Seismic Velocity Distribution Research Articles

Velocity sections of the upper mantle under the oceans

We constructed the models of the distribution of longitudinal seismic waves velocities for the upper mantle oceanic regions: mid-ocean ridges (MOR), basins, trenches, island arcs and coastal ridges, back-arc troughs (BAT). They are in line with the schemes of advection-polymorphic deepseated processes in tectonosphere. Under the island arcs and coastal ridges models meet the alpine geosyncline supplemented with recent activization. Under the MOR and BAT the models coincide. They correspond to the scheme of recent heat and mass transfer for rifting which on the mafic crust with continental thickness leads to oceanization. The basin model shows the result of smoothing of thermal anomalies under the MOR or BAT in about 50 100 million years. The trench model corresponds to the result of lateral warming-up of upper layers of the mantle of the inactive blocks from the island arc and basin

Lithospheric structure and faulting characteristics of the Helan Mountains and Yinchuan Basin: Results of deep seismic reflection profiling

The Helan Mountains and Yinchuan Basin (HM-YB) are located at the northern end of the North-South tectonic belt, and form an intraplate tectonic deformation zone in the western margin of the North China Craton (NCC). The HM-YB has a complicated history of formation and evolution, and is tectonically active at the present day. It has played a dominant role in the complex geological structure and modern earthquake activities of the region. A 135-km-long deep seismic reflection profile across the HM-YB was acquired in early 2014, which provides detailed information of the lithospheric structure and faulting characteristics from near-surface to various depths in the region. The results show that the Moho gradually deepens from east to west in the depth range of 40–48 km along the profile. Significant differences are present in the crustal structure of different tectonic units, including in the distribution of seismic velocities, depths of intra-crustal discontinuities and undulation pattern of the Moho. The deep seismic reflection profile further reveals distinct structural characteristics on the opposite sides of the Helan Mountains. To the east, The Yellow River fault, the eastern piedmont fault of the Helan Mountains, as well as multiple buried faults within the Yinchuan Basin are all normal faults and still active since the Quaternary. These faults have controlled the Cenozoic sedimentation of the basin, and display a “negative-flower” structure in the profile. To the west, the Bayanhaote fault and the western piedmont fault of the Helan Mountains are east-dipping thrust faults, which caused folding, thrusting, and structural deformation in the Mesozoic stratum of the Helan Mountains uplift zone. A deep-penetrating fault is identified in the western side of the Yinchuan Basin. It has a steep inclination cutting through the middle-lower crust and the Moho, and may be connected to the two groups of faults in the upper crust. This set of deep and shallow fault system consists of both strike-slip, thrust, and normal faults formed over different eras, and provides the key tectonic conditions for the basin-mountains coupling, crustal deformation and crust-mantle interactions in the region. The other important phenomenon revealed from the results of deep seismic reflection profiling is the presence of a strong upper mantle reflection (UMR) at a depth of 82–92 km beneath the HM-YB, indicating the existence of a rapid velocity variation or a velocity discontinuity in that depth range. This is possibly a sign of vertical structural inhomogeneity in the upper mantle of the region. The seismic results presented here provide new clues and observational bases for further study of the deep structure, structural differences among various blocks and the tectonic relationship between deep and shallow processes in the western NCC.

Hotspot volcanism in the southern South Atlantic: Geophysical constraints on the evolution of the southern Walvis Ridge and the Discovery Seamounts

The southern Atlantic hosts a variety of magmatic structures, namely the Walvis Ridge, the Discovery Seamounts and the Shona Ridge, which are believed to be related to the evolution/movement of hotspots. Although the basement of the Walvis Ridge has been sampled at different locations, geophysical data are too sparse to provide sufficient information about its deeper structure to compare it with other hotspot tracks. The Discovery Seamounts represent a completely different type feature in a way that it cannot be connected to any onshore volcanic feature. However, geological sampling of the volcanic basement indicates that the petrology of the Discovery track is very similar to Gough Island and the southern branch of Walvis Ridge. Both structures erupted into already existing seafloor and so have been seismically investigated to document how/if an associated thermal anomaly might have modified the underlying and surrounding oceanic crust. Seismic lines for both structures indicate rather normal seismic velocity distributions for oceanic crust. Both, the Walvis Ridge and the largest volcano of the Discovery Seamounts have a maximum thickness in our research area of ~13km. An interesting difference between these structures is a high velocity cone (>6km/s) at 2.4km depth in the central part of Discovery Seamount. This might indicate a primarily intrusional type of seamount such as has been reported for several similar structures. In contrast the Walvis Ridge velocity structure does not show evidences for a shallow intrusional cone, but seismic velocities typical for oceanic layer 3 at a more or less constant depth level along the entire profile. This might indicate that the ridge's present-day topography is built mainly by extrusive material.

