High-frequency borehole seismograms recorded in the San Jcinto Fault zone, Southern California Part 2. Attenuation and site effects

1991 ◽  
Vol 81 (4) ◽  
pp. 1081-1100 ◽  
Author(s):  
Richard C. Aster ◽  
Peter M. Shearer
Keyword(s):  
Fault Zone ◽  
High Frequency ◽  
Southern California ◽  
Spectral Ratio ◽  
Seismic Signals ◽  
Noise Levels ◽  
S Wave ◽  
Low Velocity ◽  
High Frequency Signal ◽  
Near Surface

Abstract We examine surface and downhole P- and S-wave spectra from local earthquakes recorded at two borehole seismometer arrays (KNW-BH and PFO-BH) installed in the Southern California Batholith region of the San Jacinto Fault zone by the U.S. Geological Survey to assess the influence of the weathered layer on the spectral content of high-frequency (2 to 200 Hz) seismic signals. Earthquake signals recorded downhole at both sites show significantly improved seismic bandwidth due to both a reduction in ambient noise levels and (especially) to dramatically increased levels of high-frequency signal. Significant seismic signal is observed up to approximately 190 Hz for P waves at KNW-BH. Stacked spectral ratios from these signals indicate that the highly weathered near-surface (between 0 and 150 m) at KNW-BH and PFO-BH exerts a much larger influence on seismic signals than deeper (between 150 and 300 m) material. Modeling of uphole/downhole spectral ratio data suggests Qα ≈ 6.5 and Qβ ≈ 9 between 0 and 150 m, increasing to Qα ≈ 27 and Qβ ≳ 26 between 150 and 300 m. An outcrop-mounted Anza network station, deployed approximately 0.4 km from KNW-BH, displays roughly similar high-frequency content to the KNW-BH downhole sensors, but it exhibits spectra that are significantly colored by directional resonances. Low-Q and low-velocity near-surface material forms a lossy boundary layer at these borehole sites that is advantageous to the high-frequency downhole environment; not only are noise levels reduced, but reflections from the surface and near-surface are greatly attenuated. As a result, high-frequency recordings from below the weathered zone more nearly resemble those recorded in a whole space than would otherwise be expected.

scholarly journals Fault Zone Guided Wave generation on the locked, late interseismic Alpine Fault, New Zealand

2021 ◽  
Author(s):  
JD Eccles ◽  
AK Gulley ◽  
PE Malin ◽  
CM Boese ◽  
John Townend ◽  
...  
Keyword(s):  
Fault Zone ◽  
Plate Boundary ◽  
Guided Wave ◽  
S Wave ◽  
Low Velocity ◽  
Catchment Scale ◽  
Near Surface ◽  
Low Velocity Zone ◽  
Alpine Fault ◽  
Velocity Zone

© 2015. American Geophysical Union. All Rights Reserved. Fault Zone Guided Waves (FZGWs) have been observed for the first time within New Zealand's transpressional continental plate boundary, the Alpine Fault, which is late in its typical seismic cycle. Ongoing study of these phases provides the opportunity to monitor interseismic conditions in the fault zone. Distinctive dispersive seismic codas (~7-35Hz) have been recorded on shallow borehole seismometers installed within 20m of the principal slip zone. Near the central Alpine Fault, known for low background seismicity, FZGW-generating microseismic events are located beyond the catchment-scale partitioning of the fault indicating lateral connectivity of the low-velocity zone immediately below the near-surface segmentation. Initial modeling of the low-velocity zone indicates a waveguide width of 60-200m with a 10-40% reduction in S wave velocity, similar to that inferred for the fault core of other mature plate boundary faults such as the San Andreas and North Anatolian Faults.

