Temporary Seismic Array Research Articles

3D crustal velocity structure beneath the broadband seismic array in the Gyeongju area of Korea by receiver function analyses

A temporary seismic array was in operation between October 2010 and March 2013 in the Gyeongju area of Korea. Teleseismic records of the seismic array appropriate for receiver function analysis were collected, and selected seismograms were split into five groups based on epicenters—the Banda-Molucca, Sumatra, Iran, Aleutian, and Vanuatu groups. 1D velocity structures beneath each seismic station were estimated by inverting the stacked receiver functions for possible groups. The inversion was done by applying a genetic algorithm, whereas surface wave dispersion data were used as constraints to avoid non-uniqueness in the inversion. The composite velocity structure was constructed by averaging the velocity structures weighted by the number of receiver functions used in stacking. The uncertainty analysis for the velocity structures showed that the average of 95% confidence intervals was ±0.1km/s. The 3D velocity structure was modeled through interpolation of 1D composite velocity structures. Moho depths were determined in each composite velocity structure based on the AK135-F S-wave velocity model, and the depths were similar to the H-κ analysis results. The deepest Moho depth in the study area was found to be 31.9km, and the shallowest, was 25.9km. The Moho discontinuity dips in a southwestward direction beneath the area. A low velocity layer was also detected between 4 and 14km depth. Adakitic intrusions and/or a high geothermal gradient appear to be the causes of this low velocity layer. The 3D velocity structure can be used to reliably assess seismic hazards in this area.

Depth variations of P-wave azimuthal anisotropy beneath Mainland China.

A high-resolution model of P-wave anisotropic tomography beneath Mainland China and surrounding regions is determined using a large number of arrival-time data recorded by the China seismic network, the International Seismological Centre (ISC) and temporary seismic arrays deployed on the Tibetan Plateau. Our results provide important new insights into the subducted Indian plate and mantle dynamics in East Asia. Our tomographic images show that the northern limit of the subducting Indian plate has reached the Jinsha River suture in eastern Tibet. A striking variation of P-wave azimuthal anisotropy is revealed in the Indian lithosphere: the fast velocity direction (FVD) is NE-SW beneath the Indian continent, whereas the FVD is arc parallel beneath the Himalaya and Tibetan Plateau, which may reflect re-orientation of minerals due to lithospheric extension, in response to the India-Eurasia collision. There are multiple anisotropic layers with variable FVDs in some parts of the Tibetan Plateau, which may be the cause of the dominant null splitting measurements in these regions. A circular pattern of FVDs is revealed around the Philippine Sea slab beneath SE China, which reflects asthenospheric strain caused by toroidal mantle flow around the edge of the subducting slab.

Studying local earthquakes in the area Baltic-Bothnia Megashear using the data of the POLENET/LAPNET temporary array

Abstract. Earthquakes in areas within continental plates are still not completely understood, and progress on understanding intraplate seismicity is slow due to a short history of instrumental seismology and sparse regional seismic networks in seismically non-active areas. However, knowledge about position and depth of seismogenic structures in such areas is necessary in order to estimate seismic hazard for such critical facilities such as nuclear power plants and nuclear waste deposits. In the present paper we address the problem of seismicity in the intraplate area of northern Fennoscandia using the information on local events recorded by the POLENET/LAPNET (Polar Earth Observing Network) temporary seismic array during the International Polar Year 2007–2009. We relocate the seismic events using the program HYPOELLIPS (a computer program for determining local earthquake hypocentral parameters) and grid search method. We use the first arrivals of P waves of local events in order to calculate a 3-D tomographic P wave velocity model of the uppermost crust (down to 20 km) for a selected region inside the study area and show that the velocity heterogeneities in the upper crust correlate well with known tectonic units. We compare the position of the velocity heterogeneities with the seismogenic structures delineated by epicentres of relocated events and demonstrate that these structures generally do not correlate with the crustal units formed as a result of crustal evolution in the Archaean and Palaeoproterozoic. On the contrary, they correlate well with the postglacial faults located in the area of the Baltic-Bothnia Megashear (BBMS). Hypocentres of local events have depths down to 30 km. We also obtain the focal mechanism of a selected event with good data quality. The focal mechanism is of oblique type with strike-slip prevailing. Our results demonstrate that the Baltic-Bothnia Megashear is an important large-scale, reactivated tectonic structure that has to be taken into account when estimating seismic hazard in northern Fennoscandia.

