within 2 km of Tarzana.

California↗
USGS1996

Landslides triggered by the 1994 Northridge, California, earthquake

The 17 January 1994 Northridge, California, earthquake ( M w = 6.7) triggered more than 11,000 landslides over an area of about 10,000 km 2 . Most of the landslides were concentrated in a 1000-km 2 area that included the Santa Susana Mountains and the mountains north of the Santa Clara River valley. We mapped landslides triggered by the earthquake in the field and from 1:60,000-nominal-scale aerial photography provided by the U.S. Air Force and taken the morning of the earthquake; these mapped landslides were subsequently digitized and plotted in a GIS-based format. Most of the triggered landslides were shallow (1- to 5-m thick), highly disrupted falls and slides within weakly cemented Tertiary to Pleistocene clastic sediment. Average volumes of these types of landslides were less than 1000 m 3 , but many had volumes exceeding 100,000 m 3 . The larger disrupted slides commonly had runout paths of more than 50 m, and a few traveled as far as 200 m from the bases of steep parent slopes. Deeper (>5-m thick) rotational slumps and block slides numbered in the tens to perhaps hundreds, a few of which exceeded 100,000 m 3 in volume. Most of these were reactivations of previously existing landslides. The largest single landslide triggered by the earthquake was a rotational slump/block slide having a volume of 8 × 10 6 m 3 . Analysis of the mapped landslide distribution with respect to variations in (1) landslide susceptibility and (2) strong shaking recorded by hundreds of instruments will form the basis of a seismic landslide hazard analysis of the Los Angeles area.

California↗
USGS1997

Fault-zone guided waves from explosions in the San Andreas fault at Parkfield and Cienega Valley, California

Fault-zone guided waves were successfully excited by near-surface explosions in the San Andreas fault zone both at Parkfield and Cienega Valley, central California. The guided waves were observed on linear, three-component seismic arrays deployed across the fault trace. These waves were not excited by explosions located outside the fault zone. The amplitude spectra of guided waves show a maximum peak at 2 Hz at Parkfield and 3 Hz at Cienega Valley. The guided wave amplitude decays sharply with observation distance from the fault trace. The explosion-excited fault-zone guided waves are similar to those generated by earthquakes at Parkfield but have lower frequencies and travel more slowly. These observations suggest that the fault-zone wave guide has lower seismic velocities as it approaches the surface at Parkfield. We have modeled the waveforms as S waves trapped in a low-velocity wave guide sandwiched between high-velocity wall rocks, resulting in Love-type fault-zone guided waves. While the results are nonunique, the Parkfield data are adequately fit by a shallow wave guide 170 m wide with an S velocity 0.85 km/sec and an apparent Q ∼ 30 to 40. At Cienega Valley, the fault-zone wave guide appears to be about 120 m wide with an S velocity 0.7 km/sec and a Q ∼ 30.

California↗
USGS1997

Transient triggering of near and distant earthquakes

We demonstrate qualitatively that frictional instability theory provides a context for understanding how earthquakes may be triggered by transient loads associated with seismic waves from near and distance earthquakes. We assume that earthquake triggering is a stick-slip process and test two hypotheses about the effect of transients on the timing of instabilities using a simple spring-slider model and a rate- and state-dependent friction constitutive law. A critical triggering threshold is implicit in such a model formulation.

Bulletin of the Seismological Society of America↗
USGS1997

Dynamic deformations of shallow sediments in the Valley of Mexico, Part I: Three-dimensional strains and rotations recorded on a seismic array

We study the spatial variation in earthquake ground motions, or equivalently the dynamic displacement gradient field, using a novel analysis procedure borrowed from geodesy. Seismic data recorded in the Valley of Mexico by a microarray of three three-component surface accelerographs and two three-component accelerographs at depths of 30 m and 102 m constrain our estimates of the dynamic displacement gradient field (from which strains and rotations derive) for four moderate earthquakes at distances of 250 to 300 km. Our study focuses on the effects of low-velocity surface materials on the deformation. At the surface, the gradients corresponding to deformation across vertical planes dominate, and vertical-axis rotations are of similar magnitudes as strains. The greatest peak surface gradient we observed was 206 μ strain for the 14 September 1995 M W 7.5 earthquake at a distance of ∼300 km. However, much larger gradients occur across horizontal planes ( ∂u/∂z , where u is a horizontal displacement and z is depth) at some depth between 0 and 30 m. These values are about a factor 10 greater than the corresponding gradient components at the surface. ∂u/∂z for the 14 September earthquake equaled or exceeded 665 μ strain at depth. The dynamic deformations experienced in Mexico City undoubtedly have occurred before and will occur again in other densely populated areas. However, in many other regions, the sediment response will not remain linear and elastic, resulting instead in liquefaction and ground failure.

