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  • 1
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    Lithospheric velocity structure of the Anatolian plateau-Caucasus-Caspian region (2011)
    Wiley
    Details
    Publication Date: 2011-05-07
    Description: The Anatolian plateau-Caucasus-Caspian region is an area of complex lithospheric structure accompanied by large variations in seismic wave velocities. Despite the complexity of the region, little is known about the detailed lithospheric structure. Using data from 31 new, permanent broadband seismic stations along with results from a previous 29 temporary seismic stations and 3 existing global seismic stations in the region, a 3-D velocity model is developed using joint inversion of teleseismic receiver functions and surface waves. Both group and phase dispersion curves (Love and Rayleigh) were derived from regional and teleseismic events. Additional Rayleigh wave group dispersion curves were determined using ambient noise correlation. Receiver functions were calculated using P arrivals from 789 teleseismic (30°–90°) earthquakes. The stacked receiver functions and surface wave dispersion curves were jointly inverted to yield the absolute shear wave velocity to a depth of 100 km at each station. The depths of major discontinuities (sediment-basement, crust-mantle, and lithosphere-asthenosphere) were inferred from the velocity-depth profiles at the location of each station. Distinct spatial variations in crustal and upper mantle shear velocities were observed. The Kura basin showed slow (∼2.7–2.9 km/s) upper crustal (0–11 km) velocities but elevated (∼3.8–3.9 km/s) velocities in the lower crust. The Anatolian plateau varied from ∼3.1–3.2 in the upper crust to ∼3.5–3.7 in the lower crust, while velocities in the Arabian plate (south of the Bitlis suture) were slightly faster (upper crust between 3.3 and 3.4 km/s and lower crust between 3.8 and 3.9 km/s). The depth of the Moho, which was estimated from the shear velocity profiles, was 35 km in the Arabian plate and increased northward to 54 km at the southern edge of the Greater Caucasus. Moho depths in the Kura and at the edge of the Caspian showed more spatial variability but ranged between 35 and 45 km. Upper mantle velocities were slow under the Anatolian plateau but increased to the south under the Arabian plate and to the east (4.3–4.4 km/s) under the Kura basin and Greater Caucasus. The areas of slow mantle coincided with the locations of Holocene volcanoes. Differences between Rayleigh and Love dispersions at long wavelengths reveal a pronounced variation in anisotropy between the Anatolian plateau and the Kura basin.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Ambient-noise tomography of north Tibet limits geological terrane signature to upper-middle crust (2013)
    Wiley
    Details
    Publication Date: 2013-02-16
    Description: : [1]  We use ambient-noise tomography to map regional differences in crustal Rayleigh-wave group velocities with periods of 8-40 s across north Tibet using the INDEPTH IV arrays (132 stations, deployed for 10-24 months). For periods of 8-24 s (sensitive to mid-crustal depths of ~5-30 km), we observe striking velocity changes across theBangong-Nujiang and Jinsha suture zonesaswell as the Kunlun-Qaidam boundary. From south to north, we see higher velocities beneath the Lhasa terrane, lower velocities beneath the Qiangtang, higher velocities in the Songpan-Ganzi and Kunlun terranes, and the lowest velocities beneath the Qaidam Basin. Maps at periods of 34 and 40 s (sensitive to the middle and lower crust at depths of ~30-60 km) do not show evidence of changes across those boundaries. Any differences between the Tibetan terrane lower crusts that were present at accretion have been erased or displaced by Cenozoic processes and replaced almost ubiquitously by uniformly low velocities.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
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    Evidence from Earthquake Data for a Partially Molten Crustal Layer in Southern Tibet (1996)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 1996-12-06
    Description: Earthquake data collected by the INDEPTH-II Passive-Source Experiment show that there is a substantial south to north variation in the velocity structure of the crust beneath southern Tibet. North of the Zangbo suture, beneath the southern Lhasa block, a midcrustal low-velocity zone is revealed by inversion of receiver functions, Rayleigh-wave phase velocities, and modeling of the radial component of teleseismic P-waveforms. Conversely, to the south beneath the Tethyan Himalaya, no low-velocity zone was observed. The presence of the midcrustal low-velocity zone in the north implies that a partially molten layer is in the middle crust beneath the northern Yadong-Gulu rift and possibly much of southern Tibet.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Kind -- Ni -- Zhao -- Wu -- Yuan -- Sandvol -- Reese -- Nabelek -- Hearn -- New York, N.Y. -- Science. 1996 Dec 6;274(5293):1692-4.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉R. Kind and Xiaohui Yuan, GeoForschungsZentrum Potsdam, 14473 Potsdam, Germany. James Ni, Jianxin Wu, C. Reese, T. Hearn, Department of Physics, New Mexico State University, Las Cruces, NM 88003, USA. Wenjin Zhao, Chinese Academy of Geological Sciences, Beijing, China Lianshe Zhao, Institute for Geophysics, University of Texas at Austin, Austin, TX 78759, USA. E. Sandvol, Department of Geological Sciences, Cornell University, Ithaca, NY 14853, USA. J. Nabelek, College of Oceanography, Oregon State University, Corvalis, OR 97331, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/8939854" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 4
