scholarly journals Seismic attenuation structure in central Mexico: Image of a focused high-attenuation zone in the mantle wedge

2009 ◽  
Vol 114 (B7) ◽  
Author(s):  
Ting Chen ◽  
Robert W. Clayton
Keyword(s):  
Mantle Wedge ◽  
Seismic Attenuation ◽  
Central Mexico ◽  
High Attenuation ◽  
Attenuation Structure

3-D seismic attenuation structure of Long Valley caldera: looking for melt bodies in the shallow crust

2019 ◽  
Vol 220 (3) ◽  
pp. 1677-1686 ◽  
Author(s):  
Janire Prudencio ◽  
Michael Manga
Keyword(s):  
Earthquake Swarm ◽  
Vertical Component ◽  
Seismic Attenuation ◽  
Hydrothermal Systems ◽  
Magma Body ◽  
Long Valley Caldera ◽  
High Attenuation ◽  
Coda Normalization ◽  
Long Valley ◽  
Attenuation Structure

SUMMARY Unrest at Long Valley caldera (California) during the past few decades has been attributed to the ascent of hydrothermal fluids or magma recharge. The difference is critical for assessing volcanic hazard. To better constrain subsurface structures in the upper crust and to help distinguish between these two competing hypotheses for the origin of unrest, we model the 3-D seismic attenuation structure because attenuation is particularly sensitive to the presence of melt. We analyse more than 47 000 vertical component waveforms recorded from January 2000 through November 2016 obtained from the Northern California Earthquake Data Center. We then inverted the S-to-coda energy ratios using the coda normalization method and obtained an average Q of 250. Low attenuation anomalies are imaged in the fluid-rich western and eastern areas of the caldera, one of which corresponds to the location of an earthquake swarm that occurred in 2014. From a comparison with other geophysical images (magnetotellurics, seismic tomography) we attribute the high attenuation anomalies to hydrothermal systems. Average to high attenuation values are also observed at Mammoth Mountain (southwest of the caldera), and may also have a hydrothermal origin. A large high attenuation anomaly within the caldera extends from the surface to the depths we can resolve at 9 km. Shallow rocks here are cold and this is where earthquakes occur. Together, these observations imply that the high attenuation region is not imaging a large magma body at shallow depths nor do we image any isolated high attenuation bodies in the upper ≈8 km that would be clear-cut evidence for partially molten bodies such as sills or other magma bodies.

scholarly journals Seismic attenuation of regional phases in the northern Middle East and eastern Tibetan plateau

2017 ◽  
Author(s):  
◽  
Wenfei Ku
Keyword(s):  
Middle East ◽  
Tibetan Plateau ◽  
Continental Collision ◽  
Seismic Attenuation ◽  
Eastern Tibetan Plateau ◽  
Uppermost Mantle ◽  
Iranian Plateau ◽  
High Attenuation ◽  
Attenuation Structure ◽  
Q Values

Temperature and composition are two major causes for subsurface seismic anomalies. Positive temperature anomalies will lead to a reduction in both attenuation and velocity; however, compositional anomalies should not necessarily produce a strong correlation in attenuation and velocity. As a result, by combining velocity and attenuation structure we can distinguish between compositional and temperature anomalies. Using efficiency tomography and Q tomography, I have constructed Sn attenuation models for two continental-continental collision zones, the northern Middle East and the eastern Tibetan Plateau. The Tibetan Plateau was formed by the continental collision between Indian and Eurasian plates that has been going on at least since [about]50Ma. Two tomographic techniques have been used to determine the attenuation structure of the uppermost mantle beneath the eastern Tibetan Plateau. I observe lateral heterogeneity of Sn attenuation beneath the southern Tibetan Plateau that indicates a complex geometry of the underthrusting Indian continental lithosphere (UICL). Sn is blocked with relative low Q values across the Qiangtang block and Songpan-Ganzi block indicating a hot and weak lithosphere. This observation can be caused by mantle upwellings induced by the sinking slab detached from the UICL. The Turkish-Iranian plateau and Zagros, the main tectonic feature of the northern Middle East, was formed as a result of the continental collision between the Arabian and Eurasian plates since Early Cenozoic (23-35Ma). I have collected a large Sn waveform data set in the northern Middle East that I have quality controlled using both automated and manual approaches. Two tomographic techniques have been used to determine the attenuation structure of the uppermost mantle. I observe inefficient/blocked Sn and low Q values in the Turkish-Iranian plateau indicating a hot and thin mantle lithosphere. Intrinsic attenuation is the dominant uppermost mantle shear wave attenuation mechanism beneath the eastern Anatolian plateau and Lesser Caucasus. Partial melting appears to be the main cause of high attenuation in two of the regions. Scattering attenuation appears to be the dominant mechanism in the Zagros. The high attenuation in the Iranian plateau is likely not caused by partial melting thus the seismic anomalies in the uppermost mantle are likely compositional. Data censorship is a common problem in seismic attenuation studies. Discarding blocked Sn paths will cause left censored data problem and the resulting model will be biased to high Q values. Using Level of Detection Divided by Two (LOD/2) technique, I am able to obtain lower Q values and smoother variations in the resulting models comparing with censored models.

