ISC Catalogue Research Articles

Site Characterization In Champhai Town Area Of Mizoram State In Northeast India: Geological And Geophysical Studies

The Mizoram state of northeast region (NER), India located in close proximity to Indo-Burma subduction zone, recently experienced an earthquake swarm; eight severely felt earthquakes (Mw 5.0–5.9) occurred during April–October, 2020 around Champhai town that caused damages to several buildings and ground cracks with maximum reported MSK intensity VIII. The largest event Mw 5.9, at depth 15 km, occurred on April 16 (ISC catalog). The present study deals with site characterization of the Champhai town area by geological and geophysical investigations. The geological investigation involves detailed mapping of the lithounits, and the geophysical investigation includes shallow (down to 30 m) shear wave velocity (Vs 30) and site amplification (SA) measurements in an about 50 sq km area in the town. Some 23 sites are selected for Vs 30 measurements using the Multichannel Analysis of Surface Wave (MASW) technique. The observed Vs 30 ranges from 200 m/s to 1060 m/s classifying the area into three categories: Class-B (hard rock), class-C (soft rock), and class-D (stiff soil) as per National Earthquake Hazard Reduction Program (NEHRP) guidelines. Geologically, the class B and C sites are represented by arenaceous and argillaceous Tertiary rocks, respectively, whereas the class D is represented by the Quaternary sediments. Site amplification (SA) studies, on the other hand, are carried out by making ambient noise surveys at some 52 selected stations using a digital seismograph. The ambient noise data are analyzed by the Nakamura method of H/V spectral ratio. The overall variation of peak amplification of the sites ranges from 1.26 to 6.65. A good correlation is obtained; the observed Vs 30 and site amplification (SA) maps are fairly comparable with the detailed geological or lithounit map. These maps are much useful for seismic hazard microzonation of the town area to mitigate seismic hazard-risk

Integrated Earthquake Catalog III: Gakkel Ridge, Knipovich Ridge, and Svalbard Archipelago

This paper represents the final part of a series of studies aimed at creating the most reliable and representative earthquake catalog covering the Russian and European Arctic. The earthquake catalog of the Gakkel and Knipovich ridges, as well as the Svalbard Archipelago with a unified magnitude scale, was formed based on the combination of four regional Russian catalogs and the ISC catalog. The merging of catalogs was carried out using the modification of the author’s methodology, which allowed for the identification of records in different datasets related to the same seismic event. The modification was introduced due to significant changes over time in the source catalogs. The unified proxy moment magnitude scale was formed based on regression analysis of the different magnitude estimates provided by various agencies. The integrated catalog included 17,922 earthquakes that occurred during the period 1962–2022. Analysis of the integrated catalog showed that the level of registration in the studied area significantly varies over space and time. Before 1995, the catalog contained only strong and moderate earthquakes, and the magnitude of complete registration Mc was 5.0 in the Gakkel Ridge, 4.7 in the Knipovich Ridge, and 4.5 in the Svalbard Archipelago. The number of recorded events increased in the period 1995–2011, and Mc decreased to 4.0 in the Gakkel and Knipovich ridges and to 2.8 in the Svalbard Archipelago. The best level of registration in the Svalbard Archipelago and the Knipovich Ridge was achieved after 2012, when Mc reached 1.7 and 2.8, respectively. In the Gakkel Ridge, despite a noticeable increase in the number of reported events from 2012, the magnitude of complete registration did not improve and was 4.0. The presented integrated earthquake catalog is intended for a wide range of studies of the seismic regime of the Arctic.

Integrated Earthquake Catalog II: The Western Sector of the Russian Arctic

The article is a continuation of the research on creating the most complete and representative earthquake catalogs by combining all available data from regional, national, and international seismological agencies and reducing magnitudes to a uniform scale. The task of identifying and removing duplicates that arise during the merging process is solved using the authors’ modification of the nearest neighbor method. It is evident that the intelligent merging of different earthquake catalogs for the same territory will improve the completeness and representativeness of events in the final integrated catalog. In this article, the earthquake catalog of the western sector of the Arctic zone of the Russian Federation (AZRF) covering the period 1962–2022 was created by merging three regional Russian catalogs and the ISC catalog. The ratio of magnitude types in the catalog for different seismic networks was analyzed, and magnitude estimates were unified based on the obtained ratios. For analyzing seismic activity in the western AZRF, it is recommended to use earthquakes from the period 1998–2020 when the catalog was significantly cleaned from explosions and other events of the “non-earthquake” type.