Valley of Mexico↗
USGS1997

Dynamic deformations of shallow sediments in the Valley of Mexico, Part II: Single-station estimates

We develop simple relations to estimate dynamic displacement gradients (and hence the strains and rotations) during earthquakes in the lake-bed zone of the Valley of Mexico, where the presence of low-velocity, high-water content clays in the uppermost layers cause dramatic amplification of seismic waves and large strains. The study uses results from a companion article (Bodin et al. , 1997) in which the data from an array at Roma, a lake-bed site, were analyzed to obtain displacement gradients. In this article, we find that the deformations at other lake-bed sites may differ from those at Roma by a factor of 2 to 3. More accurate estimates of the dominant components of the deformation at an individual instrumented lake-bed site may be obtained from the maximum horizontal velocity and displacement, ν max and u max , at the surface. The maximum surface strain ɛ max is related to ν max by ɛ max = ν max / C , with C ∼ 0.6 km/sec. From the analysis of data from sites equipped with surface and borehole sensors, we find that the vertical gradient of peak horizontal displacement (Δ u max /Δ z ) computed from sensors at 0 and 30 m equals ( u max ) z=0 /Δ z , Δ z = 30 m, within a factor of 1.5. This is the largest gradient component, and the latter simple relation permits its estimation from surface records alone. The observed profiles of u max versus depth suggest a larger gradient in some depth range of 10 to 20 m, in agreement with synthetic calculations presented in Bodin et al. (1997).

Valley of Mexico↗
USGS1997

Regional Lg attenuation for the continental United States

Measurements of the Fourier amplitude spectra of Lg phases recorded at high frequency (0.5 to 14.0 Hz) by broadband seismic stations are used to determine regional attenuation relationships for southern California, the Basin and Range Province, the central United States, and the northeastern United States and southeastern Canada. Fourier spectral amplitudes were measured every quarter octave from Lg phases windowed between 3.0 and 3.7 km sec −1 and recorded in the distance range of 150 to 1000 km. Attenuation at each frequency is determined by assuming a geometrical spreading exponent of 0.5 and inverting for Q and source and receiver terms. Both southern California and the Basin and Range Province are well described by low Lg Q and frequency-dependent attenuation. Lg spectral amplitudes in southern California are fit at low frequencies (0.625 to 0.875 Hz) by a constant Lg Q of 224 and by a frequency-dependent Lg Q function Q = 187 −7 +7 f 0.55(±0.03) in the frequency band 1.0 to 7.0 Hz. The Basin and Range Province is characterized by a constant Lg Q of 192 for frequencies of 0.5 to 0.875 Hz and by the frequency-dependent Lg Q function Q = 235 −11 +11 f 0.56(±0.04) in the frequency band 1.0 to 5.0 Hz. A change in frequency dependence above 5.0 Hz is possible due to contamination of the Lg window by Pn and Sn phases. Lg spectral amplitudes in the central United States are fit by a mean frequency-independent Lg Q of 1291 for frequencies of 1.5 to 7.0 Hz, while a frequency-dependent Lg Q of Q = 1052 −83 +91 ( f /1.5) 0.22(±0.06) fits the Lg spectral amplitudes for the northeastern United States and southeastern Canada over the passband 1.5 to 14.0 Hz. Attenuation measurements for these areas were restricted to frequencies >1.5 Hz due to larger microseismic noise levels at the lower frequencies.

Bulletin of the Seismological Society of America↗
USGS1997

Determination of site amplification in the Los Angeles urban area from inversion of strong-motion records

The amplification of strong ground motion at sites in the greater Los Angeles, California, region is determined using the generalized-inverse method of Andrews (1986). Site-amplification estimates are determined at 281 strong-motion sites that provided horizontal-component accelerograms from the 1971 San Fernando, 1987 Whittier Narrows, 1991 Sierra Madre, or 1994 Northridge mainshocks. The estimates are determined relative to the spectral level recorded at a single reference site. In a second inversion, a source-site interaction term is added to Andrews's (1986) model to quantify the effect selected mainshock records have on site-amplification estimates. The source-site interaction term is applied to the San Fernando Valley sites' records of the Northridge earthquake and to three Los Angeles basin sites' records of the Whittier Narrows mainshock.

