scholarly journals Seismic images of the Northern Chilean subduction zone at 19°40′S, prior to the 2014 Iquique earthquake

2021 ◽  
Vol 225 (2) ◽  
pp. 1048-1061
Author(s):  
Ina Storch ◽  
Stefan Buske ◽  
Pia Victor ◽  
Onno Oncken
Keyword(s):  
Subduction Zone ◽  
Root Zone ◽  
Accretionary Prism ◽  
Depth Image ◽  
Plate Interface ◽  
Data Set ◽  
Nazca Plate ◽  
Depth Migration ◽  
Splay Faults ◽  
Splay Fault

SUMMARY The Northern Chilean subduction zone is characterized by long-term subduction erosion with very little sediment input at the trench and the lack of an accretionary prism. Here, multichannel seismic reflection (MCS) data were acquired as part of the CINCA (Crustal Investigations off- and onshore Nazca Plate/Central Andes) project in 1995. These lines cover among others the central part of the MW 8.1 Iquique earthquake rupture zone before the earthquake occurred on 1 April 2014. We have re-processed one of the lines crossing the updip parts of this earthquake at 19°40′S, close to its hypocentre. After careful data processing and data enhancement, we applied a coherency-based pre-stack depth migration algorithm, yielding a detailed depth image. The resulting depth image shows the subduction interface prior to the Iquique megathrust earthquake down to a depth of approximately 16 km and gives detailed insight into the characteristics of the seismogenic coupling zone. We found significantly varying interplate reflectivity along the plate interface which we interpret to be caused by the comparably strong reflectivity of subducted fluid-rich sediments within the grabens and half-grabens that are predominant in this area due to the subduction-related bending of the oceanic plate. No evidence was found for a subducted seamount associated to the Iquique Ridge along the slab interface at this latitude as interpreted earlier from the same data set. By comparing relocated fore- and aftershock seismicity of the Iquique earthquake with the resulting depth image, we can divide the continental wedge into two domains. First, a frontal unit beneath the lower slope with several eastward dipping back-rotated splay faults but no seismicity in the upper plate as well as along the plate interface. Secondly, a landward unit beneath the middle slope with differing reflectivity that shows significant seismicity in the upper plate as well as along the plate interface. Both units are separated by a large eastward dipping mega splay fault, the root zone of which shows diffuse seismicity, both in the upper plate and at the interface. The identification of a well-defined nearly aseismic frontal unit sheds new light on the interplate locking beneath the lower continental slope and its controls.

Ultrashallow depth imaging of a channel stratigraphy with first-arrival traveltime inversion and prestack depth migration: A case history from Bull Creek, Oklahoma

Geophysics ◽  
2012 ◽  
Vol 77 (2) ◽  
pp. B87-B96 ◽  
Author(s):  
Ammanuel Fesseha Woldearegay ◽  
Priyank Jaiswal ◽  
Alexander R. Simms ◽  
Hanna Alexander ◽  
Leland C. Bement ◽  
...  
Keyword(s):  
Velocity Model ◽  
Depth Image ◽  
Prestack Depth Migration ◽  
Data Set ◽  
Depth Imaging ◽  
Traveltime Inversion ◽  
Time Processing ◽  
Depth Migration ◽  
First Arrival ◽  
Bull Creek

Depth imaging in ultrashallow ([Formula: see text]) environments presents twofold challenge: (1) coda available for depth migration is very limited; and (2) conventional time processing with limited coda generally fails to estimate reliable velocity models for depth migration. We studied the combining of first-arrival traveltime inversion and prestack depth migration (PSDM) for depth imaging of ultrashallow paleochannel stratigraphy associated with the Bull Creek drainage system, Oklahoma. Restricted by a limited number of geophones (24) we acquired data for inversion and migration through two coincident profiles. The first profile for inversion has a wider survey-aperture (115-m maximum shot-receiver spacing) and consequently sparse CMP spacing (2.5 m), whereas the second profile for PSDM has denser CMP spacing (1 m) and consequently a narrower survey aperture (46-m maximum shot-receiver spacing). We also found that the velocity model from traveltime inversion of the wider-aperture data set is more preferable for depth-migration than the velocity model from time processing of the denser data set. The preferred depth image showed three episodes of incision whose chronological order is resolved through radio-carbon dating of terrace sediments. Results suggested that even with limited geophones, depth imaging of ultrashallow targets can be achieved by combining first-arrival traveltime inversion and PSDM through coincident wide- and narrow-aperture acquisitions.