High-frequency borehole seismograms recorded in the San Jacinto Fault zone, Southern California. Part 1. Polarizations

1991 ◽  
Vol 81 (4) ◽  
pp. 1057-1080 ◽  
Author(s):  
Richard C. Aster ◽  
Peter M. Shearer
Keyword(s):  
Shear Wave ◽  
Fault Zone ◽  
Particle Motion ◽  
Southern California ◽  
Shear Waves ◽  
P Wave ◽  
S Wave ◽  
Near Surface ◽  
San Jacinto Fault ◽  
San Jacinto Fault Zone

Abstract Two borehole seismometer arrays (KNW-BH and PFO-BH) have been established in the Southern California Batholith region of the San Jacinto Fault zone by the U.S. Geological Survey. The sites are within 0.4 km of Anza network surface stations and have three-component seismometers deployed at 300 m depth, at 150 m depth, and at the surface. Downhole horizontal seismometers can be oriented to an accuracy of about 5° using regional and near-regional initial P-wave particle motions. Shear waves recorded downhole at the KNW-BH indicate that the strong alignment of initial S-wave particle motions previously observed at the (surface) KNW Anza site (KNW-AZ) is not generated in the near-surface weathered layer. The KNW-BH surface instrument, which sits atop a highly weathered zone, displays a significantly different (≈ 20°) initial S-wave polarization direction from that observed downhole and at KNW-AZ, which is bolted to an outcrop. Although downhole initial shear-wave particle motion directions are consistent with a shear-wave splitting hypothesis, observations of orthogonally polarized slow shear waves are generally elusive, even in seismograms recorded at 300 m. A cross-correlation measure of the apparent relative velocities of Sfast and Sslow horizontally polarized S waves suggests shallow shear-wave anisotropy, consistent with the observed initial S-wave particle motion direction, of 2.3 ± 1.7 per cent between 300 and 150 m and 7.5 ± 3.5 per cent between 150 and 0 m.

scholarly journals Fault Zone Guided Wave generation on the locked, late interseismic Alpine Fault, New Zealand

2021 ◽  
Author(s):  
JD Eccles ◽  
AK Gulley ◽  
PE Malin ◽  
CM Boese ◽  
John Townend ◽  
...  
Keyword(s):  
Fault Zone ◽  
Plate Boundary ◽  
Guided Wave ◽  
S Wave ◽  
Low Velocity ◽  
Catchment Scale ◽  
Near Surface ◽  
Low Velocity Zone ◽  
Alpine Fault ◽  
Velocity Zone

© 2015. American Geophysical Union. All Rights Reserved. Fault Zone Guided Waves (FZGWs) have been observed for the first time within New Zealand's transpressional continental plate boundary, the Alpine Fault, which is late in its typical seismic cycle. Ongoing study of these phases provides the opportunity to monitor interseismic conditions in the fault zone. Distinctive dispersive seismic codas (~7-35Hz) have been recorded on shallow borehole seismometers installed within 20m of the principal slip zone. Near the central Alpine Fault, known for low background seismicity, FZGW-generating microseismic events are located beyond the catchment-scale partitioning of the fault indicating lateral connectivity of the low-velocity zone immediately below the near-surface segmentation. Initial modeling of the low-velocity zone indicates a waveguide width of 60-200m with a 10-40% reduction in S wave velocity, similar to that inferred for the fault core of other mature plate boundary faults such as the San Andreas and North Anatolian Faults.

Fault zone trapped seismic waves

1990 ◽  
Vol 80 (5) ◽  
pp. 1245-1271 ◽  
Author(s):  
Y.-G. Li ◽  
P. C. Leary
Keyword(s):  
Fault Zone ◽  
Seismic Waves ◽  
Fault Plane ◽  
Body Wave ◽  
Velocity Model ◽  
Trapped Waves ◽  
Low Velocity ◽  
Near Surface ◽  
San Andreas ◽  
Borehole Seismic

Abstract Two instances of fault zone trapped seismic waves have been observed. At an active normal fault in crystalline rock near Oroville in northern California, trapped waves were excited with a surface source and recorded at five near-fault borehole depths with an oriented three-component borehole seismic sonde. At Parkfield, California, a borehole seismometer at Middle Mountain recorded at least two instances of the fundamental and first higher mode seismic waves of the San Andreas fault zone. At Oroville recorded particle motions indicate the presence of both Love and Rayleigh normal modes. The Love-wave dispersion relation derived for an idealized wave guide with velocity structure determined by body-wave travel-time inversion yields seismograms of the fundamental mode that are consistent with the observed Love-wave amplitude and frequency. Applying a similar velocity model to the Parkfield observations, we obtain a good fit to the trapped wavefield amplitude, frequency, dispersion, and mode time separation for an asymmetric San Andreas fault zone structure—a high-velocity half-space to the southwest, a low-velocity fault zone, a transition zone containing the borehole seismometer, and an intermediate velocity half-space to the northeast. In the Parkfield borehole seismic data set, the locations (depth and offset normal to fault plane) of natural seismic events which do or do not excite trapped waves are roughly consistent with our model of the low velocity zone. We conclude that it is feasible to obtain near-surface borehole records of fault zone trapped waves. Because trapped modes are excited only by events close to the fault zone proper—thereby fixing these events in space relative to the fault—a wider investigation of possible trapped mode waveforms recorded by a borehole seismic network could lead to a much refined body-wave/tomographic velocity model of the fault and to a weighting of events as a function of offset from the fault plane. It is an open question whether near-surface sensors exist in a stable enough seismic environment to use trapped modes as an earth monitoring device.

