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Results for “Bulletin of the Seismological Society of America”

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At least 793 records · Page 44Linked to original sources
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

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

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↗
USGS1998

The seismic response of the Los Angeles basin, California

Using strong-motion data recorded in the Los Angeles region from the 1992 ( M w 7.3) Landers earthquake, we have tested the accuracy of existing three-dimensional (3D) velocity models on the simulation of long-period (≧2 sec) ground motions in the Los Angeles basin and surrounding San Fernando and San Gabriel Valleys. First, the overall pattern and degree of long-period excitation of the basins were identified in the observations. Within the Los Angeles basin, the recorded amplitudes are about three to four times larger than at sites outside the basins; amplitudes within the San Fernando and San Gabriel Valleys are nearly a factor of 3 greater than surrounding bedrock sites. Then, using a 3D finite-difference numerical modeling approach, we analyzed how variations in 3D earth structure affect simulated waveforms, amplitudes, and the fit to the observed patterns of amplification. Significant differences exist in the 3D velocity models of southern California that we tested (Magistrale et al. , 1996; Graves, 1996a; Hauksson and Haase, 1997). Major differences in the models include the velocity of the assumed background models; the depth of the Los Angeles basin; and the depth, location, and geometry of smaller basins. The largest disparities in the response of the models are seen for the San Fernando Valley and the deepest portion of the Los Angeles basin. These arise in large part from variations in the structure of the basins, particularly the effective depth extent, which is mainly due to alternative assumptions about the nature of the basin sediment fill. The general ground-motion characteristics are matched by the 3D model simulations, validating the use of 3D modeling with geologically based velocity-structure models. However, significant shortcomings exist in the overall patterns of amplification and the duration of the long-period response. The successes and limitations of the models for reproducing the recorded ground motions as discussed provide the basis and direction for necessary improvements to earth structure models, whether geologically or tomographically derived. The differences in the response of the earth models tested also translate to variable success in the ability to successfully model the data and add uncertainty to estimates of the basin response given input “scenario” earthquake source models.

California↗
USGS1998

Use of fault striations and dislocation models to infer tectonic shear stress during the 1995 Hyogo-Ken Nanbu (Kobe) earthquake

Dislocation models of the 1995 Hyogo-ken Nanbu (Kobe) earthquake derived by Yoshida et al. (1996) show substantial changes in direction of slip with time at specific points on the Nojima and Rokko fault systems, as do striations we observed on exposures of the Nojima fault surface on Awaji Island. Spudich (1992) showed that the initial stress, that is, the shear traction on the fault before the earthquake origin time, can be derived at points on the fault where the slip rake rotates with time if slip velocity and stress change are known at these points. From Yoshida's slip model, we calculated dynamic stress changes on the ruptured fault surfaces. To estimate errors, we compared the slip velocities and dynamic stress changes of several published models of the earthquake. The differences between these models had an exponential distribution, not gaussian. We developed a Bayesian method to estimate the probability density function (PDF) of initial stress from the striations and from Yoshida's slip model. Striations near Toshima and Hirabayashi give initial stresses of about 13 and 7 MPa, respectively. We obtained initial stresses of about 7 to 17 MPa at depths of 2 to 10 km on a subset of points on the Nojima and Rokko fault systems. Our initial stresses and coseismic stress changes agree well with postearthquake stresses measured by hydrofracturing in deep boreholes near Hirabayashi and Ogura on Awaji Island. Our results indicate that the Nojima fault slipped at very low shear stress, and fractional stress drop was complete near the surface and about 32% below depths of 2 km. Our results at depth depend on the accuracy of the rake rotations in Yoshida's model, which are probably correct on the Nojima fault but debatable on the Rokko fault. Our results imply that curved or cross-cutting fault striations can be formed in a single earthquake, contradicting a common assumption of structural geology.

Bulletin of the Seismological Society of America↗
USGS1998

Dynamic stress changes during earthquake rupture

We assess two competing dynamic interpretations that have been proposed for the short slip durations characteristic of kinematic earthquake models derived by inversion of earthquake waveform and geodetic data. The first interpretation would require a fault constitutive relationship in which rapid dynamic restrengthening of the fault surface occurs after passage of the rupture front, a hypothesized mechanical behavior that has been referred to as “self-healing.” The second interpretation would require sufficient spatial heterogeneity of stress drop to permit rapid equilibration of elastic stresses with the residual dynamic friction level, a condition we refer to as “geometrical constraint.” These interpretations imply contrasting predictions for the time dependence of the fault-plane shear stresses. We compare these predictions with dynamic shear stress changes for the 1992 Landers ( M 7.3), 1994 Northridge ( M 6.7), and 1995 Kobe ( M 6.9) earthquakes. Stress changes are computed from kinematic slip models of these earthquakes, using a finite-difference method. For each event, static stress drop is highly variable spatially, with high stress-drop patches embedded in a background of low, and largely negative, stress drop. The time histories of stress change show predominantly monotonic stress change after passage of the rupture front, settling to a residual level, without significant evidence for dynamic restrengthening. The stress change at the rupture front is usually gradual rather than abrupt, probably reflecting the limited resolution inherent in the underlying kinematic inversions. On the basis of this analysis, as well as recent similar results obtained independently for the Kobe and Morgan Hill earthquakes, we conclude that, at the present time, the self-healing hypothesis is unnecessary to explain earthquake kinematics.

