Earthquake relocation in the wester termination of the North Aegean Trough

The area of North Aegean Trough exhibits complex tectonic characteristics as a consequence of the presence of complicated active structures. Exploitation of accurately determined earthquake data considerably contributes in the investigation of these structures and such accuracy is seeking through certain procedures. The determination of focal parameters of earthquakes that occurred in this area during 1964-2003 was performed by collecting all the available data for Ρ and S arrivals. After selecting the best solutions from an initial hypocentral location, 739 earthquakes were found that fulfilled certain criteria for the accuracy and used for further processing. The study area was divided in 16 sub regions and by the use of the HYPOINVERSE computer program, the travel time curves were constructed, and were used to define the velocity models for each one of them. For each sub region the time delays were calculated and used as time corrections in the arrival times of the seismic waves. The Vp/Vs ratio, necessary for S—wave velocity models, was calculated with two different methods and was found equal to 1.76. The velocity models and the time delays were used to relocate the events of the whole data set. The relocation resulted in significant improvement of the accuracy in the focal parameters determination.

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Velocity changes across the Ganos segment of the North Anatolian Fault Zone in NW Turkey from systematic variations in body wave arrival times

10.5194/egusphere-egu21-8152 ◽

2021 ◽

Author(s):

Esref Yalcinkaya ◽

Marco Bohnhoff ◽

Patricia Martinez-Garzon ◽

Ethem Görgün ◽

Ali Pınar ◽

...

Keyword(s):

Body Wave ◽

Waveform Analysis ◽

Marmara Region ◽

Rupture Directivity ◽

Data Set ◽

High Resolution Data ◽

Arrival Times ◽

The North ◽

Nw Turkey ◽

Geological Units

Imaging and characterizing transform fault sections that are capable to produce large earthquakes is crucial for evaluating seismic hazard and subsequent risk for nearby population centers. The Marmara Fault near the megacity of Istanbul is one of the best defined seismic gaps in the world and its complexity is captured by seismological, geodetic and geological data. A local dense seismic array (MONGAN) provides a high resolution data set allowing to image the Ganos fault separating two different geological units in the western Marmara region. First results of the waveform analysis from this array present systematic early-phase arrivals at the seismic stations located on the northern block of the Ganos fault which comprises geological units including older and more compact materials than that of the southern block. This difference in the arrival times causes the earthquake epicenters to shift further north than the real locations. In this preliminary results, the early-arrivals will be evaluated according to source azimuths and distances, and possible earth models and wave paths will be discussed. The results have implications for rupture directivity during future earthquakes as input for hazard and risk models for the Marmara region.

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Shear‐wave velocity and density estimation from PS-wave AVO analysis: Application to an OBS dataset from the North Sea

Geophysics ◽

10.1190/1.1444833 ◽

2000 ◽

Vol 65 (5) ◽

pp. 1446-1454 ◽

Cited By ~ 33

Author(s):

Side Jin ◽

G. Cambois ◽

C. Vuillermoz

Keyword(s):

Wave Velocity ◽

North Sea ◽

Random Noise ◽

P Wave ◽

S Wave ◽

Data Set ◽

Avo Analysis ◽

The North ◽

Avo Inversion ◽

The North Sea

S-wave velocity and density information is crucial for hydrocarbon detection, because they help in the discrimination of pore filling fluids. Unfortunately, these two parameters cannot be accurately resolved from conventional P-wave marine data. Recent developments in ocean‐bottom seismic (OBS) technology make it possible to acquire high quality S-wave data in marine environments. The use of (S)-waves for amplitude variation with offset (AVO) analysis can give better estimates of S-wave velocity and density contrasts. Like P-wave AVO, S-wave AVO is sensitive to various types of noise. We investigate numerically and analytically the sensitivity of AVO inversion to random noise and errors in angles of incidence. Synthetic examples show that random noise and angle errors can strongly bias the parameter estimation. The use of singular value decomposition offers a simple stabilization scheme to solve for the elastic parameters. The AVO inversion is applied to an OBS data set from the North Sea. Special prestack processing techniques are required for the success of S-wave AVO inversion. The derived S-wave velocity and density contrasts help in detecting the fluid contacts and delineating the extent of the reservoir sand.