Retracted: Self-consistent models of Earth’s mantle and core from long-period seismic and tidal constraints

SUMMARYIn this study, we inverted a large set of normal-mode centre-frequencies and quality (attenuation) factors, including astronomic-geodetic data (mass, moment of inertia and tidal response), using self-consistently built models of the radial elastic and anelastic seismic structure of the Earth. The mantle models are constructed using petrologic phase equilibria in combination with a laboratory-based viscoelastic model that connects dissipation from seismic to tidal periods, whereas seismic properties for a well-mixed and homogeneous core are computed using equations-of-state. Relative to the preliminary seismic reference model (PREM), we find that for the models to fit the observations, mantle P- and S-wave velocities have to be slightly faster and slower, respectively, while outer-core P-wave velocity is slower on account of a different velocity gradient, whereas inner-core velocity structure is similar, within the uncertainties of the inferred model parameters. In terms of density, we find that the lower mantle is less dense and the outer core more dense than PREM, while the inner core is similar to PREM. To study the impact of the inferred mantle seismic velocity structure, we computed P- and S-wave traveltimes and compared these to the observations of globally-averaged P- and S-wave traveltimes from the reprocessed ISC catalogue that resulted in an excellent match. In an attempt to further refine the seismic P-wave velocity structure of the outer core, we also considered multiple core–mantle-boundary underside-reflected body wave traveltime data. Although the match to the underside reflections clearly improves as a result of a steeper velocity gradient in the outer core relative to the normal-modes- and astronomic-geodetic-data-only case, subtle differences nevertheless persist that appear to support a change in velocity gradient in the outermost core, evocative of a stably-stratified layer. The laboratory-based viscoelastic model considered here resolves the anelastic response of Earth’s mantle from long-period seismic (∼100 s) to tidal (18.6 yr) periods, accounting for both normal-mode and tidal dissipation measurements. Finally, as a potential means of refining core composition, we considered the density contrast across the inner-core boundary (ICB) based on our inverted models. The most probable ICB density difference found here is 0.3–0.45 g cm−3, which is in the lower range of earlier body-wave- and normal-mode-based predictions. This suggests that the compositional heterogeneity associated with light-element partitioning, which is considered the principal driving mechanism for the compositional convection that powers the geodynamo, may be less effective than previously thought, calling for exsolution of solids from the liquid outer core as a possible additional source of energy. This would also help address the problem of a young inner core.

Stress dissipation and seismic potential in the central seismic gap of the north-west Himalaya

