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

By integrating data and constraints available in the literature, we defined a new &#8220;a-priori&#8221; 3D seismic velocity model imaging the lithospheric structure of Southern Italy, a highly complex area in the Mediterranean region where the Africa-Europe plate convergence and the residual rollback of the Ionian slab coexist. Involving the integration of multiple datasets and constraints (e.g. velocity patterns from seismic profiles and/or tomographies, moho depth estimates, subduction interface geometries) and following a procedure derived to the one already successfully applied in the area about a decade ago, we obtained the simplest 3D velocity structure consistent with all the available collected data. Studies and analyses performed in recent years allowed us to enlarge and improve the previous estimated model by adding further data and useful constraints. The so obtained "a-priori" velocity model has then been used as starting model for a new earthquake tomographic inversion of the study region. Dataset used for the velocity model computation has been selected from the Italian seismic database (www.ingv.it) and consists of ca. 10000 earthquakes with magnitude equal or greater than 2 and occurred in the time period 2000-2020 at depth less than 60 km and with at least 10 station readings. The obtained 3D velocity structure and the related hypocenter locations have been compared with other geophysical and geological observations and interpreted in the frame of the geodynamic models proposed for the region.

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Seismic tomography at a fire‐flood site

Geophysics ◽

10.1190/1.1442743 ◽

1989 ◽

Vol 54 (9) ◽

pp. 1082-1090 ◽

Cited By ~ 13

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.

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

Acta Geophysica ◽

10.1007/s11600-021-00594-6 ◽

2021 ◽

Author(s):

Francesca D’Ajello Caracciolo ◽

Rodolfo Console

Keyword(s):

Central Europe ◽

Velocity Model ◽

Moho Depth ◽

Seismic Sequence ◽

Tectonic Structures ◽

Study Region ◽

Waveform Data ◽

Seismic Stations ◽

Velocity Models ◽

Master Event

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

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Travel Time Tomography to Delineate 3-D Regional Seismic Velocity Structure in the Banyumas Basin, Central Java, Indonesia, Using Dense Borehole Seismographic Stations

Frontiers in Earth Science ◽

10.3389/feart.2021.639271 ◽

2021 ◽

Vol 9 ◽

Author(s):

Hidayat Hidayat ◽

Andri Dian Nugraha ◽

Awali Priyono ◽

Marjiyono Marjiyono ◽

Januar H. Setiawan ◽

...

Keyword(s):

Velocity Structure ◽

Seismic Velocity ◽

Velocity Model ◽

Crustal Layer ◽

Central Java ◽

Resolution Element ◽

Passive Seismic ◽

Dextral Strike Slip Fault ◽

Southern Central ◽

Well Data

The Banyumas Basin is a tertiary sedimentary basin located in southern Central Java, Indonesia. Due to the presence of volcanic deposits, 2-D seismic reflection methods cannot provide a good estimation of the sediment thickness and the subsurface geology structure in this area. In this study, the passive seismic tomography (PST) method was applied to image the 3-D subsurface Vp, Vs, and Vp/Vs ratio. We used 70 seismograph borehole stations with a recording duration of 177 days. A total of 354 events with 9, 370 P and 9, 368 S phases were used as input for tomographic inversion. The checkshot data of a 4, 400-meter deep exploration well (Jati-1) located within the seismic network were used to constrain the shallow crustal layer of the initial 1-D velocity model. The model resolution of the tomographic inversions was assessed using the checkerboard resolution test (CRT), the diagonal resolution element (DRE), and the derivative weight sum (DWS). Using the obtained Vp, Vs, and Vp/Vs ratio, we were able to sharpen details of the geological structures within the basin from previous geological studies, and a fault could be well-imaged at a depth of 4 km. We interpreted this as the main dextral strike-slip fault that controls the pull apart process of the Banyumas Basin. The thickness of the sediment layers, as well as its layering, were also could be well determined. We found prominent features of the velocity contrast that aligned very well with the boundary between the Halang and Rambatan formations as observed in the Jati-1 well data. Furthermore, an anticline structure, which is a potential structural trap for the petroleum system in the Banyumas Basin, was also well imaged. This was made possible due to the dense borehole seismographic stations which were deployed in the study area.

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Seismic Velocity Structure in the Area of the 2007, Mw 8.0, Pisco-Peru Earthquake: Implications for the Mechanics of Subduction in the Vicinity of the Nazca Ridge

International Journal of Geophysics ◽

10.1155/2020/3874524 ◽

2020 ◽

Vol 2020 ◽

pp. 1-8

Author(s):

I. Bernal ◽

H. Tavera

Keyword(s):

Velocity Structure ◽

Seismic Velocity ◽

Velocity Model ◽

Double Difference ◽

Seismic Velocity Structure ◽

Uplift Rates ◽

Northern Edge ◽

Nazca Ridge ◽

Geomorphological Features ◽

Postseismic Slip

In this study, we present a velocity model for the area of the 2007 Pisco-Peru earthquake ( Mw = 8.0 ) obtained using a double-difference tomography algorithm that considers aftershocks acquired for 6 months. The studied area is particularly interesting because it lies on the northern edge of the Nazca Ridge, in which the subduction of a large bathymetric structure is the origin of geomorphological features of the central coast of Peru. Relocated seismicity is used to infer the geometry of the subduction slab on the northern flank of the Nazca Ridge. The results prove that the geometry is continuous but convex because of the subduction of the ridge, thereby explaining the high uplift rates observed in this area. Our inferred distribution of seismicity agrees with both the coseismic and postseismic slip distributions.

