scholarly journals Seismic model of central and eastern Lesser Himalaya of Nepal

1985 ◽  
Vol 3 ◽  
Author(s):  
M. R. Pandey
Keyword(s):  
Velocity Distribution ◽  
Seismic Waves ◽  
Seismic Velocity ◽  
Velocity Model ◽  
Moho Depth ◽  
Seismic Events ◽  
Apparent Velocity ◽  
Lesser Himalaya ◽  
Origin Time ◽  
Layer Velocity

ABSTRACT The apparent velocity distribution of the local seismic of lesser Himalaya of central and Eastern Nepal allows to derive a three layered local seismic velocity model with first layer velocity of 5.6 Km/Sec, second layer of 6.5 Km/sec. and Moho discontinuity with 8.1 Km/sec. The first arrivals of different local phases of seismic waves are consistent with 20-23 Km thickness of the first layer and with crustal thickness of 55Km. The seismic events are confined to the first layer. Local velocity model derived after the seismic event of 6 Oct 1981, origin time 19 hr 18 mn 17 sec, by modelling the first arrivals and PMP (Moho reflection) arrivals within the interval of distance 138-218 Km confirms the velocity model derived from apparent velocity distribution. However, apparent velocity distribution of local seismic events occurring south of the line joining approximately Pokhara to Udayapur in plan does not seem to fit the theoretical distribution corresponding to the above three layered model with events within first layer. The apparent velocity of these events may be explained either (a) by the confinement of the focus of the events to the second layer or, (b) by the variation of the seismic velocity model with Moho depth at 35- 40 Km. i.e. with a normal Indian peninsular crust thickness.

Seismic Velocity Model Building Using Neural Networks: Training Data Design and Learning Generalization

Geophysics ◽  
2021 ◽  
pp. 1-73
Author(s):  
Hani Alzahrani ◽  
Jeffrey Shragge
Keyword(s):  
Seismic Data ◽  
Model Building ◽  
Seismic Velocity ◽  
Velocity Model ◽  
Training Data ◽  
Training Process ◽  
Training Models ◽  
Velocity Models ◽  
Velocity Model Building ◽  
Layer Velocity

Data-driven artificial neural networks (ANNs) offer a number of advantages over conventional deterministic methods in a wide range of geophysical problems. For seismic velocity model building, judiciously trained ANNs offer the possibility of estimating high-resolution subsurface velocity models. However, a significant challenge of ANNs is training generalization, which is the ability of an ANN to apply the learning from the training process to test data not previously encountered. In the context of velocity model building, this means learning the relationship between velocity models and the corresponding seismic data from a set of training data, and then using acquired seismic data to accurately estimate unknown velocity models. We ask the following question: what type of velocity model structures need be included in the training process so that the trained ANN can invert seismic data from a different (hypothetical) geological setting? To address this question, we create four sets of training models: geologically inspired and purely geometrical, both with and without background velocity gradients. We find that using geologically inspired training data produce models with well-delineated layer interfaces and fewer intra-layer velocity variations. The absence of a certain geological structure in training models, though, hinders the ANN's ability to recover it in the testing data. We use purely geometric training models consisting of square blocks of varying size to demonstrate the ability of ANNs to recover reasonable approximations of flat, dipping, and curved interfaces. However, the predicted models suffer from intra-layer velocity variations and non-physical artifacts. Overall, the results successfully demonstrate the use of ANNs in recovering accurate velocity model estimates, and highlight the possibility of using such an approach for the generalized seismic velocity inversion problem.

Characterisation of seismic events using time-reverse imaging 

2021 ◽  
Author(s):  
Claudia Finger ◽  
Erik H. Saenger
Keyword(s):  
Moment Tensor ◽  
A Priori ◽  
Velocity Model ◽  
Geothermal Field ◽  
Time Dependent ◽  
Seismic Events ◽  
Origin Time ◽  
Moment Tensors ◽  
Stress Components ◽  
True Source