Issues on crustal and upper-mantle structures associated with geodynamics in the northeastern Tibetan Plateau

The northeastern Tibetan Plateau is one of the key regions to explore the geodynamics of the Tibetan Plateau. In 1958, a research team led by Prof. Zeng Rongsheng did low-frequency seismic exploration in the Qaidam Basin, which was a prelude to deep geophysical research in the northeastern Tibetan Plateau. Since 1960s, a series of scientific projects have been carried out, including 27 deep seismic sounding profiles, which provided good coverage and fundamental constraints on velocity structure of the crust and upper mantle of tectonic units in the region. Among them, a number of deep seismic reflection profilings were used to reveal fine crustal structures of key tectonic areas. Moreover, crustal electrical structure and density structure were inferred from magnetotelluric sounding profiling and Bouguer gravity anomaly data, respectively. 3-D P-wave and S-wave velocity structures were determined using body-wave travel time tomography, surface-wave group velocity and phase velocity tomography based on seismic data. Since 2000, with the dramatically increased number of broadband seismic stations, application of modern seismology methods, such as teleseismic receiver function inversion, ambient noise imaging, as well as shear-wave splitting and seismic anisotropy, led to further understanding of structure and deformation of crust and upper mantle in the northeastern Tibetan Plateau. Among published research papers, most results on the deep structure are compatible. For example, the ambient noise imaging, surface wave tomography and receiver function inversion jointly show a wide distribution of low shear wave velocity in the middle and lower crust. Deep geophysical explorations show coexistence of crustal thickening, low P-wave velocities, low resistivity and high heat flow values. H- κ stacking analysis of receiver functions shows low-to- moderate crustal Poisson’s ratios in the Qilian fold system, the northern Songpan-Garze block and the west Qinling orogenic belt. The crustal Poisson’s ratios in the northeastern Tibetan Plateau are obviously lower than those in the central plateau. These compatibilities are crucial to understanding basic features of the deep structure. However, several issues related to the dynamics of the region remain in debate: (1) low velocity-high conductivity layer in the upper-middle crust; (2) crustal thickening mode; (3) crustal and mantle anisotropy; (4) lower crustal channel flow; (5) southward subduction of the Eurasian lithosphere. No consensus has been reached for these issues yet at present. Although several deep seismic sounding profiles and magnetotelluric sounding profiles collectively displayed evidence of existence of a low velocity-high conductive layer in the upper-middle crust, results of some other profiles did not show such layer. Mechanism of crustal thickening in the northeastern Tibetan Plateau can be summarized in the following three end-member hypotheses: (1) uniform crustal thickening; (2) lower crustal thickening; (3) upper crustal thickening. A prevailing view is that the crustal shortening generates folding and deformation of the upper crust, and fragment stacking is the main mode of crustal thickening. However, this is inconsistent with the crustal model from deep seismic sounding profiles. Different deep tectonic models and interpretations are causes of the on-going debate on issues such as the “lower crustal channel flow” and “southward subduction of Eurasian continent”. One of the reasons for lack of consensus might be that the resolution power of the existing seismic data is still not high enough to identify the details in deep crust and upper mantle. The national and regional seismic networks, and large-scale temporary seismic array observation, which is currently being implemented, will greatly improve the reliability and resolution of the target model. This is an effective way to enhance the knowledge of crustal and upper mantle structures and geodynamics in the northeastern Tibetan Plateau.

