Upper crustal structure of NW Iran revealed by regional 3-D Pg velocity tomography

2020 ◽  
Vol 222 (2) ◽  
pp. 1093-1108
Author(s):  
Mehdi Maheri-Peyrov ◽  
Abdolreza Ghods ◽  
Stefanie Donner ◽  
Maryam Akbarzadeh-Aghdam ◽  
Farhad Sobouti ◽  
...  
Keyword(s):  
Velocity Structure ◽  
Historical Earthquakes ◽  
Velocity Model ◽  
Central Iran ◽  
Medium Size ◽  
Depth Range ◽  
Upper Crust ◽  
Low Velocity ◽  
Nw Iran ◽  
Velocity Region

SUMMARY We present the result of a 3-D Pg tomography in NW Iran to better understand the relationship between seismicity and velocity structure within the young continental collision system. In this regard, we have collected 559 07 Pg traveltime readings from 3963 well located earthquakes recorded by 353 seismic stations including 121 stations from four new temporary seismic networks. The most prominent feature of our Pg velocity model is a high correlation between the location of majority of large magnitude events and the location of low velocity regions within the seismogenic layer. The large instrumental and historical earthquakes with some limited exceptions tends to happen close to the borders of the low velocity regions. The Lorestan arc of Zagros has the thickest (∼20 km) low velocity region and Central Iran has the thinnest (less than 10 km) low velocity region where little seismicity is observed. Despite the relative increase of thickness of low velocity region in the uppermost part of the upper crust of Alborz, the average Pg velocity of the upper crust increases from Central Iran towards Alborz and reaches to its climax in the northern hills of Alborz, where the catastrophic Rudbar-Tarom 1990 event happened. The Pg velocity map shows presence of a low angle basement ramp in the Lorestan arc at the depth range of ∼10–20 km. The large low angle thrust Ezgele-Sarpolzahab 2017 earthquake and medium size high angle thrust events happened at the base and updip part of the velocity ramp, respectively. The calculated Pg velocity map shows low velocity regions at depths deeper than 11 and 20 km beneath the Sahand and Sabalan volcanoes, respectively.

Structure beneath Queen Charlotte Sound from seismic-refraction and gravity interpretations

1992 ◽  
Vol 29 (7) ◽  
pp. 1509-1529 ◽  
Author(s):  
Tianson Yuan ◽  
G. D. Spence ◽  
R. D. Hyndman
Keyword(s):  
Crustal Structure ◽  
Velocity Structure ◽  
Sedimentary Basin ◽  
Single Channel ◽  
Velocity Variation ◽  
Moho Depth ◽  
High Porosity ◽  
Upper Crust ◽  
Low Velocity ◽  
Data Set

A combined multichannel seismic reflection and refraction survey was carried out in July 1988 to study the Tertiary sedimentary basin architecture and formation and to define the crustal structure and associated plate interactions in the Queen Charlotte Islands region. Simultaneously with the collection of the multichannel reflection data, refractions and wide-angle reflections from the airgun array shots were recorded on single-channel seismographs distributed on land around Hecate Strait and Queen Charlotte Sound. For this paper a subset of the resulting data set was chosen to study the crustal structure in Queen Charlotte Sound and the nearby subduction zone.Two-dimensional ray tracing and synthetic seismogram modelling produced a velocity structure model in Queen Charlotte Sound. On a margin-parallel line, Moho depth was modelled at 27 km off southern Moresby Island but only 23 km north of Vancouver Island. Excluding the approximately 5 km of the Tertiary sediments, the crust in the latter area is only about 18 km thick, suggesting substantial crustal thinning in Queen Charlotte Sound. Such thinning of the crust supports an extensional mechanism for the origin of the sedimentary basin. Deep crustal layers with velocities of more than 7 km/s were interpreted in the southern portion of Queen Charlotte Sound and beneath the continental margin. They could represent high-velocity material emplaced in the crust from earlier subduction episodes or mafic intrusion associated with the Tertiary volcanics.Seismic velocities of both sediment and upper crust layers are lower in the southern part of Queen Charlotte Sound than in the region near Moresby Island. Well velocity logs indicate a similar velocity variation. Gravity modelling along the survey line parallel to the margin provides additional constraints on the structure. The data require lower densities in the sediment and upper crust of southern Queen Charlotte Sound. The low-velocity, low-density sediments in the south correspond to high-porosity marine sediments found in wells in that region and contrast with lower porosity nonmarine sediments in wells farther north.

