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USGS1982

Design concepts for a Global Telemetered Seismograph Network

This study represents a first step in developing an integrated, real-time global seismic data acquisition system a Global Telemetered Seismograph Network (GTSN). The principal objective of the GTSN will be to acquire reliable, high-quality, real-time seismic data for rapid location and analysis of seismic events. A secondary, but important, objective of the GTSN is to augment the existing off-line seismic data base available for research. The deployment of the GTSN will involve a variety of interrelated activities development of the data acquisition and receiving equipment, establishment of satellite and terrestrial communication links, site selection and preparation, training of station personnel, equipment installation, and establishment of support facilities. It is a complex program and the development of a sound management plan will be essential. The purpose of this study is not to fix design goals or dictate avenues of approach but to develop working concepts that may be used as a framework for program planning. The international exchange of seismic data has been an important factor in the progress that has been made during the past two decades in our understanding of earthquakes and global tectonics. The seismic data base available for analysis and research is derived principally from the Global Seismograph Network (GSN), which is funded and managed by the U.S. Geological Survey (USGS). The GSN comprises some 120 seismograph stations located in more than 60 countries of the world. Established during the 1960 s with the installation of the World-Wide Standardized Seismograph Network (WWSSN) , the GSN has been augmented in recent years by the installation of more advanced data systems, such as the Seismic Research Observatories (SRO), the modified High-Gain LongPeriod (ASRO) seismographs, and the digital WWSSN (DWWSSN). The SRO, ASRO, and DWWSSN stations have the common, distinctive feature of digital data recording, so they are known collectively as the Global Digital Seismograph Network (GDSN). The fundamental objective in operating the GSN is to create and update a seismic data base that is accessible without restrictions to organizations and research scientists throughout the world. The USGS provides cooperating stations with instrumentation, training, and continuing support, including supplies and on-site maintenance. In return, the host organization operates the equipment and sends the recorded data to the USGS. Analog data (seismograms) are microfilmed and about four million copies are requested annually by researchers. Digital data, which are recorded on magnetic tape, are organized by the USGS Albuquerque Seismological Laboratory (ASL) into networkday tapes and copies of the day tapes are furnished to data users through national and regional data centers. After copying, original data are returned to the stations and used for local research. Most of the stations in the GSN also provide the USGS with seismic readings « phase arrival times and amplitudes scaled from the seismograms. These readings are transmitted on a daily or biweekly basis via commercial or diplomatic communication channels. They are used by the USGS National Earthquake Information Service (NEIS) to determine the location and magnitude of earthquakes occurring throughout the world. The results are published monthly in bulletins that are distributed to the participating stations and virtually all scientific organizations that are involved in seismological studies. It is a much-valued service that provides a current, updated catalog of seismic activity on a global scale. The NEIS also has the responsibility for rapid reporting of large and potentially destructive earthquakes. The NEIS issues news bulletins as soon as possible after the occurrence of magnitude 6.5 or greater earthquakes (magnitude 5 or greater in the conterminous United States). The news bulletins are sent to disaster relief, public safety, and other interested organizations. Tsunami warnings issued to countries bordering the Pacific Ocean are based initially on earthquake location and magnitude data. Rapid reporting of earthquakes requires real-time waveform data or readings. Currently, signals are being telemetered from more than thirty stations in the United States to the NEIS, which is located in Golden, Colorado. An extension of the telemetry network to other countries will provide the seismological community with a significantly improved means of monitoring earthquake activity in real time; it will lower the response time for determining the location and magnitude of potentially destructive or tsunamigenic earthquakes and it will provide more timely information that may be needed by governments to respond promptly.

Open-File Report↗

USGS1982

A study of surface and subsurface ground motions at Calico Hills, Nevada Test Site

A study of earthquake ground motions recorded at depth in a drill hole and at the ground surface has derived the surface to subsurface transfer functions such as might be expected at a potential nuclear waste repository in a similar setting. The site under investigation has small seismic velocity contrasts in the layers of rock between the surface and the subsurface seismometer location. The subsurface seismic motions were similar in spectral characteristics to the surface motions and were lower in amplitude across the recorded band-width by a factor of 1.5.

