Three-dimensional Seismic Structure Research Articles

Mantle heterogeneity caused by trapped water in the Southwest Basin of the South China Sea

Water is the most common volatile component inside the Earth. A substantial amount of water can be carried down to the interior of the Earth by subducting plates. However, how the subducted water evolves after the subducting slab breaks off remains poorly understood. Here we use the data from a passive seismic experiment using ocean bottom seismometers (OBSs) together with the land stations to determine the high-resolution, three-dimensional seismic structure of the Southwest Sub-basin (SWSB) of the South China Sea (SCS). At depths below 40 km, the mantle shear velocity (Vsv) beneath the northern side of the SWSB is similar to that of the conventional oceanic pyrolite mantle, but roughly 3% shear-velocity reduction is found beneath the southern side of the SWSB. Results of thermal dynamic modeling reveal that the observed shear-velocity reduction could be explained by the presence of 150–300 ppm of water and 5–10% of lower continental crust. The inferred high-water content at the southern side of the SWSB is consistent with a model in which the Proto-SCS plate subducted southward prior to and during the formation of the SCS basin, releasing water into the upper mantle of the SWSB.

Role of fluids and seamount subduction in interplate coupling and the mechanism of the 2021 Mw 7.1 Fukushima-Oki earthquake, Japan

The frequent occurrence of large thrust earthquakes and subsequent tsunamis in a subduction zone are attributed to multiple factors, including structural heterogeneity, fluid saturation, and topographic variations of the subducting oceanic plate. To investigate these impacts on the generation of the 2021 Mw 7.1 Fukushima-Oki earthquake and interplate seismic coupling of the northeastern (NE) Japan subduction zone, high resolution three-dimensional (3-D) seismic structures of velocities (Vp, Vs) and Poisson's ratio (σ) were determined through a multi-parameter joint inversion method using 328,625 travel times of P- and S-wave receiver-source pairs from the earthquakes recorded by onshore seismic stations and ocean bottom seismometers. We found that more than 92.5% of M 6+ thrust earthquakes occurred in and around high-V zones, while less than 7.5% of the quakes were located in low-V patches. The high-V zones with high density M ≥ 6.0 thrust earthquakes (including the 2011 Mw 9.0 Tohoku mainshock) were thought to indicate strong interplate seismic coupling (asperities), while the low-V patches lacking large thrust-earthquakes reflected weak interplate coupling or decoupling. The present results revealed that seamount (including the Joban seamount chain and the bulgy Kitamachi volcanic batholith) subduction had a significant influence on seismography and the mechanical strength variation of interplate coupling in the forearc seismogenic zone. It was demonstrated that structural heterogeneities associated with fluids and seamount subduction played a fundamental role in interplate seismic coupling and large thrust-earthquake generation along the slab upper boundary in the NE Japan subduction zone.

Seismic Attenuation Tomography From 2018 Lombok Earthquakes, Indonesia

Local earthquake data was used to determine a three-dimensional (3D) seismic attenuation structure around the aftershock source region of the 2018 Lombok earthquake in Indonesia. The aftershocks were recorded by 13 seismic stations from August 4 to September 9, 2018. The selected data consist of 6,281 P-wave t∗ values from 914 events, which had good t∗ quality in at least four stations. Our results show that the two aftershock clusters northwest and northeast of Lombok Island have different attenuation characteristics. A low P-wave quality factor (low-Qp), low P-wave velocity (Vp), and high ratio of P-wave velocity and S-wave velocity (Vp/Vs), which coincide with a shallower earthquake (<20 km) northwest of Lombok Island, might be associated with a brittle area of basal and imbricated faults influenced by high fluid content. At the same time, the high-Qp, low Vp, and low Vp/Vs, which coincide with a deeper earthquake (>20 km) northeast of Lombok Island, might be associated with an area that lacks fluid content. The difference in fluid content between the northwest and northeast regions might be the cause of the early generation of aftershocks in the northwest area. The significant earthquake that happened on August 5, 2018, took place in a region with moderate Qp, close to the contrast of high and low-Qp and high Vp, which suggests that the earthquake started in a strong material before triggering the shallower aftershocks occurring in an area affected by fluid content. We also identified an old intrusive body on the northeast flank of the Rinjani volcano, which was characterized by a high-Qp, high-velocity, and a high Bouguer anomaly.

