scholarly journals Characteristics of local earthquake seismograms of varying dislocation sources in a stratified upper crust and modeling for P and S velocity structure: comparison with observations in the Koyna-Warna region, India

2016 ◽  
Vol 58 (6) ◽  
Author(s):  
V. G. Krishna
Keyword(s):  
Velocity Structure ◽  
Source Parameters ◽  
Velocity Model ◽  
Strike Slip ◽  
Synthetic Seismograms ◽  
Structure Comparison ◽  
Earthquake Sources ◽  
Crustal Velocity ◽  
Source Mechanisms ◽  
Local Earthquake

<p>Vertical component record sections of local earthquake seismograms from a state-of-the-art Koyna-Warna digital seismograph network are assembled in the reduced time versus epicentral distance frame, similar to those obtained in seismic refraction profiling. The record sections obtained for an average source depth display the processed seismograms from nearly equal source depths with similar source mechanisms and recorded in a narrow azimuth range, illuminating the upper crustal P and S velocity structure in the region. Further, the seismogram characteristics of the local earthquake sources are found to vary significantly for different source mechanisms and the amplitude variations exceed those due to velocity model stratification. In the present study a large number of reflectivity synthetic seismograms are obtained in near offset ranges for a stratified upper crustal model having sharp discontinuities with 7%-10% velocity contrasts. The synthetics are obtained for different source regimes (e.g., strike-slip, normal, reverse) and different sets of source parameters (strike, dip, and rake) within each regime. Seismogram sections with dominantly strike-slip mechanism are found to be clearly favorable in revealing the velocity stratification for both P and S waves. In contrast the seismogram sections for earthquakes of other source mechanisms seem to display the upper crustal P phases poorly with low amplitudes even in presence of sharp discontinuities of high velocity contrasts. The observed seismogram sections illustrated here for the earthquake sources with strike-slip and normal mechanisms from the Koyna-Warna seismic region substantiate these findings. Travel times and reflectivity synthetic seismograms are used for 1-D modeling of the observed virtual source local earthquake seismogram sections and inferring the upper crustal velocity structure in the Koyna-Warna region. Significantly, the inferred upper crustal velocity model in the region reproduces the synthetic seismograms comparable to the observed sections for earthquake sources with differing mechanisms in the Koyna and Warna regions.</p>

scholarly journals Determining 1D velocity model from local earthquake data in the South Hangay region, central Mongolia

2020 ◽  
pp. 1-14
Author(s):  
Mungunsuren Dashdondog ◽  
Odonbaatar Chimed ◽  
Anne Meltzer ◽  
Nomin-Erdene Erdenetsogt ◽  
Josh Stachnik
Keyword(s):  
Velocity Structure ◽  
Velocity Model ◽  
The South ◽  
S Wave ◽  
Central Mongolia ◽  
Hypocenter Determination ◽  
Velocity Models ◽  
Crustal Velocity ◽  
Local Earthquake ◽  
Earthquake Data

One dimensional (1D) velocity models are still widely used for computing earthquake locations at seismological centers. The location accuracy of an earthquake strongly depends on the velocity model used to compute the location. In the past, the local velocity model developed for the Hangay region was lacking precision due to insufficient data. Within the framework of the “Intracontinental Deformation and Surface Uplift- Geodynamic Evolution of the Hangay Dome, Mongolia, Central Asia” project [15], 72 seismic Broadband stations network were deployed in the Hangay Dome. This gives us an opportunity to estimate the crustal velocity structure of the South Hangay region using recorded local earthquake data. For this purpose, available velocity models for the South Hangay region have been re-evaluated.  By simultaneous invertion P- and S-wave arrival times using VELEST algorithm, we estimated minimum 1D velocity models, station corrections, hypocentre locations, and origin times for the south Hangay region. Consequently, 1D crustal velocity model is proposed for the South Hangay region. This new model is expected to improve the accuracy of the routine hypocenter determination and as initial reference models for seismic tomography study.

scholarly journals Three-dimensional shear velocity structure of the Mauleon and Arzacq basins (Western Pyrenees)

2021 ◽  
Author(s):  
Maximilien Lehujeur ◽  
Sébastien Chevrot ◽  
Antonio Villaseñor ◽  
Emmanuel Masini ◽  
Nicolas Saspiturry ◽  
...  
Keyword(s):  
Velocity Structure ◽  
Hanging Wall ◽  
Three Dimensional ◽  
Sedimentary Basins ◽  
Shear Velocity ◽  
Velocity Model ◽  
Tomographic Images ◽  
Reflection Seismic ◽  
Local Earthquake Tomography ◽  
Local Earthquake

We present a 3-D shear wave velocity model of the Mauleon and Arzacq basins from the surface down to 10~km depth. This model is obtained by inverting phase velocity maps for periods from 2 to 9~s measured on coherent surface wavefronts extracted from ambient seismic noise by matched filtering. This new model, which is found in good agreement with local earthquake tomography, reveals the architecture of the Mauleon and Arzacq basins which were poorly imaged by conventional reflection seismic data. Combining these new tomographic images with surface and subsurface geological information allows us to trace major orogenic structures from the basement to the surface. In the basin, the models are successfully imaging first-order folds and thrusts at kilometric scale. The velocity structure within the basement and its geometrical relationship with the base of inverted rift basins supports a progressive northward exhumation of deep crustal and mantle rocks in the hanging wall of north-vergent Pyrenean thrusts. Our tomographic models image in 3-D orogen-perpendicular structures responsible for crustal segmentation as the Saison and Barlanes transfer zones. We propose that these steep structures consist in tear faults that accommodate the deepening of the Mauleon basin basement from west to east. To the west, this basement made of former hyper-extended rift domains (including mantle rocks) is anomalously sampled within the hanging-wall of north-directed orogenic thrusts, explaining its shallow attitude and its best preservation in comparison to the eastern segment of the study area. Eastward, the vertical shift of the basement makes that the former Mauleon basin hyper-extended rift basement remained in a footwall situation in respect of orogenic thrust and was underthrust. The comparison of the tomographic models obtained with surface wave tomography and local earthquake tomography shows that each approach has its own advantages and shortcomings but also that they are very complementary in nature, which would suggest to combine them in joint inversions to further improve passive imaging of the shallow crust and sedimentary basins.