Open-File Report↗

USGS1988

The record of major quaternary sea-level changes in a large coastal plain estuary, Chesapeake Bay, Eastern United States

Seismic-reflection surveys of the Chesapeake Bay, combined with geologic mapping and analysis of boreholes on the Delmarva Peninsula, provide evidence of at least three generations of the Susquehanna River system and three generations of the Chesapeake Bay. The evidence for ancient courses of the Susquehanna River is preserved as three distinct paleochannels, and evidence for ancient versions of the Chesapeake Bay is preserved as three sets of paleochannel fill beneath the bay and three generations of barrier-spit deposits on the southern Delmarva Peninsula. The paleochannels represent relative sea-level minima and the channel-fill and barrier-spit deposits represents relative sea-level maxima. A history of three major marine transgressions is recorded in the stratigraphy preserved in the filled paleochannels and in the overlying barrier-spit complexes: three systematic progressions from fluvial to estuarine to bay or nearshore marine environments. This sea-level record seems to be compatible with the saw-toothed pattern of the marine oxygen-isotope record and with the concept of glacial-interglacial terminations. It also seems to have a climax character in which most of the preserved evidence is related to the largest terminations and to the extreme sea-level positions that bound those terminations. The three paleochannel-fill and barrier-spit complexes appear to correspond to oxygen-isotope stages 1,5, and either 7 or 11; the three related paleochannels correspond to stages 2, 6, and either 8 or 12.

Delaware, Maryland, Virginia↗

USGS1996

Urban seismology - Northridge aftershocks recorded by multi-scale arrays of portable digital seismographs

We deployed portable digital seismographs in the San Fernando Valley (SFV), the Los Angeles basin (LAB), and surrounding hills to record aftershocks of the 17 January 1994 Northridge California earthquake. The purpose of the deployment was to investigate factors relevant to seismic zonation in urban areas, such as site amplification, sedimentary basin effects, and the variability of ground motion over short baselines. We placed seismographs at 47 sites (not all concurrently) and recorded about 290 earthquakes with magnitudes up to 5.1 at five stations or more. We deployed widely spaced stations for profiles across the San Fernando Valley, as well as five dense arrays (apertures of 200 to 500 m) in areas of high damage, such as the collapsed Interstate 10 overpass, Sherman Oaks, and the collapsed parking garage at CalState Northridge. Aftershock data analysis indicates a correlation of site amplification with mainshock damage. We found several cases where the site amplification depended on the azimuth of the aftershock, possibly indicating focusing from basin structures. For the parking garage array, we found large ground-motion variabilities (a factor of 2) over 200-m distances for sites on the same mapped soil unit. Array analysis of the aftershock seismograms demonstrates that sizable arrivals after the direct S waves consist of surface waves traveling from the same azimuth as that of the epicenter. These surface waves increase the duration of motions and can have frequencies as high as about 4 Hz. For the events studied here, we do not observe large arrivals reflected from the southern edge of the San Fernando Valley.

California↗

USGS2012

Using micro-seismicity and seismic velocities to map subsurface geologic and hydrologic structure within the Coso geothermal field, California

