Evidence for a fluid flow triggered spatio-temporal migration of seismicity in the 2001 M<Subscript>w</Subscript> 7.7 Bhuj earthquake region,Gujarat, India,during 2001–2013

We studied the variations in spatial and temporal clustering of earthquake activity (during 2001–2013) in the Kachchh seismic zone, Gujarat, India, by precisely relocating 3478 events using a joint hypocentral determination (JHD) relocation technique, and high-quality arrival times of 21032 P- and 20870 S-waves. Temporal disposition of estimated station corrections of P- and S-waves suggests that the fluid flow in the causative fault zone of the 2001 Bhuj mainshock increased during 2001–2010, while it reduced during 2011–2013, due to the healing process associated with the perturbed Kachchh fault zone. We also estimated the isotropic seismic diffusivities from epicentral growth patterns, which are found to be much lower than those observed for reservoir-induced seismicity sites in the world. Finally, we analysed the spatial and temporal evolution of this earthquake sequence by solving the diffusion equation of pore-pressure relaxation caused by co- and post-seismic stress changes associated with earthquakes. The value of the isotropic diffusivity is estimated to be 100 m²/s for the Kachchh rift zone. This gives a higher permeability (after a lapse time of 14 years from the occurrence of the 2001 Bhuj mainshock) in comparison to those observed for other intraplate regions in the world. Our results suggest that the observed spatio-temporal migration of seismicity is consistent with the shallow (meteoric water circulation at 0–10 km depths) and deeper (metamorphic fluid and volatile CO₂ circulation at 10–40 km depths) fluid flows in the permeable and fractured causative fault zone of the 2001 Bhuj earthquake.     

Possible evidence of remotely triggered and delayed seismicity due to the 2001 Bhuj earthquake (Mw?=?7.6) in western India

Remote triggering by large earthquakes at regional distances is a globally observed phenomenon. However, there are no reports of observations of dynamic triggering at regional distances of several source lengths associated with the large Mw?=?7.6 Bhuj earthquake of January 26, 2001, in western India. In the present study, a swarm of over 140 microearthquakes that occurred about 500?km southeast of Bhuj, in the geothermal province of the Western Ghats in the Deccan volcanic province (DVP) of India, immediately after the occurrence of the Bhuj earthquake in 2001 is investigated. The post-Bhuj seismicity (M?<?2.0) occurred in three bursts spread over 2?months with each burst of intense activity lasting for 2?C3?days. All the three bursts of seismicity occurred in the same volume along a 5-km-long NW?CSE trending fault. The temporal coincidence and the sudden rise in seismicity that interrupts the characteristically low background seismicity strongly suggest that the Bhuj earthquake may have remotely triggered this activity. The triggered seismicity began approximately 2.5?h after the onset of the Bhuj mainshock and continued well after the passage of the surface waves, suggesting that the dynamic stresses possibly gave rise to secondary time-dependent mechanisms leading to the triggering. It is proposed that the triggered and delayed seismicity is possibly a consequence of the redistribution in pore fluid pressure due to the Bhuj earthquake. This is the first documented observation of remotely triggered seismicity at regional distances due to the Bhuj earthquake.     

Spatial analysis of the frequency–magnitude distribution of aftershock activity of 2003 Bam earthquake: southeast Iran

The Bam earthquake (2003 December 26, M _W = 6.6) was one of the largest earthquakes that occurred in southeast of Iran during last century. It took place along an N–S trending right-lateral strike-slip fault, almost near the southern end of Nyband–Gowk fault. In this study, we mapped the frequency–magnitude distribution of aftershock events spatially across the Bam aftershock zone. The b-value varies between 0.6 and 1.1 across the Bam rupture zone. The overall depth distribution of b-value in Bam aftershock zone reveals two distinct increases in b-value: (1) at depths of 8–10 km and (2) shallower than 4 km beneath the Bam city. There is no correlation between high b- value anomalies found in this study and the region of largest slip, whereas the spatial correlation between high b-value anomalies and the zone of low V _s and high σ (in earlier tomography study) is obvious. This correlation reveals that material properties and increasing heterogeneity are more important in controlling b-value distribution in Bam earthquake rupture zone. The high b-value anomaly near the surface of northern part of rupture zone may be related to unconsolidated and water-rich quaternary alluvial sediments and probable low-strength rocks beneath them. The high b-value anomaly at depth range 8–10 km can be correlated with fractured and fluid-filled mass, which may result from the movement of magma during Eocene volcanism in the Bam area. In this study, the induced changes in pore fluid pressure due to main shock are suggested as a mechanism for aftershock generation.     