Focal spot imaging based on zero lag cross‐correlation amplitude fields: Application to dense array data at the San Jacinto fault zone

AbstractWe image the subsurface below a dense seismic array straddling the Clark branch of the San Jacinto fault zone in Southern California. The analysis is based on focal spots of surface waves associated with the zero lag amplitudes of noise cross‐correlations computed between all stations of the dense array. Local medium properties are inferred from the spatially variable focal spot size and shape based on the first zero crossing of amplitude versus distance distributions. The method provides simultaneous estimates of wave speed, apparent attenuation, and anisotropy without solving a tomographic inverse problem. The obtained images of the frequency dependent seismic velocity distributions are consistent with independent estimates from a far‐field Rayleigh wave tomography. We observe an anticorrelation between our apparent attenuation coefficient and seismic velocity, and a fault‐parallel alignment of fast propagation directions with greater structural complexity to the southwest of the fault. The results imply a complex fault zone structure including a waveguide to the northeast of the fault that is continuous across the observed depth range and a low‐velocity structure to the southwest associated with a shallow sedimentary basin.

GEOPHYSICAL REGIONAL GRAVITY MAP of TURKEY and ITS GENERAL ASSESSMENT

In this study, the apparent gravity density data was measured for Turkey using apparent density filter, and the seismic velocity distribution map was generated from this data cluster. By interpreting these two measured data clusters, three dimensional structure of the Conrad discontinuity was investigated. The apparent gravity filter is a kind of filter which is used in the apparent gravity measurements for different depth levels different than gravity data. In this study, considering previously measured distribution of the continental crustal thickness of the Anatolia, the density maps for different depth levels were formed and interpreted. The lowest and highest densities in different levels of Turkey are 2.6 gr/cm 3 , and 3 gr/cm 3 , respectively; and the mean density is 2.698 gr/cm 3 . The lowest and highest seismic velocities for different thicknesses were measured as 5.13 km/sec and 6.86 km/sec, respectively. However; the mean seismic velocity of Turkey for depths increasing up to 10 km until MOHO discontinuity was estimated as 5.66 km/s. The density and seismic velocity in the first 20 km of the continental crust have reached its highest values as 2.74 gr/cm 3 and 5.86 km/sec, respectively. This zone is also the Conrad discontinuity between the lower and upper crusts, and its average depth is 16 km in Turkey. The Conrad discontinuity boundary, which developed between SIAL-SIMA, not to be observed in the East Anatolian High Plateau made us consider that SIMA had disappeared as a result of the geological evolution, and the available crust could only be SIAL in origin.

Landslide characterization using P- and S-wave seismic refraction tomography — The importance of elastic moduli

In the broad spectrum of natural hazards, landslides in particular are capable of changing the landscape and causing significant human and economic losses. Detailed site investigations form an important component in the landslide risk mitigation and disaster risk reduction process. These investigations usually rely on surface observations, discrete sampling of the subsurface, and laboratory testing to examine properties that are deemed representative of entire slopes. Often this requires extensive interpolations and results in large uncertainties. To compliment and extend these approaches, we present a study from an active landslide in a Lias Group clay slope, North Yorkshire, UK, examining combined P- and S-wave seismic refraction tomography (SRT) as a means of providing subsurface volumetric imaging of geotechnical proxies.The distributions of seismic wave velocities determined from SRT at the study site indicated zones with higher porosity and fissure density that are interpreted to represent the extent and depth of mass movements and weathered bedrock zones. Distinguishing the lithological units was facilitated by deriving the Poisson's ratio from the SRT data as saturated clay and partially saturated sandy silts showed distinctively different Poisson's ratios. Shear and Young's moduli derived from the SRT data revealed the weak nature of the materials in active parts of the landslide (i.e. 25kPa and 100kPa respectively). The SRT results are consistent with intrusive (i.e. cone penetration tests), laboratory, and additional geoelectrical data from this site. This study shows that SRT forms a cost-effective method that can significantly reduce uncertainties in the conceptual ground model of geotechnical and hydrological conditions that govern landslide dynamics.