Geothermal reservoirs derive their capacity for fluid and heat transport in large part from faults and fractures. Micro-seismicity generated on such faults and fractures can be used to map larger fault structures as well as secondary fractures that add access to hot rock, fluid storage and recharge capacity necessary to have a sustainable geothermal resource. Additionally, inversion of seismic velocities from micro-seismicity permits imaging of regions subject to the combined effects of fracture density, fluid pressure and steam content, among other factors. We relocate 14 years of seismicity (1996-2009) in the Coso geothermal field using differential travel times and simultaneously invert for seismic velocities to improve our knowledge of the subsurface geologic and hydrologic structure. We utilize over 60,000 micro-seismic events using waveform cross-correlation to augment to expansive catalog of P- and S-wave differential travel times recorded at Coso. We further carry out rigorous uncertainty estimation and find that our results are precise to within 10s of meters of relative location error. We find that relocated micro-seismicity outlines prominent, through-going faults in the reservoir in some cases. We also find that a significant portion of seismicity remains diffuse and does not cluster into more sharply defined major structures. The seismic velocity structure reveals heterogeneous distributions of compressional (Vp) and shear (Vs) wave speed, with Vp generally lower in the main field when compared to the east flank and Vs varying more significantly in the shallow portions of the reservoir. The Vp/Vs ratio appears to outline the two main compartments of the reservoir at depths of -0.5 to 1.5 km (relative to sea-level), with a ridge of relatively high Vp/Vs separating the main field from the east flank. In the deeper portion of the reservoir this ridge is less prominent. Our results indicate that high-precision relocations of micro-seismicity can provide useful insight into: 1) prominent structural features, faults and fractures that contribute to the flow of fluid and heat in the reservoir; 2) diffuse seismicity throughout the reservoir representing fractures that likely contribute to the overall permeability, storage and heat exchange capacity of the reservoir, but which are not confined to prominent faults; and 3) seismic velocities that outline the major hydrologic compartments within the Coso geothermal field.

Coso geothermal field↗

USGS1963

Crustal structure in the western United States; study of seismic propagation paths and regional traveltimes in the California-Nevada region

The U.S. Geological Survey, with the assistance of United ElectroDynamics, Inc., completed ten weeks of seismic-refraction field work during the summer of 1962 in the southwestern part of the United States. This work was a continuation of a program initiated in 1961 to study traveltimes and seismic propagation paths in the earth?s crust and upper mantle in the western United States. A total of 761 seismograms were recorded along 10 profiles from 86 explosions at 18 shotpoints. Analysis of the data is continuing, but a few conclusions can be made from a preliminary study: (1) Variations in traveltimes in the Basin and Range province are large but measurable, and perhaps predictable. (2) Traveltimes of seismic waves in adjacent geologic provinces are usually significantly different. (3) The velocity of P g along all of the profiles recorded in 1962 ranges from 5.0 to 6.5 km/sec, and averages 6.0 km/sec. (4) The average velocity of P g in extreme northern Nevada and southern Idaho is 5.6 km/sec, and it is 6.1 km/sec in most of Nevada and California. (5) The average velocity of P n is 7–9 km/ sec and ranges from 7.85 to 7.95 km/sec on reversed profiles where the true P n velocity could be computed. (6) A shallow "intermediate" layer with a velocity of approximately 6.8 km/sec was found in the Snake River Plain. (7) Refraction arrivals from the mantle (P n ) were recorded in the Sierra Nevada. They indicate that the thickness of the crust in the Sierra Nevada is much greater than that in the Basin and Range province. (8) Many refinements in field techniques were made during the 1962 field season.

California;Nevada;Idaho↗

USGS2021

Quantifying eruptive and background seismicity, deformation, degassing, and thermal emissions at volcanoes in the United States during 1978–2020

An important aspect of volcanic hazard assessment is determination of the level and character of background activity at a volcano so that deviations from background (called unrest) can be identified. Here, we compile the instrumentally recorded eruptive and noneruptive activity for 161 US volcanoes between 1978 and 2020. We combine monitoring data from four techniques: seismicity, ground deformation, degassing, and thermal emissions. To previous work, we add the first comprehensive survey of US volcanoes using medium-spatial resolution satellite thermal observations, newly available field surveys of degassing, and new compilations of seismic and deformation data. We report previously undocumented thermal activity at 30 volcanoes using data from the spaceborne ASTER sensor during 2000–2020. To facilitate comparison of activity levels for all US volcanoes, we assign a numerical classification of the Activity Intensity Level for each monitoring technique, with the highest ranking corresponding to an eruption. There are 96 US volcanoes (59%) with at least one type of detected activity, but this represents a lower bound: For example, there are 12 volcanoes where degassing has been observed but has not yet been quantified. We identify dozens of volcanoes where volcanic activity is only measured by satellite (45% of all thermal observations), and other volcanoes where only ground-based sensors have detected activity (e.g., all seismic and 62% of measured degassing observations). Our compilation provides a baseline against which future measurements can be compared, demonstrates the need for both ground-based and remote observations, and serves as a guide for prioritizing future monitoring efforts.

