Results for “The Seismic Record” — Page 23

USGS1927

Hawaiian Volcano Observatory bulletins - 1927

The Hawaiian Volcano Observatory (HVO) Bulletin series was an informal publication issued between the years 1913 to 1929. Individual issues contain information on volcanic and earthquake activity, volcano research, and volcano monitoring in Hawaii, and issues often included photographs, sketches, and data plots. Information on volcanic activity at other locations is also occasionally included. The Bulletin series was published by HVO through the Hawaiian Volcano Research Association. Weekly Bulletins (initially called Reports) were issued between June 28, 1913, and July 1, 1914. Bulletins were issued monthly after July 1, 1914, though they were still named Weekly Bulletins until January 1919. Starting with the February 1919 issue, the Bulletins were named Monthly Bulletins, and they continued to be issued monthly until the series ceased in July 1929. During a single year, 1915, Seismometric Bulletins were also issued quarterly. These four issues contain summaries of seismic observations recorded by the then-nascent Whitney Laboratory of Seismology, located underground in a vault near the summit of Kīlauea. Two of the bulletins are misnumbered: • April 1916 volume IV no. 4 incorrectly says April 1915 volume III no. 4 • September 1916 volume IV no. 9 incorrectly says September 1916 volume VI no. 9

Hawaii↗

USGS1928

Hawaiian Volcano Observatory bulletins - 1928

The Hawaiian Volcano Observatory (HVO) Bulletin series was an informal publication issued between the years 1913 to 1929. Individual issues contain information on volcanic and earthquake activity, volcano research, and volcano monitoring in Hawaii, and issues often included photographs, sketches, and data plots. Information on volcanic activity at other locations is also occasionally included. The Bulletin series was published by HVO through the Hawaiian Volcano Research Association. Weekly Bulletins (initially called Reports) were issued between June 28, 1913, and July 1, 1914. Bulletins were issued monthly after July 1, 1914, though they were still named Weekly Bulletins until January 1919. Starting with the February 1919 issue, the Bulletins were named Monthly Bulletins, and they continued to be issued monthly until the series ceased in July 1929. During a single year, 1915, Seismometric Bulletins were also issued quarterly. These four issues contain summaries of seismic observations recorded by the then-nascent Whitney Laboratory of Seismology, located underground in a vault near the summit of Kīlauea. Two of the bulletins are misnumbered: • April 1916 volume IV no. 4 incorrectly says April 1915 volume III no. 4 • September 1916 volume IV no. 9 incorrectly says September 1916 volume VI no. 9

Hawaii↗

USGS1929

Hawaiian Volcano Observatory bulletins - 1929

The Hawaiian Volcano Observatory (HVO) Bulletin series was an informal publication issued between the years 1913 to 1929. Individual issues contain information on volcanic and earthquake activity, volcano research, and volcano monitoring in Hawaii, and issues often included photographs, sketches, and data plots. Information on volcanic activity at other locations is also occasionally included. The Bulletin series was published by HVO through the Hawaiian Volcano Research Association. Weekly Bulletins (initially called Reports) were issued between June 28, 1913, and July 1, 1914. Bulletins were issued monthly after July 1, 1914, though they were still named Weekly Bulletins until January 1919. Starting with the February 1919 issue, the Bulletins were named Monthly Bulletins, and they continued to be issued monthly until the series ceased in July 1929. During a single year, 1915, Seismometric Bulletins were also issued quarterly. These four issues contain summaries of seismic observations recorded by the then-nascent Whitney Laboratory of Seismology, located underground in a vault near the summit of Kīlauea. Two of the bulletins are misnumbered: • April 1916 volume IV no. 4 incorrectly says April 1915 volume III no. 4 • September 1916 volume IV no. 9 incorrectly says September 1916 volume VI no. 9

