Deep array electromagnetic sounding on the Baltic Shield: External excitation model and implications for upper mantle conductivity studies

The BEAR array of simultaneous electromagnetic (EM) observations probes the deep crustal and upper mantle conductivity structure of the Baltic Shield searching for the lithosphere–asthenosphere boundary beneath. The adequate interpretation of the results of this unique high latitude natural field EM sounding requires proper understanding of the actual external excitation conditions because conventionally used plane wave model assumptions may be substantially violated in the vicinity of inhomogeneous polar sources. The paper presents an overview of the morphology and statistics of source distortions in the BEAR EM field transfer functions (TF) and the ways of their suppression. The stability of the final TF estimates obtained with the exclusion of intensive non-stationary auroral effects is further justified. The external excitation model effective for the whole BEAR observation period is inferred from the array distribution of the inter-station geomagnetic transfer functions. The model is supported by the results of polar ionosphere–magnetosphere current system studies, based on the simultaneous ground and satellite geomagnetic observations, and sets bounds for the “plane wave” approach in the BEAR data interpretation to avoid unfounded inferences on the upper mantle electrical properties. The signatures of the lithosphere–asthenospere boundary under Fennoscandia derived from the BEAR data are summarized and its resolution within the traditional plane wave interpretational paradigm is analysed assuming the presented external source pattern and estimated TF uncertainties caused by the source inhomogeneity.     

Holocene deformation and crustal movements in some type areas of India

Recent crustal movements have been observed and studied in several parts of India including the Himalayan and sub-Himalayan regions, the Precambrian shield of peninsular India and also the coastal tracts. The results of studies of Holocene deformation and crustal movements in two type areas are presented, one in the extreme southeastern part of the peninsula and the other in northeastern India.The Precambrian shield in the extreme southeastern part is characterised by a major NE—SW trending fault zone in the Tirupattur—Mattur areas of Tamil Nadu with some major extended faults, one of which apparently cuts through the entire crust and Moho as indicated by gravity data and which is associated with occurrences of alkaline and basic intrusions and carbonatite complex. Evidence of Recent crustal movements in this zone is afforded by geomorphic features and recent and current seismicity of a mild nature which is apparently to be attributed to slow movements along the fault plane.The Shillong plateau in northeastern India occurs as block-uplifted horst, comprising for the most part Archaean crystalline rocks with plateau basalts and Cretaceous and Tertiary sediments occurring on its southern margin. The plateau is bounded by major faults and is located in a zone of high seismicity lying astride and parallel to the eastern Himalayas intervened by the alluvium of the Brahmaputra Valley. Geomorphic features such as raised terraces, straight-edged scarps, etc., provide evidence for Recent crustal movements with dominant vertical movements along the fault planes which have continued through Tertiary and Recent times. Repeated precision levelling measurements conducted by the Survey of India indicate a rate of uplift of 4–5 cm per 100 years during the period 1910–1975.The gravity data pertaining to this region are also discussed in relation to the crustal movements.     

Magnetometer array studies in India: Present status,data interpretation and assessment of numerical modelling results

Significant results from several array of magnetometers deployed in India to probe deep geoelectrical structures of the crust and the upper mantle are reviewed in this paper. Emphasis is on critical appraisal of earlier results so that the article summarizes what has been done so far and what caution is to be taken on future work. Two large-scale arrays over northwest and peninsular India during 1979–80, have been followed up with six more linear or two-dimensional arrays over different parts of the country. “Trans-Himalayan” conductor aligned along the strike of Aravalli range, delineated by arrays over northwest India, essentially represents one of the major continental induction anomalies mapped by electromagnetic methods. Efforts for quantifying the induction effects through numerical models are shown to be constrained due to the large inter-station spacing, lack of information on the regional background conductivity distribution and the non-inclusion of the frequency dependence of induction effects. A more comprehensive modelling, not biased by these factors, enables approximating the Trans-Himalayan conductor as an asymmetric domal upwarp in the middle and lower crust located between Delhi-Hardwar ridge and Moradabad fault. Numerical modelling results for southern peninsular, despite the constraints, indicate that the strong and complex induction pattern can be adequately attributed to the combination of conductors connected with triple junction between Indo-Ceylon Graben, Comorin ridge and the west coast rifting. Induction features derived from the Valsad array, operated over basalt-covered region of western India, demarcate an enhanced conducting zone beneath Plume-associated triple junction in the Gulf of Cambay, apart from characterizing the presently active seismic zone as a resistive block.     

