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At least 901 records · Page 50Linked to original sources
USGS1991

The 1954 and 1980 Algerian earthquakes: Implications for the characteristic-displacement model of fault behavior

Joint epicenter determination of earthquakes that occurred in northern Algeria near Ech Cheliff (named Orleansville in 1954 and El Asnam in 1980) shows that the earthquake of 9 September 1954 (M=6.5) occurred at nearly the same location as the earthquake of 10 October 1980 (M=7.3). The 1954 main shock and earliest aftershocks were concentrated close to the boundaries of segment B (nomenclature of Deschamps et al., 1982; King and Yielding, 1984) of the 1980 fault system, which was to experience approximately 8 m of slip in the 1980 earthquake. Later aftershocks of the 1954 earthquake were spread over a broad area, notably in a region north of the 1980 fault system that also experienced many aftershocks to the 1980 earthquake. The closeness of the 1954 main shock and earliest aftershocks to the 1980 segment B implies that the 1954 earthquake involved either 1) rupture of segment B proper, or 2) rupture of a distinct fault in the hanging wall of footwall block of segment B.

Bulletin of the Seismological Society of America↗
USGS1991

Estimates of velocity structure and source depth using multiple P waves from aftershocks of the 1987 Elmore Ranch and Superstition Hills, California, earthquakes

Event record sections, which are constructed by plotting seismograms from many closely spaced earthquakes recorded on a few stations, show multiple free-surface reflections (PP, PPP, PPPP) of the P wave in the Imperial Valley, California. The relative timing of these arrivals is used to estimate the strength of the P-wave velocity gradient within the upper 5 km of the sediment layer. Consistent with previous studies, a velocity model with a value of 1.8 km/sec at the surface increasing linearly to 5.8 km/sec at a depth of 5.5 km fits the data well. The relative amplitudes of the P and PP arrivals are used to estimate the source depth for the aftershock distributions of the Elmore Ranch and Superstition Hills main shocks. Although the depth determination has large uncertainties, both the Elmore Ranch and Superstition Hills aftershock sequences appear to have similar depth distribution in the range of 4 to 10 km.

California↗
USGS1991

Criticism of some forecasts of the National Earthquake Prediction Evaluation Council

The Working Group on California Earthquake Probabilities has assigned probabilities for rupture in the interval from 1988 to 2018 to various segments of the San Andreas fault on the basis of the lognormal distribution of recurrence times of characteristic earthquakes postulated by Nishenko and Buland (1987). I question the validity of those probabilities on the basis of three separate arguments: (1) The distributions of recurrence times of the four, best-observed, characteristic-earthquake sequences are each only marginally consistent with the Nishenko - Buland Iognormal distribution. (2) The range of possible 30-year conditional probabilities for many of the fault segments is so great due to uncertainty in the average recurrence time for that segment that the assigned probability is virtually meaningless. (3) The 1988 forecasts not subject to the foregoing objection are those in which there is a low probability of an earthquake in the near future (e.g., only a 5 per cent chance of rupture of the North Coast segment before the year 2049 and of the Carrizo segment before the year 2018). The same reasoning would assign only a 5 per cent chance of rupture before mid-1993 to the southern Santa Cruz Mountains segment, the segment that failed in October 1989. Finally, the forecast of the next Parkfield earthquake (95 per cent probability before 1993.0) by Bakun and Lindh (1985) depends upon an ad hoc explanation of the out-of-sequence 1934 earthquake. A less-contrived forecast would have assigned a conditional probability of about 60 ± 20 per cent to the 1985.0 to 1993.0 interval and 30 ± 15 per cent to the 1990.0 to 1993.0 interval.

California↗
USGS1991

Off-fault ground ruptures in the Santa Cruz Mountains, California: Ridge-top spreading versus tectonic extension during the 1989 Loma Prieta earthquake

