3-D Viscoelastic FEM Modeling of Crustal Deformation in Northeast Japan

-- As a first step toward establishing a standard earthquake cycle model in Japan, we simulate the crustal deformation during the past 100 years in northeast Japan, using a 3-D FEM based on the kinematic model. Then, we compare the computed results with the observed long-term leveling data and the recent GPS data. On the whole, although the effect of the subducting PAC is dominant, coseismic deformation of the interplate earthquakes can be clearly seen in the inland. Moreover, the postseismic deformation of the earthquakes due to the viscoelastic upper mantle seriously affects the inland movements, and continues for a few decades. Our modeling, including the effects of the interplate earthquakes and the three-dimensional viscoelastic inhomogeneity, reasonably explains the observed movement. Finally, we stress that the viscoelastic effect should be taken into consideration in the analyses, even if no earthquakes occur in the analyzed period.     

Evaluation of Subsurface Structure at Soultz Hot Dry Rock Site by the AE Reflection Method in Time-frequency Domain

—?Because the Soultz Hot Dry Rock (HDR) site, France, is to be expanded to a scientific pilot plant of greater depth, measurement of the deep area below the predeveloped artificial reservoir is gaining importance. In this paper, we present estimates of deep subsurface structure at the Soultz HDR site, obtained by a reflection method using acoustic emission (AE) signals, that is, induced seismicity, as the wave source. First, we briefly describe the AE reflection method in the time-frequency domain with wavelet transform. Then we show estimates of the subsurface structure by using 101 high-energy AE events observed in 1993. We compare and discuss the results obtained, using the AE reflection method in two wells with other independent borehole observations. Furthermore, we present the results of an investigation of the frequency dependence of reflectors identified by hodogram linearity as a possible means of further characterizing detected structures.     

Microseismicity Induced by Heavy Rainfall Around Flooded Vertical Ore Veins

—?Microseismicity (M?M?M?M??6 were also monitored at a crustal movement monitoring station located several hundred meters from the veins. It was found that the opening of the vertical ore veins primary led to significant strain and tilt, but not to seismicity, because the delay and the longer duration of the seismicity were significant. Most seismic events involve thrusting mechanisms that are consistent with the present stress state of E-W-oriented tectonic compression, but are not consistent with the opening of the deepest ore vein. Interstingly, all the events within a few months of the heavy rainfall occurred near the faults that offset the deepest ore veins, wheareas all those events located away from the deepest ore veins occurred many months after the heavy rainfall. Consequently, the delayed diffusion of water appears to have played a dominant role in reducing rock strength, which led to seismicity in the Ikuno mine.     

Some Aspects of Resource Uncertainty and Their Economic Consequences in Assessment of the 1002 Area of the Arctic National Wildlife Refuge

Natural Resources Research - Exploration ventures in frontier areas have high risks. Before committing to them, firms prepare regional resource assessments to evaluate the potential payoffs. With...     

A method to reduce the influence of ice-rafted debris on a grain size record from northern Fram Strait, Arctic Ocean

The granulometric composition of terrigenous deep-sea sediments provides information on current speed if certain frame conditions are fulfilled. These include that current transport is the only transport process. At high latitudes this type of investigation is impaired due to the influence of ice-rafted debris (IRD) which contaminates the current-sorted grain size fractions. This study presents a new method that addresses this problem by setting the ice-rafted sand in relation to the silt of both current- and ice-transported origin. Deviations from the resulting regression function are then used to determine the behaviour of the silt mean grain size as a function of current speed largely independent from IRD bias. The study is based on sediments from the Yermak Plateau, Arctic Ocean, a region influenced by IRD brought with the south-headed Transpolar Drift and by north-directed bottom currents. The IRD correction results in displaying changes of current speed at much higher clarity; climate forcing of the currents becomes more evident. For example, the 8200 year cold event shows up as a major event in the corrected record whereas it is hardly visible in the original record.     

Tectono-sedimentary processes along an active marine/lacustrine half-graben margin: Alkyonides Gulf, E. Gulf of Corinth, Greece

ABSTRACT The Alkyonides half‐graben is separated from the Gerania Range to the south by active faults whose offshore traces are mapped in detail. The East Alkyonides and Psatha Faults have well‐defined, Holocene‐active tip zones and cannot be extrapolated from the onshore Skinos Fault into a single continuous surface trace. During the late Quaternary, catchments draining the step‐faulted range front have supplied sediment to alluvial fans along a subsiding marine ramp margin in the hangingwall of the Skinos Fault, to shelf ledge fans on the uplifting footwall to the East Alkyonides Fault and to the Alepochori submarine fan in the hangingwall of the latter. During late Pleistocene lowstand times (c. 70–12 ka), sediment was deposited in Lake Corinth as fan deltas on the subsiding Skinos shelf ramp which acted as a sediment trap for the adjacent 360 m deep submarine basin plain. At the same time, the uplifting eastern shelf ledge was exposed, eroded and bypassed in favour of deposition on the Alepochori submarine fan. During Holocene times, the Skinos bajada was first the site of stability and soil formation, and then of substantial deposition before modern marine erosion cut a prominent cliffline. The uplifting eastern shelf ledge has developed substantial Holocene fan lobe depositional sequences as sediment‐laden underflows have traversed it via outlet channels. We estimate mean Holocene displacement rates towards the tip of the Psatha Fault in the range 0.7–0.8 mm year^?1. Raised Holocene coastal notches indicate that this may be further partitioned into about 0.2 mm year^?1 of footwall uplift and hence 0.5–0.6 mm year^?1 of hangingwall subsidence. Holocene displacement rates towards the tip of the active East Alkyonides Fault are in the range 0.2–0.3 mm year^?1. Any uplift of the West Alkyonides Fault footwall is not keeping pace with subsidence of the Skinos Fault hangingwall, as revealed by lowstand shelf fan deltas which show internal clinoforms indicative of aggradational deposition in response to relative base‐level rise due to active hangingwall subsidence along the Skinos Fault. Total subsidence here during the last 58 kyr lowstand interval of Lake Corinth was some 20 m, indicating a reduced net displacement rate compared to estimates of late Holocene (< 2000 bp ) activity from onshore palaeoseismology. This discrepancy may be due to the competition between uplift on the West Alkyonides Fault and subsidence on the onshore Skinos Fault, or may reflect unsteady rates of Skinos Fault displacement over tens of thousands of years.