Depth Of Moho Discontinuity Research Articles

Moho Depth Estimation and The Presence of Subducting Slab in West Sumatra, Jambi, and Riau Islands Regions Using Teleseismic Receiver Function Method: A Preliminary Result

The teleseismic receiver function method was used to investigate the crustal structure in West Sumatra, Jambi, and Riau Islands regions. These regions have a high level of seismicity because it is in an active tectonic zone where the oblique subduction between the Indo-Australian and Eurasian plates occurs. This study aims to determine the depth of Moho discontinuity and the presence of subducting slab beneath 6 BMKG seismic stations that form a line perpendicular to the trench and cover 3 different geological zones. PPSI and RASM are in the forearc zone, SKSM is in the volcanic arc zone, and MBBI, JMBI, and DSRI are in the back-arc zone. This study used teleseismic earthquake record data with epicentral distance between 30°-90° from the receiver and magnitude 6 or more. The iterative time-domain deconvolution and receiver function migration techniques were applied to estimate the depth of Moho discontinuity and the presence of subducting slab. The depth of Moho discontinuity in West Sumatra, Jambi, and Riau Islands regions ranges from 21-39 km and generally deeper in the volcanic arc zone possibly due to the isostatic effect. Moho is at a depth of 21-29 km in the forearc zone, 36-39 km in the volcanic arc zone, and 30-36 km in the back-arc zone. Then the subducting slab was observed at a depth of 20 km under PPSI station to 200 km under MBBI station.

Identification of Moho Discontinuity Depth and Subduction Slab in Bengkulu and South Sumatra Region using Receiver Function Method: A Preliminary Result

Bengkulu and South Sumatra regions are areas with high seismic activity due to faults and subduction zones. This study aims to analyze the depth of the Moho discontinuity layer and subduction slab under 4 BMKG seismic stations that form a perpendicular line of trench in the Bengkulu and South Sumatra areas. The distribution of stations used are Enggano Station (EGSI) in the front arc islands, Manna Station (MNAI) in the front arc basin, Pagar Alam Station (PKSI) in the volcanic-arc zone, and Lahat Station (LHSI) in the proto-volcanic zone. We selected teleseismic earthquake data with a distance of 30°-90° from the stations and magnitudes above 6 (M>6). For the identification of Moho and slab depths, we used analysis of receiver functions with iterative time domain deconvolution and migration of receiver functions with the AK-135 velocity model. The Oceanic Moho layer and the subduction slab under the EGSI Station were identified at depths of 34 km and 23 km respectively, under the MNAI Station the Moho layer and the subduction slab were identified at a depth of 18 km and 87 km respectively, under the PKSI Station the Moho layer and the subduction slab were identified at a depth of 34 km and 129 km respectively, and under the LHSI Station the Moho layer and subduction slab were identified at depths of 35 km and 175 km respectively.

Heat flow modelling of the Punta del Este Basin (offshore Uruguay) and its correlation with structural crustal domains

The Punta del Este Basin is located at the offshore of Uruguay, and it is considered an underexplored basin with a high potential for holding hydrocarbon plays. Geologically, it constitutes an aborted rift related to the Gondwana fragmentation process, and subsequent opening of the Atlantic Ocean (Upper Jurassic - Lower Cretaceous). The stratigraphic and tectonic evolution comprises four phases from the Paleozoic pre-rift stage to present-day post-rift stage (Neogene/Quaternary). Due to the estimated hydrocarbon potential of this basin, exploratory activities have been intensified in the last decades, based on the acquisition of a significant amount of geophysical data which allowed a better comprehension of the basin's geology. Previous studies related to this basin were mainly oriented to the stratigraphy, structural framework, and petroleum systems comprehension. This research is focused on the thermal evolution of the Punta del Este Basin and its correlation with the crustal geological structure. For this purpose, a heat flow model was developed in order to understand thermal evolution through geological time. A 2D heat flow modelling based on pseudo wells was carried out to research and reconstruct the paleo heat flow. Bottom-hole temperatures (BHTs), seabed heat flow measurements, and Moho discontinuity depth interpretations have been employed in the reconstruction of heat flow evolution through time. A structural analysis based on seismic interpretation allowed to identify the Moho discontinuity and define four crustal domains in the study area: proximal (unattenuated crust), necking, hyperextended (stretched and thinned transition crust), and oceanic crust. The thermal model shows a strong correlation between heat flow and crustal thickness. In this way, the proximal domain develops low heat flow values while the distal sector shows higher values, due to the strong influence of the rifting event in crust thinning.

