Location of the pilot borehole for investigations of reservoir triggered seismicity at Koyna,India

Artificial water reservoir triggered earthquakes are now known to have occurred at over 120 sites globally. The part played by the reservoirs in triggering is not exactly known due to lack of near field observations of triggered earthquakes. Koyna, located near the west coast of India, where triggered earthquakes have been occurring since 1962 provides an excellent site for near field observations of the target M ≥ 2 earthquakes. A 6 borehole seismic network has been deployed recently in the Koyna region at depths of 981–1522 m to improve the hypocenter locations. During May–December 2015, a total of 1039 earthquakes of M_L ≥ 0.5 were located using the borehole seismic network. The region is also monitored through a dense network of 23 surface broad-band stations. Our analysis indicates a significant improvement in the estimation of absolute locations of earthquakes with errors of the order of ± 300 m, combining both the networks. Based on seismicity, and logistics, a block of 2 × 2 km² area has been chosen for drilling the first pilot borehole of ~ 3 km depth, where M ≥ 2 earthquakes have been occurring frequently since 2005.     

5.9 Mw, 18th June 2010 earthquake and fault segment linkage at Andaman – A study based on macroseismic survey,GPS geodesy and Coulomb stress changes

18th June, 2010 5.9 M_w earthquake at North Andaman triggered along NW–SE pre-existing fault with reverse fault mechanism. Macroseismic survey and GPS geodesy reveal maximum damages following NE–SW trend due to normal fault mechanism. Coulomb stress modeling for post- and inter-seismic earthquakes after the 2004 mega-earthquake show different stages of fault segment linkage at North Andaman. The present earthquake has been explained as co-shock due to asiesmic soft linkage of fault propagation.     

Influence of the 2008 Wenchuan earthquake (Mw 7.9) on the occurrence probability of future earthquakes on neighboring faults

The Longquan–Shan fault and the Huya fault are two major neighboring faults of the Longmen–Shan fault zone where the 12 May 2008 Wenchuan earthquake (M_w 7.9) occurred. To study the influence of the Wenchuan event on these two active faults, we calculate changes of Coulomb stress on the Longquan–Shan fault and the Huya fault caused by the Wenchuan mainshock. Our results indicate that the Coulomb stress in the northern section (Zone A) of the Longquan–Shan fault is increased by 0.07–0.10 bars, that in the middle section (Zone B) by 0.04–0.11 bars, and that in the southern section (Zone C) shows almost no change. For the Huya fault, the Coulomb stress is decreased by 0.01–0.03 bars in the northern section (Zone A), 0.10–0.35 bars in the middle section (Zone B), and nearly 0.5 bars in the southern section (Zone C). The epicenter distribution of small earthquakes (M_L ⩾ 1.5) on the Longquan–Shan fault and the Huya fault after the Wenchuan earthquake is consistent with the distribution of the Coulomb stress change. This implies that the Wenchuan earthquake may have triggered small events on the Longquan–Shan fault, but inhibited those on the Huya fault. We then use the rate/state friction law to calculate the occurrence probability of future earthquakes in the study region for the next decade. They include the distribution of b-values, magnitude of completeness (M_c), the background seismicity rate, a value of Aσ_n and the duration for the transient effect (t_a) in the study region. We also estimate the earthquake occurrence probabilities on the neighboring faults after the Wenchuan earthquake. Our results show that, the occurrence probability of future earthquakes in the Longquan–Shan has a slight increase, being 7% for M ⩾ 5.0 shocks during the next decade, but the earthquake probability in the Huya region is reduced obviously, being 5–20%, 7–26% and 3–9% for M ⩾ 5.0 shocks during the next decade in sections A, B and C of the Huya fault, respectively.     

Geodetic and seismological investigation of crustal deformation near Izmir (Western Anatolia)