scholarly journals Structural analysis of S-wave seismics around an urban sinkhole: evidence of enhanced dissolution in a strike-slip fault zone

2017 ◽  
Vol 17 (12) ◽  
pp. 2335-2350 ◽  
Author(s):  
Sonja H. Wadas ◽  
David C. Tanner ◽  
Ulrich Polom ◽  
Charlotte M. Krawczyk
Keyword(s):  
Fault Zone ◽  
Fracture Network ◽  
Strike Slip ◽  
Strike Slip Fault ◽  
Normal Faults ◽  
S Wave ◽  
Seismic Profiles ◽  
Near Surface ◽  
Reflection Seismic ◽  
Enhanced Dissolution

Abstract. In November 2010, a large sinkhole opened up in the urban area of Schmalkalden, Germany. To determine the key factors which benefited the development of this collapse structure and therefore the dissolution, we carried out several shear-wave reflection-seismic profiles around the sinkhole. In the seismic sections we see evidence of the Mesozoic tectonic movement in the form of a NW–SE striking, dextral strike-slip fault, known as the Heßleser Fault, which faulted and fractured the subsurface below the town. The strike-slip faulting created a zone of small blocks ( < 100 m in size), around which steep-dipping normal faults, reverse faults and a dense fracture network serve as fluid pathways for the artesian-confined groundwater. The faults also acted as barriers for horizontal groundwater flow perpendicular to the fault planes. Instead groundwater flows along the faults which serve as conduits and forms cavities in the Permian deposits below ca. 60 m depth. Mass movements and the resulting cavities lead to the formation of sinkholes and dissolution-induced depressions. Since the processes are still ongoing, the occurrence of a new sinkhole cannot be ruled out. This case study demonstrates how S-wave seismics can characterize a sinkhole and, together with geological information, can be used to study the processes that result in sinkhole formation, such as a near-surface fault zone located in soluble rocks. The more complex the fault geometry and interaction between faults, the more prone an area is to sinkhole occurrence.

Utility of the horizontal-to-vertical spectral ratio passive seismic method for estimating thickness of Quaternary sediments in Minnesota and adjacent parts of Wisconsin

2016 ◽  
Vol 4 (3) ◽  
pp. SH71-SH90 ◽  
Author(s):  
Val W. Chandler ◽  
Richard S. Lively
Keyword(s):  
Cross Sections ◽  
Spectral Ratio ◽  
Sediment Thickness ◽  
S Wave ◽  
Widespread Application ◽  
Near Surface ◽  
Seismic Method ◽  
Calibration Curves ◽  
Passive Seismic ◽  
Hvsr Method