• One-third of the earthquakes in CSG show stress drop <1 bar. • High stress drop events are absent in the upper 10 km of the crust. • Source parameter values are lower in Kumaon than CSG. • Kangra and Kumaon segments can be tipped to host a future great earthquake. The central seismic gap (CSG) of the Himalaya continues to pose the ever more intriguing question of whether it has the potential to host a devastating earthquake. To address the issue, we examine the distribution and dissipation of post-seismic stress in the northwest segment of the CSG. This study computes the source parameters of 47 local earthquake events (3.5 ≤ M L ≤ 5.5) in the Garhwal-Kumaon Himalaya during the period 2016–2018 using P-wave spectral analysis. In addition, we compile the stress drop results for 326 earthquake events from numerous studies carried out across the seismic gap, to estimate the source parameter variation. The estimated seismic moments (10 12 ≤ M o ≤ 10 16 N-m), source radii (0.3 ≤ r ≤ 1.3 km) and stress drops (0.1 ≤ σ ≤ 40.6 bar) for Kumaon are observed to be significantly lower than the overall values for the CSG (10 10 ≤ Mo ≤ 10 20 N-m; 0.1 ≤ r ≤ 13.2 km; 0.01 ≤ σ ≤ 77 bar), showing incomplete dissipation of the accumulated stress. About one-third of the earthquakes in the CSG show very low stress drop (σ < 1 bar). The high stress drop events are predominant along the Main Himalayan Thrust at mid-crustal depths, with the upper ∼ 10 km of the brittle crust rarely hosting any big event. Based on the analysis, the scaling relation M o f c 3.677 = 6E + 15 N-m/s 3 has been proposed for the Kumaon region. Other scaling relations have been developed between important source parameters (M L , M w , M o , and σ) that may serve as valuable inputs for assessing the earthquake hazard in the region. The estimated b-value for the Kumaon Himalaya estimated from the local earthquake catalog is 0.64 ± 0.08, which is low compared to the Garhwal and rest of the NW Himalaya. The overall b-value of the CSG region computed from the ISC catalog is 0.62 ± 0.06. From the stress drop and b-value variation in the CSG, the Kangra and the Kumaon segments are identified to be potentially hazardous zones for a future great earthquake.

Seismicity and influence of coulomb stress on the risk of earthquakes in South Sulawesi

This study aims to analyze the distribution of earthquake events, analyze seismicity based on the parameters of b value, analyze changes in Coulomb stress and analyze the risk of earthquake events in South Sulawesi based on seismicity levels and changes in Coulomb stress in the range of 1991-2021. The data used in this study is data on earthquake events in 1991-2021 obtained from the IRIS and ISC catalogues. Data from the IRIS catalogue is mapped using ArcGIS software to see the distribution of the next earthquake spread processed by using MATLAB-based ZMapp 7 software to obtain the value of seismicity parameter (b value). Data from the ISC catalogue is mapped using Google Earth software to see the spread of earthquakes and then processed using MATLAB-based Coulomb 3.1 software to obtain analysis of stress Coulomb changes. Based on the results of the analysis obtained a value of b between 0.9-1.5 shows the value of b obtained is relatively low which correlates with a high level of stress. Based on the results of the analysis of changes in Coulomb stress obtained the movement of increased stress towards the red lobe with a value of 0.1 to 1 bar and decreased stress towards the blue lobe with a value of -0.1 to -1 bar. In general, earthquake-prone areas are located in the northern to central parts of South Sulawesi.

Comparison of P-Wave Tomography Model in Sunda-Banda Arc by Using Data from BMKG and ISC

The tectonic setting of our study area is located between the Island of Java and Timor Leste. The complexity of this region is started with two different plates, The Indo-Australian plate and the Eurasian plate that move with different orientations and convergence rate. This area also shows active seismic activity and has a series of active volcanoes as a product of subduction and collision. To deepen understand this area, we perform delay time tomography using FMTOMO package that includes 3-D finite-difference based ray tracing and sub-space inversion procedure. We used two different sets of data, the first one is 4 years data catalog from the Indonesian Agency of Meteorology, Climatology and Geophysics, and the second one is 47 years of data from the International Seismological Centre. Data from the local Indonesian show agency shows a fewer number of events but more focus clusters. Meanwhile, the data from ISC catalog has more events and evenly distributed data. However, we also noticed that data from ISC has cluster events located at the same depth that can be improved with events relocation for better depth estimation. The Checkerboard models from both data set show a comparable result, though data from ISC show a better recovered model at a deeper depth and shallow part in the eastern area. The checkerboard from the local Agency shows slightly better results in the shallow part. Next, we invert delay time for each data set using we optimized damping and smoothing parameters. Final tomogram models show that data from the local Agency show a more continuous fast velocity band representing a downgoing subducting slab and possible back-arc thrust while results from the ISC data show a more detached fast velocity band that could be contributed from fixed depth problem in the data set. However, we noticed that data from ISC show a higher amplitude low-velocity anomaly especially in the shallow depth

Hydroacoustic Observations of Two Contrasted Seismic Swarms along the Southwest Indian Ridge in 2018

