Seismicity off the coast of Northern California determined from ocean bottom seismic measurements

1969 ◽  
Vol 59 (5) ◽  
pp. 2001-2015
Author(s):  
Bruce Auld ◽  
Gary Latham ◽  
Ali Nowroozi ◽  
Leonardo Seeber
Keyword(s):  
San Francisco ◽  
Shear Velocity ◽  
Ocean Bottom ◽  
Sediment Layer ◽  
Fracture System ◽  
Resonance Effects ◽  
Average Shear ◽  
Land Station ◽  
Coastal Land ◽  
Seismic Measurements

abstract Approximately 190 local earthquakes (Δ < 4°) with suboceanic epicenters have been recorded over a 14-month period by the Ocean Bottom Geophysical Station of the Lamont-Doherty Geological Observatory. The station is located 200 km west-northwest of San Francisco and 220 km south of the Mendocino Fracture Zone at a water depth of 3.9 km. Most of the observed earthquakes are spatially related to the Mendocino fracture system. Only 20 per cent of the earthquakes recorded at the ocean bottom station were observed at an equidistant land station at Point Arena, California. P and S phases detected at the ocean bottom bottom have greater amplitudes and higher frequencies than the same phases recorded at the nearby land station. This is ascribed to a combination of near-receiver crustal resonance effects and attenuation on transmission across the continental/oceanic interface. Observations of Sn and epicentral locations support the hypothesis that the Mendocino fracture system is a transform fault. Phases converted from P to S at the base of the sediment layer beneath the ocean bottom station are used to compute the average shear-wave velocity and thickness of the sediment layer. The resulting values are 0.34-0.40 km/sec for the average shear velocity and a sediment thickness of 0.78-0.88 km. A phase tentatively identified as the False S of Byerly is observed on records from a coastal land station, but is not observed on the ocean bottom records.

Converted shear waves as seen by ocean bottom seismometers and surface buoys

1977 ◽  
Vol 67 (5) ◽  
pp. 1291-1302 ◽  
Author(s):  
Brian T. R. Lewis ◽  
James McClain
Keyword(s):  
Shear Waves ◽  
Shear Velocity ◽  
Ocean Bottom ◽  
Sediment Layer ◽  
S Waves ◽  
High Attenuation ◽  
Ocean Bottom Seismometers ◽  
Sedimentary Layer ◽  
Low Shear

abstract It is found that ocean bottom seismometers (O.B.S.) deployed in sedimented areas produce markedly different seismograms from surface hydrophones. These differences are found to be due to ringing on the O.B.S. records produced by converted shear waves trapped in the sediment layer. These shear waves do not propagate into the water and hence the hydrophone record is much “cleaner” than the O.B.S. record. It is also shown that the presence of refracted shear waves like P-S-P and P-S-S may be related to the presence of a sedimentary layer in some areas. It is suggested that the disappearance of refracted S waves in some areas without sediment is related to high attenuation and/or very low shear velocities caused by cracks and inhomogeneities in the crust. Under sedimented areas the cracks may be sufficiently filled so as to substantially reduce the attenuation and/or increase the bulk shear velocity.

Shear attenuation and anelastic mechanisms in the central Pacific upper mantle

2020 ◽  
Author(s):  
Zhitu Ma ◽  
Colleen Dalton ◽  
Joshua Russell ◽  
James Gaherty ◽  
Greg Hirth ◽  
...  
Keyword(s):  
Grain Boundary ◽  
Rayleigh Wave ◽  
Rayleigh Waves ◽  
Plane Waves ◽  
Shear Velocity ◽  
Grain Boundary Sliding ◽  
Absorption Peak ◽  
Ocean Bottom ◽  
Central Pacific ◽  
Transition Depth