A new approach to construct 3-D crustal shear-wave velocity models: method description and application to the Central Alps

We develop a new inversion approach to construct a 3-D structural and shear-wave velocity model of the crust based on teleseismic P-to-S converted waves. The proposed approach does not require local earthquakes such as body wave tomography, nor a large aperture seismic network such as ambient noise tomography, but a three-component station network with spacing similar to the expected crustal thickness. The main features of the new method are: (1) a novel model parametrization with 3-D mesh nodes that are fixed in the horizontal directions but can flexibly vary vertically; (2) the implementation of both sharp velocity changes across discontinuities and smooth gradients; (3) an accurate ray propagator that respects Snell’s law in 3-D at any interface geometry. Model parameters are inverted using a stochastic method composed of simulated annealing followed by a pattern search algorithm. The first application is carried out over the Central Alps, where long-standing permanent and the temporary AlpArray Seismic Network stations provide an ideal coverage. For this study we invert 4 independent parameters, which are the Moho discontinuity depth, the Conrad discontinuity depth, the P-velocity change at the Conrad and the average Vp/Vs of the crust. The 3-D inversion results clearly image the roots of the Alpine orogen, including the Ivrea Geophysical Body. The lower crust's thickness appears fairly constant. Average crustal Vp/Vs ratios are relatively higher beneath the orogen, and a low-Vp/Vs area in the northern foreland seems to correlate with lower crustal earthquakes, which can be related to mechanical differences in rock properties, probably inherited. Our results are in agreement with those found by 3-D ambient noise tomography, though our method inherently performs better at localizing discontinuities. Future developments of this technique can incorporate joint inversions, as well as more efficient parameter space exploration.

Comparison Study of Crustal Structure in Aceh Region based on Volcanic Arc System using Receiver Function Method

Aceh region has a very complex crustal structure from the forearc ridge to the backarc basin. This study aims to determine the velocity model of P and S waves and the depth of Moho discontinuity. This research was conducted using teleseismic earthquake data (30°-90° from the station) with M>6 from four seismic stations belonging to the BMKG in Aceh region. The stations are qualified based on the volcanic arc system zone. Furthermore, the velocity model determined by result of forward modelling, while the depth of the Moho layer estimated by migrated receiver function from time domain to the depth domain. At station SNSI that represented the forearc ridge zone, the depth of Moho is ±28 km, at station TPTI represent the forearc basin is ±16 km, while at zone with higher topography, namely volcanic arc zone represented by station KCSI, the Moho depth was identified at ±38 km, and the backarc basin represented by station LASI with ±40 km depth of Moho. This variation occurs because the composition of the earth’s layers below the station is diverse also different topography for each station.

The study of crustal structures in the southwestern part of the Baltic Sea by modeling of gravity data

The southwestern part of the Baltic Sea, including parts of the central European system, the Torne European Suture Zone, and the Ringkobing-Fyn High, is one of the most attractive areas for geoscientists due to its complicated geological structure caused by different tectonic events. In this paper, a local three-dimensional model of the crustal structures was developed based on interactive forward modeling of gravity data in the southwestern part of the Baltic Sea. The model considers sedimentary layers, upper crust, lower crust, and part of the upper mantle and applies the information of the area, including crustal thickness and Moho topography. Seismic sections and existing models in the area were used as the primary information to generate an appropriate initial model to reduce the problem of non-uniqueness, which is recognized as an inherent problem in potential fields modeling. The results indicated that the crustal thickness in the study area varies from 20 to 35 km. The thickest part is located in the Precambrian Baltic crust in the northeastern part of the region, and the thinnest part is in the margin of the Northern German Basin. The depth of Moho discontinuity in this area also varies from 26 to 42 km and shows significant changes on the sides of the Tornquist Zone.