Q estimation from vertical seismic profile data and anomalous variations in the central North Sea

Geophysics ◽  
1985 ◽  
Vol 50 (4) ◽  
pp. 615-626 ◽  
Author(s):  
S. D. Stainsby ◽  
M. H. Worthington
Keyword(s):  
North Sea ◽  
Pulse Width ◽  
Spectral Ratio ◽  
Seismic Profile ◽  
Seismic Attenuation ◽  
Recording Equipment ◽  
Vertical Seismic Profile ◽  
High Attenuation ◽  
Vsp Data ◽  
Central North Sea

Four different methods of estimating Q from vertical seismic profile (VSP) data based on measurements of spectral ratios, pulse amplitude, pulse width, and zeroth lag autocorrelation of the attenuated impulse are described. The last procedure is referred to as the pulse‐power method. Practical problems concerning nonlinearity in the estimating procedures, uncertainties in the gain setting of the recording equipment, and the influence of structure are considered in detail. VSP data recorded in a well in the central North Sea were processed to obtain estimates of seismic attenuation. These data revealed a zone of high attenuation from approximately 4 900 ft to [Formula: see text] ft with a value of [Formula: see text] Results of the spectral‐ratio analysis show that the data conform to a linear constant Q model. In addition, since the pulse‐width measurement is dependent upon the dispersive model adopted, it is shown that a nondispersive model cannot possibly provide a match to the real data. No unambiguous evidence is presented that explains the cause of this low Q zone. However, it is tentatively concluded that the seismic attenuation may be associated with the degree of compaction of the sediments and the presence of deabsorbed gases.

scholarly journals Squirt flow due to interfacial water films in hydrate bearing sediments

2017 ◽  
Author(s):  
Kathleen Sell ◽  
Beatriz Quintal ◽  
Michael Kersten ◽  
Erik H. Saenger
Keyword(s):  
Flow Model ◽  
Pore Space ◽  
Seismic Attenuation ◽  
Interfacial Water ◽  
Seismic Velocities ◽  
High Attenuation ◽  
Water Films ◽  
Direct Model ◽  
In Situ Experiments ◽  
Hydrate Bearing Sediments

Abstract. Sediments containing gas hydrate dispersed in the pore space are known to show a characteristic seismic anomaly which is a high attenuation along with increasing seismic velocities. Currently, this observation cannot be fully explained albeit squirt-flow type mechanisms at the microscale have been speculated to be the cause. Recent major findings from in-situ experiments coupled with high-resolution synchrotron-based X-ray micro-tomography revealed a systematic presence of thin water films between the quartz grains and the encrusting hydrate. In this study, the data was obtained from the experiments and underwent an image processing procedure to quantify the thicknesses and geometries of the aforementioned interfacial water films. Overall, the water films vary from sub-μm to a few μm in thickness where some of them are interconnected by water bridges. This geometrical analysis is then used to propose a new conceptual squirt flow model for hydrate bearing sediments. Subsequently the established model acts as a direct model input to obtain seismic attenuation. Our results support previous speculations that squirt flow can explain high attenuation at seismic frequencies in hydrate bearing sediments, but based on a conceptual squirt flow model which is different than those previously considered.