Performance of structures in the rural area during the March 8, 2010 Elazığ-Kovancılar earthquake

Numerous studies demonstrated the possibility of utilizing fly ash in the construction of embankments, road subgrades and stabilization of a wide range of soils. The present investigation aims at determining the optimum fly ash (OFA) for mechanical stabilization of expansive soils. Four different soils were tested for compaction characteristics and unconfined compressive strength with and without the addition of fly ash to determine the OFA. The liquid limit (LL) and the fraction coarser than 425 μ (CF) of these soils range from 50 to 120 and 25 to 70%, respectively. An experimental strategy called two-factor factorial design was adopted in the conducting experiments. LL and CF present in the soil are the two factors considered to influence the OFA content. Factorial experimentation enables relative quantification of the effect of each factor as well as their interaction with the OFA. The OFA was found to range from 5 to 40% depending upon the two factors. The LL and the CF were found to have dominating influence on OFA content in that order, whereas the interaction effect of these two factors was marginal to fair. A statistical regression model was developed for determination of the OFA in terms of the influencing factors. The validity of the model developed was verified by conducting laboratory tests on two more soils that were not used in the development of the model. Swell potential and swelling pressure of expansive soils were reduced to non-critical levels when treated with OFA.     

Formation of the disk structure and jets in the symbiotic system Z and during periods of activity in 2006–2010

We present an analysis of spectrophotometric observations of the latest cycle of activity of the symbiotic binary Z And from 2006 to 2010. We estimate the temperature of the hot component of Z And to be ≈150 000−170 000 K at minimum brightness, decreasing to ≈90 000 K at the brightness maximum. Our estimate of the electron density in the gaseous nebula is N _e = 10¹⁰−10¹² cm⁻³ in the region of formation of lines of neutral helium and N _e = 10⁶−10⁷ cm⁻³ in the region of formation of the [OIII] and [NeIII] nebular lines. A trend for the gas density derived from helium lines to increase and the gas density derived from [OIII] and [NeIII] lines to simultaneously decrease with increasing brightness of the system was observed. Our estimates show that the ratios of the theoretical and observed fluxes in the [OIII] and [NeIII] lines agree best when the O/Ne ratio is similar to its value for planetary nebulae. The model spectral energy distribution showed that, in addition to a cool component and gaseous nebula, a relatively cool pseudophotosphere (5250–11 500 K) is present in the system. The simultaneous presence of a relatively cool pseudophotosphere and high-ionization spectral lines is probably related to a disk-like structure of the pseudophotosphere. The pseudophotosphere formed very rapidly—over several weeks—during a period of increasing brightness of Z And. We infer that in 2009, as in 2006, the activity of the system was accompanied by a collimated bipolar ejection of matter (jets). In contrast to the situation in 2006, the jets were detected even before the system reached its maximum brightness. Moreover, components with velocities close to 1200 km/s disappeared at the maximum, while those with velocities close to 1800 km/s appeared.     

Trend analysis of rainfall pattern over the Central India during 1901–2010

In the present study, trends of rainfall of the Central India were evaluated in monthly, seasonal, and annual time scales using the Revised Mann-Kendall (RMK) test, Sen’s slope estimator, and innovative trend method (ITM). For this purpose, the monthly rainfall data for 20 stations in Madhya Pradesh (MP) and Chhattisgarh (CG) states in Central India during 1901–2010 was used. The Sen’s slope estimator was utilized for calculating the slope of rainfall trend line. Based on the obtained results of RMK test, there is no significant trend in the stations for the January and October months. The results also showed that for MP, two out of 15 considered stations indicate significant annual trend, while the CG has four out of five stations with significant trend. The results of applying ITM test indicated that most of the stations have decreasing trends in annual (16 stations), summer (16 stations), and monsoon (11 stations) seasons, while the winter (12 stations) and post monsoon (11 stations) seasons generally show increasing trend. Unlike the RMK, the ITM shows significant increasing trend in rainfall of November and December months. The finding of current study can be used for irrigation and water resource management purpose over the Central India.     