Evolution of Meso-Cenozoic subduction zones in the ocean-continent connection zone of the eastern South China Block: Insights from gravity and magnetic anomalies

The evolution of unified East Asian continental margin was accompanied by large-scale magmatic and tectonic events. Here we investigate the tectonic evolution history of the ocean-continent connection zone in the eastern South China Block (SCB) during the Mesozoic-Cenozoic subduction of the (Paleo-) Pacific plates based on gravity and magnetic methods that provide a window to the deep crustal structure. The decomposed multi-scale gravity wavelets reveal subduction-related intra-crustal structures and the deep structure of the Mesozoic Greater South China Block (GSCB). The depth of Moho discontinuity was inverted by gravity data, constrained by the latest global crustal model of CRUST 1.0, and the inverted depth is closely consistent with and slightly larger than the depth on seismic profiles. The undulation of the Moho discontinuity beneath the South China Block and the deep supra-crustal faults along the continental margin are considered to be associated with the subduction of the (Paleo-) Pacific plates. In general, the Moho depth in the eastern SCB gets shallower from west to east. The oblique subduction-related strike-slip faults can be traced on the 7.4 km-upward continued field of the magnetic anomaly. The geologically-indistinct Jurassic magmatic arc along the SCB continental margin is verified by residual crustal field of the Bouguer gravity anomaly. The southern Ryukyu-Taroko line is speculated as the inland segment of the Early Jurassic paleo-subduction zone, and represents the southeastern paleo-plate boundary of the GSCB in the Jurassic. As the backarc-arc-forearc paleo-system migrated eastwards since the Early Cretaceous, the subduction zone retreated to the current Taiwan-Okinawa Trough, and the Jurassic magmatic arc-forearc was transformed to a backarc, indicating an eastward migration or jumping of the Yanshanian subduction along the continental margin of the SCB. The tectonic boundary of the Mesozoic GSCB was also identified through the magnetic anomaly field and the high-order gravity field.

A New Contribution to the (IPC) Bouguer Map of a Gap Area in Mosul and Duhok Region (Iraq)

The purpose of the current study was to complete an important gap within the Bouguer map of Iraq implemented by the Iraq Petroleum Company (IPC). In addition, the collected data were processed with recent gravity concepts and methods to establish a new regional geological image. A total of 868 gravity points were measured and corrected. The Bouguer anomaly map of the study area was initially drawn up with the parameters and equations used to construct the IPC map. The newly-produced map shows a very strong correlation with the old IPC results. A new Bouguer map of the study area was then constructed using modern parameters and equations. A comparison between the old and new maps illustrates the difference between recent and previous techniques. The isostatic residual map shows the presence of over-compensated areas close to Chia Gara and Mateen and spreads to the Aqra zone with a negative value of up to -20 mGal. In contrast, there is under-compensation in the southern regions with positive isostatic anomalies of about 80 mGal. In the researched area there are zero compensation lines around the larger structures (Chia Gara, Mateen, Mangesh, Aqra, Piris, and Perat). In order to draw up the crustal thickness map, the Moho discontinuity depth was determined using topographic and Bouguer anomaly grids of the study area. In addition, the upper mantle anomalies were calculated by continuing the isostatic residual anomaly to 45 km upward and then removing the past isostatic residual anomaly, that was continued upward to 35 km. Several qualitative approaches were used to isolate the regional and remaining anomalies to analyze the gravity data, with an optimal upward continuation level of 14 kilometers generated for the large and small maps.