California↗
USGS1997

Estimating the diminution of shear-wave amplitude with distance: Application to the Los Angeles, California, urban area

The rate of decay with distance of shear-wave amplitude, computed from 20-sec S -wave spectra, is determined from TERRA scope records of small earthquakes in the greater Los Angeles area. Piecewise log-linear interpolation functions and traditional diminution functions are used to fit spectral decay to a maximum distance of 150 km. Simultaneously, isotropic source and receiver terms are determined. Separate branches of the spectral decay function are found for two categories of source depth: greater than 10 km and less than 10 km. In the hypocentral distance range of 20 to 150 km and in the frequency range of 0.5 to 8.0 Hz, an important result of the investigation is that the horizontal-component decay rate associated with deeper-crustal sources is generally greater than that associated with shallower sources and is greater than that which is estimated using more traditional models of spectral decay with distance. The same behavior generally holds for vertical-component spectra. The variation in apparent attenuation rate with source depth should affect seismic-hazard estimates associated with the rupture of blind thrust faults in the Los Angeles basin and vicinity. The results of the inversions suggest that interpolation function representations of spectral decay are sensitive to perturbations of S -wave amplitude due to crustal reflectors, such as post-critical S -wave arrivals from mid-crustal to deep-crustal velocity interfaces.

California↗
USGS1997

One minute after: Strong-motion map, effective epicenter, and effective magnitude

This article reports the recent progress on real-time seismic monitoring in Taiwan, particularly the real-time strong-motion monitoring by the Taiwan Central Weather Bureau's telemetered seismic network ( CWBSN ), which is presently aiming at rapid reporting immediately after a large earthquake occurrence. If rapid reporting can be achieved before the arrival of the strong shaking, earthquake early warning will become possible. CWBSN has achieved the generation of the intensity map, epicenter, and magnitude within 1 min of the occurrence of a large earthquake. Both rapid reporting and early warning are principally applied to large ( M ≫ 5) events; the requirement of on-scale waveform recording prompted CWBSN in 1995 to integrate strong-motion sensors (e.g., force-balance accelerometers) into its telemetered seismic monitoring system. Time-domain recursive processing is applied to the multi-channel incoming seismic signals by a group of networked personal computers to generate the intensity map. From the isoseismal contours, an effective epicenter is immediately identified that resides in the middle of the largest (usually the 100-gal) contour curve of the intensity map. An effective magnitude is also defined that can be derived immediately from the surface area covered by the largest (usually the 100-gal) contour curve. For a large event with a finite rupture surface, the epicenter and magnitude so derived are more adequate estimates of the source location and of the strength of destruction. The effective epicenter gives the center of the damage area; it stands in contrast with the conventional epicenter location, which only gives the initial point of rupture nucleation. The effective magnitude reflects more closely the earthquake damage potential, instead of the classical magnitude definition that emphasizes the total energy release. The CWBSN has achieved in obtaining the above crucial source information well within 1 min. This time can further be reduced to better than 30 sec, as illustrated by the example in this article, showing that earthquake early warning is indeed an achievable goal. The rapid reporting and early warning information is electronically transmitted to users to allow rapid response actions, with or without further human intervention.

Bulletin of the Seismological Society of America↗
USGS1997

Surface strain accumulation and the seismic moment tensor

Although the scalar moment accumulation rate within the seismogenic zone beneath a given area is sometimes deduced from the observed average surface strain accumulation rate over that same area (e.g., Working Group on California Earthquake Probabilities, 1995), the correspondence between the two is very uncertain. The equivalence between surface strain accumulation and scalar moment accumulation is based on Kostrov's (1974) relation between the average strain rate over a volume and the moment-rate tensor for that volume. The average strain rate over the volume is replaced by the average strain rate measured at the free surface to deduce an approximate moment-rate tensor. Only in exceptional circumstances will that moment-rate tensor correspond to a double-couple mechanism, a mechanism that can be represented by a scalar moment accumulation rate. More generally, the moment tensor must be resolved into the superposition of two or more double-couple mechanisms, and that resolution can be done in many ways, each with its own scalar moment rate. Thus the resolution is not unique. This is demonstrated by deducing scalar moment accumulation rates for a GPS network that covers most of California south of San Francisco. It is shown that resolutions into different double-couple mechanisms lead to scalar moment accumulation rates differing by factors of ∼2. We suggest that the minimum scalar moment rate equivalent to principal surface strain rates ɛ 1 and ɛ 2 acting over the area A is M 0 (min) = 2 μHA Max (¦ ɛ 1 ¦, ¦ ɛ 2 ¦, ¦ ɛ 1 + ɛ 2 ¦), where μ is the rigidity and H the depth of seismogenic zone, and the function Max is equal to the largest of its arguments. Within the uncertainites of measurement, the scalar moment accumulation rate in southern California based on that approximation is in balance with the average historic seismic moment release rate so that no current earthquake deficit need be accumulating.

