The 2019 Mw 5.4 East Lombok Earthquake, West Nusa Tenggara: A Shallow Fault Reactivation in the Sembalun Caldera Area of Lombok Island

Shallow fault reactivation has been identified, likely related to the 5.69 M_w East Lombok earthquake on March 17, 2019. Using relocation hypocenters and a focal mechanism analysis, the structure can be analyzed. The relocation results showed a more geologically meaningful hypocenter distribution than before relocating, which enabled a structural analysis. In this case, the mainshock clearly showed a dipping slip (a strike angle of ∼97°, a dipping angle of ∼66°, and a rake of ∼102°) extending ∼25 km from the northeast to the southwest. After cross-correlation, the largest relocated aftershock (M_w 5.1) either occurred on the same ruptured plane as the mainshock or was not caused by a different earthquake source. From the analysis of geological data, the mainshock was related to an inland fault close to the Sembalun caldera. Additionally, the high stress of the 2018 Lombok earthquake may have triggered the 2019 East Lombok earthquake according to Coulomb stress changes. This study provides valuable information about the addition of the active fault zones on Lombok Island, improving our understanding of this unique seismic activity in an unusual place. Moreover, the Sembalun region is one of the most popular tourist destinations on Lombok Island. Consequently, this study is crucial for developing future seismic risk mitigation strategies.