<p>In addition to stable and accurate hypocenters of seismic events, the characterisation of events is crucial for the investigation of seismicity in the context of geothermal reservoirs, CO2-sequestration and other geotechnical applications. Since the origin and nature of the seismicity in such cases is still under investigation, tools should rely on as few a priori assumptions about the sources as possible. Here, an approach is presented to determine the time-dependent moment tensor and origin time in addition to commonly derived hypocenter locations of seismic events using time-reverse imaging (TRI). The full six component moment tensor is derived and may be used to display for example focal mechanisms. The workflow consists of determining the location of potential sources, discriminating artificial and true source locations and obtaining the time-dependent moment tensors by recording the stress components at the derived source locations. Since TRI does not rely on the identification of seismic phases but on the simulation of the time-reversed wavefield through an adequate velocity model, no assumptions about the source location or the type of source mechanism is made. TRI is less affected by low signal-to-noise ratios and is thus promising for noisier sites and quasi-simultaneous events. However, a sufficient number of seismic stations are needed to accurately sample the wavefield spatially. The proposed workflow is demonstrated by locating and characterising microseismic events in the geothermal field of Los Humeros, Mexico. Although higher levels of noise are present and only a one-dimensional velocity model is available at this time, selected events could be located and characterised.</p>

A new a-priori velocity model for seismic tomography investigation of the Southern Italy region

2021 ◽  
Author(s):  
Cristina Totaro ◽  
Giancarlo Neri ◽  
Barbara Orecchio ◽  
Debora Presti ◽  
Silvia Scolaro
Keyword(s):  
Velocity Structure ◽  
Southern Italy ◽  
Seismic Velocity ◽  
A Priori ◽  
Velocity Model ◽  
Moho Depth ◽  
Seismic Profiles ◽  
Study Region ◽  
Plate Convergence ◽  
Geodynamic Models

<p>By integrating data and constraints available in the literature, we defined a new “a-priori” 3D seismic velocity model imaging the lithospheric structure of Southern Italy, a highly complex area in the Mediterranean region where the Africa-Europe plate convergence and the residual rollback of the Ionian slab coexist. Involving the integration of multiple datasets and constraints (e.g. velocity patterns from seismic profiles and/or tomographies, moho depth estimates, subduction interface geometries) and following a procedure derived to the one already successfully applied in the area about a decade ago, we obtained the simplest 3D velocity structure consistent with all the available collected data. Studies and analyses performed in recent years allowed us to enlarge and improve the previous estimated model by adding further data and useful constraints. The so obtained "a-priori" velocity model has then been used as starting model for a new earthquake tomographic inversion of the study region. Dataset used for the velocity model computation has been selected from the Italian seismic database (www.ingv.it) and consists of ca. 10000 earthquakes with magnitude equal or greater than 2 and occurred in the time period 2000-2020 at depth less than 60 km and with at least 10 station readings. The obtained 3D velocity structure and the related hypocenter locations have been compared with other geophysical and geological observations and interpreted in the frame of the geodynamic models proposed for the region.</p>

scholarly journals Analysis of Local Seismic Events near a Large-N Array for Moho Reflections

2020 ◽  
Vol 92 (1) ◽  
pp. 408-420
Author(s):  
Qicheng Zeng ◽  
Robert L. Nowack
Keyword(s):  
Poisson’S Ratio ◽  
Velocity Model ◽  
Poisson's Ratio ◽  
Moho Depth ◽  
Seismic Events ◽  
Wide Angle ◽  
Large N ◽  
Static Corrections ◽  
The Difference ◽  
The Individual

Abstract Local seismic events recorded by the large-N Incorporated Research Institutions for Seismology Community Wavefield Experiment in Oklahoma are used to estimate Moho reflections near the array. For events within 50 km of the center of the array, normal moveout corrections and receiver stacking are applied to identify the PmP and SmS Moho reflections on the vertical and transverse components. Corrections for the reported focal depths are applied to a uniform event depth. To stack signals from multiple events, further static corrections of the envelopes of the Moho reflected arrivals from the individual event stacks are applied. The multiple-event stacks are then used to estimate the pre-critical PmP and SmS arrivals, and an average Poisson’s ratio of 1.77±0.02 was found for the crust near the array. Using a modified Oklahoma Geological Survey (OGS) velocity model with this Poisson’s ratio, the time-to-depth converted PmP and SmS arrivals resulted in a Moho depth of 41±0.6  km. The modeling of wide-angle Moho reflections for selected events at epicenter-to-station distances of 90–135 km provides additional constraints, and assuming the modified OGS model, a Moho depth of 40±1  km was inferred. The difference between the pre-critical and wide-angle Moho estimates could result from some lateral variability between the array and the wide-angle events. However, both estimates are slightly shallower than the original OGS model Moho depth of 42 km, and this could also result from a somewhat faster lower crust. This study shows that local seismic events, including induced events, can be utilized to estimate properties and structure of the crust, which, in turn, can be used to better understand the tectonics of a given region. The recording of local seismicity on large-N arrays provides increased lateral phase coherence for the better identification of precritical and wide-angle reflected arrivals.