Upper mantle structure of the northeastern Arabian Platform from teleseismic body-wave tomography

A thick Infra-Cambrian to Phanerozoic sedimentary sequence in eastern Arabia conceals a poorly known Neoproterozoic basement that once separated the currently exposed Arabian Shield (western Arabia) from NW India in Gondwana. In this study, the Arabian Platform (eastern Arabia) is targeted by relatively high-resolution passive-source seismic tomography for the first time, which aims to illuminate crust and upper mantle structure in order to make inferences on their nature and origin. This is facilitated by a new broadband seismic network of 55 stations in the United Arab Emirates (UAE), which has recorded sufficient teleseismic body wave data to allow for the determination of three-dimensional seismic structure. Using a grid-based eikonal solver and a subspace inversion technique implemented in FMTOMO, relative arrival-time residuals from teleseismic earthquakes are mapped as 3-D P-wave velocity perturbations in the upper mantle beneath the seismic network. The resultant tomographic images reveal a marked transition from higher velocities in the Gulf of Oman to lower velocities in the west that we interpret to indicate the lithospheric boundary between Cretaceous Tethyan oceanic lithosphere and the Arabian passive continental margin. The northern extent of Tethyan oceanic lithosphere appears to terminate against a possible offshore continuation of the Dibba fault, until now only identified onshore. Further inland, the tomographic model exhibits a number of low- and high- velocity anomalies with a predominant NW orientation. Significantly, one well-defined high velocity anomaly is situated along strike of the Neoproterozoic exposure of volcanic-arc granodiorites found in northern Oman. We relate this anomaly to an intra-oceanic arc created during the Tonian subduction of the Mozambique Ocean, which strongly support the idea of continental growth in the Arabian Platform via magmatic arc accretion.

Characteristics of relocated hypocenters of the 2018 M6.7 Hokkaido Eastern Iburi earthquake and its aftershocks with a three-dimensional seismic velocity structure

I relocated the hypocenters of the 2018 M6.7 Hokkaido Eastern Iburi earthquake and its surrounding area, using a three-dimensional seismic structure, the double-difference relocation method, and the JMA earthquake catalog. After relocation, the focal depth of the mainshock became 35.4 km. As previous studies show, in south-central Hokkaido, the Hidaka collision zone is formed, and anomalous deep and thickened forearc crust material is subducting at depths of less than 70 km. The mainshock and its aftershocks are located at depths of approximately 10 to 40 km within the lower crust of the anomalous deep and thickened curst near the uppermost mantle material intrusions in the northwestern edge of this Hidaka collision zone. Like the two previous large events, the aftershocks of this event incline steeply eastward and appear to be distributed in the deeper extension of the Ishikari-teichi-toen fault zone. The highly inclined fault in the present study is consistent with a fault model by a geodetic analysis with InSAR. The aftershocks at depths of 10 to 20 km are located at the western edge of the high-attenuation (low-Qp) zone. These kinds of relationships between hypocenters and materials are the same as the 1970 and 1982 events in the Hidaka collision zone. The anomalous large focal depths of these large events compared with the average depth limit of inland earthquakes in Japan could be caused by the locally lower temperature in south-central Hokkaido. This event is one of the approximately M7 large inland earthquakes that occurred repeatedly at a recurrence interval of approximately 40 years and is important in the collision process in the Hidaka collision zone.

Combination of Least Square and Monte Carlo Methods for OBS Relocation in 3D Seismic Survey Near Bashi Channel

The relocation of ocean bottom seismometers (OBSs) is a key step in analyzing the three-dimensional seismic tomographic structure of crust and mantle. In order to get the accurate location of OBSs on the seafloor, we analyze the travel times of direct water waves emitted by air-guns. The Monte Carlo and least square methods have been adopted to calculate the true OBS location. The secondary time correction is necessary if the arrivals of direct water waves show overall time drift during relocation which maybe originates from remnant of linear clock drift correction and average errors of travel time picking, mean water velocity assumption, and experiment geometry. We have improved the original OBS relocation procedure which we used previously for other experiments by deliberateness of a secondary time correction and automatically approaching the really mean water velocity. A series of synthetic tests are carried out firstly to document the feasibility of our procedure and then it is applied on a real experiment. In here, we relocate 28 OBSs in total were relocated in 3D seismic survey near Bashi Channel. Relocation results show that the drifting distances for the 28 OBSs range from 65 to 1136 m between the deployed and relocated locations deduced by relocation results. The Pearson correlation coefficient between OBS drifting direction and sea current direction is 0.79, indicating that the two sets of data are highly linearly related and further manifest the sea current as the most possible driving force for OBS drifting during landing on the seafloor but its detailed influence mechanism is unclear by now. This research is necessary and critical for velocity structure modeling, and the optimal relocation program provides valuable experiences for 3D seismic survey in other area.

