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USGS1989

A satellite-based digital data system for low-frequency geophysical data

A reliable method for collection, display, and analysis of low-frequency geophysical data from isolated sites, which can be throughout North and South America and the Pacific Rim, has been developed for use with the Geostationary Operational Environmental Satellite (GOES) system. Geophysical data primarily intended for earthquake hazard and crustal deformation monitoring are digitized with either 12-bit or 16-bit resolution and transmitted every 10 min through a satellite link to a bank of UNIX-based computers in Menlo Park, California. There the data are available for analysis and display within a few seconds of their transmit time. This system provides real-time monitoring of crustal deformation parameters such as tilt, strain, fault displacement, local magnetic field, crustal geochemistry, and water levels, as well as meteorological and other parameters, along faults in California and Alaska, and in volcanic regions in the western United States, Rabaul, and other locations in the New Britain region of the South Pacific. Various mathematical, statistical, and graphical algorithms process the incoming data to detect changes in crustal deformation and fault slip that may indicate the first stages of catastrophic fault failure. Alert trigger levels based on physical models, signal resolution, and previous history have been defined for particular instrument types. Computer-driven remote paging and mail systems are used to notify appropriate personnel when alarm status is reached. The system supports continuous historical records of low-frequency geophysical data, software for extensive analysis of these data, and programs for modeling fault rupture with and without seismic radiation, as well as providing an environment for real-time attempts at earthquake prediction.

Bulletin of the Seismological Society of America↗

USGS2000

Strong motion from surface waves in deep sedimentary basins

It is widely recognized that long-period surface waves generated by conversion of body waves at the boundaries of deep sedimentary basins make an important contribution to strong ground motion. The factors controlling the amplitude of such motion, however, are not widely understood. A study of pseudovelocity response spectra of strong-motion records from the Los Angeles Basin shows that late-arriving surface waves with group velocities of about 1 km/sec dominate the ground motion for periods of 3 sec and longer. The rate of amplitude decay for these waves is less than for the body waves and depends significantly on period, with smaller decay for longer periods. The amplitude can be modeled by the equation log y = f(M, RE) + c + bRB where y is the pseudovelocity response, f(M, RE) is an attenuation relation based on a general strong-motion data set, M is moment magnitude, RE is the distance from the source to the edge of the basin, RB is the distance from the edge of the basin to the recording site, and b and c are parameters fit to the data. The equation gives values larger by as much as a factor of 3 than given by the attenuation relationships based on general strong-motion data sets for the same source-site distance. It is clear that surface waves need to be taken into account in the design of long-period structures in deep sedimentary basins. The ground-motion levels specified by the earthquake provisions of current building codes, in California at least, accommodate the long-period ground motions from basin-edge-generated surface waves for periods of 5 sec and less and earthquakes with moment magnitudes of 7.5 or less located more than 20 km outside the basin. There may be problems at longer periods and for earthquakes located closer to the basin edge. The results of this study suggest that anelastic attenuation may need to be included in attempts to model long-period motion in deep sedimentary basins. To obtain better data on surface waves in the future, operators of strong-motion networks should take special care for the faithful recording of the long-period components of ground motion. It will also be necessary to insure that at least some selected recorders, once triggered, continue to operate for a time sufficient for the surface waves to traverse the basin. With velocities of about 1 km/sec, that time will be as long as 100 sec for a basin the size of the Los Angeles Basin.

Bulletin of the Seismological Society of America↗

USGS2015

Time‐dependent renewal‐model probabilities when date of last earthquake is unknown

We derive time-dependent, renewal-model earthquake probabilities for the case in which the date of the last event is completely unknown, and compare these with the time-independent Poisson probabilities that are customarily used as an approximation in this situation. For typical parameter values, the renewal-model probabilities exceed Poisson results by more than 10% when the forecast duration exceeds ~20% of the mean recurrence interval. We also derive probabilities for the case in which the last event is further constrained to have occurred before historical record keeping began (the historic open interval), which can only serve to increase earthquake probabilities for typically applied renewal models.We conclude that accounting for the historic open interval can improve long-term earthquake rupture forecasts for California and elsewhere.

