The B-Value determination of the ambarawa earthquake swarm from 22 October, 2021 to 9 November, 2021 using the least squares method

The earthquake swarm that occurred in Ambarawa, Central Java, from 23 October 2021 to 5 November 2021 recorded 42 earthquakes with a magnitude of less than 5 and were at shallow depths. Even though the earthquake shaking did not cause damage to strong buildings, the community could feel the shaking on II-III MMI. Ambarawa is flanked by 4 active faults, namely the Ungaran-1 fault in the north, the Ungaran-2 fault in the north, the Merapi-Merbabu fault in the south, and the Baribis-Kendeng Fold Thrust Zone in the east. Determination of the b-value is carried out to determine the level of stress and heterogeneity in the crust of Ambarawa, and the determination of the a-value is carried out to determine the frequency of earthquakes that occur in Ambarawa. The method used is the Least Squares Method (LSM) concerning the Gutenberg-Richter Law and using earthquake data with magnitudes above Mc (Magnitude of Completeness). The b-value obtained is 1.6 ± 0.15, which indicates that Ambarawa has a crust with low stress and high heterogeneity. The a-value obtained is 5.8, which indicates a high level of seismic frequency in Ambarawa, so that the earthquake return period is short.