Foreshock activity and its probabilistic relation to earthquake occurrence in Albania and the surrounding area

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.