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

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

10.26686/wgtn.13818791.v1 ◽

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.

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The April 29, 1965, Puget Sound earthquake and the crustal and upper mantle structure of western Washington

Bulletin of the Seismological Society of America ◽

10.1785/bssa0670030693 ◽

1977 ◽

Vol 67 (3) ◽

pp. 693-711 ◽

Cited By ~ 2

Author(s):

Charles A. Langston ◽

David E. Blum

Keyword(s):

Upper Mantle ◽

Source Parameters ◽

Puget Sound ◽

P Wave ◽

Low Velocity ◽

Crust And Upper Mantle ◽

Low Velocity Zone ◽

Crustal Model ◽

Short Period ◽

Velocity Zone

abstract Simultaneous modeling of source parameters and local layered earth structure for the April 29, 1965, Puget Sound earthquake was done using both ray and layer matrix formulations for point dislocations imbedded in layered media. The source parameters obtained are: dip 70° to the east, strike 344°, rake −75°, 63 km depth, average moment of 1.4 ± 0.6 × 1026 dyne-cm, and a triangular time function with a rise time of 0.5 sec and falloff of 2.5 sec. An upper mantle and crustal model for southern Puget Sound was determined from inferred reflections from interfaces above the source. The main features of the model include a distinct 15-km-thick low-velocity zone with a 2.5-km/sec P-wave-velocity contrast lower boundary situated at approximately 56-km depth. Ray calculations which allow for sources in dipping structure indicate that the inferred high contrast value can trade off significantly with interface dip provided the structure dips eastward. The effective crustal model is less than 15 km thick with a substantial sediment section near the surface. A stacking technique using the instantaneous amplitude of the analytic signal is developed for interpreting short-period teleseismic observations. The inferred reflection from the base of the low-velocity zone is recovered from short-period P and S waves. An apparent attenuation is also observed for pP from comparisons between the short- and long-period data sets. This correlates with the local surface structure of Puget Sound and yields an effective Q of approximately 65 for the crust and upper mantle.

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A comparison of the receiver structure beneath stations of the Canadian National Seismograph Network

Canadian Journal of Earth Sciences ◽

10.1139/e95-079 ◽

1995 ◽

Vol 32 (7) ◽

pp. 938-951 ◽

Cited By ~ 42

Author(s):

John F. Cassidy

Keyword(s):

Northwest Territories ◽

Upper Mantle ◽

Velocity Structure ◽

Receiver Functions ◽

Canadian Shield ◽

Low Velocity ◽

Low Velocity Zone ◽

Appalachian Orogen ◽

Seismograph Network ◽

Velocity Zone

Three-component broadband data from the recently deployed Canadian National Seismograph Network provide a new opportunity to examine the structure of the crust and upper mantle beneath the Canadian landmass. Receiver function analysis is an ideal method to use with this data set, as it can provide constraints on the S-velocity structure beneath each station of this seismograph network. This analysis method is particularly useful in that it provides site-specific information (i.e., within 5–15 km of the station), low-velocity layers can be identified, and it is possible to examine structure to upper mantle depths. In this study, receiver functions were computed for each of the 19 stations that made up the seismograph network in June 1994. Five stations, sampling a variety of tectonic environments, including the Appalachian Orogen, the Canadian Shield, the Western Canadian Sedimentary Basin, and the Cascadia subduction zone, were chosen for detailed modelling. The results presented here are the first estimates of the S-velocity structure beneath these five stations. For those stations where comparisons can be made with seismic reflection and refraction results, there is excellent agreement. In eastern Canada, simple receiver functions and clear Moho Ps conversions at most stations indicate a relatively transparent crust and a Moho depth of 40–45 km. In northwestern Canada, Moho Ps phases indicate a crustal thickness of 33–38 km. Beneath Inuvik, Northwest Territories, the Moho is interpreted as two velocity steps separated in depth by 5 km, and an upper mantle low-velocity zone is near 47 km depth. In western Canada, the data indicate a mid-crustal low-velocity zone beneath Edmonton. The Moho of the subducting Juan de Fuca plate is interpreted at 52 km depth beneath southern Vancouver Island. Several stations exhibiting complex receiver functions warrant further study. They include stations at Schefferville, Quebec, in the Canadian Shield; Deer Lake, Newfoundland, on the boundary of the Grenville Province and the Appalachian Orogen; and Yellowknife, Northwest Territories, at the intersection of the Churchill and Slave provinces and the Western Plains.

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Rupture Branching Structure of the 2014 Mw 6.0 South Napa, California, Earthquake Inferred from Explosion‐Generated Fault‐Zone Trapped Waves

Bulletin of the Seismological Society of America ◽

10.1785/0120180181 ◽

2019 ◽

Vol 109 (5) ◽

pp. 1907-1921

Author(s):

Yong‐Gang Li ◽

Rufus D. Catchings ◽

Mark R. Goldman

Keyword(s):