The M s 7.1 Loma Prieta earthquake of 18 October 1989 produced abundant ground ruptures in an 8 by 4 km area along Summit Road and Skyland Ridge in the Santa Cruz Mountains. Predominantly extensional fissures formed a left-stepping, crudely en echelon pattern along ridges of the hanging-wall block southwest of the San Andreas fault, about 12 km northwest of the epicenter. The fissures are subparallel to the San Andreas fault and appear to be controlled by bedding planes, faults, joints, and other weak zones in the underlying Tertiary sedimentary strata of the hanging-wall block. The pattern of extensional fissures is generally consistent with tectonic extension across the crest of the uplifted hanging-wall block. Also, many displacements in Laurel Creek canyon and along the San Andreas and Sargent faults are consistent with right-lateral reverse faulting inferred for the mainshock. Additional small tensile failures along the axis of the Laurel anticline may reflect growth of the fold during deep-seated compression. However, the larger ridge-top fissures commonly have displacements that are parallel to the north-northeast regional slope directions and appear inconsistent with east-northeast extension expected from this earthquake. Measured cumulative displacements across the ridge crests are at least 35 times larger than that predicted by the geodetically determined surface deformation. These fissures also occur in association with ubiquitous landslide complexes that were reactivated by the earthquake to produce the largest concentration of co-seismic slope failures in the epicentral region. The anomalously large displacements and the apparent slope control of the geometry and displacement of many co-seismic surface ruptures lead us to conclude that gravity is an important driving force in the formation of the ridge-top fissures. Shaking-induced gravitational spreading of ridges and downslope movement may account for 90¿ or more of the observed displacements on the linear fissures. Similar fissures occurred in the same area and elsewhere near the San Andreas fault during the predominantly right-lateral 1906 San Francisco earthquake and suggest that the Loma Prieta ground ruptures may, in large part, be independent of fault kinematics.

California↗
USGS1991

The Marina District, San Francisco, California: Geology, history, and earthquake effects

A northwest-trending valley in the bedrock surface is buried by firm Pleistocene bay clay, a dense Pleistocene sand layer, soft Holocene bay sediments, loose to dense Holocene beach and dune sands, and artificial fill that have an aggregate maximum thickness of about 90 m (300 ft). Artificial filling of a cove at the site of The Marina District proceeded gradually from the late 1860s to 1912, when major hydraulic filling was done for the Panama-Pacific International Exposition. The remains of thousands of piles driven for the Exposition very probably still exist and have had unknown effects on long-term ground settlement and earthquake-related ground displacements. Intensity maps of the 1906 earthquake, and seismic recordings and severe building damage in 1989, reported by others, indicate that ground motion was amplified on both natural and artificial ground. This suggests that the configuration of the bedrock surface and the location and thickness of various clay and sand deposits underlying the fill had an important effect on the shaking. However, most of the settlement and liquefaction and the damage to pipelines, building foundations, streets, side-walks, and curbs occurred in areas of artificial fill consisting mainly of loose sand.

California↗
USGS1991

The Loma Prieta earthquake, ground motion, and damage in Oakland, Treasure Island, and San Francisco

The basis of this study is the acceleration, velocity, and displacement wave-forms of the Loma Prieta earthquake (18 October 1989; M = 7.0) at two rock sites in San Francisco, a rock site on Yerba Buena Island, an artificial-fill site on Treasure Island, and three sites in Oakland underlain by thick sections of poorly consolidated Pleistocene sediments. The waveforms at the three rock sites display a strong coherence, as do the three sedimentary sites in Oakland. The duration of strong motion at the rock sites is very brief, suggestive of an unusually short source duration for an earthquake of this size, while the records in Oakland show strong amplification effects due to site geology. The S -wave group at Treasure Island is phase coherent with the Oakland records, but at somewhat diminished amplitudes, until the steps in acceleration at approximately 15 sec, apparently signaling the onset of liquefaction. All seven records clearly show shear-wave first motion opposite to that expected for the mainshock radiation pattern and peak amplitudes greater than expected for sites at these distances (95 ± 3 km) from an earthquake of this magnitude. While the association between these ground motion records and related damage patterns in nearby areas has been easily and eagerly accepted by seismological and engineering observers of them, we have had some difficulty in making such relationships quantitative or even just clear. The three Oakland records, from sites that form a nearly equilateral triangle about the Cypress Street viaduct collapse, are dominated by a long-period resonance (≃ 1 1/2-sec period) far removed from the natural frequency of the structure to transverse motion (2.5 Hz) or from high-frequency amplification bands observed in aftershock studies. A spectral ratio arbiter of this discrepancy confuses it further. The failure of the East Bay crossing of the San Francisco-Oakland Bay Bridge cannot be attributed to relative displacements of the abutments in Oakland and Yerba Buena Island, but the motions of the Bay Bridge causing failure remain unknown. The steps in acceleration at Treasure Island present unusual strong-motion accelerogram processing problems, and modeling suggests that the velocity and displacement waveforms are contaminated by a spurious response of the filtering operations to the acceleration steps. A variety of coincidences suggests that the Treasure island accelerogram is the most likely strong-motion surrogate for the filled areas of the Marina District, for which no mainshock records are available, but the relative contributions of bad ground, poor construction and truly strong ground motion to damage in the Marina District will never by known in any quantitative way. The principal lesson of all of this is that until a concerted effort is mounted to instrument ground and structures that are likely to fail during earthquakes, our understanding of the very complex relationships between strong ground motion and earthquake damage will, in general, remain rudimentary, imprecise, and vague.