An application of induced event interferometry approach at The Geysers Geothermal Field, California, USA

2021 ◽  
Author(s):  
Taghi Shirzad ◽  
Stanisław Lasocki ◽  
Beata Orlecka‐Sikora
Keyword(s):  
Velocity Structure ◽  
General Structure ◽  
Clean Energy ◽  
Velocity Model ◽  
Geothermal Field ◽  
Small Scale ◽  
Low Velocity ◽  
Study Region ◽  
Resultant Velocity ◽  
The Geysers

<p>While the classical tomography approaches, e.g., P-, S-, and/or surface-wave traveltime tomography, provide a general structure of the Earth’s interior, new developments in signal processing of interferometry approaches are needed to obtain a high-resolution velocity structure. If the number of earthquakes is adequate, the virtual seismometer method may be a solution in regions with sparse instrumental coverage. Theoretically, the empirical Green’s functions between a pair of events can be retrieved using earthquake’s cross-correlations. Here, an event interferometry approach was used on a very small scale around Prati-9 and Prati-29 injection wells in the NW of The Geysers Geothermal Field. The study region experienced intense injection-induced seismicity. We selected all events with location uncertainties less than 50 m in a cuboid of the horizontal side ~1 × ~2 km and the vertical edge at depths between 1.0 and 2.0 km. The cuboid was cut into 100m thick layers, and we applied to events from each layer criteria enabling a quasi 2D approach. After calculating the Rayleigh wave group velocity dispersion curves, further processing was performed at a 0.2s period, selected based on the sensitivity kernel criterion. Finally, the relative velocity model of each layer at the depth z was obtained by subtracting the velocity model of the just overlying layer (at the depth z-100m) from the model of this layer. Our resultant velocity model in the study area indicated four low-velocity anomalies. The first one can be linked by the two layers interface topography variation at the top of the cuboid (depth 1000 m). The secondary faults can cause the second low-velocity anomaly. The other two anomalies look to result from fluid injection into Prati-9 and Prati-29 wells. <br>This work was supported under the S4CE: "Science for Clean Energy" project, which has received funding from the European Union’s Horizon 2020 research and innovation program, under grant agreement No 764810.</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>

Characteristics of Conrad Discontinuity in the Northern Margin of Tibet Plateau Obtained from Regional Seismic Data

2020 ◽  
Author(s):  
Biao Yang ◽  
Yanbin Wang
Keyword(s):  
Travel Time ◽  
Qaidam Basin ◽  
Lower Boundary ◽  
Historical Earthquakes ◽  
Tibet Plateau ◽  
Upper Crust ◽  
Low Velocity ◽  
Western Basin ◽  
Northern Margin ◽  
Waveform Simulation

<p>Qaidam Basin, located in the northern margin of the Tibet Plateau, is the junction of several tectonic blocks. The blocks’ extrusion resulted in large faults and strong historical earthquakes. Previous studies have shown that the crustal structures of the eastern and the western Qaidam Basin are obviously different. In this study, the seismic reflection and refraction phases from Conrad and Moho discontinuity in Qaidam Basin are distinguished by waveform simulation and travel time fitting of 3 regional earthquakes on 32 stations. The results of travel time fitting and waveform simulation show that the first arrivals in the epicenter range of 90km ~ 260km are the P* phases from the Conrad discontinuity. The depth of Conrad discontinuity under the eastern basin is about 4 km shallower than that in the western basin, which can be attributed to different crust thickening models between the eastern and western basin. In addition, the focal depths of regional earthquakes occurred within recent 5 years in Qaidam region also shows the difference of the Conrad discontinuity. The Conrad discontinuity is considered to be the lower boundary of the low velocity layer in the upper crust. The upper crust thickening in the western basin led to the sinking of the layer, while the multiple thrusts resulted in the rise of the lower crust in the east. The two different effects could interpret the depth change of the Conrad discontinuity in the basin from the west to the east. </p>