Joint Inversion of Body-Wave Arrival Times and Surface-Wave Dispersion for Three-Dimensional Seismic Structure Around SAFOD

We incorporate body-wave arrival time and surface-wave dispersion data into a joint inversion for three-dimensional P-wave and S-wave velocity structure of the crust surrounding the site of the San Andreas Fault Observatory at Depth. The contributions of the two data types to the inversion are controlled by the relative weighting of the respective equations. We find that the trade-off between fitting the two data types, controlled by the weighting, defines a clear optimal solution. Varying the weighting away from the optimal point leads to sharp increases in misfit for one data type with only modest reduction in misfit for the other data type. All the acceptable solutions yield structures with similar primary features, but the smaller-scale features change substantially. When there is a lower relative weight on the surface-wave data, it appears that the solution over-fits the body-wave data, leading to a relatively rough V s model, whereas for the optimal weighting, we obtain a relatively smooth model that is able to fit both the body-wave and surface-wave observations adequately.

Methods for inversion of body-wave waveforms for localized three-dimensional seismic structure and an application to D′′ structure beneath Central America

Methods for inversion of body-wave waveforms for localized three-dimensional seismic structure and an application to D′′ structure beneath Central America

A cloud-based synthetic seismogram generator implemented using Windows Azure

Synthetic seismograms generated by solving the seismic wave equation using numerical methods are being widely used in seismology. For fully three-dimensional seismic structure models, the generation of these synthetic seismograms may require large amount of computing resources. Conventional high-performance computer clusters may not provide a cost-effective solution to this type of applications. The newly emerging cloud-computing platform provides an alternative solution. In this paper, we describe our implementation of a synthetic seismogram generator based on the reciprocity principle using the Windows Azure cloud application framework. Our preliminary experiment shows that our cloud-based synthetic seismogram generator provides a cost-effective and numerically efficient approach for computing synthetic seismograms based on the reciprocity principle.

Structure Evolution of Qinjiatun-Qindong Fault System in Lishu Subbasin

Three-dimensional(3-D) seismic data and structure analysis of the Lishu subasin in Songliao basin indicates that Qinjiatun fault zone is composed of two faults: East-Qin and West-Qin fault. This fault system initially formed at Huoshiling stage, peaked at Shahezi stage and faded dramatically from Yingcheng stage. The Qinjiatun fault was important in controlling strata thickness and distribution of the Huoshiling formation. Qindong fault, a typical strike-slip fault, developed relatively later, cutting the Qinjiatun fault, The major active stage was in Denglouku-Quantou stage, and weakened in the end of late Cretaceous. Qinjiatun fault zone was reversed at Denglouku stage when the regional stress went compressive, generating a structure nose that was potentially beneficial for hydrocarbon to accumulate. The strike-slip Qindong fault became active relatively later, cutting through the previous strata and proving pathways for both accumulation and effusion of hydrocarbon.

Three-dimensional seismic attenuation structure beneath the Taiwan region and its tectonic implication

Three-dimensional (3-D) attenuation structures were determined for Taiwan region by inversion of earthquake intensity data set. The seismic intensity which is assumed to be a measure of the maximum acceleration of the S-wave at the seismic station is used to estimate the attenuation structure. The intensity data set consists of about 5500 intensity readings for 1410 earthquakes reported by the Central Weather Bureau of Taiwan from 1987 to 2010. The 3-D attenuation maps consisting of five layers were constructed to a depth of 74km.The obtained model reveals the following features: first, a high absorption zone exists in the westernmost Okinawa trough. These high-attenuation anomalies seem to extend from upper crust to mantle beneath the Yilan Plain and Kueishantao Island. We interpret the high absorption zone as the presence of the relatively hot lower crust and uppermost mantle in response to the local opening of the Okinawa trough. Second, strong lateral variation of seismic attenuation is observed from the source region of the Chi–Chi earthquake in the central Taiwan. Most aftershocks were occurred beneath the Western Foothills which is characterized by a transition from high attenuation to low attenuation between the Chelungpu fault and Chuchih fault. However, the region east to the Chuchih fault is relatively quiescent. This study suggests that the high Vp/Vs and lateral variation of attenuation features existing beneath the Western Foothills could be associated with porous materials containing fluid which might affect the generation of the Chi–Chi earthquake.