Seismic Sensor Misorientation Measurement UsingP‐Wave Particle Motion: An Application to the NECsaids Array

Seismic sensor orientation is one of the most critical parameters for modern three‐component seismological observation. However, this parameter is easily subject to error imposed by strong magnetic anomalies near the station or by human error in declination calibration. It is therefore very important to inspect and correct for sensor misorientation before utilizing three‐component waveform data. In this study, we measured the epoch‐dependent sensor misorientation for our temporary seismic array (NorthEast China Seismic Array to Investigate Deep Subduction, or NECsaids) by analyzing P ‐wave particle motions. We applied principal component analysis and the minimizing transverse energy method to study earthquakes with epicentral distance between 5° and 90° to estimate the sensor misorientation. Our results show high consistency with the direct gyrocompass measurements, with a correlation coefficient of 0.95. Our statistical analysis suggests that we can estimate robust sensor misorientation utilizing 10 earthquakes with high signal‐to‐noise ratio records and highly linear P ‐wave polarizations. We also find that the influence of anisotropy or a dipping interface produces a periodical pattern with back azimuth and is relatively small for our misorientation estimation. By analyzing the amplitude change of synthetic seismograms due to misorientation and taking into account the influence of anisotropy and dipping interfaces, as well as the measurement errors, we expect engineers to be able to orient seismic sensors with error smaller than 3°.

3D imaging of subducting and fragmenting Indian continental lithosphere beneath southern and central Tibet using body-wave finite-frequency tomography

Abstract We perform a finite-frequency tomographic inversion to image 3D velocity structures beneath southern and central Tibet using teleseismic body-wave data recorded by the TIBET-31N passive seismic array as well as waveforms from previous temporary seismic arrays. High-velocity bodies dip ∼40° northward beneath the Himalaya and the Lhasa Terrane. We interpret these high-velocity anomalies as subducting Indian Continental Lithosphere (ICL). The ICL appears to extend further north in central Tibet than in eastern Tibet, reaching 350 km depth at ∼31°N along 85°E but at ∼30°N along 91°E. Low P- and S-wave velocity anomalies extend from the lower crust to ≥180 km depth beneath the Tangra Yum Co Rift, Yadong-Gulu Rift, and the Cona Rift, suggesting that rifting in southern Tibet may involve the entire lithosphere. The anomaly beneath Tangra Yum Co Rift extends down to about 180 km, whereas the anomalies west of the Yadong-Gulu Rift and east of the Cona Rift extend to more than 300 km depth. The low-velocity upper mantle west of the Yadong-Gulu Rift extends furthest north and appears to connect with the extensive upper-mantle low-velocity region beneath central Tibet. Thus the northward-subducting Indian Plate is fragmented along north–south breaks that permit or induce asthenospheric upwellings indistinguishable from the upper mantle of northern Tibet.

Seismic images of the mantle transition zone beneath Northeast China and the Sino-Korean craton from P-wave receiver functions

Seismic data from northeast (NE) China and the Sino-Korean craton were combined to image the upper mantle discontinuities at 410 and 660km. Fine-scale topographic variations on these two discontinuities provide important clues for both delineating geometry of the subducting Pacific slab particularly at arc–arc junction and interpreting regional Cenozoic intraplate volcanism. We used over 90,000 receiver functions from 1916 teleseismic earthquakes recorded by 584 broadband seismic stations, primary those of temporary seismic arrays. We found the average depths of the two discontinuities to be 410km and 672km, respectively, beneath the study area. Results show that the 660-km discontinuity is strongly depressed by about 20–30km in a narrow region beneath and around the Changbaishan volcano, consistent with the results of previous receiver function studies. In contrast, much of the Sino-Korean craton exhibits typical transition zone thickness (~260km) and thus offers no evidence of a stagnated Pacific slab. Our results also reveal an elevated 660-km discontinuity and a thinner transition zone both to the west of the observed depression region and beneath the Kuril–Japan arc junction. This feature is most likely due to a tearing of the descending Pacific plate at both its leading and junction edges. An additional elevated 660-km discontinuity together with a thinner transition zone appears in the vicinity of the Dariganga lava field, supporting interpretations of a deep-rooted mantle plume. Our observations of an elevated 410-km discontinuity and a thicker transition zone correlate spatially with the diffuse distribution of volcanism around Hannuoba, Aershan and Wudalianchi. This correlation may suggest lithospheric removal as a mechanism for these magmatic activities.