Evaluation of crustal and upper mantle structures using receiver function analysis: ISM broadband observatory data

A three-component broadband seismograph is in operation since January 2007 at the Indian School of Mines (ISM) campus, Dhanbad. We have used the broadband (BB) seismograms of 17 teleseismic events (M ≥ 5.8) recorded by this single BB station during 2008–09 to estimate the crust and upper mantle discontinuities in Dhanbad area which falls in the peninsular India shield. The converted wave technique and the Receiver function analysis are used. A 1-D velocity model has been derived using inversion. The Mohorovicic (Moho) discontinuity (crustal thickness) below the ISM observatory is estimated to be ∼41 km, with an average Poisson ratio of ∼0.28, suggesting that the crust below the Dhanbad area is intermediate to mafic in nature. The single station BB data shed new light to the estimate of crustal thickness beneath the eastern India shield area, which was hitherto elusive. Further, it is observed that the global upper mantle discontinuity at 410 km is delayed by ∼0.6 sec compared to the IASP-91 global model; this may be explained by a slower/hotter upper mantle; while the 660 km discontinuity is within the noise level of data.     

Gravity anomalies and dynamics of the Swiss Alps

An interpretation of the gravity observations in Switzerland is presented. The gravity anomalies are mainly caused by crustal effects. Taking 0.5 g/cm³ as the average density contrast between crust and upper mantle, the remaining positive residual field of + 50 mGal can be explained as the effect of the so-called “lithospheric root” underneath the Swiss Alps. This “root” must be considered as a relatively cold body with increased density. In order to calculate the thermally induced density distribution in the upper mantle, a kinematic and geothermal model has been constructed simulating the Alpine history for a time span covering the last 40 m.y.At the same time the moving lithospheric body (crustal uplift and subsidence of the lower lithosphere) is analysed from a dynamical point of view in order to test the kinematic conditions. The calculation leads to a mantle subsidence rate of 1 mm/y for the present period.     

Fission track geochronology of India

The fission track ages of cogenetic/co-existing minerals namely garnet, muscovite and apatite from three mica beltsi.e., Bihar, Rajasthan, Nellore of peninsular India and Himalayan region, coupled with the corresponding closing temperatures of the minerals have been used to reveal the thermal and uplift histories of these regions. The data show that the extra-peninsular part of the subcontinent during Himalayan orogenic cycle (upper cretaceous-tertiary) witnessed the highest cooling and uplift rates in comparison to the older cycles in peninsular India.     

Teleseismic p-wave traveltime residuals and deep structure of the Aegean region

Teleseismic P-wave traveltime residuals have been measured at the Greek seismic stations with respect to the Herrin 68 tables. In spite of the large scatter, some insight into crustal and upper-mantle structure of the Aegean region can be gained. The average absolute residuals (observed minus Herrin traveltimes) are of the order of + 2 s. The most plausible interpretation is an efficient low-velocity zone in the upper mantle. Simple estimates of densities and subsequently gravity with the aid of Birch's law suggest that the Aegean region is underlain by hot expanded upper mantle, perhaps involving partial melting. The relative P residuals (observed minus Herrin traveltime differences between a station and Athens) are generally positive and can be interpreted with lateral variations of the LVZ or of the crust. The latter interpretation is supported in some cases by seismic refraction data. The azimuthal variation of the relative residuals at stations on the non-volcanic arc bears a distinct relation with the arc orientation. At Archangelos (Rhodes) where we “see” through the Benioff zone, the residuals from N to W are between -1 and -2 s and indicate a high-velocity slab sinking below the Aegean sea. At Vamos (Crete), Valsamata (Kephallenia), and Joanina (Pindus Range) the largest (smallest) residuals are along directions parallel (perpendicular) to the arc. This can be interpreted by crustal thickening under the sedimentary arc and/or by velocity anisotropy with the maximum perpendicular to the arc. On the whole, our study supports the hypothesis that the Aegean region is a trench—island-arc—marginal-sea system.     