Source parameters of earthquakes in the reservoir-triggered seismic (RTS) zone of Koyna–Warna, Western India

New empirical relations are derived for source parameters of the Koyna–Warna reservoir-triggered seismic zone in Western India using spectral analysis of 38 local earthquakes in the magnitude range M _L 3.5–5.2. The data come from a seismic network operated by the CSIR-National Geophysical Research Institute, India, during March 2005 to April 2012 in this region. The source parameters viz. seismic moment, source radius, corner frequency and stress drop for the various events lie in the range of 10¹³–10¹⁶ Nm, 0.1–0.4 km, 2.9–9.4 Hz and 3–26 MPa, respectively. Linear relationships are obtained among the seismic moment (M ₀), local magnitude (M _L), moment magnitude (M _w), corner frequency (fc) and stress drop (?σ). The stress drops in the Koyna–Warna region are found to increase with magnitude as well as focal depths of earthquakes. Interestingly, accurate depths derived from moment tensor inversion of earthquake waveforms show a strong correlation with the stress drops, seemingly characteristic of the Koyna–Warna region.     

Variability of the blazar 3C 454.3 during 2007–2010

Results are presented of multicolor observations of the blazar 3C 454.3 carried out at the Astronomical Institute of St. Petersburg State University and the Central Astronomical Observatory of the Russian Academy of Sciences in 2007–2010. The color variability of the blazar is analyzed. Several outbursts were observed. The existence of two variable synchrotron sources is inferred. The first is responsible for the small-amplitude flux variability, and the second for flares. In each flare, the relative spectral energy distribution (SED) of the variable source is found to be constant. All the SEDs are power laws, but with different spectral indexes in different flares. This indicates the impossibility of explaining the global variability only via a difference in Doppler boosting due to variations of the angle between the line of sight and the velocity of the electrons responsible for the synchrotron radiation. The polarimetric and photometric observations are used to derive the absolute SED of constant component. A comparison of the observed SEDs for different brightness levels with the SED of the constant componentmakes it possible to explain the observed color variability as due to the superposition of a bluer variable source with a constant SED and variable flux onto the constant component.     

Occurrence of népouite in the serpentinite of the Manipur ophiolite belt,Northeastern India: Implication for melt-rock interaction in a supra-subduction zone

Serpentinization is pervasive in the ultramafic rocks of Manipur ophiolite belt (MOB), Northeastern India. Electron microprobe data of a serpentinite from the Ukhrul-Nungbi sector of MOB shows Ni-rich serpentine mineral (NiO = 33.4-33.9 wt %, SiO₂= 37.55-38.96 wt %, MgO= 14.83-16.89 wt %). The composition and X-ray diffraction pattern characterize this Ni-rich serpentine mineral as népouite which is suggested to be a hydrothermal alteration product of NiO-rich olivine in a fore-arc peridotite. The genesis of this NiO-rich olivine is attributed to the melt-rock interaction in a supra-subduction zone setting.     

Characterization of C‐isotope variability in the Delhi Supergroup,northwestern India and the Mesoproterozoic ocean

Marine carbonate rocks of the Delhi Supergroup of northwestern India show little deviation in whole‐rock δ ¹³Ccarb and δ ¹⁸Ocarb values, which generally are around 0 and –10‰ respectively. These narrow ranges and almost constant δ ¹³Ccarb values persist despite close sampling through long sections. The data suggest that the global rate of organic carbon burial was probably constant during deposition of the Delhi Supergroup. The nearly invariant C isotopic profile of the Delhi Supergroup is similar to C isotopic profiles of Mesoproterozoic carbonates older than 1.3 Ga, as reported from different parts of world. Carbonate units on the western margin of the Delhi Supergroup however, have on average moderately positive δ ¹³C values (from 2 to +4.96‰). These high δ ¹³C carbonates may represent the Mesoproterozoic–Neoproterozoic transition (from ～1.25 to ～0.85 Ga), a period characterized by high positive δ ¹³C values globally.     