Barren Acidic Soil Assessment using Seismic Refraction Survey

Seismic refraction method is one of the geophysics subsurface exploration techniques used to determine subsurface profile characteristics. From past experience, seismic refraction method is commonly used to detect soil layers, overburden, bedrock, etc. However, the application of this method on barren geomaterials remains limited due to several reasons. Hence, this study was performed to evaluate the subsurface profile characteristics of barren acidic soil located in Ayer Hitam, Batu Pahat, Johor using seismic refraction survey. The seismic refraction survey was conducted using ABEM Terraloc MK 8 (seismograph), a sledge hammer weighing 7 kg (source) and 24 units of 10 Hz geophones (receiver). Seismic data processing was performed using OPTIM software which consists of SeisOpt@picker (picking the first arrival and seismic configureuration data input) and SeisOpt@2D (generating 2D image of barren acidic soil based on seismic velocity (primary velocity, Vp) distribution). It was found that the barren acidic soil profile consists of three layers representing residual soil (Vp= 200-400 m/s) at 0-2 m, highly to completely weathered soil (Vp= 500-1800 m/s) at 3-8 m and shale (Vp= 2100-6200 m/s) at 9-20 m depth. Furthermore, result verification was successfully done through the correlation of seismic refraction data based on physical mapping and the geological map of the study area. Finally, it was found that the seismic refraction survey was applicable for subsurface profiling of barren acidic soil as it was very efficient in terms of time, cost, large data coverage and sustainable.

Seismic velocity estimation from post-critical wide-angle reflections in layered structures

The travel time inversion of wide-angle seismic data is a technique commonly used in the deep seismic sounding. We propose an application of this technique to a smaller scale of a sedimentary layer, where the characteristics of seismic observations changes significantly. Field observations confirmed by synthetic analysis recognize the dominant amplitudes of wide-angle post-critical reflections. A case study is presented in this paper, of a joint interpretation of conventional reflection seismic with reflection imaging, combined with the wide-angle travel time inversion of additional full-spread observations. A joint interpretation results in a precise recognition of the seismic velocity distribution, that is further used for the seismic depth conversion with the uncertainty analysis of the depth of the reflecting horizons. Despite the salt layer in the studied structure this method is able to precisely recognize the seismic velocities of the sub-salt structures.

The role of continental margins in the final stages of arc formation: Constraints from teleseismic tomography of the Gibraltar and Calabrian Arc (Western Mediterranean)

The deep seismicity and lateral distribution of seismic velocity in the Central Western Mediterranean, point to the existence under the Alboran and Tyrrhenian Seas of two lithospheric slabs reaching the mantle transition zone. Gibraltar and Calabrian narrow arcs correspond to the slabs. Similarities in the tectonic and mantle structure of the two areas have been explained by a common subduction and roll-back mechanism, in which the two arcs are symmetrical end members.We present a new 3-D tomographic model at mantle scale for the Calabrian Arc and compare it with a recently published model for the Gibraltar Arc by Monna et al. (2013a). The two models, calculated with inversion of teleseismic phase arrivals, have a scale and parametrization that allow for a direct comparison. The inclusion in both inversions of ocean bottom seismometer broadband data improves the resolution of the areas underlying the seafloor networks. This additional information is used to resolve the deep structure and constrain the reconstruction of the Central Western Mediterranean geodynamic evolution. The Gibraltar tomography model suggests that the slab is separated from the Atlantic oceanic domain by a portion of African continental margin, whereas the Calabrian model displays a continuous oceanic slab that is connected, via a narrow passage (~350km), to the Ionian basin oceanic domain. Starting from the comparison of the two models we propose the following interpretation: within the Mediterranean geodynamic regime (dominated by slab rollback) the geometry of the African continental margin, located on the lower plate, represents a critical control on the evolution of subduction. As buoyant continental lithosphere entered the subduction zones, slab pull caused tears in the subducted lithosphere. This tectonic response, which occurred in the final stages of arc evolution and was strongly controlled by the paleogeography of the subducted plates, explains the observed differences between the Gibraltar and Calabrian Arcs.