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Lithospheric structure models applied for locating the Romanian seismic events

Annals of Geophysics ◽

10.4401/ag-4215 ◽

1994 ◽

Vol 37 (3) ◽

Author(s):

M. Rizescu ◽

E. Popescu ◽

V. Oancea ◽

D. Enescu

Keyword(s):

Constant Velocity ◽

Seismic Waves ◽

Seismic Events ◽

Lithospheric Structure ◽

Structural Units ◽

Data Set ◽

Velocity Models ◽

Seismic Sequences ◽

Geological Information ◽

Propagation Velocities

The paper presents our attempts made for improving the locations obtained for local seismic events, using refined lithospheric structure models. The location program (based on Geiger method) supposes a known model. The program is run for some seismic sequences which occurred in different regions, on the Romanian territory, using for each of the sequences three velocity models: 1) 7 layers of constant velocity of seismic waves, as an average structure of the lithosphere for the whole territory; 2) site dependent structure (below each station), based on geophysical and geological information on the crust; 3) curves deseribing the dependence of propagation velocities with depth in the lithosphere, characterizing the 7 structural units delineated on the Romanian territory. The results obtained using the different velocity models are compared. Station corrections are computed for each data set. Finally, the locations determined for some quarry blasts are compared with the real ones.

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Azimuth and slowness anomalies of seismic waves measured on the central California seismographic array. Part I. Observations

Bulletin of the Seismological Society of America ◽

10.1785/bssa0560010223 ◽

1966 ◽

Vol 56 (1) ◽

pp. 223-239 ◽

Cited By ~ 10

Author(s):

Michio Otsuka

Keyword(s):

Upper Mantle ◽

Measurement Errors ◽

Seismic Waves ◽

Crust And Upper Mantle ◽

Arrival Times ◽

Central California ◽

The North ◽

Upper Mantle Structure ◽

Coast Ranges ◽

The University

abstract Arrays of seismographs are usually considered to be detectors which give enhanced signals from distant earthquakes. They also provide, however, a new way of learning more about the structure of the crust and upper mantle. The deviation of the seismic-wave surface from its expected configuration may be regarded as a consequence of non-homogeneous and anisotropic conditions in the earth. The operations of the University of California network of telemetry stations in the Coast Ranges of California provides an opportunity to discover the practicality of this approach. The situation of this network near the continental margin gives the study particular interest. The differences in arrival-times between array elements of coherent peaks or troughs of P and pP phases from 28 teleseisms in the period of 1963-1964 were read from the telemetry records of the central California seismographic array. The direction of approach and velocities of the wave fronts were then determined and compared with the great circle azimuths and with the apparent velocities calculated from the Jeffreys-Bullen tables. The observed anomalies in direction of approach and apparent velocites are found to be cyclic functions of the direction of the source. The amplitudes of these functions are almost 10 degrees in azimuth anomaly and 1.0 sec/deg in slowness anomaly. Error analyses show that the anomaly functions cannot be attributed to the measurement errors. The derived anomaly functions provide a powerful means of examining crustal and upper mantle structure under the array and perhaps at the source. Variations between subsets of the array indicate significant differences in structure between portions of the Coast Ranges to the north and to the south of Hollister.

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Modeling guided waves to constrain the velocity structure of the oceanic crust in the subduction zone of eastern Alaska

10.5194/egusphere-egu2020-3268 ◽

2020 ◽

Author(s):

Xiaoyu Guan ◽

Yuanze Zhou ◽

Takashi Furumura

Keyword(s):

Subduction Zone ◽

Oceanic Crust ◽

High Frequency ◽

Subduction Zones ◽

Guided Waves ◽

Velocity Structure ◽

Guided Wave ◽

S Wave ◽

Arrival Times ◽

Velocity Models

Fitting subduction zone guided waves with synthetics is an ideal choice for studying the velocity structure of the oceanic crust. After an earthquake occurs in subduction zones, seismic waves can be trapped in the low-velocity oceanic crust and propagated as guided waves. The arrival time and frequency characteristics of the guided waves can be used to image the velocity structure of the oceanic crust. The analysis and modeling based on guided wave observations provide a rare opportunity to understand the velocity structure of the oceanic crust and the variations in oceanic crustal materials during the subduction process.High-frequency guided waves have been observed in the subduction zone of eastern Alaska. On several sections, observed seismograms recorded by seismic stations show low-frequency (<2Hz) onsets ahead of the main high-frequency (>2Hz) guided waves. Differences in the arrival times and dispersion characteristics of seismic phases are related to the velocity structure of the oceanic crust, and the characteristics of coda waves are related to the distribution of elongated scatters in the oceanic crust. Through fitting the observed broadband waveforms and synthetics modeled with the 2-D FDM (Finite Difference Method), we obtain the preferred oceanic crustal velocity models for several sections in the subduction zone of eastern Alaska. The preferred models can explain the seismic phase arrival times, dispersions, and coda characteristics in the observed waveforms. With the obtained P- and S- wave models of velocity structures on several sections, the material compositions they represent are deduced, and the variations of oceanic crustal materials during subducting can be understood. This provides new evidence for studying the details of the subduction process in the subduction zone of eastern Alaska.