In 2018, two earthquake swarms occurred along spreading ridge segments of the ultra-slow Southwest Indian Ridge (SWIR). The first swarm was located at the spreading-ridge intersection with the Novara Fracture Zone, comprising 231 events (ISC catalogue) and spanning over 6 days (10 July to 15 July). The second swarm was more of a cluster of events focusing near a discontinuity, 220 km west of the Rodrigues Triple Junction, composed of 92 events and spanning over 31 days (27 September to 27 October). We examined these two swarms using hydroacoustic records from the OHASISBIO network with seven to nine autonomous hydrophones moored on either side of the SWIR. We detected 1109 hydroacoustic events spanning over 13 days (6 July to 18 July) in the first swarm and 4880 events spanning over 33 days in the second swarm (25 September to 27 October). The number of events per day was larger, and the hydroacoustic magnitude (source level) was, on average, smaller during the second swarm than the first. The spatio-temporal distribution of events from both swarms indicates a magmatic origin initiated by dike intrusions and followed by a readjustment of stresses in the surrounding crust.

Mapping of Eastern North Atlantic Ocean seismicity from Po/So observations at a mid-aperture seismological broad-band deep sea array

SUMMARYA mid-aperture broad-band test array (OBS array DOCTAR) was deployed from June 2011 to April 2012 about 100 km north of the Gloria fault in the Eastern North Atlantic in about 5000 m water depth. In addition arrays were installed on Madeira Island and in western Portugal mainland. For the first time in the Eastern North Atlantic, we recorded a large number of high frequency Po and So waves from local and regional small and moderate earthquakes (ML &amp;lt; 4). An incoherent beamforming method was adapted to scan continuous data for such Po and So arrivals applying a sliding window waveform migration and frequency–wavenumber technique. We identify about 320 Po and 1550 So arrivals and compare the phase onsets with the ISC catalogue (ISC 2015) for the same time span. Up to a distance of 6° to the DOCTAR stations all events listed in the ISC catalogue could be associated to Po and So phases. Arrivals from events in more than 10° distance could be identified only in some cases. Only few Po and/or So arrivals were detected for earthquakes from the European and African continental area, the continental shelf regions and for earthquakes within or northwest of the Azores plateau. Unexpectedly, earthquake clusters are detected within the oceanic plates north and south of the Gloria fault and far from plate boundaries, indicating active intraplate structures. We also observe and locate numerous small magnitude earthquakes on the segment of the Gloria fault directly south of DOCTAR, which likely coincides with the rupture of the 25 November 1941 event. Local small magnitude earthquakes located beneath DOCTAR show hypocentres up to 30 km depth and strike-slip focal mechanisms. A comparison with detections at temporary mid-aperture arrays on Madeira and in western Portugal shows that the deep ocean array performs much better than the island and the continental array regarding the detection threshold for events in the oceanic plates.We conclude that sparsely distributed mid-aperture seismic arrays in the deep ocean could decrease the detection and location threshold for seismicity with ML &amp;lt; 4 in the oceanic plate and might constitute a valuable tool to monitor oceanic plate seismicity.

Предварительная оценка сейсмической опасности района Арктического хребта Гаккеля и окрестностей

This study describes primary data, methods of estimation and final results of the preliminary seismic hazard assessment in the region of the Gakkel Ridge that is a northernmost seismogenic zone of the Earth. According to geological data, the region is considered potentially oil and gas, but its industrial development has not yet begun. These authors for the Baltic Sea did the similar work earlier. At the first stage of this study, the earthquake catalog unified in magnitude Mw was compiled for the period from 1912 to 2014. Information on seismic events from historical sources and the ISC catalog was used, as well as the results of observations of the Arkhangelsk seismic network in the Arctic for 2014–2018. The representative part of earthquake data was revealed and the seismic regime has been studied. By seismicity origin, the region is divided into the highly active rift zone of the Gakkel Ridge and the continental slopes of the Barents, Kara and eastern Laptev seas with weaker activity, separated and framed by aseismic areas like the Nansen, Amundsen basins and the Lomonosov Ridge. The seismic zoning of the study region was carried out based on structural analysis of geological and geophysical data. The mapping of probabilistic seismic hazard in terms of maximum accelerations of PGA soil movements for a return period of 500 and 100 years (10% probability of exceedance in the next 50 and 10 years) was conducted using the CRISIS program. As expected, the most dangerous was the Gakkel zone about 200 km wide.

