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Geophysicist Tanimoto Speaks about Earthquake and Tsunami

• Geophysicist Toshiro Tanimoto, Professor of the University of California-Santa Barbara, spoke about the earthquake at the Chicago Japanese Mission Church Southern Baptist Convention (CJMC) on May 15.
• Tanimoto, who majored in physics at Tokyo University, decided to study geophysics when he entered graduate school because he wanted to contribute the society by his research. After graduating from Tokyo University, he obtained his Ph. D at the University of California-Berkeley. He taught at the California Institute of Technology as an Assistant Professor and currently he as been Professor of Seismology, Department of Earth Science at UC-Santa Barbara since 1992. He met Rev. Yugo Kobari of CJMC during his stay in Pasadena, and their long-time friendship brought him to the church in Arlington Heights.

Plates and Earthquake

• The earth is covered by jigsaw-puzzle like plates, and each plate has been moving toward a certain direction. The movements have been accurately measured by GPS since late 1990s. For example, the North American continent, which is on the North American Plate, moves to rotate several inches from west to southwest every year.

• Most severe earthquakes occur at the border of plates where the plates conflict with each other.
• Japan has been influenced by four plates, Pacific Plate, North American Plate, Philippine Sea Plate, and Eurasian Plate. Pacific Plate, which is located just east of Japan, moves about four inches toward Japan, while Eurasian Plate, which is located just west of Japan, moves about an inch toward Japan. Pacific Plate sinks under North American Plate, and Philippine Sea Plate sinks under Eurasian Plate. More complicatedly, the west half of Japan is on the Eurasian Plate, and the other half is on the North American Plate. Tanimoto said, “Japan is located in a very scary place, and Japanese people have always lived in such a place.”

• Tremendous energy is accumulated when Pacific Plate sinks under North American Plate and pulls the rim of the crust down. The pulled rim eventually bounces back and triggers earthquakes and tsunamis. That was the cause of the Great Earthquake and Tsunami of Tohoku in 2011.

The Great Earthquake and Tsunami of Tohoku

• Pacific Plate dives deep under North American Plate, and earthquakes occur between the plates as deep as 440 miles, but big ones happen at shallow places. Japan has had several earthquakes of magnitude 8, and seismologists predicted that a magnitude 9 level earthquake would never occur in Japan. However, the scale of the Great Earthquake was beyond the prediction. Even three earthquakes of M7.9 happened within 24 hours of the main quake of M9.

Tsunami

• The size of the fault sliding at the Great Earthquake in Tohoku was about 312 miles from north to south and 125 miles from west to east. The trench created by the crust sinking was 25 miles deep. The fault sliding started at the middle of the area and spread across the entire area within three minutes. The rim of the crust elevated 164 feet and caused the great tsunami. Again, the mega scale of the fault sliding was far beyond seismologists’ prediction.

• A year before the Great Earthquake, Japan’s National Research Institute for Earth Science and Disaster Prevention didn’t forecast big earthquakes in Tohoku area because M7 level of earthquakes occasionally occurred in the area to release some degrees of accumulated energy. Seismologists came to understand that M7 level of earthquakes were not enough to release the accumulated energy in a subduction zone.

Seismology

• Seismology began in the early 1900s, so its history is a little more than 100 years. Many geological phenomena were not limited to a 100 or 150-year cycle, and seismologists encounter a significant phenomenon for the first time. Tanimoto said, “That was the Great Earthquake and Tsunami in Tohoku.”

• A key factor to infer a big earthquake cycle is searching for traits of previous tsunami. The Great tsunami in Tohoku rolled on 5-kilometers (3.125 miles) to the inland where the ancient “Namiwake” shrine was located. “Namiwake” means dividing waves, and as the name implies, the shrine has some folklore related to tsunamis.
• An important rediscovery was a tsunami record written by Michizane Sugawara and others in an ancient document “Sandai Jitsuroku”. It reported that there was a tsunami in 896 which reached “Namiwake” shrine.
• Seismologists reexamined the old document and came to conclude 896 tsunamis truly happened and the 2011 tsunami was an extraordinary earthquake that would happen every 1000 years or so.

Earthquake in Kumamoto

• On April 14, two M6 earthquakes hit Kumamoto City and its suburbs. Tanimoto wondered why the same level of quakes came three hours apart. Two days later a M7 quake hit Kumamoto again and was the main earthquake. An M7-quake level has 1/100th of the power of an M9; however, it occurred just beneath the city and caused about 50 deaths and mass evacuation.
• The series of earthquakes moved to Oita Prefecture and Aso villages. Tanimoto said that fault corruptions were identified and those faults were a part of the Median Tectonic Line.

• The Median Tectonic Line has been inactive and has not produced quakes. However, Tanimoto said that he recognized for the first time that something happened along with the tectonic line. There might be a possibility that the faults would corrupt more.

Is Geologic Change Active?

• Tanimoto said that north half of Japan moved 16.4 feet to eastward after the 2011 Great Earthquake. “Relating to the surprising phenomenon, I feel that the geologic change has become a little active,” he said. However, he emphasized that the total number of earthquakes were almost the same.

How can we prepare for an earthquake?

• Thanimoto said that the earthquake was not avoidable and the best preparation was building quake-resistance homes and buildings. Tokyo has made efforts to build such buildings, so they will sustain if an M6 quake hits the city.


Geophysicist Dr. Toshiro Tanimoto, Professor
of the University of California-Santa Barbara