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Research Group Division of GeoEnvironmental Science

 

Geomorphology Research Group

•Professor Toshifumi Imaizumi, specializing in Tectonic geomorphology, and Active faults
•Assoc. Professor Shinichi Hirano, specializing in Geomorphology
•Asst. Professor Yoshinori Otsuki, specializing in Geomorphology, and Natural Environmental Geography
 
 

Investigating earthquakes from the fault shape

Investigating earthquakes from the fault shape Why does an earthquake occur just exactly “at that place”? It does because an active fault exists just exactly there. Such an earthquake as occurred in southern Hyogo prefecture of Japan in 1995 and a series of earthquakes as occurred in northern Miyagi prefecture of Japan in 2003 were all generated right above active faults. When it comes to earthquakes, an active fault is, so to speak, always at fault.

 

We continuously strive to evaluate the distribution of and activity levels of active faults to make our evaluations increasingly accurate.

 

What is the structure of an active fault underground? To know it, it is quite practical to conduct an elastic wave probe using a vibroseismograph, for example, in addition to concentrating on the morphology and geology type of work for another. Figure 3 presents the underground structure of the Nagamachi–Rifu line fault zone that passes right under the Sendai city suburbs. In 1998, an earthquake of M5.0 occurred near the Ayashi area of Sendai city. That active fault happened to move at 12 km deep underneath Ayashi itself.

 

When would the active fault move? That presents an extremely difficult question to answer. There is a tendency by which an earthquake of large scale according to the length of the active fault would occur periodically within practically the same time interval. The scale and the timing of the next unique earthquake to occur can be determined from investigation of the associated trenches, for example. We estimate that the time to the next earthquake to come from the Nagamachi–Rifu line fault is almost up, so we are predicting an earthquake of about magnitude 7 should occur at some time soon.

 

Where would an active fault start, and where would it end? The answer is closely associated with a complex geometry of the fault in terms of a fault segment, a fault jog, and so on. We tackled this problem from a fractal geometry perspective, and were able to understand not only the fault size distribution successfully but also its spatial distribution comprehensively. Our analyses proved that a relation existed between the earthquake core size and the main shock coming from that earthquake.

 

It is important to determine exactly how the Earth’s ground––the platform for our normal living––is constructed. As an example, let us consider a coastal plain. It is built such that the sea level rose as warm weather came after the last glacial period ended; as a result, coastal areas are piled with earth and sand. Such areas have become important places on the earth where people conduct their lives and activities, Investigating the earthquake from the shape of fault but once such an area is hit by an earthquake, because it is made of the soft base stratum, the seismic vibration is amplified––even doubled––possibly causing a huge disaster. Including subjects such as landslides, rock falls or slope failures, and debris avalanches, how our ground base is constructed is an important field that we, the Department of Geoenvironmental Science, must work on.