Submeridional boundary zone in Asia: seismicity, lithosphere structure, and the distribution of convective flows in the upper mantle

The study is focused on the submeridional transregional boundary that stretches as a wide band along 105°E in Central Asia. In modern seismic models, it is traceable to a depth of ~600 km. In the continental area to the west of this boundary, seismic activity is increased. Following the study of the origin of the transregional boundary zone, it becomes possible to assess its contribution to the current geodynamic processes in Asia. This article presents a comprehensive analysis based on comparison of the available data with the results obtained in our study using independent methods. The distribution of earthquakes was analyzed by depth. We revealed a correlation between the characteristics of seismotectonic deformation (STD) reconstructed from earthquake focal mechanisms, the structure of P-velocity anomalies, and the distribution of convection flows in the upper mantle. The pattern of seismic velocity anomalies in the upper mantle was investigated on the basis of the data from the ISC catalogue for the period of 1964–2011. The modeling was carried out for two regional tomographic schemes, using the first arrivals of P-waves from [Koulakov et al.,2002and PP-phases from [Bushenkova et al.,2002, with the subsequent summation with weight coefficients depending on the distribution of the input data in each scheme. A similar approach was applied in [Koulakov, Bushenkova,2010for the territory of Siberia; however, that model only partially covered the submeridional transregional boundary zone and was based on fewer ISC data (until 2001). The parameters of the combined model were used to estimate variations in the lithosphere thickness, which can significantly influence the structure of convection flows in the upper mantle [Chervov et al., 2014; Bushenkova et al., 2014, 2016. The thickness variations were taken into account when setting boundary conditions in the numerical modeling of thermal convection, which followed the algorithm described in [Chervov, Chernykh,2014. The STD field was reconstructed from the earthquake focal mechanisms (M≥4.6) which occurred in Central Asia in 1976–2017. The analysis shows that the zone, wherein the seismic regime changes, correlates with the band wherein the STD principal axes are turning, the submeridional high/low velocity elongated boundary in the seismotomographic model, as well as with the submeridionally elongated descending convective flow in the upper mantle. Shortening of the STD principal axes is observed in the submeridional direction in the western part and in the sublatitudinal direction in the eastern part of the study area. The directions of the principal axes turn in the 93–105°E zone. It is thus probable that the submeridionally elongated descending convective flow in the upper mantle of this region, which results from the superposition of the lithosphere thickness heterogeneities, is a barrier to propagation of seismically manifested active geodynamic processes caused by lithospheric plates collision.

Active tectonics in the Gulf of California and seismicity (M > 3.0) for the period 2002–2014

We present a catalog of accurate epicenter coordinates of earthquakes located in the Gulf of California (GoC) in the period 2002–2014 that permits us to analyze the seismotectonics and to estimate the depth of the seismogenic zone of this region. For the period April 2002 to December 2014 we use body-wave arrival times from regional stations of the Broadband Seismological Network of the GoC (RESBAN) operated by CICESE to improve hypocenter locations reported by global catalogs. For the northern region of the GoC (30°N–32°N) we added relocated events from the 2011-Hauksson-Yang-Shearer, Waveform Relocated Earthquake Catalog for Southern California (Hauksson et al., 2012; Lin et al., 2007). From October 2005 to October 2006 we incorporated hypcenters located by Sumy et al. (2013) in the southern GoC combining an array of ocean-bottom seismographs, of the SCOOBA experiment, with onshore stations of the NARS-Baja array. This well constrained catalog of seismicity highlights zones of active tectonics and seismic deformation within the North America-Pacific plate boundary. We estimate that the minimum magnitude of completeness of this catalog is Mc=3.3±0.1 and the b=0.92±0.04 value of the Gutenberg-Richter relation. We find that most earthquakes in the southern GoC are generated by transform faults and this region is more active than the central GoC region. However, the northern region, where most deformation is generated by oblique faults is as active as the southern region. We used the ISC catalog to evaluate the size distribution of seismicity of these regions, and the b value of the Gutenberg-Richter relation and found that b is slightly lower in the central GoC (b=0.86±0.02) compared to the northern (b=1.14±0.04) and the southern (b=1.11±0.04) regions. We observed seismicity that occurs in the Stable Central Peninsular Province, despite the fact that significant active deformation has not been identified in this region.

