The 1998 Hida Mountain, Central Honshu, Japan, earthquake swarm: Double-difference event relocation, frequency–magnitude distribution and Coulomb stress changes Bogdan Enescu a,b, * , Kiyoshi Ito a a Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan b National Institute for Earth Physics, P.O. Box MG-2, 76900, Bucharest-Magurele, Romania Received 11 March 2004; accepted 16 August 2005 Available online 17 October 2005 Abstract By using the double-difference relocation technique, we have determined the fine structure of seismicity during the 1998 Hida Mountain earthquake swarm. The distribution of seismic activity defines two main directions (N–S and E–W) that probably correspond to the regional stress pattern. The detailed structure of seismicity reveals intense spatio-temporal clustering and earthquake lineations. Each cluster of events contains a mainshock and subsequent aftershock activity that decays according to the Omori law. The seismicity and the b -value temporal and spatial patterns reflect the evolution of the static stress changes during the earthquake swarm. About 80% of the swarm’s best-relocated events occur in regions of increased DCFF. The smaller value of b found in the northern part of the swarm region and a larger b -value observed to the south, for the same period of time, could be well explained by the static stress changes caused by the larger events of the sequence. We argue that the state of stress in the crust is the main factor that controls the variation of b-value. D 2005 Elsevier B.V. All rights reserved. Keywords: 1998 Hida Mountain earthquake swarm; Double-difference event relocation; Seismicity; b-value; Static stress changes 1. Introduction The discrimination between different physical mechanisms responsible for the occurrence of earth- quakes in a certain region is generally a difficult task. Besides, a reliable interpretation of the results depends heavily on the quality and accuracy of the data to be analyzed. Therefore, we have considered a very good opportunity to attempt such an investigation in the case of the 1998 Hida Mountain sequence, for which the amount and quality of data is high. The 1998 Hida Mountain, central Honshu, Japan, earthquake swarm started on August 7 and its most active period lasted for about two months. In this period, 18 shocks with M z 4.0 (Earthquake Research Institute (ERI), Tokyo University, data) occurred, the largest one (M = 5.0) being recorded on August 16. The swarm started in the south and migrated toward the north, reaching the northern most point in about one month. Most of the largest events in the sequence (fifteen) occurred during this South-to-North migration period. For the following month, an opposite, North-to- South migration of seismicity was observed. The aver- age speed of migration was of about 1–2 km/day (Wada 0040-1951/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2005.08.013 * Corresponding author. Earthquake Hazards Devision, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan. E-mail addresses: benescu@eqh.dpri.kyoto-u.ac.jp (B. Enescu), ito@rcep.dpri.kyoto-u.jp (K. Ito). Tectonophysics 409 (2005) 147 – 157 www.elsevier.com/locate/tecto