California↗
USGS1992

On the characteristics of local geology and their influence on ground motions generated by the Loma Prieta earthquake in the San Francisco Bay region, California

Strong ground motions recorded at 34 sites in the San Francisco Bay region from the Loma Prieta earthquake show marked variations in characteristics dependent on crustal structure and local geological conditions. Peak horizontal acceleration and velocity inferred for sites underlain by “rock” generally occur on the transverse component of motion. They are consistently greater with lower attenuation rates than the corresponding mean value predicted by empirical curves based on previous strong-motion data. Theoretical amplitude distributions and synthetic seismograms calculated for 10-layer models suggest that “bedrock” motions were elevated due in part to the wide-angle reflection of S energy from the base of a relatively thin (25 km) continental crust in the region. Characteristics of geologic and geotechnical units as currently mapped for the San Francisco Bay region show that average ratios of peak horizontal acceleration, velocity and displacement increase with decreasing mean shear-wave velocity. Ratios of peak acceleration for sites on “soil” (alluvium, fill/Bay mud) are statistically larger than those for sites on “hard rock” (sandstone, shale, Franciscan Complex). Spectral ratios establish the existence of predominant site periods with peak amplifications near 15 for potentially damaging levels of ground motion at some sites underlain by alluvium and fill/bay mud. Average spectral amplifications inferred for vertical and the mean horizontal motion are, respectively, (1,1) for sites on the Franciscan Complex (KJf), (1.4, 1.5) for sites on Mesozoic and Tertiary rocks (TMzs), (2.1, 2.0) for sites on the Santa Clara Formation (QTs), (2.3, 2.9) for sites on alluvium (Qal), and (2.1, 4.0) for sites on fill/Bay mud (Qaf/Qhbm). These mean values are not statistically different at the 5% significance level from those inferred from previous low-strain data. Analyses suggest that soil amplification and reflected crustal shear energy were major contributors to levels of ground motion sufficient to cause damage to vulnerable structures at distances near 100 km in the cities of San Francisco and Oakland.

California↗
USGS1993

The Parkfield prediction fallacy

The Parkfield earthquake prediction is generally stated as a 95% probability that the next moderate earthquake there should occur before January 1993. That time limit is based on a two-sided 95% confidence interval. Because at the time of the prediction (1985) it was already clear that the earthquake had not occurred prior to 1985, a one-sided 95% confidence interval would have been more appropriate. That confidence interval ended in October 1991. The Parkfield prediction was based on an extrapolation of five of the six events in the 1857 to 1966 earthquake sequence; the 1934 event was omitted because it did not fit the regularity exhibited by the other data. The fallacy in the prediction is that it did not take account of other less-contrived explanations of the Parkfield seismicity (e.g., not excluding the 1934 event). Even if the Parkfield earthquake should occur in the near future, it would be better explained by less-contrived hypotheses.