Bulletin of the Seismological Society of America↗

USGS2023

Comment on “A new decade in seismoacoustics (2010–2022)” by Fransiska Dannemann Dugick, Clinton Koch, Elizabeth Berg, Stephen Arrowsmith, and Sarah Albert

An increase in seismic stations also having microbarographs has led to increased interest in the field of seismoacoustics. A review of the recent advances in this field can be found in Dannemann Dugick et al. (2023) . The goal of this note is to draw the attention of the readers of Dannemann Dugick et al. (2023) to several additional interactions between the solid Earth and atmosphere that have not been classically considered in the field of seismoacoustics. The 15 January 2022 Hunga Tonga–Hunga Ha‘api eruption produced acoustic gravity waves that were recorded globally. For example, the Lamb wave from this eruption produced early‐arriving and long‐lasting tsunami waves. This eruption also provided globally recorded coupling of atmospheric modes with solid Earth modes, providing another example of the complex interactions that can occur at the boundary between the atmosphere and the solid Earth. Even in the absence of large atmospheric signals, collocated pressure sensors at seismic stations can be a useful tool for estimating the local substructure, such at V S 30 ⁠ , the average shear velocity of the upper 30 m. Finally, at low frequencies, it is possible to use pressure records to correct out atmospheric disturbances recorded on seismometers. We briefly review the aforementioned, nontraditional seismoacoustic topics that we feel are important to consider as part of the full suite of interactions occurring between the solid Earth and atmosphere.

Bulletin of the Seismological Society of America↗

USGS2020

Revision of Boore (2018) Ground‐motion predictions for Central and Eastern North America: Path and offset adjustments and extension to 200 m/s <= Vs30 <= 3000 m/s

The three sets of ground‐motion predictions (GMPs) of Boore (2018 ; hereafter, B18) are compared with a much larger dataset than was used in deriving the predictions. The B18 GMPs work well for response spectra at periods between ∼ 0.15 "> ∼ 0.15 ∼0.15 and 4.0 s after an adjustment accounting for a path bias at distances beyond 200 km—this was the maximum distance used to derive the stress parameters on which the simulations in B18 are based. An additional offset adjustment is needed in the B18 predictions for short and long periods. The adjustment at short periods may be because the κ 0 "> κ 0 κ0 of 0.006 s stipulated by the Next Generation Attenuation‐East (NGA‐East) project to be used in deriving the GMPs is inconsistent with the observations on rock sites. The explanation for the offset adjustment at long periods is not clear, but it could be a combination of limitations of the point‐source stochastic model for longer period motions, as well as a decreasing number of observations at longer periods available to constrain the simulations on which the predictions are based. The predictions of B18, developed for very‐hard‐rock sites ( ⁠ V S 30 "> V S 30 VS30 of 2000 and 3000    m / s "> 3000 m / s 3000 m/s ⁠ ), have here been extended down to V S 30 "> V S 30 VS30 values as low as 200    m / s "> 200 m / s 200 m/s ⁠ . I find, as have others, that for a given V S 30 "> V S 30 VS30 ⁠ , there is generally less site amplification for central and eastern North America (CENA) than for the active crustal region dataset used for the Boore, Stewart, et al. (2014 ; hereafter, BSSA14) GMP equations. This might have an impact on conclusions of several previous studies of CENA GMPs that used the site amplifications in BSSA14 in comparing data and predictions. An additional finding is that the κ 0 "> κ 0 κ0 implied by recordings on a subset of stations in the Charlevoix region located on rock (data from these stations were not used in the analysis described earlier) is more consistent with a value near 0.014 s than the 0.006 s value used in B18 and the NGA‐East project.

Seismological Research Letters↗

USGS1992

Preliminary seismological results

On April 25 and April 26, 992, three earthquakes with magnitude (M) equal to or greater than 6.6 occurred near Cape Mendocino, California. the sequence began with a relatively shallow earthquake (M=7.1) on Saturday, April 25, at 11:06 a.m. local time. The two most most powerful aftershocks has magnitudes of 6.6 and 6.7. They occurred at 00:41 a.m. and 4:18 a.m. local times, respectively, on Sunday, April 26. The mainshock had a focal depth of about 10 km and the hypocenters of the strong afterschokcs were about 20 km deep. During the next three weeks, approximately 1500 aftershocks were recorded by the seismographic networks of the U.S Geological Survey. Some 200 of these events had M ≥3.0.

California↗

USGS2017

Uncertainty, variability, and earthquake physics in ground‐motion prediction equations

