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USGS2019

Variability in synthetic earthquake ground motions caused by source variability and errors in wave propagation models

Numerical simulations of earthquake ground motions are used both to anticipate the effects of hypothetical earthquakes by forward simulation and to infer the behaviour of the real earthquake source ruptures by the inversion of recorded ground motions. In either application it is necessary to assume some Earth structure that is necessarily inaccurate and to use a computational method that is also inaccurate for simulating the wavefield Green's functions. We refer to these two sources of error as ‘propagation inaccuracies’, which might be considered to be epistemic. We show that the variance of the Fourier spectrum of the synthetic earthquake seismograms caused by propagation inaccuracies is related to the spatial covariance on the rupture surface of errors in the computed Green's functions, which we estimate for the case of the 2009 L'Aquila, Italy, earthquake by comparing erroneous computed Green's functions with observed L'Aquila aftershock seismograms (empirical Green's functions). We further show that the variance of the synthetic seismograms caused by the rupture variability (aleatory uncertainty) is related to the spatial covariance on the rupture surface of aleatory variations in the rupture model, and we investigate the effect of correlated variations in Green's function errors and variations in rupture models. Thus, we completely characterize the variability of synthetic earthquake seismograms induced by errors in propagation and variability in the rupture behaviour. We calculate the spectra of the variance of the ground motions of the L'Aquila main shock caused by propagation inaccuracies for two specific broad-band stations, the AQU and the FIAM stations. These variances are distressingly large, being comparable or in some cases exceeding the data amplitudes, suggesting that the best-fitting L'Aquila rupture model significantly overfits the data and might be seriously in error. If these computed variances are typical, the accuracy of many other rupture models for past earthquakes may need to be reconsidered. The results of this work might be useful in seismic hazard estimation because the variability of the computed ground motion, caused both by propagation inaccuracies and variations in the rupture model, can be computed directly, not requiring laborious consideration of multiple Earth structures.

Geophysical Journal International↗
USGS2004

Acceleration and volumetric strain generated by the Parkfield 2004 earthquake on the GEOS strong-motion array near Parkfield, CA

An integrated array of 11 General Earthquake Observation System (GEOS) stations installed near Parkfield, CA provided on scale broad-band, wide-dynamic measurements of acceleration and volumetric strain of the Parkfield earthquake (M 6.0) of September 28, 2004. Three component measurements of acceleration were obtained at each of the stations. Measurements of collocated acceleration and volumetric strain were obtained at four of the stations. Measurements of velocity at most sites were on scale only for the initial P-wave arrival. When considered in the context of the extensive set of strong-motion recordings obtained on more than 40 analog stations by the California Strong-Motion Instrumentation Program (Shakal, et al., 2004 http://www.quake.ca.gov/cisn-edc) and those on the dense array of Spudich, et al, (1988), these recordings provide an unprecedented document of the nature of the near source strong motion generated by a M 6.0 earthquake. The data set reported herein provides the most extensive set of near field broad band wide dynamic range measurements of acceleration and volumetric strain for an earthquake as large as M 6 of which the authors are aware. As a result considerable interest has been expressed in these data. This report is intended to describe the data and facilitate its use to resolve a number of scientific and engineering questions concerning earthquake rupture processes and resultant near field motions and strains. This report provides a description of the array, its scientific objectives and the strong-motion recordings obtained of the main shock. The report provides copies of the uncorrected and corrected data. Copies of the inferred velocities, displacements, and Psuedo velocity response spectra are provided. Digital versions of these recordings are accessible with information available through the internet at several locations: the National Strong-Motion Program web site (http://agram.wr.usgs.gov/), the COSMOS Virtual Data Center Web site (http://www.cosmos-eq.org), and the CISN Engineering and Berkeley data centers (http://www.quake.ca.gov/cisn-edc). They are also accessible together with recordings on the GEOS Strong-motion Array near Parkfield, CA since its installation in 1987 through the USGS GEOS web site ( http://nsmp.wr.usgs.gov/GEOS).

