scholarly journals Seismic velocity anomalies beneath SE Brazil fromPandSwave travel time inversions

2003 ◽  
Vol 108 (B4) ◽  
Author(s):  
M. Schimmel ◽  
M. Assumpção ◽  
J. C. VanDecar
Keyword(s):  
Travel Time ◽  
Seismic Velocity ◽  
Se Brazil ◽  
Velocity Anomalies

Lateral inhomogeneities in P velocity under the Tarbela array of the lesser Himalayas of Pakistan

1977 ◽  
Vol 67 (3) ◽  
pp. 725-734
Author(s):  
William H. Menke
Keyword(s):  
Travel Time ◽  
Upper Mantle ◽  
Lower Crust ◽  
Seismic Velocity ◽  
Three Dimensional ◽  
Horizontal Layer ◽  
Main Central Thrust ◽  
Crust And Upper Mantle ◽  
Lesser Himalayas ◽  
Velocity Anomalies

abstract Three-dimensional seismic-velocity heterogeneities (to a depth of 125 km) under the Tarbela array are determined by the Aki et al. (1976a) method of inverting teleseismic travel-time residuals. Velocity anomalies are clearly present and are elongated in the northwest direction. An overall 2 to 3 per cent decrease in velocity to the northeast is observed across any horizontal layer. These features result from a 4° dip of geologic structures in the direction N41°E. This direction is similar to some observed trends of seismicity in the Tarbela area and to the trend of the Himalayan Main Central Thrust (MCT) east and north of Tarbela, but not to the trend of the fault traces nor the strike of geologic structures in the Tarbela area. Just to the southeast of the Tarbela array, these faults bend sharply westward, forming a mountainous loop. In this study it is concluded that the westerly trending fault traces and westerly striking geology are both only surficial and not representative of structures at greater depth. These deep structures within the lower crust and upper mantle preserve a strike similar to more eastern areas along the MCT. They are shown to be volumetrically and tectonically the more important features.

Single-station seismic monitoring of permafrost on Mt. Zugspitze (Germany) over the past 15 years

2021 ◽  
Author(s):  
Fabian Lindner ◽  
Joachim Wassermann
Keyword(s):  
Travel Time ◽  
Seismic Velocity ◽  
Temperature Record ◽  
Lapse Time ◽  
Resistivity Tomography ◽  
Single Station ◽  
Velocity Variations ◽  
Spatio Temporal ◽  
Velocity Changes ◽  
Permafrost Thawing

<p>Permafrost thawing affects mountain slope stability and can trigger hazardous rock falls. As rising temperatures promote permafrost thawing, spatio-temporal monitoring of long-term and seasonal variations in the perennially frozen rock is therefore crucial in regions with high hazard potential. With various infrastructure in the summit area and population in the close vicinity, Mt. Zugspitze in the German/Austrian Alps is such a site and permafrost has been monitored with temperature logging in boreholes and lapse-time electrical resistivity tomography. Yet, these methods are expensive and laborious, and are limited in their spatial and/or temporal resolution.</p><p>Here, we analyze continuous seismic data from a single station deployed at an altitude of 2700 m a.s.l. in a research station, which is separated by roughly 250 m from the permafrost affected ridge of Mt. Zugspitze. Data are available since 2006 (with some gaps) and reveal high-frequency (>1 Hz) anthropogenic noise likely generated by the cable car stations at the summit. We calculate single-station cross-correlations between the different sensor components and investigate temporal coda wave changes by applying the recently introduced wavelet-based cross-spectrum method. This approach provides time series of the travel time relative to the reference stack as a function of frequency and lag time in the correlation functions. In the frequency and lag range of 1-10 Hz and 0.5-5 s respectively, we find various parts in the coda that show clear annual variations and an increasing trend in travel time over the past 15 years of consideration. Converting the travel time variations to seismic velocity variations (assuming homogeneous velocity changes affecting the whole mountain) results in seasonal velocity changes of up to a few percent and on the order of 0.1% decrease per year. Yet, estimated velocity variations do not scale linearly with lag time, which indicates that the medium changes are localized rather than uniform and that the absolute numbers need to be taken with caution. The annual velocity variations are anti-correlated with the temperature record from the summit but delayed by roughly one month.</p><p>The phasing of the annual seismic velocity change (relative to the temperature record) is in agreement with a previous study employing lapse-time electrical resistivity tomography. Furthermore, the decreasing trend in seismic velocity happens concurrently with an increasing trend in temperature. The results therefore suggest that the velocity changes are related to seasonal thaw and refreeze and permafrost degradation and thus highlight the potential of seismology for permafrost monitoring. By adding additional receivers and/or a fiber-optic cable for distributed acoustic sensing, hence increasing the spatial resolution, the presented method holds promise for lapse-time imaging of permafrost bodies with high spatio-temporal resolution from passive measurements.</p>

Comparing Higher-dimensional Velocity Models for Seismic Location Accuracy using a Consistent Travel Time Framework

2021 ◽  
Author(s):  
Michael Begnaud ◽  
Sanford Ballard ◽  
Andrea Conley ◽  
Patrick Hammond ◽  
Christopher Young
Keyword(s):  
Travel Time ◽  
Ray Tracing ◽  
Seismic Velocity ◽  
3D Seismic ◽  
Location Accuracy ◽  
Velocity Models ◽  
Higher Dimensional ◽  
1D Model ◽  
Tracing Algorithm ◽  
2D And 3D

