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At least 199 records · Page 11Linked to original sources
USGS2004

Evidence for liquefaction identified in peeled slices of Holocene deposits along the Lower Columbia River, Washington

Peels made from 10 geoslices beneath a riverbank at Washington's Hunting Island, 45 km inland from the Pacific coast, aid in identifying sand that liquefied during prehistoric earthquakes of estimated magnitude 8-9 at the Cascadia subduction zone. Each slice was obtained by driving sheetpile and a shutter plate to depths of 6-8 m. The resulting sample, as long as 8 m, had a trapezoidal cross section 42-55 cm by 8 cm. The slicing created few artifacts other than bending and smearing at slice edges. Each slice is dominated by well-stratified sand and mud deposited by the tidal Columbia River. Nearly 90% of the sand is distinctly laminated. The sand contains mud beds as thick as 0.5 m and at least 20 m long, and it is capped by a mud bed that contains a buried soil that marks the 1700 Cascadia earthquake of estimated magnitude 9. Every slice intersected sills and dikes of fluidized sand, and many slices show folds and faults as well. Sills, which outnumber dikes, mostly follow and locally invade the undersides of mud beds. The mud beds probably impeded diffuse upward flow of water expelled from liquefied sand. Trapped beneath mud beds, this water flowed laterally, destroyed bedding by entraining (fluidizing) sand, and locally scoured the overlying mud. Horizontal zones of folded sand extend at least 10 or 20 m, and some contain low-angle faults. Many of the folds probably formed while sand was weakened by liquefaction. The low-angle faults may mark the soles of river-bottom slumps or lateral spreads. As many as four great Cascadia earthquakes in the past 2000 yr contributed to the intrusions, folds, and faults. This subsurface evidence for fluid escape and deformation casts doubt on maximum accelerations that were previously inferred from local absence of liquefaction features at the ground surface along the Columbia River. The geosliced evidence for liquefaction abounds not only beneath banks riddled with dikes but also beneath banks in which dikes are absent. Such dike-free banks of the Columbia River, if interpreted without study of postdepositional structures in deposits beneath them, provide insufficient basis for setting upper bounds on the strength of shaking from great Cascadia earthquakes.

Washington↗
USGS2003

On the expected relationships among apparent stress, static stress drop, effective shear fracture energy, and efficiency

We consider expected relationships between apparent stress τ a and static stress drop Δτ s using a standard energy balance and find τ a = Δτ s (0.5 - ξ), where ξ is stress overshoot. A simple implementation of this balance is to assume overshoot is constant; then apparent stress should vary linearly with stress drop, consistent with spectral theories (Brune, 1970) and dynamic crack models (Madariaga, 1976). Normalizing this expression by the static stress drop defines an efficiency η sw = τ s a/Δτ s as follows from Savage and Wood (1971). We use this measure of efficiency to analyze data from one of a number of observational studies that find apparent stress to increase with seismic moment, namely earthquakes recorded in the Cajon Pass borehole by Abercrombie (1995). Increases in apparent stress with event size could reflect an increase in seismic efficiency; however, η sw for the Cajon earthquakes shows no such increase and is approximately constant over the entire moment range. Thus, apparent stress and stress drop co-vary, as expected from the energy balance at constant overshoot. The median value of η sw for the Cajon earthquakes is four times lower than η sw for laboratory events. Thus, these Cajon-recorded earthquakes have relatively low and approximately constant efficiency. As the energy balance requires η sw = 0.5 - ξ, overshoot can be estimated directly from the Savage-Wood efficiency; overshoot is positive for Cajon Pass earthquakes. Variations in apparent stress with seismic moment for these earthquakes result primarily from systematic variations in static stress drop with seismic moment and do not require a relative decrease in sliding resistance with increasing event size (dynamic weakening). Based on the comparison of field and lab determinations of the Savage-Wood efficiency, we suggest the criterion η sw > 0.3 as a test for dynamic weakening in excess of that seen in the lab.

