Unveiling the dynamics of deep-focus earthquakes: Insights from the 2020 Jepara event Mw 6.7 and subduction processes beneath Java

The 2020 Jepara earthquake (Mw 6.7) represents a significant deep-focus seismic event within the subducting Indo-Australian Plate beneath Java Island. This study investigates its characteristics using automatic moment tensor inversion and double-difference relocation based on seismic data from the Meteorology, Climatology, and Geophysics Agency (BMKG). A total of 1,899 earthquakes recorded between 2010 and 2022 were analyzed, focusing on events with a depth of up to 600 km. Moment tensor analysis revealed a normal faulting mechanism with two nodal planes: the first with a strike of 310º, dip of 59º, and rake of -68º, and the second with a strike of 92º, dip of 37º, and rake of -121º. The centroid depth was refined to 544 km, and the variance reduction (VR) exceeded 60%, which classifies the results as high-quality (Category A). Approximately 85% of the hypocenters were successfully relocated, improving root mean square (RMS) travel times to less than 1 second. The earthquake is associated with slab dehydration and the polymorphic phase transition of olivine to spinel under high pressure and temperature. These processes contribute to changes in fault mechanisms and the structural variation of the mantle transition zone. The findings enhance understanding of the deep-focus earthquakes beneath Java and can significantly contribute to future earthquake mitigation in Indonesia.