Hawaii↗

USGS1915

Hawaiian Volcano Observatory seismometric bulletins - 1915

The Hawaiian Volcano Observatory (HVO) Bulletin series was an informal publication issued between the years 1913 to 1929. Individual issues contain information on volcanic and earthquake activity, volcano research, and volcano monitoring in Hawaii, and issues often included photographs, sketches, and data plots. Information on volcanic activity at other locations is also occasionally included. The Bulletin series was published by HVO through the Hawaiian Volcano Research Association. Weekly Bulletins (initially called Reports) were issued between June 28, 1913, and July 1, 1914. Bulletins were issued monthly after July 1, 1914, though they were still named Weekly Bulletins until January 1919. Starting with the February 1919 issue, the Bulletins were named Monthly Bulletins, and they continued to be issued monthly until the series ceased in July 1929. During a single year, 1915, Seismometric Bulletins were also issued quarterly. These four issues contain summaries of seismic observations recorded by the then-nascent Whitney Laboratory of Seismology, located underground in a vault near the summit of Kīlauea. Two of the bulletins are misnumbered: • April 1916 volume IV no. 4 incorrectly says April 1915 volume III no. 4 • September 1916 volume IV no. 9 incorrectly says September 1916 volume VI no. 9

Hawaii↗

USGS2019

The LArge-n Seismic Survey in Oklahoma (LASSO) experiment

In 2016, the U.S. Geological Survey deployed more than 1,800 vertical-component nodal seismometers in Grant County, Oklahoma to study induced seismic activity associated with production of the Mississippi Limestone Play. The LArge-n Seismic Survey in Oklahoma (LASSO) array operated for approximately one month, covering a 25-km-by-32-km region with a nominal station spacing of ~400 m. Primary goals of the deployment were to detect microseismic events not captured by the sparser regional network stations and to provide nearly unaliased records of the seismic wavefield. A more complete record of earthquakes allows us to map the spatiotemporal evolution of induced event sequences and illuminates the structures on which the events occur. Dense records of the seismic wavefield also provide improved measurements of the earthquake source, including focal mechanisms and stress drops. Taken together, we can use these findings to glean insights into the processes that induce earthquakes. Here, we describe the array layout, features of the nodal sensors, data recording configurations, and the field deployment. We also provide examples of earthquake waveforms recorded by the array to illustrate data quality and initial observations. LASSO array data provide a significant resource for understanding the occurrence of earthquakes induced by wastewater disposal.

Kansas, Oklahoma↗

USGS1988

Crustal velocities near Coalinga, California, modeled from a combined earthquake/explosion refraction profile

Crustal velocity structure for the region near Coalinga, California, has been derived from both earthquake and explosion seismic phase data recorded along a NW-SE seismic-refraction profile on the western flank of the Great Valley east of the Diablo Range. Comparison of the two data sets reveals P-wave phases in common which can be correlated with changes in the velocity structure below the earthquake hypocenters. In addition, the earthquake records reveal secondary phases at station ranges of less than 20 km that could be the result of S- to P-wave conversions at velocity interfaces above the earthquake hypocenters. Two-dimensional ray-trace modeling of the P-wave travel times resulted in a P-wave velocity model for the western flank of the Great Valley comprised of: (a) a 7- to 9-km-thick section of sedimentary strata with velocities similar to those found elsewhere in the Great Valley (1.6 to 5.2 km/sec); (b) a middle crust extending to about 14 km depth with velocities comparable to those reported for the Franciscan assemblage in the Diablo Range (5.6 to 5.9 km/sec); and (c) a 13- to 14-km-thick lower crust with velocities similar to those reported beneath the Diablo Range and the Great Valley (6.5 to 7.30 km/sec). This lower crust may have been derived from subducted oceanic crust that was thickened by accretionary underplating or crustal shortening.

California↗

USGS1988

Data report for 1980-1981 seismic-refraction profiles in the western Mojave Desert, California

During 1980 and 1981, the U.S. Geological Survey collected seismic refraction data in the western Mojave Desert (fig. 1). The seismic refraction data extends from north of the Garlock Fault, south through the Mojave Desert, and across the San Andreas Fault into the San Gabriel Mountains. This report describes and documents the recorded data, which was collected over three field sessions. In total thirteen shots were fired at 11 different shotpoints (Table 1). The experiment consisted of five deployments of a set of 100 portable seismic recorders, during which seismic energy from one or more shots was recorded. The data set was acquired with five deployments of a set of 100 portable seismic recorders.