Sensitivity of prestack depth migration to the velocity model

Geophysics ◽  
1993 ◽  
Vol 58 (6) ◽  
pp. 873-882 ◽  
Author(s):  
Roelof Jan Versteeg
Keyword(s):  
Seismic Data ◽  
Synthetic Data ◽  
Model Space ◽  
Velocity Model ◽  
Depth Image ◽  
Necessary Condition ◽  
Prestack Depth Migration ◽  
Data Set ◽  
Depth Migration ◽  
Model Determination

To get a correct earth image from seismic data acquired over complex structures it is essential to use prestack depth migration. A necessary condition for obtaining a correct image is that the prestack depth migration is done with an accurate velocity model. In cases where we need to use prestack depth migration determination of such a model using conventional methods does not give satisfactory results. Thus, new iterative methods for velocity model determination have been developed. The convergence of these methods can be accelerated by defining constraints on the model in such a way that the method only looks for those components of the true earth velocity field that influence the migrated image. In order to determine these components, the sensitivity of the prestack depth migration result to the velocity model is examined using a complex synthetic data set (the Marmousi data set) for which the exact model is known. The images obtained with increasingly smoothed versions of the true model are compared, and it is shown that the minimal spatial wavelength that needs to be in the model to obtain an accurate depth image from the data set is of the order of 200 m. The model space that has to be examined to find an accurate velocity model from complex seismic data can thus be constrained. This will increase the speed and probability of convergence of iterative velocity model determination methods.

Depth image focusing in traveltime map‐based wide‐angle migration

Geophysics ◽  
2002 ◽  
Vol 67 (6) ◽  
pp. 1903-1912 ◽  
Author(s):  
Igor B. Morozov ◽  
Alan Levander
Keyword(s):  
Velocity Model ◽  
Depth Image ◽  
Velocity Spectrum ◽  
Prestack Depth Migration ◽  
Data Set ◽  
Arrival Times ◽  
Depth Migration ◽  
Traveltime Tomography ◽  
Interactive Analysis ◽  
The Common

Wide‐aperture, prestack depth migration requires application of challenging and time‐consuming velocity analysis and depth focusing, collectively referred to here as depth focusing. We present an approach to depth focusing using (1) a detailed starting velocity model obtained by a 1‐D transformation of the first‐arrival times, followed iteratively by (2) interactive analysis of the common‐image gathers, (3) computation of coherency attributes of the wavefield in the depth domain, and (4) 2‐D traveltime tomography to update the background velocity model. We employ two interactive method of migration velocities refinement. In the first method (similar to the common‐midpoint velocity spectrum approach), residual velocity updates are picked directly from the common‐image gathers. In another method (analogous to the common velocity stacks), we pick the velocity updates from the areas of maximum coherency in depth sections that are migrated using rescaled traveltime maps. Both types of migration velocity picks, optionally combined with the first arrivals, are inputs for a 2‐D traveltime inversion scheme that uses either the infinite‐frequency or a finite‐bandwidth approximation. This flexible and versatile depth focusing approach is implemented for several prestack depth migration algorithms and illustrated on an application to a real, ultrashallow seismic data set. The technique resolves overburden velocity variations and facilitates reliable high‐resolution reflection imaging of a paleochannel that was the target of the study.