喜马拉雅造山带东段错那裂谷的地壳结构

喜马拉雅造山带由印度与欧亚大陆板块的陆陆碰撞而形成。为何在挤压造山的碰撞前缘形成代表垮塌的藏南裂谷系存在巨大的争议。回答这个问题需要对裂谷的地壳结构有一个全面的认识。各裂谷带的起始活动年代自西向东逐渐年轻。本研究选取喜马拉雅东部较为年轻的错那裂谷,利用密集台阵接收的远震数据,通过P波接收函数方法,揭示错那裂谷的精细地壳结构,进而通过地壳结构分析裂谷的形成。结果显示错那裂谷为全地壳尺度结构,裂谷下方莫霍面发生明显错断,且壳内结构侧向不连续发育显著。本研究表明裂谷的形成可能关联更大尺度的区域构造运动,单一的重力垮塌是否能形成地壳尺度的裂谷需要进一步研究。综合前人对藏南裂谷系区域的超钾岩和埃达克岩研究以及深部地球物理观测结果,推断因俯冲的印度板片撕裂导致软流圈物质上涌弱化了错那裂谷区域下地壳,并且结合研究区内喜马拉雅淡色花岗岩研究显示中上地壳也存在弱化现象。因此,结合本研究结果推测全地壳尺度裂谷的形成需要不同深度的地壳弱化。     

Lithosphere structure of European sedimentary basins from regional three-dimensional gravity modelling

A three-dimensional (3D) density model, approximated by two regional layers—the sedimentary cover and the crystalline crust (offshore, a sea-water layer was added), has been constructed in 1° averaging for the whole European continent. The crustal model is based on simplified velocity model represented by structure maps for main seismic horizons—the “seismic” basement and the Moho boundary. Laterally varying average density is assumed inside the model layers. Residual gravity anomalies, obtained by subtraction of the crustal gravity effect from the observed field, characterize the density heterogeneities in the upper mantle. Mantle anomalies are shown to correlate with the upper mantle velocity inhomogeneities revealed from seismic tomography data and geothermal data. Considering the type of mantle anomaly, specific features of the evolution and type of isostatic compensation, the sedimentary basins in Europe may be related into some groups: deep sedimentary basins located in the East European Platform and its northern and eastern margins (Peri-Caspian, Dnieper–Donets, Barents Sea Basins, Fore–Ural Trough) with no significant mantle anomalies; basins located on the activated thin crust of Variscan Western Europe and Mediterranean area with negative mantle anomalies of −150 to −200×10⁻⁵ ms⁻² amplitude and the basins associated with suture zones at the western and southern margins of the East European Platform (Polish Trough, South Caspian Basin) characterized by positive mantle anomalies of 50–150×10⁻⁵ ms⁻² magnitude. An analysis of the main features of the lithosphere structure of the basins in Europe and type of the compensation has been carried out.     

Crustal structure of the Indian subcontinent

Studies carried out by various investigators up to 1971 to delineate the Indian crustal structure using body wave travel times, surface wave dispersion and gravity methods are summarised and reviewed. The average crustal thickness is found to be 35–40 km in the Indian peninsular shield, 30–35 km in the Indo Gangetic plains and 60–80 km in the Himalayas and the Tibetan plateau region. The limitations of the various methods used and the errors in the estimation of crustal thickness by them are discussed. As no deep refraction work for crustal studies has been carried out so far in India, this topic is not covered in this study.     

A tentative 2D thermal model of central India across the Narmada-Son Lineament (NSL)

This work deals with 2D thermal modeling in order to delineate the crustal thermal structure of central India along two Deep Seismic Sounding (DSS) profiles, namely Khajuriakalan–Pulgaon and Ujjan–Mahan, traversing the Narmada-Son-Lineament (NSL) in an almost north–south direction. Knowledge of the crustal structure and P-wave velocity distribution up to the Moho, obtained from DSS studies, has been used for the development of the thermal model. Numerical results reveal that the Moho temperature in this region of central India varies between 500 and 580 °C. The estimated heat flow density value is found to vary between 46 and 49 mW/m². The Curie depth varies between 40 and 42 km and is in close agreement with the Curie depth (40±4 km) estimated from the analysis of MAGSAT data. Based on the present work and previous work, it is suggested that the major part of peninsular India consisting of the Wardha–Pranhita Godavari graben/basin, Bastar craton and the adjoining region of the Narmada Son Lineament between profiles I and III towards the north and northwest of the Bastar craton are characterized with a similar mantle heat flow density value equal to 23 mW/m². Variation in surface heat flow density values in these regions are caused by variation in the radioactive heat production and fluid circulation in the upper crustal layer.     

Crustal and mantle strengths in continental lithosphere: is the jelly sandwich model obsolete?

