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The latest observations, data on the March 11 Japan earthquake

April 14th, 2011

The Seismological Society of America will convene its annual meeting in Memphis, Tennessee, April 13-15. In response to the recent March 11 great earthquake in Japan, SSA has organized a special session to discuss the wealth of data captured by the extensive seismic network in Japan

The latest observations, data on the March 11 Japan earthquake

Time: 1:10 p.m. (Eastern)

Call this number: 800-894-5910, 785-424-1052 (for international reporters)

Gavin Hayes, U.S. Geological Survey (Moderator)
Hiroo Kanamori, California Institute of Technology
Eric Kiser, Harvard University
Zhigang Peng, Georgia Tech
Justin Rubinstein, U.S. Geological Survey
Takeshi Sagiya, Nagoya University

The First 25 Minutes of the 2011 Tohoku Earthquake

Scientists have analyzed the first 25 minutes of the massive March 11 earthquake in Japan, providing a closer look at the stunning details behind the magnitude 9.0 event along the Japan Trench. Recreating the earthquake from seismic wave data collections, Eric Kiser and colleagues show that the rupture associated with the quake's mainshock was about 250 kilometers long and 175 kilometers wide. The mainshock lasted 220 seconds, although most of the energy released in the mainshock occurred in the first 110 seconds. Over the next 20 or so minutes, several other ruptures occurred away from the mainshock rupture area, many of them with a magnitude of 6.4 or larger. After comparing their data with the historical catalogue of earthquakes in the area, the researchers think that the 2011 earthquake and its main aftershocks ruptured five areas within the region that have previously ruptured as individual earthquakes. The size of the March 2011 quake was unprecedented in the recent 200-300-year history of the Japan Trench, and is one of the largest earthquake events ever recorded.

In a second analysis, Guangfu Shao and colleagues use several models with seismic and global positioning data to trace the great rupture as it formed and moved through the fault plane. They note that the first 3-4 seconds of the rupture were so weak that they were not correctly identified at many seismic stations. Their models suggest that the rupture began at a depth of 23 kilometers, began moving slowly in the first 20 seconds, and then accelerated rapidly.

Eric Kiser, Harvard Univeristy, kiser@fas.harvard.edu

Guangfu Shao, University of California, Santa Barbara, shao@umail.ucsb.edu

Tohoku's Surprising Size

The 2011 Tohoku earthquake took many seismologists by surprise, since there have been few other earthquakes larger than magnitude 8.0 in the area for the past 1200 years. But there were some signs of seismic strain for the area that may have been considered in assessing the possibility of a rare and devastating earthquake, says Hiroo Kanamori. In this overview of the Tohoku event, Kanamori outlines the broad details of the earthquake based on seismic, tsunami and global positioning system data. Preliminary analyses suggest that there was large ocean bottom deformation of the crust along the Japan Trench, where the Pacific ocean basin tectonic plate collides and is pulled under the continental crust. The earthquake probably began in the Miyagi area, triggering a large slip in the up-thrust region near the trench. The large slip may have then triggered smaller slips in the Fukushima, Ibaraki, and possibly Sanriku areas.

Hiroo Kanamori, California Institute of Technology, hiroo@gps.caltech.edu

Massive Crust Movements in Japan Earthquake

The Tohoku earthquake was the largest earthquake ever recorded in and around Japan. But it was also one of the most closely observed earthquakes in history, thanks to a dense array of seismic and other sensing stations in the region. Takeshi Sagiya and colleagues used this wealth of data to trace the massive deformation of the earth's crust in Japan, including some startling large movements. Global positioning data recorded a horizontal movement of 5.4 meters to east-southeast -- and a sinking of 1.1 meters near the quake's epicenter at Ojika station. The entire northern Honshu island moved a meter to the east during the event. The pattern of fault slips during the earthquake suggest that historical records from the last 400 years might not provide a complete picture of the seismic possibilities along this tectonic plate boundary, the researchers note.

Takeshi Sagiya, Nagoya University, Nagoya, Japan, sagiya@nagoya-u.jp

USGS Response to Japan Earthquake

The US Geological Survey's National Earthquake Information Center runs a 24 hour/ 7 days a week service to rapidly determine the location and size of all major global earthquakes and rapidly disseminate this information to national and international agencies, scientists, and the general public. Gavin Hayes and colleagues offer a behind-the-scenes look at the NEIC response to the Tohoku earthquake. Much of the NEIC's early information focused on estimates of the earthquake's magnitude, maps of shaking, aftershock locations and other factors necessary to guide a humanitarian and scientific response to the massive quake.

Gavin Hayes, U.S. Geological Survey, ghayes@usgs.gov

Global Tremors Triggered by Tohoku Earthquake

The massive Tohoku earthquake in Japan triggered seismic tremors and microearthquakes around the world, according to analyses by Justin Rubinstein and colleagues. The Japanese earthquake triggered tremors mostly in places where they have previously been identified, including Southwest Japan, Vancouver Island, Washington and Oregon, Central California, Alaska, the Aleutians, Taiwan, and for the first time in Cuba. Zhigang Peng and colleagues further analyze these triggered tremors in several regions, discussing the timings of the triggered activities and the seismic waves from the Japan event. Most of the triggered earthquakes occurred in areas with high background levels of seismic activity, such as California's Geysers and Coso Geothermal Fields. Some triggered earthquakes also occurred in low-activity areas, such as central Nebraska, eastern Missouri, central Arkansas, and near Beijing, China. Several large earthquakes (magnitude 6.0 or higher) in Japan occurred well beyond the rupture zone of the Tohoku quake, and may have also been triggered by the giant seismic event.

Justin Rubinstein, U.S. Geological Survey, jrubinstein@usgs.gov

Zhigang Peng Georgia Institute of Technology, zpeng@gatech.edu;

Provided by Seismological Society of America

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