Residuals between ground‐motion data and ground‐motion prediction equations (GMPEs) can be decomposed into terms representing earthquake source, path, and site effects. These terms can be cast in terms of repeatable (epistemic) residuals and the random (aleatory) components. Identifying the repeatable residuals leads to a GMPE with reduced uncertainty for a specific source, site, or path location, which in turn can yield a lower hazard level at small probabilities of exceedance. We illustrate a schematic framework for this residual partitioning with a dataset from the ANZA network, which straddles the central San Jacinto fault in southern California. The dataset consists of more than 3200 1.15≤ M ≤3 earthquakes and their peak ground accelerations (PGAs), recorded at close distances ( R ≤20 km). We construct a small‐magnitude GMPE for these PGA data, incorporating V S 30 site conditions and geometrical spreading. Identification and removal of the repeatable source, path, and site terms yield an overall reduction in the standard deviation from 0.97 (in ln units) to 0.44, for a nonergodic assumption, that is, for a single‐source location, single site, and single path. We give examples of relationships between independent seismological observables and the repeatable terms. We find a correlation between location‐based source terms and stress drops in the San Jacinto fault zone region; an explanation of the site term as a function of kappa, the near‐site attenuation parameter; and a suggestion that the path component can be related directly to elastic structure. These correlations allow the repeatable source location, site, and path terms to be determined a priori using independent geophysical relationships. Those terms could be incorporated into location‐specific GMPEs for more accurate and precise ground‐motion prediction.

Bulletin of the Seismological Society of America↗

USGS1994

Triggering of the Ms = 5.4 Little Skull Mountain, Nevada, earthquake with dynamic strains

We have developed an approach to test the viability of dynamic strains as a triggering mechanism by quantifying the dynamic strain tensor at seismogenic depths. We focus on the dynamic strains at the hypocenter of the M s = 5.4 Little Skull Mountain ( LSM ), Nevada, earthquake. This event is noteworthy because it is the largest earthquake demonstrably triggered at remote distances (∼280 km) by the M s = 7.4 Landers, California, earthquake and because of its ambiguous association with magmatic activity. Our analysis shows that, if dynamic strains initiate remote triggering, the orientation and modes of faulting most favorable for being triggered by a given strain transient change with depth. The geometry of the most probable LSM fault plane was favorably oriented with respect to the geometry of the dynamic strain tensor. We estimate that the magnitude of the peak dynamic strains at the hypocentral depth of the LSM earthquake were ∼4 μstrain (∼.2 MPa) which are ∼50% smaller than those estimated from velocity seismograms recorded at the surface. We suggest that these strains are too small to cause Mohr-Coulomb style failure unless the fault was prestrained to near failure levels, the fault was exceptionally weak, and/or the dynamic strains trigger other processes that lead to failure.

Nevada↗

USGS2016

Continuity of the West Napa–Franklin fault zone inferred from guided waves generated by earthquakes following the 24 August 2014 Mw 6.0 South Napa Earthquake

We measure peak ground velocities from fault‐zone guided waves (FZGWs), generated by on‐fault earthquakes associated with the 24 August 2014 M w 6.0 South Napa earthquake. The data were recorded on three arrays deployed across north and south of the 2014 surface rupture. The observed FZGWs indicate that the West Napa fault zone (WNFZ) and the Franklin fault (FF) are continuous in the subsurface for at least 75 km. Previously published potential‐field data indicate that the WNFZ extends northward to the Maacama fault (MF), and previous geologic mapping indicates that the FF extends southward to the Calaveras fault (CF); this suggests a total length of at least 110 km for the WNFZ–FF. Because the WNFZ–FF appears contiguous with the MF and CF, these faults apparently form a continuous Calaveras–Franklin–WNFZ–Maacama (CFWM) fault that is second only in length (∼300 km) to the San Andreas fault in the San Francisco Bay area. The long distances over which we observe FZGWs, coupled with their high amplitudes (2–10 times the S waves) suggest that strong shaking from large earthquakes on any part of the CFWM fault may cause far‐field amplified fault‐zone shaking. We interpret guided waves and seismicity cross sections to indicate multiple upper crustal splays of the WNFZ–FF, including a northward extension of the Southhampton fault, which may cause strong shaking in the Napa Valley and the Vallejo area. Based on travel times from each earthquake to each recording array, we estimate average P ‐, S ‐, and guided‐wave velocities within the WNFZ–FF (4.8–5.7, 2.2–3.2, and 1.1–2.8 km/s, respectively), with FZGW velocities ranging from 58% to 93% of the average S ‐wave velocities.

California↗

USGS2019

Rapid station and network quality analysis for temporary deployments

Seismic station data quality is commonly defined by metrics such as data completeness or background seismic noise levels in specific frequency bands. However, for temporary networks such as aftershock deployments or induced seismicity monitoring, the most critical metric is often how well the station performs when recording events of interest. A timely measure of station performance can be used for real-time network maintenance and to help make decisions about which stations may need to be moved or are redundant. We develop new event-based methods to assess station and network performance, including estimating network magnitude of completeness, determining station signal-to-noise ratios as a function of earthquake magnitude, and computing relative station amplitudes. At times, a complete catalog of local seismic events may not exist, such as in an aftershock deployment where hundreds to thousands of small earthquakes may be happening and catalog generation efforts cannot keep up. To overcome this, we use an envelope of the average energy recorded by the network to identify events of interest. We find that the log amplitude of events identified using this technique scales linearly with local earthquake magnitudes. This suggests that this approach can be used to determine seismicity rates and detection thresholds.

