scholarly journals Seismicity in colombian llanos foothills: Characterization, relocation and local seismic tomography

2015 ◽  
pp. 14-24 ◽  
Author(s):  
Francisco Javier Muñoz-Burbano ◽  
Carlos Alberto Vargas-Jiménez ◽  
German Chicangana
Keyword(s):  
Velocity Model ◽  
Fault System ◽  
Low Velocity ◽  
Simultaneous Inversion ◽  
Velocity Anomaly ◽  
3D Velocity Model ◽  
New Locations ◽  
Double Differences ◽  
Southwest Region ◽  
Santa Maria

An earthquake relocation by seismic simultaneous inversion and double differences methods were done in the Colombian Llanos Foothills from 3° to 5°N and from 73° to 75°W. The data used in this work take account 483 earthquakes registered by The Colombia National Seismological Network (RSNC) between 1993 and 2012. For the events relocation the root mean square (RMS) was reduced and several earthquake clusters were identified. The new locations shows principally at southwestern zone are related with the Eastern Frontal Fault System with faults as the Servitá-Santa Maria fault and the Algeciras Fault. In addition this work shows a 3D velocity model indicating an anomaly in the wave behavior related mainly to the low velocity zone under the Eastern Cordillera and minimum variations in average velocity toward southeast zone related with the Amazon Craton. Finally in southwest region where located the faults shows a Vp high velocity anomaly.

Velocity estimation by beam stack

Geophysics ◽  
1992 ◽  
Vol 57 (8) ◽  
pp. 1034-1047 ◽  
Author(s):  
Biondo Biondi
Keyword(s):  
Estimation Method ◽  
Velocity Model ◽  
Velocity Estimation ◽  
Gradient Algorithm ◽  
Stratified Medium ◽  
Detailed Knowledge ◽  
Low Velocity ◽  
Velocity Anomaly ◽  
Interval Velocity ◽  
Lateral Variations

Imaging seismic data requires detailed knowledge of the propagation velocity of compressional waves in the subsurface. In conventional seismic processing, the interval velocity model is usually derived from stacking velocities. Stacking velocities are determined by measuring the coherency of the reflections along hyperbolic moveout trajectories in offset. This conventional method becomes inaccurate in geologically complex areas because the conversion of stacking velocities to interval velocities assumes a horizontally stratified medium and mild lateral variations in velocity. The tomographic velocity estimation proposed in this paper can be applied when there are dipping reflectors and strong lateral variations. The method is based on the measurements of moveouts by beam stacks. A beam stack measures local coherency of reflections along hyperbolic trajectories. Because it is a local operator, the beam stack can provide information on nonhyperbolic moveouts in the data. This information is more reliable than traveltimes of reflections picked directly from the data because many seismic traces are used for computing beam stacks. To estimate interval velocity, I iteratively search for the velocity model that best predicts the events in beam‐stacked data. My estimation method does not require a preliminary picking of the data because it directly maximizes the beam‐stack’s energy at the traveltimes and surface locations predicted by ray tracing. The advantage of this formulation is that detection of the events in the beam‐stacked data can be guided by the imposition of smoothness constraints on the velocity model. The optimization problem of maximizing beam‐stack energy is solved by a gradient algorithm. To compute the derivatives of the objective function with respect to the velocity model, I derive a linear operator that relates perturbations in velocity to the observed changes in the beam‐stack kinematics. The method has been successfully applied to a marine survey for estimating a low‐velocity anomaly. The estimated velocity function correctly predicts the nonhyperbolic moveouts in the data caused by the velocity anomaly.

An integrated 3D velocity inversion—joint hypocentral determination relocation analysis of events in the Northridge area

1996 ◽  
Vol 86 (1B) ◽  
pp. S138-S155
Author(s):  
Jose Pujol
Keyword(s):  
Weighted Average ◽  
Velocity Model ◽  
Actual Data ◽  
Low Velocity ◽  
Lateral Velocity ◽  
Epicentral Area ◽  
Velocity Inversion ◽  
Station Corrections ◽  
3D Velocity Model ◽  
Velocity Zone