The Aegean region including western Turkey, mainland Greece, and the Hellenic Arc is the most seismological and geodynamical active domain in the Alpine Himalayan Belt. In this study, we processed 3 years of survey-mode GPS data and present the analysis of a combination of geodetic and seismological data around Izmir, which is the third most populated city in Turkey. The velocities obtained from 15 sites vary between 25 mm/yr and 28 mm/yr relative to the Eurasian plate. The power law exponent of earthquake size distribution (b-value) ranges from 0.8 to 2.8 in the Izmir region between 26.2°E and 27.2°E. The lowest b-value zones are found along Karaburun Fault (b = 0.8) and, between Seferihisar and Tuzla Faults (b = 0.8). A localized stress concentration is expected from numerical models of seismicity along geometrical locked fault patches. Therefore, areas with lowest b-values are considered to be the most likely location for a strong earthquake, a prediction that is confirmed by the 2005 M_w = 5.9 Seferihisar earthquake sequences, with epicentres located to the south of the Karaburun Fault. The north–south extension of the Izmir area is corroborated by extension rates up to 140 nanostrain/yr as obtained from our GPS data. We combined the 3-year GPS velocity field with the published velocity field to determine the strain rate pattern in the area. The spatial distribution of b-value reflects the normal background due to the tectonic framework and is corroborated by the geodetic data. b-Values correlate with strain pattern. This relationship suggests that decrease of b-values signifies accumulating strain.     

Rapid Eocene erosion,sedimentation and burial in the eastern Himalayan syntaxis and its geodynamic significance

The lower Bomi Group of the eastern Himalayan syntaxis comprises a lithological package of sedimentary and igneous rocks that have been metamorphosed to upper amphibolite-facies conditions. The lower Bomi Group is bounded to the south by the Indus–Yarlung Suture and to the north by unmetamorphosed Paleozoic sediments of the Lhasa terrane. We report U–Pb zircon dating, geochemistry and petrography of gneiss, migmatite, mica schist and marble from the lower Bomi Group and explore their geological implications for the tectonic evolution of the eastern Himalaya. Zircons from the lower Bomi Group are composite. The inherited magmatic zircon cores display ²⁰⁶Pb/²³⁸U ages from ~ 74 Ma to ~ 41.5 Ma, indicating a probable source from the Gangdese magmatic arc. The metamorphic overgrowth zircons yielded ²⁰⁶Pb/²³⁸U ages ranging from ~ 38 Ma to ~ 23 Ma, that overlap the anatexis time (~ 37 Ma) recorded in the leucosome of the migmatites. Our data indicate that the lower Bomi Group do not represent Precambrian basement of the Lhasa terrane. Instead, the lower Bomi Group may represent sedimentary and igneous rocks of the residual forearc basin, similar to the Tsojiangding Group in the Xigaze area, derived from denudation of the hanging wall rocks during the India–Asia continental collision. We propose that following the Indian–Asian collision, the forearc basin was subducted, together with Himalayan lithologies from the Indian continental slab. The minimum age of detrital magmatic zircons from the supracrustal rocks is ~ 41.5 Ma and their metamorphism had happened at ~ 37 Ma. The short time interval (< 5 Ma) suggests that the tectonic processes associated with the eastern Himalayan syntaxis, encompassing uplift and erosion of the Gangdese terrane, followed by deposition, imbrication and subduction of the forearc basin, were extremely rapid during the Late Eocene.     

Estimation of strain distribution using GPS measurements in the Kumaun region of Lesser Himalaya

The continuous process of continent–continent collision between the Indian and the Eurasian plates has led to the formation of the Himalayan range and continuously caused earthquakes in the region. Large earthquakes with magnitudes of 8 and above occur in this region infrequently, releasing the elastic strain accumulated over years around the plate boundary. Geodetic measurements can help estimate the strain distribution along the fault system. These measurements provide information on active deformations and associated potential seismic hazards along the Himalayan arc. In order to understand the present deformation around the plate boundary, we collected GPS data during three campaigns in the years of 2005–2007 at 16 sites in the Kumaun region of the Lesser Himalaya. Horizontal velocity vectors estimated in ITRF2000 are found to be in the range of 41–50 mm/yr with an uncertainty level of the order of 1 mm/yr. The velocity field indicates that the present convergence of around 15 mm/yr takes place in the Kumaun Himalaya. Further, we estimate the strain components in the study area for understanding the currently active tectonic process in the region. The estimated dilatational strain indicates that the northern part near the Main Central Thrust (MCT) is more compressional than the southern part. Maximum shear strain is mostly accommodated in the northern part too. The maximum shear and dilatational strain rates are about 1.0 and 0.5 μstrain/yr. It is seen that the distribution of high shear strain spatially correlates with seismicity. The maximum of extensional and compressional strains due to the force acting along the Main Central Thrust (MCT) in the NW–SE direction are found to be 0.4 and 0.1 μstrain/yr, respectively. The maximum shear strain in the northern part of the Himalaya appears to be associated with the convergence of the region proposed by other geophysical studies.     