Structural heterogeneity of the midcrust adjacent to the central Alpine Fault, New Zealand: Inferences from seismic tomography and seismicity between Harihari and Ross

AbstractDetermining the rates and distributions of microseismicity near major faults at different points in the seismic cycle is a crucial step toward understanding plate boundary seismogenesis. We analyze data from temporary seismic arrays spanning the central section of the Alpine Fault, New Zealand, using double‐difference seismic tomography. This portion of the fault last ruptured in a large earthquake in 1717 AD and is now late in its typical 330 year cycle of Mw∼8 earthquakes. Seismicity varies systematically with distance from the Alpine Fault: (1) directly beneath the fault trace, earthquakes are sparse and largely confined to the footwall at depths of 4–11 km; (2) at distances of 0–9 km southeast of the trace, seismicity is similarly sparse and shallower than 8 km; (3) at distances of 9–20 km southeast of the fault trace, earthquakes are much more prevalent and shallower than 7 km. Hypocenter lineations here are subparallel to faults mapped near the Main Divide of the Southern Alps, confirming that those faults are active. The region of enhanced seismicity is associated with the highest topography and a high‐velocity tongue doming at 3–5 km depth. The low‐seismicity zone adjacent to the Alpine Fault trace is associated with Vp and Vs values at midcrustal depths about 8 and 6% lower than further southeast. We interpret lateral variations in seismicity rate to reflect patterns of horizontal strain rate superimposed on heterogeneous crustal structure, and the variations in seismicity cutoff depth to be controlled by temperature and permeability structure variations.

Complex Structure beneath the Southeastern Tibetan Plateau from Teleseismic P-Wave Tomography

Abstract A high‐resolution image of the deep structures in the southeastern Tibetan plateau and Yangtze block was revealed by teleseismic inversion based on the dense teleseismic data collected from the permanent seismic stations and a recently deployed temporary seismic array. The results show that low‐velocity anomaly is imaged mainly in the eastern and southeastern Tibetan plateau (except the southern Panzhihua area) above 200 km depth in the upper mantle, whereas a high‐velocity anomaly exists to its east especially beneath the Sichuan basin. We infer the cold crustal and upper mantle of the Sichuan basin prevents the extrusion of mantle materials of the Tibetan plateau, while the cold lithosphere of the western Yangtze block is eroded by the upper‐mantle materials with high temperature from the extrusion of the Tibetan plateau. Under the Tengchong volcanic region, a low‐velocity anomaly extends to 300 km depth. A high‐velocity anomaly is shown beneath the region from the southern Sichuan to Yunnan in the mantle transition zone, and the anomaly reaches deeper to the lower mantle in the western Yunnan area. The widespread low velocity above the high velocity in the Yunnan region indicates the dehydration of the Indian plate may occur in the mantle transition zone, which causes fast‐rising heat and lower melting temperature in the upper mantle. A small‐scale, high‐velocity anomaly in the southern Panzhihua region is probably related to the late Paleozoic mantle plume activity. It plays an important role in impeding the southward extrusion of the plateau materials, which might be one of the causes for the rapid change of the polarization direction of the fast shear waves around the area.

Evidence for bathymetric control on the distribution of body wave microseism sources from temporary seismic arrays in Africa

Evidence for bathymetric control on the distribution of body wave microseism sources from temporary seismic arrays in Africa

Pn anisotropic tomography and dynamics under eastern Tibetan plateau

AbstractWe present a new anisotropic tomographic model of the uppermost mantle around eastern Tibet using Pn traveltime data from a newly deployed temporary seismic array and recent observation bulletins of Chinese provincial networks. Our results are generally consistent with previous results but provide new insights into the dynamics of Tibetan plateau. Prominent high‐velocity (high‐V) anomalies are visible under Alashan block and Qaidam and Sichuan basins, which clearly outline their tectonic margins. A distinct high‐V zone representing the double‐sided subduction of Indo‐Eurasian plates is imaged from Lhasa block to the south of Qaidam basin. A pronounced low‐velocity (low‐V) zone is observed from Songpan‐Ganzi block to southern Chuan‐Dian diamond block, suggesting the existence of hot material upwelling there. Crustal strong earthquakes frequently occurred around high‐V anomalies or transition zones from high‐V to low‐V anomalies, suggesting that these earthquakes could be related to lateral heterogeneities in the mantle. The Pn fast direction approximately rotates around Eastern Himalayan Syntaxis, and it is tangential to the margins of Sichuan basin, suggesting that the mantle material flow of Tibetan plateau may have affected east China. In the Yunnan region to the south of 26°N, the Pn fast direction is different from SKS splitting results, indicating that the mantle lithosphere could be mechanically decoupled at certain depth below the uppermost mantle, which might be attributable to the subduction of Indian (or Burma) slab. Although the correlation between anisotropy and velocity is complicated, anisotropy strength could be associated with the pattern of velocity anomalies in the region.