KAJIAN STATISTIK SEISMISITAS KAWASAN SUMATERA

The research was conducted in order to identify the distribution of seismicity, repeated periods of earthquakes and the probability of earthquake in Sumatra using Likelihood Maximum methods. Earthquake data from the ISC and USGS catalogue were used. Earthquake data from 1914-2014 (100 year span) with magnitude of (M)≥5 SR and depth (h)≤100 km that is located at 60LS – 100LU and 920BT – 1080BT. The latitude and longitude coordinates were divided into 21 regions. The results showed distribution of seismicity in Sumatran as much as 3634 events of earthquake during the 100 last years. The distribution of seismicity in Sumatra is quite high. From data shown, there was value a, which is between 4.81 - 9.94 and value b which is between 0.54 - 1.32. This condition indicated an index value of M≥5 SR, which is between 0.006 - 0.076 and repeated periods between 13 - 175 years with the probability of earthquake M≥5 SR between 5.6% - 99.9% for T (time) = 10, 50 and 100 years.

Matched-filter detection of the missing pre-mainshock events and aftershocks in the 2015 Gorkha, Nepal earthquake sequence

The 25 April 2015 Mw 7.8 Gorkha, Nepal earthquake occurred at the bottom edge of the locked portion of the Main Himalayan Thrust, where the Indian plate underthrusts the Himalayan wedge. The earthquake was followed by a number of large aftershocks, but not preceded by any foreshocks within ~3weeks according to the NEIC or ISC catalog. However, due to the limited station coverage of the local seismic network, a large portion of events may not be reported in routine catalogs. Here, we employ the matched filter technique to recover the undocumented earthquakes for the period beginning 80days before through 30days after the 2015 Gorkha earthquake. We detect twice as many aftershocks as those listed in the ISC catalog. We observe an along-strike aftershock expansion after the mainshock and before the largest Mw 7.3 aftershock that occurred on 12 May 2015. Repeating earthquakes are found to the ESE of the mainshock rupture area, suggesting that the expansion may be partially driven by afterslip. In addition, we observe a significant increase in seismicity rate ~3–4days prior to the mainshock, initiating ~6h after a M 5.2 earthquake, located ~240km to the NW of the mainshock. The increase of seismic activities occurs in a wide region around the Gorkha principal slip zone, indicating the effect of delayed dynamic triggering may contribute to the large-scale unloading process prior to the 2015 Gorkha mainshock.

Segmentation of the Izu-Bonin and Mariana slabs based on the analysis of the Benioff seismicity distribution and regional tomography results