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    Partially Molten Middle Crust Beneath Southern Tibet: Synthesis of Project INDEPTH Results (1996)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 1996-12-06
    Description: INDEPTH geophysical and geological observations imply that a partially molten midcrustal layer exists beneath southern Tibet. This partially molten layer has been produced by crustal thickening and behaves as a fluid on the time scale of Himalayan deformation. It is confined on the south by the structurally imbricated Indian crust underlying the Tethyan and High Himalaya and is underlain, apparently, by a stiff Indian mantle lid. The results suggest that during Neogene time the underthrusting Indian crust has acted as a plunger, displacing the molten middle crust to the north while at the same time contributing to this layer by melting and ductile flow. Viewed broadly, the Neogene evolution of the Himalaya is essentially a record of the southward extrusion of the partially molten middle crust underlying southern Tibet.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Nelson -- Zhao -- Brown -- Kuo -- Che -- Liu -- Klemperer -- Makovsky -- Meissner -- Mechie -- Kind -- Wenzel -- Ni -- Nabelek -- Leshou -- Tan -- Wei -- Jones -- Booker -- Unsworth -- Kidd -- Hauck -- Alsdorf -- Ross -- Cogan -- Wu -- Sandvol -- Edwards -- New York, N.Y. -- Science. 1996 Dec 6;274(5293):1684-8.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉K. D. Nelson, M. Cogan, C. Wu, Department of Earth Sciences, Syracuse University, Syracuse, NY 13244, USA. W. Zhao, J. Che, X. Liu, Chinese Academy of Geological Sciences, Beijing 100037, China. L. D. Brown, M. Hauck, D. Alsdorf, A. Ross, Institute for the Study of the Continents, Cornell University, Ithaca, NY 14853, USA. J. Kuo, Lamont Doherty Geological Observatory, Palisades, NY, 10964, USA. S. L. Klemperer and Y. Makovsky, Department of Geophysics, Stanford University, Stanford, CA 94305, USA. R. Meissner, Institut fur Geophysik, Christian-Albrechts-Universitaet zu Kiel, 24098 Kiel, Germany. J. Mechie and R. Kind, GeoForschungsZentrum Potsdam (GFZ), 14473 Potsdam, Germany. F. Wenzel, Geophysikalisches Institut, Universitaet Karlsruhe, 76187 Karlsruhe, Germany. J. Ni and E. Sandvol, Department of Physics, New Mexico State University, Las Cruces, NM 88003, USA. J. Nabelek, College of Oceanography, Oregon State University, Corvallis, OR 97331, USA. L. Chen, H. Tan, W. Wei, China University of Geosciences, Beijing, China. A. G. Jones, Geological Survey of Canada, 1 Observatory Crescent, Ottawa, Ontario, Canada. J. Booker and M. Unsworth, Geophysics Program, University of Washington, Seattle, WA 98195, USA. W. S. F. Kidd and M. Edwards, Department of Geosciences, SUNY-Albany, Albany, NY 12222, USA〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/8939851" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 5
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    Investigation of the Regional Site Response in the Central and Eastern United States (2019)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2019
    Description: 〈span〉〈div〉Abstract〈/div〉We estimated the site amplification of regional high‐frequency 〈span〉Lg〈/span〉 seismic phases by a reverse two‐station (RTS) method using seismic events (Mw 4–6) recorded by Earthscope’s Transportable Array from 2010 to 2013. We compare regional site amplification estimates (horizontal and vertical) from the RTS technique with horizontal‐to‐vertical spectral ratio (HVSR) estimates derived from ambient noise and earthquake records. We compare the RTS results with (1) shallow shear‐wave velocity estimates from near‐surface (horizontal/vertical) ratios of the local body‐wave (initial 〈span〉P〈/span〉‐wave) method, and (2) high topography, basins, and sediment thicknesses. Our RTS results show a strong positive correlation between regional site amplification and basins such as the Michigan basin, the Illinois basin, and the Mississippi embayment. In the case of the Illinois and Michigan basins, the higher the frequency, the higher the horizontal and vertical amplification. Waves passing through the Appalachian and Ozark plateaus are deamplified on both vertical and horizontal ground components; however, the variation in amplification with frequency is larger for horizontal motion than vertical motion. In some regions, such as the western edge of the Appalachian basin and southern Illinois basin, vertical amplification decreases with frequency but horizontal amplification is essentially invariant with respect to frequency. Topography and sediment thickness are likely to affect amplification and both factors likely frequency dependent. There is a negative correlation between the RTS‐measured amplification and shallow shear‐wave velocity, whereas HVSR shows a negative correlation only for low frequencies 〈2.0  Hz. We conclude that regional ground‐motion amplification is clearly a function of more than one variable. In general, it appears that both regional topography (i.e., long‐wavelength topography) and deeper subsurface seismic structures (basins and sediments) have a large impact on site amplification.〈/span〉
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America (SSA)
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  • 6
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    The structure of the crust in the Turkish-Iranian Plateau and Zagros using Lg Q and velocity (2015)
    Oxford University Press
    Details
    Publication Date: 2015-01-11