<p>We determine the mantle attenuation (1/Q) structure beneath 70 Myr seafloor in the central Pacific. We use long-period (33-100 sec) Rayleigh waves recorded by the NoMelt array of broadband ocean-bottom seismometers. After the removal of tilt and compliance noise, we are able to measure Rayleigh wave phase and amplitude for 125 earthquakes. The compliance correction for ocean wave pressure on the seafloor is particularly important for improving signal-to-noise at periods longer than 55 sec. Attenuation and azimuthally anisotropic phase velocity in the study area are determined by approximating the wavefield as the interference of two plane waves. We find that the amplitude decay of Rayleigh waves across the NoMelt array can be adequately explained using a two-layer model: in the shallow layer, in the deeper layer, and a transition depth at 70 km, although the sharpness of the transition is not well resolved by the Rayleigh wave data. Notably, observed in the NoMelt lithosphere is significantly higher than values in this area from global attenuation models. When compared with lithospheric measured at higher frequency (~3 Hz), the frequency dependence of attenuation is very slight, revising previous interpretations. The effect of anelasticity on shear velocity (V<sub>S</sub>) is estimated from the ratio of observed velocity to the predicted anharmonic value. We use laboratory-based parameters to predict attenuation and velocity-dispersion spectra that result from the superposition of a weakly frequency dependent high-temperature background and an absorption peak. We test a large range of frequencies for the position of the absorption peak (<em>f</em><sub>e</sub>) and determine, at each depth, which values of <em>f</em><sub>e</sub> predict and V<sub>S</sub> that can fit the NoMelt and V<sub>S </sub>values simultaneously. We show that between depths of 60 and 80 km the seismic models require an increase in <em>f</em><sub>e</sub> by at least 3-4 orders of magnitude. Under the assumption that the absorption peak is caused by elastically accommodated grain-boundary sliding, this increase in <em>f</em><sub>e</sub> reflects a decrease in grain-boundary viscosity of 3-4 orders of magnitude. A likely explanation is an increase in the water content of the mantle, with the base of the dehydrated lid located at ~70-km depth.   </p>

RECONSTRUCTION OF LAYERED ELASTIC BOTTOM CHARACTERISTICS BY THE FREQUENCY DEPENDENCE OF SOUND REFLECTIVITY

2001 ◽  
Vol 09 (04) ◽  
pp. 1475-1484 ◽  
Author(s):  
VLADIMIR N. FOKIN ◽  
MARGARETE S. FOKINA
Keyword(s):  
Frequency Dependence ◽  
Half Space ◽  
Simulated Data ◽  
Elastic Half Space ◽  
Ocean Bottom ◽  
Sediment Layer ◽  
Elastic Bottom ◽  
Sea Bottom ◽  
Sound Reflection ◽  
Grazing Angles

The relation between the sound reflection losses measured at fixed grazing angles and the characteristics of the sediment layer and underlying half-space is considered. Based on this relation, a method of the reconstruction of the sea bottom characteristics is developed for a ocean bottom consisting of a single sediment layer overlaying a semi-infinite elastic half-space. Using this bottom model, the reconstruction of the characteristics of a layered elastic bottom is performed from the numerically simulated data with induced synthetic error.

COLLECTION AND ANALYSIS OF PACIFIC OCEAN‐BOTTOM SEISMIC DATA

Geophysics ◽  
1964 ◽  
Vol 29 (5) ◽  
pp. 745-771 ◽  
Author(s):  
William A. Schneider ◽  
Patrick J. Farrell ◽  
Ross E. Brannian
Keyword(s):  
Pacific Ocean ◽  
Seismic Data ◽  
Ambient Noise ◽  
Power Spectra ◽  
Simultaneous Recording ◽  
Distinct Advantage ◽  
Noise Power ◽  
Ocean Bottom ◽  
Detection And Identification ◽  
Land Station

A total of 500 hours of usable ocean‐bottom seismic data recorded on pressure and three components of velocity has been collected in three geographically separate areas of the Pacific Ocean at depths to 20,000 ft. These data are presently being analyzed to determine the extent to which monitoring seismic motion on the ocean floor can assist Project VELA UNIFORM goals of detection and identification of underground and underwater nuclear blasts. Analysis of three earthquakes and ambient noise recorded simultaneously on the ocean bottom and land reveals: 1. Ocean‐bottom signal‐to‐noise ratios are equal to or less than those seen at a comparative land station; 2. Ocean‐bottom signal and noise levels are higher than those obtained at the land station; and 3. Ocean‐bottom ambient noise power spectra increase in level towards the microseismic 6‐ to 8‐sec peak as do the land data. No strong directional ocean‐bottom noise components have been observed. Simultaneous recording of pressure and particle velocity affords the ocean‐bottom station a distinct advantage over its land counterpart, through exploitation of the relationships between pressure and vertical velocity which exist for various types of arrivals and modes.