Journal of Geophysical Research↗

USGS2012

A teleseismic analysis of the New Brunswick earthquake of January 9, 1982

The analysis of the New Brunswick earthquake of January 9, 1982, has important implications for the evaluation of seismic hazards in eastern North America. Although moderate in size ( m b 5.7), it was well-recorded teleseismically. Source characteristics of this earthquake have been determined from analysis of data that were digitally recorded by the Global Digital Seismograph Network. From broadband displacement and velocity records of P waves, we have obtained a dynamic description of the rupture process as well as conventional static properties of the source. The depth of the hypocenter is estimated to be 9 km from depth phases. The focal mechanism determined from the broadband data corresponds to predominantly thrust faulting. From the variation in the waveforms the direction of slip is inferred to be updip on a west dipping NNE striking fault plane. The steep dip of the inferred fault plane suggests that the earthquake occurred on a preexisting fault that was at one time a normal fault. From an inversion of bodywave pulse durations, the estimated rupture length is 5.5 km. Average properties of the rupture process were examined by a moment tensor analysis of long-period P and SH body waves. The long-period moment of this earthquake was 5.3 × 10 24 dyne cm. The static and dynamic stress drops are 41 and 65 bars, respectively, similar to those of many earthquakes with similar moment in regions that are more seismically active. The joint epicenter determination algorithm was used to locate, relative to the mainshock, the three teleseismically recorded aftershocks that occurred through March 31, 1982. The relocated hypocenters of the aftershocks are significantly different from each other and from that of the mainshock; they provide additional support for the source dimensions inferred from the waveform analysis.

Journal of Geophysical Research Solid Earth↗

USGS2019

Chesapeake Bay impact structure—Development of "brim" sedimentation in a multilayered marine target

The late Eocene Chesapeake Bay impact structure was formed in a multilayered target of seawater underlain sequentially by a sediment layer and a rock layer in a continental-shelf environment. Impact effects in the “brim” (annular trough) surrounding and adjacent to the transient crater, between the transient crater rim and the outer margin, primarily were limited to the target-sediment layer. Analysis of published and new lithostratigraphic, biostratigraphic, sedimentologic, petrologic, and mineralogic studies of three core holes, and published studies of a fourth core hole, provided information for the interpretation of the impact processes, their interactions and relative timing, their resulting products, and sedimentation in the brim. Most studies of marine impact-crater materials have focused on those found in the central crater. There are relatively few large, complex marine craters, of which most display a wide brim around the central crater. However, most have been studied using minimal data sets. The large number of core holes and seismic profiles available for study of the Chesapeake Bay impact structure presents a special opportunity for research. The physical and chronologic records supplied by study of the sediment and rock cores of the Chesapeake Bay impact indicate that the effects of the initial, short-lived contact and compression and excavation stages of the impact event primarily were limited to the transient crater. Only secondary effects of these processes are evident in the brim. The preserved record of the brim was created primarily in the subsequent modification stage. In the brim, the records of early impact processes (e.g., outgoing tsunamis, overturned flap collapse) were modified or removed by later processes. Transported and rotated, large and small clasts of target sediments, and intervals of fluidized sands indicate that seismic shaking fractured and partially fluidized the Cretaceous and Paleogene target sediments, which led to their inward transport by collapse and lateral spreading toward the transient crater. The succeeding inward seawater-resurge flow quickly overtook and interacted with the lateral spreading, further facilitating sediment transport across the brim and into the transient crater. Variations in the cohesion and relative depth of the target sediments controlled their degree of disaggregation and redistribution during these events. Melt clasts and shocked and unshocked rock clasts in the resurge sediments indicate fallout from the ejecta curtain and plume. Basal parautochthonous remnant sections of target Cretaceous sediments in the brim thin toward the collapsed transient crater. Overlying seawater-resurge deposits consist primarily of diamictons that vary laterally in thickness, and vertically and laterally in maximum grain size. After cessation of resurge flow and re-establishment of pre-impact sea level, sandy sediment gravity flows moved from the margin to the center of the partially filled impact structure (shelf basin). The uppermost unit consists of stratified sediments deposited from suspension. Postimpact clayey silts cap the crater fill and record the return to shelf sedimentation at atypically large paleodepths within the shelf basin. An unresolved question involves a section of gravel and sand that overlies Neoproterozoic granite in the inner part of the brim in one core hole. This section may represent previously unrecognized, now parautochthonous Cretaceous sediments lying nonconformably above basement granite, or it may represent target sediments that were moved significant distances by lateral spreading above basement rocks or above a granite megaclast from the overturned flap. The Chesapeake Bay impact structure is perhaps the best documented example of the small group of multilayer, marine-target impacts formed in continental shelves or beneath epeiric seas.

