Seismology and Volcanology Research Department, Meteorological Research Institute, Tsukuba, Ibaraki, Japan

A precursory seismic quiescence lasting 1.5 ± 0.5 years was observed prior to the 1987 M 6.7 Chiba-toho-oki earthquake, Central Japan. This event was the largest mainshock to occur in the region in 60 years. A quantitative analysis of the seismicity rates, using two independent catalogs provided by the NIED and JMA networks, shows that the precursory seismic quiescence is centered in the shallower part of the rupture zone of the subsequent mainshock, at a depth of 20-40 km. At the hypocenter of the 1987 Chiba-toho-oki mainshock, a 50% increase in the seismicity rate was detected in the NIED data, coinciding in time with the onset of quiescence (1986.4 ± 0.5). The simultaneous appearance of both quiescence in the shallow part of the rupture zone observed in two catalogs, and a rate increase in the immediate hypocenter region, suggest that these phenomena are causally linked to the subsequent mainshock. However, a quantitative analysis of both catalogs reveals that the precursory quiescence and rate increase are not unique, since rate changes of this duration and significance often occur in the data. A rate change of this significance rating could probably not be detected as a precursor in a real time approach. For the aid of real time monitoring of seismicity rate changes, we introduce the method to calculate the 95-percentile of confidence level for the significant rate changes.

We investigate some characteristics of foreshock activity of moderate and large earthquakes which occurred in the present century in Albania and the surrounding area. Using a prediction algorithm, based on possible foreshocks, we obtained a probabilistic relation between possible foreshocks and mainshocks. From documentary and instrumental data for the period 1901-1994 for the area between 39.0°- 43.0°N and 18.5°-21.5°E we evaluated the probability of the occurrence of mainshocks immediately after their possible foreshocks. The result shows that the probability that mainshocks with magnitude M ³ 6.0 are preceded by a foreshock with magnitude M ³ 4.4, distance £ about 50 km and time £ 10 days is 38% (6/16). The probability that one earthquake with M ³ 4.4 will be followed by a larger earthquake with M ³ 6.0 within about 50 km and 10 days is 1.3% (6/468), but the probability increases to 33% (1/3) if 7 earthquakes with M ³ 4.4 occur within about 50 km and 10 days. From instrumental data for the period 1971-1994, the probability that mainshocks with M ³ 5.0 are preceded by a foreshock with magnitude M ³ 4.0 is 33% (5/15). The probability that one earthquake with M ³ 4.0 will be followed by a larger earthquake with M ³ 5.0 within about 50 km and 10 days is 1.9% (5/262), but the probability increase to 5.6% (1/18) if 3 earthquakes with M ³ 4.0 occur within about 50 km and 10 days. We also found a regional variation of foreshock activity with activity decreasing from the Vlora-Elbasani-Dibra transversal seismic belt to the Ionian-Adriatic seismic zone to the interior part of Albania seismic zone.