Abstract. We present a new model of P and S velocity anomalies in the mantle down to a depth of 1300 km beneath the Izu-Bonin and Mariana (IBM) arcs. This model is derived based on tomographic inversion of global travel time data from the revised ISC catalogue. The results of inversion are thoroughly verified using a series of different tests. The obtained model is generally consistent with previous studies by different authors. We also present the distribution of relocated deep events projected to the vertical surface along the IBM arc system. Unexpectedly, the seismicity forms elongated vertical clusters instead of horizontal zones indicating phase transitions in the slab. We propose that these vertical seismicity zones mark zones of intense deformation and boundaries between semi-autonomous segments of the subducting plate. The P and S seismic tomography models consistently display the slab as prominent high-velocity anomalies coinciding with the distribution of deep seismicity. We can distinguish at least four segments which subduct differently. The northernmost segment of the Izu-Bonin arc has the gentlest angle of dipping which is explained by backward displacement of the trench. In the second segment, the trench stayed at the same location, and we observe the accumulation of the slab material in the transition zone and its further descending to the lower mantle. In the third segment, the trench is moving forward causing the steepening of the slab. Finally, for the Mariana segment, despite the backward displacement of the arc, the subducting slab is nearly vertical. Between the Izu-Bonin and Mariana arcs we clearly observe a gap which can be traced down to about 400 km in depth. Based on joint consideration of the tomography results and the seismicity distribution, we propose two different scenarios of the subduction evolution in the IBM zone during the recent time, depending on the reference frame of plate displacements. In the first case, we consider the movements in respect to the Philippine Plate, and explain the different styles of the subduction by the relative backward and forward migrations of the trench. In the second case, all the elements of the subduction system move westward in respect to the stable Asia. Different subduction styles are explained by the "anchoring" of selected segments of the slab, different physical properties of the subducting plate and the existence of buoyant rigid blocks related to sea mount and igneous provinces.

Seismotectonics and Earth tides

The results of the processing and analysis of the global earthquake distribution (more than 250000 events based on the ISC catalog) and the study of moonquakes distribution (about 900 events based on the published materials) are presented. It was found that the number of events and the energy for both cases show a bimodal distribution with maximums in the middle latitudes, zero values at the polarcaps, and a local minimum in the vicinity of the equator. The probable influence of tectonic processes on the revealed character of the seismic event distribution is analyzed, and the role of Earth tides in the activation of the seismicity in the symmetric zones on both sides of the equator is shown.

High-frequencyPandSvelocity anomalies in the upper mantle beneath Asia from inversion of worldwide traveltime data

[1] A model of seismic P and S anomalies in the upper mantle beneath Asia (in limits of 35°E–140°E, 12°N–57°N) was constructed based on the tomographic inversion of traveltime data from the revised ISC catalog for the years 1964–2004. The inversions were performed independently in 32 overlapping circular windows that cover the entire study area. The free inversion parameters in each window were defined individually depending on the available data based on synthetic modeling. Such adaptive tuning of parameters enables more optimal usage of the input data in areas with inhomogeneous ray coverage compared to global inversions. This approach resolves high-frequency patterns but is less sensitive to large anomalies with sizes comparable to the window diameter. Thus, this approach is somewhat similar to high-frequency filtration of the velocity distribution. The resolution capacity of the model was tested using checkerboard tests with various pattern sizes. To assess the role of random noise, independent test inversions of two data subsets (with odd and even numbers of events) were performed. Clear reconstructions of known structures, such as subducting plates beneath the Japan and Ryukyu arcs whose locations and shapes have been constrained by other studies, further indicate the reliability of the model. The 3-D models of P and S anomalies presented in horizontal and vertical sections show complex interactions of the lithospheric segments beneath the Alpine-Himalayan orogenic belts. Particular attention is focused on the collisional areas of Iran, Pamir-Hindukush, Tien-Shan, and Burma. The digital version of the 3-D P and S models is available at http://www.ivan-art.com/science/REGIONAL.

General regularities in the distribution of seismic events on the Earth and on the Moon