    Description: We present a new approach for understanding the origin and nature of seismic anomalies in the continental crust of the Northern Middle East. We have created detailed attenuation ( Q Lg ) and velocity ( V Lg ) models for the Northern Middle East based on the analysis of waveforms of regional seismic phase Lg from 3171 regional earthquakes recorded at 578 stations in Turkish and Iranian Plateaus and surrounding regions. The attenuation and velocity models are assumed to serve as proxies for the bulk average crustal shear wave attenuation ( Q β ) and velocities ( V s ). 31 232 reliable Lg spectra were collected and used to measure the two-station method (TSM) and reverse two-station/event method (RTM) Lg Q at 1 Hz ( Q 0 ) and its frequency-dependence factor ( ). The Lg Q 0 and values are measured over the individual TSM and RTM paths and are then used to perform an LSQR tomographic inversion for lateral variations in Q 0 and . The Lg Q 0 and models both correlate well with the major tectonic boundaries in the region. The tomographic models as well as the individual TSM and RTM measurements show lower values of Lg Q 0 over the Turkish-Anatolian Plateau (〈150) than those observed over the Iranian Plateau (150–400). Furthermore, we obtained the Lg group velocity model by inverting the time of the first arrival of the Lg waveform on each seismogram. Our Q measurements are strongly correlated with the measurements of Lg group velocity ( V Lg ) suggesting that the source of many of the low Q and velocity anomalies is likely the same. The regions where we see negative correlations are likely a result of Sn to Lg converted energy. Our results also have implications for the far field ground motions, suggesting that large earthquakes in eastern Iran could have a significant far field ground motions due to relatively low crustal attenuation within the Iranian plateau.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 7
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    Investigation of the Regional Site Response in the Central and Eastern United States (2019)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2019
    Description: 〈span〉〈div〉Abstract〈/div〉We estimated the site amplification of regional high‐frequency 〈span〉Lg〈/span〉 seismic phases by a reverse two‐station (RTS) method using seismic events (Mw 4–6) recorded by Earthscope’s Transportable Array from 2010 to 2013. We compare regional site amplification estimates (horizontal and vertical) from the RTS technique with horizontal‐to‐vertical spectral ratio (HVSR) estimates derived from ambient noise and earthquake records. We compare the RTS results with (1) shallow shear‐wave velocity estimates from near‐surface (horizontal/vertical) ratios of the local body‐wave (initial 〈span〉P〈/span〉‐wave) method, and (2) high topography, basins, and sediment thicknesses. Our RTS results show a strong positive correlation between regional site amplification and basins such as the Michigan basin, the Illinois basin, and the Mississippi embayment. In the case of the Illinois and Michigan basins, the higher the frequency, the higher the horizontal and vertical amplification. Waves passing through the Appalachian and Ozark plateaus are deamplified on both vertical and horizontal ground components; however, the variation in amplification with frequency is larger for horizontal motion than vertical motion. In some regions, such as the western edge of the Appalachian basin and southern Illinois basin, vertical amplification decreases with frequency but horizontal amplification is essentially invariant with respect to frequency. Topography and sediment thickness are likely to affect amplification and both factors likely frequency dependent. There is a negative correlation between the RTS‐measured amplification and shallow shear‐wave velocity, whereas HVSR shows a negative correlation only for low frequencies 〈2.0  Hz. We conclude that regional ground‐motion amplification is clearly a function of more than one variable. In general, it appears that both regional topography (i.e., long‐wavelength topography) and deeper subsurface seismic structures (basins and sediments) have a large impact on site amplification.〈/span〉
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 8
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    Lg attenuation in northeast China using NECESSArray data (2014)
    Oxford University Press
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    Publication Date: 2014-11-07
    Description: The 127 station NorthEast China Extended SeiSmic Array (NECESSArray) provides large quantities of high quality seismic data in northeast China that allow us to resolve lateral variations of Lg Q or crustal attenuation at 1 Hz ( Q o ) to 2.0° or greater. Using the reverse two-station/event method with 11 642 Lg path-amplitudes from 78 crustal earthquakes, we obtain a 2-D tomographic image of Lg Q o with values ranging from ~50 to 1400. A high degree of detail in the lateral variation of Lg attenuation is revealed in our tomographic image. High Q o regions are found in the Great Xing'an, Lesser Xing'an and Songen-Zhangguangcai Ranges. Low Q o regions are observed in the Songliao, Sanjiang and Erlian Basins. The lowest Q o is found near the Wudalianchi volcanic field and other Quaternary volcanic fields, the southern Songliao Basin, the western edge of the Erlian Basin and the Sanjiang Basin. Low Q o values are measured for paths that cross sedimentary basins with thick, unconsolidated sediments. Most of the high Lg attenuation in the Songliao Basin correlates reasonably well with low crustal Rayleigh wave phase velocity anomalies. The highest attenuating regions also correlate well with regions of Holocene volcanism.