Identification of Moho Discontinuity Depth Variations and Subduction Slab in North Sumatra Region Using Receiver Function Method

Northern Sumatra is an area with a complex structure of the earth’s crust. This area is very suitable for studying the state of the lithosphere in the subduction zone, where the average distribution of earthquakes with magnitudes greater than 5.0 occurs due to the movement of the subduction zone. This study aims to map the depth of the Moho discontinuity and subduction slab under 3 seismic stations in northern Sumatra, where 3 broadband sensors are 3 components of the BMKG seismograph network (Meteorology, Climatology and Geophysics) using the receiver function method. This study used teleseismic earthquake data with a distance of 30° - 90°. Inversion of the neighbourhood algorithm is used to get the S wave velocity model and the Vp/Vs value used to migrate the amplitude of the receiver function from the time domain to the depth domain. The depth of the Moho in northern Sumatra varies, under the GSI station the Moho depth is shallow ∼ 9 km, while the Moho depth for the other stations is on average 19 - 47 km. The slab is identified at a depth of ∼ 35.5 - 192.54 km below the GSI station to the TSI station where the position of the linear station is towards the Indo-Australian subduction zone. The low speed zone can be identified in the study and found to be in the range of ∼ 10 - 35 km below the surface of the Toba caldera.

Crustal nature and lithospheric structure of the Okinawa Trough

The Okinawa Trough (OT) is a back‐arc basin located in the eastern edge of the Eurasian Plate. The stage of back‐arc spreading and the exact crustal thickness of the OT are still controversial. To determine the crustal nature and lithospheric structure of the OT, the Moho discontinuity depth of the OT is calculated through the Parker–Oldenburg iterative inversion method based on the Bouguer gravity anomaly data in this article. The result shows that the crustal thickness of the OT is 15–27 km, thinner than those of the typical continental crust, and thicker than those of the normal oceanic crust. In this study, the formation and evolution of marginal basins or back‐arc basins is divided into four stages based on the crustal thickness of the basin. By comparing crustal thickness and heat flow of typical marginal basins located in global passive continental margins and continental rift, it can be inferred that the OT is at a stage later than continental drift. We suggest that the southern and middle OT are at the early stage of back‐arc spreading, while the northern OT is still at the ocean–continent transitional stage. There may have been initial oceanic crust developed at the spreading center in the southern OT. The tomographic images of the OT show that there is low‐velocity anomalous mantle beneath the OT, providing deep geophysical evidence for back‐arc spreading of the OT.

Deep structures underneath the Sihong Segment of the Tan-Lu Fault Zone, Eastern China: Interpretations of gravity anomaly and seismic profiles

The Sihong Segment of the Tan-Lu Fault Zone (SSTLFZ) is located in the Sihong county of the Jiangsu province and composed of 5 branches, which are mainly buried faults. The Tan-Lu Fault Zone is characterized by segmentation and its regional distribution, deep structures and geodynamic evolution in the Sihong area remains controversial. We provided evidence of gravity anomaly and seismic reflection profiles suggesting that the Xinyi-Xindian fault (F2) and Qiaobeizhen-Suqian fault (F5) of the SSTLFZ are segmented, which appear as thrust faults in the north and normal faults in the south. The F2, F5 and Mohe-Lingcheng fault (F3) are inferred as Quaternary faults. Based on the inversion of gravity anomaly, the tectonic structure of the bedrock consists of two grabens and one horst and the Moho discontinuity depth in the Sihong area shows an obvious uplift. We conclude that a fault-depression superimposed basin developed inner the SSTLFZ. The northern segments of the F2 and F5 were inverted due to the continuous compression since Quaternary.

The refined Moho depth map in the Carpathian-Pannonian region

We present a new digital Moho depth map of the Carpathian-Pannonian region. The map was produced by compiling Moho discontinuity depth data, which were obtained by interpretation of seismic measurements taking into account the results of 2-D and 3-D integrated geophysical modelling. The resultant map is characterized by significant Moho-depth variations. The trends and features of the Moho in this region were correlated with tectonic units.