Seismological Research Letters↗

USGS2022

Microtremor array method using spatial autocorrelation analysis of Rayleigh‑wave data

Microtremor array measurements (MAM) and passive surface wave methods in general, have been increasingly used to non-invasively estimate shear-wave velocity structures (Vs) for various purposes. The methods estimate dispersion curves and invert them for retrieving S-wave velocity profiles. This paper summarizes principles, limitations, data collection and processing methods. It intends to enable students and practitioners to understand the principles needed to plan a microtremor array investigation, record and process the data, and evaluate the quality of investigation result. The paper focuses on the spatial autocorrelation (SPAC) processing method among microtremor array processing methods because of its relatively simple calculation and stable applicability.

Journal of Seismology↗

USGS1990

The effect of S-wave arrival times on the accuracy of hypocenter estimation

Well-constrained hypocenters (latitude, longitude, depth, and origin time) are required for nearly all studies that use earthquake data. We have examined the theoretical basis behind some of the widely accepted “rules of thumb” for obtaining accurate hypocenter estimates that pertain to the use of S phases and illustrate, in a variety of ways, why and when these “rules” are applicable. Results of experiments done for this study show that epicentral estimates (latitude and longitude) are typically far more robust with respect to data inadequacies; therefore, only examples illustrating the relationship between S phase arrival time data and focal depth and origin time estimates are presented. Most methods used to determine earthquake hypocenters are based on iterative, linearized, least-squares algorithms. Standard errors associated with hypocenter parameters are calculated assuming the data errors may be correctly described by a Gaussian distribution. We examine the influence of S-phase arrival time data on such algorithms by using the program HYPOINVERSE with synthetic datasets. Least-squares hypocenter determination algorithms have several shortcomings: solutions may be highly dependent on starting hypocenters, linearization and the assumption that data errors follow a Gaussian distribution may not be appropriate, and depth/origin time trade-offs are not readily apparent. These shortcomings can lead to biased hypocenter estimates and standard errors that do not always represent the true error. To illustrate the constraint provided by S-phase data on hypocenters determined without some of these potential problems, we also show examples of hypocenter estimates derived using a probabilistic approach that does not require linearization. We conclude that a correctly timed S phase recorded within about 1.4 focal depth's distance from the epicenter can be a powerful constraint on focal depth. Furthermore, we demonstrate that even a single incorrectly timed S phase can result in depth estimates and associated measures of uncertainty that are significantly incorrect.

Bulletin of the Seismological Society of America↗

USGS1977

Use of a hybrid computer in engineering-seismology research

A hybrid computer is an important tool in the seismological research conducted by the U.S. Geological Survey in support of the Energy Research and Development Administration nuclear explosion testing program at the Nevada Test Site and the U.S. Geological Survey Earthquake Hazard Reduction Program. The hybrid computer system, which employs both digital and analog computational techniques, facilitates efficient seismic data processing. Standard data processing operations include: (1) preview of dubbed magnetic tapes of data; (2) correction of data for instrument response; (3) derivation of displacement and acceleration time histories from velocity recordings; (4) extraction of peak-amplitude data; (5) digitization of time histories; (6) rotation of instrumental axes; (7) derivation of response spectra; and (8) derivation of relative transfer functions between recording sites. Catalog of time histories and response spectra of ground motion from nuclear explosions and earthquakes that have been processed by the hybrid computer are used in the Earthquake Hazard Research Program to evaluate the effects of source, propagation path, and site effects on recorded ground motion; to assess seismic risk; to predict system response; and to solve system design problems.

Journal of Research of the U.S. Geological Survey↗

USGS2016

Adjusting central and eastern North America ground-motion intensity measures between sites with different reference-rock site conditions