Quaternary glacial and fluvial sediments in Minnesota and adjacent areas are a major source for groundwater, and they are important to many geologic investigations. Unfortunately, the thicknesses of these widespread sediments are often poorly known. The horizontal-to-vertical-spectral ratio (HVSR) passive seismic method is being increasingly used in the region to assist in mapping the thickness of these sediments and the topography of the underlying bedrock surface. HVSR results when control sites with known sediment thickness are used to derive calibration curves that in turn are used to estimate sediment thicknesses in areas lacking control. The HVSR method must be used with appropriate caution. Extreme variations in S-wave velocities of near-surface materials can complicate or even negate the use of calibration curves, and very irregular bedrock surfaces can produce weak, multipeak HVSR spectra that are difficult to interpret. Over soft bedrock, HVSR peaks may reflect intrabedrock features, and velocity contrasts within the glacial sequence can overwhelm HVSR bedrock signatures. Nonetheless, the HVSR method has proven a useful tool to investigate Quaternary geology and depth to bedrock in areas of the state where subsurface data are limited. Thickness estimate errors are usually less than 25%, which is adequate for many geologic applications. The addition of multilocation spectral profiles and cross sections anchored with bedrock control points has allowed us to use data with poor signal quality and to achieve good continuity away from control stations. HVSR methods provide a suitable and cheaper alternative to conventional seismic studies, and they help to target sites for Quaternary drilling programs, bedrock topography evaluation, and determining glacial sediment thickness for 3D mapping. In areas where conventional seismic profiling may be necessary, the HVSR method is useful in selecting and prioritizing targets. The HVSR method should have widespread application in the glaciated areas of the north-central midcontinent.

Estimation of near‐surface shear‐wave velocity by inversion of Rayleigh waves

Geophysics ◽  
1999 ◽  
Vol 64 (3) ◽  
pp. 691-700 ◽  
Author(s):  
Jianghai Xia ◽  
Richard D. Miller ◽  
Choon B. Park
Keyword(s):  
Dispersion Curve ◽  
Shear Wave ◽  
Wave Velocity ◽  
High Frequency ◽  
Solution Technique ◽  
High Frequency Range ◽  
Frequency Range ◽  
S Wave ◽  
Near Surface ◽  
Levenberg Marquardt

The shear‐wave (S-wave) velocity of near‐surface materials (soil, rocks, pavement) and its effect on seismic‐wave propagation are of fundamental interest in many groundwater, engineering, and environmental studies. Rayleigh‐wave phase velocity of a layered‐earth model is a function of frequency and four groups of earth properties: P-wave velocity, S-wave velocity, density, and thickness of layers. Analysis of the Jacobian matrix provides a measure of dispersion‐curve sensitivity to earth properties. S-wave velocities are the dominant influence on a dispersion curve in a high‐frequency range (>5 Hz) followed by layer thickness. An iterative solution technique to the weighted equation proved very effective in the high‐frequency range when using the Levenberg‐Marquardt and singular‐value decomposition techniques. Convergence of the weighted solution is guaranteed through selection of the damping factor using the Levenberg‐Marquardt method. Synthetic examples demonstrated calculation efficiency and stability of inverse procedures. We verify our method using borehole S-wave velocity measurements.

Horizontal-to-vertical spectral ratio and geological conditions: The case of Garner Valley Downhole Array in southern California

1996 ◽  
Vol 86 (2) ◽  
pp. 306-319 ◽  
Author(s):  
N. Theodulidis ◽  
P.-Y. Bard ◽  
R. Archuleta ◽  
M. Bouchon
Keyword(s):  
Southern California ◽  
Transfer Functions ◽  
High Sensitivity ◽  
Spectral Ratio ◽  
S Wave ◽  
Absolute Level ◽  
Data Set ◽  
Geological Conditions ◽  
Different Depths ◽  
Downhole Array

Abstract The aim of the present article is to further check the use of the horizontal-to-vertical (h/v) spectral ratio, which has been recently suggested as an indicator of site effects. The data set consists of 110, three-component, high sensitivity accelerograms, recorded at five different depths by the Garner Valley Downhole Array (GVDA), in southern California, with peak ground accelerations 0.0002 g ≦ ag ≦ 0.04 g, magnitudes 3.0 ≦ ML ≦ 4.6, and hypocentral distances 16 km ≦ R ≦ 107 km. First, the stability of the (h/v) spectral ratio is investigated by computing the mean for the whole data set in different depths. The (h/v) spectral ratio on the surface is compared with the surface-to-depth standard spectral ratio, with theoretical S-wave transfer functions derived from the vertical geotechnical profile, as well as with the (h/v) spectral ratio of synthetic accelerograms generated by the discrete wavenumber method. Both theoretical and experimental data show a good stability of the (h/v) spectral ratio shape, which is in good agreement with the local geological structure and is insensitive to the source location and mechanism. However, the absolute level of the (h/v) spectral ratio depends on the wave field and is different from the surface-to-depth spectral ratio. Consequently the (h/v) spectral ratio technique provides only partially the information that can be obtained from a downhole array. But surface-to-depth ratios may also be misleading because they combine effects at surface and at depth.