Structure of the Crust Beneath the South Western Cameroon, from Gravity Data Analysis

The study area is located in the south western Cameroon and includes part of the Cameroon Volcanic Line (CVL). Volcanic activity has been recorded in this area, precisely on the Mount Cameroon which recently erupted in 2002. In addition, deadly carbonic gas was emitted by crater lakes (Monoun and Nyos) in 1984 and 1986 respectively. Potential field model EGM2008 has been used to investigate the structure of the crust. A regional/residual separation is performed using upward continuation and polynomial separation methods. The results from this operation show a similarity between the regional anomalies resulting from both methods. The regional anomaly maps present an increasing gradient trending ENE-WSW above and below latitude 5?N. Moreover, six nearly parallel profiles were drawn on the CVL in addition to two other profiles at the northern edge of Congo craton. These profiles were used to estimate the depths of the Moho discontinuity and some shallow sources by the means of the Bouguer and the residual anomalies respectively. The results show that the Moho discontinuity depths vary from 19 - 25 km (under Mount Cameroon) to 28 - 34 km (in Kumbo), while the southern neighbouring zone presents a Moho discontinuity depth ranging between 23 - 31 km (in Ngambe) and 22 - 32 km (in Eseka). These findings agree with the previous seismic and gravity researches lead in the area. EGM2008 is therefore a reliable tool to investigate the subsurface structures.

Seismotectonic-Geologic Hazards Zoning of Iran

<p class="MsoNormal"><span style="font-size: 10.0pt; font-family: Times; mso-fareast-font-family: 'Times New Roman'; mso-bidi-font-family: 'Times New Roman';">Seismotectonic-geologic hazards zoning map of Iran is drawn based on deterministic seismic hazards evaluation using the seismicity records, structural trends, tectonic settings, fault ruptures and neotectonics activities in Iran. This map has been prepared to indicate the seismotectonic-geologic hazards of Iran. It contains the earthquake hazards parameters such as b value and M<sub>max</sub> for the nineteen seismotectonic provinces. Furthermore, Moho discontinuity depth, seismogenic layer depths and seismic rates for all provinces are investigated. The majority of deformation in Iran has been concentrated in the continental crust of the country. Zagros and Alborz experience deep earthquakes that are an indication of existence of the thick-skinned tectonics. Finally, some provinces such as Piranshahr- Borojen, East Iran, Naien- Rafsanjan and East Alborz, which include main suture zones of Iran, have faced high seismic hazards.</span></p><p class="MsoNormal"><span style="font-size: 10.0pt; font-family: Times; mso-fareast-font-family: 'Times New Roman'; mso-bidi-font-family: 'Times New Roman';"><br /></span></p><p class="MsoNormal"><span style="font-size: 10.0pt; font-family: Times; mso-fareast-font-family: 'Times New Roman'; mso-bidi-font-family: 'Times New Roman';"><strong>Resumen</strong></span></p><p class="MsoNormal"><span style="font-size: 10.0pt; font-family: Times; mso-fareast-font-family: 'Times New Roman'; mso-bidi-font-family: 'Times New Roman';">Este estudio se realizó con el fin de preparar un mapa zonal de amenazas seismotectónicas y geológicas de Irán con base en la evaluación de amenazas sismícas deterministas a través de los registros sísmicos, las tendencias estructurales, las configuraciones tectónicas, las rupturas de fallas y la actividad neotectónica en Irán. Este mapa contiene los parámetros de amenaza sismológica como los valores b y M<sub>max</sub> para las 19 provincias sismotectónicas del país. Además, se investigó la profundidad de la Discontinuidad de Mohorovičić, las capas sismogénicas profundas y las clasificaciomes sísmicas para todas las provincias. La mayoría de la deformación de Irán está concentrada en la corteza continental del país. Las provincias de Zagros y Alborz experimentan terremotos profundos que indican la existencia de una configuración tectónica de capa gruesa. Finalmente, algunas provincias como Piranshahr-Borojen, Este de Irán, Naien-Rafsanjan y Este de Alborz, que incluyen las principales zonas de sutura de Irán, han enfrentado grandes riesgos sísmicos.</span></p>