scholarly journals Three dimensional velocity structure and relocated aftershocks for the 1985 Constantine, Algeria (MS = 5.9) earthquake

1998 ◽  
Vol 41 (1) ◽  
Author(s):  
M. ou A. Bounif ◽  
C. Dorbath
Keyword(s):  
Velocity Structure ◽  
Three Dimensional ◽  
Velocity Model ◽  
P Wave ◽  
Time Data ◽  
Data Set ◽  
S Waves ◽  
Travel Time Data ◽  
Velocity Image ◽  
Local Earthquake

Local earthquake travel-time data were inverted to obtain a three dimensional tomographic image of the region centered on the 1985 Constantine earthquake. The resulting velocity model was then used to relocate the events. The tomographic data set consisted of P and S waves travel-times from 653 carefully selected aftershocks of this moderate size earthquake, recorded at 10 temporary stations. A three-dimensional P-wave velocity image to a depth of 12 km was obtained by Thurber's method. At shallower depth, the velocity contrasts reflected the differences in tectonic units. Velocities lower than 4 km/s corresponded to recent deposits, velocities higher than 5 km/s to the Constantine Neritic and the Tellian nappes. The relocation of the aftershocks indicates that most of the seismicity occured where the velocity exceeded 5.5 km/s. The aftershock distribution accurately defined the three segments involved in the main shock and led to a better understanding of the rupture process.

Microearthquake study of the Elsinore fault zone, Southern California

1974 ◽  
Vol 64 (1) ◽  
pp. 187-203
Author(s):  
David Langenkam ◽  
Jim Combs
Keyword(s):  
Fault Zone ◽  
Southern California ◽  
Velocity Model ◽  
High Gain ◽  
Strike Slip ◽  
San Jacinto Fault ◽  
Crustal Velocity ◽  
Crustal Velocity Model ◽  
Slip Displacement ◽  
East West

Abstract Microearthquakes along the Elsinore fault zone, southern California, were monitored during the summer and fall of 1972. Four arrays of at least five portable, high-gain, seismographs were operated for about 12 days each from the vicinity of Corona to just north of the Mexican border. Over 5,000 hr of noisefree records were accumulated and analyzed. The recorded rates of seismic activity show a marked increase going from north to south along the fault— 0.5 events per day in the vicinity of Lake Elsinore to 3.7 events per day in the south near Monument Peak. Fifty-three events located, assuming a four-layer crustal-velocity model, show considerable scatter along the fault and are generally very shallow, averaging 3.3 km below sea level. A signal duration (D) versus magnitude (M) relationship was found: M = −1.9+2.0 log D. First motions of the located earthquakes indicate a complex pattern of faulting along the Elsinore fault zone. In comparison to the San Jacinto Fault to the east, the Elsinore Fault shows very little strike-slip displacement and is a seismically quiet area except for a localized area of east-west faulting in the far south near Vallecito Mountain.

scholarly journals A three-dimensional crustal velocity model of the southwestern Alps from local earthquake tomography

2001 ◽  
Vol 106 (B9) ◽  
pp. 19367-19389 ◽  
Author(s):  
Anne Paul ◽  
Marco Cattaneo ◽  
François Thouvenot ◽  
Daniele Spallarossa ◽  
Nicole Béthoux ◽  
...  
Keyword(s):  
Three Dimensional ◽  
Velocity Model ◽  
Local Earthquake Tomography ◽  
Crustal Velocity ◽  
Crustal Velocity Model ◽  
Local Earthquake ◽  
Southwestern Alps

scholarly journals Development of a first 3D crustal velocity model for the region of Bogotá, Colombia

2017 ◽  
Vol 37 (2) ◽  
pp. 42-51
Author(s):  
Andrea C. Riaño ◽  
Juan C. Reyes ◽  
Luis E. Yamin ◽  
Julian S. Montejo ◽  
Jose L. Bustamante ◽  
...  
Keyword(s):  
Ground Motion ◽  
Velocity Structure ◽  
Seismic Velocity ◽  
System Modeling ◽  
Velocity Model ◽  
Fault System ◽  
Crustal Velocity ◽  
Wide Range ◽  
Geological Units ◽  
The City

Knowledge regarding the characteristics of soils in Bogotá basin has been possible to get through previous microzonation studies. However, there is still insufficient knowledge of the crustal velocity structure of the region. Bogotá is located in a region prone to a significant seismic hazard. Historically, the city has been affected by strong earthquakes, reaching moment magnitudes greater than or equal to 7. Furthermore, the city was built on a lacustrine basin, with soft soils of considerable depth that may strongly amplify the ground motion during an earthquake. In this article, we describe the development of a first crustal structure and material properties model for the region of Bogotá, Colombia, covering an area of about 130 km by 102 km. This effort aims at constructing a realistic 3D seismic velocity model using geological and geotechnical information from several sources. Major geological units have been identified and mapped into the model. The Inverse Distance Weighted (IDW) interpolation was used to create continuous surfaces delimiting the geological units. Seismic-wave properties are assigned to any point in the domain using a location-based approach. We expect this model to be useful for a wide range of applications, including dynamic ground motion simulations and fault system modeling.

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