Adjustment factors are provided for converting ground‐motion intensity measures between central and eastern North America (CENA) sites with different reference‐rock site conditions ( V S 30 =760, 2000, and 3000 m/s) for moment magnitudes ranging from 2 to 8, rupture distances ranging from 2 to 1200 km, Fourier amplitude spectra (FAS) for frequencies ranging from 0.01 to 100 Hz, response spectra for periods ranging from 0.01 to 10.0 s, peak ground acceleration, and peak ground velocity. The adjustment factors are given for a wide range of the site diminution parameters ( κ 0 ) for sites with V S 30 =760 m/s and for a κ 0 of 0.006 s for two harder rock sites. Fourteen CENA velocity profiles with V S 30 values within a factor of 1.1 of 760 m/s were used to derive average FAS amplification factors as a function of frequency, which were then used in simulations of peak ground‐motion parameters and response spectra to derive the adjustment factors. The amplification function differs from that used in western North America (e.g., Campbell and Boore, 2016 ) in having a peak near 9 Hz, due to the resonance of motions in the relatively thin low‐velocity material over hard rock that characterizes many CENA sites with V S 30 near 760 m/s. We call these B/C sites, because this velocity marks the boundary between National Earthquake Hazards Reduction Program site classes B and C ( Building Seismic Safety Council, 2004 ). The adjustments for short‐period motions are sensitive to the value of κ 0 , but there are very few if any determinations of κ 0 for CENA B/C sites. For this reason, we determined κ 0 from multiple recordings at Pinyon Flat Observatory (PFO), California, which has a velocity‐depth profile similar to those of CENA B/C sites. The PFO and other results from the literature suggest that appropriate values of κ 0 for CENA B/C sites are expected to lie between 0.01 and 0.03 s.

Bulletin of the Seismological Society of America↗

USGS1989

The dependence of peak horizontal acceleration on magnitude, distance, and site effects for small-magnitude earthquakes in California and eastern North America

One-hundred and ninety free-field accelerograms recorded on deep soil (> 10 m deep) were used to study the near-source scaling characteristics of peak horizontal acceleration for 91 earthquakes (2.5 ≦ ML ≦ 5.0) located primarily in California. Of the six attenuation relationships developed in this study, the one considered most reliable is given by the expression.

Bulletin of the Seismological Society of America↗

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↗

USGS2017

Aftershocks, earthquake effects, and the location of the large 14 December 1872 earthquake near Entiat, central Washington

Reported aftershock durations, earthquake effects, and other observations from the large 14 December 1872 earthquake in central Washington are consistent with an epicenter near Entiat, Washington. Aftershocks were reported for more than 3 months only near Entiat. Modal intensity data described in this article are consistent with an Entiat area epicenter, where the largest modified Mercalli intensities, VIII, were assigned between Lake Chelan and Wenatchee. Although ground failures and water effects were widespread, there is a concentration of these features along the Columbia River and its tributaries in the Entiat area. Assuming linear ray paths, misfits from 23 reports of the directions of horizontal shaking have a local minima at Entiat, assuming the reports are describing surface waves, but the region having comparable misfit is large. Broadband seismograms recorded for comparable ray paths provide insight into the reasons why possible S – P times estimated from felt reports at two locations are several seconds too small to be consistent with an Entiat area epicenter.

Washington↗

USGS2025

Near-surface material and topography generate anomalous high-frequency ground motion amplification in Chugiak, Alaska

An ∼3 km long nodal array oriented approximately east–west was deployed in Chugiak, Alaska, by the U.S. Geological Survey during 2021. The array intersects with the permanent NetQuakes station NP.ARTY, where peak ground acceleration (PGA) value of 1.98 g was recorded during the 2018 M w 7.1 Anchorage, Alaska, earthquake, in sharp contrast to the PGA of ∼0.3 g at a site just 4 km to the west. Seismic data for M w 1.8–4.3 aftershocks from the M w 7.1 event recorded by the nodal array confirm the anomalously large ground motions obtained at NP.ARTY as well as similar amplifications at nodes within ∼1 km to the east. Here, we performed 0–10 Hz 3D finite‐difference simulations, including high‐resolution surface topography, to explore the cause of the unexpectedly large amplification. As expected, the simulations computed with a regional 3D tomography velocity model severely underpredict the 0–10 Hz acceleration records at almost all sites. Adding a near‐surface low‐velocity taper to 300 m depth amplifies the accelerations by up to a factor of 5 and enables a reasonable match between the nodal data and simulations at sites to the west of NP.ARTY. However, this model still underpredicts the spectral energy in the area covered by glacial sediments by up to an order of magnitude. The addition of a till layer using a depth‐dependent shear‐wave velocity ( ⁠⁠ V s ) profile along with a homogeneous, 8 m thick low‐velocity layer with V s = 250 m/s representing the kame terraces improves the fit to data to within a factor of 2 at nodes located on top of the glacial sediments. Our study shows that the anomalously large high‐frequency amplification recorded at and near NP.ARTY can be explained by a combination of topographic effects and near‐surface low‐velocity material with amplification effects on the high‐frequency ground motion by up to about 40% and an order of magnitude, respectively.

Alaska↗