Yoshiteru Kono

Geometry of slab, intraslab stress field and its tectonic implication in the Nankai trough, Japan

The characteristics of geometry of slabs and the intraslab stress field in the Nankai subduction zone, Japan, were analyzed based on highly accurate hypocentral data and focal mechanism solutions. The results suggest that the shallow seismic zone of the Philippine Sea slab subducts with dip angels between 10 and 22 degrees beneath Shikoku and the Kii peninsula, and between 11 and 40 degrees beneath Kyushu. Two types of seismogenic stress field exist within the slab. The stress field of down-dip compression type can be seen in the slab beneath Shikoku and the Kii peninsula, where the horizontal component of regional compression stress is NNW. On the other hand the stress field of down-dip extension type within the slab is dominant in the region from western Shikoku to Kyushu, where the direction of horizontal compressive stress is near WWN. The existence of the two types of stress field is related to the differences of slab geometry and slab age of the subduciton zone. These properties imply that slab beneath Kyushu (40 Ma) probably is older than that beneath Shikoku and the Kii peninsula (11–20 Ma). The young slab of the oceanic Philippine Sea plate subducts with a shallow angle beneath the Eurasian plate in Shikoku and the Kii peninsula. The subduction has encountered strong resistance there, resulting in a down-dip compression stress field. The down-dip extension stress field may be related to the older slab of the Philippine Sea plate which subducts beneath Kyushu with a steeper dip angle.

Buried large block revealed by gravity anomalies in the Tonankai and Nankai earthquakes regions, southwestern Japan

We confirmed existence of a buried large block beneath seafloor off Cape Shionomisaki, Kii Peninsula, southwestern Japan, by improving marine gravity data. We found that the 1944 Tonankai and 1946 Nankai earthquakes occurred close to this block. The block is characterized by high gravity anomalies of 80 mGal larger than surrounding area with a diameter of about 40 km. Center of the anomaly is located 30 km off Cape Shionomisaki, southern end of the Kii Peninsula. This high gravity anomaly can be explained by existence of a higher density body with approximate thickness of 8 km just beneath the seafloor. This block coincides with the reported high velocity zone from recent seismic refraction survey in this area. Judging from seismic velocity, density, and geologic evidences, the block presumably consists of intruded acidic rocks, not seamount.

Evolution of the Hawaiian-Emperor swell—thinning of the oceanic lithosphere

Abstract The thinning process of the oceanic lithosphere is discussed with special reference to the evolution of the Hawaiian-Emperor swell. According to the hypothesis employed in this study, the lithosphere increases its thickness depending upon the heat balance between the input heat flow from the asthenosphere, cooling by thermal conduction within the lithosphere and release or absorption of latent heat at the lithosphere-asthenosphere boundary. The input heat flow from the asthenosphere controls this process. Although under ordinary conditions the lithosphere thickens with time, an extra high heat flow thins the lithosphere. A source of excess heat flow may be recognized as a hotspot near Hawaii. Based on this theory, thinning and thickening processes of the Pacific plate beneath the Hawaiian-Emperor swell are examined. To evaluate the change in lithospheric thickness in more detail, the “residual gravity anomalies” around the Hawaiian-Emperor swell are calculated using available data or both gravity anomalies and crustal structures derived from explosion seismology. From these data, the lithospheric thickness beneath Hawaii is estimated to have been decreased by about 20 km. Extra high heat flux from the asthenosphere, in the order of 420 mW m−2 (10 HFU) is required in this region. Theoretical predictions of temporal variations of ocean-bottom topography, surface heat flow, gravity anomalies and plate thickness along the Hawaiian-Emperor swell fit well with the observations.

Gravity anomalies and the geologic block structures in and around the aftershock area of the 2007 Noto Hanto Earthquake

The 2007 Noto Hanto Earthquake occurred on 25 March. We have constructed a detailed gravity anomaly map over the Noto peninsula. Four block boundaries which are identified by morphological/geological studies are also recognized on the gravity anomaly map. Various other observations indicate that the mainshock ruptured only one of the blocks, the Kuwatsuka block, although aftershocks are distributed over two blocks, the Kuwatsuka and the Saruyama blocks. The Bouguer anomalies over the Kuwatsuka block are relatively high compared to those of the Saruyama block, i.e. the basement depth of the Kuwatsuka block is shallower than that of the Saruyama block. This difference in the basement depth is consistent with the deformation pattern observed by SAR interferometry. At the block boundary, an offset is recognized in the aftershock distribution. Based on the relationships among the gravity anomalies, the geologic structures, the aftershock distribution and the source fault, we propose that the rupture size of the earthquake was constrained by the block structure in this region.

Digital Compilation of Gravity Data Over the Japanese Islands

The Japanese Islands are situated in the most tectonically active areas of the Pacific and Philippine Sea subduction zones. The high mountain regions have many summits of 3,000 meters in elevation, comparable with the Alps. The length of the islands is about 2,000 km. The topographic relief is so great that roads are largely restricted to river courses, and large areas around high mountains remain difficult to access for gravity measurements.

A hypothesis on the heat flow similarity between oceans and continents

Abstract A simple physical model, the thickening plate model, gives a possible explanation of the fact that the averaged terrestrial heat flow over oceans and continents is roughly equal. In the model, latent heat released by solidification of asthenospheric material plays an important role: faster development (thickening) of oceanic lithosphere releases a larger amount of latent heat than does continental lithosphere, and the heat production becomes comparable with radiogenic heat production within continental crust. The model also suggests that the rough equality of the heat flows is not an essential feature of the Earth but merely coincidental under the conditions of the present plate motion. A more important feature may be the similarity of the terrestrial heat flow over the oldest regions of both continents and oceans. Terrestrial heat flow over an oceanic area may be a function of the spreading rate of the lithosphere: if the spreading rate of the organic lithosphere is slower than the present rate, the averaged terrestrial heat flow over oceans should be lower.

3D Basement Structure of the Fukui Plain, Central Japan, Inferred from Gravity Anomalies: ―福井地震およびその周辺の活断層との関係―

The Fukui Earthquake of M 7.1 was occurred in 26 June, 1948. The epicenter is located at the central part of an alluvial plain (the Fukui plain). In order to understand locations both of the epicenter of the earthquake and active faults surrounding the plain in terms of crustal structure and geology, the basement structure beneath the plain was estimated by using gravity data of about 1,000 points. Applying 2D-Talwanis method iteratively, we calculated 3D density structure beneath the plain up to a depth of 4 km. Three-layers model is assumed in this study: i.e., from top to bottom, each layer corresponds to the Quaternary sediments (density 2.1 g/cm3), the Neogene sedimentary layers (2.4) and the lowest Neogene layer and the lower strata (2.67), respectively. An elongated depression of about 3 km depth with N-S orientation is found in the central part o:f the Fukui plain. Epicenter of the Fukui earthquake is located on the deepest part of the depression. The results also imply that active faults in the Fukui plain are situated along boundaries where the basement structure steeply changes.