Crustal structure of the Northwestern part of the Arabian Shield in Saudi Arabia deduced from gravity data

The present-day tectonics of the Northwestern Region of Saudi Arabia is affected by the tectonics related to the Red Sea floor spreading. The tectonic movemets related to the Red Sea floor spreading creat great variations in the crustal structure and depth of Moho discontinuity through this region. Gravity data have been acquired from Earth Gravitional Model (EGM-2008) and analyzed to estimate the crustal thickness under this region. 2-D gravity modeling have been conducted along eleven profiles oriented ENE-WSW and NW-SE to verify the lateral variations of the crust and mantle beneath these profiles. Accordingly, the upper layer of the crust is represented by sedimentary rocks underlying by basement layer that extends until the Moho discontinuity. The crustal thickness increases eastward,and the depth to the Moho is 25 km in the western part, along the Red Sea and increased upto 38 km through the Arabian Shield. These results are in agreement with the previous deep seismic and seismological investigations. The relation between recent earthquake swarms at Harrat Al-Shaqah area and the variations of Moho depth through the Northwestern Region has been clarified. It can be concluded that, these earthquake activities are of crustal origin and occurred due to the stresses exerted upon the pre-existed faults by mantle upwelling in this region. These results can be confirmed by the presence of the recent volcanic activities that experienced the Western Region of Saudi Arabia. Key words: Western Region of Saudi Arabia, Red Sea, tectonics, Arabian Shield.

Deep-rooted “thick skinned” model for the High Atlas Mountains (Morocco). Implications for the seismic Eurasia-Africa plate boundary region

Previous crustal models of the High Atlas suppose the existence of a mid-crustal detachment where all the surface thrusts merged and below which the lower crust was continuous. However, both seismic refraction data and gravity modeling detected a jump in crustal thickness between the High Atlas and the northern plains. Here we show that this rapid and vertical jump in the depth of Moho discontinuity suggests that a thrust fault may penetrate the lower crust and offset the Moho (deep-rooted “thick skinned” model). The distribution of Neogene and Quaternary volcanisms along and at the northern part of the High Atlas lineament can be related to the beginning of a partial continental subduction of the West African plate to the north underneath Moroccan microplate. Allowing from the complex problem of the plate boundary in the western zone of the Mediterranean, we propose to interpret the South-Atlasic fault zone as the actual northwestern boundary of the stable part of the African plate rather than the Azores-Gibraltar fault currently used.

Crustal P-wave velocity structure in Lower Yangtze region: Reinterpretation of Fuliji-Fengxian deep seismic sounding profile

The finite-difference inversion method and Raylnvr technique had been employed to interpret the wide-angle seismic reflection/refraction data of the Fuliji-Fengxian deep seismic sounding (DSS) profile in Lower Yangtze region, hence the velocity structure was acquired and conclusions were summarized as follows: (1) The velocity model along this profile can be divided into three large layers vertically (upper, middle and lower crusts) and six blocks laterally, and this velocity distribution agrees with the feature of stable platform. (2) The depth of Moho discontinuity is 30–36 km. The thickness of the upper crust is 10.5–13.0 km, where the lateral velocity varies strongly, and the velocity increases to 6.2 km/s−1 at bottom. Besides, the velocity distributions in the bottom layer of middle crust and lower crust have an apparent inhomogeneity. The velocity in upper layer of middle crust, lower layer of middle crust, lower crust and uppermost mantle is 5.9–6.2, 6.3–6.4, 6.6–7.0 and 8.06–8.29 km/s−1, respectively. (3) On two sides of the Tanlu fault belt (TFB), the mid-crustal velocity structure is quite different, nevertheless no apparent discrimination in velocity distribution and boundary topography exhibits in lower crust, hence it is inferred that the Jiashan segment of TFB had probably cut through whole crust in the Mesozoic, and the fault behaviour in lower crust had disappeared due to the low viscosity produced by the orogenic extension or crustal balance, while the fault features in the rigid middle-upper crust have been preserved up to the present. (4) The moderate earthquakes with Ms > 5.0 nearby Zhenjiang are related to the deep faults extending into the lower crust, and the earthquakes were probably induced by the energy been transferred from mantle lithosphere to upper-mid crust along the deep faults, and aggregated at some preferable tectonic positions.