Bulletin of the Seismological Society of America↗
USGS1993

Site response estimates in Salt Lake Valley, Utah, from borehole seismic velocities

The general correlation of Salt Lake Valley sites located on soft, saturated unconsolidated silty and clayey deposits (that is, deposits with low S -wave velocity) with high seismic amplification at the ground surface motivated our investigation of the relationship between the P - and S -wave seismic velocity ( V p and V s ) of these units and their corresponding observed site response. We found that low, near surface V s −1.5 is proportional to seismic amplification measured on the surface, and suspected that we might be able to predict the site response if the near surface seismic velocity structure were known. With this idea in mind we constructed plane-layered seismic impedance models from borehole data in order to correlate impedance structure with variations in site response. Seismic travel times, measured in a borehole at 2-m intervals, were converted into compressional and shear wave seismic velocity profiles for 22 boreholes (average depth = 59 m) in Salt Lake Valley, Utah. Using this impedance model, we estimated site response on the ground surface of the borehole site to within 12% of the measured value in the 0.7 to 1.0 Hz frequency range for six of the twenty sites, and within 20% for 55% of the sites. All except two of our site response estimates are within a factor of two of the measured value. Thus, high values of seismic amplification appear to be partially explained by a near surface high-impedance contrast produced by the low S -wave velocities. Comparing the downhole data with published Salt Lake Valley ground motion data derived from Nevada nuclear tests shows that increased site response (sites of relative ground motion amplification) is associated with: (1) a lower value of V s (110 to 400 m/sec), and (2) high Poisson ratios (0.45 to 0.49) derived from borehole V p and V s values. The lowest S -wave velocities found in the Salt Lake Valley are comparable to other regions, such as the muds around San Francisco Bay and the lake sediments of Mexico City, with low S -wave velocities and a record of severe seismic wave amplification in previous earthquakes. The very-low-velocity surface layer in Salt Lake Valley is, however, about half as thick (10 to 14 m) as the low-velocity layers around San Francisco Bay and Mexico City. We also considered the influence of sedimentary basin fill on site response, because coincidentally, the sites of high seismic wave amplification correspond to the locations of thickest basin fill. The valley fill impedance structure, revealed in seismic reflection profiles we acquired, indicate that a more dynamic impedance structure characterizes sites near the center of the valley where the basin fill is thickest. The reflection data from mid-valley sites typically have more high-amplitude reflectors relative to basin edge sites. The sites with a more dynamic impedance structure suggest that the site response would also be correspondingly different. Thus, the broadband, deep basin effect on site response might account for some of the discrepancy between observed and predicted site response in this study.

Utah↗
USGS1993

Three-dimensional simulations of ground motions in the San Bernardino Valley, California, for hypothetical earthquakes on the San Andreas Fault

Three-dimensional finite difference simulations of elastic waves in the San Bernardino Valley were performed for two hypothetical earthquakes on the San Andreas fault: a point source with moment magnitude M5 and an extended rupture with M6.5. A method is presented for incorporating a source with arbitrary focal mechanism in the grid. Synthetics from the 3-D simulations are compared with those derived from 2-D (vertical cross section) and 1-D (flat-layered) models. The synthetic seismograms from the 3-D and 2-D simulations exhibit large surface waves produced by conversion of incident S waves at the edge of the basin. Seismograms from the flat-layered model do not contain these converted surface waves and underestimate the duration of shaking. Maps of maximum ground velocities occur in localized portions of the basin. The location of the largest velocities changes with the rupture propagation direction. Contours of maximum shaking are also dependent on asperity positions and radiation pattern.

California↗
USGS1994

The attenuation of seismic shear waves in quaternary alluvium in Santa Clara Valley, California

We used shear waves, generated by an air-powered source at the ground surface and recorded in a borehole, to estimate the shear-wave quality factor at strong-motion station Gilroy no. 2. We find similar values of Q using both the decay of the spectra with depth and the slope of the spectral ratio at two depths; we find no evidence of a frequency dependence of Q . The mean value of Q over the depth range 10 to 115 m is close to 10. The use of this value over the depth of the borehole and the observed travel time of 0.358 sec gives a cumulative attenuation factor t * of 0.036 sec for the upper 180 m of the Quaternary alluvium. This is comparable to the differential decay between Gilroy no. 2 and a rock site 1.9 km away (Gilroy no. 1), as measured from the decay of the high-frequency spectra of accelerograms from large earthquakes, plotted on a log-linear scale: t * = 0.05, 0.04, and 0.03 sec for the 1979 Coyote Lake, 1984 Morgan Hill, and 1989 Loma Prieta earthquakes, respectively. The similarity between the attenuations measured from the low-strain surface source and those from the larger amplitude earthquake sources suggests that increases of damping due to nonlinear wave propagation effects are limited.