Fault Zone ◽

Rupture Zone ◽

Seismic Velocities ◽

Trapped Waves ◽

Low Velocity ◽

Surface Rupture ◽

Multiple Fault ◽

S Waves ◽

Surface Rupture Zone ◽

Surface Ruptures

Abstract We present evidence for multiple fault branches of the West Napa fault zone (WNFZ) based on fault‐zone trapped waves (FZTWs) generated by two explosions that were detonated within the main surface rupture zone produced by the 24 August 2014 Mw 6.0 South Napa earthquake. The FZTWs were recorded by a 15‐kilometer‐long dense (100 m spacing) linear seismic array consisting of 155 4.5‐hertz three‐component seismometers that were deployed across the surface ruptures and adjacent faults in Napa Valley in the summer of 2016. The two explosions were located ∼3.5 km north and ∼5 km south of the 2016 recording array. Prominent FZTWs, with large amplitudes and long wavetrains following the P and S waves, are observed on the seismograms. We analyzed FZTW waveforms in both time and frequency domains to characterize the branching structure of subsurface rupture zones along the WNFZ. The 2014 surface rupture zone was ∼12 km in length along the main trace of the WNFZ, which appears to form an ∼400–600‐meter‐wide low‐velocity waveguide to depths in excess of 5–7 km. Seismic velocities within the main rupture are reduced by 40%–50% relative to the surrounding‐rock velocities. Within 1.5 km of the main trace of the WNFZ, there are at least two subordinate fault traces that formed 3‐ to 6‐kilometer‐long surface breaks during the 2014 mainshock. Our modeling suggests that these subordinate fault traces are also low‐velocity waveguides that connect with the main rupture at depths of ∼2–3 km, forming a flower structure. FZTWs were also recorded at seismic stations across the Carneros fault (CF), which is ∼1 km west of the WNFZ; this suggests that the CF connects with the WNFZ at shallow depths, even though the CF did not experience surface rupture during the 2014 Mw 6.0 mainshock. 3D finite‐difference simulations of recorded FZTWs imply a branching structure along multiple fault strands associated with the WNFZ.

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A Low Velocity Zone along the Chaochou Fault in Southern Taiwan: Seismic Image Revealed by a Linear Seismic Array

Terrestrial Atmospheric and Oceanic Sciences ◽

10.3319/tao.2009.11.06.01(t) ◽

2010 ◽

Vol 21 (5) ◽

pp. 781

Author(s):

Hsin-Chieh Pu ◽

Cheng-Horng Lin ◽

Kuo-Liang Wen ◽

Tao-Ming Chang ◽

Yih-Hsiung Yeh ◽

...

Keyword(s):

Seismic Array ◽

Low Velocity ◽

Low Velocity Zone ◽

Seismic Image ◽

Southern Taiwan ◽

Velocity Zone

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Thickness Variations in the Lithospheric Mantle and the Low Velocity Zone of the Adamawa Plateau (Cameroon) from Teleseismic Receiver Functions

Open Journal of Geology ◽

10.4236/ojg.2018.86032 ◽

2018 ◽

Vol 08 (06) ◽

pp. 529-542

Author(s):

Serge H. Pokam Kengni ◽

Charles T. Tabod ◽

Eric N. Ndikum ◽

Alain-Pierre Kamga Tokam ◽

Pascal Gounou Pokam

Keyword(s):

Lithospheric Mantle ◽

Receiver Functions ◽

Low Velocity ◽

Low Velocity Zone ◽

Thickness Variations ◽

Velocity Zone

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Noise attenuation in shallow holes

Bulletin of the Seismological Society of America ◽

10.1785/bssa0560030619 ◽

1966 ◽

Vol 56 (3) ◽

pp. 619-632

Author(s):

Eduard J. Douze

Keyword(s):

Background Noise ◽

Seismic Waves ◽

Noise Attenuation ◽

Signal To Noise ◽

Low Velocity ◽

Wind Noise ◽

Low Velocity Zone ◽

Short Period ◽

Normal Background ◽

Velocity Zone

abstract Significant improvements in the performance of short-period seismograph recordings are sometimes obtained in shallow holes (<300 m). Wind noise attenuates rapidly with depth and becomes insignificant at depths of 60 m or less. In the presence of low-velocity weathered layers, the normal background noise decays rapidly with depth and significant improvements in signal-to-noise ratios are obtained. In the absence of a low-velocity zone, only a small attenuation in the background noise level is obtained. Little or no wind noise is converted into traveling seismic waves.

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Temporal changes in the distinct scattered wave packets associated with earthquake swarm activity beneath the Moriyoshi-zan volcano, northeastern Japan

Earth Planets and Space ◽

10.1186/s40623-019-1115-6 ◽

2019 ◽

Vol 71 (1) ◽

Author(s):

Yuta Amezawa ◽

Masahiro Kosuga ◽

Takuto Maeda

Keyword(s):

Wave Packets ◽

Scattered Wave ◽

Temporal Changes ◽

Low Velocity ◽

Small Aperture ◽

Low Velocity Zone ◽

The North ◽

Northeastern Japan ◽

Short Period ◽

Velocity Zone

AbstractWe investigated temporal changes in the waveforms of S-coda from triggered earthquakes around the Moriyoshi-zan volcano in northeastern Japan. Seismicity in the area has drastically increased after the 2011 off the Pacific coast of Tohoku earthquake, forming the largest cluster to the north of the volcano. We analyzed distinct scattered wave packets (DSW) that are S-to-S scattered waves from the mid-crust and appeared predominantly at the high frequency range. We first investigated the variation of DSW for event groups with short inter-event distances and high cross-correlation coefficients (CC) in the time window of direct waves. Despite the above restriction, DSW showed temporal changes in their amplitudes and shapes. The change occurred gradually in some cases, but temporal trends were much more complicated in many cases. We also found that the shape of DSW changed in a very short period of time, for example, within ~ 12 h. Next, we estimated the location of the origin of the DSW (DSW origin) by applying the semblance analysis to the data of the temporary small-aperture array deployed to the north of the largest cluster of triggered events. The DSW origin is located between the largest cluster within which hypocentral migration had occurred and the low-velocity zone depicted by a tomographic study. This spatial distribution implies that the DSW origin was composed of geofluid-accumulated midway in the upward fluid movement from the low-velocity zone to the earthquake cluster. Though we could not entirely exclude the possibility of the effect of the event location and focal mechanisms, the temporal changes in DSW waveforms possibly reflect the temporal changes in scattering properties in and/or near the origin. The quick change in DSW waveforms implies that fast movement of geofluid can occur at the depth of the mid-crust.

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