Nankai-Suruga Trough Research Articles

Geochemical and heavy mineral signatures of marine incursions by a paleotsunami on the Miyazaki plain along the Nankai–Suruga trough, the Pacific coast of southwest Japan

Earthquakes and tsunami repeatedly occur in the coastal areas of the Japanese islands, which are located on the margin of continental plates. Paleotsunami deposits in these areas provide important clues regarding the reconstruction of past marine incursions by earthquakes and tsunami. The Miyazaki plain on the Pacific coast of southwest Japan along the Nankai–Suruga trough has been also affected by marine incursions by previous interplate earthquakes and tsunami. Tsunami deposits are usually identified by their geological, sedimentological, and geochemical characteristics. In the present work, portable energy dispersive X-ray fluorescence spectrometry (portable XRF), X-ray analytical microscopy (XGT), statistical analyses, and heavy mineral analyses were performed to characterize the paleotsunami deposits of the Kanbun Hyuganada earthquake (age, 1662 common era) in the SY03 core from the south Miyazaki plain. Our new findings can be summarized as follows: (1) Increases in the Ti-normalized values of S, Ca, Fe, and Sr, and depletions of K were observed in the deposits; (2) Accumulation of Zr and heavy minerals such as zircon, likely related with the high-energy tsunami, were found in the deposits; (3) Cluster analyses of the geochemical data acquired from the SY03 core by the portable XRF distinguished the paleotsunami deposits on the south Miyazaki plain. Furthermore, we also compiled the geochemical data of samples from the Shizuoka (central Japan), Sendai (Pacific coast of northeast Japan), and Miyazaki plains. Increases in Ti-normalized values (e.g., Sr/Ti) were found in these paleotsunami deposits. Our studies indicate that geochemical approaches are suitable for the characterization and exploration of paleotsunami deposits on the several coastal areas of Japan.

Tsunamis and submarine landslides in Suruga Bay, central Japan, caused by Nankai–Suruga Trough megathrust earthquakes during the last 5000 years

Magnitude 8 megathrust earthquakes that have generated large tsunamis with wave heights of >5 m in coastal areas of southwestern Japan have occurred every 100–150 years along the Nankai–Suruga Trough, which marks the convergent boundary between the Eurasian and Philippine Sea plates. Using sedimentological, geochemical, and paleontological analyses, as well as 14C dating, we examined tsunami deposits in 12 sediment cores collected from the Hamatome lowland, Yaizu Plain, Shizuoka, which faces the Suruga Trough. Four sandy tsunami deposits (Sands 1–4) are identified in sediments deposited during the last 5000 years. Sand 1 was deposited between 805 and 405 BC, suggesting that only one tsunami left a sedimentological imprint in the Hamatome lowland lagoon during the period of ∼4000 years between 3090 and 2913 BC and the AD 1096 Eicho-Tokai earthquake. This very low frequency of tsunami inundation may suggest that this tsunami was larger than those typically associated with megathrust earthquakes. Sands 2–4 are correlated with tsunamis associated with the Eicho-Tokai, AD 1361 Shohei-Koan, and AD 1498 Meio-Tokai earthquakes, respectively. Our data indicate that a sand spit, which had previously protected the lagoon from oceanic waves, was abruptly destroyed following the AD 1096 Eicho-Tokai earthquake and did not re-form at the same location. This disappearance of the sand spit is inferred to have involved an earthquake-induced submarine landslide. The data presented in this paper, together with information obtained from historical documents, indicates that M8 megathrust earthquakes in AD 1096 and AD 1498 caused submarine landslides in the study area, resulting in coastal subsidence and damage to coastal areas additional to that caused by the tsunamis.

Microfossil evidence for recurrent coseismic subsidence around Lake Hamana, near the Nankai-Suruga trough, central Japan