Tomographic imaging of hydrated crust and mantle in the subducting Pacific slab beneath Hokkaido, Japan: Evidence for dehydration embrittlement as a cause of intraslab earthquakes

We estimate detailed three-dimensional seismic velocity structures in the subducting Pacific slab beneath Hokkaido, Japan, using a large number of arrival-time data from 6902 local earthquakes. A remarkable low-velocity layer with a thickness of ~ 10 km is imaged at the uppermost part of the slab and is interpreted as hydrated oceanic crust. The layer gradually disappears at depths of 70–80 km, suggesting the breakdown of hydrous minerals there. We find prominent low-velocity anomalies along the lower plane of the double seismic zone and above the aftershock area of the 1993 Kushiro-oki earthquake (M7.8). Since seismic velocities of unmetamorphosed peridotite are much higher than the observations, hydrous minerals are expected to exist in the lower plane as well as the hypocentral area of the 1993 earthquake. On the other hand, regions between the upper and lower planes, where seismic activity is not so high compared to the both planes, show relatively high velocities comparable to those of unmetamorphosed peridotite. Our observations suggest that intermediate-depth earthquakes occur mainly in regions with hydrous minerals, which support dehydration embrittlement hypothesis as a cause of earthquake in the subducting slab.

Three-Dimensional Seismic Attenuation Structure around the SAFOD Site, Parkfield, California

Abstract We present models of the three-dimensional (3D) seismic attenuation structure, both Q p and Q s , for a 16 km 2 area centered on the San Andreas Fault Observatory at Depth (SAFOD). The P - and S -wave t * -values used in the inversion were determined from local earthquake data recorded by seismic network and portable array stations within the Parkfield region by inverting arrival spectra for source parameters, t * , and site response. Two techniques for determining the site response, the joint and alternating methods, were compared and it was found that the alternating method significantly underestimated site response variations. The t * -values were inverted to obtain 3D frequency-independent Q p and Q s models using 3D V p and V s models and associated event locations. A shallow low- Q area ( Q p and Q s about 50–75) on the southwest edge of both models is attributed to the low-velocity Cenozoic sedimentary rocks that overlie the Salinian basement rock. A high- Q feature ( Q p and Q s about 250 to 300) abuts this area and is interpreted as the Salinian basement. Adjacent to the San Andreas fault (SAF) trace, on its southwest side, there is a low- Q feature ( Q p and Q s about 50–80) attributed to a wedge of sedimentary rocks; uniformly low Q p - and Q s -values suggest that the wedge is fluid rich. A low- Q basin feature ( Q p and Q s about 50–75) on the northeast side of the SAF is interpreted as a fluid rich zone. Beneath this area there is a high- Q feature ( Q p and Q s about 220–300), which may be caused by crack closure due to increased pressure with depth in the rocks of the Franciscan formation. Given these high Q -values, it seems unlikely that this area acts as a fluid pathway for fluids entering the fault zone from the east into the seismogenic zone of the SAF.

Simulation of 3-D Teleseismic SV-waves Accelerated by the Multilevel Fast Multipole Method

We use first-order perturbation theory to represent the three-dimensional (3-D) seismic structure of the Earth. The background structure is assumed one-dimensional (1-D) with variations in only vertical direction. The perturbations are assumed 3-D volumetric inclusions with certain velocity contrast to the embedding structure. A convolutional type integral equation employing the Green’s function of the 1-D medium is utilized to solve the total wavefield at any point in the 3-D medium. The 3-D perturbations acting as secondary sources are allowed to have arbitrary shapes and sizes, and the integral equation is solved using the numerical techniques. A flat Earth structure is assumed in the mathematical formulation and the spherical-to-flat transforms are employed to account for the Earth’s sphericity. The 1-D Green’s function is extracted using the stable reflectivity method. The integral equation consists of five integrals in the spectral and spatial domains. The Multilevel Fast Multipole Method (MLFMM) employed for the two integrals on the horizontal plane greatly helps reduce the computational time. The speed gain is characterized as logarithmic versus exponential if a traditional technique instead of the MLFMM is applied. The difference between the logarithmic and exponential time complexities becomes evident especially when the data size enlarges.