Seismic tomography at a fire‐flood site

Geophysics ◽  
1989 ◽  
Vol 54 (9) ◽  
pp. 1082-1090 ◽  
Author(s):  
N. D. Bregman ◽  
P. A. Hurley ◽  
G. F. West
Keyword(s):  
Oil Recovery ◽  
Velocity Structure ◽  
Seismic Velocity ◽  
Pilot Project ◽  
Velocity Model ◽  
High Amplitude ◽  
Oil Sand ◽  
Low Velocity ◽  
Study Region ◽  
Velocity Channel

A crosshole seismic experiment was conducted to locate and characterize a firefront at an enhanced oil recovery (EOR) pilot project. The reservoir engineers involved in the project were interested in finding out why the burnfront apparently had stalled between two wells 51 m apart. In a noisy producing environment, good quality seismic data were recorded at depths ranging from 710 to 770 m. The frequency range of the data, 500 to 1500 Hz, allows resolution of the velocity structure on a scale of several meters. The moveout of first arrivals indicates that there are large velocity variations in the study region; a high‐amplitude, late arriving channel wave points to the existence of a low‐velocity channel connecting the boreholes. Using an iterative, nonlinear scheme which incorporates curved ray tracing and least‐squares inversion in each iteration, the first‐arrival times were inverted to obtain a two‐dimensional model of the compressional seismic velocity between the boreholes. The velocities range from 1.5 km/s to 3.2 km/s, with a low‐velocity channel at the depth of the producing oil sand. Sonic, core, and temperature logs lead us to conclude that the extremely low velocities in the model are probably due to gases produced by the burn. Increased velocities in an adjacent shale may be a secondary effect of the burn. The velocity model also indicates an irregularity in the topography at the bottom of the reservoir, an irregularity which may be responsible for blocking the progress of the burnfront.

scholarly journals Upper mantle velocity structure beneath Italy from direct and secondary P-wave teleseismic tomography

1997 ◽  
Vol 40 (1) ◽  
Author(s):  
G. B. Cimini ◽  
P. De Gori
Keyword(s):  
Upper Mantle ◽  
Velocity Structure ◽  
Velocity Model ◽  
P Wave ◽  
Depth Range ◽  
Tyrrhenian Sea ◽  
Tomographic Images ◽  
3D Velocity Model ◽  
Southern Tyrrhenian Sea ◽  
Velocity Anomalies

High-quality teleseismic data digitally recorded by the National Seismic Network during 1988-1995 have been analysed to tomographically reconstruct the aspherical velocity structure of the upper mantle beneath the Italian region. To improve the quality and the reliability of the tomographic images, both direct (P, PKPdf) and secondary (pP,sP,PcP,PP,PKPbc,PKPab) travel-time data were used in the inversion. Over 7000 relative residuals were computed with respect to the IASP91 Earth velocity model and inverted using a modified version of the ACH technique. Incorporation of data of secondary phases resulted in a significant improvement of the sampling of the target volume and of the spatial resolution of the heterogeneous zones. The tomographic images show that most of the lateral variations in the velocity field are confined in the first ~250 km of depth. Strong low velocity anomalies are found beneath the Po plain, Tuscany and Eastern Sicily in the depth range between 35 and 85 km. High velocity anomalies dominate the upper mantle beneath the Central-Western Alps, Northern-Central Apennines and Southern Tyrrhenian sea at lithospheric depths between 85 and 150 km. At greater depth, positive anomalies are still observed below the northernmost part of the Apenninic chain and Southern Tyrrhenian sea. Deeper anomalies present in the 3D velocity model computed by inverting only the first arrivals dataset, generally appear less pronounced in the new tomographic reconstructions. We interpret this as the result of the ray sampling improvement on the reduction of the vertical smearing effects.