California↗

USGS2003

Site response, shallow shear-wave velocity, and wave propagation at the San Jose, California, dense seismic array

Ground-motion records from a 52-element dense seismic array near San Jose, California, are analyzed to obtain site response, shallow shear-wave velocity, and plane-wave propagation characteristics. The array, located on the eastern side of the Santa Clara Valley south of the San Francisco Bay, is sited over the Evergreen basin, a 7-km-deep depression with Miocene and younger deposits. Site response values below 4 Hz are up to a factor of 2 greater when larger, regional records are included in the analysis, due to strong surface-wave development within the Santa Clara Valley. The pattern of site amplification is the same, however, with local or regional events. Site amplification increases away from the eastern edge of the Santa Clara Valley, reaching a maximum over the western edge of the Evergreen basin, where the pre-Cenozoic basement shallows rapidly. Amplification then decreases further to the west. This pattern may be caused by lower shallow shear-wave velocities and thicker Quaternary deposits further from the edge of the Santa Clara Valley and generation/trapping of surface waves above the shallowing basement of the western Evergreen basin. Shear-wave velocities from the inversion of site response spectra based on smaller, local earthquakes compare well with those obtained independently from our seismic reflection/refraction measurements. Velocities from the inversion of site spectra that include larger, regional records do not compare well with these measurements. A mix of local and regional events, however, is appropriate for determination of site response to be used in seismic hazard evaluation, since large damaging events would excite both body and surface waves with a wide range in ray parameters. Frequency-wavenumber, plane-wave analysis is used to determine the backazimuth and apparent velocity of coherent phases at the array. Conventional, high-resolution, and multiple signal characterization f-k power spectra and stacked slowness power spectra are compared. These spectra show surface waves generated/ scattered at the edges of the Santa Clara Valley and possibly within the valley at the western edge of the Evergreen basin.

California↗

USGS2000

Program to convert SUDS2ASC files to a single binary SEGY file

This program, SUDS2SEGY, converts and combines ASCII files created using SUDS2ASC Version 2.60, to a single SEGY file. SUDS2ASC has been used previously to create an ASCII file of three-component seismic data for an individual recording station. However, many seismic processing packages have difficulty reading in ASCII data. In addition, it may be cumbersome to process a separate file for each recording station, particularly if traces from different recording stations contain a different number of data samples and/or a different start time. This new program - SUDS2SEGY - combines these recording station files into a single SEGY file. In addition, SUDS2SEGY normalizes the trace times so that each trace starts at a given time and consists of a fixed number of samples. This normalization allows seismic data from many different stations to be read in as a single "data gather". SUDS2SEGY also produces a report summarizing the offset and maximum absolute amplitude for each component in a station file. These data are output separately to an ASCII file and can be subsequently input to a plotting package.

Open-File Report↗

USGS2016

Observations of seismicity and ground motion in the northeast U.S. Atlantic margin from ocean bottom seismometer data

Earthquake data from two short-period ocean-bottom seismometer (OBS) networks deployed for over a year on the continental slope off New York and southern New England were used to evaluate seismicity and ground motions along the continental margin. Our OBS networks located only one earthquake of M c ∼1.5 near the shelf edge during six months of recording, suggesting that seismic activity (M Lg >3.0) of the margin as far as 150–200 km offshore is probably successfully monitored by land stations without the need for OBS deployments. The spectral acceleration from two local earthquakes recorded by the OBS was found to be generally similar to the acceleration from these earthquakes recorded at several seismic stations on land and to hybrid empirical acceleration relationships for eastern North America. Therefore, the seismic attenuation used for eastern North America can be extended in this region at least to the continental slope. However, additional offshore studies are needed to verify these preliminary conclusions.

Seismological Research Letters↗

USGS2000

Data for Quaternary faults, liquefaction features, and possible tectonic features in the Central and Eastern United States, east of the Rocky Mountain Front