The large aftershocks triggered by the 2011 Mw 9.0 Tohoku-Oki earthquake,Japan

The Mw 9.0 Tohoku-Oki earthquake that occurred off the Pacific coast of Japan on March 11, 2011, was followed by thousands of aftershocks, both near the plate interface and in the crust of inland eastern Japan. In this paper, we report on two large, shallow crustal earthquakes that occurred near the Ibaraki-Fukushima prefecture border, where the background seismicity was low prior to the 2011 Tohoku-Oki earthquake. Using densely spaced geodetic observations (GPS and InSAR datasets), we found that two large aftershocks in the Iwaki and Kita-Ibarake regions (hereafter referred to as the Iwaki earthquake and the Kita-Ibarake earthquake) produced 2.1 m and 0.44 m of motion in the line-of-sight (LOS), respectively. The azimuth-offset method was used to obtain the preliminary location of the fault traces. The InSAR-based maximum offset and trace of the faults that produced the Iwaki earthquake are consistent with field observations. The fault location and geometry of these two earthquakes are constrained by a rectangular dislocation model in a multilayered elastic half-space, which indicates that the maximum slips for the two earthquakes are 3.28 m and 0.98 m, respectively. The Coulomb stress changes were calculated for the faults following the 2011 Mw 9.0 Tohoku-Oki earthquake based on the modeled slip along the fault planes. The resulting Coulomb stress changes indicate that the stresses on the faults increased by up to 1.1 MPa and 0.7 MPa in the Iwaki and Kita-Ibarake regions, respectively, suggesting that the Tohoku-Oki earthquake triggered the two aftershocks, supporting the results of seismic tomography.     

Co and postseismic characteristics of Indian sub-continent in response to the 2004 Sumatra earthquake

The M_w 9.3 Sumatra earthquake of December 26, 2004 caused extensive coseismic displacements globally, measurements of which were made essentially using modern geodetic techniques. This earthquake induced considerable perturbation in stress distribution as far as ∼8000 km away from the epicenteral region, which is tending to relax to its normal rates as seen from postseismic transient deformation. The monitoring of crustal displacements from strategically located sites using GPS provides coseismic as well as postseismic deformation that facilitates the understanding of the fault geometry, elastic thickness, postseismic relaxation mechanisms, rheology and earthquake recurrence time interval.We investigated coseismic and postseismic GPS derived displacements in Indian region together with the GPS data collected from Andaman and Sumatra region. It is found that while EW displacements are significantly large in peninsular India, those in the region to the north of Central India Tectonic Zone (CITZ) are relatively small. We could delineate the postseismic transients from position time series and interpreted them in terms of viscoelastic relaxation. It is inferred that the postseismic deformation is characterized by a power-law viscoelastic flow in the mantle. In Indian peninsula region, the timescale parameter of the exponential decay (τ = 250 days) would require an extremely low viscosity for the upper mantle. Relying on the prevailing coseismic and postseismic displacement fields, the present study also reflects upon the contemporary litho-tectonics of the Indian sub-continent.     

The basic parameters of acceleration spectra for M > 5 earthquakes in the Baikal Rift Zone

We consider the acceleration spectra of S waves from M = 5-6.5 earthquakes that occurred in three regions of the Baikal Rift Zone. The main characteristics of the shape and level of the spectra are given, and the difference between the spectra in the relative positions of the focus and seismic station is shown. Average acceleration spectra have been obtained for all three regions with regard to shape and the magnitude of the earthquakes. The spectra have been transformed so that they correspond to the accelerations of M = 6.5 and M = 7.5 earthquakes. The conclusion is made that the spectra not necessarily retain their shape even with the same movements in the focus.     

