Takaya Iwasaki

A crustal study off Lofoten, N. Norway, by use of 3-component Ocean Bottom Seismographs

Abstract Twenty-four 3-component Ocean Bottom Seismographs (Obs) were used in August 1988 in a combined seismic refraction and reflection study off Lofoten, N. Norway. The purpose of the experiment was to map the crustal structure from the continental shelf to the oceanic crust off Lofoten. The very high data quality is demonstrated by the strong P-wave and shear-wave reflections, as well as converted waves from the Moho observed on the continental shelf. These arrivals are observed continuously from near vertical to wide angle incidence. Very high seismic sea-floor velocities in this area (3.1 km/s to 5.1 km/s) indicates absence or very thin sequences of Mesozoic sediments. The 5.1 km/s refractor coincides with the base Cretaceous reflection interpreted from the multichannel reflection data. The crystalline continental crust is here divided in layers with velocities of 6.0 km/s, 6.4 km/s and 6.8 km/s, respectively. On the seaward side of the escarpment Tertiary sediments varying in thickness from 1.0–1.5 km are situated on top of a 1.5-2.0-km-thick layer of flood-basalt containing seaward-dipping reflectors. A layer with velocity of 6.7 km/s is observed above the lower crust, which in this area is found to have a velocity as high as 7.3 km/s. These high velocities indicate that the crust in this area is of oceanic origin or, alternatively, the high-velocity layer in the lower crust might represent a magmatic body underplating highly thinned and intruded continental crust. Seven obss were deployed in the area that was covered by landward-flowing basalt deposited during the early Eocene breakup between Norway and Greenland. The survey was partly performed to investigate whether this method can be used to map structures below the basalt, which is impenetrable with conventional seismic reflection techniques. The obs data contain considerable information about structures below the flood-basalt; pre-opening sediments up to 4.0 km thick are indicated below the 1.0–2.5-km-thick landward-flowing basalt. The velocities of the crystalline portion of the crust are found to be similar to those observed under the continental shelf (6.0–6.8 km/s), which implies that the crust east of the escarpment is of continental origin. The crystalline crust is strongly thinned in this area, showing a minimum thickness of about 6 km. The depth to the Moho increases from about 15 km in the western part of the area to about 26 km on the continental shelf. The success of the obs survey indicates that such measurements can become an important tool in investigations on passive volcanic margins, and, potentially, in other areas where highly reflective surfaces make the reflection technique inefficient.

A detailed subduction structure in the Kuril trench deduced from ocean bottom seismographic refraction studies

Abstract In 1983, we conducted an ocean bottom seismograph (OBS) experiment in the southernmost part of the Kuril trench, beneath which the Pacific Plate is subducting under Hokkaido Island, Japan. The aim of this experiment was to determine crust and upper mantle structure from the oceanic basin to the continental slope by dense seismic refraction profiling, using explosives and airguns. The total length of the profiles was 560 km, along which ten OBSs were deployed. The observed data were of good quality, which enabled us to obtain a detailed velocity structure of the active margin down to 20–30 km. We constructed a velocity structure model by ray-tracing and amplitude modeling. In the oceanic basin, the crust has a typical oceanic structure characterized by three layers with P-wave velocities of 1.8, 3.8–6.5 and 6.5–7.0 km/s. The velocity gradient in layer 3 increases downward from 0.075 to 0.10 s −1 . The Moho discontinuity was well constrained by clearly observed P n and P m P phases. The total thickness of the oceanic crust was determined to be 8 km and almost constant in the oceanic basin. The P-wave velocity is 7.9 km/s beneath the Moho discontinuity, which increases downward with a rather small velocity gradient, 0.015–0.03 s −1 . Beneath the continental slope, we found relatively low velocity (2.5 to 5.5–5.8 km/s) material. The oceanic Moho discontinuity associated with the subducting plate was traced down to a depth of 25 km. Our seismic data strongly suggest that oceanic layer 2 is smoothly subducting and does not break up to form a wedge structure. This result is in remarkable contrast with the velocity structure in the active margin of the Ryukyu trench area (about 1000 km south of the present experimental area), where we found a 12 km-thick, prominent low-velocity wedge is situated 50–150 km landward from the trench axis. On the seaward side of the wedge, the surface of the igneous basement undulates severely. The wedge was probably formed by materials of oceanic origin. Such a difference in velocity structure suggests that the subduction mechanism in the trench area differs from region to region in the northwestern Pacific.

Groundwater Microtemperature in Earthquake Regions

Microtemperature measurements of groundwater with a relative precision better than 1/1000°C have been made in several seismically active areas in Japan. The measured temperatures show clear coseismic signals as well as a correlation with atmospheric pressure. Simultaneous observations at various depths have shown that these temperature changes were not induced by simple groundwater level changes. Also, distinctive signals occurred before several earthquakes and seem to be caused by a different mechanism than the coseismic signals. The microtemperature at some observation sites shows excellent correlation with records of nearby sensitive borehole strainmeters. Simultaneous recording of microtemperature and strain has been initiated in some boreholes.