California↗
USGS1994

Early warning system for aftershocks

A prototype early warning system to provide San Francisco and Oakland, California a few tens-of-seconds warning of incoming strong ground shaking from already-occurred M ≧ 3.7 aftershocks of the magnitude 7.1 17 October 1989 Loma Prieta earthquake was operational on 28 October 1989. The prototype system consisted of four components: ground motion sensors in the epicentral area, a central receiver, a radio repeater, and radio receivers. One of the radio receivers was deployed at the California Department of Transportation ( CALTRANS ) headquarters at the damaged Cypress Street section of the I-880 freeway in Oakland, California on 28 October 1989 and provided about 20 sec of warning before shaking from the M 4.5 Loma Prieta aftershock that occurred on 2 November 1989 at 0550 UTC. In its first 6 months of operation, the system generated triggers for all 12 M > 3.7 aftershocks for which trigger documentation is preserved, did not trigger on any M ≦ 3.6 aftershocks, and produced one false trigger as a result of a now-corrected single point of failure design flaw. Because the prototype system demonstrated that potentially useful warnings of strong shaking from aftershocks are feasible, the USGS has completed a portable early warning system for aftershocks that can be deployed anywhere.

California↗
USGS1994

Implications of felt area-magnitude relations for earthquake scaling and the average frequency of perceptible ground motion

Observed relations between felt area and moment magnitude M for stable continental regions ( SCR ) and California earthquakes can be explained by a simple model with geometrical spreading, attenuation ( Q −1 ), and earthquake stress drops that remain constant from M 4 to 8. Differences in the slopes of the observed felt area- M relations between SCR and California earthquakes are consistent with differences in Q between these tectonic settings. Fitting the felt area- M data to the model yields an estimate of f/Q , where f is the average frequency of minimum perceptible ground motion. Using fits to the data and instrumentally derived values of Q , I find that f is between about 2 and 4 Hz. I present an estimate of the spectral acceleration level associated with the lower limit of perceptibility.

Bulletin of the Seismological Society of America↗
USGS1994

Shear zones formed along long, straight traces of fault zones during the 28 June 1992 Landers, California, earthquake

Surface rupturing during the 28 June 1992 Landers, California, earthquake, east of Los Angeles, accommodated right-lateral offsets up to about 6 m along segments of distinct, en-echelon fault zones with a total length of 80 km. The offsets were accommodated generally not by faults—distinct slip surfaces—but rather by shear zones , tabular bands of localized shearing. Along simple stretches of fault zones at Landers the rupture is characterized by telescoping of shear zones and intensification of shearing: broad shear zones of mild shearing, containing narrow shear zones of more intense shearing, containing even narrower shear zones of very intense shearing, which may contain a fault. Thus the ground ruptured across broad belts of shearing with clearly defined, subparallel walls, oriented NW. Each broad belt consists of a broad zone of mild shearing, extending across its entire width (50 to 200 m), and much narrower (a few meters wide) shear zones that accommodate most of the offset of the belt and are portrayed by en-echelon tension cracks. In response to right-lateral shearing, the slices of ground bounded by the tension cracks rotated in a clockwise sense, producing left-lateral shearing, and the slices were forced against the walls of the shear zone, producing thrusting. Even narrower shear zones formed within the narrow shear zones. Although these probably are guides to right-lateral fault segments below, the surface rupturing during the earthquake is characterized not by faulting, but by the formation of shear zones at various scales.

California↗
USGS1994

The co-seismic slip distribution of the Landers earthquake

We derived a model for the co-seismic slip distribution on the faults which ruptured during the Landers earthquake sequence of 28 June 1992. The model is based on the inversion of surface geodetic measurements, primarily vector displacements measured using the Global Positioning System ( GPS ). The inversion procedure assumes that the slip distribution is to some extent smooth and purely right-lateral strike slip. For a given fault geometry, a family of solutions of varying smoothness can be generated. We choose the optimal model from this family based on cross-validation, which measures the predictive power of the data, and the trade-off of misfit and roughness. Solutions which give roughly equal weight to misfit and smoothness are preferred and have certain features in common: (1) there are two main patches of slip, on the Johnson Valley fault, and on the Homestead Valley, Emerson, and Camp Rock faults; (2) virtually all slip is in the upper 10 to 12 km; and (3) the model reproduces the general features of the geologically measured surface displacements, without prior constraints on the surface slip. In all models, regardless of smoothing, very little slip is required on the fault that represents the Big Bear event, and the total moment of the Landers event is 9 · 10 19 N-m. The nearly simultaneous rupture of multiple distinct faults suggests that much of the crust in this region must have been close to failure prior to the earthquake.

