Seismic activity parameters (seismicity index, annual probability, and recurrence time) estimated for the M ≥ 4.0 earthquakes in East Kolaka, Indonesia

Abstract

The M 4.9 magnitude earthquake in East Kolaka, Indonesia, that occurred on Friday, 24 January 2025, was followed by a series of other earthquakes with magnitudes ranging from M 1.2 to M 5.1. The distribution of epicenters of the earthquake series that occurred in the East Kolaka region forms a single line of convergence. Seismic activity parameters were estimated using the Maximum Likelihood method, as implemented in the ZMap 7.1 program. Using the BMKG earthquake event catalogue for the period 2021–March 2025, the a-value for the East Kolaka region—specifically at coordinates 3.9512 °S 121.685°E to 4.1625°S 121.905°E, is 4.894, indicating that annual seismic activity in that area is quite high, while the Mc value is 2.4, indicating that the frequency of earthquakes with magnitudes greater than 2.4 (M>2.4) will weaken or decrease. The seismicity index values for earthquakes with M≥4.0 are higher than those for other large-magnitude categories and decrease as magnitude increases. The probability of large-scale earthquakes in this region decreases with increasing magnitude, whether over a 1-year or a 5-year period. The M≥4.0 magnitude earthquake also has a shorter recurrence interval than other large-magnitude categories. The presence of a new active fault in the East Kolaka region, triggered by the composition of the rock strata—which consist largely of metamorphic rock—is one of the reasons for the high number of earthquakes in the area. The earthquakes that occurred in the East Kolaka region between April and October 2025 correspond to a high b-value (1.01 to 1.23), indicating that many small-magnitude earthquakes occurred and that large-magnitude earthquakes are not expected in the near future. This real-world frequency aligns perfectly with the high b-value and probability estimates derived in this study, confirming that the region is currently characterized by frequent small-scale energy releases rather than catastrophic ruptures.