Simulation of a seismic survey with combined surface and in-mine data acquisition for imaging steeply dipping ore veins

2021 ◽  
Author(s):  
Olaf Hellwig ◽  
Stefan Buske
Keyword(s):  
Finite Difference ◽  
Message Passing Interface ◽  
Seismic Survey ◽  
Difference Operators ◽  
Mining District ◽  
Prestack Depth Migration ◽  
Data Set ◽  
Depth Migration ◽  
Difference Grid ◽  
Triangulated Surfaces

<p>The polymetallic, hydrothermal deposit of the Freiberg mining district in the southeastern part of Germany is characterised by ore veins that are framed by Proterozoic orthogneiss. The ore veins consist mainly of quarz, sulfides, carbonates, barite and flourite, which are associated with silver, lead and tin. Today the Freiberg University of Mining and Technology is operating the shafts Reiche Zeche and Alte Elisabeth for research and teaching purposes with altogether 14 km of accessible underground galleries. The mine together with the most prominent geological structures of the central mining district are included in a 3D digital model, which is used in this study to study seismic acquisition geometries that can help to image the shallow as well as the deeper parts of the ore-bearing veins. These veins with dip angles between 40° and 85° are represented by triangulated surfaces in the digital geological model. In order to import these surfaces into our seismic finite-difference simulation code, they have to be converted into bodies with a certain thickness and specific elastic properties in a first step. In a second step, these bodies with their properties have to be discretized on a hexahedral finite-difference grid with dimensions of 1000 m by 1000 m in the horizontal direction and 500 m in the vertical direction. Sources and receiver lines are placed on the surface along roads near the mine. A Ricker wavelet with a central frequency of 50 Hz is used as the source signature at all excitation points. Beside the surface receivers, additional receivers are situated in accessible galleries of the mine at three different depth levels of 100 m, 150 m and 220 m below the surface. Since previous mining activities followed primarily the ore veins, there are only few pilot-headings that cut through longer gneiss sections. Only these positions surrounded by gneiss are suitable for imaging the ore veins. Based on this geometry, a synthetic seismic data set is generated with our explicit finite-difference time-stepping scheme, which solves the acoustic wave equation with second order accurate finite-difference operators in space and time. The scheme is parallelised using a decomposition of the spatial finite-difference grid into subdomains and Message Passing Interface for the exchange of the wavefields between neighbouring subdomains. The resulting synthetic seismic shot gathers are used as input for Kirchhoff prestack depth migration as well as Fresnel volume migration in order to image the ore veins. Only a top mute to remove the direct waves and a time-dependent gain to correct the amplitude decay due to the geometrical spreading are applied to the data before the migration. The combination of surface and in-mine acquisition helps to improve the image of the deeper parts of the dipping ore veins. Considering the limitations for placing receivers in the mine, Fresnel volume migration as a focusing version of Kirchhoff prestack depth migration helps to avoid migration artefacts caused by this sparse and limited acquisition geometry.</p>

A mega-splay fault system and tsunami hazard in the southern Ryukyu subduction zone

2013 ◽  
Vol 362 ◽  
pp. 99-107 ◽  
Author(s):  
Shu-Kun Hsu ◽  
Yi-Ching Yeh ◽  
Jean-Claude Sibuet ◽  
Wen-Bin Doo ◽  
Ching-Hui Tsai
Keyword(s):  
Subduction Zone ◽  
Tsunami Hazard ◽  
Fault System ◽  
Splay Fault

scholarly journals From near-surface to root-zone soil moisture using an exponential filter: an assessment of the method based on in-situ observations and model simulations

2008 ◽  
Vol 12 (6) ◽  
pp. 1323-1337 ◽  
Author(s):  
C. Albergel ◽  
C. Rüdiger ◽  
T. Pellarin ◽  
J.-C. Calvet ◽  
N. Fritz ◽  
...  
Keyword(s):  
Soil Moisture ◽  
Soil Water ◽  
Surface Soil ◽  
Root Zone ◽  
Synthetic Data ◽  
Surface Soil Moisture ◽  
Data Set ◽  
Water Index ◽  
In Situ Observations