The relative importance of the contribution of the lower crust and of the lithospheric mantle to the total strength of the continental lithosphere is assessed systematically for realistic ranges of layer thickness, composition, and temperature. Results are presented as relative strength maps, giving the ratio of the lower crust to upper mantle contribution in terms of crustal thickness and surface heat flow. The lithosphere shows a “jelly sandwich” rheological layering for low surface heat flow, thin to average crustal thickness, and felsic or wet mafic lower crustal compositions. On the other hand, most of the total strength resides in the seismogenic crust in regions of high surface heat flow, crust of any thickness, and dry mafic lower crustal composition.     

Crustal structure of the Rhinegraben area

Numerous ge ological and geophysical investigations within the past decades have shown that the Rhinegraben is the most pronounced segment of an extended continental rift system in Europe. The structure of the upper and lower crust is significantly different from the structure of the adjacent “normal” continental crust.

Two crustal cross-sections across the central and southern part of the Rhinegraben have been constructed based on a new evaluation of seismic refraction and reflection measurements. The most striking features of the structure derived are the existence of a well-developed velocity reversal in the upper crust and of a characteristic cushion-like layer with a compressional velocity of 7.6–7.7 km/sec in the lower crust above a normal mantle with 8.2 km/sec. Immediately below the sialic low-velocity zone in the middle part of the crust, an intermediate layer with lamellar structure and of presumably basic composition could be mapped.

It is interesting to note that the asymmetry of the sedimentary fill in the central Rhinegraben seems to extend down deeper into the upper crust as indicated by the focal depths of earthquakes. The top of the rift “cushion” shows a marked relief which has no obvious relation to the crustal structure above it or the visible rift at the surface.     

Seismic ray direction anomalies and relative residuals of earthquakes recorded at Gauribidanur array

Slowness and azimuthal anomalies provide valuable information about lateral inhomogeneities within the crust and mantle of the earth. Over 300 earthquakes (distance range 14°–36° and azimuth 0°–360°) recorded at Gauribidanur seismic array (GBA) in southern India, were analysed using adaptive processing techniques. Slowness anomalies upto 1·3 sec/deg and azimuthal anomalies upto 8° have been observed in the present analysis. Slowness anomaly patterns for Java trench, Mid-Indian oceanic ridge earthquakes are more consistent as compared to the events originating in the Himalayan and Hindukush regions. A significant feature of the azimuthal anomaly pattern was the distinct absence of any positive anomalies from earthquakes occurring in mid-oceanic ridge. These anomalies have also been analysed as a function of epicentral distance and are mainly attributed to the transition zones occurring between 400–700 km depth ranges in the Indian upper mantle regions. Relative residuals between the stations of GBA have very little dependence on azimuth and distance. An anomalous structure beneath the array in the direction of the Java trench region (azimuth 116–126°) has been postulated on the basis of large systematic slowness vectors observed.     

Structure of the crust and upper mantle beneath the central european rift system

The crustal structure of the central European rift system has been investigated by seismic methods with varying success. Only a few investigations deal with the upper-mantle structure. Beneath the Rhinegraben the Moho is elevated, with a minimum depth of 25 km. Below the flanks it is a first-order discontinuity, while within the graben it is replaced by a transition zone with the strongest velocity gradient at 20–22 km depth. An anomalously high velocity of up to 8.6 km/s seems to exist within the underlying upper mantle at 40–50 km depth. A similar structure is also found beneath the Limagnegraben and the young volcanic zones within the Massif Central of France, but the velocity within the upper mantle at 40–50 km depth seems to be slightly lower. Here, the total crustal thickness reaches only 25 km. The crystalline crust becomes extremely thin beneath the southern Rhônegraben, where the sediments reach a thickness of about 10 km while the Moho is found at 24 km depth. The pronounced crustal thinning does not continue along the entire graben system. North of the Rhinegraben in particular the typical graben structure is interrupted by the Rhenohercynian zone with a “normal” West-European crust of 30 km thickness evident beneath the north-trending Hessische Senke. A single-ended profile again indicates a graben-like crustal structure west of the Leinegraben north of the Rhenohercynian zone. No details are available for the North German Plain where the central European rift system disappears beneath a sedimentary sequence of more than 10 km thickness.     