A new dawn for Australian ocean-bottom seismography

The Australian government recently acquired 20 state-of-the-art broadband ocean-bottom seismograph (OBS) units. These instruments were designed and built by Güralp Systems Ltd. (U. K.) with the ability to withstand extreme ocean depths to 6000 m and to remotely record data for as long as 12 months. The Australian national OBS fleet is part of AuScope's Australian Geophysical Observing System (AGOS) — an initiative of the Australian government funded through the Education Investment Fund (EIF). These instruments will contribute greatly to understanding of the crust beneath oceanic basins surrounding Australia. In 2014–2015, the Australian national OBS fleet was used by the petroleum industry on several seismic surveys. High-quality data were recorded at all OBS deployment sites, often to offsets sufficiently large enough to detect possible Pn phases — refractions from the upper mantle. Such information means that crustal thickness as well as the seismic-velocity distribution to the Moho can be determined. As a result, these data can provide important constraint for subsidence and hydrocarbon-maturation modeling. Analysis of earthquake data recorded during marine seismic surveys is ongoing and should provide new information on the interaction between anthropogenic signals (air-gun sources, vessel noise) and the natural environment. Recording earthquake and air-gun signals at fixed locations opens a completely new possibility for calibration and comparison of those signal strengths and spectral compositions. Access to the OBS fleet is open to industry, academia, and government organizations. Application to use Australia's OBS fleet is via a simple process through ANSIR ( www.ansir.org.au ).

Seismic monitoring of CO2 plume growth, evolution and migration in a heterogeneous reservoir: Role, impact and importance of patchy saturation

We combine reservoir simulation with 2D synthetic seismic reflection time-lapse data to assess the ability of seismic methods to image plume growth, evolution, and migration within a heterogeneous saline reservoir. The incorporation of reservoir heterogeneity results in a range of saturations due to the tortuous migration around the intra-reservoir baffles. To account for the disruptive nature of the injected CO2, and the uncertainties regarding the fluid saturation distribution, we use two end-member models, uniform and patchy, to generate the widest range of seismic velocity distributions to understand the range of velocity-saturation behaviour which could be encountered. The generated seismic sections show clear differences between the two models while also providing confidence in the ability to detect CO2 plume growth and evolution in the reservoir. A free-phase migrating front of CO2 appears to be difficult to detect, however. The ability to image a front is shown to be dependent not only on the pore-fluid saturation distribution – patchy or uniform – but also on its larger-scale spatial geometry. As the subtle change in amplitude is directly related to the concentration of CO2 within each accumulation, it suggests that the saturation model has important implications for CO2 detectability and for quantifying the volume of CO2 injected into the reservoir.

Significance of Geological Units of the Bohemian Massif, Czech Republic, as Seen by Ambient Noise Interferometry

Broadband recordings of 88 seismic stations distributed in the Bohemian Massif, Czech Republic, and covering the time period of up to 12 years were processed by a cross-correlation technique. All correlograms were analyzed by a novel approach to get both group and phase dispersion of Rayleigh and Love waves. Individual dispersion curves were averaged in five distinct geological units which constitute the Bohemian Massif (Saxothuringian, Tepla-Barrandean, Sudetes, Moravo-Silesian, and Moldanubian). Estimated error of the averaged dispersion curves are by an order smaller than the inherent variability due to the 3D distribution of seismic velocities within the units. The averaged dispersion data were inverted for 1D layered velocity models including their uncertainty, which are characteristic for each of the geological unit. We found that, overall, the differences between the inverted velocity models are of similar order as the variability inside the geological units, suggesting that the geological specification of the units is not fully reflected into the S-wave propagation velocities on a regional scale. Nevertheless, careful treatment of the dispersion data allowed us to identify some robust characteristics of the area. The vp to vs ratio is anomalously low (~1.6) for all the units. The Moldanubian is the most rigid and most homogeneous part of the Bohemian Massif. Middle crust in the depth range of ~3–15 km is relatively homogeneous across the investigated region, while both uppermost horizon (0–3 km) and lower crust (>15 km) exhibit lower degree of homogeneity.

