Faulting and other surface deformations in recent geologic time are essentially relevant to understanding present-day tectonic processes, which in turn are a key to scientific, not empirical, earthquake prediction. Geologic records are indispensable because instrumentally observed records, such as geodetic measurements and microseismicity, are not sufficient in time to cover a whole cycle of strain buildup and release in a orogenic zone.
Rheological structure of the Japan arc b ased on explosion seismology, heat-flow measurements, and laboratory experiments indicates that the western half of central and northern Honshu, including continental slopes on the Japan Sea side, is mechanically very weak; only the upper 15 kilometers of crustal rocks behaves elastic, and ductile lower crust is underlain directly by asthenospheric mantle. This zone of weakness was rifted and stretched during the early Miocene back-arc spreading event, and coincides broadly with the distribution of active faults. Since late Miocene time up to the present, the Japan arc has been subjected to east-west compression due principally to the westward convergence of the Pacific plate at Japan trench at a rate as high as ∼90 millimeters per year.
If the megathrust at the Japan trench is locked, the plate convergence is to be ac c ommodated mainly in this zone of weakness. Actually, geodetic observations in the last 100 years have revealed that strain accumulation rates over the mechanically weak zone are on the order as high as 10-7 per year. However, geologically observed strain rates, based on slip rates on active faults and folding rates, are one order of magnitude lower than the geodetic rates. A possible explanation for this discrepancy between short-term (geodetic) and long-term (geologic) observations is that the strain accumulated in the last 100 years at abnormally high rates is likely to be released by slip on the megathrust at Japan trench, which would produce big earthquake(s) with magnitude 8 or greater. Only a fraction of plate convergence may be accommodated within the Japan arc as long-term deformation.
Whether or not the above scenario is real, the process of strain buildup and release in the Japan arc-trench system is unique, and should be understood with more geologic, as well as geophysical, observations.