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 9
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    Shear wave splitting and shear wave splitting tomography of the southern Puna plateau (2014)
    Oxford University Press
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    Publication Date: 2014-08-30
    Description: We have investigated the seismic anisotropy beneath the Central Andean southern Puna plateau by applying shear wave splitting analysis and shear wave splitting tomography to local S waves and teleseismic SKS, SKKS and PKS phases. Overall, a very complex pattern of fast directions throughout the southern Puna plateau region and a circular pattern of fast directions around the region of the giant Cerro Galan ignimbrite complex are observed. In general, teleseismic lag times are much greater than those for local events which are interpreted to reflect a significant amount of sub and inner slab anisotropy. The complex pattern observed from shear wave splitting analysis alone is the result of a complex 3-D anisotropic structure under the southern Puna plateau. Our application of shear wave splitting tomography provides a 3-D model of anisotropy in the southern Puna plateau that shows different patterns depending on the driving mechanism of upper-mantle flow and seismic anisotropy. The trench parallel a -axes in the continental lithosphere above the slab east of 68W may be related to deformation of the overriding continental lithosphere since it is under compressive stresses which are orthogonal to the trench. The more complex pattern below the Cerro Galan ignimbrite complex and above the slab is interpreted to reflect delamination of continental lithosphere and upwelling of hot asthenosphere. The a -axes beneath the Cerro Galan, Cerro Blanco and Carachi Pampa volcanic centres at 100 km depth show some weak evidence for vertically orientated fast directions, which could be due to vertical asthenospheric flow around a delaminated block. Additionally, our splitting tomographic model shows that there is a significant amount of seismic anisotropy beneath the slab. The subslab mantle west of 68W shows roughly trench parallel horizontal a -axes that are probably driven by slab roll back and the relatively small coupling between the Nazca slab and the underlying mantle. In contrast, the subslab region (i.e. depths greater than 200 km) east of 68W shows a circular pattern of a -axes centred on a region with small strength of anisotropy (Cerro Galan and its eastern edge) which suggest the dominant mechanism is a combination of slab roll back and flow driven by an overlying abnormally heated slab or possibly a slab gap. There seems to be some evidence for vertical flow below the slab at depths of 200–400 km driven by the abnormally heated slab or slab gap. This cannot be resolved by the tomographic inversion due to the lack of ray crossings in the subslab mantle.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 10
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    Crustal Velocity Structure of the Northeastern Tibetan Plateau from Ambient Noise Surface-Wave Tomography and Its Tectonic Implications (2014)
    Seismological Society of America (SSA)
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    Publication Date: 2014-06-12
    Description: Broadband seismic data from the regional seismic network operated by the China Earthquake Administration and 32 temporary seismic stations are used to image the crustal velocity structure in the northeast Tibetan plateau. Empirical Rayleigh- and Love-wave Green’s functions are obtained from interstation cross correlation of continuous seismic records. Group velocity dispersion curves for Rayleigh and Love waves between 10 and 50 s are obtained using the multiple-filter analysis method with phase-matched processing. The group velocity variations of Rayleigh and Love waves overall correlate well with the major geologic structures and tectonic units in the study region. Shear-wave velocity structures were then inverted from Rayleigh- and Love-wave dispersion maps. The results show that the Songpan–Ganzi terrane is associated with a low velocity at depth greater than 20 km. The northern Qilian orogen, with higher elevation and thicker crust compared to the southern Qilian orogen, is also dominated by low velocity at depth greater than ~25 km. However, there is no clear evidence of the low-velocity mid-to-lower crust beneath the southern Qilian orogen as the crustal flow model predicts. The low-velocity zone (LVZ) beneath the northern Qilian orogen may suggest that the crustal thickening and surface uplift of the northern Qilian orogen are related to the LVZ, and the LVZ may be considered as an intracrustal response to bear the ongoing deformation in the northern Qilian orogen. Online Material: Figures of crustal topography, number of group velocity measurements, checkerboard tests for NETS stations, and 1D velocity models.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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