The Signature and Elimination of Sediment Reverberations on Submarine Receiver Functions: Imaging the Lithosphere of a Normal Ocean

2021 ◽  
Author(s):  
Ziqi Zhang ◽  
Tolulope Olugboji
Keyword(s):  
High Resolution ◽  
Receiver Function ◽  
Receiver Functions ◽  
Function Technique ◽  
Ocean Bottom ◽  
Sediment Layer ◽  
Waveform Modeling ◽  
Seismic Arrays ◽  
Filter Approach ◽  
Synthetic Waveform

<p>While the receiver function technique has been successfully applied to high-resolution imaging of sharp discontinuities within and across the lithosphere, it has been shown, however, that it suffers from severe limitations when applied to seafloor seismic recordings. This is because the water and sediment layer could strongly influence the receiver function traces, making detection and interpretation of crust and mantle layering difficult. This effect is often referred to as the singing phenomena in marine environments. Here, we show how one can silence this singing effect. We demonstrate, using analytical and synthetic waveform modeling, that this singing effect can be reversed using dereverberation filters tuned to match the elastic property of each layer. We apply the filter approach to high-quality earthquake records collected from the NoMelt seismic array deployed on normal, mature (~70 Ma) Pacific seafloor. An appropriate filter designed using the elastic properties of the underlying sediments, and obtained from prior studies, greatly improves the detection of Ps conversions generated from the moho (~8.6 km) and from a sharp discontinuity (<~ 5 km) across the lithosphere-asthenosphere transition (~72 km). Sensitivity tests show that the filter is robust to small errors in the sediment properties. Our analysis suggests that appropriately filtering out the sediment reverberations from ocean seismic data could make inferences on subsurface structure more robust. We expect that this study will enable high-resolution receiver function imaging of the base of the oceanic plate across a growing fleet of ocean bottom seismic arrays being deployed in the global oceans.</p>

Nationwide 7.5-Arc-Second Japan Engineering Geomorphologic Classification Map and Vs30 Zoning

2013 ◽  
Vol 8 (5) ◽  
pp. 904-911 ◽  
Author(s):  
Kazue Wakamatsu ◽  
◽  
Masashi Matsuoka ◽  
Keyword(s):  
Seismic Hazard ◽  
Local Governments ◽  
Seismic Hazard Assessment ◽  
Shear Velocity ◽  
Hazard Maps ◽  
Seismic Risk Analysis ◽  
Average Shear ◽  
Ground Conditions ◽  
Estimation System ◽  
Ground Condition

Local geological and ground conditions play important roles in characterizing and estimating hazards in seismic hazard assessment. The authors recently constructed the Japan Engineering Geomorphologic Classification Map (JEGM), which is a systematically standardized GIS-based ground-condition map containing attributes of geomorphologic classification in grid cells of 7.5 arc-seconds in latitude × 11.25 arc-seconds in longitude for Japan nationwide. This paper introduces the concept of developing the 7.5-arc-second JEGM, and presents sample JEGM images. As an example of the database’s application in estimating hazards, the average shear velocity of the ground in the upper 30m, Vs30 is estimated and mapped for Japan nationwide. Other applications being released include seismic hazard maps of Japan, seismic risk analysis by Central Disaster Prevention Council and local governments, and a Quick Estimation System for Earthquake Maps Triggered by Observation Records (QuiQuake).

THE INFLUENCE OF KINEMATIC CONDITION AND LUBRICANT PROPERTIES ON FRICTION IN COMPLEX SLIDE–ROLL MOTION

Tribologia ◽  
2020 ◽  
Vol 289 (1) ◽  
pp. 41-48
Author(s):  
Piotr KOWALEWSKI ◽  
Maciej PASZKOWSKI
Keyword(s):  
Variable Temperature ◽  
Slip Rate ◽  
Shear Velocity ◽  
Roll Motion ◽  
Lithium Grease ◽  
Lubricant Flow ◽  
Wide Range ◽  
Average Shear ◽  
Different Temperatures ◽  
Friction Joint

The paper presents the results of preliminary studies into the influence of selected lubricants (greases) and the variable temperature of the friction joint on the value of friction during complex slide-roll motion. The experiment was carried out for three different types of lubricants: bentonite grease (Benterm 2), lithium grease (GREASEN ŁT4S2), and calcium grease with the addition of graphite (GREASEN GRAFIT) at different temperatures. Tribological investigations were carried out in a roller-plate system at a wide range of plate dislocation velocities and at a constant slip rate; rheological investigations consisted in determining the changes in shear stress as a function of shear time at constant average shear velocity of 50 mm/s. The results showed a significant influence of kinematic conditions on the value of friction. Four different cases of lubricant flow during friction were identified. The least resistance to motion was found when the lubricant was applied in the same direction along the surface of the roller and plate. It has been shown that the greatest influence of the kinematics of working elements on friction occurs at low temperatures, i.e. at -10°C. This phenomenon is closely related to the rheological properties of the lubricant. It was also found that the lubricant thickened with bentonite is the most susceptible to kinematic changes of the friction joint.

