| dc.description.abstract | The Flores Thrust fault zone in the eastern Sunda–Banda arc (Indonesia) is one of the rare back-arc thrusting faults in the world. This southward-dipping fault is located in the back-arc region in the transition zone from the subduction zone at the Sunda/Java trench to the continent-to-arc collision at the Timor trough. There are some arguable views on the tectonic development and present kinematics of this fault zone. Despite the Flores Thrust often hosting destructive shallow earthquakes and some triggered tsunami, there is a lack of study that integrates seismic source investigations for earthquakes along the Flores Thrust fault zone. Three parts of the Flores Thrust fault have been seismically active and produced some significant M5.7+ earthquakes in the present earthquake catalog, namely, from west to east, the Lombok segment, the Sumbawa segment, and the West Flores segment. The seismicity tends to be clustered with a cascade of moderate magnitude earthquakes in these segments in the western part of the fault zone.
Using seismic data came from the global and regional seismic network, in this thesis, we investigate the source characteristics of earthquakes along the Flores Thrust fault zone, mainly for earthquakes that occurred from 1999 to 2022. We performed finite-fault rupture inversions for MW 6.2+ earthquakes constrained by the teleseismic body and surface waves and introduced jackknife resampling tests to measure the model uncertainties. In addition, we investigated two seismic sequences in Lombok and West Flores using double-difference hypocenter relocation and statistical seismology. We mainly focus on three segments, the Lombok segment that ruptured in July–August 2018 Lombok seismic sequence (with one MW 6.5 and two MW 6.9 events), the Sumbawa segment that ruptured in the 2002–2009 Sumbawa earthquakes (with five MW 6.2–6.6 events), and the West Flores segment that hosted an MW 6.4 normal-faulting event in 2003 and a seismic sequence in 2022.
We performed a comprehensive seismological analysis to characterize earthquake sources and seismicity along the Flores Thrust fault. As a result, we suggested that the earthquakes ruptured the major splay thrust fault in the zone and not in the basal detachment fault of the incipient subducting Flores sea oceanic crust. Furthermore, we highlighted the similarities of the thrusting rupture processes of Flores Thrust earthquakes, i.e., in their rupture initiation, speed, size, and static stress drops, and they are the type of thrust earthquakes that occur in a plate boundary.
In the Lombok segment, we discussed the relations of unusual, energetic seismicity in 2018 and the rupture behaviors of the significant cascading events. The mechanical discrepancy between complex fault ruptures and the spatiotemporal evolution of seismicity suggested an earthquake-volcano interaction and provides insights into how fault rupture behaves in such a fault-volcano environment. In addition, the westernmost and easternmost termination of seismicity extent indicated a strong segment barrier. In the Sumbawa segment, constructed finite-fault models of five moderate earthquakes suggest a non-overlap asperities area. The ruptures often propagated along-strike or down-dip directions with low static stress drops. We suggested that the cascade moderate-size asperities with low static stress drop may be the indication of the less mature feature of western Flores Thrust compared to its more developed eastern part. Alternatively, these earthquakes may act as asperities located at the down-dip patches of the Sumbawa segment, and its shallower section still has a potential of ruptures with MW > 7.0, similar to a historical earthquake in 1836. In the West Flores segment, the 2022 seismic sequence also ruptured the major splay thrust fault and also highlighted a complex deformation, but we confirmed that the 2003 normal faulting event was not located on the Flores Thrust; instead, it ruptured at the deeper depth, in the Flores Sea incipient subducted slab (intraslab event), below the Flores Thrust fault zone.
We proposed a new seismotectonic model and segmentation of the Flores Thrust fault zone based on our source models and seismicity analysis results. Our result indicated that the Flores Thrust is a complex discontinuous zone and that the three segments analyzed in this study behaved differently. In addition, we compiled earthquake dimensions for significant earthquakes along the Flores Thrust fault necessary to build an earthquake source scaling. This study provides a unified treatment of faulting mechanics and seismicity along the Flores Thrust fault and sheds light on the seismotectonic, earthquake nucleation, and seismicity activation in the Lombok, Sumbawa, and West Flores segments. It would be necessary for further seismic hazard analysis. | en_US |