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

10.5194/egusphere-egu21-8312 ◽

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

While the classical tomography approaches, e.g., P-, S-, and/or surface-wave traveltime tomography, provide a general structure of the Earth&#8217;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&#8217;s functions between a pair of events can be retrieved using earthquake&#8217;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 &#215; ~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.&#160; This work was supported under the S4CE: "Science for Clean Energy" project, which has received funding from the European Union&#8217;s Horizon 2020 research and innovation program, under grant agreement No 764810.

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The nature of the Albstadt Shear Zone, Germany

10.5194/egusphere-egu2020-7165 ◽

2020 ◽

Author(s):

Sarah Mader ◽

Klaus Reicherter ◽

Joachim Ritter ◽

the AlpArray Working Group

Keyword(s):

Shear Zone ◽

Seismic Velocity ◽

Active Regions ◽

Velocity Model ◽

Strike Slip ◽

Fault System ◽

Earthquake Catalog ◽

Depth Range ◽

Study Region ◽

The Town

The region around the town of Albstadt, SW Germany, is one of the most seismically active regions in Central Europe. In the last century alone three earthquakes with a magnitude greater than five happened and caused major damage. The ruptures occur along the Albstadt Shear Zone (ASZ), an approx. 20-30 km long, N-S striking fault with left-lateral strike slip. As there is no evidence for surface rupture the nature of the Albstadt Shear Zone can only be studied by its seismicity.To characterize the ASZ we continuously complement the earthquake catalog of the State Earthquake Service of Baden-W&#252;rttemberg with additional seismic phase onsets. For the latter we use the station network of AlpArray as well as 5 additional, in 2018/2019 installed seismic stations from the KArlsruhe BroadBand Array. We inverted for a new minimum 1D seismic velocity model of the study region. We use this seismic velocity model to relocalize the complemented catalog and to calculate focal mechanisms. The majority of the seismicity happens between the towns T&#252;bingen and Albstadt at around 9&#176;E in a depth range of about 1.5 to 16 km and aligns north-south. Additionally, we see a clustering of events at the towns Hechingen and Albstadt. The dominating focal mechanism is strike-slip, but we also observe minor components of normal and reverse faulting. Our results image the ASZ by its mainly micro-seismic activity between 2011 and 2018 confirming the N-S striking character, but also indicating a more complex fault system.We thank the State Earthquake Service in Freiburg for using their data (Az. 4784//18_3303). &#160;

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Tomographic determination of three‐dimensional seismic velocity structure using well logs, vertical seismic profiles, and surface seismic data

Geophysics ◽

10.1190/1.1442374 ◽

1987 ◽

Vol 52 (8) ◽

pp. 1085-1098 ◽

Cited By ~ 37

Author(s):

Stephen K. L. Chiu ◽

Robert R. Stewart

Keyword(s):

Velocity Structure ◽

Seismic Velocity ◽

Random Noise ◽

Three Dimensional ◽

Real Data ◽

Well Logs ◽

Seismic Profiles ◽

Vertical Seismic Profiles ◽

The Earth ◽

Vsp Data

A tomographic technique (traveltime inversion) has been developed to obtain a two‐ or three‐dimensional velocity structure of the subsurface from well logs, vertical seismic profiles (VSP), and surface seismic measurements. The earth was modeled by continuous curved interfaces (polynomial or sinusoidal series), separating regions of constant velocity or transversely isotropic velocity. Ray tracing for each seismic source‐receiver pair was performed by solving a system of nonlinear equations which satisfy the generalized Snell’s law. Surface‐to‐borehole and surface‐to‐surface rays were included. A damped least‐squares formulation provided the updating of the earth model by minimizing the difference between the traveltimes picked from the real data and calculated traveltimes. Synthetic results indicated the following conclusions. For noise‐free cases, the inversion converged closely from the initial guess to the true model for either surface or VSP data. Adding random noise to the observations and performing the inversion indicated that (1) using surface data alone allows reconstruction of the broad velocity structure but with some inaccuracy; (2) using VSP data alone gives a very accurate but laterally limited velocity structure; and (3) the integration of both data sets produces a more laterally extensive, accurate image of the subsurface. Finally, a field example illustrates the viability of the method to construct a velocity structure from real data.

