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.

On the relocation of earthquake clusters. A case history: The Arkansas earthquake swarm

1989 ◽  
Vol 79 (6) ◽  
pp. 1846-1862
Author(s):  
José Pujol ◽  
Jer-Ming Chiu ◽  
Arch Johnston ◽  
Byau-Heng Chin
Keyword(s):  
Seismic Activity ◽  
Earthquake Swarm ◽  
Singular Values ◽  
P Wave ◽  
Seismic Velocities ◽  
Low Velocity ◽  
Epicentral Area ◽  
Low Velocity Zone ◽  
Station Corrections ◽  
Velocity Zone

Abstract A portable digital network (the PANDA array) of 40 three-component stations with an aperture of about 35 km was deployed for 4 months in the Arkansas swarm area in 1987. Only 12 swarm events occurred during the deployment, in contrast to the intense seismic activity that characterized this region in 1982 to 1984. These events were relocated using a joint hypocentral determination technique (JHD). The JHD method used here allows for the simultaneous determination of P- and S-wave station corrections while providing information on the uniqueness of the solution based on the singular values of a matrix related to the station corrections. P-wave station corrections, determined when all nonzero singular values were used in the computations (or with the two smallest nonzero singular values deleted), show a circular pattern of positive values surrounded by negative values. The epicentral area is localized slightly displaced from the center of the pattern. Since positive and negative corrections correspond to velocities that are lower and higher, respectively, than the average, our results indicate that the swarm area is characterized by seismic velocities lower than those of its surroundings. Independent information on this region is afforded by reflection seismic lines recorded in the swarm area and its vicinity, which show that the hypocenters are located in a region where strong reflectors completely lose their coherence, indicating that this volume is anomalous when compared to surrounding crust. Additional support for a low-velocity zone comes from the results of a 3-D velocity inversion of the same PANDA data. A selected subset of data recorded digitally by the USGS in 1982 was also relocated. Comparison with the results from the PANDA data shows that the seismic activity did not migrate over a 5-yr period and that it is concentrated within a small volume between about 3 km and 6 km depth. While the results of this study do not determine the ultimate cause of the Arkansas swarm, the discovery of a pronounced localized low velocity zone is consistent with a previously proposed magmatic intrusion or a zone of highly fractured, fluid-filled crust.

Passive seismic tomography using recorded microseismicity: Application to mining-induced seismicity

Geophysics ◽  
2019 ◽  
Vol 84 (1) ◽  
pp. B41-B57 ◽  
Author(s):  
Himanshu Barthwal ◽  
Mirko van der Baan
Keyword(s):  
Seismic Tomography ◽  
Velocity Model ◽  
P Wave ◽  
Double Difference ◽  
Lateral Velocity ◽  
Study Region ◽  
Arrival Times ◽  
3D Velocity Model ◽  
Passive Seismic ◽  
Dip Angle

Microseismicity is recorded during an underground mine development by a network of seven boreholes. After an initial preprocessing, 488 events are identified with a minimum of 12 P-wave arrival-time picks per event. We have developed a three-step approach for P-wave passive seismic tomography: (1) a probabilistic grid search algorithm for locating the events, (2) joint inversion for a 1D velocity model and event locations using absolute arrival times, and (3) double-difference tomography using reliable differential arrival times obtained from waveform crosscorrelation. The originally diffusive microseismic-event cloud tightens after tomography between depths of 0.45 and 0.5 km toward the center of the tunnel network. The geometry of the event clusters suggests occurrence on a planar geologic fault. The best-fitting plane has a strike of 164.7° north and dip angle of 55.0° toward the west. The study region has known faults striking in the north-northwest–south-southeast direction with a dip angle of 60°, but the relocated event clusters do not fall along any mapped fault. Based on the cluster geometry and the waveform similarity, we hypothesize that the microseismic events occur due to slips along an unmapped fault facilitated by the mining activity. The 3D velocity model we obtained from double-difference tomography indicates lateral velocity contrasts between depths of 0.4 and 0.5 km. We interpret the lateral velocity contrasts in terms of the altered rock types due to ore deposition. The known geotechnical zones in the mine indicate a good correlation with the inverted velocities. Thus, we conclude that passive seismic tomography using microseismic data could provide information beyond the excavation damaged zones and can act as an effective tool to complement geotechnical evaluations.

scholarly journals Improved imaging of gas hydrate reservoirs and their plumbing system using 2d elastic full waveform inversion

2021 ◽  
pp. 1-61
Author(s):  
Adnan Djeffal ◽  
Ingo A. Pecher ◽  
Satish C. Singh ◽  
Gareth J. Crutchley ◽  
Jari Kaipio
Keyword(s):  
Gas Hydrate ◽  
Waveform Inversion ◽  
Hydrate Formation ◽  
Velocity Model ◽  
Plumbing System ◽  
Low Velocity ◽  
Velocity Models ◽  
Full Waveform ◽  
Hydrate Reservoirs ◽  
Velocity Zone

