Formation and shortening deformation of a back-arc rift basin revealed by deep seismic profiling, central Japan

The northern Fossa Magna (NFM) basin is a Miocene rift system produced in the final stages of the opening of the Sea of Japan. It divides the major structure of Japan into two regions, with north-trending geological structures to the NE of the basin and EW trending structures to the west of the basin. The Itoigawa-Shizuoka Tectonic Line (ISTL) bounds the western part of the northern Fossa Magna and forms an active fault system that displays one of the largest slip rates (4–9 mm/year) in the Japanese islands. Deep seismic reflection and refraction/wide-angle reflection profiling were undertaken in 2002 across the northern part of ISTL in order to delineate structures in the crust, and the deep geometry of the active fault systems. The seismic images are interpreted based on the pattern of reflectors, the surface geology and velocities derived from refraction analysis. The 68-km-long seismic section suggests that the Miocene NFM basin was formed by an east dipping normal fault with a shallow flat segment to 6 km depth and a deeper ramp penetrating to 15 km depth. This low-angle normal fault originated as a comparatively shallow brittle/ductile detachment in a high thermal regime present in the Miocene. The NFM basin was filled by a thick (>6 km) accumulation of sediments. Shortening since the late Neogene is accommodated along NS to NE–SE trending thrust faults that previously accommodated extension and produce fault-related folds on their hanging wall. Based on our balanced geologic cross-section, the total amount of Miocene extension is ca. 42 km and the total amount of late Neogene to Quaternary shortening is ca. 23 km.     

Preliminary results of a study on the responses of sedimentary rocks to shaft excavation

A shaft with a diameter of 6 m and a depth of 150 m was excavated in sedimentary rocks. Anin situ study was carried out to identify the size and rock properties of the zone disturbed by the excavation and to estimate the applicability of existing methods for measuring and modelling such a zone. The study detected displacements and changes in the properties of the rock within about 1 m of the shaft wall. Numerical analysis using a model of continuous rock mass can simulate some features of disturbance due to excavation, although slight discrepancies exist between the measured and analysed results. The excavation disturbance detected in this study is probably due to fracturing, the redistribution of stress and undersaturation. However, the relationships between the disturbance due to excavation and these processes have not been quantified. The existing methods for measurements and numerical analysis can provide important information on the disturbance due to excavation but need to be improved to understand disturbance due to excavation further.     

Cation disorder in garnets along the Mg3Al2Si3O12-Mg4Si4O12 join: an infrared,Raman and NMR study

We have obtained infrared and Raman spectra for garnets synthesized at high (static) pressures and temperatures along the join Mg₃Al₂Si₃O₁₂ (pyrope) — Mg₄Si₄O₁₂ (magnesium majorite). The vibrational spectra of Mg-majorite show a large number of additional weak peaks compared with the spectra of cubic pyrope garnet, consistent with tetragonal symmetry for the MgSiO₃ garnet phase. The Raman bands for this phase show no evidence for line broadening, suggesting that Mg and Si are ordered on octahedral sites in the garnet. The bands for the intermediate garnet compositions are significantly broadened compared with the end-members pyrope and Mg-majorite, indicating cation disorder in the intermediate phases. Solid state ²⁷Al NMR spectroscopy for pyrope and two intermediate compositions show that Al is present only on octahedral sites, so the cation disorder is most likely confined to Mg-Al-Si mixing on the octahedral sites. We have also obtained a Raman spectrum for a natural, shock-produced (Fe,Mg) majorite garnet. The sharp Raman peaks suggest little or no cation disorder in this sample.     

Full‐scale shaking table test of a base‐isolated medical facility subjected to vertical motions

Base isolation is a well known technology that has been proven to reduce structural response to horizontal ground accelerations. However, vertical response still remains a topic of concern for base‐isolated buildings, perhaps more so than in fixed‐base buildings as isolation is often used when high performance is required. To investigate the effects of vertical response on building contents and nonstructural components, a series of full‐scale shaking table tests were conducted at the E‐Defense facility in Japan. A four‐story base‐isolated reinforced concrete building was outfitted as a medical facility with a wide variety of contents, and the behavior of the contents was observed. The rubber base isolation system was found to significantly amplify vertical accelerations in some cases. However, the damage caused by the vertical ground motions was not detrimental when peak vertical floor accelerations remained below 2 g with three exceptions: (1) small items placed on shelves slid or toppled; (2) objects jumped when placed on nonrigid furniture, which tended to increase the response; and (3) equipment with vertical eccentricities rocked and jumped. In these tests, all equipment and nonstructural components remained functional after shaking. Copyright © 2013 John Wiley & Sons, Ltd.     