Abstract A subset of 3371 events recorded in the Northridge area by the Southern California Seismic Network during January to April 1994 was relocated with the joint hypocentral determination (JHD) technique. This analysis showed two unexpected results: (a) the JHD locations are shifted about 3.9 km on average in a northwest direction with respect to the locations determined using a single-event location (SEL) program, and (b) the station corrections vary between −0.55 and 1.26 sec, a rather large range. In addition, the JHD locations are less scattered than the SEL locations. For each station, the weighted average of the arrival time residuals obtained when the events are located with the SEL program (which does not apply distance or error weighting) are generally smaller than the corresponding JHD corrections. The locations determined with SEL and using the weighted average residuals as station corrections do not differ much from the SEL locations, but on average the RMS residuals become as small as those corresponding to the JHD locations. As the magnitude of the station corrections indicates the presence of large lateral velocity variations, a 3D velocity model for the area was determined using the arrival times of 1012 events recorded by at least 17 stations. The initial velocity model was that used routinely by the Southern California Earthquake Center. The first two layers (5.5- and 10.5-km thick) were subdivided into 100 blocks each (12 × 12 km). These layers show a pronounced low-velocity anomaly (24% and 16%, respectively) immediately to the northwest of the epicentral area. This low-velocity zone coincides with the west Ventura Basin. Another pronounced low-velocity zone to the southeast of the epicentral area reflects the presence of the Los Angeles Basin. The locations obtained with the 3D velocity model are consistently to the southeast of the JHD locations, 2.4 km on average. To establish the effect of these pronounced lateral velocity variations on the SEL and JHD locations, synthetic travel times were analyzed. The synthetic times were generated for event locations determined by JHD (shifted by various amounts) and the 3D velocity model and were subsequently treated as the actual data. The most important result of this analysis is that the JHD locations are affected by a quasi-systematic shift in a northwest direction (up to about 2.7 km on average, depending on the initial shift) but that the relative locations are well preserved. Therefore, both the velocity inversion of the actual data and the analysis of the synthetic data indicate that the JHD locations determined for the actual data are quasi-systematically mislocated. To account for this mislocation, an overall shift of 2.5 km to the southeast was applied to all the JHD locations. One of the most important implications of the shifted locations is the possibility that the northeasterly dipping Santa Susana fault, to the northwest of the epicentral area, was seismically active during the aftershock sequence. This feature is more diffuse in other published locations.

scholarly journals Ambient seismic noise tomography to build up a 3D shear-wave velocity model

2021 ◽  
Vol 22 (2) ◽  
pp. 1-9 ◽  
Author(s):  
Martín Cárdenas Soto ◽  
José Piña Flores ◽  
David Escobedo Zenil ◽  
Jesús Sánchez González ◽  
José Antonio Martínez González
Keyword(s):  
Rayleigh Waves ◽  
Seismic Noise ◽  
Velocity Model ◽  
Ambient Seismic Noise ◽  
Lava Tube ◽  
Low Velocity ◽  
Rectangular Array ◽  
Velocity Anomaly ◽  
Adjacent Building ◽  
Tomography Method

To explore the usefulness of the ambient seismic noise tomography method for characterizing the subsoil surface structure, in this study, we apply this method to contribute to geotechnical decision-making in the construction of a school building. We used a rectangular array (36x56 m) of 48-4.5 Hz vertical geophones and produce surface wave tomographies from the travel times of Rayleigh waves extracted by cross-correlation of seismic noise. We determined a final 3D Vs model using 1D models derived from the inversion of dispersion curves obtained from the tomography maps for different frequencies. The 3D model shows an excellent resolution (vertical and lateral); we observe critical velocity contrasts in the range of 2 to 15 m deep. At depths higher than 15 m, the velocity has values close to 900 m/s; however, we observe a low-velocity anomaly associated with a lava tube or crack that seems to continue under an adjacent building.

High-resolution teleseismic body-wave tomography with a 3D initial crustal model for crust-to-upper mantle images in highly heterogeneous media.

2020 ◽  
Author(s):  
Adeline Clutier ◽  
Stéphanie Gautier ◽  
Christel Tiberi
Keyword(s):  
Upper Mantle ◽  
Heterogeneous Media ◽  
Body Wave ◽  
A Priori ◽  
Velocity Model ◽  
Low Velocity ◽  
Local Tomography ◽  
Velocity Models ◽  
The North ◽  
3D Velocity Model