Gas hydrates are ice-like crystalline materials that form under submarine environments of moderate pressure and low temperature. Another key factor to their formation is the abundance in gas supply from depth in addition to local biogenic gas. Detailed imaging and velocity analysis of the plumbing system of gas hydrates can provide confidence that amplitude anomalies in seismic data are related to gas hydrate accumulations. We have conducted 2D elastic full-waveform inversion (FWI) along a 14 km long segment of a 2D multichannel seismic profile to obtain a high-resolution velocity model of a hydrate system on the southern Hikurangi margin. We compare the FWI velocity model to previously published semblance- and tomography-based velocity models from the same data to explore how much more can be gained from the FWI. The FWI yielded a structurally more accurate velocity model that better delineated the low-velocity zone associated with free gas beneath the bottom simulating reflector (BSR) compared to the semblance- and tomography-based velocity models. Our results also find a lateral velocity inversion, that is, a narrow low-velocity zone surrounded by bands of higher velocities at a seaward-verging protothrust fault, which the two other methodologies failed to resolve. The FWI provides an improved lateral resolution making it an important tool when imaging the “plumbing” systems of gas hydrate reservoirs. In the southeastern limb of the anticline, our results find that the closely spaced landward-vergent protothrusts provide gas-charged fluids for hydrate formation above the BSR. Moreover, at the center of the anticline, our results find that a seaward-vergent protothrust fault appears to be acting as a conduit for gas-rich fluids into strata, although there is no accumulation of any significant hydrate above the BSR at the apex of the anticline. Our finding emphasizes the significance of densely spaced faults and fractures for providing gas for hydrate formation in the hydrate stability zone.

scholarly journals The shallow structure of Mars at the InSight landing site from inversion of ambient vibrations

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
M. Hobiger ◽  
M. Hallo ◽  
C. Schmelzbach ◽  
S. C. Stähler ◽  
D. Fäh ◽  
...  
Keyword(s):  
Seismic Data ◽  
Seismic Velocity ◽  
Landing Site ◽  
Velocity Model ◽  
Low Velocity ◽  
Shallow Subsurface ◽  
Seismic Vibrations ◽  
First Time ◽  
Velocity Channel ◽  
Velocity Zone

AbstractOrbital and surface observations can shed light on the internal structure of Mars. NASA’s InSight mission allows mapping the shallow subsurface of Elysium Planitia using seismic data. In this work, we apply a classical seismological technique of inverting Rayleigh wave ellipticity curves extracted from ambient seismic vibrations to resolve, for the first time on Mars, the shallow subsurface to around 200 m depth. While our seismic velocity model is largely consistent with the expected layered subsurface consisting of a thin regolith layer above stacks of lava flows, we find a seismic low-velocity zone at about 30 to 75 m depth that we interpret as a sedimentary layer sandwiched somewhere within the underlying Hesperian and Amazonian aged basalt layers. A prominent amplitude peak observed in the seismic data at 2.4 Hz is interpreted as an Airy phase related to surface wave energy trapped in this local low-velocity channel.

Diffraction imaging and depth-velocity inversion with 3D P-Cable seismic data

2021 ◽  
Author(s):  
Alexander Bauer ◽  
Benjamin Schwarz ◽  
Dirk Gajewski
Keyword(s):  
Seismic Data ◽  
Model Building ◽  
Computational Cost ◽  
Velocity Model ◽  
Data Cube ◽  
Velocity Inversion ◽  
Velocity Models ◽  
North East ◽  
3D Velocity Model ◽  
Zero Offset

<p>Most established methods for the estimation of subsurface velocity models rely on the measurements of reflected or diving waves and therefore require data with sufficiently large source-receiver offsets. For seismic data that lacks these offsets, such as vintage data, low-fold academic data or near zero-offset P-Cable data, these methods fail. Building on recent studies, we apply a workflow that exploits the diffracted wavefield for depth-velocity-model building. This workflow consists of three principal steps: (1) revealing the diffracted wavefield by modeling and adaptively subtracting reflections from the raw data, (2) characterizing the diffractions with physically meaningful wavefront attributes, (3) estimating depth-velocity models with wavefront tomography. We propose a hybrid 2D/3D approach, in which we apply the well-established and automated 2D workflow to numerous inlines of a high-resolution 3D P-Cable dataset acquired near Ritter Island, a small volcanic island located north-east of New Guinea known for a catastrophic flank collapse in 1888. We use the obtained set of parallel 2D velocity models to interpolate a 3D velocity model for the whole data cube, thus overcoming possible issues such as varying data quality in inline and crossline direction and the high computational cost of 3D data analysis. Even though the 2D workflow may suffer from out-of-plane effects, we obtain a smooth 3D velocity model that is consistent with the data.</p>

Recent microseismicity observed at Hekla volcano and first velocity inversion results

2020 ◽  
Author(s):  
Martin Möllhoff ◽  
Meysam Rezaeifar ◽  
Christopher J. Bean ◽  
Kristin S. Vogfjörd ◽  
Bergur H. Bergsson ◽  
...  
Keyword(s):  
Real Time ◽  
Velocity Model ◽  
Velocity Inversion ◽  
The Real ◽  
Location System ◽  
Station Corrections ◽  
High Hazard ◽  
Real Time Detection ◽  
Warning Time