Bulletin of the Seismological Society of America↗
USGS2003

Lithology and shear-wave velocity in Memphis, Tennessee

We have derived a new three-dimensional model of the lithologic structure beneath the city of Memphis , Tennessee , and examined its correlation with measured shear - wave velocity profiles. The correlation is sufficiently high that the better-constrained lithologic model may be used as a proxy for shear - wave velocities, which are required to calculate site-amplification for new seismic hazard maps for Memphis . The lithologic model and its uncertainties are derived from over 1200 newly compiled well and boring logs, some sampling to 500 m depth, and a moving-least-squares algorithm. Seventy-six new shear - wave velocity profiles have been measured and used for this study, most sampling to 30 m depth or less. All log and velocity observations are publicly available via new web sites.

Tennessee↗
USGS2003

Ground cracks associated with the 1994 double spring flat earthquake, west-central Nevada

The 1994 Double Spring Flat earthquake (M w 5.8) occurred within a densely faulted step-over between the Genoa and Antelope Valley faults, two principal normal faults of the transition zone between the Basin and Range Province and the northern Sierra Nevada . The earthquake created zones of ground cracks from 0.1 to 2.8 km long along at least five northwest- to north-northwest-striking faults in the epicentral area. Individual cracks had extensional openings generally from 1 to 10 mm wide. No cracks displayed obvious vertical separation, and only one zone showed permissive evidence of right-lateral separation. Over the 8 days following the mainshock (the period over which the cracks were found), aftershocks formed a dominant northeast trend suggesting the earthquake occurred along a northeast-striking structure. However, no ground breakage was found along faults striking parallel to this northeast aftershock alignment, and subsequent aftershocks formed a conjugate northwest trend. Based on the location and character of the five zones, the observed cracks are attributed to secondary fault slip and shaking effects. The earthquake also created ground cracks along at least two faults 15-25 km from the epicenter. In both of these cases, the faults had documented histories of prior ground cracking, indicating that they are particularly susceptible to such triggered deformation.

Bulletin of the Seismological Society of America↗
USGS2003

Detonation charge size versus coda magnitude relations in California and Nevada

Magnitude-charge size relations have important uses in forensic seismology and are used in Comprehensive Nuclear-Test-Ban Treaty monitoring. I derive empirical magnitude versus detonation-charge-size relationships for 322 detonations located by permanent seismic networks in California and Nevada. These detonations, used in 41 different seismic refraction or network calibration experiments, ranged in yield (charge size) between 25 and 106 kg; coda magnitudes reported for them ranged from 0.5 to 3.9. Almost all represent simultaneous (single-fired) detonations of one or more boreholes. Repeated detonations at the same shotpoint suggest that the reported coda magnitudes are repeatable, on average, to within 0.1 magnitude unit. An empirical linear regression for these 322 detonations yields M = 0.31 + 0.50 log10(weight [kg]). The detonations compiled here demonstrate that the Khalturin et al. (1998) relationship, developed mainly for data from large chemical explosions but which fits data from nuclear blasts, can be used to estimate the minimum charge size for coda magnitudes between 0.5 and 3.9. Drilling, loading, and shooting logs indicate that the explosive specification, loading method, and effectiveness of tamp are the primary factors determining the efficiency of a detonation. These records indicate that locating a detonation within the water table is neither a necessary nor sufficient condition for an efficient shot.

California, Nevada↗
USGS2003

Phase derivatives and simulation of strong ground motions

Phase derivatives can be used to compute instantaneous frequency and envelope delay (also known as group delay). Envelope delay, in the guise of phase differences, has been used by engineers in the simulation of strong ground motion , particularly as a way of controlling the duration of motion . Simulations using the stochastic method, in which duration is a simple function of source duration and a path-dependent duration, possess envelope delay properties similar to those from simulations based on phase differences. Envelope delay provides a way of extending the standard stochastic method to produce nonstationary frequency content, as produced by ground motions containing surface waves.

Bulletin of the Seismological Society of America↗
USGS2003

Estimated ground motion from the 1994 Northridge, California, earthquake at the site of interstate 10 and La Cienega Boulevard bridge collapse, West Los Angeles, California

We have estimated ground motions at the site of a bridge collapse during the 1994 Northridge, California, earthquake. The estimated motions are based on correcting motions recorded during the mainshock 2.3 km from the collapse site for the relative site response of the two sites. Shear-wave slownesses and damping based on analysis of borehole measurements at the two sites were used in the site response analysis. We estimate that the motions at the collapse site were probably larger, by factors ranging from 1.2 to 1.6, than at the site at which the ground motion was recorded, for periods less than about 1 sec.