Deep crustal structure across a young passive margin from wide-angle and reflection seismic data (The SARDINIA Experiment) – II. Sardinia’s margin

Abstract Geophysical data acquired on the conjugate margins system of the Gulf of Lion and West Sardinia (GLWS) is unique in its ability to address fundamental questions about rifting (i.e. crustal thinning, the nature of the continent-ocean transition zone, the style of rifting and subsequent evolution, and the connection between deep and surface processes). While the Gulf of Lion (GoL) was the site of several deep seismic experiments, which occurred before the SARDINIA Experiment (ESP and ECORS Experiments in 1981 and 1988 respectively), the crustal structure of the West Sardinia margin remains unknown. This paper describes the first modeling of wide-angle and near-vertical reflection multi-channel seismic (MCS) profiles crossing the West Sardinia margin, in the Mediterranean Sea. The profiles were acquired, together with the exact conjugate of the profiles crossing the GoL, during the SARDINIA experiment in December 2006 with the French R/V L’Atalante. Forward wide-angle modeling of both data sets (wide-angle and multi-channel seismic) confirms that the margin is characterized by three distinct domains following the onshore unthinned, 26 km-thick continental crust : Domain V, where the crust thins from ~26 to 6 km in a width of about 75 km; Domain IV where the basement is characterized by high velocity gradients and lower crustal seismic velocities from 6.8 to 7.25 km/s, which are atypical for either crustal or upper mantle material, and Domain III composed of “atypical” oceanic crust. The structure observed on the West Sardinian margin presents a distribution of seismic velocities that is symmetrical with those observed on the Gulf of Lion’s side, except for the dimension of each domain and with respect to the initiation of seafloor spreading. This result does not support the hypothesis of simple shear mechanism operating along a lithospheric detachment during the formation of the Liguro-Provencal basin.

Seismic Wave Velocities in Deep Sediments in Poland: Borehole and Refraction Data Compilation

Sedimentary cover has significant influence on seismic wave travel times and knowing its structure is of great importance for studying deeper structures of the Earth. Seismic tomography is one of the methods that require good knowledge of seismic velocities in sediments and unfortunately by itself cannot provide detailed information about distribution of seismic velocities in sedimentary cover. This paper presents results of P-wave velocity analysis in the old Paleozoic sediments in area of Polish Lowland, Folded Area, and all sediments in complicated area of the Carpathian Mountains in Poland. Due to location on conjunction of three major tectonic units — the Precambrian East European Craton, the Paleozoic Platform of Central and Western Europe, and the Alpine orogen represented by the Carpathian Mountains the maximum depth of these sediments reaches up to 25 000 m in the Carpathian Mountains. Seismic velocities based on 492 deep boreholes with vertical seismic profiling and a total of 741 vertical seismic profiles taken from 29 seismic refraction profiles are analyzed separately for 14 geologically different units. For each unit, velocity versus depth relations are approximated by second or third order polynomials.

Geophysical-petrological modeling of the lithosphere beneath the Cantabrian Mountains and the North-Iberian margin: geodynamic implications

Cenozoic contractional deformation in the North-Iberian continental margin (southern Bay of Biscay) led to the uplift of the Cantabrian Mountains and the northward subduction of part of the thick continental crust, down to at least ~55km depth beneath the coastline, and perhaps even ~30–40km deeper. In order to provide a more constrained model of this unique structure and gain insight into the factors controlling its evolution, we performed an integrated geophysical-petrological modeling of the lithosphere along a 470km-long, N-S transect down to 400km depth. The methodology used allows for fitting gravity anomalies, geoid undulations, surface heat flow, elevation and seismic velocities with a realistic distribution of densities and seismic velocities in the mantle and the subducting lower crust, which are dependent on chemical composition, pressure and temperature. Two models are presented, with variable maximum depth for the crustal root: 60km (Model A) and 90km (Model B). Results indicate that both models are feasible from the geophysical point of view, but the shallower root agrees slightly better with tomographic results. The thickness of the thermal lithosphere in Model A varies from 125–145km south of the Cantabrian Mountains to 170km beneath the crustal root and 135–140km beneath the central part of the Bay of Biscay. Model B requires a thicker thermal lithosphere beneath the crustal root (205km). Low seismic velocities beneath the Bay of Biscay Moho and in the mantle wedge above the crustal root are explained by the addition of 1–2wt% of water. Input from dehydration reactions in the subducting lower crust is ruled out in Model A and has a very minor influence in Model B. We therefore interpret the water to have percolated from the seafloor during the formation of the margin in the Mesozoic. A later basaltic underplating was also inferred. A tentative evolutionary model (to a great extent governed by these petrological processes) is proposed, implying a minimum shortening close to 100km from the Latest Cretaceous to the present.