California↗
USGS2009

Offshore double-planed shallow seismic zone in the NE Japan forearc region revealed by sP depth phases recorded by regional networks

We detected the sP depth phase at small epicentral distances of about 150 km or more in the seismograms of shallow earthquakes in the NE Japan forearc region. The focal depths of 1078 M > 3 earthquakes that occurred from 2000 to 2006 were precisely determined using the time delay of the sP phase from the initial P-wave arrival. The distribution of relocated hypocentres clearly shows the configuration of a double-planed shallow seismic zone beneath the Pacific Ocean. The upper plane has a low dip angle near the Japan Trench, increasing gradually to ???30?? at approximately 100 km landward of the Japan Trench. The lower plane is approximately parallel to the upper plane, and appears to be the near-trench counterpart of the lower plane of the double-planed deep seismic zone beneath the land area. The distance between the upper and lower planes is 28-32 km, which is approximately the same as or slightly smaller than that of the double-planed deep seismic zone beneath the land area. Focal mechanism solutions of the relocated earthquakes are determined from P-wave initial motion data. Although P-wave initial motion data for these offshore events are not ideally distributed on the focal sphere, we found that the upper-plane events that occur near the Japan Trench are characterized by normal faulting, whereas lower-plane events are characterized by thrust faulting. This focal mechanism distribution is the opposite to that of the double-planed deep seismic zone beneath the land area. The characteristics of these focal mechanisms for the shallow and deep doubled-planed seismic zones can be explained by a bending-unbending model of the subducting Pacific plate. Some of relocated earthquakes took place in the source area of the 1933 Mw8.4 Sanriku earthquake at depths of 10-23 km. The available focal mechanisms for these events are characterized by normal faulting. Given that the 1933 event was a large normal-fault event that occurred along a fault plane dipping landward, the earthquakes that currently occur just beneath or oceanwards of the Japan Trench are probably its aftershocks, suggesting that aftershock activity continues to the present day, 70 years after the main shock. ?? 2009 The Authors, Journal compilation ?? 2009 RAS.

Geophysical Journal International↗
USGS1998

Rupture characteristics of the three M ∼ 4.7 (1992-1994) Parkfield earthquakes

Slip on the San Andreas fault was determined for three M ∼ 4.7 earthquakes using a tomographic inverse system [ Beroza and Spudich , 1988] to invert seismic source time functions (STFs) from S waves. STFs were obtained by deconvolving mainshock accelerograms by those from collocated smaller earthquakes. Accelerograms were from the U.S. Geological Survey Parkfield Small Aperture Array (UPSAR) and from a distributed array of digital accelerometer stations at Parkfield. Eight or nine STFs are used in each of the three inversions. STFs are typically symmetrical pulses with a duration of about 0.3–0.5 s. In the inversion, mainshock rise time was set to 0.05 s, and we allowed the rupture time to vary slightly from a constant rupture velocity of approximately 0.85β. Rupture for all three events, which are located in or close to the Middle Mountain preparation zone or box (MMB), quickly reaches a local maximum in slip and then propagates outward to peaks, ridges, or plateaus in the slip distribution. Slip for the October 20, 1992, event (located just inside the southern edge of the MMB) propagates from an initial spike north and updip along a curving ridge for about 2 km. The initial spike continued to grow in the November 14, 1993, event (located north of the October 20, 1992, event just beneath the hypocenter of the 1966 Parkfield earthquake), which shows little directivity, although there is a smaller patch of slip updip and to the south. In contrast, rupture for the December 20, 1994, event (located just south of the October 20, 1992, event) propagated north and slightly updip, creating a rough plateau in slip a few kilometers wide on a side. Directivity for this event also is to the north. Directivity for all three events points in the approximate direction of the 1966 hypocenter. Small pulses, which comprise a coda, are found on the STFs for several seconds after the initial impulsive event. Several tests based on the assumption that the average of all STFs from UPSAR for each event is an estimate of the true slip at the source suggest that the codas in the STFs are S waves from a long-duration source rather than uncorrected site response. An initiation phase is found on the array average for the November 14, 1993, and December 20, 1994, events. These precursory phases are the result of a spike in slip at the hypocenter. A value of 2.4–4 mm is obtained for D c , the slip-weakening distance, by interpreting the initial spike as a critical patch. The few aftershocks for the October 20, 1992, event are distributed to the north and updip of the mainshock, but the November 14, 1993, event had a strong burst of aftershock activity that propagated to the north of its hypocenter at roughly the same depth. Aftershocks of the December 20, 1994, event are mostly updip. The November 14, 1993, event had the simplest slip distribution, appeared to be the most impulsive, and had the most active aftershock sequence and the greatest depth. If the eventual Parkfield earthquake initiates near the 1966 hypocenter, then the directivity of the three events studied here will have pointed to it. However, it is certainly possible that both the initiation of characteristic Parkfield shocks and the directivity of smaller events are controlled by fault properties on a larger scale such as by fault bends or jogs.

