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

© 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.

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An eigenfunction representation of deep waveguides with application to unconventional reservoirs

Geophysics ◽

10.1190/geo2021-0201.1 ◽

2021 ◽

Vol 86 (6) ◽

pp. T509-T521

Author(s):

Owen Huff ◽

Bin Luo ◽

Ariel Lellouch ◽

Ge Jin

Keyword(s):

Finite Difference ◽

Energy Distribution ◽

Wave Energy ◽

High Frequency ◽

Guided Waves ◽

Guided Wave ◽

Low Velocity ◽

Eagle Ford ◽

Low Velocity Zone ◽

Velocity Zone

Guided waves that propagate in deep low-velocity zones can be described using the displacement-stress eigenfunction theory. For a layered subsurface, these eigenfunctions provide a framework to calculate guided-wave properties at a fraction of the time required for fully numerical approaches for wave-equation modeling, such as the finite-difference approach. Using a 1D velocity model representing the low-velocity Eagle Ford Shale, an unconventional hydrocarbon reservoir, we verify the accuracy of the displacement eigenfunctions by comparing with finite-difference modeling. We use the amplitude portion of the Green’s function for source-receiver eigenfunction pairs as a proxy for expected guided-wave amplitude. These response functions are used to investigate the impact of the velocity contrast, reservoir thickness, and receiver depth on guided-wave amplitudes for discrete frequencies. We find that receivers located within the low-velocity zone record larger guided-wave amplitudes. This property may be used to infer the location of the recording array in relation to the low-velocity reservoir. We also study guided-wave energy distribution between the different layers of the Eagle Ford model and find that most of the high-frequency energy is confined to the low-velocity reservoir. We corroborate this measurement with field microseismic data recorded by distributed acoustic sensing fiber installed outside of the Eagle Ford. The data contain high-frequency body-wave energy, but the guided waves are confined to low frequencies because the recording array is outside the waveguide. We also study the energy distribution between the fundamental and first guided-wave modes as a function of the frequency and source depth and find a nodal point in the first mode for source depths originating in the middle of the low-velocity zone, which we validate with the same field data. The varying modal energy distribution can provide useful constraints for microseismic event depth estimation.

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The Multiscale Structure of the Longmen Shan Central Fault Zone from Local and Teleseismic Data Recorded by Short-Period Dense Arrays

Bulletin of the Seismological Society of America ◽

10.1785/0120190292 ◽

2020 ◽

Vol 110 (6) ◽

pp. 3077-3087

Author(s):

Yafen Huang ◽

Hongyi Li ◽

Xin Liu ◽

Yuting Zhang ◽

Min Liu ◽

...

Keyword(s):

Fault Zone ◽

Receiver Functions ◽

Seismic Array ◽

Trapped Waves ◽

Low Velocity ◽

Surface Rupture ◽

P Waves ◽

Low Velocity Zone ◽

Short Period ◽

Velocity Zone

ABSTRACT The Longmen Shan fault zone (FZ), which consists of the back-range, the central, and the front-range faults, acts as the boundary between the Sichuan basin and eastern Tibet. In this study, local and teleseismic waveforms recorded by a 2D small aperture seismic array (176 temporary short-period seismometers) deployed by China University of Geosciences (Beijing) from 22 October to 20 November 2017 and a dense linear seismic array of 16 stations deployed by Geophysical Exploration Center, China Earthquake Administration during July 2008 are used to study the FZ structure by analyzing FZ-trapped waves (FZTWs), the radial-to-vertical amplitude ratio, and travel-time delays. Based on power density spectra analysis, FZTWs from local events with larger amplitudes and longer wavetrains are clearly observed at stations 6002–6003, 6013–6025, and W025–W032. The dispersion measured from trapped waves is quite weak. The near-surface shear velocity structure estimated from the radial-to-vertical amplitude ratios of local initial P waves shows a low-velocity zone around the surface rupture trace. The slight time delay of direct P waves examined from local and teleseismic events indicates a relatively shallow slow structure beneath the arrays. Through the comprehensive analysis of the central FZ, our results suggest a shallow low-velocity zone with a width of ∼150–160 m along the surface rupture trace. Moreover, our P-wave receiver functions reveal that the Moho depth beneath the Longmen Shan FZ is approximately 45 km, and receiver functions at stations located within the surface rupture zone show more complicated waveforms than those off the surface rupture.