In this work, characteristic features of seismic event distribution by latitudes and depth are compared for the Earth and the Moon. It is shown that earthquakes and moonquakes are distributed similarly by latitudi� nal belts. The problem of earthquake epicenter distribution by latitudinal belts of the Earth was stated in the 1960s. Firstly, only the distribution of strong earthquakes ( M > 7) was studied. In the early works [5, 8], hetero� geneity in events distribution by latitudes was noted. A substantial progress was achieved in [9]. On the basis of a Chinese catalogue of strong earthquakes in 1897– 1980 (1165 events), it was noted that the energy released after seismic events is almost zero for high lat� itudes and the two main peaks of seismic activity are located in middle latitudes and are divided by a zone of less activity near the equator. The papers of the authors of the present communi� cation contain the results of the analysis of earthquake latitudinal and depth distributions [2, 3]; however, comparative analysis of earthquake and moonquake distributions taking into account latitude, depth, and energy of events has not yet been undertaken. The aim of the present work is to present the results of such analysis and to comment on the possible relationship of the seismic process with exogenous effects on both the Earth and the Moon. Analysis of a wide spectrum of seismic events was carried out on the material of the ISC catalogue [6] (more than 200 000 events with M ≥ 4) on the basis of the approached elaborated in [2]. It was stated that seismic activity of the planet is almost absent in the poles and in polar caps of the Earth and reveals clearly expressed maximums in middle latitudes of the North� ern and Southern Hemispheres and the stable local minimus near the equator. These distributions by lati� tudinal belts of the Earth are characteristic for a num� ber of seismic events and for released energy as well. Because of the fact that most earthquakes are concen� trated in the boundaries of lithospheric plates, in [2] normalizing of earthquake number and released energy by length of the lithospheric plate boundaries in every single latitudinal belt was used. Such a nor� malizing gives us a power of this area of a plate bound� ary (average number of earthquakes generated per every 100 km of plate boundary). Using of this charac� teristic, the physical sense of which is clear, allows us to compare seismic activity of latitudinal belts in vari� ous parts of the world. The total number of studied events was subdivided into several subgroups by values of magnitude ranges (MRs: 4.0 ≤ Mb < 4.5; 4.5 ≤ Mb < 5.0; 5.0 ≤ M b < 5.5; 5.5 ≤ M b < 6.0; 6.0 ≤ M b ).

Upper mantle structure beneath southern Siberia and Mongolia, from regional seismic tomography

Tomographic inversion of ~130,000 P and ~11,000 S arrivals from 1045 events recorded by the world seismological network (ISC catalog data) has been applied to image the three-dimensional velocity structure of the upper mantle beneath the Baikal rift and Mongolia. The inversion-derived P and S velocity anomalies show a good agreement. At depths above 200 km, low-velocity zones occur along the contours of the high-velocity Siberian craton and in Mongolia and coincide with fields of Cenozoic volcanism. The deeper mantle appears quite homogeneous, with anomalies no greater than 0.5% and a single low-velocity feature beneath the Siberian craton. The tomographic image is poorly consistent with the hypothesis implying the existence of large mantle plumes under Mongolia which has been checked with synthetic tests. According to the tomography-based geodynamic model, volcanism in the East Sayan mountains may be induced by a hot plume rising from beneath the Siberian craton, but the source of volcanism in the area of Hangayn remains open to discussion.

Seismic tomographic imaging ofP- andS-waves velocity perturbations in the upper mantle beneath Iran

SUMMARY The inverse tomography method has been used to study the P- and S-waves velocity structure of the crust and upper mantle underneath Iran. The method, based on the principle of source‐receiver reciprocity, allows for tomographic studies of regions with sparse distribution of seismic stations if the region has sufficient seismicity. The arrival times of body waves from earthquakes in the study area as reported in the ISC catalogue (1964‐1996) at all available epicentral distances are used for calculation of residual arrival times. Prior to inversion we have relocated hypocentres based on a 1-D spherical earth’s model taking into account variable crustal thickness and surface topography. During the inversion seismic sources are further relocated simultaneously with the calculation of velocity perturbations. With a series of synthetic tests we demonstrate the power of the algorithm and the data to reconstruct introduced anomalies using the ray paths of the real data set and taking into account the measurement errors and outliers. The velocity anomalies show that the crust and upper mantle beneath the Iranian Plateau comprises a low velocity domain between the Arabian Plate and the Caspian Block. This is in agreement with global tomographic models, and also tectonic models, in which active Iranian plateau is trapped between the stable Turan plate in the north and the Arabian shield in the south. Our results show clear evidence of the mainly aseismic subduction of the oceanic crust of the Oman Sea underneath the Iranian Plateau. However, along the Zagros suture zone, the subduction pattern is more complex than at Makran where the collision of the two plates is highly seismic.