<p>Hekla is one of the most active and dangerous volcanoes in Iceland presenting a high hazard to air travel and a growing tourist population. Until now the pre-eruption warning time at Hekla is only around one hour.  In 2018 we installed the real-time seismic network HERSK directly on Hekla's edifice. If microseismicity on Hekla increases prior to the next eruption the network could possibly provide a means to improve early warning. In addition it is hoped that HERSK will better our understanding of the processes driving the evolution of pre-eruptive seismicity. The configuration and tuning of a dedicated real-time detection and location system requires the determination of a suitable velocity model and station corrections. We present a catalogue of recently detected local events that we use to invert for a 1-D velocity model. We observe significant variations in station corrections and conclude that it is important to account for these in the real-time detection and location system which we are developing based on the SeisComp3 software.</p>

Crustal velocity structure beneath the NW Dinarides from 1-D hypocenter-velocity inversion

2021 ◽  
Author(s):  
Gregor Rajh ◽  
Josip Stipčević ◽  
Mladen Živčić ◽  
Marijan Herak ◽  
Andrej Gosar
Keyword(s):  
Velocity Structure ◽  
Velocity Model ◽  
P Wave ◽  
Depth Range ◽  
Velocity Inversion ◽  
Arrival Times ◽  
Simultaneous Inversion ◽  
Velocity Modeling ◽  
Velocity Models ◽  
Station Corrections

<p>The investigated area of the NW Dinarides is bordered by the Adriatic foreland, the Southern Alps, and the Pannonian basin at the NE corner of the Adriatic Sea. Its complex crustal structure is the result of interactions among different tectonic units. Despite numerous seismic studies taking place in this region, there still exists a need for a detailed, smaller scale study focusing mainly on the brittle part of the Earth's crust. Therefore, we decided to investigate the velocity structure of the crust using concepts of local earthquake tomography (LET) and minimum 1-D velocity model. Here, we present the results of the 1-D velocity modeling and the catalogue of the relocated seismicity. A minimum 1-D velocity model is computed by simultaneous inversion for hypocentral and velocity parameters together with seismic station corrections and represents the best fit to the observed arrival times.</p><p>We used 15,579 routinely picked P wave arrival times from 631 well-located earthquakes that occurred in Slovenia and in its immediate surroundings (mainly NW Croatia). Various initial 1-D velocity models, differing in velocity and layering, were used as input for velocity inversion in the VELEST program. We also varied several inversion parameters during the inversion runs. Most of the computed 1-D velocity models converged to a stable solution in the depth range between 0 and 25 km. We evaluated the inversion results using rigorous testing procedures and selected two best performing velocity models. Each of these models will be used independently as the initial model in the simultaneous hypocenter-velocity inversion for a 3-D velocity structure in LET. Based on the results of the 1-D velocity modeling, seismicity distribution, and tectonics, we divided the study area into three parts, redefined the earthquake-station geometry, and performed the inversion for each part separately. This way, we gained a better insight into the shallow velocity structure of each subregion and were able to demonstrate the differences among them.</p><p>Besides general structural implications and a potential to improve the results of LET, the new 1-D velocity models along with station corrections can also be used in fast routine earthquake location and to detect systematic travel time errors in seismological bulletins, as already shown by some studies using similar methods.</p>

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 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.

Joint hypocentral determination and the detection of low-velocity anomalies. An example from the Phlegraean Fields earthquakes

1990 ◽  
Vol 80 (1) ◽  
pp. 129-139 ◽  
Author(s):  
Jose Pujol ◽  
Richard Aster
Keyword(s):  
Earthquake Swarm ◽  
P Wave ◽  
S Wave ◽  
Time Data ◽  
Low Velocity ◽  
Data Set ◽  
Epicentral Area ◽  
Velocity Inversion ◽  
Phlegraean Fields ◽  
Velocity Anomalies

Abstract Arrival time data from the Phlegraean Fields (Italy) earthquake swarm recorded by the University of Wisconsin array in 1983 to 1984 were reanalyzed using a joint hypocentral determination (JHD) technique. The P- and S-wave station corrections computed as part of the JHD analysis show a circular pattern of central positive values surrounded by negative values whose magnitudes increase with distance from the center of the pattern. This center roughly coincides with the point of the maximum uplift (almost 2 m) associated with the swarm. Corrections range from −0.85 to 0.10 sec for P-wave arrivals and from −1.09 to 0.70 sec for S-wave arrivals. We interpret these patterns of corrections as caused by a localized low-velocity anomaly in the epicentral area, which agrees with the results of a previous 3-D velocity inversion of the same data set. The relocated (JHD) epicenters show less scatter than the epicenters obtained in the velocity inversion, and move more of the seismic activity to the vicinity of the only presently active fumarolic feature. The capability of the JHD technique to detect low-velocity anomalies and at the same time to give reliable locations, particularly epicenters, was verified using synthetic data generated for a 3-D velocity model roughly resembling the model obtained by velocity inversion.

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