The Active Fault Research Center (AFRC) of the National Institute of Advanced Industrial Science and Technology (AIST), an independent administrative institution, has found that the Pacific Coast of Eastern Hokkaido uplifted as much as 1 to 2 meters over decades following a giant earthquake occurred in the 17th century at Kuril Trench. The coastal uplift may be attributed to aseismic slip of a fault lying deeper than the seismogenic zone of the interplate earthquakes.

While the geodesic data indicate that the Pacific Coast of Eastern Hokkaido is sinking for the past hundred years, the geological data shows slow coastal uplift for the past hundred of thousands years. In order to solve this mysterious controversy, the AFRC-AIST has investigated volcanic ash deposit, tsunami deposit and fossil diatoms in the coastal geological layers, and found a coastal uplift of 1 to 2 m over decades following multi-segment interplate earthquakes occurred in the 17th century. A model computation indicates that the coastal uplift is due to fault movement at a depth below the seismogenic zone of the interplate earthquake. Smaller, a few cm of, coastal uplift occurred following the Tokachi-Oki Earthquake of 2003, but no major uplift as large as 1 m over decades has ever been reported in Japan.

The result of this study was published in the December 10 issue of the journal Science.

The AFRC-AIST has been investigating geological evidence of past earthquakes for the purpose of making a long-term forecast of major earthquakes at active faults and oceanic trenches in and around Japan. As a part of this project, traces of earthquakes, tsunamis and crustal movements in the past have been surveyed on the Pacific Coast of Eastern Hokkaido since fiscal 2001. Since historical records for Eastern Hokkaido are available only after the beginning of 19th century, around 1800, data on earthquakes, tsunamis and crustal movements occurred before are resort to geological surveys. In fiscal 2003, a study of tsunami deposits revealed the occurrence of a giant multi-segment interplate earthquake in the 17th century.

Geodesic data have clearly shown that the Pacific Coast of Eastern Hokkaido is sinking at a rate of around 1 cm per year over the last 100 years. The Tokachi-Oki earthquakes of 1952 and 2003, as well as the Nemuro-Oki earthquake in 1973, along the Kuril Trench (Fig. 1) caused the coastal subsidence for additional 10 cm or so. These earthquakes reportedly accompanied postseismic coastal uplift, but the magnitude was just as much as to cancel the coseismic subsidence. On the other hand, the geological data discloses a slow rise of coastal level over the past hundred of thousands years, at an average rate of 0.02 to 0.05 cm per year. The coastal movement in the geodetic time scale (around 100 years) is opposite to that in geological time scale (100,000 years), and the controversy may be resolved by assuming a coastal uplift unknown yet in the history.

Fig. 1. Major interplate earthquakes have often occurred off Nemuro and Tokachi along the Kuril Trench in the south-east of Hokkaido. Evidences based on tsunami deposits and ashes from volcanic eruption of Mt. Tarumae and Mt. Komagadake suggest a giant earthquake combining these source areas.

Drilling surveys carried out by the AFRC-AIST at Lake Mochirippu and Kiritappu Mashland, Hamanaka Town, Kushiro District reveal that a sea bottom mud deposit in the past cove had been covered by a tsunami deposit, and then, the slough environment was replaced by a fresh water bog to be overlaid by a peat deposit. The environmental changes before and after the tsunami are regarded to be due to a quake-caused coastal uplift. Based on the volcanic deposit immediately above the tsunami deposit, it was concluded that the earthquake and the associated coastal uplift had occurred in the 17th century.

The study of fossil diatoms in the deposits demonstrates that the coast had been sinking before the deposition of the tsunami sand (Fig. 2). After the tsunami, the crustal movement was reversed from sinking to uplifting. Since the coastal elevation as inferred from fossil diatoms seemed to have changed very little before and after the tsunami, the coastal rise is supposed to have occurred not simultaneously with the earthquake, but very slowly following the quake. This reasoning is substantiated also by the contrast between the thickness of volcanic ash layer and that of peat layer. On the basis of such surveys at 12 sites from Akkeshi, Kushiro District to Nemuro City, the magnitude of coastal uplift was estimated to be as much as 1 to 2 m.

For the purpose of identifying the cause of the coastal uplift, the crustal movements caused by the interplate fault movement was computed and compared with the magnitude of the coastal movement as derived from changes in fossil diatoms (Fig. 3). Two fault models have been considered: one with slip at a depth where usual interplate earthquake takes place (seismogenic zone, depth 15 to 55 km) and the other with slip at a level deeper than the seismogenic zone, 55 to 85 km. In both models, it is assumed that two plates are presently coupled each other. The computation shows that the coastal level is not affected by fault movement in the seismogenic zone, but is subjected to an uplift as large as 1.5 m by the fault movement at a level deeper than the seismogenic zone. The coastal uplift inferred from the fossil diatoms is attributed only to fault movement beneath the seismogenic zone.

These results suggest that a massive postseismic slip took place after the interplate earthquake of the 17th century. In subduction zones where an oceanic plate is sinking beneath a continental plate, strain accumulated at a boundary between the two plates is released not only by a coseismic fault movement but also by postseismic fault slip lasting for a few days to a few years following the major quake. Following the Chilean Earthquake of 1960 (M = 9.5) and the Alaskan Earthquake of 1964 (M = 9.2), postseismic ground movement have been observed up to now for more than decades. The geological records show that postseismic movements comparable to those giant earthquakes of M9 class occurred in Hokkaido.