日本东京大学歌田久司教授学术报告

主讲人：

Hisashi Utada, professor, chief of Ocean Hemisphere research center, Earthquake Research Institute, University of Tokyo. Dozens of papers were published in EPSL, JGR, PEPI, GJI, et al.

歌田久司，教授，东京大学地震研究所海半球观测研究中心主任。曾在国际著名地学杂志EPSL, JGR, PEPI, GJI等发表过数十篇论文。

时间：2011年5月6日14：00

地点：同济大学海洋楼一楼报告厅

题目：

1. Exploring the oceanic mantle by electromagnetics combined with broadband seismology

结合宽频带地震学的海洋地幔电磁法探测

Hisashi Utada (Earthquake Research Institute, University of Tokyo)

Earthquake Research Institute (ERI), University of Tokyo, was established in 1925, two years after the devastating Kanto Earthquake of 1923 which caused more than 100,000 fatalities. The ERI aims to carry out basic studies on earthquakes, volcanic eruptions, and engineering to mitigate related natural disasters. Today, the ERI is composed of 4 divisions, 7 research centers, and other supporting offices and sections. The Ocean Hemisphere Research Center (OHRC) was organized in 1996, aiming to carry out observational studies on the Earth's deep interior. During the initial activity for 6 years, we built a network of global geophysical observatories (broad-band seismographs and magnetometers) in the western Pacific region, called as the OHP Network. This is a part of international collaboration to establish a global network with station spacing of 1,000 km. We also developed advanced ocean bottom instruments that can interpolate the global network. In this presentation, I will give an overview of scientific results of global electromagnetic study to estimate electrical conductivity in the mantle, combined with broad-band seismology, from the past 15 years' OHRC activity. From analysis of global data, structures in the mantle transition zone were investigated, which provided evidence for the presence of water. Ocean bottom data are useful to study the oceanic upper mantle, particularly the origin of fluidity of the asthenosphere. Our recent results suggest the presence of partial melts as a most plausible cause for the asthenosphere, although the possibility of other candidates is not rejected at all. I show our electrical conductivity results from the Philippine Sea and the western Pacific, indicating a high conductivity layer at depths of about 50-70 km and 200 km, respectively. If the cause of high conductivity is the presence of partial melts, we can estimate the conductivity of responsible melt. I will show that the volatile content will be obtained from thus estimated melt conductivity, which is an important parameter to understand dynamical condition of the asthenosphere.

2. The 2011 off the Pacific Coast of Tohoku Earthquake and Tsunami: a brief overview and preliminary result of a geomagnetic study

2011年东日本东北大地震及其引发的海啸：地磁学初步研究成果

Hisashi Utada (Earthquake Research Institute, University of Tokyo)

On March 11, 2011, at 5h46m23s UTC, a M9.0 earthquake occurred with hypocenter at 38.322°N, 142.369°E, D=32 km (USGS), which was named as the 2011 off the Pacific Coast of Tohoku Earthquake. This is the sixth over M9 earthquake after 1950 (modern seismology was established. The strong ground motion caused severe damages on facilities and buildings. The maximum acceleration observed was as high as 2,933 cm/s^2. Various models were estimated by using strong motion, tele-seismic body wave, and GPS, showing dip slip faulting as large as exceeding 20 m. The fault motion caused a large displacement of the ocean floor, which cased tsunami. The unexpectedly huge damage was caused by the tsunami, causing more than 10,000 fatalities in total and damages on buildings (including the nuclear power plant). Earthquake Research Institute installed a cabled seismic and tsunami observation  system in 1998. Two tsunami sensors (pressure gauge) recorded water height as high as 5 m at the depth of 1000m or deeper. The tsunami was already such large in open ocean and therefore it reached 30 m or more inside a narrow bay. Recent geological and historical studies indicated that a similar tsunami has happened in historical times. If this result were considered in the government policy, a lot of disasters could have been mitigated.

The M9 earthquake provides a rare opportunity to examine a question "Does the geomagnetic field change in association with or prior to an earthquake?". We collected geomagnetic total intensity time series data from 5 observation sites along the Pacific coast, and analyzed the time variation using geomagnetic data from an inland observatory. A co-seismic change in the total intensity was estimated for 5 sites. Resulting values estimated are small (not exceeding 1 nT), which are revealed to be consistent with the theoretical prediction by piezo-magnetic effect. Next we examined time variations in a time window of a few hours including the time of earthquake and tsunami. Careful correction of external disturbances, an impulsive change was obtained for every site. Considering the arrival time and phase delay to the southern stations, this phenomenon is most probably caused by the motional induction (Faraday's law) of tsunami. A rough estimate suggests the effect to be about 10 nT, in a good order-of-magnitude agreement with the observations.