<p>Local and teleseismic body wave inversions are two approaches commonly used to obtain 3D Earth velocity models for shallow and mantle scale, respectively. However, each method used separately is poorly resolved at the mantle/crust boundary while imaging that interface is important to understand the geodynamic processes (e.g. magmatic underplating, mantle delamination, crustal thinning or thickening) occurring at this depth. In order to develop a high-resolved final velocity model, the two approaches were combined. First, an irregular grid was settled, with a higher density of nodes at crustal scale (from 0 to 40 km) and an increasing node step when approaching the limits of the model. Then, an a priori 3D crustal velocity model (from an independent local tomography) was inserted within the 1D IASP91 lithospheric one. Finally, the teleseismic tomographic inversion was carried out at crust-to-upper mantle scale using this new mixed initial model and teleseismic data. We applied the method on a real case that includes both tectonic and magmatic processes, the North Tanzanian Divergence (NTD). Synthetic tests showed that we had no resolution between 0 and 35 km. However, a fine crustal grid with the 3D local model helps to better constrain ray paths, limiting the artefacts and smearing from the mantle to the crust, enhancing details, sharpening the velocity anomalies and modifying the geometry of anomalies at depth (> 150 km). Following these tests, we propose then a final scheme in which we include the a priori crustal 3D velocity model in the finer crustal grid, and we prevent the inversion from modifying it. This insertion of strong crustal constraints in teleseismic inversion provides sharper spatial resolution at both crustal and mantle scales, including areas with poor ray coverage, beneath the NTD region. Our strategy allows to counteract the degradation of the results in areas with low velocity zones (such as rift and hotspot), where the seismic rays go around these anomalies.</p>

scholarly journals Seismic Imaging and Seismicity Analysis in Beijing-Tianjin-Tangshan Region

2011 ◽  
Vol 2011 ◽  
pp. 1-13 ◽  
Author(s):  
Xiangwei Yu ◽  
Wenbo Zhang ◽  
Yun-tai Chen
Keyword(s):  
Focal Depth ◽  
Velocity Model ◽  
Double Difference ◽  
Low Velocity ◽  
Arrival Times ◽  
Velocity Anomaly ◽  
The North ◽  
Tomographic Method ◽  
Vertical Slice ◽  
Tomography Method

In this study a new tomographic method is applied to over 43,400 high-quality absolute direct P arrival times and 200,660 relative P arrival times to determine detailed 3D crustal velocity structures as well as the absolute and relative hypocenter parameters of 2809 seismic events under the Beijing-Tianjin-Tangshan region. The inferred velocity model of the upper crust correlates well with the surface geological and topographic features in the BTT region. In the North China Basin, the depression and uplift areas are imaged as slow and fast velocities, respectively. After relocation, the double-difference tomography method provides a sharp picture of the seismicity in the BTT region, which is concentrated along with the major faults. A broad low-velocity anomaly exists in Tangshan and surrounding area from 20 km down to 30 km depth. Our results suggest that the top boundary of low-velocity anomalies is at about 25.4 km depth. The event relocations inverted from double-difference tomography are clusted tightly along the Tangshan-Dacheng Fault and form three clusters on the vertical slice. The maximum focal depth after relocation is about 25 km depth in the Tangshan area.

Hotspot signatures at the North American passive margin

Geology ◽  
2020 ◽  
Author(s):  
Zhongmin Tao ◽  
Aibing Li ◽  
Karen M. Fischer
Keyword(s):  
North America ◽  
New England ◽  
Velocity Model ◽  
Passive Margin ◽  
Small Scale ◽  
Low Velocity ◽  
Velocity Anomaly ◽  
The North ◽  
New Information ◽  
Velocity Anomalies

The presence of localized low-velocity anomalies in the upper mantle beneath the passive Atlantic margin in North America is a puzzling geophysical observation. Whether the anomalies are caused by the remnant heat from past hotspots or ongoing asthenospheric upwelling is still debated. We addressed the formation of the anomalies based on a recent velocity model for eastern North America, which reveals new information on their shapes and anisotropic signatures. The low-velocity anomaly in New England appears as a narrow column above 90 km depth and broadens westward at depths of 120–200 km. Two slow anomalies are imaged under the central Appalachian Mountains between 140 km and 240 km. These low velocities correspond to pronounced positive radial anisotropy (Vsh > Vsv), indicating a dominantly horizontal asthenospheric flow. They also coincide with the tracks of the Great Meteor hotspot (140–115 Ma) and an inferred hidden hotspot (60–50 Ma). The anomalies in the central Appalachians could be due to lithospheric interaction with the second hotspot and subsequent lithospheric instabilities. The complex shape of the New England anomaly is consistent with interaction with both hotspots. The first hotspot could have eroded the base of the lithosphere, forming a channel, and the second hotspot could have further thinned the lithosphere and produced a localized cavity at shallow depths. Consequently, the indented lithosphere base would have filled with channelized asthenospheric flow or produced small-scale convection, helping to sustain the slow anomaly. Low-velocity anomalies at the North America passive margin are likely the consequences of prior hotspot interactions.

