Seismic zone par excellence, the region off the Japanese coast is the theater of complex geodynamic interactions associated with the convergence of several tectonic plates. In this context, new results show that the seismic risk of the Tokyo region may have been underestimated.
The Tokyo region is located in a complex area associated with the convergence of severaloff the Japanese coast.
Three subductions in a pocket square
The city is in fact located near what is called a, which marks the junction between three plates different: the Pacific, Philippine and Eurasian plates. In a relatively small area are therefore grouped three plate boundaries, each of which has a seismic potential.
The increased seismic risk comes from the fact that these are convergent limits, particularly seismogenic. The Philippine and Pacific plates thus plunge under the Eurasian continental plate at the level of the Sagami trench and the Japan trench, respectively. The Pacific plate plunges under the Philippine plate at the level of the Izu-Bonin trench, which continues south-east under the better known name of the Mariana trench.
The Tokyo region is therefore subject to the seismic hazard associated with these three subduction zones. The risk of occurrence of a strong tremoror tsunami is thus particularly high.
Historically, however, the seismicity of the Tokyo region is more associated with ruptures along the Eurasian / Philippine and Eurasian / Pacific plate boundaries., has thus identified the occurrence of numerous earthquakes in connection with these two subduction zones. More recently, was caused by the rupture of the northern and central parts of the trench of Japan. The rupture zone associated with this powerful earthquake of Mw 9.1 is 600 kilometers long, a record for previously documented on this plate boundary. According to the models, the Eurasia / Pacific boundary should not generate another high magnitude earthquake (class 8 or 9) before 550-1,100 years. However, many uncertainties persist regarding the seismic potential of the area surrounding the triple point, including the Philippine / Pacific boundary, which lies off the Tokyo region.
The contribution of paleoseismology
The boundary between the Pacific and Philippine plates has not until now been considered an independent source capable of generating large earthquakes.appeared to be induced by seismic events at the Eurasia / Pacific boundary. However, a recent study published in shows that the seismic risk could actually be underestimated.
The last earthquakes that affected the Tokyo region date back to December 31, 1703 and November 4, 1677. The earthquake of 1703, which also generated a devastating tsunami, is associated with the Eurasia / Philippine subduction zone. The earthquake of 1677, meanwhile, is more associated with the combined rupture of the Eurasian / Pacific and Philippine / Pacific plate boundaries, also generating a tsunami with aseveral meters high. Paleoseismological data, however, do not allow a clear determination of the exact source of this earthquake.
The authors were therefore interested in the risk represented by the Philippine / Pacific limit alone, in particular for the nearby Tokyo region, and went in search of traces left by possible tsunamis that hit the region by the pass.
Indeed, tsunamis leave marks of their passage. Their power allows the transport of sandy sediments on the coast and inland. These abnormal sandy deposits can subsequently be identified by paleoseismologists in the sedimentary series through the creation of trenches or at the level ofnatural. The researchers thus discovered two sandy deposits associated with tsunamis on the coast near Tokyo. The characteristics of these sedimentary levels as well as their dating have enabled researchers to determine the age and power of the earthquakes responsible for these two events.
An undervalued seismic risk for the Tokyo region
The first deposit could thus be associated with the earthquakes of 1703 or 1677, or else with a previous undocumented earthquake that took place between the year 800 and 1700. The second deposit, older, does not correspond to any documented seismic event. It would have taken place about 1,000 years ago. Theauthors show that it is possible that this tsunami was generated by an earthquake originating in the Philippine / Pacific limit, generally forgotten in calculations of the seismic risk of the region. The magnitude (Mw) of the earthquake may have been 8.5 or even 8.7, generating a large tsunami in present-day Tokyo.
These results suggest that the Tokyo region could be subject to a greater seismic risk than expected. The Philippine / Pacific boundary, whose role has so far been minimized, thus representing an additional threat of earthquake and tsunami.