Journal of Geophysical Research B: Solid Earth↗
USGS1966

Effects of the earthquake of March 27, 1964, in the Copper River Basin area, Alaska

The Copper River Basin area is in south-central Alaska and covers 17,800 square miles. It includes most of the Copper River Basin and parts of the surrounding Alaska Range and the Talkeetna, Chugach, and Wrangell Mountains. On March 27, 1964, shortly after 5:36 p.m. Alaska standard time, a great earthquake having a Richter magnitude of about 8.5 struck south-central Alaska. Computations by the U.S. Coast and Geodetic Survey place the epicenter of the main shock at lat 61.1° N. and long 147.7° W., and the hypocenter, or actual point of origin, from 20 to 50 kilometers below the surface. The epicenter is near the western shore of Unakwik Inlet in northern Prince William Sound; it is 30 miles from the closest point within the area of study and 180 miles from the farthest point. Releveling data obtained in 1964 after the earthquake indicates that broad areas of south-central Alaska were warped by uplift and subsidence. The configuration of these areas generally parallels the trend of the major tectonic elements of the region. Presumably a large part of this change took place during and immediately after the 1964 earthquake. The water level in several wells in the area lowered appreciably, and the water in many became turbid; generally, however, within a few days after the earthquake the water level returned to normal and the suspended sediment settled out. Newspaper reports that the Copper River was completely dammed and Tazlina Lake drained proved erroneous. The ice on most lakes was cracked, especially around the margins of the lakes where floating ice broke free from the ice frozen to the shore. Ice on Tazlina, Klutina, and Tonsina Lakes was intensely fractured by waves generated by sublacustrine landslides off the fronts of deltas. These waves stranded large blocks of ice above water level along the shores. River ice was generally cracked in the southern half of the area and was locally cracked in the northern half. In the area of study, the majority of the ground cracks occurred within a radius of 100 miles from the epicenter of the earthquake. Ground cracks formed in flood plains of rivers, in deltas, and along the toes of alluvial fans. They also occurred locally in low terraces adjacent to flood plains, in highway and other fill material, along the margins of lakes, along the faces of steep slopes of river bluffs and hillsides, and in areas cleared of vegetation for several years. The ground cracks were restricted to areas underlain by unconsolidated deposits where one or more of the following conditions existed: (1) permafrost was absent or deep lying, (2) the ground-water table was near the surface, (3) bedrock was relatively deep lying, and (4) slopes were steep. Because the earthquake occurred in March, seasonal frost was present throughout the area. Despite the diversity of local conditions, the origin of most of the ground cracks can be explained by the following mechanisms: (1) lateral extension, caused by materials moving toward an unconfined face such as a lakeshore, river bluff, hillside, or terrace escarpment; (2) horizontal compaction, caused by repeated alternate compression and dilation (in the horizontal direction) of materials in flat-lying areas where there are no unconfined faces; (3) differential vertical compaction, caused by the shaking of materials that vary laterally in thickness or character; and (4) combinations of the above. Snowslides, avalanches, and rockslides were restricted to the mountainous areas surrounding the Copper River Basin. They were especially numerous in the Chugach Mountains which are closest to the epicenter of the earthquake. The large amount of snow and rock debris that has cascaded onto the icefield and glaciers of these mountains, and, probably even more important, the overall disturbance to the ice field will affect the regimen of the glaciers. Most of the damage to manmade structures occurred in the southern half of the area, and, primarily because of the sparsity of population and manmade structures, property damage was not great and no lives were lost.