Near-surface seismic traveltime tomography using a direct-push source and surface-planted geophones

Geophysics ◽  
2009 ◽  
Vol 74 (4) ◽  
pp. G17-G25 ◽  
Author(s):  
Hendrik Paasche ◽  
Ulrike Werban ◽  
Peter Dietrich
Keyword(s):  
Velocity Distribution ◽  
Seismic Velocity ◽  
Velocity Model ◽  
Unconsolidated Sediments ◽  
Alluvial Deposits ◽  
Near Surface ◽  
Traveltime Tomography ◽  
Direct Push ◽  
Refraction Seismic ◽  
Push Technology

Information about seismic velocity distribution in heterogeneous near-surface sedimentary deposits is essential for a variety of environmental and engineering geophysical applications. We have evaluated the suitability of the minimally invasive direct-push technology for near-surface seismic traveltime tomography. Geophones placed at the surface and a seismic source installed temporarily in the subsurface by direct-push technology quickly acquire reversed multioffset vertical seismic profiles (VSPs). The first-arrival traveltimes of these data were used to reconstruct the 2D seismic velocity distribution tomographically. After testing this approach on synthetic data, we applied it to field data collected over alluvial deposits in a former river floodplain. The resulting velocity model contains information about high- and low-velocity anomalies and offers a significantly deeper penetration depth than conventional refraction tomography using surface-planted sources and receivers at the investigated site. A combination of refraction seismic and direct-push data increases resolution capabilities in the unsaturated zone and enables reliable reconstruction of velocity variations in near-surface unconsolidated sediments. The final velocity model structurally matches the results of cone-penetration tests and natural gamma-radiation data acquired along the profile. The suitability of multiple rapidly acquired reverse VSP surveys for 2D tomographic velocity imaging of near-surface unconsolidated sediments was explored.

scholarly journals Autocorrelation of the ground vibration recorded by the SEIS-InSight seismometer on Mars for imaging and monitoring applications

2021 ◽  
Author(s):  
Nicolas Compaire ◽  
Ludovic Margerin ◽  
Raphaël F. Garcia ◽  
Marie Calvet ◽  
Baptiste Pinot ◽  
...  
Keyword(s):  
Seismic Waves ◽  
Signal To Noise Ratio ◽  
Ground Vibration ◽  
Velocity Model ◽  
Temporal Variations ◽  
Seismic Events ◽  
Ambient Vibrations ◽  
Ground Response ◽  
Arrival Times ◽  
Single Station

<p>Since early February 2019, the SEIS seismometer deployed at the surface of Mars in the framework of the NASA-InSight mission has been continuously recording the ground motion at Elysium Planitia. In this work, we take advantage of this exceptional dataset to put constraints on the crustal properties of Mars using seismic interferometry (SI). This method use the seismic waves, either from background vibrations of the planet or from quakes, that are scattered in the medium in order to recover the ground response between two seismic sensors. Applying the principles of SI to the single-station configuration of SEIS, we compute, for each Sol (martian day) and each local hour, all the components of the time-domain autocorrelation tensor of random ambient vibrations in various frequency bands. A similar computation is performed on the diffuse waveforms generated by more than a hundred Marsquakes. For imaging application a careful signal-to-noise ratio analysis and an inter-comparison between the two datasets are applied. These analyses suggest that the reconstructed ground responses are most reliable in a relatively narrow frequency band around 2.4Hz, where an amplification of both ambient vibrations and seismic events is observed. The average Auto-Correlation Functions (ACFs) from both ambient vibrations and seismic events contain well identifiable seismic arrivals, that are very consistent between the two datasets. We interpret the vertical and horizontal ACFs as the ground reflection response below InSight for the compressional waves and the shear waves respectively. We propose a simple stratified velocity model of the crust, which is most compatible with the arrival times of the detected phases, as well as with previous seismological studies of the SEIS record. The hourly computation of the ACFs over one martian year also allows us to study the diurnal and seasonal variations of the reconstructed ground response with a technique call Passive Image Interferometry (PII). In this study we present measurements of the relative stretching coefficient between consecutive ACF waveforms and discuss the potential origins of the observed temporal variations.</p>

Kinematically equivalent velocity distributions

Geophysics ◽  
2008 ◽  
Vol 73 (5) ◽  
pp. VE369-VE375 ◽  
Author(s):  
Alexey Stovas
Keyword(s):  
Average Velocity ◽  
Layered Medium ◽  
Seismic Velocity ◽  
Velocity Model ◽  
Model Parameters ◽  
Inversion Problem ◽  
Heterogeneous Model ◽  
Velocity Models ◽  
Stripping Method ◽  
Layer Velocity