Molluscan fauna from the Miocene sediments of Kachchh,Gujarat, India — Part 4, Indarca,a new anadaroid genus

An interesting bivalve fossil obtained from the Claystone Member of the Chhasra Formation (Burdigalian), Kachchh District, India was found conspecific with Anadara gourae Dey (Arcidae, Anadarinae) from the Quilon Beds (Burdigalian), Kerala. However, this form is feebly inequivalve and has dissimilar ornamentation on the two valves, a trapezoidal form and its maximum height is at the posterior. Also, there is a small but distinct median sulcus on the exterior of the umbonal region. These characteristics do not agree with definition of the genus Anadara. Hence it is described here under a new genus, Indarca. Subgenus Cunearca Dall of genus Scapharca Gray (Arcidae, Anadarinae), also shows dissimilar ornamentation on the two valves, but differs from Indarca in having an inflated form, left valve slightly larger and absence of a sulcus on the umbonal region. Indarca resembles Bentharca Verrill and Bush (Arcidae, Anadarinae) in certain respects, but differs from it in not having a markedly reduced anterior. Occurrence of Indarca gourae strengthens the faunal affinity between the Chhasra Formation of Kachchh and the Quilon Beds of Kerala.     

Space–time analysis, faulting and triggering of the 2010 earthquake doublet in western Corinth Gulf

Two moderate magnitude earthquakes (M5.5 and M5.4) occurred in January 2010 with their epicenters at a distance of about 5?km between them, in the western part of the Corinth Gulf. The recordings of the regional seismological network, which is dense locally, were used for the location of the two main events and aftershocks, which are concentrated in three clusters beneath the northern coasts of the Gulf. The first two clusters accompany each one of the two stronger earthquakes, whereas the third cluster comprises only low magnitude aftershocks, located westward of the two stronger events. Seismic excitation started in January 18, 2010, with the M?=?5.5 earthquake in the area occupied by the central cluster. Seismicity immediately jumped to the east with numerous aftershocks and the M?=?5.4 earthquake which occurred four days later (January 22, 2010). Cross sections normal to the long axis of each cluster show ruptures on north dipping faults at depths of 7?C11?km. Focal mechanisms of the stronger events of the sequence support the results obtained from the spatial distribution of the aftershocks that three different fault segments activated in this excitation. The slip vectors of all the events have an NNW?CSSE to NNE?CSSW orientation almost parallel to the direction of extension along the Corinth Gulf. Calculation of the Coulomb stress changes supports an interaction between the different clusters, with the major activity being coincided with the area of positive induced stress changes after the first earthquake.     

Complexity in hydro-seismicity of the Koyna–Warna region,India

Koyna–Warna region in western India is known to be the largest case of the reservoir-triggered seismicity in the world with M6.3 earthquake in 1967. This region continues to be seismically active even after 45 years with occurrences of earthquakes up to M5.0. The porous crustal rocks of Koyna–Warna region respond to changes in the prevailing stress/strain regime. This crustal section is highly fractured and is being fed by rivers and reservoirs. It is also subjected to fluctuating plate boundary forces and significant gravity-induced stresses due to crustal inhomogeneities. These changes induce variations in the water level in bore wells before, during and after an earthquake, and their study can help in understanding the earthquake genesis in the region. The ongoing seismicity thus requires understanding of coupled hydrological and tectonic processes in the region. Water table fluctuations are a reflection of the ongoing hydro-tectonics of the region. The fractal dimension of water levels in the bore wells of the region can be used as measure of the nonlinear characteristics of porous rock, revealing the underlying complexity. In this paper, we present values of correlation dimensions of the water level data in the bore wells using the nonlinear time series methodology. The spatiotemporal changes in the fractal dimensions have also been determined. The results show that hydro-seismically the region behaves as a low-dimensional nonlinear dynamical system.     