Alaska↗
USGS1967

Surface faults on Montague Island associated with the 1964 Alaska earthquake

Two reverse faults on southwestern Montague Island in Prince William Sound were reactivated during the earthquake of March 27, 1964. New fault scarps, fissures, cracks, and flexures appeared in bedrock and unconsolidated surficial deposits along or near the fault traces. Average strike of the faults is between N. 37° E. and N. 47° E.; they dip northwest at angles ranging from 50° to 85°. The dominant motion was dip slip; the blocks northwest of the reactivated faults were relatively upthrown, and both blocks were upthrown relative to sea level. No other earthquake faults have been found on land. The Patton Bay fault on land is a complex system of en echelon strands marked by a series of spectacular landslides along the scarp and (or) by a zone of fissures and flexures on the upthrown block that locally is as much as 3,000 feet wide. The fault can be traced on land for 22 miles, and it has been mapped on the sea floor to the southwest of Montague Island an additional 17 miles. The maximum measured vertical component of slip is 20 to 23 feet and the maximum indicated dip slip is about 26 feet. A left-lateral strike-slip component of less than 2 feet occurs near the southern end of the fault on land where its strike changes from northeast to north. Indirect evidence from the seismic sea waves and aftershocks associated with the earthquake, and from the distribution of submarine scarps, suggests that the faulting on and near Montague Island occurred at the northeastern end of a reactivated submarine fault system that may extend discontinuously for more than 300 miles from Montague Island to the area offshore of the southeast coast of Kodiak Island. The Hanning Bay fault is a minor rupture only 4 miles long that is marked by an exceptionally well defined almost continuous scarp. The maximum measured vertical component of slip is 16⅓ feet near the midpoint, and the indicated dip slip is about 20 feet. There is a maximum left-lateral strike-slip component of one-half foot near the southern end of the scarp. Warping and extension cracking occurred in bedrock near the midpoint on the upthrown block within about 1,000 feet of the fault scarp. The reverse faults on Montague Island and their postulated submarine extensions lie within a tectonically important narrow zone of crustal attenuation and maximum uplift associated with the earthquake. However, there are no significant lithologic differences in the rock sequences across these faults to suggest that they form major tectonic boundaries. Their spatial distribution relative to the regional uplift associated with the earthquake, the earthquake focal region, and the epicenter of the main shock suggest that they are probably subsidiary features rather than the causative faults along which the earthquake originated. Approximately 70 percent of the new breakage along the Patton Bay and the Hanning Bay faults on Montague Island was along obvious preexisting active fault traces. The estimated ages of undisturbed trees on and near the fault trace indicate that no major disc placement had occurred on these faults for at least 150 to 300 years before the 1964 earthquake.

Alaska↗
USGS1995

Source parameters and crustal Q for four earthquakes in South Carolina

Two three-component seismometers (one surface and one borehole) were re-installed on the Savannah River Site (SRS), South Carolina in July 1992 to determine attenuation in the Coastal Plain sediment wedge and source parameters of local earthquakes. Four earthquakes M ∼ 1.8 to 3.6 were recorded during the next 6 months. The largest event was located near Summerville within the meizoseismal area for the 1886 Charleston earthquake. Two shocks were located 50 km to the east near Neeses, and one was located 20 km north of the SRS near Aiken. Although source parameters have been determined from strong motion data and short-period regional networks for east coast earthquakes, such as the Saguenay, Nahanni, and Mt. Laurier earthquakes (e.g., Atkinson, 1993 ), these are some of the first source parameters determined from broad-band digital recorders. Seismograms for the Summerville event are also available from Chapel Hill, North Carolina and Blacksburg, Virginia, providing estimates of t * beyond 200 km. Here we determine source parameters such as moment, stress drop, and the attenuation parameter t * using a non-linear least-squares algorithm. We do not correct for site response because the deepest borehole is not below the Coastal Plain sediments (about 300m thick at this site) and because only one station is available for most of the data. Values of t* are marginally higher from seismograms recorded at the surface when compared to records from the 91m depth borehole seismograph. A value of 170-200 bars was determined for the Brune stress drop of the Summerville event using the borehole data, which is high compared to a value of 50 bars usually specified for modeling strong motion in western North America, but similar to other estimates for eastern North America. A higher stress drop leads to a higher seismic risk because peak acceleration is approximately proportional to stress drop. Moreover, mid- to upper-crustal Qs are in the range of 2,000 to 3,000, which would permit the propagation of high frequency seismic waves. A comparison of the surface records from the SRS with a record from the USGS dense array at Parkfield, CA for an event at about the same distance range and moment as the Summerville event-SRS case shows that the peak acceleration of the Summerville event is 16 times higher than that for the event from California (stress drop of 21 bars).

South Carolina↗
USGS1988

Retardations in fault creep rates before local moderate earthquakes along the San Andreas fault system, central California