scholarly journals Ground motion simulations for finite-fault earthquake scenarios on the Húsavík-Flatey Fault, North Iceland

2021 ◽  
Author(s):  
Claudia Abril ◽  
Martin Mai ◽  
Benedikt Halldórsson ◽  
Bo Li ◽  
Alice Gabriel ◽  
...  
Keyword(s):  
Seismic Hazard ◽  
Ground Motion ◽  
Large Scale ◽  
Velocity Model ◽  
Fault System ◽  
Fault Rupture ◽  
Ground Shaking ◽  
Earthquake Scenarios ◽  
3D Velocity Model ◽  
Finite Fault

<p>The Tjörnes Fracture Zone (TFZ) in North Iceland is the largest and most complex zone of transform faulting in Iceland, formed due to a ridge-jump between two spreading centers of the Mid-Atlantic Ridge, the Northern Volcanic Zone and Kolbeinsey Ridge in North Iceland. Strong earthquakes (Ms>6) have repeatedly occurred in the TFZ and affected the North Icelandic population. In particular the large historical earthquakes of 1755 (Ms 7.0) and 1872 (doublet, Ms 6.5), have been associated with the Húsavı́k-Flatey Fault (HFF), which is the largest linear strike-slip transform fault in the TFZ, and in Iceland. We simulate fault rupture on the HFF and the corresponding near-fault ground motion for several potential earthquake scenarios, including scenario events that replicate the large 1755 and 1872 events. Such simulations are relevant for the town of Húsavı́k in particular, as it is located on top of the HFF and is therefore subject to the highest seismic hazard in the country. Due to the mostly offshore location of the HFF, its precise geometry has only recently been studied in more detail. We compile updated seismological and geophysical information in the area, such as a recently derived three-dimensional velocity model for P and S waves. Seismicity relocations using this velocity model, together with bathymetric and geodetic data, provide detailed information to constrain the fault geometry. In addition, we use this 3D velocity model to simulate seismic wave propagation. For this purpose, we generate a variety of kinematic earthquake-rupture scenarios, and apply a 3D finite-difference method (SORD) to propagate the radiated seismic waves through Earth structure. Slip distributions for the different scenarios are computed using a von Karman autocorrelation function whose parameters are calibrated with slip distributions available for a few recent Icelandic earthquakes. Simulated scenarios provide synthetic ground motion and time histories and estimates of peak ground motion parameters (PGA and PGV) at low frequencies (<2 Hz) for Húsavík and other main towns in North Iceland along with maps of ground shaking for the entire region [130 km x 110 km]. Ground motion estimates are compared with those provided by empirical ground motion models calibrated to Icelandic earthquakes and dynamic fault-rupture simulations for the HFF. Directivity effects towards or away from the coastal areas are analyzed to estimate the expected range of shaking. Thick sedimentary deposits (up to ∼4 km thick) located offshore on top of the HFF (reported by seismic, gravity anomaly and tomographic studies) may affect the effective depth of the fault's top boundary and the surface rupture potential. The results of this study showcase the extent of expected ground motions from significant and likely earthquake scenarios on the HFF. Finite fault earthquake simulations complement the currently available information on seismic hazard for North Iceland, and are a first step towards a systematic and large-scale earthquake scenario database on the HFF, and for the entire fault system of the TFZ, that will enable comprehensive and physics-based hazard assessment in the region.</p>

Use of refraction, reflection, and wave-equation-based tomography for imaging beneath shallow gas: A Trinidad field data example

Geophysics ◽  
2008 ◽  
Vol 73 (5) ◽  
pp. VE281-VE289 ◽  
Author(s):  
Nurul Kabir ◽  
Uwe Albertin ◽  
Min Zhou ◽  
Vishal Nagassar ◽  
Einar Kjos ◽  
...  
Keyword(s):  
Wave Equation ◽  
Velocity Field ◽  
Velocity Model ◽  
Surface Velocity ◽  
Low Velocity ◽  
Shallow Gas ◽  
Near Surface ◽  
Velocity Anomaly ◽  
Refraction Tomography ◽  
Reflection Tomography