Crustal attenuation from USArray ML amplitude tomography

2020 ◽  
Vol 224 (1) ◽  
pp. 199-206
Author(s):  
Thomas M Hearn
Keyword(s):  
Seismic Velocity ◽  
Gulf Coast ◽  
Sedimentary Basins ◽  
Volcanic Field ◽  
Seismic Attenuation ◽  
High Attenuation ◽  
The North ◽  
Western Us ◽  
Geothermal Activity ◽  
The Us

SUMMARY Seismic attenuation across the US is estimated using station ML magnitude data from the USArray. Station magnitudes are recalibrated back to amplitude and back projected in a 2-D tomography. Data represent the amplitudes of the horizontal components of the Lg phase. The western US shows regions of very high attenuation and contrasts with the lesser attenuation of the eastern US. Individual attenuation anomalies can be clearly tied to regional geology. Station gains show broad regional variations that match geographic regions. Most of the high-attenuation areas are regions of high geothermal activity suggesting that intrinsic attenuation dominates over scattering attenuation. An exception is the central San Andreas Fault zone because it lacks any localized heat-flow anomaly. The US east of the Rocky Mountains is bland and contains none of the high-attenuation regions of the western US. Instead, the central US has low-attenuation patches that do not obviously correspond to geologic province. Sediments of the Gulf Coast Plain, Willison Basin and Michigan Basin do show up as intermediate attenuation while the Illinois Basin, Appalachian Basin and other basins are not apparent. In Alaska, attenuation is generally less than the western US, but still much greater than the eastern US. In southeast Alaska, the Wrangell Volcanic Field causes a sizeable high-attenuation zone. The volcanic Aleutian Mountains also have high attenuation. However, moderate to high attenuation also correlates with the tertiary sedimentary basins in Alaska. The North Slope Basin does not seem to attenuate. Thicker crust and mountain roots tend to show less attenuation, if anything, but this correspondence is most likely due to differences in temperature and seismic velocity. Heat, scattering and young sedimentary basins create seismic attenuation in the continental crust.

Seismic attenuation structure of southern Peruvian subduction system

2019 ◽  
Vol 771 ◽  
pp. 228203 ◽  
Author(s):  
Hyoihn Jang ◽  
YoungHee Kim ◽  
Hobin Lim ◽  
Robert W. Clayton
Keyword(s):  
Seismic Attenuation ◽  
Attenuation Structure ◽  
Subduction System

scholarly journals Squirt flow due to interfacial water films in hydrate bearing sediments

Solid Earth ◽  
2018 ◽  
Vol 9 (3) ◽  
pp. 699-711 ◽  
Author(s):  
Kathleen Sell ◽  
Beatriz Quintal ◽  
Michael Kersten ◽  
Erik H. Saenger
Keyword(s):  
Flow Model ◽  
Pore Space ◽  
Seismic Attenuation ◽  
Interfacial Water ◽  
Seismic Velocities ◽  
High Attenuation ◽  
Water Films ◽  
In Situ Experiments ◽  
Micro Tomography ◽  
Hydrate Bearing Sediments

Abstract. Sediments containing gas hydrate dispersed in the pore space are known to show a characteristic seismic anomaly which is a high attenuation along with increasing seismic velocities. Currently, this observation cannot be fully explained albeit squirt-flow type mechanisms on the microscale have been speculated to be the cause. Recent major findings from in situ experiments, using the gas in excess and water in excess formation method, and coupled with high-resolution synchrotron-based X-ray micro-tomography, have revealed the systematic presence of thin water films between the quartz grains and the encrusting hydrate. The data obtained from these experiments underwent an image processing procedure to quantify the thicknesses and geometries of the aforementioned interfacial water films. Overall, the water films vary from sub-micrometer to a few micrometers in thickness. In addition, some of the water films interconnect through water bridges. This geometrical analysis is used to propose a new conceptual squirt flow model for hydrate bearing sediments. A series of numerical simulations is performed considering variations of the proposed model to study seismic attenuation caused by such thin water films. Our results support previous speculation that squirt flow can explain high attenuation at seismic frequencies in hydrate bearing sediments, but based on a conceptual squirt flow model which is geometrically different than those previously considered.

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