Geochronological problems related to polymetamorphism in the Limpopo Complex,South Africa

The integration of new and published geochronologic data with structural, magmatic/anatectic and pressure–temperature (P–T) process information allow the recognition of high-grade polymetamorphic granulites and associated high-grade shear zones in the Central Zone (CZ) of the Limpopo high-grade terrain in South Africa. Together, these two important features reflect a major high-grade D₃/M₃ event at ~ 2.02 Ga that overprinted the > 2.63 Ga high-grade Neoarchaean D₂/M₂ event, characterized by SW-plunging sheath folds. These major D₂/M₂ folds developed before ~ 2.63 Ga based on U–Pb zircon age data for precursors to leucocratic anatectic gneisses that cut the high-grade gneissic fabric. The D₃/M₃ shear event is accurately dated by U–Pb monazite (2017.1 ± 2.8 Ma) and PbSL garnet (2023 ± 11 Ma) age data obtained from syntectonic anatectic material, and from sheared metapelitic gneisses that were completely reworked during the high-grade shear event. The shear event was preceded by isobaric heating (P = ~ 6 kbar and T = ~ 670–780 °C), which resulted in the widespread formation of polymetamorphic granulites. Many efforts to date high-grade gneisses from the CZ using PbSL garnet dating resulted in a large spread of ages (~ 2.0–2.6 Ga) that reflect the polymetamorphic nature of these complexly deformed high-grade rocks.     

The Yarlung suture mélange,Lopu Range,southern Tibet: Provenance of sandstone blocks and transition from oceanic subduction to continental collision

With the aim of better understanding the history of ocean closure and suturing between India and Asia, we conducted a geologic investigation of a siliciclastic matrix tectonic mélange within the western Yarlung suture zone of southern Tibet (Lopu Range region, ~ 50 km northwest of Saga). The siliciclastic matrix mélange includes abundant blocks of ocean plate stratigraphy and sparse blocks of sandstone. Metapelite and metabasite blocks in the mélange exhibit lower greenschist facies mineral assemblages, indicating that they were not deeply subducted. We obtained detrital zircon U-Pb geochronologic and sandstone petrographic data from sandstone blocks in the mélange and sandstone beds from Tethyan Himalayan strata exposed to the south of the suture. The sandstones from both units are all similar in U-Pb detrital zircon age spectra and petrography to the nearby Tethyan Cretaceous–Paleocene Sangdanlin section, which records the earliest appearance (at ~ 59 Ma) of arc-affinity strata deposited conformably on Indian-affinity strata. Two Paleocene sandstones, one of which is a schistose block incorporated in the siliciclastic matrix mélange, yielded indistinguishable maximum depositional ages of ~ 59 Ma. Mesozoic Asian-affinity sandstone blocks previously documented in the siliciclastic matrix mélange 200–500 km along strike to the east are notably absent in the Lopu Range region. We documented a gradational transition in structural style from the block-in-matrix mélange in the northeast to the south-vergent Tethyan thrust belt in the southwest. Blocks of Tethyan Himalayan strata increase in size and the volumetric proportion of matrix decreases from northeast to southwest. We conclude that no arc-affinity sandstone blocks were incorporated into the subduction complex until India-Asia collision at ~ 59 Ma when the Xigaze forearc basin became overfilled and Tethyan Himalayan strata entered the trench. As collision progressed, there was a gradual transition in structural style from block-in-matrix mélange formation to imbricate-style thrust belt formation.     

Crustal shear-wave velocity structure beneath northeast India from teleseismic receiver function analysis

We investigated the seismic shear-wave velocity structure of the crust beneath nine broadband seismological stations of the Shillong–Mikir plateau and its adjoining region using teleseismic P-wave receiver function analysis. The inverted shear wave velocity models show ∼34–38 km thick crust beneath the Shillong Plateau which increases to ∼37–38 km beneath the Brahmaputra valley and ∼46–48 km beneath the Himalayan foredeep region. The gradual increase of crustal thickness from the Shillong Plateau to Himalayan foredeep region is consistent with the underthrusting of Indian Plate beyond the surface collision boundary. A strong azimuthal variation is observed beneath SHL station. The modeling of receiver functions of teleseismic earthquakes arriving the SHL station from NE backazimuth (BAZ) shows a high velocity zone within depth range 2–8 km along with a low velocity zone within ∼8–13 km. In contrast, inversion of receiver functions from SE BAZ shows high velocity zone in the upper crust within depth range ∼10–18 km and low velocity zone within ∼18–36 km. The critical examination of ray piercing points at the depth of Moho shows that the rays from SE BAZ pierce mostly the southeast part of the plateau near Dauki fault zone. This observation suggests the effect of underthrusting Bengal sediments and the underlying oceanic crust in the south of the plateau facilitated by the EW-NE striking Dauki fault dipping 30⁰ toward northwest.     