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Discontinuity of the Mozumi–Sukenobu fault low-velocity zone, central Japan, inferred from 3-D finite-difference simulation of fault zone waves excited by explosive sources

Tectonophysics ◽

10.1016/j.tecto.2003.09.008 ◽

2004 ◽

Vol 378 (3-4) ◽

pp. 209-222 ◽

Cited By ~ 10

Author(s):

Yutaka Mamada ◽

Yasuto Kuwahara ◽

Hisao Ito ◽

Hiroshi Takenaka

Keyword(s):

Finite Difference ◽

Fault Zone ◽

Low Velocity ◽

Central Japan ◽

Low Velocity Zone ◽

Velocity Zone ◽

Fault Zone Waves

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

Bulletin of the Seismological Society of America ◽

10.1785/bssa0810041081 ◽

1991 ◽

Vol 81 (4) ◽

pp. 1081-1100 ◽

Cited By ~ 1

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.

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Seismic characteristics of a Precambrian pluton and its adjacent rocks

Geophysics ◽

10.1190/1.1441487 ◽

1983 ◽

Vol 48 (5) ◽

pp. 569-581 ◽

Cited By ~ 13

Author(s):

Z. Hajnal ◽

M. R. Stauffer ◽

M. S. King ◽

P. F. Wallis ◽

H. F. Wang ◽

...

Keyword(s):

High Frequency ◽

Low Frequency ◽

Seismic Energy ◽

Seismic Profile ◽

Open Fractures ◽

Low Velocity ◽

Near Surface ◽

Low Velocity Zone ◽

Plutonic Rocks ◽

Velocity Zone

Surface, borehole, and laboratory acoustic measurements all confirm the existence of a near‐surface low‐velocity zone in metavolcanic, metasedimentary, and plutonic rocks of the Flin Flon region of Canada. This zone is caused by a high frequency of open fractures and extends from the surface to depths of between 5 and 44 m, although occasional open fractures extend to at least 60 m. There is a linear decrease in sonic velocity with increasing frequency of large fractures; the details, however, vary for different sites, depending upon several geologic features including rock type and nonfracture porosity. Laboratory sonic data indicate very low microcrack densities in the volcanic and plutonic rocks. Synthetic seismograms derived from sonic log information from the center of the granitic pluton have been compared with a nearby multifold seismic profile. This shows that the near‐surface low‐velocity zone attenuates most of the high‐frequency seismic energy. However, the remaining low‐frequency portion of the seismic spectrum can be used to map some features of the pluton.

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Sub-vertical low-velocity zone from P and S wave local tomography of the aftershock sequence of the Mw5.9 Chenoua (Algeria) earthquake of October 29, 1989: relict of the Miocene Eurasia–Africa suture zone?

Journal of Seismology ◽

10.1007/s10950-019-09817-2 ◽

2019 ◽

Vol 23 (3) ◽

pp. 455-471

Author(s):

Maya Aouad ◽

Jérôme Van der Woerd ◽

Catherine Dorbath ◽

Abdallah Bounif

Keyword(s):

Aftershock Sequence ◽

Suture Zone ◽

S Wave ◽

Low Velocity ◽

Low Velocity Zone ◽

Local Tomography ◽

Velocity Zone

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Modeling fault‐zone guided waves of microearthquakes in a geothermal reservoir

Geophysics ◽

10.1190/1.1444229 ◽

1997 ◽

Vol 62 (4) ◽

pp. 1278-1284 ◽

Cited By ~ 7

Author(s):

Min Lou ◽

José A. Rial ◽

P. E. Malin

Keyword(s):

Fault Zone ◽

Guided Waves ◽

Velocity Structure ◽

Geothermal Field ◽

Guided Wave ◽

S Wave ◽

Low Velocity ◽

Geothermal Fields ◽

Coso Geothermal Field ◽

Fault Zone Structures

Fault‐zone guided waves have been identified in microearthquake seismograms recorded at the Coso Geothermal Field, California. The observed guided waves have particle motions and propagation group velocities similar to Rayleigh wave modes. A numerical method has been employed to simulate the guided‐wave propagation through the fault zone. By comparing observed and synthetic waveforms the fault‐zone width and its P‐ and S‐wave velocity structure have been estimated. It is suggested here that the identification and modeling of such guided waves is an effective tool to locate fracture‐induced, low‐velocity fault‐zone structures in geothermal fields.

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