Records of shallow aseismic slip (fault creep) obtained along parts of the San Andreas and Calaveras faults in central California demonstrate that significant changes in creep rates often have been associated with local moderate earthquakes. An immediate postearthquake increase followed by gradual, long-term decay back to a previous background rate is generally the most obvious earthquake effect on fault creep. This phenomenon, identified as aseismic afterslip, usually is characterized by above-average creep rates for several months to a few years. In several cases, minor step-like movements, called coseismic slip events, have occurred at or near the times of mainshocks. One extreme case of coseismic slip, recorded at Cienega Winery on the San Andreas fault 17.5 km southeast of San Juan Bautista, consisted of 11 mm of sudden displacement coincident with earthquakes of ML=5.3 and ML=5.2 that occurred 2.5 minutes apart on 9 April 1961. At least one of these shocks originated on the main fault beneath the winery. Creep activity subsequently stopped at the winery for 19 months, then gradually returned to a nearly steady rate slightly below the previous long-term average. The phenomena mentioned above can be explained in terms of simple models consisting of relatively weak material along shallow reaches of the fault responding to changes in load imposed by sudden slip within the underlying seismogenic zone. In addition to coseismic slip and afterslip phenomena, however, pre-earthquake retardations in creep rates also have been observed. Onsets of significant, persistent decreases in creep rates have occurred at several sites 12 months or more before the times of moderate earthquakes. A 44-month retardation before the 1979 ML=5.9 Coyote Lake earthquake on the Calaveras fault was recorded at the Shore Road creepmeter site 10 km northwest of Hollister. Creep retardation on the San Andreas fault near San Juan Bautista has been evident in records from one creepmeter site for the past 5 years. Retardations with durations of 21 and 19 months also occurred at Shore Road before the 1974 and 1984 earthquakes of ML=5.2 and ML=6.2, respectively. Although creep retardation remains poorly understood, several possible explanations have been discussed previously. (1) Certain onsets of apparent creep retardation may be explained as abrupt terminations of afterslip generated from previous moderate-mainshock sequences. (2) Retardations may be related to significant decreases in the rate of seismic and/or aseismic slip occurring within or beneath the underlying seismogenic zone. Such decreases may be caused by changes in local conditions related to growth of asperities, strain hardening, or dilatancy, or perhaps by passage of stress-waves or other fluctuations in driving stresses. (3) Finally, creep rates may be lowered (or increased) by stresses imposed on the fault by seismic or aseismic slip on neighboring faults. In addition to causing creep-rate increases or retardations, such fault interactions occasionally may trigger earthquakes. Regardless of the actual mechanisms involved and the current lack of understanding of creep retardation, it appears that shallow fault creep is sensitive to local and regional effects that promote or accompany intermediate-term preparation stages leading to moderate earthquakes. A strategy for more complete monitoring of fault creep, wherever it is known to occur, therefore should be assigned a higher priority in our continuing efforts to test various hypotheses concerning the mechanical relations between seismic and aseismic slip. ?? 1988 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗
USGS2001

Borehole P- and S-wave velocity at thirteen stations in Southern California

The U.S. Geological Survey (USGS), as part of a program to acquire seismic velocity data at locations of strong-ground motion in earthquakes (e.g., Gibbs et al., 2000), has investigated thirteen additional sites in the Southern California region. Of the thirteen sites, twelve are in the vicinity of Whittier, California, and one is located in San Bernardino, California. Several deployments of temporary seismographs were made after the Whittier Narrows, California earthquake of 1 October 1987 (Mueller et al., 1988). A deployment, between 2 October and 9 November 1987, was the motivation for selection of six of the drill sites. Temporary portable seismographs at Hoover School (HOO), Lincoln School (LIN), Corps of Engineers Station (NAR), Olive Junior High School (OLV), Santa Anita Golf Course (SAG), and Southwestern Academy (SWA) recorded significant aftershock data. These portable sites, with the exception of Santa Anita Golf Course, were co-sited with strong-motion recorders. Stations at HOO, Lincoln School Whittier (WLB), Saint Paul High School (STP), Alisos Adult School (EXC), Cerritos College Gymnasium (CGM), Cerritos College Physical Science Building (CPS), and Cerritos College Police Building (CPB) were part of an array of digital strong-motion stations deployed from "bedrock" in Whittier to near the deepest part of the Los Angeles basin in Norwalk. Although development and siting of this new array (partially installed at the time of this writing) was generally motivated by the Whittier Narrows earthquake, these new sites (with the exception of HOO) were not part of any Whittier Narrows aftershock deployments. A similar new digital strong-motion site was installed at the San Bernardino Fire Station during the same time frame. Velocity data were obtained to depths of about 90 meters at two sites, 30 meters at seven sites, and 18 to 25 meters at four sites. Lithology data from the analysis of cuttings and samples was obtained from the two 90-meter deep holes and from five of the shallower holes to supplement the velocity interpretation. The two 90-meter boreholes (SB1, CPB) have been instrumented with borehole seismometers for continuous monitoring of earthquake activity (Rogers et al., 1998). No drill samples or cuttings were obtained from the other six sites, but driller's logs were scanned for major changes noted there. The velocity models at those sites were interpreted using only the measured data and major changes in the driller's log as noted above. The sites are shown in Figure 1 and listed in Table 1, which gives references to information regarding the strong-motion data. Several hundred strong-motion records of the main-shock were written by this moderate size earthquake (ML = 5.9), making it important from a scientific and engineering prospective (Brady et al., 1988; Shakal et al., 1988).

California↗