California↗
USGS2003

Near-field survey of the 1946 Aleutian tsunami on Unimak and Sanak Islands

The 1946 Aleutian earthquake stands out among tsunamigenic events because it generated both very high run-up near the earthquake source region and a destructive trans-Pacific tsunami . We obtained new data on the distribution of its tsunami in the near field along south-facing coasts between Unimak Pass on the west and Sanak Island on the east by measuring the height of driftwood and beach materials that were deposited by the tsunami above the extreme storm tide level. Our data indicate that (1) the highest measured run-up, which is at the Scotch Cap lighthouse, was 42 m above tide level or about 37 m above present storm tide elevation; (2) run-up along the rugged coast from Scotch Cap for 12 km northwest to Sennett Point is 12-18 m, and for 30 km east of Scotch Cap to Cape Lutke it is 24-42 m; (3) run-up along the broad lowlands bordering Unimak Bight is 10-20 m, and in-undation is locally more than 2 km; (5) run-up diminishes to 8 m or less at the southeast corner of Unimak Island ; (6) no evidence was found for run-up above present storm tides (about 4-5 m above MLLW) on the Ikatan Peninsula or areas along the coast to the west; and (7) run-up above storm tide level in the Sanak Island group is restricted to southwest-facing coasts of Sanak , Long, and Clifford Islands , where it is continuous and locally up to 24 m high. Generation of the tsunami by one or more major earthquake-triggered submarine landslides near the shelf edge south of Unimak Island seems to be the only viable mechanism to account for the data on wave arrival time, run-up heights, and distribution, as well as for unconfirmed anecdotal reports of local postquake increases in water depth and diminished bottom-fisheries productivity. A preliminary hydrodynamic simulation of the local tsunami propagation and run-up using a dipolar model of a possible landslide off Davidson Bank provides an acceptable fit to the characteristics of the distribution of local run-up, with a value at 34 m at the Scotch Cap lighthouse.

Bulletin of the Seismological Society of America↗
USGS2003

The dependence of PGA and PGV on distance and magnitude inferred from Northern California ShakeMap data

We analyze peak ground velocity (PGV) and peak ground acceleration (PGA) data from 95 moderate (3.5 ??? M < 5.5) and 9 large (5.5 ??? M ??? 7.1) earthquakes in northern California. The 95 moderate earthquakes occurred from August 1998 through December 2002, and their peak motions were compiled and mapped by ShakeMap. The nine large earthquakes include the M 6.2 Morgan Hill earthquake in 1984, the M 6.9 Loma Prieta earthquake in 1989, and the M 7.1 Petrolia earthquake in 1992. For r > 100 km, the peak motions attenuate more rapidly than a simple power law (that is, r-??) can fit. Instead, we use an attenuation function that combines a fixed power law (r-0.7) with a fitted exponential dependence on distance, which is estimated as expt(-0.0063r) and exp(-0.0073r) for PGV and PGA, respectively, for moderate earthquakes. We regress log(PGV) and log(PGA) as functions of distance and magnitude. We assume that the scaling of log(PGV) and log(PGA) with magnitude can differ for moderate and large earthquakes, but must be continuous. Because the frequencies that carry PGV and PGA can vary with earthquake size for large earthquakes, the regression for large earthquakes incorporates a magnitude dependence in the exponential attenuation function. We fix the scaling break between moderate and large earthquakes at M 5.5; log(PGV) and log(PGA) scale as 1.06M and 1.00M, respectively, for moderate earthquakes and 0.58M and 0.31M for large earthquakes.