Bulletin of the Seismological Society of America↗
USGS1997

Variability of site response in the Los Angeles urban area

This article addresses the variability of site response in the Los Angeles area and possible structural causes for the observations. Aftershock records from 231 sites in the San Fernando and Los Angeles basins and the surrounding mountains are used in this study. Spectral ratios, taken with respect to a low-amplitude reference site, are used to document the variation in site amplification in the frequency range 2 to 6 Hz, both spatially and with backazimuth to the source. At higher frequencies (6 to 10 Hz), spectral ratios are shown to have greater spatial variability. Interstation spectral ratios are used to measure the standard deviation among source as a function of station separation. An increase in the variation in ground motion is shown to take place at a station separation of 1 km. Relative site-response estimates between nearby stations are used to demonstrate that preferred directions of motion can exist even in areas with no surface topographic effects.

California↗
USGS1998

Seismicity alert probabilities at Parkfield, California, revisited

For a decade, the U.S. Geological Survey has used the Parkfield Earthquake Prediction Experiment scenario document to estimate the probability that earthquakes observed on the San Andreas fault near Parkfield will turn out to be foreshocks followed by the expected magnitude 6 mainshocks. During this time, we have learned much about the seismogenic process at Parkfield, about the long-term probability of the Parkfield mainshock, and about the estimation of these types of probabilities. The probabilities for potential foreshocks at Parkfield are reexamined and revised in light of these advances. As part of this process, we have confirmed both the rate of foreshocks before strike-slip earthquakes in the San Andreas physiographic province and the uniform distribution of foreshocks with magnitude proposed by earlier studies. Compared to the earlier assessment, these new estimates of the long-term probability of the Parkfield mainshock are lower, our estimate of the rate of background seismicity is higher, and we find that the assumption that foreshocks at Parkfield occur in a unique way is not statistically significant at the 95% confidence level. While the exact numbers vary depending on the assumptions that are made, the new alert probabilities are lower than previously estimated. Considering the various assumptions and the statistical uncertainties in the input parameters, we also compute a plausible range for the probabilities. The range is large, partly due to the extra knowledge that exists for the Parkfield segment, making us question the usefulness of these numbers.

California↗
USGS1998

Faulting along the southern margin of Reelfoot Lake, Tennessee

Approximately 320 km of deep seismic-reflection profiles in northwestern Tennessee reveal the structure of a major portion of the southeastern margin of the Reelfoot Rift. This rift margin consists of at least two major down-to-the-west late Precambrian to Cambrian normal faults. Maximum fault displacement at one location is 3 km. These two faults strike N50°E, in their northern portions; over their southern extent they trend N30°E however. Numerous faults in these reflection lines displace Paleozoic, Cretaceous, and Tertiary reflectors. The dominantly reverse faulting, folding, and positive flower structures in the shallower section indicate Eocene or younger transpression. We believe the late Tertiary faulting is due to reactivation of the basement faults, because on some of the reflection lines basement normal faults can be traced into Tertiary reverse faults, the Tertiary faults parallel the basement faults, and the Tertiary faults overlie or are adjacent to the basement faults. Numerous faults displace the highest (youngest) reflectors and therefore we do not know how recently faulting has occurred. Previous studies have identified Quaternary faulting within the southeastern Reelfoot Rift margin of western Tennessee, however. Thus, we believe all of the late Tertiary faults identified in this research should be evaluated for possible Quaternary movement.

Tennessee↗
USGS1998

Determination of Δσ and κ0 from response spectra of large earthquakes in Greece

We fit an ω −2 model to response spectra from eight recent Greek earthquakes ranging in size from M = 5.8 to M = 6.9. The diminution parameter κ 0 was determined for each site, with a value near 0.06 for a typical soil site. The stress parameter (Δσ) showed little variation from earthquake to earthquake and had a mean value of 56 bars over all earthquakes. Predictions of peak velocity, peak acceleration, rupture duration, and fault length using the derived stress parameters are consistent with observations. Frequency-dependent site amplifications were included in all estimates; the combined effect of amplification and attenuation had a maximum value close to a factor of 2.5 for a typical soil site, relative to the motions at the surface of a perfectly elastic uniform half-space composed of materials near the source. The results form the foundation for predictions of strong motions in Greece for distances and magnitudes other than those for which data are available.

Bulletin of the Seismological Society of America↗