The USGS is currently leading an effort to compile published geological information on Quaternary faults, folds, and earthquake-induced liquefaction in order to develop an internally consistent database on the locations, ages, and activity rates of major earthquake-related features throughout the United States. This report is the compilation for such features in the Central and Eastern United States (CEUS), which for the purposes of the compilation, is defined as the region extending from the Rocky Mountain Front eastward to the Atlantic seaboard. A key objective of this national compilation is to provide a comprehensive database of Quaternary features that might generate strong ground motion and therefore, should be considered in assessing the seismic hazard throughout the country. In addition to printed versions of regional and individual state compilations, the database will be available on the World-Wide Web, where it will be readily available to everyone. The primary purpose of these compilations and the derivative database is to provide a comprehensive, uniform source of geological information that can by used to complement the other types of data that are used in seismic-hazard assessments. Within our CEUS study area, which encompasses more than 60 percent of the continuous U.S., we summarize the geological information on 69 features that are categorized into four classes (Class A, B, C, and D) based on what is known about the feature's Quaternary activity. The CEUS contains only 13 features of tectonic origin for which there is convincing evidence of Quaternary activity (Class A features). Of the remaining 56 features, 11 require further study in order to confidently define their potential as possible sources of earthquake-induced ground motion (Class B), whereas the remaining features either lack convincing geologic evidence of Quaternary tectonic faulting or have been studied carefully enough to determine that they do not pose a significant seismic hazard (Classes C and D). The correlation between historical seismicity and Quaternary faults and liquefaction features in the CEUS is generally poor, which probably reflects the long return times between successive movements on individual structures. Some Quaternary faults and liquefaction features are located in aseismic areas or where historical seismicity is sparse. These relations indicate that the record of historical seismicity does not identify all potential seismic sources in the CEUS. Furthermore, geological studies of some currently aseismic faults have shown that the faults have generated strong earthquakes in the geologically recent past. Thus, the combination of geological information and seismological data can provide better insight into potential earthquake sources and thereby, contribute to better, more comprehensive seismic-hazard assessments.

Open-File Report↗

USGS2003

Location of long-period events below Kilauea Volcano using seismic amplitudes and accurate relative relocation

We present methods for improving the location of long-period (LP) events, deep and shallow, recorded below Kilauea Volcano by the permanent seismic network. LP events might be of particular interest to understanding eruptive processes as their source mechanism is assumed to directly involve fluid transport. However, it is usually difficult or impossible to locate their source using traditional arrival time methods because of emergent wave arrivals. At Kilauea, similar LP waveform signatures suggest the existence of LP multiplets. The waveform similarity suggests spatially close sources, while catalog solutions using arrival time estimates are widely scattered beneath Kilauea's summit caldera. In order to improve estimates of absolute LP location, we use the distribution of seismic amplitudes corrected for station site effects. The decay of the amplitude as a function of hypocentral distance is used for inferring LP location. In a second stage, we use the similarity of the events to calculate their relative positions. The analysis of the entire LP seismicity recorded between January 1997 and December 1999 suggests that a very large part of the LP event population, both deep and shallow, is generated by a small number of compact sources. Deep events are systematically composed of a weak high-frequency onset followed by a low-frequency wave train. Aligning the low-frequency wave trains does not lead to aligning the onsets indicating the two parts of the signal are dissociated. This observation favors an interpretation in terms of triggering and resonance of a magmatic conduit. Instead of defining fault planes, the precise relocation of similar LP events, based on the alignment of the high-energy low-frequency wave trains, defines limited size volumes. Copyright 2003 by the American Geophysical Union.

Journal of Geophysical Research B: Solid Earth↗

USGS2003

The Sidebar Computer Program, a seismic-shaking intensity meter: users' manual and software description

The SideBar computer program provides a visual display of seismic shaking intensity as recorded at one specific seismograph. This software allows a user to tap into the seismic data recorded on that specific seismograph and to display the overall level of shaking at the single location where that seismograph resides (usually the same place the user is). From this shaking level, SideBar also estimates the potential for damage nearby. SideBar cannot tell you the “Richter magnitude” of the earthquake (see box), only how hard the ground shook locally and this estimate of how much damage is likely in the neighborhood. This combination of local effects is called the “seismic intensity”. SideBar runs on a standard desktop or laptop PC, and is intended for the media, schools, emergency responders, and any other group hosting a seismograph and who want to know immediately after an earthquake the levels of shaking measured by that instrument. These local values can be used to inform the public and help initiate appropriate local emergency response activities in the minutes between the earthquake and availability of the broader coverage provided by the USGS over the Web, notably by ShakeMap. For example, for instruments installed in schools, the level of shaking and likely damage at the school could immediately be Web broadcast and parents could quickly determine the likely safety of their children—their biggest postearthquake concern. Also, in the event of a Web outage, SideBar may be a continuing primary source of local emergency response information for some additional minutes. Specifically, SideBar interprets the peak level of acceleration (that is, the force of shaking, as a percentage of the force of gravity) as well as the peak velocity, or highest speed, at which the ground moves. Using these two basic measurements, SideBar computes what is called Instrumental Intensity—a close approximation of the Modified Mercalli Intensity scale, or “MMI” (using the Wald et al., 1999a, relationships between acceleration, velocity, and shaking intensity). Intensity is a measure of local shaking strength and the potential for damage—of how bad the earthquake effects were locally. The intensity level is what SideBar displays most prominently on the PC monitor. Intensity is shown as a large, colored bar that gets taller and changes color up a rainbow from blues toward reds as the shaking level increases. As opposed to earthquake magnitudes, which are reported as decimal values (like “7.6”), intensity is traditionally given as a Roman numeral, with “I” to “X+” assigned to levels of potential damage and perceived shaking strength. For good measure, SideBar shows the actual values of the force of shaking (peak ground acceleration as a percentage of gravity) and the speed of ground motion (peak ground velocity in inches per second, by default, or in centimeters per second, if you wish), both these values as decimal numbers. SideBar also remembers the most recent earthquakes (for up to one week), and can store as many of these previous earthquakes as the user allows (and as the user’s PC has room for)—typically thousands. SideBar also remembers forever the three largest earthquakes it has seen and all earthquakes over intensity IV so that one never loses particularly important events.