Chesapeake Bay↗

USGS2016

Differences in coastal subsidence in southern Oregon (USA) during at least six prehistoric megathrust earthquakes

Stratigraphic, sedimentologic (including CT 3D X-ray tomography scans), foraminiferal, and radiocarbon analyses show that at least six of seven abrupt peat-to-mud contacts in cores from a tidal marsh at Talbot Creek (South Slough, Coos Bay), record sudden subsidence (relative sea-level rise) during great megathrust earthquakes at the Cascadia subduction zone. Data for one contact are insufficient to infer whether or not it records a great earthquake—it may also have formed through local, non-seismic, hydrographic processes. To estimate the amount of subsidence marked by each contact, we expanded a previous regional modern foraminiferal dataset to 174 samples from six Oregon estuaries. Using a transfer function derived from the new dataset, estimates of coseismic subsidence across the six earthquake contacts vary from 0.31 m to 0.75 m. Comparison of subsidence estimates for three contacts in adjacent cores shows within-site differences of ≤0.10 m, about half the ±0.22 m error, although some estimates may be minimums due to uncertain ecological preferences for Balticammina pseudomacrescens in brackish environments and almost monospecific assemblages of Miliammina fusca on tidal flats. We also account for the influence of taphonomic processes, such as infiltration of mud with mixed foraminiferal assemblages into peat, on subsidence estimates. Comparisons of our subsidence estimates with values for correlative contacts at other Oregon sites suggest that some of our estimates are minimums and that Cascadia's megathrust earthquake ruptures have been heterogeneous over the past 3500 years.

Oregon↗

USGS2018

Induced earthquake families reveal distinctive evolutionary patterns near disposal wells

The timing of events in seismic sequences can provide insights into the physical processes controlling fault slip. In southern Kansas, the rate of earthquakes rose rapidly starting in 2013 following expansion of energy production into the area, demanding the disposal of large volumes of wastewater into deep wells. Seismicity catalogs that are complete to low magnitudes can provide insights into the physical processes that induce seismicity near wastewater disposal. We develop a catalog of over 130,000 earthquakes recorded in southern Kansas from mid‐March 2014 through December 2017 by applying a matched filter algorithm to an original catalog of 5,831 template earthquakes. Detections have nearly identical waveforms to their associated template event and represent slip on nearly co‐located sections of a fault. We select template events with at least 100 associated detections and examine the characteristics of these prolific families of earthquakes. We find that families located close (<10 km) to areas with significant volumes of injected fluids have near‐Poissonian interevent times and the families remain active over longer durations. Families farther from high‐volume injection wells show strong clustering of interevent times and shorter sequence durations. We conclude that increasing pore fluid pressures from nearby disposal of large volumes of wastewater is the primary driver of these long duration episodes, with earthquake‐earthquake interactions driving sequences at greater distance from the wells.