California↗
USGS2005

Subsurface structure and kinematics of the Calaveras-Hayward fault stepover from three-dimensional V p and seismicity, San Francisco Bay region, California

The Calaveras and Hayward faults are major components of the San Andreas fault system in the San Francisco Bay region. Dextral slip is presumed to transfer from the Calaveras fault to the Hayward fault in the Mission Hills region, an area of uplift in the contractional stepover between the two faults. Here the estimated deep slip rates drop from 15 to 6 mm/yr on the Calaveras fault, and slip begins on the Hayward fault at an estimated 9 mm/yr. A lineament of microseismicity near the Mission fault links the seismicity on the Calaveras and Hayward faults and is presumed to be related directly to this slip transfer. However, geologic and seismologic evidence suggest that the Mission fault may not be the source of the seismicity and that the Mission fault is not playing a major role in the slip transfer. We perform a joint inversion for hypocenters and the 3D P -wave velocity structure of the stepover region using 477 earthquakes. We find strong velocity contrasts across the Calaveras and Hayward faults, corroborated by geologic, gravity, and aeromagnetic data. Detailed examination of two seismic lineaments in conjunction with the velocity model and independent geologic and geophysical evidence suggests that they represent the southern extension of a northeasterly dipping Hayward fault that splays off the Calaveras fault, directly accounting for the deep slip transfer. The Mission fault appears to be accommodating deformation within the block between the Hayward and Calaveras faults. Thus, the Calaveras and Hayward faults need to be considered as a single system for developing rupture scenarios for seismic hazard assessments.

Bulletin of the Seismological Society of America↗
USGS2003

Site-specific seismic-hazard analysis that is completely probabilistic

When a site - specific probabilistic ground-motion estimate is required, the full site -amplification distribution should be used instead of a single deterministic median value. A probabilistic methodology using site -amplification distributions to modify rock ground-motion attenuation relations into site - specific relations prior to calculating seismic hazard has been developed and applied at two selected sites in the central United States: Memphis, Tennessee, and Paducah, Kentucky. The use of a completely probabilistic approach can make about a 10% difference in ground-motion estimates over simply multiplying a bedrock probabilistic ground motion by a median site -amplification factor at a 1 in 2475 annual probability of exceedance and even larger differences at smaller probabilites of exceedance. The value of this approach is that a probabilistic answer incorporating the uncertainty in our knowledge of site amplification of ground motions can be calculated.

Tennessee, Kentucky↗
USGS2004

Comparison of four moderate-size earthquakes in southern California using seismology and InSAR

Source parameters determined from interferometric synthetic aperture radar (InSAR) measurements and from seismic data are compared from four moderate-size (less than M 6) earthquakes in southern California. The goal is to verify approximate detection capabilities of InSAR, assess differences in the results, and test how the two results can be reconciled. First, we calculated the expected surface deformation from all earthquakes greater than magnitude 4 in areas with available InSAR data (347 events). A search for deformation from the events in the interferograms yielded four possible events with magnitudes less than 6. The search for deformation was based on a visual inspection as well as cross-correlation in two dimensions between the measured signal and the expected signal. A grid-search algorithm was then used to estimate focal mechanism and depth from the InSAR data. The results were compared with locations and focal mechanisms from published catalogs. An independent relocation using seismic data was also performed. The seismic locations fell within the area of the expected rupture zone for the three events that show clear surface deformation. Therefore, the technique shows the capability to resolve locations with high accuracy and is applicable worldwide. The depths determined by InSAR agree with well-constrained seismic locations determined in a 3D velocity model. Depth control for well-imaged shallow events using InSAR data is good, and better than the seismic constraints in some cases. A major difficulty for InSAR analysis is the poor temporal coverage of InSAR data, which may make it impossible to distinguish deformation due to different earthquakes at the same location.

Bulletin of the Seismological Society of America↗
USGS2003

Inferences drawn from two decades of alinement array measurements of creep on faults in the San Francisco Bay Region

We summarize over 20 years of monitoring surface creep on faults of the San Andreas system in the San Francisco Bay region using alinement arrays. The San Andreas fault is fully locked at five sites northwest from San Juan Bautista, the southern end of the 1906 earthquake rupture, that is, no creep (< 1 mm/yr) is observed. Likewise, the San Gregorio, Rodgers Creek, and West Napa faults show no creep . The measured creep rate on the Calaveras-Paicines fault from Hollister southward is either 6 or ∼ 10 mm/yr, depending on whether the arrays cross all of the creeping traces. Northward of Hollister, the central Calaveras creep rate reaches 14 ± 2 mm/yr but drops to ∼ 2 mm/yr near Calaveras Reservoir, where slip transfers to the southern Hayward fault at a maximum creep rate of 9 mm/yr at its south end. However, the Hayward fault averages only 4.6 mm/yr over most of its length. The Northern Calaveras fault , now creeping at 3-4 mm/yr, steps right to the Concord fault , which has a similar rate, 2.5-3.5 mm/yr, which is slightly slower than the 4.4 mm/yr rate on its northward continuation, the Green Valley fault . The Maacama fault creeps at 4.4 mm/yr near Ukiah and 6.5 mm/yr in Willits. The central and southern segments of the Calaveras fault are predominantly creeping, whereas the Hayward, Northern Calaveras, and Maacama faults are partly locked and, along with the Rodgers Creek and San Andreas, have high potential for major earthquakes.

