Kumamoto Earthquakes Reveal the Importance of Active Fault Studies in Sumatra and the Southeast Asian Region

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Kumamoto Earthquakes Reveal the Importance of Active Fault Studies in Sumatra and the Southeast Asian Region

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Japan is known for its earthquakes and tsunami hazards due to the active collision involving three tectonic plates; the Philippine Sea plate, the Pacific plate and the Eurasian plate. These plate convergences not only created the giant trench system found off the eastern Japanese coastline, they also generated a series of active inland faults close to densely populated cities like Osaka, Nagoya, and Tokyo (Fig. 1).

Figure 1. Plate tectonics of Japan Islands. Figure from NUMO, Japan (https://www.numo.or.jp/en/reports/pdf/Level3_SF_Final-06.pdf)

Active faults found on land are usually shorter in length, and move more slowly than those found under the sea in the giant subduction zone where tectonic plates meet. The earthquakes generated by these inland faults are infrequent, and often smaller in magnitude and intensity than the earthquakes in the offshore subduction zone. However, because these inland faults are closer to the ground surface, these rare and smaller earthquakes can still result in great destruction to populated Japanese cities. Such destruction was observed in the Great Hanshin earthquake in 1995, where a magnitude-6.9 earthquake struck the city of Kobe and the nearby area. Although the energy released in the Hanshin earthquake was 1,000 times smaller than the magnitude-9.0 Tohoku-Oki earthquake in March 2011, it caused over 6,000 casualties and more than US$100 billion in economic losses, comparable to the losses from the Tohoku-Oki earthquake in the subduction zone[1] (Fig. 2).

Figure 2. The comparison of loss from 1995 Kobe and 2011 Tohoku-Oki earthquake (http://www.rieti.go.jp/jp/publications/pdp/12p003.pdf)

The recent Kumamoto earthquakes on 14 and 16 April 2016 are similar to the Great Hanshin earthquake. The magnitude-6.2 foreshock and magnitude-7.0 mainshock occurred along the well-mapped Futagawa-Hinagu fault zone southeast of the Kumamoto city, and was accompanied by a nearly 2-metre dextral offset along its 30- to 50-km-long fault rupture[2],[3]. Post-earthquake field surveys confirmed that the site of the surface rupture approximately matched the location of the Futagawa fault, which has not ruptured in thousands of years as determined by paleoseismological studies. Before this earthquake event, the Futagawa-Hinagu fault zone was already considered a candidate fault for a future inland earthquake that could measure greater than magnitude-7.0, if any segment of the fault ruptured[4].

Indeed, the two recent Kumamoto earthquakes have proven the scientists’ predictions right – predictions that were based on the research studies in earthquake science that extended over several decades.

The Median Tectonic Line and Nankai Trough

On the plate-tectonic scale, the Futagawa-Hinagu fault zone that caused the recent Kumamoto earthquakes belongs to the western extension of the Median Tectonic Line (MTL), which extends from central Honshu to Shikoku and the Kyushu island (Fig. 1 and 3). The MTL runs approximately parallel to the Nankai Trough for more than 650 km. This active fault, located in the western part of the Japan islands, is considered to be an important geological structure.

Figure 3. The median tectonic line in western Japan (http://www.osk.janis.or.jp/~mtl-muse/)

The current kinematics of the MTL is deeply affected by the plate motion between the Philippine Sea Plate and the Eurasian Plate (Fig. 1). When the Philippine Sea Plate moves ~4.5 cm/yr northwestward beneath the Eurasian Plate along the Nankai Trough, part of its plate motion (~ 5 mm/yr) that moves parallel to the subduction trench translates into the Eurasian Plate, and accumulates along the right-lateral MTL fault[5]. This relationship makes the Nankai Trough and the MTL a classic example of the slip partitioning system at the convergent plate boundary. This relationship is similar to the one between the Sunda megathrust and the Sumatran Fault in Indonesia, as well as the Rakhine megathrust and Sagaing fault in Myanmar (Fig. 4).

Figure 4. A simplified tectonic diagram explaining the slip partition between the Nankai and MTL fault, analog to the Sunda megathrust and Sumatran Fault.

Like the Nankai Trough, the home of many destructive earthquakes in Japan’s thousand-year-long history, the MTL produced many notable earthquakes in the past 1,500 years, with some behaving as a chain reaction of triggered earthquakes. One of the most prominent is the earthquake sequence from 1-5 September 1596, when a magnitude-7.0 earthquake in eastern Kyushu Island triggered subsequent quakes greater than magnitude-7.0, generated local tsunamis, and resulted in heavy destruction in ancient Japan[6].

It is interesting to note that the 2016 and 1596 earthquake sequences in Japan share many characteristics: Both occurred along the MTL fault line, where a few known historical earthquakes have taken place at that section of the MTL (there is a long lapse in time from the last major event there). Both mainshocks were triggered by a smaller foreshock from the adjacent fault segment, with a similar amount of right-lateral slip (2 to 3 m)[7]. The knowledge of these factors have increased researchers’ concern that another major event could occur at a nearby active fault where no historical earthquakes have taken place in the past few hundred to a thousand years. 

The implications on Southeast Asia

The relationship between the Kumamoto earthquake sequence and the known active faults in Kyushu serve as a real-life example of the fault-rupture interaction and its role in seismic hazards. Thanks to the geological research on earthquake studies in Japan that stretched over several decades, we were able to narrow down the potential hazard area before the Kumamoto earthquake sequence happened, and to alert the local residents so that they were prepared. Their preparation greatly reduced the damage that usually happens during magnitude-7 earthquakes. A preliminary survey of the damage shows that most of it occurred along the ruptured fault, and that the most damaged buildings were old structures[8].

The fault that generated the Kumamoto earthquakes may only awaken once in every three to four thousand years. Without the detailed research being done on the fault’s behavior in the past decades, local residents would have no way to prepare for such sudden and unexpected events.

Many of the active faults in Southeast Asia have higher fault slip rates than the fault that moved during the Kumamoto earthquakes, and will generate an earthquake every few hundred years. Some of them have not ruptured in hundreds of years. This means that the risk of a hazardous earthquake may strike nearby cities in the foreseeable future (e.g. the Valley fault system in the Philippines, the Sagaing fault in Myanmar, and the Sumatran fault in Indonesia). However, our understanding of these active fault systems are far less than what Japanese scientists have learned about the active faults in their region. For example, many of above-mentioned active faults in Southeast Asia have not been mapped in detail due to the tropical climate and the lack of funding to support elaborate research in such a broad region. This leads to an inadequate understanding of the geometry and linkages of faults, which is needed to estimate future earthquake interactions and seismic hazards. Likewise, the insufficient paleoseismological studies in this region hinder our progress in estimating the return times of earthquakes along specific segments of these active faults. This has resulted in ineffective enforcement of many seismic regulations in places like Nepal. If an earthquake similar to Japan’s were to occur in Southeast Asia, the lack of understanding of the nearby active faults and their behavior would certainly hinder the scientists’ ability to assess potential seismic hazards in the near future. This knowledge is much needed for the advanced preparation for such earthquakes events.

Just like the 2010 Christchurch earthquake in New Zealand, it is the second strong shock that usually results in greater damage and fatalities. Therefore, it is important that the governments be able to provide critical real-time information to the people. By being well-prepared for such situations, lives and infrastructure can be better protected during an earthquake event with two or more strong shocks. For this reason, the fault systems in Southeast Asia must be better mapped and studied, and the 2016 Kumamoto earthquakes is a stark reminder of that.

 
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