Abstract
Temporal changes of the hypocentral distances of earthquakes around the major shocks in and around Japan were studied based on the JMA (Japan Meteorological Agency) hypocenter catalog to examine a hypothesis that the area of the precursory seismic gap expands with time until a mainshock occurs at the center. Most of the inland large earthquakes occur in the interior of long-term seismic gaps. The most noticeable case is that the gap expands after the occurrence of a small-scale seismic activity around the center of the gap. The radius of the gap is approximately represented by an exponential function of time with the time-constant being 3?4 years for major inland large earthquakes in Japan. The conditional probability that the above case occurs prior to a large earthquake is 26% for events of the inland earthquakes with magnitude 5.8 or more. The probability is raised up to 50% for the events with magnitude 7.0 or more. The gap expands 4?5 times as large as the radius of the source region of the main shock. For the regions close to a plate boundary such as the Japan trench east off north Honshu, the time-constant is as small as 1?2 years and the gap area is confined to the same range of the mainshock source region. The above time-constant is inversely proportional to the expansion rate of seismicity gap and is considered to be closely related to stress relaxation process and/or hardening of the medium. Although the expansion phenomenon of a seismicity gap is not always followed by a large earthquake, it may be an important factor for estimating the possibility of a large earthquake.