Seismic and mechanical anisotropy and the past and present deformation of the Australian lithosphere

We interpret the three-dimensional seismic wave-speed structure of the Australian upper mantle by comparing its azimuthal anisotropy to estimates of past and present lithospheric deformation. We infer the fossil strain field from the orientation of gravity anomalies relative to topography, bypassing the need to extrapolate crustal measures, and derive the current direction of mantle deformation from present-day plate motion. Our observations provide the depth resolution necessary to distinguish fossil from contemporaneous deformation. The distribution of azimuthal seismic anisotropy is determined from multi-mode surface-wave propagation. Mechanical anisotropy, or the directional variation of isostatic compensation, is a proxy for the fossil strain field and is derived from a spectral coherence analysis of digital gravity and topography data in two wavelength bands. The joint interpretation of seismic and tectonic data resolves a rheological transition in the Australian upper mantle. At depths shallower than ∼150–200 km strong seismic anisotropy forms complex patterns. In this regime the seismic fast axes are at large angles to the directions of principal shortening, defining a mechanically coupled crust–mantle lid deformed by orogenic processes dominated by transpression. Here, seismic anisotropy may be considered ‘frozen’, which suggests that past deformation has left a coherent imprint on much of the lithospheric depth profile. The azimuthal seismic anisotropy below ∼200 km is weaker and preferentially aligned with the direction of the rapid motion of the Indo-Australian plate. The alignment of the fast axes with the direction of present-day absolute plate motion is indicative of deformation by simple shear of a dry olivine mantle. Motion expressed in the hot-spot reference frame matches the seismic observations better than the no-net-rotation reference frame. Thus, seismic anisotropy supports the notion that the hot-spot reference frame is the most physically reasonable. Independently from plate motion models, seismic anisotropy can be used to derive a best-fitting direction of overall mantle shear.

Three-dimensional seismic structure beneath the Australasian region from refracted wave observations

The earthquakes in the seismicity belt extending through Indonesia, New Guinea, Vanuatu and Fiji to the Tonga-Kermadec subduction zone recorded at the 65 portable broad-band stations deployed during the Skippy experiment from 1993-1996 provide good coverage of the lithosphere and mantle under the Australian continent, Coral Sea and Tasman Sea. The variation in structure in the upper part of the mantle is characterized by deter-mining a suite of 1-D structures from stacked record sections utilizing clear P and S arrivals, prepared for all propagation paths lying within a 10° azimuth band. The azimuth of these bands is rotated by 20° steps with four parallel corridors for each azimuth. This gives 26 separate azimuthal corridors for which 15 independent 1-D seismic velocity structures have been derived, which show significant variation in P and S structure. The set of 1-D structures is combined to produce a 3-D representation by projecting the velocity values along the ray path using a turning point approximation and stacking into 3-D cells (5° by 50 km in depth). Even though this procedure will tend to underestimate wave-speed perturbations, S-velocity deviations from the ak135 reference model exceed 6 per cent in the lithosphere. In the uppermost mantle the results display complex features and very high S-wave speeds beneath the Precambrian shields with a significant low-velocity zone beneath. High velocities are also found towards the base of the transition zone, with highS-wave speeds beneath the continent and high P-wave speeds beneath the ocean. The wave-speed patterns agree well with independent surface wave studies and delay time tomography studies in the zones of common coverage.

Three-dimensional seismic structure beneath Shillong Plateau and Assam Valley, Northeast India

Abstract Northeast India is bounded by the Himalayan arc to the north and the Burmese arc to the east. The Shillong Plateau and Assam Valley lie at the boundary zone of the two arcs. Three microearthquake surveys were conducted in this area from 1984 to 1986. We have applied a tomographic method to about 2800 high-quality P- and S-wave arrival times from 364 local earthquakes that were recorded in the magnitude range 2.0 to 4.0 by 22 temporary seismic stations during the surveys to determine the 3D velocity structure of the crust and upper mantle in this region. The result reveals significant lateral heterogeneities in the study area. The tomographic images obtained in this study are compatible with the major tectonic features such as active faults and seismicity trends.