scholarly journals Velocity Structure at the Source Physics Experiment Phase I Site Obtained with the Large‐N Array Data

2019 ◽  
Vol 91 (1) ◽  
pp. 304-309
Author(s):  
Ting Chen ◽  
Catherine M. Snelson ◽  
Robert Mellors
Keyword(s):  
Velocity Structure ◽  
Volcanic Rocks ◽  
Velocity Model ◽  
Seismic Array ◽  
P Wave ◽  
Low Velocity ◽  
Large N ◽  
Chemical Explosions ◽  
Physics Experiment ◽  
Source Physics

Abstract The Source Physics Experiment (SPE) consists of a series of chemical explosions at the Nevada National Security Site. The goal of the SPE is to understand and model seismic‐wave generation and propagation from these explosions. To achieve this goal, we need an accurate velocity model of the SPE site. A large‐N seismic array deployed at the SPE site during one of the chemical explosions (SPE‐5) provides great data for this purpose. The array consists of 996 geophones and covers an area of approximately 2×2.5  km. In addition to the SPE‐5 explosion, the array recorded 53 large weight drops. Using the large‐N seismic array recordings, we perform first‐arrival analysis and obtain a 2D P‐wave velocity model of the SPE site. We image a sharp transition from high‐velocity Cretaceous granite to low‐velocity Quaternary alluvium. Other geological units such as the Tertiary volcanic rocks and Paleozoic sedimentary rocks are also clearly shown. The results of this work provide important local geological information and will be incorporated into the larger 3D modeling effort of the SPE program to validate the predictive models developed for the site.

scholarly journals From tomographic images to fault heterogeneities

1994 ◽  
Vol 37 (6) ◽  
Author(s):  
C. Chiarabba ◽  
A. Amato
Keyword(s):  
Velocity Structure ◽  
Sedimentary Cover ◽  
Central Italy ◽  
Fault Zones ◽  
Fault System ◽  
Upper Crust ◽  
Low Velocity ◽  
South Central ◽  
Velocity Anomalies ◽  
Lateral Heterogeneities

Local Earthquake Tomography (LET) is a useful tool for imaging lateral heterogeneities in the upper crust. The pattern of P- and S-wave velocity anomalies, in relation to the seismicity distribution along active fault zones. can shed light on the existence of discrete seismogenic patches. Recent tomographic studies in well monitored seismic areas have shown that the regions with large seismic moment release generally correspond to high velocity zones (HVZ's). In this paper, we discuss the relationship between the seismogenic behavior of faults and the velocity structure of fault zones as inferred from seismic tomography. First, we review some recent tomographic studies in active strike-slip faults. We show examples from different segments of the San Andreas fault system (Parkfield, Loma Prieta), where detailed studies have been carried out in recent years. We also show two applications of LET to thrust faults (Coalinga, Friuli). Then, we focus on the Irpinia normal fault zone (South-Central Italy), where a Ms = 6.9 earthquake occurred in 1980 and many thousands of attershock travel time data are available. We find that earthquake hypocenters concentrate in HVZ's, whereas low velocity zones (LVZ’ s) appear to be relatively aseismic. The main HVZ's along which the mainshock rupture bas propagated may correspond to velocity weakening fault regions, whereas the LVZ's are probably related to weak materials undergoing stable slip (velocity strengthening). A correlation exists between this HVZ and the area with larger coseismic slip along the fault, according to both surface evidence (a fault scarp as high as 1 m) and strong ground motion waveform modeling. Smaller wave-length, low-velocity anomalies detected along the fault may be the expression of velocity strengthening sections, where aseismic slip occurs. According to our results, the rupture at the nucleation depth (~ 10-12 km) is continuous for the whole fault lenoth (~ 30 km), whereas at shallow depth, different fault segments are activated due to lateral heterogeneities in the sedimentary cover. This finding confirms that the rupture process is controlled by lithologic and structural discontinuities in the upper crust, and emphasizes the contribution that LET can make to the study of fault mechanics.