California↗
USGS1998

Shallow velocity structure of Stromboli Volcano, Italy, derived from small-aperture array measurements of Strombolian tremor

The properties of the tremor wave field at Stromboli are analyzed using data from small-aperture arrays of short-period seismometers deployed on the north flank of the volcano. The seismometers are configured in two semi-circular arrays with radii of 60 and 150 m and a linear array with length of 600 m. The data are analyzed using a spatiotemporal correlation technique specifically designed for the study of the stationary stochastic wave field of Rayleigh and Love waves generated by volcanic activity and by scattering sources distributed within the island. The correlation coefficients derived as a function of frequency for the three components of motion clearly define the dispersion characteristics for both Rayleigh and Love waves. Love and Rayleigh waves contribute 70% and 30%, respectively, of the surface-wave power. The phase velocities of Rayleigh waves range from 1000 m/sec at 2 Hz to 350 m/sec at 9 Hz, and those for Love waves range from 800 to 400 m/sec over the same frequency band. These velocities are similar to those measured near Puu Oo on the east rift of Kilauea Volcano, Hawaii, although the dispersion characteristics of Rayleigh waves at Stromboli show a stronger dependence on frequency. Such low velocities are consistent with values expected for densely cracked solidified basalt. The dispersion curves are inverted for a velocity model beneath the arrays, assuming those dispersions represent the fundamental modes of Rayleigh and Love waves.

Stromboli volcano↗
USGS1998

Spectral structure in the Earth's microseismic background between 20 and 40 seconds

The long-term behavior of a narrow band peak near 26 sec in the microseismic background of the Earth has been studied over many years at several sites. The amplitude of this peak has been determined to be a function of the Earth's weather seasons; it is larger during the southern hemisphere winter. In addition, the microseismic background between 20 and 40 sec has been scanned for peaks in the spectra; three more lower-level peaks have been identified with the possible presence of more.

Bulletin of the Seismological Society of America↗
USGS1998

Bayesian statistics applied to the location of the source of explosions at Stromboli Volcano, Italy

We present a method for determining the location and spatial extent of the source of explosions at Stromboli Volcano, Italy, based on a Bayesian inversion of the slowness vector derived from frequency-slowness analyses of array data. The method searches for source locations that minimize the error between the expected and observed slowness vectors. For a given set of model parameters, the conditional probability density function of slowness vectors is approximated by a Gaussian distribution of expected errors. The method is tested with synthetics using a five-layer velocity model derived for the north flank of Stromboli and a smoothed velocity model derived from a power-law approximation of the layered structure. Application to data from Stromboli allows for a detailed examination of uncertainties in source location due to experimental errors and incomplete knowledge of the Earth model. Although the solutions are not constrained in the radial direction, excellent resolution is achieved in both transverse and depth directions. Under the assumption that the horizontal extent of the source does not exceed the crater dimension, the 90% confidence region in the estimate of the explosive source location corresponds to a small volume extending from a depth of about 100 m to a maximum depth of about 300 m beneath the active vents, with a maximum likelihood source region located in the 120- to 180-m-depth interval.

Stromboli Volcano↗
USGS1998

Seismic-reflection evidence that the hayward fault extends into the lower crust of the San Francisco Bay Area, California

This article presents deep seismic-reflection data from an experiment across San Francisco Peninsula in 1995 using large (125 to 500 kg) explosive sources. Shot gathers show a mostly nonreflective upper crust in both the Franciscan and Salinian terranes (juxtaposed across the San Andreas fault), an onset of weak lower-crustal reflectivity beginning at about 6-sec two-way travel time ( TWTT ) and bright southwest-dipping reflections between 11 and 13 sec TWTT . Previous studies have shown that the Moho in this area is no deeper than 25 km (∼8 to 9 sec TWTT ). Three-dimensional reflection travel-time modeling of the 11 to 13 sec events from the shot gathers indicates that the bright events may be explained by reflectors 15 to 20 km into the upper mantle, northeast of the San Andreas fault. However, upper mantle reflections from these depths were not observed on marine-reflection profiles collected in San Francisco Bay, nor were they reported from a refraction prifile on San Francisco Peninsula. The most consistent interpretation of these events from 2D raytracing and 3D travel-time modeling is that they are out-of-plane reflections from a high-angle (dipping ∼70° to the southwest) impedance contrast in the lower crust that corresponds with the surface trace of the Hayward fault. These results suggest that the Hayward fault truncates the horizontal detachment fault suggested to be active beneath San Francisco Bay.

California↗