Mapping crustal structure beneath southern Tibet: Seismic evidence for continental crustal underthrusting

Receiver function imaging along a temporary seismic array (ANTILOPE-2) reveals detailed information of the underthrusting of the Indian crust in southern Tibet. The Moho dips northward from ~50km to 80km beneath the Himalaya terrane, and locally reaches ~85km beneath the Indus–Yalung suture. It remains at ~80km depth across the Lhasa terrane, and shallows to ~70km depth under the Qiangtang terrane. An intra-crustal interface at ~60km beneath the Lhasa terrane can be clearly followed southward through the Main Himalaya Thrust and connects the Main Boundary Thrust at the surface, which represents the border of the Indian crust that is underthrusting until south of the Bangong–Nujiang Suture. A mid-crustal low velocity zone is observed at depths of 14–30km beneath the Lhasa and Himalaya terranes probably formed by partial melt and/or aqueous fluids.

Upper mantle deformation beneath central-southern Tibet revealed by shear wave splitting measurements

Abstract Seismic anisotropy in the upper mantle has been inferred from shear wave splitting measurements at the temporary seismic array (ANTILOPE-2) in the central-southern Tibet from September 2005 to October 2006. Teleseismic shear waves (mainly SKS and SKKS) are used to determine the splitting parameters (fast polarization direction and delay time). Weak splitting with average delay time of 0.3 s is observed at stations in the southern Lhasa terrane and northernmost Tethyan Himalaya. Significant seismic polarization anisotropy with delay times from 0.8 to 1.5 s and NE-oriented fast direction is obtained for stations located farther north in the northern Lhasa terrane. The transition in shear wave splitting characteristics occurs at 30.5°N and suggests distinct upper mantle deformation from south to north. The negligible anisotropy south of this transition indicates that there is no appreciable large-scale mantle seismic polarization anisotropy. The substantial anisotropy in the north may be caused by the eastward flow in a lithospheric crush zone which is squeezed and sheared between the advancing Indian plate to the south and Eurasian plate to the north.

Wavefront distortion across Mt. Vesuvius observed by a seismic array

<p>Local and regional earthquakes recorded at Mt. Vesuvius in 1997 and 1998 by a temporary seismic array have been analyzed with array methods to estimate slowness and backazimuth of correlated phases. The backazimuth observed for the direct P-wave has been compared with the expected direction, and results show surprising large differences in most of the cases. Since the observed difference is not systematic but depends on the epicenter backazimuth, we ascribe it to a wavefront distortion produced by the propagation through a non-homogenous velocity structure. To check this hypothesis we computed synthetic wavefront propagation in two different velocity models by applying a finite-difference method. The first model was homogeneous, while the second one was heterogeneous, with 2D symmetry in the horizontal plane given by a positive velocity anomaly centered at the volcano crater. We observe that the heterogeneous model gives results in good qualitative agreement with the observations. Such model is supported by the results of tomographic studies carried out for Mt. Vesuvius.</p>

Receiver function structures beneath the deep large faults in the northeastern margin of the Tibetan Plateau