Seismic behavior in central Taiwan: Response to stress evolution following the 1999 Mw 7.6 Chi-Chi earthquake

Following the 1999 M_w 7.6 Chi-Chi earthquake, a large amount of seismicity occurred in the Nantou region of central Taiwan. Among the seismic activities, eight M_w ⩾ 5.8 earthquakes took place following the Chi-Chi earthquake, whereas only four earthquakes with comparable magnitudes took place from 1900 to 1998. Since the seismicity rate during the Chi-Chi postseismic period has never returned to the background level, such seismicity activation cannot simply be attributed to modified Omori’s Law decay. In this work, we attempted to associate seismic activities with stress evolution. Based on our work, it appears that the spatial distribution of the consequent seismicity can be associated with increasing coseismic stress. On the contrary, the stress changes imparted by the afterslip; lower crust–upper mantle viscoelastic relaxation; and sequent events resulted in a stress drop in most of the study region. Understanding seismogenic mechanisms in terms of stress evolution would be beneficial to seismic hazard mitigation.     

Paleoearthquakes in the Uimon basin (Gorny Altai)

Paleoseismological studies confirm that the Uimon basin is thrust by its northern mountain border along the active South Terekta fault. The latest motion along the fault in the 7-8th centuries AD induced an earthquake with a magnitude of M_w= 7.4-7.7 and a shaking intensity of I = 9-11 on the MSK-64 scale. The same fault generated another event (M > 7, I = 9-10), possibly, about 16 kyr ago, which triggered gravity sliding. The rockslide dammed the Uimon valley and produced a lake, where lacustrine deposition began about 14 ± 1 kyr ago, and a later M > 7 (I = 9-10) earthquake at ~ 6 ka caused the dam collapse and the lake drainage. Traces of much older earthquakes that occurred within the Uimon basin are detectable from secondary deformation structures (seismites) in soft sediments deposited during the drainage of a Late Pleistocene ice-dammed lake between 100 and 90 ka and in ~ 77 ka alluvium. The magnitude and intensity of these paleoearthquakes were at least M > 5.0-5.5 and I > 6-7.     

Paleomagnetic versus GPS determined tectonic rotation around eastern Himalayan syntaxis in East Asia

Northward indentation of the Indian Plate has brought about significant tectonic deformation into East Asia. A record of long-term tectonic deformation in this area for the past 50 M yr, particularly the vertical axis rotation, is available through paleomagnetic data. In order to depict rotational deformation in this area with respect to Eurasia, we compiled reliable paleomagnetic data sets from 79 localities distributed around eastern Himalayan syntaxis in East Asia. This record delineates that a zone affected by clockwise rotational deformation extends from the southern tip of the Chuan Dian Fragment to as far as the northwestern part of the Indochina Peninsula. A limited zone that experienced a significant amount of clockwise rotation after an initial India–Asia collision is now located at 23.5°N, 101°E, far away from an area (27.5°N, 95.5°E) where an intense rotational motion has been viewed by a snapshot of GPS measurements. This discrepancy in clockwise rotated positions is attributed to southeastward extrusion of the tectonic blocks within East Asia as a result of ongoing indentation of the Indian Plate. A quantitative comparison between the GPS and paleomagnetically determined clockwise rotation further suggests that following an initial India–Asia collision the crust at 30°N, 94°E paleoposition was subjected to southeastward displacement together with clockwise rotation, which eventually reached to present-day position of 23.5°N, 101°E, implying a crustal displacement of about 1000 km during the past 50 M yr.     

Present-day crustal motion around the Pamir Plateau from GPS measurements

We have used geodetic techniques to improve constraints on the crustal motion of the Pamir Plateau. Three campaigns of Global Position System data acquisition between 2011 and 2015 demonstrate that, in association with the India–Asia collision, a complex pattern of crustal motion exists in the Pamir Plateau. In a north–south direction from the Indian Plate to the Hazak Block, the crust has absorbed ~ 35 mm/yr of shortening, of which ~ 35% is distributed around the Hindu Kush region (~ 12 mm/yr), and another ~ 35% is taken up around the Alai Valley (also ~ 12 mm/yr). Global Position System measurements also show ~ 5 mm/yr of shortening between the Pamir Plateau and the Tajik Basin, whereas between the Pamir and the Tarim Basin, an ~ 10 mm/yr extension rate is observed. With respect to the stable Eurasian Plate, the Pamir rotates counterclockwise at a rate of ~ 1.822°Myr^− 1, with an Euler pole positioned about the west end of the Tajik Basin (37.03 ± 0.74°N, 65.89 ± 0.12°E). The strain rate field calculated from Global Position System velocities reveals that the crustal motion is consistent with localized deformation around the Hindu Kush and the Alai Valley, the latter representing a zone with strong shallow seismic activity.     