We reconstructed recurrent coseismic subsidence during the middle Holocene from analyses of sedimentary facies and fossil diatom assemblages in a small drowned valley, the Shinjo lowland, bordering Lake Hamana, near the Nankai–Suruga trough, central Japan. The Lake Hamana region is known for coseismic subsidence during the 1707 Hoei (M8.6) and 1854 Ansei-Tokai (M8.4) earthquakes along the Nankai–Suruga trough subduction zone.A coring survey revealed three widely distributed peat layers in the lowland, with a mud layer overlying each of the two lower peats with a sharp contact. We traced the alternating peat–mud beds for over 300 m along the long axis of a valley. The fossil diatom assemblage analysis revealed that the peat layers were deposited in a freshwater pond/marsh environment, which changed abruptly to an estuarine to inner bay environment, recorded by the mud layers. From the sudden environmental change at the two peat–mud contacts and our radiocarbon dating results, we inferred that coseismic subsidence followed by an invasion of seawater into the lowland occurred at least twice, during a great earthquake that occurred between 5645 and 6635 cal yr BP and during another great earthquake at ca. 5700 cal yr BP.Within the mud layers, the dominant diatom species changed gradually upward from Nitzschia frustulum and Cocconeis scutellum in the lower part to Cyclotella striata and Achnanthes submarina in the upper part. These changes suggest that water depth and salinity gradually increased after the coseismic subsidence. A possible cause of this gradual subsidence is postseismic subsidence following great Nankai-Suruga trough earthquakes.

Stress drop as a criterion to differentiate subduction zones where Mw9 earthquakes can occur

Abstract I propose a hypothesis that might be used to differentiate zones that produce Mw ≥ 9 earthquakes from zones that do not. I calculate stress drop (Δσ) values, for Mw ≥ 7 thrust-type earthquakes over worldwide subduction zones, compiling the studies that obtained well-constrained slip distributions by inverting seismic, geodetic or tsunami data. Earthquakes are grouped into class 1: Mw ≥ 9 earthquakes, class 2: Mw Δσ ¯ ) values of the class 1, 2, and 3 events are 4.6, 3.4 and 1.6 MPa, respectively. In individual subduction zones, Δσ ¯ values of the class 2 events are by more than twice greater than those of the class 3 events, except Kuril–Hokkaido. Based on these results, I propose a hypothesis that if Δσ ¯ is greater than 3 MPa in a subduction zone segment, this segment possibly produces Mw ≥ 9 earthquakes, and if Δσ ¯ is less than 2 MPa, the segment would not produce Mw ≥ 9 earthquakes. I examine the fault parameters obtained in this study based on the newly derived scaling relations that take into account the variation in Δσ. The rupture of subduction zone earthquakes, from Mw ≥ 9 through Mw ~ 7, can be understood on the basis of the same scale-invariant physics, if Mo is normalized by Δσ1.5 in the scaling relation between L and Mo. Using this relation, I estimate the maximum magnitude of an earthquake which may rupture the entire Nankai–Suruga Trough off SW Japan, and obtain Mw = 8.6–8.4.

Plate convergence and long‐term crustal deformation in central Japan

Surveys by continuous Global Positioning System in and around Japan revealed that the Amurian Plate collides with the North American Plate in central Japan by ∼2 cm/yr. Long‐term crustal deformation seems to be influenced mainly by this collision although subduction of oceanic plates governs short‐term elastic deformation over the arc. Here we study the long‐term deformation field by carefully removing the short‐term signals inferred from a‐priori plate convergence vectors and coupling strengths predicted by a thermal model. The obtained field shows that the change in velocities occurs along the longitude 135° ∼ 137°, and there exist a relatively rigid block and zones accommodating strains. Characteristic compressional deformation is found northwest of Izu due possibly to the collision of the Izu‐Bonin arc with Honshu. Plate convergence rate along the Nankai‐Suruga Trough is considerably smaller in eastern parts, due partly to the transition from the Amurian to the North American Plate of the landward side, and partly to the motion of the Izu Microplate relative to the Philippine Sea Plate. This accounts for longer recurrence intervals of interplate earthquakes in the Suruga Trough where the Tokai earthquake is anticipated to occur.

Microearthquakes and tectonics around an outer rise: the Zenisu ridge, Japan

To investigate the activity and tectonic implications of intraplate microearthquakes occurring within the deflecting oceanic plate along the Nankai-Suruga trough, Japan, we deployed eight ocean bottom seismometers (OBS's) on the Zenisu ridge, for a period of 25 days in 1985. Hypocenters of approximately 40 microearthquakes were identified around the Zenisu ridge, using the OBS data. The OBS-located hypocenters were distributed mainly on the southern half flank of the ridge. There are no earthquakes located by the OBS data within the upper layer of the Philippine Sea plate. The focal depths range from 18 to 35 km, indicating that the earthquakes occurred within the lower layer of the mechanically strong part of the lithosphere. The composite focal plane solution of the microearthquakes that occurred on the Zenisu ridge was a strike-slip or reverse fault type, having P-axis in the WNW. The tectonic origin of the earthquakes located on the ridge was demonstrated to be found in the bending, with a regional compression, of the Philippine Sea plate along the eastern Nankai-Suruga through.