Using the teleseismic P- and S-wave receiver functions of the dense linear temporary seismic array, the crust and uppermost mantle structures beneath the deep large faults in the northeastern margin of the Tibetan Plateau were imaged. The images of the first converted wave and the multiples indicated that the North Fault Zone of West Qinling (NWQ) Mountain and Diebu–Lueyang (DBL) faults cut the Mohorovicic (Moho) Discontinuity and cause an obvious difference feature for the Moho in the two sides of the faults. The higher Vp/Vs ratio and lower velocity layer is found beneath the west portion of the array near the Tibetan Plateau, which implies a lower crust channel flow coming from the Tibetan Plateau. The weak Moho and higher Vp/Vs ratio beneath the eastern portion of the array near the Ordos suggest the upwelling of the hot mantle material. The results also indicate an obvious deformation in the upper crust with the lower Vp/Vs ratio beneath the middle of the array. Such upper crust deformation is closely related to the topography of the surface; therefore, we deduce that the deformation of the brittle upper crust is accompanied by the formation of the local topography during the uplift of the Tibetan Plateau, which is also the primary reason for the active seismicity in the study region. The deformation of the lithosphere–asthenosphere boundary (LAB) can also be associated with the formation of the diapir caused by the upwelling hot materials in the upper mantle due to the uprising of the thrusting plate caused by the subduction of the India Plate. The existence of the lower crust channel flow, the crust shortening, and the mantle diapir in the local region simultaneously implies that the elevation and formation of the Tibetan Plateau cannot be explained with a single model. The higher resolution results for the crust and the mantle, especially beneath the block boundary region, are necessary to construct the completed geodynamic model to understand the formation of the Tibetan Plateau.

Imaging fluid-related subduction processes beneath Central Java (Indonesia) using seismic attenuation tomography

We use local earthquake data observed by the amphibious, temporary seismic MERAMEX array to derive spatial variations of seismic attenuation (Qp) in the crust and upper mantle beneath Central Java. The path-averaged attenuation values (t∗) of a high quality subset of 84 local earthquakes were calculated by a spectral inversion technique. These 1929 t∗-values inverted by a least-squares tomographic inversion yield the 3D distribution of the specific attenuation (Qp). Analysis of the model resolution matrix and synthetic recovery tests were used to investigate the confidence of the Qp-model. We notice a prominent zone of increased attenuation beneath and north of the modern volcanic arc at depths down to 15km. Most of this anomaly seems to be related to the Eocene–Miocene Kendeng Basin (mainly in the eastern part of the study area). Enhanced attenuation is also found in the upper crust in the direct vicinity of recent volcanoes pointing towards zones of partial melts, presence of fluids and increased temperatures in the middle to upper crust. The middle and lower crust seems not to be associated with strong heating and the presence of melts throughout the arc. Enhanced attenuation above the subducting slab beneath the marine forearc seems to be due to the presence of fluids.

1-D P- and S-wave velocity models for the collision zone between the northern Tianshan mountain and the Junggar basin based on local earthquake data

We have developed crustal minimum 1-D P- and S-wave velocity models of the collision zone between the northern Tianshan mountain and the Junggar basin (86°E–89°E, 43°N–44.5°N). These two models were created through inversion of 1 370 P- and 1 396 S-wave travel times from 173 well-constrained local earthquakes recorded by the Urumqi sparse local seismic network and temporary seismic arrays. In contrast to previous models, our results indicate relatively low velocity at both shallow (<10 km) and deep (30–45 km) depths. The shallow zone is interpreted to be the result of thick surficial sedimentary deposits, whereas the deeper anomaly is interpreted to result from ductile shearing and lower crustal flow. Additionally, we detected several transition layers under the lower crust, which may imply structural complexity of the uppermost mantle in this region. The improved models reduce the RMS residual of earthquake locations by 41.7% from 1.2 to 0.5 seconds. The more accurately located hypocenters appear to correlate with prominent local over-thrusts, which underlie an anticlinal fold belt and several blind faults. Positive station corrections are observed near the Junggar basin, which likely reflects low wave velocity; negative corrections near the Tianshan mountain and Bogda mountain suggest high wave velocity.

Structure of central and southern Mexico from velocity and attenuation tomography