Open-File Report↗

USGS1980

Single-channel seismic-reflection profiles and sidescan sonar records collected during May 15-20, 1978, on the southern New England continental shelf

The U.S. Geological Survey completed a cruise aboard the R/V CAPE HENLOPEN during May 15-20, 1978, to map the surface character, thickness and extent of the fine-grained.sediment deposit that covers an area 100 x 200 km on the southern New England Continental Shelf. The study area lies between Great South Channel to the east and Black Channel to the west, and extends from the 50-m isobath to the shelf edge. Single-channel high-resolution seismic-reflection profiles and echo-sounding profiles were collected along 941 km of trackline, sidescan sonar records were collected along 673 km of trackline. The subbottom profiles were collected by using a Huntec*system that was towed at midwater depths. Filters were set at 1 to 7 kHz. Echo-sounding records were collected by using a 60 kHz EDO Western system. A Klein stdescan sonar, set to scan 100 m to either side of the towed fish, was used to collect the sonographs. Navigation during the survey was done by the scientific staff using Loran-C equipment. Fixes were recorded and logged at least every 15 minutes; after the cruise, they were digitized and stored on magnetic tape. The original records can be seen and studied at the U.S. Geological Survey Data Library at Woods Hole, MA 02543. Microfilm copies of the subbottom, echosounding, and sidescan sonar records collected during the cruise can be purchased from the National Geophysical and Solar-Terrestrial Data Center, NOAA (National Oceanic ancl Atmosphere Administration), Boulder, CO 80302.

Open-File Report↗

USGS1978

The utility of petroleum seismic exploration data in delineating structural features within salt anticlines

The Salt Valley anticline, in the Paradox Basin of southeastern Utah, is under investigation for use as a location for storage of solid nuclear waste. Delineation of thin, nonsalt interbeds within the upper reaches of the salt body is extremely important because the nature and character of any such fluid- or gas-saturated horizons would be critical to the mode of emplacement of wastes into the structure. Analysis of 50 km of conventional seismic-reflection data, in the vicinity of the anticline, indicates that mapping of thin beds at shallow depths may well be possible using a specially designed adaptation of state-of-the-art seismic oil-exploration procedures. Computer ray-trace modeling of thin beds in salt reveals that the frequency and spatial resolution required to map the details of interbeds at shallow depths (less than 750 m) may be on the order of 500 Hz, with surface-spread lengths of less than 350 m. Consideration should be given to the burial of sources and receivers in order to attenuate surface noise and to record the desired high frequencies. Correlation of the seismic-reflection data with available well data and surface geology reveals the complex, structurally initiated diapir, whose upward flow was maintained by rapid contemporaneous deposition of continental clastic sediments on its flanks. Severe collapse faulting near the crests of these structures has distorted the seismic response. Evidence exists, however, that intrasalt thin beds of anhydrite, dolomite, and black shale are mappable on seismic record sections either as short, discontinuous reflected events or as amplitude anomalies that result from focusing of the reflected seismic energy by the thin beds; computer modeling of the folded interbeds confirms both of these as possible causes of seismic response from within the salt diapir. Prediction of the seismic signatures of the interbeds can be made from computer-model studies. Petroleum seismic-reflection data are unsatisfactory for mapping the thin beds because of the lack of sufficient resolution to provide direct evidence of the presence of the thin beds. However, indirect evidence, present in these data as discontinuous seismic events, suggests that two geophysical techniques designed for this specific problem would allow direct detection of the interbeds in salt. These techniques are vertical seismic profiling and shallow, short-offset, high-frequency, seismic-reflection recording.