Kansas↗

USGS2012

Intrusion of horizontal dikes: tectonic significance of Middle Proterozoic diabase sheets widespread in the upper crust of the southwestern United States

Initially horizontal sheet intrusions of Middle Proterozoic diabase are abundant in a region 650 by 300 km across in Arizona and California. The diabase forms discordant sheets in basement granite and gneiss and sills in overlying shelf sedimentary sequences. Massive granite is the most common basement host for the sheets, probably because it fractured more easily than foliated hosts during sheet emplacement. Steep feeder dikes are rare compared to the sheets. The diabase in many places is exposed in fault blocks that were tilted during Tertiary tectonic extension. Structure sections restored from the map patterns of upended blocks show that the sheets were intruded at levels throughout the upper crust, to depths of at least 13 km. Sheet intrusion implies a vertical orientation of the least compressive stress, so I conclude that the crust was under tectonic compression or in an isotropic state of stress at the time of diabase intrusion about 1.1 Ga. Magma overpressures, water encountered by rising magma, vertical changes in the crustal stress regime, and flotation of low-density granite all may be important factors for sheet intrusion. The stress conditions suggested by the presence of the sheets argue against an extensional tectonic regime earlier proposed for the diabase event. The diabase province contrasts structurally with similar-age provinces of basaltic magmatism elsewhere in North America that show evidence of tectonic extension, such as the midcontinent rift. The consistent orientations of the sheets commonly allow them to be used as structural markers for postdiabase deformation of basement blocks. Sheet intrusions in the geologic record may be seriously underreported. Their recognition is important for the interpretation of seismic reflection profiles of continental crust. Sheets in Arizona may be responsible for the Bagdad reflection sequence, which extends to depths of at least 15 km.

Journal of Geophysical Research↗

USGS2002

Implications for evaporite tectonism in the Carbondale and Eagle collapse centers of west-central Colorado, based on reprocessed seismic reflection data

Reprocessing of approximately 64 km of seismic reflection data along five seismic lines has led to new interpretations of evaporite tectonism associated with the Pennsylvanian Eagle Valley Evaporite of the central Colorado trough. Evidence of the oldest evaporite tectonism in the region, imaged by the seismic data, occurs in the Eagle collapse center at Hardscrabble Mountain. Here, the Triassic-Permian State Bridge Formation is abnormally thick, reflecting an outflow of underlying evaporite. On the west side of the Carbondale collapse center, structural wedging formed the west-dipping Grand Hogback monocline during early Tertiary Laramide deformation close to the western margin of the late Paleozoic central Colorado trough. Uplimb thrusts beneath the steep face of the monocline provide evidence of backthrusting. The presence of the Roaring Fork diapir proximal to the point of greatest curvature of the Grand Hogback monocline at the Leadville reflector level suggests that diapirism may have begun during formation of the monocline. Evidence for late Cenozoic evaporite tectonism is observed in all the seismic lines in spite of the difficulty of imaging shallow, young deposits that record evidence of abundant late Cenozoic evaporite tectonism observed during geologic mapping.