California↗
USGS2003

Structure and mechanics of the Hayward-Rodgers Creek Fault step-over, San Francisco Bay, California

A dilatational step - over between the right-lateral Hayward and Rodgers Creek faults lies beneath San Pablo Bay in the San Francisco Bay area. A key seismic hazard issue is whether an earthquake on one of the faults could rupture through the step - over , enhancing its maximum possible magnitude. If ruptures are terminated at the step - over , then another important issue is how strain transfers through the step . We developed a combined seismic reflection and refraction cross section across south San Pablo Bay and found that the Hayward and Rodgers Creek faults converge to within 4 km of one another near the surface, about 2 km closer than previously thought. Interpretation of potential field data from San Pablo Bay indicated a low likelihood of strike-slip transfer faults connecting the Hayward and Rodgers Creek faults. Numerical simulations suggest that it is possible for a rupture to jump across a 4-km fault gap, although special stressing conditions are probably required (e.g., Harris and Day, 1993, 1999). Slip on the Hayward and Rodgers Creek faults is building an extensional pull-apart basin that could contain hazardous normal faults. We investigated strain in the pull-apart using a finite-element model and calculated a ∼0.02-MPa/yr differential stressing rate in the step - over on a least-principal-stress orientation nearly parallel to the strike-slip faults where they overlap. A 1- to 10-MPa stress-drop extensional earthquake is expected on normal faults oriented perpendicular to the strike-slip faults every 50-500 years. The last such earthquake might have been the 1898 M 6.0-6.5 shock in San Pablo Bay that apparently produced a small tsunami. Historical hydrographic surveys gathered before and after 1898 indicate abnormal subsidence of the bay floor within the step - over , possibly related to the earthquake. We used a hydrodynamic model to show that a dip-slip mechanism in north San Pablo Bay is the most likely 1898 rupture scenario to have caused the tsunami. While we find no strike-slip transfer fault between the Hayward and Rodgers Creek faults, a normal- fault link could enable through-going segmented rupture of both strike-slip faults and may pose an independent hazard of M ∼6 earthquakes like the 1898 event.

California↗
USGS2008

Near-surface location, geometry, and velocities of the Santa Monica Fault Zone, Los Angeles, California

High-resolution seismic-reflection and seismic-refraction imaging, combined with existing borehole, earthquake, and paleoseismic trenching data, suggest that the Santa Monica fault zone in Los Angeles consists of multiple strands from several kilometers depth to the near surface. We interpret our seismic data as showing two shallow-depth low-angle fault strands and multiple near-vertical (???85??) faults in the upper 100 m. One of the low-angle faults dips northward at about 28?? and approaches the surface at the base of a topographic scarp on the grounds of the Wadsworth VA Hospital (WVAH). The other principal low-angle fault dips northward at about 20?? and projects toward the surface about 200 m south of the topographic scarp, near the northernmost areas of the Los Angeles Basin that experienced strong shaking during the 1994 Northridge earthquake. The 20?? north-dipping low-angle fault is also apparent on a previously published seismic-reflection image by Pratt et al. (1998) and appears to extend northward to at least Wilshire Boulevard, where the fault may be about 450 m below the surface. Slip rates determined at the WVAH site could be significantly underestimated if it is assumed that slip occurs only on a single strand of the Santa Monica fault or if it is assumed that the near-surface faults dip at angles greater than 20-28??. At the WVAH, tomographic velocity modeling shows a significant decrease in velocity across near-surface strands of the Santa Monica fault. P-wave velocities range from about 500 m/sec at the surface to about 4500 m/sec within the upper 50 m on the north side of the fault zone at WVAH, but maximum measured velocities on the south side of the low-angle fault zone at WVAH are about 3500 m/sec. These refraction velocities compare favorably with velocities measured in nearby boreholes by Gibbs et al. (2000). This study illustrates the utility of com- bined seismic-reflection and seismic-refraction methods, which allow more accurate reflection imaging and compositional estimations across areas with highly variable velocities, a property that is characteristic of most fault zones.