California↗
USGS1994

Extremal bounds on earthquake movement from geodetic data: Application to the Landers earthquake

We present a technique to place quantifiable bounds on the moment of an earthquake from geodetic data, assuming known fault geometry. Application of this technique to the 1992 Landers earthquake shows that the moment must have been between 0.84 and 1.15 × 10 20 Nm with 90% confidence ( M 7.25 to 7.34). We also find that to satisfy the data to this same level of confidence, the slip on the fault must have exceeded 7 m in at least one location, in good agreement with field mapping of the surface rupture.

California↗
USGS1994

Local observations of the onset of a large earthquake: 28 June 1992 Landers, California

The Landers earthquake ( M W 7.3) of 28 June 1992 had a very emergent onset. The first large amplitude arrivals are delayed by about 3 sec with respect to the origin time, and are preceded by smaller-scale slip. Other large earthquakes have been observed to have similar emergent onsets, but the Landers event is one of the first to be well recorded on nearby stations. We used these recordings to investigate the spatial relationship between the hypocenter and the onset of the large energy release, and to determine the slip function of the 3-sec nucleation process. Relative location of the onset of the large energy release with respect to the initial hypocenter indicates its source was between 1 and 4 km north of the hypocenter and delayed by approximately 2.5 sec. Three-station array analysis of the P wave shows that the large amplitude onset arrives with a faster apparent velocity compared to the first arrivals, indicating that the large amplitude source was several kilometers deeper than the initial onset. An M L 2.8 foreshock, located close to the hypocenter, was used as an empirical Green's function to correct for path and site effects from the first 3 sec of the mainshock seismogram. The resultant deconvolution produced a slip function that showed two subevents preceding the main energy release, an M W 4.4 followed by an M W 5.6. These subevents do not appear anomalous in comparison to simple moderate-sized earthquakes, suggesting that they were normal events which just triggered or grew into a much larger earthquake. If small and moderate-sized earthquakes commonly “detonate” much larger events, this implies that the dynamic stresses during earthquake rupture are at least as important as long-term static stresses in causing earthquakes, and the prospects of reliable earthquake prediction from premonitory phenomena are not improved.

California↗
USGS1994

Magnetic field observations in the near-field the 28 June 1992 Mw 7.3 Landers, California, earthquake

Recent reports suggest that large magnetic field changes occur prior to, and during, large earthquakes. Two continuously operating proton magnetometers, LSBM and OCHM , at distances of 17.3 and 24.2 km, respectively, from the epicenter of the 28 June 1992 M w 7.3 Landers earthquake, recorded data through the earthquake and its aftershocks. These two stations are part of a differentially connected array of proton magnetometers that has been operated along the San Andreas fault since 1976. The instruments have a sensitivity of 0.25 nT or better and transmit data every 10 min through the GOES satellite to the USGS headquarters in Menlo Park, California. Seismomagnetic offsets of −1.2 ± 0.6 and −0.7 ± 0.7 nT were observed at these sites. In comparison, offsets of −0.3 ± 0.2 and −1.3 ± 0.2 nT were observed during the 8 July 1986 M L 5.9 North Palm Springs earthquake, which occurred directly beneath the OCHM magnetometer site. The observations are generally consistent with seismomagnetic models of the earthquake, in which fault geometry and slip have the same from as that determined by either inversion of the seismic data or inversion of geodetically determined ground displacements produced by the earthquake. In these models, right-lateral rupture occurs on connected fault segments in a homogeneous medium with average magnetization of 2 A/m. The fault-slip distribution has roughly the same form as the observed surface rupture, and the total moment release is 1.1 × 10 20 Nm. There is no indication of diffusion-like character to the magnetic field offsets that might indicate these effects result from fluid flow phenomena. It thus seems unlikely that these earthquake-generated offsets and those produced by the North Palm Springs earthquake were generated by electrokinetic effects. Also, there are no indications of enhanced low-frequency magnetic noise before the earthquake at frequencies below 0.001 Hz.

California↗