<p>Historically, location algorithms have relied on simple, one-dimensional (1D, with depth) velocity models for fast, seismic event locations. The speed of these 1D models made them the preferred type of velocity model for operational needs, mainly due to computational requirements. Higher-dimensional (2D-3D) seismic velocity models are becoming more readily available from the scientific community and can provide significantly more accurate event locations over 1D models. The computational requirements of these higher-dimensional models tend to make their operational use prohibitive. The benefit of a 1D model is that it is generally used as travel-time lookup tables, one for each seismic phase, with travel-time predictions pre-calculated for event distance and depth. This simple, lookup structure makes the travel-time computation extremely fast.</p><p>Comparing location accuracy for 2D and 3D seismic velocity models tends to be problematic because each model is usually determined using different inversion parameters and ray-tracing algorithms. Attempting to use a different ray-tracing algorithm than used to develop a model almost always results in poor travel-time prediction compared to the algorithm used when developing the model.</p><p>We will demonstrate that using an open-source framework (GeoTess, www.sandia.gov/geotess) that can easily store 3D travel-time data can overcome the ray-tracing algorithm hurdle. Travel-time lookup tables (one for each station and phase) can be generated using the exact ray-tracing algorithm that is preferred for a specified model. The lookup surfaces are generally applied as corrections to a simple 1D model and also include variations in event depth, as opposed to legacy source-specific station corrections (SSSCs), as well as estimates of path-specific travel-time uncertainty. Having a common travel-time framework used for a location algorithm allows individual 2D and 3D velocity models to be compared in a fair, consistent manner.</p>

scholarly journals Imaging of the Upper Mantle Beneath Southeast Asia: Constrained by Teleseismic P-Wave Tomography

2020 ◽  
Vol 12 (18) ◽  
pp. 2975
Author(s):  
Huiyan Shi ◽  
Tonglin Li ◽  
Rongzhe Zhang ◽  
Gongcheng Zhang ◽  
Hetian Yang
Keyword(s):  
South China Sea ◽  
Southeast Asia ◽  
Travel Time ◽  
South China ◽  
High Velocity ◽  
Velocity Model ◽  
P Wave ◽  
Deep Part ◽  
Low Velocity ◽  
Velocity Anomalies

It is of great significance to construct a three-dimensional underground velocity model for the study of geodynamics and tectonic evolution. Southeast Asia has attracted much attention due to its complex structural features. In this paper, we collected relative travel time residuals data for 394 stations distributed in Southeast Asia from 2006 to 2019, and 14,011 seismic events were obtained. Then, teleseismic tomography was applied by using relative travel time residuals data to invert the velocity where the fast marching method (FMM) and subspace method were used for every iteration. A novel 3D P-wave velocity model beneath Southeast Asia down to 720 km was obtained using this approach. The tomographic results suggest that the southeastern Tibetan Plateau, the Philippines, Sumatra, and Java, and the deep part of Borneo exhibit high velocity anomalies, while low velocity anomalies were found in the deep part of the South China Sea (SCS) basin and in the shallow part of Borneo and areas near the subduction zone. High velocity anomalies can be correlated to subduction plates and stable land masses, while low velocity anomalies can be correlated to island arcs and upwelling of mantle material caused by subduction plates. We found a southward subducting high velocity body in the Nansha Trough, which was presumed to be a remnant of the subduction of the Dangerous Grounds into Borneo. It is further inferred that the Nansha Trough and the Dangerous Grounds belong to the same tectonic unit. According to the tomographic images, a high velocity body is located in the deep underground of Indochina–Natuna Island–Borneo–Palawan, depth range from 240 km to 660 km. The location of the high velocity body is consistent with the distribution range of the ophiolite belt, so we speculate that the high velocity body is the remnant of thee Proto-South China Sea (PSCS) and Paleo-Tethys. This paper conjectures that the PSCS was the southern branch of Paleo-Tethys and the gateway between Paleo-Tethys and the Paleo-Pacific Ocean. Due to the squeeze of the Australian plate, PSCS closed from west to east in a scissor style, and was eventually extinct under Borneo.

Deep origin of mid-ocean-ridge seismic velocity anomalies

Nature ◽  
1992 ◽  
Vol 360 (6400) ◽  
pp. 149-152 ◽  
Author(s):  
Wei-jia Su ◽  
Robert L. Woodward ◽  
Adam M. Dziewonski
Keyword(s):  
Seismic Velocity ◽  
Mid Ocean Ridge ◽  
Velocity Anomalies ◽  
Ocean Ridge

Temporal and spatial variations of travel-time residuals in central California for Novaya Zemlya events

1976 ◽  
Vol 66 (5) ◽  
pp. 1733-1747 ◽  
Author(s):  
Russell Robinson ◽  
H. M. Iyer
Keyword(s):  
Travel Time ◽  
Seismic Velocity ◽  
Temporal Variations ◽  
P Wave ◽  
Central California ◽  
Nuclear Explosions ◽  
Novaya Zemlya ◽  
San Andreas ◽  
Temporal And Spatial

abstract Eight large nuclear explosions in Novaya Zemlya, from October 1969, through November 1974, were used to monitor long-term variations in crustal seismic velocity near the San Andreas fault in central California. Relative P-wave travel-time residuals appear to be accurate to approximately ±0.1 sec. Of the over 100 stations used, none show clearly significant temporal variations in residual greater than this amount, corresponding to about a 4 per cent change in velocity in the upper crust. Average relative residuals at individual stations show a large spatial variation of about 1.5 sec. These variations reflect both a complex crustal geology and changes in crustal thickness and provide a potentially powerful tool for studying crustal structure.

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