Yuichi Nishimura - Academia.edu (original) (raw)

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Papers by Yuichi Nishimura

Page 1. Initial field survey report of the 2011 East Japan Tsunami in Sendai, Natori and Iwanuma ... more Page 1. Initial field survey report of the 2011 East Japan Tsunami in Sendai, Natori and Iwanuma Cities 2011/07/10 UNESCO-IOC International Tsunami Survey Team Daisuke Sugawara1, Kazuhisa Goto2, Catherine Chagué-Goff3, Shigehiro ...

Pure and Applied Geophysics, Jul 25, 2012

Agu Fall Meeting Abstracts, 2007

On April 1, 2007, a large earthquake occurred off the Solomon Islands along the Solomon subductio... more On April 1, 2007, a large earthquake occurred off the Solomon Islands along the Solomon subduction zone. The earthquake generated a large tsunami that killed more than 40 people in Gizo and Simbo Islands near the epicenter. The one day aftershock distribution showed that the source region was located in the subduction zone where the Woodlark ridge system subducted beneath the Pacific plate. Because of the subduction of the ridge, no trench exists near the plate boundary. Instead, two Islands, Simbo and Ranongga Islands, exist unusually close to the plate boundary. About two week after the earthquake, the Japanese survey team was in the Solomon Islands to conduct the coseismic deformation survey in Gizo, Simbo, Ranongga, Vella Lavella, and Kilimbangara Islands near the source area of the 2007 Solomon earthquake. A whole island of Ranongga was uplifted by the earthquake because a large area of coral flats around the island was now appeared above high tide levels. In Simbo Island, located about 20km south of Ranongga Island, we found small subsidence. In Vella Lavella Island, the most part of the island was subsided except the most southeast tip of the island. In Gizo Island, the small subsidence was found along the most part of the coast. Using those crustal deformation data, we estimated that the strike of the fault is 315 degree, the width of the fault is 35 km, the dip of the fault is about 35 degree which is much larger than a typical dip of the plate interface near a trench, the shallowest edge of the fault is located between Simbo and Renongga Islands, and the depth of the shallowest edge is less than 5km. This unusual splay fault type earthquake occurred in this area where the Woodlark ridge subducted beneath the Pacific plate.

Agu Fall Meeting Abstracts, Dec 1, 2003

The North Island GeopHysical Transect (NIGHT) was conducted in 2001 to study the Hikurangi subduc... more The North Island GeopHysical Transect (NIGHT) was conducted in 2001 to study the Hikurangi subduction zone. The study included a multichannel seismic (MCS) reflection and a wide-angle seismic survey with ocean bottom seismometers (OBS) and offshore-onshore stations. We will present stacked and time-migrated images of the MCS. We used velocity information form the wide-angle experiment to stretch theses images to depth. A subducted seamount is displayed about 40 km landward of the trench. A small accretionary prism is forming seaward of this seamount. An up to 10-km-thick sedimentary basin seems to be present landward of the seamount. The margin is underlain by oceanic crust of the Hikurangi Plateau, a large igneous province. We estimate its thickness at 10-12 km based on wide-angle arrivals in the OBS records. The Hikurangi Plateau is being subducted at a dip of 2-3 degrees close to the trench. A pronounced increase of subduction dip about 120 km away from the trench is displayed in MCS sections. The change of dip of the subducted plate appears to coincide with the eastern termination of the sedimentary basin as well as a stratigraphic high; the Lachlan Ridge. From the offshore-onshore data, we infer the presence of a low velocity zone above the subducted plate at this location. We speculate that the increase of subduction dip may be linked to a major tectonic process shaping the East Coast of New Zealand's North Island, perhaps normal faulting of the Hikurangi Plateau caused by loading from the overriding plate. The low velocity layer may mark sediments entering the subduction zone.

Agu Fall Meeting Abstracts, Dec 1, 2006

Based on resent studies on tsunami deposits, eastern Hokkaido was known to be affected by large t... more Based on resent studies on tsunami deposits, eastern Hokkaido was known to be affected by large tsunamis caused by earthquakes along the Kuril Trench more than 10 times during the last 5,000 years. To consider whether these historical tsunamis are similar in size or not, we identified plural tsunami deposits lie between peat deposits and compare their thickness and grain size distribution. At Tokachibuto, six major tsunami layers were traced along a channel up to 1,500 meter inland from the beach. The tsunami deposits are sheet- like distributed and their mean grain sizes are getting finer landward, as seen in most modern tsunami deposits. However, the size-decreasing patterns are significantly different among the tsunami deposits. We speculate that these sedimentary faces of the tsunami deposits are mostly linked to the wave height, wavelength and wave velocity of each tsunami. It means that we can evaluate relative sizes of the ancient tsunamis by tracing grain sizes of associated tsunami deposits. This information must be useful to estimate source size and location of the earthquakes that occurred repeatedly in time. If tsunami deposit is traced over the invaded area, such as a valley facing the beach, run-up heights of the tsunami can be determined well. We identified the 17th century tsunami deposit at wetland near Seika-numa, eastern Hokkaido, and observed sedimentary characteristics of the deposits along the artificial channels at the wetland and traced them inland by hand excavation. Grain sizes of the deposits decrease not only along the valley extend from the beach but also along the slopes of both sides of the valley. On both valley slopes, the mean grain size of the deposits decreased from very coarse to coarse sand at the base of the valley to fine sand at the distribution limit. Based on these thickness and grain size distribution of the deposit, we determine the minimum run-up at the site as about 14 m at the eastern slope of the valley and 12 m at the western slope.

The Holocene, May 1, 2004

Earth and Planetary Science Letters, 2002

Usu volcano, one of the most active volcanoes in Japan, erupted at regular intervals of ca. 30 ye... more Usu volcano, one of the most active volcanoes in Japan, erupted at regular intervals of ca. 30 years during the 1900s. The 2000 eruption took place on March 31 following 23 years of dormancy. An increase in seismic activity and remarkable ground deformation preceded the eruption. Many small-scale craters were successively formed in two areas at the foot of the

Advances in Natural and Technological Hazards Research, 2005

Earth, Planets and Space, 2002

The Quaternary Research (daiyonki-kenkyu), 2000

Earth, Planets and Space, 2005

The Quaternary Research (Daiyonki-Kenkyu), 2007

Journal of Japan Society of Civil Engineers, Ser. B2 (Coastal Engineering), 2010

During the Polish−Norwegian−Japanese Polar Expedition in August 2005 a deep seismic sounding of t... more During the Polish−Norwegian−Japanese Polar Expedition in August 2005 a deep seismic sounding of the Earth's crust was performed in the southern Svalbard area. It was an introduction to the international Cluster Program 77: Plate Tectonics and Polar Gateways in Earth History, closely related to the International Polar Year. During the expedition onboard r/v Horyzont II, a seismic transect Horsted'05 along