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Eastern Indonesia suffers from significant tsunami hazards due to its complex tectonic setting characterized by several subduction zones, numerous active volcanoes, as well as submarine landslides. Following the two destructive tsunamis in Indonesia in 2018 in Palu and Sunda Strait, it has been urgent to address large tsunami hazards in this unique tsunamigenic zone. This presentation outlines the outcomes of an international collaborative project on “Building Earthquake and Tsunami Resilience in East Indonesia”, supported by the Royal Society (UK) for the period 2019-2023. The project brought together research teams from UK and Indonesia, working together towards safer communities from tsunamis and earthquakes. The project mapped and studied seismogenic and tsunamigenic zones in East Indonesia, modelled scenarios of potential future tsunamis, and developed community resilience to tsunamis. A marine seismic survey was conducted in August 2022 to obtain seismic and bathymetric data from the region. The project also studied several recent earthquakes and tsunamis in the region, including the 14th November 2019 Molucca Sea tsunami following an Mw 7.2 earthquake, the 16th of June 2021 tsunami following an Mw 5.9 earthquake, and the 2018 Palu earthquake and tsunamis. It is believed that this project has generated a large database for earthquake and tsunami resilience studies in East Indonesia, and will inspire future research works. This research is funded by TheRoyal Society(the United Kingdom), grant number CHL/R1/180173 and Lloyd’s Tercentenary Research Foundation, the Lighthill Risk Network, and the Lloyd’s Register Foundation. &nbsp, The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

An important tool for understanding deformation occurring within a subduction zone is the measurement of seismic anisotropy through observations of shear wave splitting (SWS). In Sumatra, two temporary seismic networks were deployed between December 2007 and February 2009, covering the fore arc between the fore-arc islands to the back arc. We use SKS and local SWS measurements to determine the type, amount, and location of anisotropy. Local SWS measurements from the fore-arc islands exhibit trench-parallel fast directions which can be attributed to shape preferred orientation of cracks/fractures in the overriding sediments. In the Sumatran Fault region, the predominant fast direction is fault/ trench parallel, while in the back-arc region it is trench perpendicular. The trench-perpendicular measurements exhibit a positive correlation between delay time and raypath length in the mantle wedge, while the fault-parallel measurements are similar to the fault-parallel fast directions observed for two crustal events at the Sumatran Fault. This suggests that there are two layers of anisotropy: one due to entrained flow within the mantle wedge and a second layer within the overriding crust due to the shear strain caused by the Sumatran Fault. SKS splitting results show a NNW-SSE fast direction with delay times of 0.8–3.0 s. The fast directions are approximately parallel to the absolute plate motion of the subducting Indo-Australian Plate. The small delay times exhibited by the local SWS (0.05–0.45 s), in combination with the large SKS delay times, suggest that the anisotropy generating the teleseismic SWS is dominated by entrained flow in the asthenosphere below the slab.

[1] The combination of the Sunda megathrust and the (strike-slip) Sumatran Fault (SF) represents a type example of slip-partitioning. However, superimposed on the SF are geometrical irregularities that disrupt the local strain field. The largest such feature is in central Sumatra where the SF splits into two fault strands up to 35 km apart. A dense local network was installed along a 350 km section around this bifurcation, registering 1016 crustal events between April 2008 and February 2009. 528 of these events, with magnitudes between 1.1 and 6.0, were located using the double-difference relative location method. These relative hypocentre locations reveal several new features about the crustal structure of the SF. Northwest and southeast of the bifurcation, where the SF has only one fault strand, seismicity is strongly focused below the surface trace, indicating a vertical fault that is seismogenic to ∼15 km depth. By contrast intense seismicity is observed within the bifurcation, displaying streaks in plan and cross-section that indicate a complex system of faults bisecting the bifurcation. In combination with analysis of topography and focal mechanisms, we propose that the bifurcation is a strike-slip duplex system with complex faulting between the two main fault branches.

The Sumatra subduction zone is located on the eastern side of the Sunda Arc between the Sunda Strait and the Andaman Islands, where the Indo-Australian plate is subducting beneath the Eurasian plate. An important tool in understanding the style and geometry of deformation within a subduction zone is the measurement of seismic anisotropy, through observations of shear wave splitting, which provides information about the mantle flow. In Sumatra two temporary seismic networks were deployed within the Mentawai and Northern region, between December 2007 and October 2008 and April 2008 and February 2009, respectively. Here we use new splitting measurements from SKS and local S phases from these networks, to characterize the type and amount of anisotropy within the region. High quality SKS splitting results obtained from 16 stations show a coherent fast direction of NNW (350°) to SSE (170°) with delay times of 1.0 to 3.0s. The observations do not conform to the standard classification of trench parallel and trench perpendicular, instead they are trench oblique, approximately parallel to the direction of motion of the subducting Indo-Australian plate. The magnitude and direction of splitting suggests that the anisotropy is due too entrained flow in the asthenosphere beneath the subducting lithosphere. Local S wave splitting measurements were obtained from 85 rays, generated by earthquakes at focal depths of 15 km-200 km and recorded across 39 stations. The polarization of the fast shear wave is trench parallel along the islands and can be contributed to shape preferred orientation of cracks in the top of the subducting slab and overriding crust. In the forearc a rotation in fast direction to trench perpendicular is observed, with a clear positive correlation between the distance the ray has traveled in the mantle and delay time, suggesting anisotropy originates from entrained flow within the mantle wedge. In the Sumatra fault region and the back arc, events originating at the plate interface show both trench perpendicular and trench parallel fast directions with delay times of up to 0.4 s, while shallow events on the Sumatra fault indicate a clear fault parallel direction with delay times of 0.1 s to 0.18 s. This pattern of shear wave splitting suggests layers of different anisotropy, one deeper layer due to entrained flow within the Sumatran mantle wedge causing trench perpendicular direction, and the second shallower one within the overriding crust due to shape preferred orientation possibly caused by the Sumatran Fault.

For the first time, ambient noise tomography is used to clearly image the magma chamber beneath Lake Toba caldera, one of the largest Quaternary calderas on Earth. Using data from 40 seismic stations deployed between May and October 2008 around Lake Toba, empirical Green's functions are extracted from long term cross-correlations of continuous records. These functions are dominated by Rayleigh waves, whose group velocities can be measured in the period range from 2.5 to 12 seconds. Arrival times of these waves are picked for a given period and inverted using 2-D tomography to calculate lateral variations in velocity for the given period. This was done for six different periods, which all correspond to different sampling depths. Thus the six 2-D models presented together provide information on velocity variations with depth. The results show a low-velocity body coincident with the Lake Toba caldera, representing the magma chamber under the volcano. The chamber is observed to have a complex 3-D geometry, with at least two separate sub-chambers underlying the caldera. Other results include a deep low velocity body, possibly another magma chamber, south west of the lake with an upper limit of ∼7 km depth. The maximum depth to which this body reaches could not be resolved. The Sumatra Fault marks a velocity contrast, but only down to depths not greater than 5 km. The reliability of the results was further confirmed by checkerboard recovery tests.