When a larger earthquake than the preceding ones occurs successively in a clustered activity, it seems a further larger earthquake is apt to be observed afterwards (Yoshikawa et al., 2000). We investigate alarm rate, success rate, and probability gain for the occurrence of an earthquake with M5 or larger in such sequential activities. We use the JMA catalogue during the period from 1995 to 2000. First, we select out clusters of shallow (depth≤30km) earthquakes withM≥2.5 in and around Japanese islands. We name the first earthquake in a cluster E₀, and the one larger than E₀ that occurs first in the succeeding activity E₁, and term such clusters that contain E₁ earthquake E₁-class clusters. E₂ earthquake and E₂-class clusters are defined in the similar way. We calculate the alarm rate, the success rate and the probability gain, when M≥5 earthquakes are forecasted using E₁-class clusters and E₂-class clusters, respectively. We also examine how the alarm and success rates change when criterion of selecting clusters and upper limit of distance and time interval are varied. It is shown that, for E₁-class clusters, the alarm rate is 24 to 30% and the success rate is about 5%. For E₂-class clusters, the alarm rate is 10 to 15% and the success rate is 10 to 14%, and we get a value as large as 5702.2 for the probability gain in the best case that both the criterion and the upper limit of selecting clusters are set to be 5km-5days. The large values of the alarm and success rates and the probability gain indicate that the algorithm for forecasting M5 earthquakes proposed in this paper is practically effective as well as statistically significant.

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The Median Tectonic Line (MTL) active fault system in Shikoku, which is a 180km-long right-lateral strike-slip fault system, is composed of several active faults. The Iyo fault zone, which is composed of the Iyo, Kominato and Hongu faults, is distributed in the westernmost parts of Shikoku. We conducted a seismic reflection survey, arrayed boring researches and a trench study to reveal the fault structure and fault activity of the Iyo fault zone. The seismic reflection survey, which traverses the Kominato and Hongu faults, reveals that the Kominato fault is a south-dipping reverse fault, and the Hongu fault is a high-angle fault with upheaval on the northern side. The Kominato and Hongu faults merge about 600m below the surface. The Kominato fault may converge with the Iyo fault at a grater depth, because some previous research suggests that the Iyo fault may be a high-angle fault, based on geological and geomorphological data. In addition to this result, the Kominato and Hongu faults have no strike-slip component detectable on fault topography, and the average vertical-slip rate of the Kominato fault is roughly estimated to be 0.6mm/yr from the result of boring research. The fault mechanism of both of the Kominato and Hongu faults is different from that of the Iyo fault, because the Iyo fault is the same typical right-lateral fault as other members of the MTL active fault system. The Kominato and Hongu faults have formed a tectonic bulge through Pliocene to Quaternary compressional regimes in the northern area of the Iyo fault. These geophysical and geological facts indicate that the Iyo fault is a part of the main fault, and the Kominato and Hongu faults have developed as spray faults in the northern area of the Iyo fault. The developing process of the Iyo fault zone may be explained by tectonic setting such as a contractional bend or slip partitioning by the Iyo fault. On the other hand, we found evidence for two surface-faulting events on the trench walls at the Hongu fault. The most recent surface-faulting event of the Hongu fault is inferred to have occurred between 3, 660 and 2, 010 y. B. P. The timing of penultimate faulting event of the Hongu fault is constrained to have occurred between 7, 160 and 3, 580 y. B. P. It was proposed from the previous research that the most recent surface-faulting event of the Iyo fault occurred some time after the 14th century. The penultimate faulting event of the Iyo fault is assumed between 11, 000 and 7, 000 y. B. P. The Iyo fault zone, therefore, have occurred four earthquake events during the past 10, 000 years. But the faulting history does not coincide between the Iyo and Hongu faults under the present data. This fact suggests that the timing of faulting event is different between the main fault and spray fault.

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