Elimination of the overburden response from multicomponent source and receiver seismic data, with source designature and decomposition into PP-, PS-, SP-, and SS-wave responses

Geophysics ◽  
2005 ◽  
Vol 70 (2) ◽  
pp. S43-S59 ◽  
Author(s):  
Egil Holvik ◽  
Lasse Amundsen
Keyword(s):  
Shear Wave ◽  
Local Density ◽  
Compressional Wave ◽  
Single Component ◽  
Common Source ◽  
Ocean Bottom ◽  
Depth Level ◽  
Seismic Experiment ◽  
Wavefield Decomposition ◽  
Seismic Measurements

This paper shows that Betti's reciprocity theorem gives an integral equation procedure to eliminate from the physical multicomponent-source, multicomponent-receiver seismic measurements the effect of the physical source radiation pattern and the response of the physical overburden (that is, the medium above the receiver plane). The physical multicomponent sources are assumed to be orthogonally aligned anywhere above the multicomponent-receiver depth level. Other than the position of the sources, no source characteristics are required. The method, denoted the Betti designature/elastic demultiple, has the following additional characteristics: it preserves primary amplitudes while eliminating all waves scattered from the overburden; it requires no knowledge of the medium below the receiver level; it requires no knowledge of the medium above the receiver level; it requires information only of the local density and elastic wave propagation velocities at the receiver level to decompose the physical seismic measurements into upgoing and downgoing waves. Following the Betti designature/elastic demultiple step is an elastic wavefield decomposition step that decomposes the data into PP-, PS-, SP-, and SS-wave responses that would be recorded from idealized compressional-wave and shear-wave sources and receivers. The combined elastic wavefield decomposition on the source and receiver side gives data equivalent to data from a hypothetical survey with overburden absent, with single-component compressional and shear-wave sources, and single-component compressional and shear-wave receivers. When the medium is horizontally layered, the Betti designature/elastic demultiple scheme followed by the elastic source-receiver decomposition scheme greatly simplifies and is conveniently implemented as deterministic multidimensional deconvolution and elastic source-receiver wavefield decomposition of common-source gathers (or common-receiver gathers when source array variations are negligible). Betti designature/elastic demultiple followed by source-receiver wavefield decomposition applies to three different seismic experiments: a 9-component (9C) land seismic experiment, a 12-component (12C) ocean-bottom seismic experiment, and an 18-component (18C) borehole seismic experiment. For the land and ocean-bottom seismic experiments, an additional geophone should be deployed below the zero-offset geophone to predict the source-induced vertical traction vector at the source location. A numerical example for the 12C ocean-bottom seismic experiment over a horizontally layered medium validates the Betti designature/elastic demultiple scheme.

RESONANCES OF ACOUSTIC WAVES INTERACTING WITH AN ELASTIC SEABED

2001 ◽  
Vol 09 (03) ◽  
pp. 1079-1093 ◽  
Author(s):  
MARGARETE S. FOKINA ◽  
VLADIMIR N. FOKIN
Keyword(s):  
Reflection Coefficient ◽  
Half Space ◽  
Acoustic Waves ◽  
Elastic Half Space ◽  
Exact Expression ◽  
Ocean Bottom ◽  
Sediment Layer ◽  
Resonance Theory ◽  
Space Characteristic ◽  
Exact Calculations

An exact expression for the reflection coefficient is obtained with the Thomson–Haskell technique for the geoacoustical model of an ocean bottom consisting of an elastic homogeneous sediment layer overlying an elastic half-space. Characteristic equations for explicit determination of the position of each individual resonance contribution to the reflection coefficient are derived. Analytical expressions for the angular and frequency resonance positions are found. The resonance expression for the reflection coefficient is written in the form of a sum of resonance terms. Comparison between resonance theory and exact calculations for the elastic layer covering the elastic half-space is presented. The results of resonance formalism show excellent agreement with exact theory in all the cases.

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