An analysis of split shear waves observed above crustal and uppermost mantle earthquakes beneath Shikoku, Japan: Implications in effective depth extent of seismic anisotropy

1989 ◽  
Vol 94 (B10) ◽  
pp. 14077-14092 ◽  
Author(s):  
Satoshi Kaneshima ◽  
Masataka Ando
Keyword(s):  
Seismic Anisotropy ◽  
Shear Waves ◽  
Uppermost Mantle ◽  
Effective Depth ◽  
Depth Extent ◽  
Mantle Earthquakes

Shear‐wave splitting in cross‐hole surveys: Modeling

Geophysics ◽  
1989 ◽  
Vol 54 (1) ◽  
pp. 57-65 ◽  
Author(s):  
Enru Liu ◽  
Stuart Crampin ◽  
David C. Booth
Keyword(s):  
Shear Wave ◽  
Seismic Anisotropy ◽  
Pore Space ◽  
Shear Waves ◽  
Shear Wave Splitting ◽  
Surface Layers ◽  
Near Surface ◽  
Crustal Rocks ◽  
Wave Splitting ◽  
Almost All

Shear‐wave splitting, diagnostic of some form of effective seismic anisotropy, is observed along almost all near‐vertical raypaths through the crust. The splitting is caused by propagation through distributions of stress‐aligned vertical parallel fluid‐filled cracks, microcracks, and preferentially oriented pore space that exist in most crustal rocks. Shear waves have severe interactions with the free surface and may be seriously disturbed by the surface and by near‐surface layers. In principle, cross‐hole surveys (CHSs) should be free of much of the near‐surface interference and could be used for investigating shear waves at higher frequencies and greater resolution along shorter raypaths than is possible with reflection surveys and VSPs. Synthetic seismograms are examined to estimate the effects of vertical cracks on the behavior of shear waves in CHS experiments. The azimuth of the CHS section relative to the strike of the cracks is crucial to the amount of information about seismic anisotropy that can be extracted from such surveys. Interpretation of data from only a few boreholes located at azimuths chosen from other considerations is likely to be difficult and inconclusive. Application to interpreting acoustic events generated by hydraulic pumping is likely to be more successful.

Interpreting non-orthogonal split shear waves for seismic anisotropy in multicomponent VSPS

1998 ◽  
Vol 46 (1) ◽  
pp. 1-27 ◽  
Author(s):  
Xiang-Yang Li ◽  
Colin MacBeth ◽  
Stuart Crampin
Keyword(s):  
Seismic Anisotropy ◽  
Shear Waves

A multiscale study of the mechanisms controlling shear velocity anisotropy in the San Andreas Fault Observatory at Depth

Geophysics ◽  
2006 ◽  
Vol 71 (5) ◽  
pp. F131-F146 ◽  
Author(s):  
Naomi L. Boness ◽  
Mark D. Zoback
Keyword(s):  
Seismic Anisotropy ◽  
San Andreas Fault ◽  
Shear Waves ◽  
Shear Velocity ◽  
Granitic Rocks ◽  
Velocity Anisotropy ◽  
Bedding Planes ◽  
San Andreas ◽  
State Of Stress ◽  
Sonic Logs

We present an analysis of shear velocity anisotropy using data in and near the San Andreas Fault Observatory at Depth (SAFOD) to investigate the physical mechanisms controlling velocity anisotropy and the effects of frequency and scale. We analyze data from borehole dipole sonic logs and present the results from a shear-wave-splitting analysis performed on waveforms from microearthquakes recorded on a downhole seismic array. We show how seismic anisotropy is linked either to structures such as sedimentary bedding planes or to the state of stress, depending on the physical properties of the formation. For an arbitrarily oriented wellbore, we model the apparent fast direction that is measured with dipole sonic logs if the shear waves are polarized by arbitrarily dipping transversely isotropic (TI) structural planes (bedding/fractures). Our results indicate that the contemporary state of stress is the dominant mechanism governing shear velocity anisotropy in both highly fractured granitic rocks and well-bedded arkosic sandstones. In contrast, within the finely laminated shales, anisotropy is a result of the structural alignment of clays along the sedimentary bedding planes. By analyzing shear velocity anisotropy at sonic wavelengths over scales of meters and at seismic frequencies over scales of several kilometers, we show that the polarization of the shear waves and the amount of anisotropy recorded are strongly dependent on the frequency and scale of investigation. The shear anisotropy data provide constraints on the orientation of the maximum horizontal compressive stress [Formula: see text] and suggest that, at a distance of only [Formula: see text] from the San Andreas fault (SAF), [Formula: see text] is at an angle of approximately 70° to the strike of the fault. This observation is consistent with the hypothesis that the SAF is a weak fault slipping at low levels of shear stress.