For a layered medium, the seismic velocity model can be vertically heterogeneous within the layers. The traveltime parameters estimated from each reflection must be converted into layer traveltime parameters by using the layer-stripping method. The layer traveltime parameters must be inverted into layer velocity model parameters. Interpretation or inversion of layer traveltime parameters depends on the chosen velocity model within the layer. Different or kinematically equivalent velocity distributions can result in the same traveltime parameters. The inversion problem for traveltime parameters is strongly nonunique even if they are estimated accurately. To evaluate the accuracy of a velocity model, one can choose the phase for the two-way propagator. The discrepancy in this phase factor between the kinematically equivalent velocity models depends on the number of traveltime parameters estimated and increases with spatial frequency. By estimating two traveltime parameters, we approximately preserve the average velocity, regardless of the complexity of the vertically heterogeneous model. By estimating three traveltime parameters, we approximately preserve the average velocity gradient.

scholarly journals Earthquake location in tectonic structures of the Alpine Chain: the case of the Constance Lake (Central Europe) seismic sequence

2021 ◽  
Author(s):  
Francesca D’Ajello Caracciolo ◽  
Rodolfo Console
Keyword(s):  
Central Europe ◽  
Velocity Model ◽  
Moho Depth ◽  
Seismic Sequence ◽  
Tectonic Structures ◽  
Study Region ◽  
Waveform Data ◽  
Seismic Stations ◽  
Velocity Models ◽  
Master Event

AbstractA set of four magnitude Ml ≥ 3.0 earthquakes including the magnitude Ml = 3.7 mainshock of the seismic sequence hitting the Lake Constance, Southern Germany, area in July–August 2019 was studied by means of bulletin and waveform data collected from 86 seismic stations of the Central Europe-Alpine region. The first single-event locations obtained using a uniform 1-D velocity model, and both fixed and free depths, showed residuals of the order of up ± 2.0 s, systematically affecting stations located in different areas of the study region. Namely, German stations to the northeast of the epicenters and French stations to the west exhibit negative residuals, while Italian stations located to the southeast are characterized by similarly large positive residuals. As a consequence, the epicentral coordinates were affected by a significant bias of the order of 4–5 km to the NNE. The locations were repeated applying a method that uses different velocity models for three groups of stations situated in different geological environments, obtaining more accurate locations. Moreover, the application of two methods of relative locations and joint hypocentral determination, without improving the absolute location of the master event, has shown that the sources of the four considered events are separated by distances of the order of one km both in horizontal coordinates and in depths. A particular attention has been paid to the geographical positions of the seismic stations used in the locations and their relationship with the known crustal features, such as the Moho depth and velocity anomalies in the studied region. Significant correlations between the observed travel time residuals and the crustal structure were obtained.

Super-Resolution of Seismic Velocity Model Guided by Seismic Data

2021 ◽  
pp. 1-12
Author(s):  
Yinshuo Li ◽  
Jianyong Song ◽  
Wenkai Lu ◽  
Patrice Monkam ◽  
Yile Ao
Keyword(s):  
Seismic Data ◽  
Seismic Velocity ◽  
Super Resolution ◽  
Velocity Model

Reconnaissance seismic refraction-reflection surveys in northwestern New Mexico

1984 ◽  
Vol 74 (4) ◽  
pp. 1263-1274
Author(s):  
Lawrence H. Jaksha ◽  
David H. Evans
Keyword(s):  
New Mexico ◽  
Wave Velocity ◽  
Lower Crust ◽  
Seismic Waves ◽  
Seismic Refraction ◽  
Velocity Model ◽  
P Wave ◽  
Uppermost Mantle ◽  
P Wave Velocity ◽  
Crustal Thinning

Abstract A velocity model of the crust in northwestern New Mexico has been constructed from an interpretation of direct, refracted, and reflected seismic waves. The model suggests a sedimentary section about 3 km thick with an average P-wave velocity of 3.6 km/sec. The crystalline upper crust is 28 km thick and has a P-wave velocity of 6.1 km/sec. The lower crust below the Conrad discontinuity has an average P-wave velocity of about 7.0 km/sec and a thickness near 17 km. Some evidence suggests that velocity in both the upper and lower crust increases with depth. The P-wave velocity in the uppermost mantle is 7.95 ± 0.15 km/sec. The total crustal thickness near Farmington, New Mexico, is about 48 km (datum = 1.6 km above sea level), and there is evidence for crustal thinning to the southeast.

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