Detrital zircon U–Pb age and Hf isotope constrains on the generation and reworking of Precambrian continental crust in the Cathaysia Block,South China: A synthesis

The South China Block, consisting of the Yangtze and the Cathaysia blocks, is one of the largest Precambrian blocks in eastern Asia. However, the early history of the Cathaysia Block is poorly understood due largely to intensive and extensive reworking by Phanerozoic polyphase orogenesis and magmatism which strongly overprinted and obscured much of the Precambrian geological record. In this paper, we use the detrital zircon U–Pb age and Hf isotope datasets as an alternative approach to delineate the early history of the Cathaysia Block. Compilation of published 4041 Precambrian detrital zircon ages from a number of (meta)sedimentary samples and river sands exhibits a broad age spectrum, with three major peaks at ~ 2485 Ma, ~ 1853 Ma and ~ 970 Ma (counting for ~ 10%, ~ 16% and ~ 24% of all analyses, respectively), and four subordinate peaks at ~ 1426 Ma, ~ 1074 Ma, ~ 780 Ma and ~ 588 Ma. Five of seven detrital zircon age peaks are broadly coincident with the crystallisation ages of ~ 1.89–1.83 Ga, ~ 1.43 Ga, ~ 1.0–0.98 Ga and ~ 0.82–0.72 Ga for known igneous rocks exposed in Cathaysia, whereas, igneous rocks with ages of ~ 2.49 Ga and ~ 0.59 Ga have not yet been found. The Hf isotopic data from 1085 detrital zircons yield Hf model ages (T_DM^C) between ~ 4.19 Ga and ~ 0.81 Ga, and the calculated εHf(t) values between − 40.2 and 14.4. The Archean detrital zircons are exclusively oval in shape with complicated internal textures, indicating that they were sourced by long distance transportations and strong abrasion from an exotic Archean continent. In contrast, the majority of detrital zircons in age between ~ 1.9 and ~ 0.8 Ga are euhedral to subhedral crystals, indicative of local derivation by short distance transportations from their sources. The oldest crustal basement rocks in Cathaysia were most likely formed by generation of juvenile crust and reworking of recycled Archean components in Late Paleoproterozoic at ~ 1.9–1.8 Ga, rather than in the Archean as previously speculated. Reworking and recycling of the continental crust are likely the dominant processes for the crustal evolution of Cathaysia during the Mesoproterozoic to Neoproterozoic time, with an intervenient period of significant generation of juvenile crust at ~ 1.0 Ga.Precambrian crustal evolutions of the Cathaysia Block are genetically related to the supercontinent cycles. By comparing detrital zircon data from Cathaysia with those for other continents, and integrating multiple lines of geological evidence, we interpret the Cathaysia Block as an orogenic belt located between East Antarctica, Laurentia and Australia during the assembly of supercontinent Columbia/Nuna at ~ 1.9–1.8 Ga. The Cathaysia Block amalgamated with the Yangtze Block to form the united South China Block during the Sibao Orogeny at ~ 1.0–0.89 Ga. The Laurentia–Cathaysia–Yangtze–Australia–East Antarctica connection gives the best solution to the paleo-position of Cathaysia in supercontinent Rodinia. The significant amount of ~ 0.6–0.55 Ga detrital zircons in Cathaysia and West Yangtze have exclusively high crustal incubation time of > 300 Ma, indicating crystallisation from magmas generated dominantly by crustal reworking. This detrital zircon population compares well with the similar-aged zircon populations from a number of Gondwana-derived terranes including Tethyan Himalaya, High Himalaya, Qiangtang and Indochina. The united South China–Indochina continent was likely once an integral part of Gondwanaland, connected to northern India by a “Pan-African” collisional orogen.     