Subsurface modeling of mud volcanoes, using density model and analysis of seismic velocity

Detection of subsurface structures by means of gravity method can be used to determine mass distribution and density contrast of rock units. This distribution could be detected by different geophysical methods, especially gravity method. However, gravity techniques have some drawbacks and can't be always successful in distinguishing subsurface structures. Performance of the gravity technique could be further improved by simultaneous combination and introducing additional information from other geophysical data. This study used existing relation between seismic and gravity methods to better clarify subsurface structures. This relationship relates mass distribution of the medium to velocity of wave propagation in that media. This method was applied on an area that consists of three mud volcanoes. After completion of the primary model by forward modeling, mass distribution and analysis of seismic velocity were provided on a 2-D profile. Bouguer anomaly map of gravity data of the area was obtained and negative anomalies were identified. These negative anomalies could be related to the existence of mud volcanoes. A 2-D seismic line was also acquired over the greatest mud volcano, as additional information for direct modeling. The Gardner equation was used for further velocity estimation by density values. This velocity model also compared with seismic velocity analysis for evaluation. The final results indicated that density modeling and the use of seismic velocity model increases the resolution of subsurface structures imaging. Separation of subsurface layers was implemented correctly in the velocity model resulting from gravity data and subsurface discontinuities of the area that become more obvious by this technique.

Evolution of a block cave from time-lapse passive source body-wave traveltime tomography

Most underground mines are equipped with microseismic monitoring systems that allow the detection, location, and characterization of microseismic events. Microseismic events can be exploited to understand the rock mass response to mining. However, seismicity provides information only for regions that are seismically active. Although some information on nonseismically active regions can be obtained from point measurements and numerical modeling, these methods suffer from limitations of their own. Passive source traveltime body-wave tomography (passive source tomography [PST]) uses information readily collected by microseismic monitoring systems, namely, the P- and/or S-wave traveltimes and microseismic event hypocenter locations. This technique allowes the simultaneous estimation of the velocity distribution between sensors and microseismic events and the correction of microseismic event hypocenter locations. In this paper, we present an application of time-lapse PST to the Northparkes Mines E26 Lift 2 block cave showing that PST can be used to obtain information on evolution and distribution of seismic velocities, leading to a better understanding of stress distribution and redistribution and of rock mass behavior during the development and production phases. In particular, we found that (1) the magnitude of the velocity perturbation varied through time and appeared to be strongly correlated with the intensity of microseismic activity, the mining rate, and the nature of the mining activity, (2) the velocity models provided information that allowed for the inference of the cave geometry and its evolution through time, (3) the stress distributions inferred from the velocity model were not fully consistent with a widely accepted conceptual stress redistribution model, which may reflect the significant influence of rock mass inhomogeneities and the mining sequence, (4) seismicity was found in regions in which velocity was higher and lower than the background velocity, and (5) there was no obvious correlation between geology and velocity distribution and evolution.

Regularized seismic velocity inversion based on inverse scattering theory

In this paper, the regularized finite difference contrast source inversion algorithm based on inverse scattering theory is utilized to invert the seismic velocity distribution in the underground medium, and this algorithm is a waveform inversion method in frequency domain based on wave equation. Velocity distribution is updated iteratively by minimizing the cost function using the nonlinear conjugate gradient method. The geophysical inversion problem has the characters of ill-posedness and instability, we handle this problem by adding an additional regularization item based on total variation of inversion parameter, making the inversion problem become a well-posed problem, and the algorithm has the ability of anti-noise interference. In the inversion process we use the frequency-space domain 9-point difference propagation operator under PML absorption boundary condition. Compared with other inversion algorithm, the construction of forward modeling operator and other matrix factorization calculation process can be avoided in each iterative step because the background medium remains unchanged throughout the inversion process, this makes the algorithm more suitable for large-scale three-dimensional inversion. In addition, the MPI parallel computation is applied, by which the efficiency of inversion process is improved greatly. The two-dimensional CSEG model inversion results show that this algorithm can obtain high resolution seismic velocity reconstruction, and provide an accurate velocity information for seismic data processing and interpretation.