Study of cloudburst and flash floods around Leh, India, during August 4–6, 2010

Leh and surrounding region of the Ladakh mountain range in the trans-Himalaya experienced multiple cloudbursts and associated flash floods during August 4–6, 2010. However, 12.8 mm/day rainfall recorded at the nearest meteorological station at Leh did not corroborate with the flood severity. For better understanding of this event, hydrological analysis and atmospheric modeling are carried out in tandem. Two small catchments (<3 km²) were studied along the stream continuum to assess the flood characteristics to identify the cloudburst impact zones. Peak flood discharges were estimated close to the head wall region and at the catchment outlet of the Leh town and the Sabu eastern tributary catchments. Storm runoff depth is estimated by developing a triangular hydrograph by using the known time base of the flood hydrograph. This triangular hydrographs have been transformed further into storm hydrographs to gain a better understanding of the storm duration by using the dimensionless hydrograph method at selected cross sections. Storm duration is estimated by using the relationship between time to peak and time of concentration of the catchment. The peak flood estimates ranged from 122(±35 %) m³/s for Leh town catchment (2.393 km²), 545(±35 %) m³/s for Sabu eastern tributary catchment (2.831 km²) to 1,070(±35 %) m³/sec for Sabu catchment (64.95 km²). To assess the atmospheric processes associated with this event, a triple nest simulation (27, 9 and 3 km) is performed using Advanced Research Weather Research and Forecasting (WRF) modeling system. The simulation does show the evolution of the event from August 4 to 6, 2010. Observation constraints, orographic responses, etc. make such analysis complex at such scale. Independent estimate by the atmospheric process model and the hydrological method shows the storm depth of 70 mm and 91.8(±35 %) mm, respectively, in catchment scale. Hydrological evaluation further refined the spatial and temporal extents of the cloudbursts in the respective catchments with an estimated storm depth of 209(±35 %) mm in 11.9 min and 320(±35 %) in 8.8 min occurring in an area of 0.842–1.601 km², respectively. This study shows that the insight developed on the cloudburst phenomena by the atmospheric and the hydrological modeling is hugely constrained by the spatial and temporal scales of data used for the analysis. Apart from this, study also highlighted the regular occurrence of cloudburst events over this region in the recent past. Most of such events go unreported due to lack of monitoring mechanisms in the region and weaken our ability to understand these events in complete perspective.     

Sedimentation of particulate matter in the Dona Paula Bay, West Coast of India during November to May 1995–1997

Data on hydrography, nutrients, suspended particles, and sedimented particles were collected at weekly intervals from November to May during 1995 to 1997 at a station in the coastal waters of Dona Paula Bay, India. Suspended and sedimented particles were analyzed for total suspended matter (SPM), total sedimented particulate matter (TPM), particulate organic carbon (POC), particulate organic nitrogen (PON), chlorophylla (chla), and diatom abundance. Variations in hydrography and nutrients influenced the quantity and composition of sedimented particles. The TPM, POC, PON, and chla fluxes showed small-scale seasonal variations and were higher in the summer (February to May) than in the winter (November to January). Resuspension of carbon accounted for approximately 25% of the gross POC and was highest in April 1997 (45%). The mean net POC flux was 197±90 mg C m⁻² d⁻¹ and accounts for 4.6% of the TPM. The average C∶N (w∶w) ratio of the sedimented material was 13.2±6.6. The POC:chla ratio was relatively higher in the sedimented material as compared to the suspended material. The particulate carbon reaching the bottom sediment was 39% of the primary production. The low organic carbon concentration (approximately 0.1% of dry sediment) in the sediments implies that about 98% of the sedimented carbon was either consumed at the sedimentwater interface or resuspended/advected before it was finally buried into the sediments.     

