Java, Indonesia, sits within the tectonically active Sunda Arc, where the Indo-Australian Plate subducts beneath the Eurasian Plate at rates of approximately 6–7 cm per year. This convergence drives frequent seismicity and volcanism across the island, which forms part of the Pacific Ring of Fire. The subduction zone produces earthquakes at varying depths, with shallow events often linked to megathrust interfaces or crustal faults. Historical records document major events, including the 2006 Yogyakarta earthquake of magnitude 6.3 and the 1994 Java tsunami earthquake of magnitude 7.8, underscoring the region's persistent hazard.
Seismic swarms represent clusters of earthquakes occurring in a localized area over a short period without a single dominant mainshock. Swarm S20221121.1, recorded in Java, began at 06:21 on 21 November 2022 and concluded at 14:49 on 23 November 2022, spanning 56 hours and 27 minutes. During this interval, 49 earthquakes were detected, with magnitudes ranging from 2.5 to 5.6 and focal depths predominantly at 10 km, alongside one event at 12 km. The sequence initiated with a magnitude 5.6 shock, followed by numerous smaller events that declined in frequency over the subsequent days.
The temporal distribution shows the highest activity on 21 November, with 28 events concentrated in the first 12 hours after onset. Magnitudes tapered rapidly, with most aftershocks below 3.5. On 22 November, activity decreased to 11 events, and the final day recorded 10 events, culminating in a magnitude 3.4 shock. Depths remained shallow throughout, consistent with upper-crustal faulting in the overriding plate. This pattern aligns with fluid-driven or stress-transfer mechanisms often observed in subduction-related swarms, where pore-pressure changes or aseismic slip can trigger distributed seismicity.
Geological context indicates the swarm likely occurred along secondary faults associated with the subduction interface. Java's volcanic arc, featuring active systems such as Mount Merapi, contributes to crustal heterogeneity that can localize such sequences. Updated regional monitoring by the Indonesian Agency for Meteorology, Climatology, and Geophysics (BMKG) confirms ongoing microseismicity in similar zones, with no significant surface rupture or damage reported from this particular swarm.
Analysis of the magnitude-frequency distribution reveals a typical Gutenberg-Richter relationship, with smaller events dominating. The absence of events exceeding magnitude 5.6 after the initial shock suggests rapid stress release and limited fault dimensions. Such swarms provide valuable data for refining seismic hazard models in Indonesia, where population density amplifies risk from even moderate events.