scholarly journals POLENET/LAPNET teleseismic P-wave traveltime tomography model of the upper mantle beneath northern Fennoscandia

2015 ◽  
Vol 7 (3) ◽  
pp. 2527-2562 ◽  
Author(s):  
H. Silvennoinen ◽  
E. Kozlovskaya ◽  
E. Kissling
Keyword(s):  
Upper Mantle ◽  
Velocity Model ◽  
Fennoscandian Shield ◽  
P Wave ◽  
Significant Feature ◽  
Depth Range ◽  
Low Velocity ◽  
Karelian Craton ◽  
International Polar Year ◽  
Northern Fennoscandia

Abstract. The POLENET/LAPNET broadband seismic array was deployed in northern Fennoscandia (Finland, Sweden, Norway, and Russia) during the third International Polar Year 2007–2009. The array consisted of roughly 60 seismic stations. In our study we estimate the 3-D architecture of the upper mantle beneath the northern Fennoscandian shield using high-resolution teleseismic P-wave tomography. For this purpose 111 clearly recorded teleseismic events were selected and the data from the stations handpicked and analysed. Our study reveals a highly heterogeneous lithospheric mantle beneath the northern Fennoscandian shield though without any large high P-wave velocity area that may indicate presence of thick depleted lithospheric "keel". The most significant feature seen in the velocity model is a large elongated negative velocity anomaly (up to −3.5 %) in depth range 100–150 km in the central part of our study area that can be followed down to a depth of 200 km in some local areas. This low-velocity area separates three high-velocity regions corresponding to the cratons and it extends to greater depth below the Karelian craton.

scholarly journals 1-D velocity structure modelling of the Earth's Crust in the NW Dinarides

2021 ◽  
Author(s):  
Gregor Rajh ◽  
Josip Stipčević ◽  
Mladen Živčić ◽  
Marijan Herak ◽  
Andrej Gosar ◽  
...  
Keyword(s):  
Crustal Structure ◽  
Velocity Structure ◽  
Pannonian Basin ◽  
Stable Solution ◽  
P Wave ◽  
Small Scale ◽  
Depth Range ◽  
Eurasian Plate ◽  
Low Velocity ◽  
Velocity Models

Abstract. The investigated area of the NW Dinarides is located at the NE corner of the Adriatic microplate and is bordered by the Adriatic foreland, the Southern Alps, and the Pannonian basin. Its complex crustal structure is the result of interactions among different tectonic units, mainly the Eurasian plate and the Adriatic microplate. Despite numerous seismic studies in this tectonically complex area, there is still a need for a detailed, small scale study focusing mainly on the upper, brittle part of the crust. We investigated the crustal velocity structure with 1-D simultaneous hypocenter-velocity inversion using routinely picked P wave arrival times. Most of the computed models converged to a stable solution in the depth range between 0 and 26 km. We further evaluated the inversion results with hypocenter shift tests, high and low velocity tests, and relocations. This helped us to select two best performing velocity models for the whole study area. Based on these results and the seismicity distribution, we further divided the study area into three parts, redefined the earthquake-station geometry, and performed inversion for each part separately to gain better insight into the crustal structure of each subregion. Median velocities in the upper 20 km of the crust in the eastern subregion are lower compared to the regional median and the median of the other two subregions. The northwestern and southwestern subregions are very similar in terms of crustal structure between about 8 and 23 km depth. The largest difference between them is observed in the upper 8 km, with higher median velocities in the southwestern subregion. Compared to the model currently used at Slovenian Environment Agency to locate earthquakes, the velocity models obtained show higher velocities in the upper 30 km depth and agree very well with some of the previous studies. In addition to general structural implications and a potential for improving seismic tomography results, the new 1-D velocity models can also be used for fast routine earthquake location and for detecting systematic travel time errors in seismological bulletins.

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