Colorado↗

USGS2015

Aftershocks illuminate the 2011 Mineral, Virginia, earthquake causative fault zone and nearby active faults

Deployment of temporary seismic stations after the 2011 Mineral, Virginia (USA), earthquake produced a well-recorded aftershock sequence. The majority of aftershocks are in a tabular cluster that delineates the previously unknown Quail fault zone. Quail fault zone aftershocks range from ~3 to 8 km in depth and are in a 1-km-thick zone striking ~036° and dipping ~50°SE, consistent with a 028°, 50°SE main-shock nodal plane having mostly reverse slip. This cluster extends ~10 km along strike. The Quail fault zone projects to the surface in gneiss of the Ordovician Chopawamsic Formation just southeast of the Ordovician–Silurian Ellisville Granodiorite pluton tail. The following three clusters of shallow (<3 km) aftershocks illuminate other faults. (1) An elongate cluster of early aftershocks, ~10 km east of the Quail fault zone, extends 8 km from Fredericks Hall, strikes ~035°–039°, and appears to be roughly vertical. The Fredericks Hall fault may be a strand or splay of the older Lakeside fault zone, which to the south spans a width of several kilometers. (2) A cluster of later aftershocks ~3 km northeast of Cuckoo delineates a fault near the eastern contact of the Ordovician Quantico Formation. (3) An elongate cluster of late aftershocks ~1 km northwest of the Quail fault zone aftershock cluster delineates the northwest fault (described herein), which is temporally distinct, dips more steeply, and has a more northeastward strike. Some aftershock-illuminated faults coincide with preexisting units or structures evident from radiometric anomalies, suggesting tectonic inheritance or reactivation.

Virginia↗

USGS1987

Seismic stratigraphy and facies of continental slope and rise seaward of Baltimore Canyon Trough

As part of a survey of the United States continental rise seaward of the northern Baltimore Canyon Trough, four major depositional sequences were mapped on a grid of 2,350 km of multichannel seismic reflection profiles. The sequences, which range in age from Jurassic (?) to Quaternary, record a gradual sedimentary buildup of fine-grained onlapping and slope-front fill. A broad wedge of Jurassic-age (?) sediment up to 5 km thick was deposited seaward of a conspicuous platform. During the Cretaceous, the slope-rise transition became much gentler, and sequences are more blanket-like because the declivity seaward of the platform was smoothed and filled by fine-grained clastic sediments and thin-bedded limestones. The main constructional phase for the continental rise was during the Cenozoic, when a thick (0.1-2.4 km) wedge formed seaward of the shelf edge in response to major fluctuations in sea level and erosion of the gentle, ancestral continental slope. The Cenozoic rise section can be subdivided into two main sequences separated by a conspicuous unconformity. The lower sequence is mostly a blanket (0.2-0.8 km thick) of Paleogene hemipelagic ooze and claystone. The sequence above the unconformity is a complex association of Neogene slump deposits, turbidites, hemipelagic clays, and channel fill that thickens seaward to 2.2 km under the middle continental rise. The final phase of rise construction was caused by widespread fluctuations in coastal onlap. These regressions resulted in deltaic outward-building on the shelf, extensive Pleistocene landward erosion of the slope, and the accumulation of a broad sedimentary apron on the rise.

American Association of Petroleum Geologists Bulle↗

USGS1986

Preliminary report on aftershock sequence for earthquake of January 31, 1986, near Painesville, Ohio (time period: 2/1/86-2/10/86)

A ten-station array of broad-band digital instrumentation (GEOS) was deployed by the U. S. Geological Survey with partial support provided by Electric Power Research Institute to record the aftershock sequence of the moderate (mb ~ 4.9) earthquake that occurred on January 31, 1986 (16:46:43 UTC) near Painesville, Ohio. The occurrence of the event has raised questions concerning possible contributory factors to the occurrence of the event and questions concerning the character of earthquake-induced high-frequency ground motions in the area. To aid in the timely resolution of the implications of some of these questions, this preliminary report provides copies of the ground motion time-histories and corresponding spectra for the six identified aftershocks and two events, thought to be quarry blasts, recorded as of February 10, 1986. Recording station locations and epicenter locations based on two preliminary estimates of local seismic velocity structure are provided.

