On April 11, 2011, a M7 shallow earthquake occurred in Iwaki City, Fukushima Prefecture, Japan. Here, I present the results of temporary observations of groundwater level changes in two deep boreholes (SMK and HLN) in the aftershock area. Comparison of the tidal components of water level changes with theoretical volumetric strain changes due to the earth tides indicates that 1 mm water level change correlates to 2.94 × 10−10 strain in the SMK borehole, and to 2.42 × 10−10 strain in the HLN borehole. Extrapolation of 50days observation data up to the occurrence of the main shock indicates an 8 m coseismic drop in the SMK borehole, and a 10 m rise in the HLN borehole, which is situated only 6.2 km to the east of SMK. Water level changes were also detected for as many as 27 M3.3 to M5.9 aftershocks. Focal mechanism solutions are available for 24 of these aftershocks, with 21 of these aftershocks having volumetric strain changes calculated by dislocation models that have good linear correlations with water level changes (R2 = 0.96), suggesting that the latter may be caused by the former. About 30% of the 27 aftershocks were associated with small water level changes between 20 minutes to 3 hours before coseismic water level changes. These are interesting signals, but cannot be identified as definitive seismic precursors.
An early Middle Pleistocene marker tephra has been found in sediment cores and outcrop in and around the Chikushi Plain of southwestern Japan. This tephra has been correlated to the Yufugawa pyroclastic flow deposits that erupted from East Kyushu at about 0.6 Ma using mineral assemblages, refractive indices, chemical components, and fission track ages.
The Poroshiri ophiolite is exposed on the western side of the Hidaka metamorphic belt in central Hokkaido, Japan. Here, we present a study of metamorphism in the northern part of the ophiolite, in the Chiroro River area, to determine the relationship between metamorphism and large-scale folding and faulting. Along the northern E–W transect, the ophiolite succession in the western area forms an isoclinal anticline, whereas the eastern area of the transect shows an open syncline. Along the southern E–W transect, the upper sequence of the ophiolite is exposed in the eastern part of the transect along a NW–SE trending thrust fault. The metamorphic grades in these areas change sharply from greenschist to amphibolite facies at the western margin, then gently increases to 600 °C towards the central fault in the northern transect, whereas the metamorphic temperature to the east of this central fault is around 700 °C, indicating a temperature gap of about 100 °C at the central fault. The folded structure of the ophiolite along the northern transect is discordant with the metamorphic thermal structure, indicating that peak metamorphism occurred after folding. This also indicates that the central fault must have been active after peak metamorphism to cause the temperature gap recorded in the northern transect. The metamorphic temperature of the upper sequence of the ophiolite in the eastern section of the southern transect is about 600 °C, similar to the eastern margin of the western section of the ophiolite within the northern transect. This suggests that the upper sequence along the southern transect was transported from the west after peak metamorphism along the NW–SE trending thrust fault. Amphibole porphyroclasts record pervasive prograde but no retrograde zoning, probably resulting from rapid exhumation after peak metamorphism. The metamorphism observed in the Poroshiri ophiolite is vastly different from ordinary regional metamorphism.
The Naradani Formation is located to the south of the Torinosu Group in the Sakawa district of Kochi, southwest Japan, and consists of a linear, narrowly distributed series of marine clastic deposits that contain allochthonous limestone blocks. This formation was previously correlated to the Bajocian–Bathonian Middle Jurassic using brachiopods extracted from the limestone blocks, with later radiolarian correlations suggesting an Oxfordian–Kimmeridgian Upper Jurassic age. The formation has also been interpreted to underlie the Upper Jurassic–Lower Cretaceous Torinosu Group. However, the discovery of two stage-diagnostic aspidoceratid ammonoids, Aspidoceras sp. and Hybonoticeras sp., from muddy sandstones in the stratotype area of the Naradani Formation casts doubt on these previous interpretations. Aspidoceras was widespread during the latest Jurassic, and the presence of Hybonoticeras limits this short stratigraphic interval to the Kimmeridgian–Tithonian boundary in various Tethyan sections. This clearly suggests a latest Jurassic age for the Naradani Formation and generally supports the radiolarian biostratigraphy age-assignment. However, the ranges of ammonoids from the Naradani Formation overlap those from the Torinosu Group, meaning that it is not possible to determine an age-difference between these two lithostratigraphic units. Further research is needed to test other hypotheses, for example to determine whether heterotopic facies relationships exist between the Naradani Formation and the Torinosu Group.
Collision between the eastern part of southwestern Japan (SWJ) and the Izu−Bonin Arc caused lateral bending of pre-Miocene geologic belts and boundary faults in central Honshu, including the Median Tectonic Line (MTL), producing a prominent cuspate structure. Paleomagnetic studies suggest that these geologic belts and the MTL were linear prior to that collision; however, a number of previously determined paleomagnetic directions have significant uncertainties and thus provide only poor constraints on the timing and amount of rotation that occurred during collision. Here, we present the results of a paleomagnetic study of sediments from the Hokusetsu Subgroup in Shitara, on the western limb of the cuspate structure, with the aim of obtaining a precise paleodirection. Samples were collected from 14 sites within the uppermost Kadoya Formation, with principal component analysis of the results of stepwise demagnetization providing reliable site-mean directions for 6 of these sites. Rock magnetic experiments determined that magnetite is the main remanence carrier, and these directions pass a fold test, indicating that magnetization predated tilting at 15 Ma and is probably primary. The resultant overall mean direction has a NE declination: D (declination) = 30.1°, I (inclination) = 48.3°, α95 (95% confidence limit) = 3.9°. The most likely cause of paleomagnetic deflection from the north is the clockwise rotation of SWJ relative to the Asian continent during the Early to Middle Miocene. The angle of deflection measured in these sediments is smaller than within the main part of SWJ, suggesting counterclockwise rotation in the western limb of the cuspate structure relative to the main part during or after clockwise rotation of SWJ. The Shitara paleodirection is deflected 25° counterclockwise relative to the strike of the nearby MTL; this is also the case for the Chichibu area of the eastern limb, indicating that the MTL had the same strike direction in these two areas in the late Early Miocene.
Laser ablation–inductively coupled plasma-mass spectrometry (LA-ICP-MS) is a powerful tool for rapid and precise U-Pb zircon age dating. Here, we report U-Pb ages for detrital zircons determined using 213 nm Nd-YAG LA-ICP-MS analysis, and compare these results with those determined using a sensitive high-resolution ion microprobe (SHRIMP) to evaluate approaches for detrital zircon age dating during provenance analysis. Consistency of the system was assessed by measuring four well-characterized standard zircon samples (Plešovice, SL13, AS3, QGNG), with weighted mean measured 206Pb/238U and 207Pb/206Pb ages of the samples only slightly offset from the reference ages and with acceptable MSWD values, barring the 207Pb/206Pb age of the Plešovice zircon, which had a low 207Pb signal intensity. Detrital zircon grains were re-measured by LA-ICP-MS to assess consistency between the LA and SHRIMP systems, with 207Pb/206Pb ages for zircons older than 1000 Ma in excellent agreement with SHRIMP ages; typical offsets were between −3.9% and 0.3%. In addition, zircons younger than 1000 Ma had more accurate 206Pb/238U than 207Pb/206Pb ages. Both LA-ICP-MS and SHRIMP techniques produce results that are in excellent agreement, barring a few analyses that were affected by zircon heterogeneity or zoning.