Is the Long-Term Probability of the Occurrence of Large Earthquakes along the Nankai Trough Inflated?—Scientific Review

The Earthquake Research Committee (ERC), the Headquarters for Earthquake Research Promotion, Japan, issued a new report on long-term evaluation of the occurrence of large earthquakes along the Nankai trough, southwestern Japan, in 2013, which was made along the line of the revision of national earthquake counter measures according to the 2011 Great Eastern Japan Earthquake Disaster. The central issue was the 30-yr probability, but the method to estimate probability was not changed from the previous one issued in 2001 based on the time-predictable model, which led to a 60%–70% chance of M ≥ 8 event during the coming 30 yr. However, there are several drawbacks in this evaluation such as neglect of errors in uplift data, inconsistency between uplift rate and geodetic and geomorphological data, mechanical interpretation of time-predictable model, and contradiction between the assumptions in Brownian passage time model and time-predictable model. Because ERC calculates probability with average recurrence intervals for other subduction and active fault zones, the same scheme should be adopted to the Nankai trough.