Genesis of the boron potential of the Taukha metallogenic zone,Sikhote Alin,and boron sources during formation of the Dal’negorsk boron deposit

It is shown that the formation of borosilicate skarn in the Taukha metallogenic zone completes a series of successive stages of the formation and transformation of the folded sequences of the Taukha accretionary wedge. The Early Cretaceous sedimentary stage, including accumulation of detrital tourmaline-rich sedimentary rocks, was implemented in the marginal sea of the Paleopacific Sino-Korean segment. In the Turonian–Campanian, the boron-bearing folded sequences of the accretionary wedge were involved in anatexis to generate siliceous S-type boron-bearing melts. The thus-formed magmatic chambers were emptied during catastrophic volcanic eruptions. At the final Middle Campanian volcanic stage, fluid-magmatic differentiation of the melt in the residual chambers generated fluid flow. The infiltration interaction of the fluids, which inherited enrichment in boron, with limestones of the olistostrome sequence resulted in the formation of a giant zone of grossular–wollastonite skarns and danburite lodes. The boron potential of the Taukha boron–lead–zinc metallogenic zone may be considered as a reproduction of the Precambrian boron metallogeny of the eastern Eurasian margin, which was implemented in the Late Mesozoic during recycling of the continental crust.     

Evolution of continental crust of the Aravalli craton,NW India,during the Neoarchaean–Palaeoproterozoic: evidence from geochemistry of granitoids

Neoarchaean–Palaeoproterozoic granitoids of the Aravalli craton, represented by four plutons with different ages, viz. Gingla (2.6–2.4 Ga), Ahar River (2562 Ma), Untala (2505 Ma), and Berach (2440 Ma) granitoids, are classified into three suites: TTG-like, Sanukitoid, and High-K Granitoid suite, all exhibiting negative Nb and Ti anomalies. The TTG-like suite is characterized by high contents of SiO₂, Na₂O, and LREEs, high (La/Yb)_N, low contents of K₂O, MgO, Cr, and Ni, and low (Dy/Yb)_N, suggesting that this suite formed by partial melting of a subducted basaltic slab without interacting with a mantle wedge. In contrast, the calc-alkaline Sanukitoid suite is marked by a high content of LILEs and mantle-compatible elements, which indicate that this suite formed by partial melting of a slab-fluid metasomatized mantle wedge in a subduction-related arc environment. On the other hand, the High-K Granitoid suite is characterized by high contents of SiO₂ and K₂O, and low contents of Na₂O, MgO, Cr, and Ni with variable Eu anomaly, along with high (La/Sm)_N and (La/Yb)_N, and low (Dy/Yb)_N and Nb/Th. Some high-K granitoids also exhibit A-type characteristics. These features indicate that the High-K Granitoid suite formed by melting of crustal rocks. Early Neoarchaean continental crust formation reflected a slab-melting-dominated magmatic process as evidenced by the TTG-like suite, whereas Palaeoproterozoic petrogenesis was governed by the interaction of slab melt with mantle wedge as demonstrated by the Sanukitoid suite. The High-K Granitoid suite formed during the waning stages of subduction. This study reveals that granitic rocks of the Aravalli craton evolved from slab melting in the Neoarchaean to melting of mantle wedge in the Palaeoproterozoic. Melting of older crust led to the formation of the High-K Granitoid suite.     

Tectonic evolution of the India–Asia suture zone since Middle Eocene time,Lopukangri area,south-central Tibet