Abstract. A long term data acquisition effort of profile soil moisture is under way in southwestern France at 13 automated weather stations. This ground network was developed in order to validate remote sensing and model soil moisture estimates. In this paper, both those in situ observations and a synthetic data set covering continental France are used to test a simple method to retrieve root zone soil moisture from a time series of surface soil moisture information. A recursive exponential filter equation using a time constant, T, is used to compute a soil water index. The Nash and Sutcliff coefficient is used as a criterion to optimise the T parameter for each ground station and for each model pixel of the synthetic data set. In general, the soil water indices derived from the surface soil moisture observations and simulations agree well with the reference root-zone soil moisture. Overall, the results show the potential of the exponential filter equation and of its recursive formulation to derive a soil water index from surface soil moisture estimates. This paper further investigates the correlation of the time scale parameter T with soil properties and climate conditions. While no significant relationship could be determined between T and the main soil properties (clay and sand fractions, bulk density and organic matter content), the modelled spatial variability and the observed inter-annual variability of T suggest that a weak climate effect may exist.

scholarly journals The Imbert Formation of northern Hispaniola: a tectono-sedimentary record of arc-continent collision and ophiolite emplacement in the northern Caribbean subduction-accretionary prism

2015 ◽  
Vol 7 (2) ◽  
pp. 1827-1876 ◽  
Author(s):  
J. Escuder-Viruete ◽  
A. Suárez-Rodríguez ◽  
J. Gabites ◽  
A. Pérez-Estaún
Keyword(s):  
Subduction Zone ◽  
Continental Margin ◽  
Middle Eocene ◽  
Accretionary Prism ◽  
Island Arc ◽  
Caribbean Island ◽  
Stratigraphic Sequence ◽  
Oblique Collision ◽  
The Caribbean ◽  
Ophiolitic Complex

Abstract. In northern Hispaniola, the Imbert Formation (Fm) has been interpreted as an orogenic "mélange" originally deposited as trench-fill sediments, an accretionary (subduction) complex formed above a SW-dipping subduction zone, or the sedimentary result of the early oblique collision of the Caribbean plate with the Bahama Platform in the middle Eocene. However, new stratigraphical, structural, geochemical and geochronological data from northern Hispaniola indicate that the Imbert Fm constitutes a coarsening-upward stratigraphic sequence that records the transition of the sedimentation from a pre-collisional forearc to a syn-collisional piggy-back basin. This piggy-back basin was transported on top of the Puerto Plata ophiolitic complex slab and structurally underlying accreted units of the Rio San Juan complex, as it was emplaced onto the North America continental margin units. The Imbert Fm unconformably overlies different structural levels of the Caribbean subduction-accretionary prism, including a supra-subduction zone ophiolite, and consists of three laterally discontinuous units that record the exhumation of the underlying basement. The distal turbiditic lower unit includes the latest volcanic activity of the Caribbean island arc; the more proximal turbiditic intermediate unit is moderately affected by syn-sedimentary faulting; and the upper unit is a (caotic) olistostromic unit, composed of serpentinite-rich polymictic breccias, conglomerates and sandstones, strongly deformed by syn-sedimentary faulting, slumping and sliding processes. The Imbert Fm is followed by subsidence and turbiditic deposition of the overlying El Mamey Group. The 40Ar / 39Ar plagioclase plateau ages obtained in gabbroic rocks from the Puerto Plata ophiolitic complex indicate its exhumation at ∼ 45–40 Ma (lower-to-middle Eocene), contemporaneously to the sedimentation of the overlying Imbert Fm. These cooling ages imply the uplift to the surface and submarine erosion of the complex to be the source of the ophiolitic fragments in the Imbert Fm, during of shortly after the emplacement of the intra-oceanic Caribbean island-arc onto the continental margin.