Shallow localized gas pockets cause challenging problems in seismic imaging because of sags and wipe-out zones they produce on imaged reflectors deep in the section. In addition, the presence of shallow gas generates strong surface-related and interbed multiples, making velocity updating very difficult. When localized gas pockets are very shallow, we have limited information to build a near-surface velocity model using ray-based reflection tomography alone. Diving-wave refraction tomography successfully builds a starting model for the very shallow part. Usual ray-based reflection tomography using single-parameter hyperbolic moveout might need many iterations to update the deeper part of the velocity model. In addition, the method generates a low-velocity anomaly in the deeper part of the model. We present an innovative method for building the final velocity model by combining refraction, reflection, and wave-equation-based tomography. Wave-equation-based tomography effectively generates a detailed update of a shallow velocity field, resolving the gas-sag problem. When applied as the last step, following the refraction and reflection tomography, it resolves the gas-sag problem but fails to remove the low-velocity anomaly generated by the reflection tomography in the deeper part of the model. To improve the methodology, we update the shallow velocity field using refraction tomography followed by wave-equation tomography before updating the deeper part of the model. This step avoids generating the low-velocity anomaly. Refraction and wave-equation-based tomography followed by reflection tomography generates a simpler velocity model, giving better focusing in the deeper part of the image. We illustrate how the methodology successfully improves resolution of gas anomalies and significantly reduces gas sag from an imaged section in the Greater Cassia area, Trinidad.

Pitfalls in the seismic interpretation of fault shadow events — Vicksburg formation of south Texas

2015 ◽  
Vol 3 (1) ◽  
pp. SB17-SB22 ◽  
Author(s):  
Richard C. Bain
Keyword(s):  
Seismic Data ◽  
Seismic Velocity ◽  
Velocity Model ◽  
South Texas ◽  
Velocity Anomaly ◽  
3D Velocity Model ◽  
Production Wells ◽  
Hidalgo County ◽  
Velocity Changes ◽  
Depositional Units

Reliance on prestack time-migrated seismic data to define structural highs without incorporating all subsurface data and without taking into account the regional and local lateral depositional trends may result in dry holes or poorly positioned production wells due to local velocity changes, which are usually caused by some depositional or structural phenomenon. Tying check-shot control to depositional units may reveal those phenomena and permit assumptions to be made about velocities in areas beyond check-shot control points. We discovered a significant gas accumulation in an area surrounded by dry holes and marginal wells in the Vicksburg Formation in McAllen Ranch Field, Hidalgo County, Texas, by treating a seismic velocity anomaly as a geologic problem and by simple application of arithmetic and geometry to a 3D velocity model. Due to the effects of the anomaly, seismic data displayed in time gave no indication of the existence of a 325 ha (800 ac), 150 BCFG anticlinal structure. A subsurface model that accounted for the velocity anomaly was able to predict its extent and severity by readily identifiable thickness changes in the anomalous units. The resulting discovery yielded a sevenfold increase in field production within a two-year time span.

scholarly journals Pre-stack 3-D Tau migration and velocity analysis: application to 3-D seismic data from offshore Abu Dhabi, United Arab Emirates

GeoArabia ◽  
2006 ◽  
Vol 11 (3) ◽  
pp. 43-60
Author(s):  
Tariq Alkhalifah ◽  
Saif Al Sharif ◽  
Kamal Belaid
Keyword(s):  
United Arab Emirates ◽  
Seismic Data ◽  
Velocity Model ◽  
Migration Velocity ◽  
Abu Dhabi ◽  
Velocity Analysis ◽  
Low Velocity ◽  
Data Set ◽  
Velocity Anomaly ◽  
Migration Velocity Analysis

ABSTRACT A pre-stack 3-D Tau migration was applied to a 3-D seismic data set acquired in offshore Abu Dhabi, United Arab Emirates. The velocity model was built through an initial series of 2-D Tau migration velocity analysis, and supplemented by 3-D subset migration. A 3-D Tau migration velocity analysis was used for the final two updates of the model. The final interval velocity model provided low residuals in the common-image gathers from different offsets and was consistent with velocities from four wells located in the region. This velocity model included the main known features of the region including a low-velocity zone and a major fault. A final 3-D pre-stack Tau migration was applied using the velocity model and a relatively moderate aperture. This migration imaged the region including part of the critical poor data quality region, which includes the reservoir as well as reflections from the fault. Based on the derived velocity model, we concluded that the major cause for the poor image is the presence of a shallow high-velocity anomaly separated by a fault from a low-velocity anomaly.

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