Seismic Anisotropy of the Uppermost Mantle beneath Mid-ocean Ridges

Nature ◽  
1968 ◽  
Vol 218 (5141) ◽  
pp. 558-559 ◽  
Author(s):  
R. B. WHITMARSH
Keyword(s):  
Seismic Anisotropy ◽  
Uppermost Mantle ◽  
Ocean Ridges

Seismic Anisotropy of the Uppermost Mantle under Oceans

Nature ◽  
1964 ◽  
Vol 203 (4945) ◽  
pp. 629-631 ◽  
Author(s):  
H. H. HESS
Keyword(s):  
Seismic Anisotropy ◽  
Uppermost Mantle

Seismic anisotropy of the uppermost mantle beneath the Rio Grande rift: Evidence from Kilbourne Hole peridotite xenoliths, New Mexico

2011 ◽  
Vol 311 (1-2) ◽  
pp. 172-181 ◽  
Author(s):  
Takako Satsukawa ◽  
Katsuyoshi Michibayashi ◽  
Elizabeth Y. Anthony ◽  
Robert J. Stern ◽  
Stephen S. Gao ◽  
...  
Keyword(s):  
New Mexico ◽  
Seismic Anisotropy ◽  
Rio Grande ◽  
Rio Grande Rift ◽  
Uppermost Mantle ◽  
Peridotite Xenoliths

scholarly journals Shear-wave polarization alignment on the eastern flank of Mt. Etna volcano (Sicily, Italy)

1996 ◽  
Vol 39 (2) ◽  
Author(s):  
F. Bianco ◽  
M. Castellano ◽  
G. Milano ◽  
G. Vilardo
Keyword(s):  
Shear Wave ◽  
Seismic Anisotropy ◽  
Physical Phenomenon ◽  
Shear Waves ◽  
Polarization Vector ◽  
Volcanic Area ◽  
Etna Volcano ◽  
Complex Situation ◽  
Mt Etna ◽  
Seismic Networks

Recently, with the improvement of three-component seismic networks, studies revealing anisotropic characteristics in different regions have assumed great interest. In a complex volcanic area like Mt. Etna (Sicily, Italy), the existence of both iso-oriented fault systems and intrusive bodies consisting of olivine and clinopyroxene suggest the presence of anisotropic structures. In order to investigate this we analyzed the physical phenomenon of shear-wave splitting since under certain constraints, shear waves are less sensitive to local heterogeneity. The aims of this paper are: 1) to evaluate if in a structural complex situation like that at Mt. Etna the signal crossing an anisotropic volume could be enhanced in spite of effects due to undirectional properties along the source-receiver path; 2) to investigate the correlations, if any, between polarization direction of the leading shear wave and the patterns of compressive stress acting on the investigated area. Therefore we measured time-delays between the S-onsets on the horizontal components of 3D seismograms to reveal the possible seismic anisotropy in the Etnean region; moreover, we analyzed the polarization vector of shear-waves seismic data recorded during a survey carried out in the spring-summer 1988. We found clear evidence of splitting that we attributed to the presence of an anisotropic volume not homogeneously distributed on the eastern slope of Mt. Etna volcano.

scholarly journals Earthquakes and crustal structure of Himalaya from Himalayan Nepal-Tibet seismic experiment (HIMNT)

2008 ◽  
Vol 38 ◽  
pp. 1-8
Author(s):  
Anne F. Sheehan ◽  
Thomas De La Torre ◽  
Gaspar Monsalve ◽  
Vera Schulte-Pelkum ◽  
Roger Bilham ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Seismic Anisotropy ◽  
Velocity Structure ◽  
Receiver Functions ◽  
Moho Depth ◽  
Upper Crust ◽  
Southern Tibet ◽  
Seismic Experiment ◽  
3D Velocity Model ◽  
Mantle Earthquakes

The Himalayan Nepal - Tibet PASSCAL Seismic Experiment (HIMNT) included the deployment of 28 broadband seismometers throughout eastern Nepal and southern Tibet in 2001- 2002. The main goals of the project were to better understand the mountain building processes of this region through studies of seismicity and Earth structure determined from local and teleseismic earthquakes. The seismic deployment was in collaboration with the National Seismological Centre, Department of Mines and Geology, Nepal, and the Institute of Geology and Geophysics of the Chinese Academy of Sciences. Our new subsurface images from HIMNT teleseismic receiver functions and local earthquake tomography show evidence of the basal decollement of the Himalaya (Main Himalayan Thrust, MHT) and an increase in Moho depth from - 45 km beneath Nepal to -75 km beneath Tibet. We find strong seismic anisotropy above the decollement, likely developed in response to shear on the MHT. The shear may be taken up as slip in great earthquakes at shallower depths. Many local earthquakes were recorded during the deployment, and the large contrast in crustal thickness and velocity structure over a small lateral distance makes the use of a 3D velocity model important to determine accurate hypocentres. Large north-south variations are found in P and S wave velocity structure across the array. High Pn velocities are found beneath southern Tibet. Seismicity shows strong alignment of shallow (15-25 km depth) events beneath the region of highest relief along the Himalayan Front, and a cluster of upper mantle earthquakes beneath southern Tibet (70-90 km depth). Weak-mantle models do not expect the upper mantle earthquakes. Focal mechanisms of these upper mantle earthquakes beneath southern Tibet are mostly strike-slip, markedly different from the norm al faulting mechanisms observed for earthquakes in the mid and upper crust beneath Tibet. This change in the orientation of the major horizontal compression axis from vertical in the upper crust to horizontal in the upper mantle suggests a transition from deformation driven by body forces in the crust to plate boundary forces in the upper mantle. Several lines of evidence point to a decoupling zone in the Tibetan mid or lower crust, which may be related to the presence of a previously suggested flow channel in the Tibetan mid crust.

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