Three-dimensional seismic structure and moment tensors of non-double-couple earthquakes at the Hengill-Grensdalur volcanic complex, Iceland

SUMMARY The volcanic and geothermal areas of Iceland are rich sources of non-double-couple (non-DC) earthquakes. A state-of-the-art digital seismometer network deployed at the Hengill‐Grensdalur volcanic complex in 1991 recorded 4000 small earthquakes. We used the best recorded of these to determine 3-D V P and V P /V S structure tomographically and accurate earthquake moment tensors. The V P field is dominated by high seismic wave speed bodies interpreted as solidified intrusions. A widespread negative (’4 per cent) V P /V S anomaly in the upper 4 km correlates with the geothermal field, but is too strong to be caused solely by the eVect of temperature upon liquid water or the presence of vapour, and requires in addition mineralogical or lithological diVerences between the geothermal reservoir and its surroundings. These may be caused by geothermal alteration. Well-constrained moment tensors were obtained for 70 of the best-recorded events by applying linear programming methods to P- and S-wave polarities and amplitude ratios. About 25 per cent of the mechanisms are, within observational error, consistent with DC mechanisms consistent with shear faulting. The other 75 per cent have significantly non-DC mechanisms. Many have substantial explosive components, one has a substantial implosive component, and the deviatoric component of many is strongly non-DC. Many of the non-DC mechanisms are consistent, within observational error, with simultaneous tensile and shear faulting. However, the mechanisms occupy a continuum in source-type parameter space and probably at least one additional source process is occurring. This may be fluid flow into newly formed cracks, causing partial compensation of the volumetric component. Studying non-shear earthquakes such as these has great potential for improving our understanding of geothermal processes and earthquake source processes in general.

Three-dimensional seismic structure of the upper mantle beneath the central part of the Eurasian continent

This work is a study of the upper-mantle seismic structure beneath the central part of the Eurasian continent, including the northern Mongolia, Altai and Sayan orogenic areas and the Baikal rift zone. Seismic velocity models are reconstructed using the inverse teleseismic scheme. This scheme uses information from earthquakes located within the study area recorded by the Worldwide Network. The seismic anomaly structure is obtained for different volumes in the study area that partially overlap one another. Special attention has been paid to the reliability of the results: several noise and resolution comparisons are made. The main results are as follows. (1) A cell structure of anomalies is observed beneath the Altai–Sayan region: positive, cold anomalies correspond to regions of recent orogenesis, negative anomalies are located beneath the depression of the Great Lakes in Mongolia and Hubsugul Lake. (2) A large negative anomaly is observed beneath the Hangai dome in Mongolia. (3) Strong velocity variations are obtained in a zone around Baikal Lake. A large negative anomaly is traced beneath the southern margin of the Siberian craton down to a depth of 700km. Contrasting positive anomalies (4–5 per cent) are observed at a depth of 100–300km beneath the Baikal rift. Our geodynamical interpretation of the velocity structure obtained beneath central Asia involves the existence of two processes in the mantle: thermal convection with regular cells, and a narrow plume beneath the southern border of the Siberian plate.

Three-dimensional seismic crustal structure of the Monashee hinterland core complex on the Lithoprobe Southern Canadian Cordillera Transect

Three-dimensional seismic coverage in an approximately 12 km × 12 km area of the southern Monashee metamorphic complex in the south-central portion of the Canadian Cordillera reveals a complex geometry to the Mesozoic–early Tertiary contractional Monashee décollement. Data were acquired as part of the Lithoprobe Southern Canadian Cordillera Transect where two approximately perpendicular lines intersected on the south flank of the Monashee mountains in the hinterland of the Cordillera. Stacks of traces within 100 m × 100 m bins are nominally 6-fold, but range from zero to 108-fold due to the crooked nature of the lines. Both migrated and unmigrated data have been examined for interpretation, but the highly variable data quality and discontinuous reflectivity cause excessive added noise during the migration process.