Ohio↗

USGS2004

The global short-period wavefield modelled with a Monte Carlo seismic phonon method

At high frequencies (∼1 Hz), much of the seismic energy arriving at teleseismic distances is not found in the main phases (e.g. P , PP , S , etc.) but is contained in the extended coda that follows these arrivals. This coda results from scattering off small-scale velocity and density perturbations within the crust and mantle and contains valuable information regarding the depth dependence and strength of this heterogeneity as well as the relative importance of intrinsic versus scattering attenuation. Most analyses of seismic coda to date have concentrated on S -wave coda generated from lithospheric scattering for events recorded at local and regional distances. Here, we examine the globally averaged vertical-component, 1-Hz wavefield (>10° range) for earthquakes recorded in the IRIS FARM archive from 1990 to 1999. We apply an envelope-function stacking technique to image the average time–distance behavior of the wavefield for both shallow (≤50 km) and deep (≥500 km) earthquakes. Unlike regional records, our images are dominated by P and P coda owing to the large effect of attenuation on PP and S at high frequencies. Modelling our results is complicated by the need to include a variety of ray paths, the likely contributions of multiple scattering and the possible importance of P -to- S and S -to- P scattering. We adopt a stochastic, particle-based approach in which millions of seismic phonons are randomly sprayed from the source and tracked through the Earth. Each phonon represents an energy packet that travels along the appropriate ray path until it is affected by a discontinuity or a scatterer. Discontinuities are modelled by treating the energy normalized reflection and transmission coefficients as probabilities. Scattering probabilities and scattering angles are computed in a similar fashion, assuming random velocity and density perturbations characterized by an exponential autocorrelation function. Intrinsic attenuation is included by reducing the energy contained in each particle as an appropriate function of traveltime. We find that most scattering occurs in the lithosphere and upper mantle, as previous results have indicated, but that some lower-mantle scattering is likely also required. A model with 3 to 4 per cent rms velocity heterogeneity at 4-km scale length in the upper mantle and 0.5 per cent rms velocity heterogeneity at 8-km scale length in the lower mantle (with intrinsic attenuation of Q α = 450 above 200 km depth and Q α = 2500 below 200 km) provides a reasonable fit to both the shallow- and deep-earthquake observations, although many trade-offs exist between the scale length, depth extent and strength of the heterogeneity.

Geophysical Journal International↗

USGS1989

On the use of volumetric strain meters to infer additional characteristics of short-period seismic radiation

Volumetric strain meters (Sacks-Evertson design) are installed at 15 sites along the San Andreas fault system, to monitor long-term strain changes for earthquake prediction. Deployment of portable broadband, high-resolution digital recorders (GEOS) at several of the sites extends the detection band for volumetric strain to periods shorter than 5 × 10 −2 sec and permits the simultaneous observation of seismic radiation fields using conventional short-period pendulum seismometers. Simultaneous observations establish that the strain detection bandwidth extends from periods greater than 10 7 seconds to periods near 5 × 10 −2 sec with a dynamic range exceeding 140 dB. Measurements of earth-strain noise for the period band, 10 7 to 10 −2 sec, show that ground noise, not instrument noise, currently limits the measurement of strain over a bandwidth of more than eight orders of magnitude in period. Comparison of the short-period portion of earth-strain, noise spectra (20 to 5 × 10 −2 sec) with average spectra determined from pendulum seismometers, suggest that observed noise is predominantly dilatational energy. Recordings of local and regional earthquakes indicate that dilatometers respond to P energy but not direct shear energy and that straingrams can be used to resolve superimposed reflected P and S waves for inference of wave characteristics not permitted by either sensor alone. Simultaneous measurements of incident P - and S -wave amplitudes are used to introduce a technique for single-station estimates of wave field inhomogeneity, free-surface reflection coefficients and local material P velocity. Estimates of these parameters derived for the North Palm Springs earthquake ( M w 5.9) respectively for an incident P wave of 29° are −85°, 1.71, 2.9 km/sec, and for an incident S wave of 17° are 79°, 0.85, 2.9 km/sec. The empirical estimates of reflection coefficients are consistent with model estimates derived using an anelastic half-space model with incident inhomogeneous wave fields.

California↗