Geomagnetic field anomalies over the Lombok Island region: an attempt to understand the local tectonic changes

Abstract

During the last years, several investigations on the earthquakes and related tectonic structures along the Java trench have been conducted. In this study, we focus on the Lombok Island -West Nusa Tenggara (Indonesia), which lies between the centres of the highest intensity of contiguous negative-positive geomagnetic anomalies in this region. The geological and tectonic structures, however, are not known in detail for this island, whereas a better knowledge of these conditions could lead to better estimate the potential earthquake risks and thus a possible improvement of the existing early warning system.We have performed a ground-based geomagnetic survey at 56 stations in the Lombok Island during October-November 2006 for a detailed mapping and interpretation of geomagnetic anomalies related to the geological and tectonic characteristics. The 2D and 3D magnetic maps show a general geomagnetic anomaly pattern in the Lombok Island which consists of repeated contiguous negative-positive anomalies. Two forward models have been proposed for a profile connecting a strongest apparent dipolar structure. The first model assumes a uniformly magnetized sphere as the source of the anomaly, and could be interpreted as a specific local structure composed by a quite large magnetic body. The second model considers several rocks with different susceptibilities and magnetizations, and could be interpreted as a discontinuity in the geological structures. This model agrees with the local geological surface conditions and the known large scale regional tectonic structure. Therefore, it is used to interpret our results in terms of tectonic characteristics, which suggests the potential existence of a new tectonic element (e.g. a local normal fault) in this region.

Coastal sedimentation and topographic changes in the Mataram Plain, Lombok (Indonesia) following the 1257 CE eruption of Samalas volcano

“…The schematic model in Figure 12 extends a similar model on the Lombok Island that describes the topographic evolution of this island from the Late Oligocene to the Holocene (Zubaidah, 2010). Although only focused on the western part (SW direction from caldera), our model further develops the previously proposed model yet with a higher resolution, especially the recent evolution over the last ~700 years.…”

Seismic Attenuation Tomography From 2018 Lombok Earthquakes, Indonesia

“…Based on the 1978 Flores earthquake and gravity data, McCaffrey and Nabelek (1984) found evidence of a 30 • dipping southward active thrusting of the FOC beneath Lombok, which also confirmed by seismic velocity tomography of Widiyantoro et al (2011). From the gravity regional study done on Lombok Island, a high Bouguer gravity anomaly of around 140 mGal was found northeast of the Rinjani volcano, which can be interpreted as the expression of an igneous rock or magmatic conduit (Sukardi, 1979;Zubaidah et al, 2010). Afif et al (2020) in their recent tomographic velocity study revealed that most of the significant earthquakes occurred at the edge of a high-velocity region where the stress concentration is expected to be the highest.…”

Delineation of Upper Crustal Structure Beneath the Island of Lombok, Indonesia, Using Ambient Seismic Noise Tomography

“…Limited geophysical surveys have been conducted for Lombok to constrain crustal structure using geomagnetic field observation (Zubaidah et al, 2009) and integrated modeling of gravity and magnetic anomalies (Zubaidah et al, 2014). Because the physical origin associated with tectonic activities which causes the observed potential field anomalies is not as obvious and direct as seismic waves induced by tectonic events such as earthquakes, studies using seismological methods are often better suited to obtaining hazard-relevant information.…”