Geologic hazards associated with seismogenic faulting in southern Siberia and Mongolia: forms and location patterns

The forms and location patterns of geologic hazards induced by earthquakes in southern Siberia, Mongolia, and northern Kazakhstan in1950 through 2008 have been investigated statistically, using a database of coseismic effects created as a GIS MapInfo application, with a handy input box for large data arrays. The database includes 689 cases of macroseismic effects from M_S = 4.1–8.1 events at 398 sites. Statistical analysis of the data has revealed regional relationships between the magnitude of an earthquake and the maximum distance of its environmental effects (soil liquefaction and subsidence, secondary surface rupturing, and slope instability) to the epicenter and to the causative fault. Thus estimated limit distances to the fault for the M_S = 8.1 largest event are 40 km for soil subsidence (sinkholes), 80 km for surface rupture, 100 km for slope instability (landslides etc.), and 130 km for soil liquefaction. These distances are 3.5–5.6 times as short as those to the epicenter, which are 150, 450, 350, and 450 km, respectively. Analysis of geohazard locations relative to nearest faults in southern East Siberia shows the distances to be within 2 km for sinkholes (60% within 1.5 km), 4.5 km for landslides (90% within 1.5 km), 8 km for liquefaction (69% within 1 km), and 35.5 km for surface rupture (86% within 2 km). The frequency of hazardous effects decreases exponentially away from both seismogenic and nearest faults. Cases of soil liquefaction and subsidence are analyzed in more detail in relation to rupture patterns. Equations have been suggested to relate the maximum sizes of secondary structures (sinkholes, dikes, etc.) with the earthquake magnitude and shaking intensity at the site. As a result, a predictive model has been created for locations of geohazard associated with reactivation of seismogenic faults, assuming an arbitrary fault pattern. The obtained results make basis for modeling the distribution of geohazards for the purposes of prediction and estimation of earthquake parameters from secondary deformation.     

Combined U-Pb,Lu-Hf,Sm-Nd and Ar-Ar multichronometric dating on the Bailang eclogite constrains the closure timing of the Paleo-Tethys Ocean in the Lhasa terrane,Tibet

The Lhasa terrane, the main tectonic component of the Himalayan–Tibetan orogen, has received much attention as it records the entire history of the orogeny. The occurrence of Permian to Triassic high-pressure eclogites has a significant bearing on the understanding of the Paleo-Tethys subduction and plate suturing processes in this area. An eclogite from the Bailang, eastern Lhasa terrane, was investigated with a combined metamorphic P–T and U–Pb, Lu–Hf, Sm–Nd and Ar–Ar multichronometric approach. Pseudosection modeling combined with thermobarometric calculations indicate that the Bailang eclogite equilibrated at peak P–T conditions of ~ 2.6 GPa and 465–503 °C, which is much lower than those of Sumdo and Jilang eclogites in this area. Garnet–whole rock–omphacite Lu–Hf and Sm–Nd ages of 238.1 ± 3.6 Ma and 230.0 ± 4.7 Ma were obtained on the same sample, which are largely consistent with the corresponding U–Pb age of 227.4 ± 6.4 Ma for the metamorphic zircons within uncertainty. The peak metamorphic temperature of the sample is lower than the Lu–Hf and Sm–Nd closure temperatures in garnet. This, combined with the core-to-rim decrease in Mn and HREE concentrations, the slightly U-shaped Sm zonation across garnet and the exclusive occurrence of omphacite inclusion in garnet rim, are consistent with the Lu–Hf system skewing to the age of the garnet core and the Sm–Nd system favoring the rim age. The Sm–Nd age was thus interpreted as the age of eclogite-facies metamorphism and the Lu–Hf age likely pre-dated the eclogite-facies metamorphism. ⁴⁰Ar/³⁹Ar dating of hornblende from the eclogite yielded ages about 200 Ma, which is interpreted as a cooling age and is probably indicative of the time of exhumation to the middle crust. The difference of peak eclogite-facies metamorphic conditions and the distinct metamorphic ages for the Bailang eclogite (~ 2.6 GPa and ~ 480 °C; ca. 230 Ma), the Sumdo eclogite (~ 3.4 GPa and ~ 650 °C; ca. 262 Ma) and Jiang eclogite (~ 3.6 GPa and ~ 750 °C; ca. 261 Ma) in the same (ultra)-high-pressure belt indicate that this region likely comprises different slices that had distinct P–T histories and underwent (U)HP metamorphism at different times. The initiation of the opening the Paleo-Tethys Ocean in the Lhasa terrane could trace back to the early Permian. The ultimate closure of the Paleo-Tethys Ocean in the Lhasa terrane was no earlier than ca. 230 Ma.