Bulletin of the Seismological Society of America↗
USGS2004

Optimizing correlation techniques for improved earthquake location

Earthquake location using relative arrival time measurements can lead to dramatically reduced location errors and a view of fault-zone processes with unprecedented detail. There are two principal reasons why this approach reduces location errors. The first is that the use of differenced arrival times to solve for the vector separation of earthquakes removes from the earthquake location problem much of the error due to unmodeled velocity structure. The second reason, on which we focus in this article, is that waveform cross correlation can substantially reduce measurement error. While cross correlation has long been used to determine relative arrival times with subsample precision, we extend correlation measurements to less similar waveforms, and we introduce a general quantitative means to assess when correlation data provide an improvement over catalog phase picks. We apply the technique to local earthquake data from the Calaveras Fault in northern California. Tests for an example streak of 243 earthquakes demonstrate that relative arrival times with normalized cross correlation coefficients as low as ∼70%, interevent separation distances as large as to 2 km, and magnitudes up to 3.5 as recorded on the Northern California Seismic Network are more precise than relative arrival times determined from catalog phase data. Also discussed are improvements made to the correlation technique itself. We find that for large time offsets, our implementation of time-domain cross correlation is often more robust and that it recovers more observations than the cross spectral approach. Longer time windows give better results than shorter ones. Finally, we explain how thresholds and empirical weighting functions may be derived to optimize the location procedure for any given region of interest, taking advantage of the respective strengths of diverse correlation and catalog phase data on different length scales.

Bulletin of the Seismological Society of America↗
USGS2003

Analog-to-digital conversion as a source of drifts in displacements derived from digital recordings of ground acceleration

Displacements obtained from double integration of digitally recorded ground accelerations often show drifts much larger than those expected for the true ground displacements. These drifts might be due to many things, including dynamic elastic ground tilt, inelastic ground deformation, hysteresis in the instruments, and cross feed due to misalignment of nominally orthogonal sensors. This article shows that even if those effects were not present, the analog-to-digital conversion (ADC) process can produce apparent "pulses" and offsets in the acceleration baseline if the ground motion is slowly varying compared with the quantization level of the digitization. Such slowly varying signals can be produced by constant offsets that do not coincide with a quantization level and by near- and intermediate-field terms in the wave field radiated from earthquakes. Double integration of these apparent pulses and offsets leads to drifts in the displacements similar to those found in processing real recordings. These effects decrease in importance as the resolution of the ADC process increases.

Bulletin of the Seismological Society of America↗
USGS2004

Spatial variations in the frequency-magnitude distribution of earthquakes at Mount Pinatubo volcano

The frequency-magnitude distribution of earthquakes measured by the b-value is mapped in two and three dimensions at Mount Pinatubo, Philippines, to a depth of 14 km below the summit. We analyzed 1406 well-located earthquakes with magnitudes MD ???0.73, recorded from late June through August 1991, using the maximum likelihood method. We found that b-values are higher than normal (b = 1.0) and range between b = 1.0 and b = 1.8. The computed b-values are lower in the areas adjacent to and west-southwest of the vent, whereas two prominent regions of anomalously high b-values (b ??? 1.7) are resolved, one located 2 km northeast of the vent between 0 and 4 km depth and a second located 5 km southeast of the vent below 8 km depth. The statistical differences between selected regions of low and high b-values are established at the 99% confidence level. The high b-value anomalies are spatially well correlated with low-velocity anomalies derived from earlier P-wave travel-time tomography studies. Our dataset was not suitable for analyzing changes in b-values as a function of time. We infer that the high b-value anomalies around Mount Pinatubo are regions of increased crack density, and/or high pore pressure, related to the presence of nearby magma bodies.

Bulletin of the Seismological Society of America↗