The shortest recurrence interval of Genroku-type great Kanto earthquakes revised from 2000 to 500 years
Overview of the press release
A research team from the University of Tokyo and the Geological Survey of Japan revealed that great earthquakes similar to the 1703 Genroku Kanto earthquake (a Genroku-type earthquake) occurred at least five times during the past 6300 years with intervals ranging from 500 to 2800 years on the plate interface near the Tokyo metropolitan area.
Since subduction zone great earthquakes are accompanied by coastal uplift, geomorphological features called marine terraces and shell fossils in the terrace deposits may contain records of past earthquakes. Therefore, the formation ages of marine terraces are usually determined by radiocarbon-dating of fossil samples collected from natural outcrops; however, the accuracy of the estimated ages have been unclear due to limited samples. To attain higher accuracy, the research team constructed a 50-cm resolution digital elevation model by using airborne laser ranging technology and analyzed the geomorphology to identify the detailed structures of the marine terraces in the Chikura lowland, which is located southeast of the Boso peninsula. The team then intensively drilled into the depositional layers constituting the terraces at a number of locations in a narrow area. From the drilling cores, they collected many samples of shell fossils that could correlate with the ages of the uplift events precisely. By applying state-of-the-art accelerometer mass spectrometry to the fossil samples, they determined the ages of the terrace formations at an unprecedented level of accuracy. The newly obtained ages were found to be nearly a thousand of years younger than previously estimated ages; consequently, the intervals of the great earthquakes that occurred along the Sagami Trough are estimated to be much shorter and more varied than those of previous estimations.
The results of this study suggest the need for HERP, a Japanese governmental committee, to revise their current recurrence intervals for Genroku-type earthquakes from the minimum of 2,000 years (with an average interval of 2,300 years for the long-term evaluation of earthquake activities) to 500 years.
Figure: Distribution of marine terraces and their formation ages in the Chikura area southeast of the Boso peninsula (inset). The hatched lines indicate the locations of the paleo-shorelines. The numbers correspond to the previously estimated formation ages (upper) and the revised ages by the current study (lower).
- Junki Komori (2nd year Master’s student, Department of Earth and Planetary Science, School of Science, The University of Tokyo)
- Ryosuke Ando (Assoc. Professor, Department of Earth and Planetary Science, School of Science, The University of Tokyo)
- Masanobu Shishikura (Research Group Leader, Research Institute of Earthquake and Volcano Geology, National Institute of Advanced Industrial Science and Technology)
- Yusuke Yokoyama (Professor, Atmospheric and Oceanography Research Institute, The University of Tokyo)
- Yousuke Miyairi (Project Researcher, Atmospheric and Oceanography Research Institute, The University of Tokyo)
Journal Earth and Planetary Science Letters, 471C (2017) pp. 74-84 Title History of the great Kanto earthquakes inferred from the ages of Holocene marine terraces revealed by a comprehensive drilling survey Authors Junki Komori; Masanobu Shishikura; Ryosuke Ando; Yusuke Yokoyama; Yosuke Miyairi DOI 10.1016/j.epsl.2017.04.044 Paper link http://www.sciencedirect.com/science/article/pii/S0012821X17302388
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