Earth Planets Space, Vol. 64 (No. 8), pp. 683-691, 2012
Ken'ichiro Yamashina and Kazuyoshi Z. Nanjo
Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyoku, Tokyo 113-0032, Japan
(Received July 27, 2010; Revised December 27, 2010; Accepted February 2, 2011; Online published August 27, 2012)
In a basic forecast model, where the expected activity in a future period is in proportion to the observed activity in the past, i.e. a relative intensity model, there are important options which can improve the performance of the model: (1) The expected number of earthquake occurrences at places where no earthquakes have been observed in the past, and (2) the extent of the area of the past reference activity, are considered to be two of the most significant factors. However, these issues have not been fully examined in previous studies of Japan. Taking into consideration the forecast of the expected number of earthquakes with a magnitude of 5 or greater in the Japan area, as designated by the Japanese test center of the Collaboratory for the Study of Earthquake Predictability (CSEP), retrospective experiments result in an optimized value of 0.00085 per year per unit cell in which no earthquakes were observed in the previous 43 years. Here, the size of a unit cell is 0.1° by 0.1° in latitude and longitude, with depths from the surface to 100 km. In addition, an area of 0.3° by 0.3° in latitude and longitude was found to be the best spatial extent for the reference activity.
Key words: Earthquake forecast, CSEP, long-term average, relative intensity, zero activity.