A comparison of the high-frequency (>1 Hz) surface and subsurface noise environment at three sites in the United States

1996 ◽  
Vol 86 (5) ◽  
pp. 1516-1528 ◽  
Author(s):  
Christopher J. Young ◽  
Eric P. Chael ◽  
Mitchell M. Withers ◽  
Richard C. Aster
Keyword(s):  
United States ◽  
Wind Speed ◽  
High Frequency ◽  
Wind Profile ◽  
The United States ◽  
Noise Levels ◽  
Wind Noise ◽  
Near Surface ◽  
Unconsolidated Material ◽  
Noise Data

Abstract Surface and subsurface high-frequency (&gt;1 Hz) noise data were recorded using nearly identical instrumentation at three widely separated sites in the United States (Amarillo, Texas; Datil, New Mexico; and Pinedale, Wyoming) for extended periods of time under varying wind conditions. While the sites are geologically distinct, the near-surface noise environments have many common features that we believe may be due in large part to the presence of a surficial layer of highly attenuative unconsolidated material at each site. Noise levels seen at or near the surface (5 m or less) are much higher (up to 30 dB) and much more variable (power range up to 44 dB) than those seen at depth (the smallest range was 9 dB for 1951 m at Amarillo). The greatest gains in noise level reduction are realized within the first 100 m and probably much shallower (&lt; ∼ 10 m). Regardless of the wind profile or local lithology, all sites show an excellent correlation between increased noise levels and higher wind speed, even at significant depths (367 m at Amarillo). Wind-generated noise is broadband (at least 15 to 60 Hz) and apparently nonlinear, increasing dramatically when a wind speed threshold is exceeded (3 to 4 m/sec within a few meters of the surface; as high as 8 m/sec at a depth of a few hundred meters). It is possible to be essentially completely shielded from the wind-generated component of seismic noise by deploying instruments at sufficient depth, but we observed this only for the two deepest deployments (1219 and 1951 m, both at Amarillo). Reducing the wind profile at the surface, however, can yield similar reductions for a much smaller cost. Cultural or “workday” noise, if present (depending on the remoteness of the site), is typically much weaker (10 dB or less) than wind noise but may propagate very effectively to great depths and therefore could be of concern for very deep deployments where wind is not a factor.

scholarly journals Near surface structure of the North Anatolian Fault Zone from Rayleigh and Love wave tomography using ambient seismic noise

2018 ◽  
Author(s):  
George Taylor ◽  
Sebastian Rost ◽  
Gregory Houseman ◽  
Gregor Hillers
Keyword(s):  
Phase Velocity ◽  
Fault Zone ◽  
North Anatolian Fault ◽  
Fault System ◽  
Velocity Variation ◽  
S Wave ◽  
North Anatolian Fault Zone ◽  
Near Surface ◽  
The North ◽  
Wave Tomography

Abstract. We use observations of surface waves in the ambient noise field recorded at a dense seismic array to image the North Anatolian Fault Zone (NAFZ) in the region of the 1999 magnitude 7.6 Izmit earthquake in western Turkey. The NAFZ is a major strike slip fault system extending ~ 1200 km across northern Turkey and poses a high level of seismic hazard, particularly to the city of Istanbul. Assuming isotropy, we obtain maps of phase velocity variation using surface wave tomography applied to Rayleigh and Love waves and construct high resolution images of S-wave velocity in the upper 10 km of a 70 km by 30 km region around Lake Sapanca. We observe low S-wave velocities (< 2.5 km s−1) associated with the Adapazari and Pamukova sedimentary basins, as well as the northern branch of the NAFZ. In the Armutlu Block, between the two major branches of the NAFZ, we detect higher velocities (> 3.2 km s−1) associated with a shallow crystalline basement. We measure azimuthal anisotropy in our phase velocity observations, with the fast direction seeming to align with the direction of maximum extension for the region (~ 45°). The signatures of both the northern and southern branches of the NAFZ are clearly associated with strong gradients in seismic velocity that also denote the boundaries of major tectonic units. Our results suggest that the development of the NAFZ has exploited this pre-existing contrast in physical properties.

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