Hypocenter migration and crustal seismic velocity distribution observed for the inland earthquake swarms induced by the 2011 Tohoku‐Oki earthquake in NE Japan: implications for crustal fluid distribution and crustal permeability

AbstractAfter the occurrence of the 2011 magnitude 9 Tohoku earthquake, the seismicity in the overriding plate changed. The seismicity appears to form distinct belts. From the spatiotemporal distribution of hypocenters, we can quantify the evolution of seismicity after the 2011 Tohoku earthquake. In some earthquake swarms near Sendai (Nagamachi‐Rifu fault), Moriyoshi‐zan volcano, Senya fault, and the Yamagata–Fukushima border (Aizu‐Kitakata area, west of Azuma volcano), we can observe temporal expansion of the focal area. This temporal expansion is attributed to fluid diffusion. Observed diffusivity would correspond to the permeability of about 10−15 (m2). We can detect the area from which fluid migrates as a seismic low‐velocity area. In the lower crust, we found seismic low‐velocity areas, which appear to be elongated along N–S or NE–SW, the strike of the island arc. These seismic low‐velocity areas are located not only beneath the volcanic front but also beneath the fore‐arc region. Seismic activity in the upper crust tends to be high above these low‐velocity areas in the lower crust. Most of the shallow earthquakes after the 2011 Tohoku earthquake are located above the seismic low‐velocity areas. We thus suggest fluid pressure changes are responsible for the belts of seismicity.

Teleseismic Tomography When Stations Follow Profiles: Pitfalls and Remedies

Research Article| September 01, 2014 Teleseismic Tomography When Stations Follow Profiles: Pitfalls and Remedies Babak Hejrani; Babak Hejrani Department of Geoscience, Aarhus University, Høegh‐Guldbergs Gade 2, Denmark‐8000 Aarhus Cbabak.hejrani@geo.au.dkbo@geo.au.dkniels.balling@geo.au.dk Search for other works by this author on: GSW Google Scholar Bo Holm Jacobsen; Bo Holm Jacobsen Department of Geoscience, Aarhus University, Høegh‐Guldbergs Gade 2, Denmark‐8000 Aarhus Cbabak.hejrani@geo.au.dkbo@geo.au.dkniels.balling@geo.au.dk Search for other works by this author on: GSW Google Scholar Niels Balling Niels Balling Department of Geoscience, Aarhus University, Høegh‐Guldbergs Gade 2, Denmark‐8000 Aarhus Cbabak.hejrani@geo.au.dkbo@geo.au.dkniels.balling@geo.au.dk Search for other works by this author on: GSW Google Scholar Author and Article Information Babak Hejrani Department of Geoscience, Aarhus University, Høegh‐Guldbergs Gade 2, Denmark‐8000 Aarhus Cbabak.hejrani@geo.au.dkbo@geo.au.dkniels.balling@geo.au.dk Bo Holm Jacobsen Department of Geoscience, Aarhus University, Høegh‐Guldbergs Gade 2, Denmark‐8000 Aarhus Cbabak.hejrani@geo.au.dkbo@geo.au.dkniels.balling@geo.au.dk Niels Balling Department of Geoscience, Aarhus University, Høegh‐Guldbergs Gade 2, Denmark‐8000 Aarhus Cbabak.hejrani@geo.au.dkbo@geo.au.dkniels.balling@geo.au.dk Publisher: Seismological Society of America First Online: 14 Jul 2017 Online ISSN: 1938-2057 Print ISSN: 0895-0695 © 2014 by the Seismological Society of America Seismological Research Letters (2014) 85 (5): 997–1003. https://doi.org/10.1785/0220130199 Article history First Online: 14 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Babak Hejrani, Bo Holm Jacobsen, Niels Balling; Teleseismic Tomography When Stations Follow Profiles: Pitfalls and Remedies. Seismological Research Letters 2014;; 85 (5): 997–1003. doi: https://doi.org/10.1785/0220130199 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search When geoscientists study the structure and history of old orogens, basins, and cratons, information from teleseismic data may provide important constraints on the present distribution of seismic velocities. The existing networks of permanent observatories are typically quite sparse, so temporary arrays are installed for a limited period of time with carefully designed measurement geometry. A dense grid of stations should ideally be deployed, but logistics, like road access and coastlines, combined with limited resources, commonly result in a distribution of the available seismometers along profiles perpendicular to the supposed strike direction of the target structures. This study focuses on the... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.