Suture zones often archive complex geologic histories underscored by episodes of varying style of deformation associated with intercontinental collision. In the Lopukangri area of south-central Tibet (29°54′N, 84°24′E) field relationships between tectonic units juxtaposed by the India–Asia suture are well exposed, including Indian passive margin rocks (Tethyan Sedimentary Sequence), forearc deposits (Xigaze Group), magmatic arc rocks (Gangdese batholith and Linzizong Formation) and syncollision deposits (Eocene–Miocene conglomerates). To better understand the structural history of this area, we integrated geologic mapping with biotite ⁴⁰Ar/³⁹Ar thermochronology and zircon U–Pb geochronology. The first-order structure is a system of north-directed thrusts which are part of the Great Counter thrust (GCT) that places Indian passive margin rocks and forearc deposits on top of magmatic arc rocks and syn-tectonic conglomerates. We infer the south-directed Late Oligocene Gangdese Thrust (GT) exists at unexposed structural levels based on field mapping, cross sections, and regional correlations as it has been documented immediately to the east. A granite in the footwall has a U–Pb zircon age of 38.4 ± 0.4 Ma, interpreted to be the age of emplacement of the granite, and a younger ⁴⁰Ar/³⁹Ar biotite age of 19.7 ± 0.1 Ma. As the granite sample is situated immediately below a nonconformity with low grade greenschist facies rocks, we interpret the younger age to reflect Miocene resetting of the biotite Ar system. Syn-tectonic deposits in the Lopukangri area consist of three conglomerate units with a total thickness of ∼1.5 km. The lower two units consist of cobble gravel pebble conglomerates rich in volcanic and plutonic clasts, transitioning to conglomerates with only sedimentary clasts in the upper unit. We correlate the syncollision deposits to the Eocene–Oligocene Qiuwu Formation based on field relationships, stratigraphy and petrology. Petrology and clast composition suggest the lower two units of the Qiuwu Formation had a northern provenance (Lhasa block and magmatic arc) and the upper unit had a southern provenance (Tethyan Sedimentary Sequence). Our observations are consistent with paleocurrent data from other studies which suggest a predominant south-directed paleoflow for this formation. We propose a model in which: (1) granites intrude at 38.4 ± 0.4 Ma; (2) are exhumed by erosion; (3) and buried due to regional subsidence and initial deposition of a conglomerate unit; (4) exposed by the GT at ∼27–24 Ma to provide detritus; (5) buried a second time by hanging wall-derived sedimentary deposits and the GCT, then (6) exposed from a depth of ∼12–10 km by a blind thrust at ∼19 Ma. An alternate model describes: (1) intrusion of the granites at 38.4 ± 0.4 Ma, followed by (2) exhumation of the granites via normal faulting to provide detritus; (3) then burial by the GCT at ∼24 Ma, followed by (4) exhumation via regional erosional denudation at ∼19 Ma. Exposure of the GT west of Xigaze has not been confirmed. We suggest that shallower structural levels of the India-Asia suture zone are exposed to the west of the study area, compared to the east, where the GT has been previously documented. The GCT in the area is short-lived, as it is cut and offset by a Middle Miocene ∼N-striking W-dipping oblique normal fault system.     

Massive landslide triggered by 2008 Iwate�CMiyagi inland earthquake in the Aratozawa Dam area, Tohoku, Japan

On June 14 2008, an Iwate–Miyagi inland earthquake that had a magnitude of 7.2 hit the eastern foot of the Ohu Mountains in Tohoku district, Japan. The seismic peak ground acceleration was greater than 1,000 gal in the Aratozawa Dam area. The earthquake triggered a massive landslide at the upper reach of the dam. The landslide had the sediment volume of over 67 million cubic meters and is considered the largest catastrophic landslide in Japan during the last 100 years. This report presents a summary of our findings pertinent to the landslide’s activities based on our field investigations that started the day after the landslide. This report covers: (1) details of the land deformations caused by the landslide, (2) geological background pertinent to landslide development, and (3) estimation of the slip surface and the other physical properties of the landslide based on the analysis of the boring core specimens and landform features. The landslide is roughly divided into two sections, a lower and an upper half. The lower half moved almost simultaneously as one massive block of 700 m long, 800 m wide, and 70–80 m thick. The slip surface had developed on the very fine sand of the alternate layer of fine-grained sandstone and siltstone. The slickensided slip surface has a gradient of only 2°. This feature indicates that the type of the landslide movement is considered to be a block glide. The landslide body is nearly identical to the topography of the landslide area that was developed about 50,000 years ago. This shows the possibility that the landslide was reactivated. The upper half consists of two large ridges and the broad debris field and is 600 m long, 900 m wide, and 70–100 m thick. The maximum height of the main scarp is over 150 m.