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Recent advances in rock physics

Geophysics ◽

10.1190/1.1441879 ◽

1985 ◽

Vol 50 (12) ◽

pp. 2480-2491 ◽

Cited By ~ 26

Author(s):

David P. Yale

Keyword(s):

Electrical Properties ◽

Seismic Waves ◽

Seismic Velocity ◽

Pore Space ◽

Rock Physics ◽

Rock Properties ◽

Bright Spot ◽

Geometrical Parameters ◽

S Wave ◽

Velocity Models

The need to extract more information about the subsurface from geophysical and petrophysical measurements has led to a great interest in the study of the effect of rock and fluid properties on geophysical and petrophysical measurements. Rock physics research in the last few years has been concerned with studying the effect of lithology, fluids, pore geometry, and fractures on velocity; the mechanisms of attenuation of seismic waves; the effect of anisotropy; and the electrical and dielectric properties of rocks. Understanding the interrelationships between rock properties and their expression in geophysical and petrophysical data is necessary to integrate geophysical, petrophysical, and engineering data for the enhanced exploration and characterization of petroleum reservoirs. The use of amplitude offsets, S‐wave seismic data, and full‐waveform sonic data will help in the discrimination of lithology. The effect of in situ temperatures and pressures must be taken into account, especially in fractured and unconsolidated reservoirs. Fluids have a strong effect on seismic velocities, through their compressibility, density, and chemical effects on grain and clay surfaces. S‐wave measurements should help in bright spot analysis for gas reservoirs, but theoretical considerations still show that a deep, consolidated reservoir will not have any appreciable impedance contrast due to gas. The attenuation of seismic waves has received a great deal of attention recently. The idea that Q is independent of frequency has been challenged by experimental and theoretical findings of large peaks in attenuation in the low kHz and hundreds of kHz regions. The attenuation is thought to be due to fluid‐flow mechanisms and theories suggest that there may be large attenuation due to small amounts of gas in the pore space even at seismic frequencies. Models of the effect of pores, cracks, and fractures on seismic velocity have also been studied. The thin‐crack velocity models appear to be better suited for representing fractures than pores. The anisotropy of seismic waves, especially the splitting of polarized S‐waves, may be diagnostic of sets of oriented fractures in the crust. The electrical properties of rocks are strongly dependent upon the frequency of the energy and logging is presently being done at various frequencies. The effects of frequency, fluid salinity, clays, and pore‐grain geometry on electrical properties have been studied. Models of porous media have been used extensively to study the electrical and elastic properties of rocks. There has been great interest in extracting geometrical parameters about the rock and pore space directly from microscopic observation. Other models have focused on modeling several different properties to find relationships between rock properties.

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The impact of common‐offset migration on porosity estimation by AVO inversion

Geophysics ◽

10.1190/1.1444965 ◽

2001 ◽

Vol 66 (3) ◽

pp. 755-762 ◽

Cited By ~ 9

Author(s):

Arild Buland ◽

Martin Landrø

Keyword(s):

P Wave ◽

High Porosity ◽

S Wave ◽

Seismic Line ◽

Data Set ◽

The North ◽

Avo Inversion ◽

Time Migration ◽

The Impact ◽

Porosity Estimation

The impact of prestack time migration on porosity estimation has been tested on a 2-D seismic line from the Valhall/Hod area in the North Sea. Porosity is estimated in the Cretaceous chalk section in a two‐step procedure. First, P-wave and S-wave velocity and density are estimated by amplitude variation with offset (AVO) inversion. These parameters are then linked to porosity through a petrophysical rock data base based on core plug analysis. The porosity is estimated both from unmigrated and prestack migrated seismic data. For the migrated data set, a standard prestack Kirchhoff time migration is used, followed by simple angle and amplitude corrections. Compared to modern high‐cost, true amplitude migration methods, this approach is faster and more practical. The test line is structurally fairly simple, with a maximum dip of 5°; but the results differ significantly, depending on whether migration is applied prior to the inversion. The maximum difference in estimated porosity is of the order of 10% (about 50% relative change). High‐porosity zones estimated from the unmigrated data were not present on the porosity section estimated from the migrated data.

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Full-waveform matching of VP and VS models from surface waves

Geophysical Journal International ◽

10.1093/gji/ggz279 ◽

2019 ◽

Vol 218 (3) ◽

pp. 1873-1891 ◽

Cited By ~ 2

Author(s):

Farbod Khosro Anjom ◽

Daniela Teodor ◽

Cesare Comina ◽

Romain Brossier ◽

Jean Virieux ◽

...