The 3D Vp, Vp/Vs, P‐ and S‐wave attenuation structure of the Cocos subduction zone in Mexico is imaged using earthquakes recorded by two temporary seismic arrays and local stations. Direct P wave arrivals on vertical components and direct S wave arrivals on transverse components from local earthquakes are used for velocity imaging. Relative delay times for P and PKP phases from teleseismic events are also used to obtain a deeper velocity structure beneath the southern seismic array. Using a spectral‐decay method, we calculate a path attenuation operatort* for each P and S waveform from local events, and then invert for 3D spatial variations in attenuation (Qp−1 and Qs−1). Inversion results reveal a low‐attenuation and high‐velocity Cocos slab. The slab dip angle increases from almost flat in central Mexico near Mexico City to about 30° in southern Mexico near the Isthmus of Tehuantepec. High attenuation and low velocity in the crust beneath the Trans‐Mexico Volcanic Belt correlate with low resistivity, and are probably related to dehydration of the slab and melting processes. The most pronounced high‐attenuation, low‐Vpand high‐Vp/Vsanomaly is found in the crust beneath the Veracruz Basin. A high‐velocity structure dipping into the mantle from the side of Gulf of Mexico coincides with a discontinuity from a receiver functions study, and provides an evidence for the collision between the Yucatán Block and Mexico in the Miocene.

East Asia mantle tomography: New insight into plate subduction and intraplate volcanism

We present a new P-wave tomographic model beneath East Asia by inverting 1,401,797 arrival-time data recorded by the regional seismic networks in East Asia and temporary seismic arrays deployed on the Tibetan Plateau. Our high-resolution tomography shows that the subducted Pacific slab is revealed clearly as a high-velocity (high-V) zone and it becomes stagnant in the mantle transition zone beneath eastern China. Prominent high-V anomalies atop the 410km discontinuity are detected beneath the eastern North China Craton and Northeast China, suggesting that lithospheric delamination may have occurred and greatly affected the thermal state, intraplate magmatism and surface topography of this region. The Indian lithosphere is characterized by a high-V anomaly and it is subducting nearly horizontally beneath the entire or most parts of western Tibet and with a small dipping angle to the southernmost part of eastern Tibet. The intraplate magmatism in different parts of East Asia has different origins. The active Tengchong volcano is underlain by a prominent low-velocity (low-V) anomaly in the shallow mantle, which may be caused by the subduction and dehydration of the Burma microplate plate (or Indian plate). The Hainan volcano is underlain by a plume-like low-V anomaly that extends down to at least 1000km depth and seems to be related to the deep subductions of the Burma microplate (or Indian plate) in the west and the Philippine Sea plate in the east. Prominent low-V anomalies are detected beneath the Changbai, Longgang and Xianjindao volcanoes in Northeast Asia, which may be related to the rollback of the subducting Pacific slab as well as the convective circulation process in the big mantle wedge above the stagnant slab. The origin of the Wudalianchi volcano in Northeast China seems associated with the upwelling of asthenospheric materials caused by the subduction-induced lithospheric delamination.

Study of Local Seismic Events in Lithuania and Adjacent Areas Using Data from the PASSEQ Experiment

The territory of Lithuania and adjacent areas of the East European Craton have always been considered a region of low seismicity. Two recent earthquakes with magnitudes of more than 5 in the Kaliningrad District (Russian Federation) on 21 September 2004 motivated re-evaluation of the seismic hazard in Lithuania and adjacent territories. A new opportunity to study seismicity in the region is provided by the PASSEQ (Pasive Seismic Experiment) project that aimed to study the lithosphere–asthenosphere structure around the Trans-European Suture Zone. Twenty-six seismic stations of the PASSEQ temporary seismic array were installed in the territory of Lithuania. The stations recorded a number of local and regional seismic events originating from Lithuania and adjacent areas. This data can be used to answer the question of whether there exist seismically active tectonic zones in Lithuania that could be potentially hazardous for critical industrial facilities. Therefore, the aim of this paper is to find any natural tectonic seismic events in Lithuania and to obtain more general view of seismicity in the region. In order to do this, we make a manual review of the continuous data recorded by the PASSEQ seismic stations in Lithuania. From the good quality data, we select and relocate 45 local seismic events using the well-known LocSAT and VELEST location algortithms. In order to discriminate between possible natural events, underwater explosions and on-shore blasts, we analyse spatial distribution of epicenters and temporal distribution of origin times and perform both visual analysis of waveforms and spectral analysis of recordings. We show that the relocated seismic events can be grouped into five clusters (groups) according to their epicenter coordinates and origin and that several seismic events might be of tectonic origin. We also show that several events from the off-shore region in the Baltic Sea (at the coasts of the Kaliningrad District of the Russian Federation) are non-volcanic tremors, although the origin of these tremor-type events is not clear.