The crustal structure of the Guayana Shield, Venezuela, from seismic refraction and gravity data

We present results from a seismic refraction experiment on the northern margin of the Guayana Shield performed during June 1998, along nine profiles of up to 320 km length, using the daily blasts of the Cerro Bolívar mines as energy source, as well as from gravimetric measurements. Clear Moho arrivals can be observed on the main E–W profile on the shield, whereas the profiles entering the Oriental Basin to the north are more noisy. The crustal thickness of the shield is unusually high with up to 46 km on the Archean segment in the west and 43 km on the Proterozoic segment in the east. A 20 km thick upper crust with P-wave velocities between 6.0 and 6.3 km/s can be separated from a lower crust with velocities ranging from 6.5 to 7.2 km/s. A lower crustal low velocity zone with a velocity reduction to 6.3 km/s is observed between 25 and 25 km depth. The average crustal velocity is 6.5 km/s. The changes in the Bouguer Anomaly, positive (30 mGal) in the west and negative (−20 mGal) in the east, cannot be explained by the observed seismic crustal features alone. Lateral variations in the crust or in the upper mantle must be responsible for these observations.     

Crustal and upper mantle seismic structure of the Australian Plate, South Island, New Zealand

Seismic reflection and refraction data were collected west of New Zealand's South Island parallel to the Pacific–Australian Plate boundary. The obliquely convergent plate boundary is marked at the surface by the Alpine Fault, which juxtaposes continental crust of each plate. The data are used to study the crustal and uppermost mantle structure and provide a link between other seismic transects which cross the plate boundary. Arrival times of wide-angle reflected and refracted events from 13 recording stations are used to construct a 380-km long crustal velocity model. The model shows that, beneath a 2–4-km thick sedimentary veneer, the crust consists of two layers. The upper layer velocities increase from 5.4–5.9 km/s at the top of the layer to 6.3 km/s at the base of the layer. The base of the layer is mainly about 20 km deep but deepens to 25 km at its southern end. The lower layer velocities range from 6.3 to 7.1 km/s, and are commonly around 6.5 km/s at the top of the layer and 6.7 km/s at the base. Beneath the lower layer, the model has velocities of 8.2–8.5 km/s, typical of mantle material. The Mohorovicic discontinuity (Moho) therefore lies at the base of the second layer. It is at a depth of around 30 km but shallows over the south–central third of the profile to about 26 km, possibly associated with a southwest dipping detachment fault. The high, variable sub-Moho velocities of 8.2 km/s to 8.5 km/s are inferred to result from strong upper mantle anisotropy. Multichannel seismic reflection data cover about 220 km of the southern part of the modelled section. Beneath the well-layered Oligocene to recent sedimentary section, the crustal section is broadly divided into two zones, which correspond to the two layers of the velocity model. The upper layer (down to about 7–9 s two-way travel time) has few reflections. The lower layer (down to about 11 s two-way time) contains many strong, subparallel reflections. The base of this reflective zone is the Moho. Bi-vergent dipping reflective zones within this lower crustal layer are interpreted as interwedging structures common in areas of crustal shortening. These structures and the strong northeast dipping reflections beneath the Moho towards the north end of the (MCS) line are interpreted to be caused by Paleozoic north-dipping subduction and terrane collision at the margin of Gondwana. Deeper mantle reflections with variable dip are observed on the wide-angle gathers. Travel-time modelling of these events by ray-tracing through the established velocity model indicates depths of 50–110 km for these events. They show little coherence in dip and may be caused side-swipe from the adjacent crustal root under the Southern Alps or from the upper mantle density anomalies inferred from teleseismic data under the crustal root.     

Lithospheric structure and dynamic processes of the Tianshan orogenic belt and the Junggar basin