Keyword(s):

Surface Waves ◽

Wave Velocity ◽

Real Data ◽

Test Site ◽

P Wave ◽

S Wave ◽

Surface Wave Dispersion ◽

Data Set ◽

Near Surface ◽

Velocity Models

SUMMARY The analysis of surface wave dispersion curves (DCs) is widely used for near-surface S-wave velocity (VS) reconstruction. However, a comprehensive characterization of the near-surface requires also the estimation of P-wave velocity (VP). We focus on the estimation of both VS and VP models from surface waves using a direct data transform approach. We estimate a relationship between the wavelength of the fundamental mode of surface waves and the investigation depth and we use it to directly transform the DCs into VS and VP models in laterally varying sites. We apply the workflow to a real data set acquired on a known test site. The accuracy of such reconstruction is validated by a waveform comparison between field data and synthetic data obtained by performing elastic numerical simulations on the estimated VP and VS models. The uncertainties on the estimated velocity models are also computed.

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A site effect study in Acapulco, Guerrero, Mexico: Comparison of results from strong-motion and microtremor data

Bulletin of the Seismological Society of America ◽

10.1785/bssa0820020642 ◽

1992 ◽

Vol 82 (2) ◽

pp. 642-659 ◽

Cited By ~ 1

Author(s):

Carlos Gutierrez ◽

Shri Krishna Singh

Keyword(s):

Seismic Waves ◽

Site Response ◽

Nonlinear Behavior ◽

Strong Motion ◽

Site Effect ◽

Seismic Gap ◽

S Wave ◽

Data Set ◽

S Waves ◽

The City

Abstract The city of Acapulco is located near or above the mature seismic gap of Guerrero along the Mexican subduction zone. With the purpose of studying the character of strong ground motion on soft sites, four digital accelerographs have been installed in the city on such sites. These instruments have been in operation since 1988. Two additional instruments, part of the Guerrero Accelerograph Array, are located on hard sites in the area. One of these, VNTA, has been in operation since 1985 and the other, ACAN, since 1989. These stations have recorded several earthquakes. We use data from eight events (4.2 ≤ M ≤ 6.9) to study spectral amplification of seismic waves at the soft sites with respect to VNTA. The S waves are amplified by a factor of 6 to 25 at the soft sites in a fairly broad range of frequencies; both the amplification and the frequency band over which it occurs depend upon the site. Although the largest earthquake in our data set (M = 6.9) gave rise to a peak horizontal acceleration exceeding 0.3 g at one of the soft sites, no clear evidence of nonlinear behavior of the subsoil is found. Spectral amplifications of S-wave coda are very similar to those of S waves. We also measured microtremors at the strong-motion sites. The microtremor spectra were interpreted, using reasonable assumptions, to test the feasibility of this technique in reproducing the spectral amplifications observed during earthquakes. Our results show that only a rough estimate of site response can be obtained from this technique, at least in Acapulco; caution is warranted in its use elsewhere.

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SEISMOLOGICAL AND ENGINEERING PARAMETERS OF 24 and 26 SEPTEMBER, 2019 MARMARA SEA EARTHQUAKES

10.5194/egusphere-egu2020-22107 ◽

2020 ◽

Author(s):

Eser Çakti ◽

Fatma Sevil Malcioğlu ◽

Hakan Süleyman

Keyword(s):

Earthquake Engineering ◽

Prediction Models ◽

Source Parameters ◽

Strong Motion ◽

P Wave ◽

Corner Frequency ◽

Marmara Sea ◽

S Wave ◽

Data Set ◽

The North

On 24th&#160;and 26th&#160; September 2019, two earthquakes of Mw=4.5 and Mw=5.6 respectively took place in the Marmara Sea. They were associated with the Central Marmara segment of the North Anatolian Fault Zone, which is pinpointed by several investigators as the most likely segment to rupture in the near future giving way to an earthquake larger than M7.0. Both events were felt widely in the region. The Mw=5.6 event, in particular, led to a number of building damages in Istanbul, which were larger than expected in number and severity. There are several strong motion networks in operation in and around Istanbul. We have compiled a data set of recordings obtained at the stations of the Istanbul Earthquake Rapid Response and Early Warning operated by the Department of Earthquake Engineering of Bogazici University and of the National Strong Motion Network operated by AFAD. It consists of 148 three component recordings, in total. &#160;444 records in the data set, after correction, were analyzed to estimate the source parameters of these events, such as corner frequency, source duration, radius and rupture area, average source dislocation and stress drop. Duration characteristics of two earthquakes were analyzed first by considering P-wave and S-wave onsets and then, focusing on S-wave and significant durations. PGAs, PGVs and SAs were calculated and compared with three commonly used ground motion prediction models (i.e &#160;Boore et al., 2014; Akkar et al., 2014 and Kale et al., 2015). Finally frequency-dependent Q models were estimated using the data set and their validity was dicussed by comparing with previously developed models.

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