Features of Deep Crustal Structure Beneath the Wutai Mountain Area of Shanxi Province

AbstractIn this paper, the deep crustal structures in Wutai mountain area are studied by using deep seismic sounding (DSS) data obtained from wide‐angle reflection/refraction profiles which pass through Shanxi down‐faulted zone and Wutai mountain area. The results show that the crust in this area presents obviously layered structures. The boundary interface between upper and lower crust is determined by means of reliable intra‐crust reflected waves P3 that can be traced continuously. The thickness of upper crust is 23~28km, and the depth of Moho discontinuity is 39~43 km. The Moho discontinuity in Wutai mountain area is the deepest in this region. In Wutai mountain and its adjacent areas, the crust structures have strong inhomogeneous characteristics. The large‐scale low velocity bodies in upper crust and strong intra‐crust reflected wave groups can be regarded as a sign of magma activity in deep crust. The breaking of intra‐crust interfaces and the local discontinuation of seismic wave group Pm as well as existence of deep crustal faults and so on indicate that the crust in this region suffered violent squeeze, deformation, and tectonic activities. Therefore the complex topography and anomalous structural characteristics in deep and shallow crust have been formed in this region.

Deep Structural Features of the Sanhe–Pinggu Strong Earthquake Area Imaged by Wide Angle Reflection / Refraction and Deep Seismic Reflection Profiling

AbstractIn this paper, the data from deep seismic reflection and wide angle reflection profiling completed in Sanhe‐Pinggu strong earthquake (M=8.0) region are comprehensively studied and interpreted. The result shows that the layered crustal structures obtained by the two investigation methods are basically consistent. In Sanhe‐Pinggu strong earthquake region, the fluctuation of basement structures is considerable, shallow faults well developed, and the thickness of upper crust and the depth of Moho discontinuity are 21–23km and 36–37km, respectively. Among several faults determined by this study, the Xiadian fault is a tectonic zone with obvious characteristic and coexistence of both shallow and deep faults. Surrounding the focus, anomalous velocity structures and complex geological tectonic environment mean that these positions are favorable for strong earthquake developing and occurring. The phenomena of Moho discontinuity fluctuation and rather thick overlapping reflection layers and existence of locally complex cuneiform reflection zones indicate that the strong compression and deformation of the crustal structures had taken place in this region, and these phenomena also reflect that magmatic activity put strong reformation on material and structures in lower crust to result in complicated deep crustal structures in the region which may be considered as deep background for the development and occurrence of Sanhe‐Pinggu strong earthquake (M=8.0).

Helium-3 evidence for a spot of thinned lithosphere in the eastern part of the Balkan Peninsula

The presence of a mantle component in the crust-to-atmosphere volatile flux may be considered as evidence for increased crustal permeability. The air-corrected 3He/4He = R ratio, usually normalized to Rair, is well known as a starting point for such an evaluation.The relative abundance of 3He, however, may be underestimated especially in the case of helium-rich fluids. The 3He/20Ne ratio is proposed as an alternative geochemical tracer for mantle exhalation, normalizing the 3He abundance to a small and comparatively constant atmospheric component: neon. The 3He/20Ne ratio for 21 mineral water sites from the eastern part of the Balkan peninsula are calculated on the bases of previous measurements. The regional distribution of the values found is compared with the data available on the depth of Moho discontinuity and lithosphere/asthenosphere boundary. An evident correlation between 3He/20Ne maximum values and an area with thin lithosphere is observed.