Integrated prestack depth migration of vertical seismic profile and surface seismic data from the Rocky Mountain Foothills of southern Alberta, Canada

Geophysics ◽  
2003 ◽  
Vol 68 (6) ◽  
pp. 1782-1791 ◽  
Author(s):  
M. Graziella Kirtland Grech ◽  
Don C. Lawton ◽  
Scott Cheadle
Keyword(s):  
Seismic Data ◽  
Rocky Mountain ◽  
Velocity Model ◽  
Seismic Profile ◽  
Vertical Seismic Profile ◽  
Prestack Depth Migration ◽  
Data Set ◽  
Depth Migration ◽  
Southern Alberta ◽  
Vsp Data

We have developed an anisotropic prestack depth migration code that can migrate either vertical seismic profile (VSP) or surface seismic data. We use this migration code in a new method for integrated VSP and surface seismic depth imaging. Instead of splicing the VSP image into the section derived from surface seismic data, we use the same migration algorithm and a single velocity model to migrate both data sets to a common output grid. We then scale and sum the two images to yield one integrated depth‐migrated section. After testing this method on synthetic surface seismic and VSP data, we applied it to field data from a 2D surface seismic line and a multioffset VSP from the Rocky Mountain Foothills of southern Alberta, Canada. Our results show that the resulting integrated image exhibits significant improvement over that obtained from (a) the migration of either data set alone or (b) the conventional splicing approach. The integrated image uses the broader frequency bandwidth of the VSP data to provide higher vertical resolution than the migration of the surface seismic data. The integrated image also shows enhanced structural detail, since no part of the surface seismic section is eliminated, and good event continuity through the use of a single migration–velocity model, obtained by an integrated interpretation of borehole and surface seismic data. This enhanced migrated image enabled us to perform a more robust interpretation with good well ties.

The influence of fluids in the unusually high-rate seismicity in the Ometepec segment of the Mexican subduction zone

2021 ◽  
Author(s):  
D Legrand ◽  
A Iglesias ◽  
S K Singh ◽  
V Cruz-Atienza ◽  
C Yoon ◽  
...  
Keyword(s):  
Subduction Zone ◽  
Temporal Distribution ◽  
Aftershock Sequence ◽  
High Rate ◽  
Earth Tides ◽  
P Value ◽  
Plate Interface ◽  
Flow Measurements ◽  
Slow Slip Events ◽  
Mexican Subduction Zone

Summary The rate of earthquakes with magnitudes Mw ≤ 7.5 in the Ometepec segment of the Mexican subduction zone is relatively high as compared to the neighboring regions of Oaxaca and Guerrero. Although the reason is not well understood, it has been reported that these earthquakes give rise to a large number of aftershocks. Our study of the aftershock sequence of the 2012 Mw7.4 Ometepec thrust earthquake suggests that it is most likely due to two dominant factors: (1) The presence of an anomalously high quantity of over-pressured fluids near the plate interface, and (2) the roughness of the plate interface. More than 5,400 aftershocks were manually detected during the first ten days following the 2012 earthquake. Locations were obtained for 2,419 events (with duration magnitudes Md ≥ 1.5). This is clearly an unusually high number of aftershocks for an earthquake of this magnitude. Furthermore, we generated a more complete catalog, using an unsupervised fingerprint technique, to detect more smaller events (15,593 within one month following the mainshock). For this catalog, a high b-value of 1.50 ± 0.10 suggests the presence of fluid release during the aftershock sequence. A low p-value (0.37 ± 0.12) of the Omori law reveals a slow decaying aftershock sequence. The temporal-distribution of aftershocks shows peaks of activity with two dominant periods of 12h and 24h that correlate with the Earth tides. To explain these observations, we suggest that the 2012 aftershock sequence is associated with the presence of over-pressured fluids and/or a heterogeneous and irregular plate interface related to the subduction of the neighboring seamounts. High fluid content has independently been inferred by magneto-telluric surveys and deduced from heat flow measurements in the region. The presence of fluids in the region has also been proposed to explain the occurrence of slow slip events, low frequency earthquakes, and tectonic tremors.

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