A study of microearthquake seismicity and focal mechanisms within the Sea of Marmara (NW Turkey) using ocean bottom seismometers (OBSs)

We have carried out seismological observations within the Sea of Marmara (NW Turkey) in order to investigate the seismicity induced after Gölcük–İzmit (Kocaeli) earthquake (M_w 7.4) of August 17, 1999, using ocean bottom seismometers (OBSs). High-resolution hypocenters and focal mechanisms of microearthquakes have been investigated during this Marmara Sea OBS project involving deployment of 10 OBSs within the Çınarcık (eastern Marmara Sea) and Central-Tekirdağ (western Marmara Sea) basins during April–July 2000. Little was known about microearthquake activity and their source mechanisms in the Marmara Sea. We have detected numerous microearthquakes within the main basins of the Sea of Marmara along the imaged strands of the North Anatolian Fault (NAF). We obtained more than 350 well-constrained hypocenters and nine composite focal mechanisms during 70 days of observation. Microseismicity mainly occurred along the Main Marmara Fault (MMF) in the Marmara Sea. There are a few events along the Southern Shelf. Seismic activity along the Main Marmara Fault is quite high, and focal depth distribution was shallower than 20 km along the western part of this fault, and shallower than 15 km along its eastern part. From high-resolution relative relocation studies of some of the microearthquake clusters, we suggest that the western Main Marmara Fault is subvertical and the eastern Main Marmara Fault dips to south at 45°. Composite focal mechanisms show a strike-slip regime on the western Main Marmara Fault and complex faulting (strike-slip and normal faulting) on the eastern Main Marmara Fault.     

Prediction of pile response to lateral spreading by 3-D soil–water coupled dynamic analysis: Shaking in the direction perpendicular to ground flow

The 1995 Kobe earthquake seriously damaged numerous buildings with pile foundations adjacent to quay walls. The seismic behavior of a pile group is affected by movement of quay walls, pile foundations, and liquefied backfill soil. For such cases, a three-dimensional (3-D) soil–water coupled dynamic analysis is a promising tool to predict overall behavior. We report predictions of large shake table test results to validate 3-D soil–water coupled dynamic analyses, and we discuss liquefaction-induced earth pressure on a pile group during the shaking in the direction perpendicular to ground flow. Numerical analyses predicted the peak displacement of footing and peak bending moment of the group pile. The earth pressure on the pile in the crustal layer is most important for the evaluation of the peak bending moment along the piles. In addition, the larger curvatures in the bending moment distribution along the piles at the water side in the liquefied ground were measured and predicted.     

Subduction–accretion–collision history along the Gondwana suture in southern India: A laser ablation ICP-MS study of zircon chronology

We report the petrological characteristics and preliminary zircon geochronology based on laser ablation ICP mass spectrometry of the various units in an accretionary belt within the Palghat-Cauvery Shear/Suture Zone in southern India, a trace of the Cambrian Gondwana suture. Zircons extracted from a plagiogranite in association with an ophiolite suite within this suture possess internal structure that suggests magmatic crystallization, and yield mid Neoproterozoic ²⁰⁶Pb/²³⁸U age of 817 ± 16 Ma (error: 1σ) constraining the approximate timing of birth of the Mozambique Ocean floor. Compiled age data on zircons separated from a quartzite and metamorphosed banded iron formation within the accretionary belt yields a younger intercept age of 759 ± 41 Ma (error: 1σ) which we relate to a mid Neoproteozoic magmatic arc. Detrital zircons extracted from the quartzite yield ²⁰⁷Pb/²⁰⁶Pb age peaks of about 1.9–2.6 Ga suggesting that they were sourced from multiple protolithis of Neoarchean and Paleoproterozoic. Metamorphic overgrowths on some zircon grains record ca. 500–550 Ma ages which are in good harmony with the known ages for the timing of high-grade metamorphism in this zone during the final stage of continent collision associated with the birth of the Gondwana supercontinent in the latest Neoproterozoic-Cambrian. The preliminary geochronological results documented in our study correlate with the subduction–accretion–collision history associated with the closure of the Mozambique Ocean and the final amalgamation of the Gondwana supercontinent.     

Ground subsidence in Semarang-Indonesia investigated by ALOS–PALSAR satellite SAR interferometry

The focus of this study is investigation of land subsidence in Semarang city Indonesia with the use of Interferometry Synthetic Aperture Radar (InSAR) of ALOS–PALSAR satellite. We processed 22 ascending SAR images during January 2007 to January 2009 plus two descending SAR images acquired on 6 June 2006 and 17 June 2007. The time series analysis of interferometry was performed by using 12 pairs of interferogram relative to 21 January 2007 and 8 pairs of interferogram relative 24 January 2008. The topographic phase contribution was removed using the 3-arcsec (90 m) Shuttle Radar Topography Mission (SRTM), Digital Elevation Model (DEM). We performed precision baseline estimation to vanish the fringes from baseline effect between master and slave data. In order to investigate the contribution of horizontal movement in our analysis, we constructed two interferograms of ascending orbit and descending orbit. The time series results exhibited that the area is subsiding continuously without a significant seasonal effect during January 2007 to January 2009. The land subsidence observed from InSAR data is approximately up to 8 cm/year. Three cross sections on image displacement show the extreme land subsidence occurred especially along the coastal area and lowland area where this area is considered as industrial with high-density settlements, consuming a lot of groundwater, and land is changed from agriculture and cultivation purposes to industrial estates and house. Our result also shows a consistency with historical pattern of subsidence measured by leveling data. The results highlight the potential use of InSAR measurements to provide better constraints for land subsidence in Semarang city Indonesia.     

Groundwater-level Changes Due to Pressure Gradient Induced by Nearby Earthquakes off Izu Peninsula, 1997

Anomalous water level changes were observed at two wells associated with seismic swarm activity off Izu Peninsula on March, 1997. These are coseismic water level drops followed by gradual postseismic water level rise at the time of large earthquakes during the swarm activity. The post-seismic water level rises, which can be fitted by an exponential function with a time constant of about six hours, are explained in terms of the horizontal pressure diffusion due to the pressure gradient in the aquifer induced by the coseismic static strain.