Open-File Report↗

USGS1979

Notes on the acquisition of high resolution seismic reflection profiles, side-scanning sonar records, and sediment samples from lower Cook Inlet and Kodiak Shelf, R/V Sea Sounder Cruise S8-78-WG, August 1978

The third U.S. Geological Survey geo-environmental cruise in lower Cook Inlet and on the Kodiak shelf and adjacent continental slope, Gulf of Alaska, was conducted aboard the R/V SEA SOUNDER from 2 August to 22 August, 1978 (Fig. 1, 2, and 3). The objectives of the cruise were to study in detail specific potentially hazardous environmental conditions identified as a result of the first reconnaissance cruise conducted in June and July of 1976 and from work by other investigators, and to initiate reconnaissance characterization studies on the continental slope. High-resolution seismic reflection profiling (sparker, Uniboam: 3.5 kHz, 12 kHz) and side-scanning sonar surveys formed the basis for selecting stations for observation with bottom television and 70 mm bottom camera as well as for sampling of surficial sediments (gravity corer, grab sampler), Generalized trackline charts are given in Figures 2 and 3. Detailed shot-point charts could not be constructed clearly, because of the overlap and coincidence of many of the lines. Station locations are shown in Figure 4 and 5, and sampling information is given in Table 4. Table 5 contains the navigation records from the cruise. The results of our investigations to date can be found in the references listed at the end of this text, Background information in lower Cook Inlet with several references is given in Open-File Report 75-429 (Magoon and others, 1975), and on the Kodiak shelf in Open-File Report 76-325 (von Huene and others, 1976). In addition, this report accompanies the basic seismic-reflection and side-scanning sonar records acquired on the cruise. The seismic-reflection records are publicly available from the National Geophysical and Solar Terrestrial Data Center EDS/NOAA, Boulder, Colorado 80302. These records can be inspected at U.S. Geological Survey offices at Rm B-164, Deer Creek Facility, 3475 Deer Creek Road, Palo Alto, California 94303.

Alaska↗

USGS1982

High-resolution seismic-reflection profiles and sidescan-sonar records collected on Block Island Sound by U.S. Geological Survey, R/V ASTERIAS, cruise AST 81-2

Cruise AST 81-2 was conducted aboard the R/V ASTERIAS during September 10-18, 1981, in Block Island Sound by the U.S. Geological Survey. It was funded in part by the Connecticut Geological and Natural History Survey. The purpose of the study was to define and map the geology and shallow structure, to determine the geologic framework and late Tertiary to Holocene history, and to identify and map any potential geologic hazards of Block Island Sound. The survey was conducted using an EG&G Uniboom seismic system and an EDO Western sidescan-sonar system. Seismic signals were band-passed between 400 and 4,000 Hz and were recorded at a quarter-second sweep rate. Sidescan sonographs were collected at a 100-m scan range to each side of the ship track. In all, 702 km of seismic-reflection profiles and 402 km of sidescan-sonar records were collected. Navigation was by Loran-C, and the ship position was recorded at 5-minute intervals. Seismic-reflection profiling is continuous and good in quality. Sidescan-sonar records are varied in quality; coverage was intermittent and eventually terminated owing to difficulties with the recorder. Original records can be seen and studied at the U.S. Geological Survey Data Library at Woods Hole, MA 02543. Microfilm copies of the seismic-reflection profiles and the sidescan sonographs can be purchased only from the National Geophysical and Solar-Terrestrial Data Center, NOAA/EDIS/NGSDC, Code D621, 325 Broadway, Boulder, CO 80303 (telephone 303-497-6338).

Open-File Report↗

USGS1980

Coincident sediment slump/clathrate complexes on the U.S. Atlantic continental slope

High-resolution seismic reflection data recorded on the continental slope off the east coast of the United States have revealed instances of sediment mass movement (slumps) which appear to occur above clathrate accumulations. The slumping is believed to be related to the liberation of free gas by clathrate decomposition and consequent weakening of unconsolidated sediments above the clathrate. Pleistocene sea-level lowering and/or post-Pleistocene bottom water temperature increases may have had a significant role in this process.

Geo-Marine Letters↗