Search for mantle seismic discontinuities beneath northern Kalimantan, central Indonesia: A preliminary result of employing SS precursors
Kalimantan, located in the island of Borneo, represents a rare region in Indonesia, which has a relatively low seismic activity as it is located far from subduction zones. The number of existing seismic stations in this region is also quite low. Therefore, in order to investigate the mantle discontinuities beneath Kalimantan, we employed SS precursor data with SS bounce points within the study region (northern Borneo and its surroundings), located more or less halfway between earthquakes and receivers. In this study, we used data from several events in Fiji and Tonga recorded by the seismographic stations UOSS (the United Arab Emirates) and RAYN (Saudi Arabia). We used SS precursors resulting from a strict data selection with criteria: (i) earthquake depths ≤ 75 km; (ii) earthquake magnitudes > 5.8; and (iii) epicentral distances between 120o and 145o. There are two main steps in the data processing: (i) seismogram stacking by employing a Radon transform and (ii) ray tracing by using a shooting method. From the stacking results we obtained a strong energy focus in the Radon domain associated with a discontinuity at ~690 km depth. Another energy focus – which is weak – was also observed that is related to a discontinuity at ~290 km depth. We did not observe the global 410-km discontinuity beneath the study region. We focused our discussion on the strong discontinuity at the depth of 690 km, which is quite significantly deeper than the global average of seismic discontinuity; i.e., at 660 km depth. This observation is related to the subducted oceanic lithospheric slab that penetrates into the lower mantle and is then deflected horizontally below the 660 km depth beneath the study region before going down to a depth of at least 1500 km as depicted by previous tomographic models.
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