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Seismic velocity model of the crust in the northern Canadian Cordillera from Rayleigh wave dispersion data

Canadian Journal of Earth Sciences ◽

10.1139/cjes-2016-0115 ◽

2017 ◽

Vol 54 (2) ◽

pp. 163-172 ◽

Cited By ~ 2

Author(s):

Shutian Ma ◽

Pascal Audet

Keyword(s):

Rayleigh Wave ◽

Velocity Structure ◽

Seismic Velocity ◽

Group Velocity Dispersion ◽

Velocity Model ◽

Canadian Cordillera ◽

Seismic Velocities ◽

Crustal Layer ◽

Velocity Models ◽

Moderate Earthquake

Models of the seismic velocity structure of the crust in the seismically active northern Canadian Cordillera remain poorly constrained, despite their importance in the accurate location and characterization of regional earthquakes. On 29 August 2014, a moderate earthquake with magnitude 5.0, which generated high-quality Rayleigh wave data, occurred in the Northwest Territories, Canada, ∼100 km to the east of the Cordilleran Deformation Front. We carefully selected 23 seismic stations that recorded the Rayleigh waves and divided them into 13 groups according to the azimuth angle between the earthquake and the stations; these groups mostly sample the Cordillera. In each group, we measured Rayleigh wave group velocity dispersion, which we inverted for one-dimensional shear-wave velocity models of the crust. We thus obtained 13 models that consistently show low seismic velocities with respect to reference models, with a slow upper and lower crust surrounding a relatively fast mid crustal layer. The average of the 13 models is consistent with receiver function data in the central portion of the Cordillera. Finally, we compared earthquake locations determined by the Geological Survey of Canada using a simple homogenous crust over a mantle half space with those estimated using the new crustal velocity model, and show that estimates can differ by as much as 10 km.

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Seismic velocity structure of the Queen Charlotte Basin beneath Hecate Strait

Canadian Journal of Earth Sciences ◽

10.1139/e93-065 ◽

1993 ◽

Vol 30 (4) ◽

pp. 787-805 ◽

Cited By ~ 19

Author(s):

G. D. Spence ◽

I. Asudeh

Keyword(s):

Lower Crust ◽

Constant Velocity ◽

Velocity Structure ◽

Seismic Velocity ◽

Moho Depth ◽

Moderate Increase ◽

Central Basin ◽

Velocity Models ◽

Brittle Ductile Transition ◽

Numerical Predictions

Seismic refraction data across Hecate Strait in the northern Queen Charlotte Basin were collected in a coincident reflection and refraction survey. Crustal velocity models provide a framework to help understand the formation of the sedimentary basin and the processes occurring near the Queen Charlotte Fault, a major ocean–continent transform fault. Beneath the sediments, which have a maximum thickness of 6 km, a velocity gradient extends from about 5 to 8 km depth, within which velocities increase typically from 6.3 to 6.4 km∙s−1. A thick constant-velocity region was found down to a depth varying from 14 to 22 km, with the smallest depths located beneath the central basin. The base of the constant-velocity layer was marked by a distinct mid-crustal interface, across which velocities increased from 6.4–6.5 km∙s−1 to approximately 6.8–6.9 km∙s−1. Moho was interpreted to be at a near-uniform depth of 26–28 km beneath Hecate Strait and the eastern Queen Charlotte Islands. The associated variation in crustal thickness beneath the basin implies crustal thinning, perhaps caused by extension, of 30% or more.The mid-crustal interface may mark the change to a more mafic and perhaps ductile lower crust. The interface appears to be about 1–4 km deeper than the brittle–ductile transition, as indicated by the estimated depth to the 450 °C isotherm and by the moderate increase in reflectivity on the seismic reflection sections. Ductile flow may also occur in the lower crust near the Queen Charlotte Fault, where the relative motion of the oceanic plate induces lithospheric flow and thinning beneath both the ocean and the continent. The observed decrease in Moho depth from 28 to 21 km near the fault is consistent with recent (1989) numerical predictions of I. Reid for lithospheric flow near ocean–continent transforms.

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One dimensional seismic velocity structure beneath Western Nepal

Journal of Nepal Geological Society ◽

10.3126/jngs.v27i0.31981 ◽

2002 ◽

Vol 27 ◽

Author(s):

S. Rajaure

Keyword(s):

Arrival Time ◽

Velocity Structure ◽

Seismic Velocity ◽

Velocity Model ◽

Compressional Wave ◽

S Wave ◽

Time Data ◽

Seismic Velocity Structure ◽

One Dimensional ◽

Wave Velocities

An attempt has been made to study the velocity structure of western Nepal. Arrival time data of local earthquakes occurring in that region were used to derive the model. A three layered velocity model both for the P- as well as S-wave velocity has been estimated. The compressional wave velocities in the first, second and the third layers have been estimated to be 5.53 km/sec. 6.29 km/sec and 8.13 km/sec respectively. Similarly the corresponding S-wave velocities are 3.18, 3.62, 4.66 km/sec respectively. The model for the Western Nepal and that for the Centre-East Nepal are almost same. The crust and the mantle beneath west and center-east are homogeneous.

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