Located at the center of the Eurasian continent and accommodating as much as 44% of the present crustal shortening between India and Siberia, the Tianshan orogenic belt (TOB) is one of the youngest (<20 Ma) and highest (elevation>7000 m) orogenic belts in the world. It provides a natural laboratory for examining the processes of intracontinental deformation. In recent years, wide angle seismic reflection/refraction profiling and magnetotelluric sounding surveys have been carried out along a geoscience transect which extends northeastward from Xayar at the northern margin of the Tarim basin (TB), through the Tianshan orogenic belt and the Junggar basin (JB), to Burjing at the southern piedmont of the Altay Mountain. We have also obtained the 2D density structure of the crust and upper mantle of this area by using the Bouguer anomaly data of Northwestern Xinjiang. With these surveys, we attempt to image the 2D velocity and the 2D electric structure of the crust and upper mantle beneath the Tianshan orogenic belt and the Junggar basin. In order to obtain the small-scale structure of the crust–mantle transitional zone of the study area, the wavelet transform method is applied to the seismic wide angle reflection/refraction data. Combining our survey results with heat flow and other geological data, we propose a model that interprets the deep processes beneath the Tianshan orogenic belt and the Junggar basin.Located between the Tarim basin and the Junggar basin, the Tianshan orogenic belt is a block with relatively low velocity, low density, and partially high resistivity. It is tectonically a shortening zone under lateral compression. A detachment exists in the upper crust at the northern margin of the Tarim basin. Its lower part of the upper crust intruded into the lower part of the upper and the middle crust of the Tianshan, near the Korla fault; its middle crust intruded into the lower crust of the Tianshan; and its lower crust and lithospheric mantle subducted into the upper mantle of the Tianshan. In these processes, the mass of the lower crust of the Tarim basin was carried down to the upper mantle beneath the Tianshan, forming a 20-km-thick complex crust–mantle transitional zone composed of seven thin layers with a lower than average velocity. The thrusting and folding of the sedimentary cover, the intrusive layer in the upper and middle crust, and the mass added by the subduction of the Tarim basin into the upper mantle of the Tianshan are probably responsible for the crustal thickening of the Tianshan. Due to the important mass deficiency in the crust and the upper mantle of the Tianshan, buoyancy must occur and lead to rapid ascent of the Tianshan.The episodic tectonic uplift of the Tianshan and tectonic subsidence of the Junggar basin are closely related to the evolution of the Paleozoic, Mesozoic, and Cenozoic Tethys.     

2D model of the crust and uppermost mantle along rift profile, Siberian craton

A two-dimensional model of the crust and uppermost mantle for the western Siberian craton and the adjoining areas of the Pur-Gedan basin to the north and Baikal Rift zone to the south is determined from travel time data from recordings of 30 chemical explosions and three nuclear explosions along the RIFT deep seismic sounding profile. This velocity model shows strong lateral variations in the crust and sub-Moho structure both within the craton and between the craton and the surrounding region. The Pur-Gedan basin has a 15-km thick, low-velocity sediment layer overlying a 25-km thick, high-velocity crystalline crustal layer. A paleo-rift zone with a graben-like structure in the basement and a high-velocity crustal intrusion or mantle upward exists beneath the southern part of the Pur-Gedan basin. The sedimentary layer is thin or non-existent and there is a velocity reversal in the upper crust beneath the Yenisey Zone. The Siberian craton has nearly uniform crustal thickness of 40–43 km but the average velocity in the lower crust in the north is higher (6.8–6.9 km/s) than in the south (6.6 km/s). The crust beneath the Baikal Rift zone is 35 km thick and has an average crustal velocity similar to that observed beneath the southern part of craton. The uppermost mantle velocity varies from 8.0 to 8.1 km/s beneath the young West Siberian platform and Baikal Rift zone to 8.1–8.5 km/s beneath the Siberian craton. Anomalous high Pn velocities (8.4–8.5 km/s) are observed beneath the western Tunguss basin in the northern part of the craton and beneath the southern part of the Siberian craton, but lower Pn velocities (8.1 km/s) are observed beneath the Low Angara basin in the central part of the craton. At about 100 km depth beneath the craton, there is a velocity inversion with a strong reflecting interface at its base. Some reflectors are also distinguished within the upper mantle at depth between 230 and 350 km.     

Seismological features of island arc crust as inferred from recent seismic expeditions in Japan

Crustal studies within the Japanese islands have provided important constraints on the physical properties and deformation styles of the island arc crust. The upper crust in the Japanese islands has a significant heterogeneity characterized by large velocity variation (5.5–6.1 km/s) and high seismic attenuation (Qp=100–400 for 5–15 Hz). The lateral velocity change sometimes occurs at major tectonic lines. In many cases of recent refraction/wide-angle reflection profiles, a “middle crust” with a velocity of 6.2–6.5 km/s is found in a depth range of 5–15 km. Most shallow microearthquakes are concentrated in the upper/middle crust. The velocity in the lower crust is estimated to be 6.6–7.0 km/s. The lower crust often involves a highly reflective zone with less seismicity, indicating its ductile rheology. The uppermost mantle is characterized by a low Pn velocity of 7.5–7.9 km/s. Several observations on PmP phase indicate that the Moho is not a sharp boundary with a distinct velocity contrast, but forms a transition zone from the upper mantle to the lower crust. Recent seismic reflection experiments revealed ongoing crustal deformations within the Japanese islands. A clear image of crustal delamination obtained for an arc–arc collision zone in central Hokkaido provides an important key for the evolution process from island arc to more felsic continental crust. In northern Honshu, a major fault system with listric geometry, which was formed by Miocene back arc spreading, was successfully mapped down to 12–15 km.