A conjugate fault revealed by the destructive Mw 5.6 (November 21, 2022) Cianjur earthquake, West Java, Indonesia

Abstract

On 21 November 2022, a destructive earthquake (Mw 5.6) struck Cianjur, West Java, Indonesia, resulting in at least 321 deaths, damage to 47,000 buildings, and economic losses of up to 7.7 trillion Indonesian Rupiahs (∼US $546 million). Prior to this earthquake, the fault on which slip occurred had not been mapped, thus making further analysis crucial for assessing future seismic hazard in the region. We constructed a detailed earthquake catalogue, which spanned the period from 10 days before to 48 days after the mainshock, using waveform migration and stacking, followed by relative relocation using a double-difference method. Source mechanisms for selected aftershocks were estimated using waveform inversion. Our results show three clear foreshocks preceding the mainshock, while the aftershocks reveal the presence of a conjugate fault pair trending NNW-SSE with a length of ∼8 km and WSW-ENE with a length of ∼5 km. Directivity analysis highlights bilateral rupture of the main shock toward N20°E and N200°E, although based on the focal mechanism solutions, it is likely that there was some slip on the conjugate fault. Analysis of the Coulomb stress change induced by the mainshock shows that areas to the NNW and WSW experienced an increase in stress, consistent with the observed aftershock pattern. The nearby fault to the south (the Rajamandala Fault) experienced an increase in stress, which likely elevates the risk of it rupturing in the future.