Field Insights and Analysis of the 2018 Mw 7.5 Palu, Indonesia Earthquake, Tsunami and Landslides

A devastating Mw 7.5 earthquake and tsunami struck northwestern Sulawesi, Indonesia on 28 September 2018, causing over 4000 fatalities and severe damage to several areas in and around Palu City. Severe earthquake-induced soil liquefaction and landslides claimed hundreds of lives in three villages within Palu. The mainshock occurred at 18:03 local time at a depth of 10 km on a left-lateral strike-slip fault. The hypocenter was located 70 km north of Palu City and the rupture propagated south, under Palu Bay, passing on land on the west side of Palu City. The surface rupture of the earthquake has been mapped onshore along a 30 km stretch of the Palu-Koro fault. We present results of field surveys on the effects of the earthquake, tsunami and liquefaction conducted between 1–3 and 12–19 of October 2018. Seismic intensities on the Modified Mercalli Intensity (MMI) scale are reported for 375 sites and reach a maximum value of 10. We consolidate published tsunami runup heights from several field studies and discuss three possible interrelated tsunami sources to explain the variation in observed tsunami runup heights. Due to limited instrumentation, PGA and PGV values were recorded at only one of our field sites. To compensate, we use our seismic intensities and Ground Motion to Intensity Conversion Equations (GMICEs) and Ground Motion Prediction Equations (GMPEs) developed for similar tectonic regions. Our results indicate that the maximum predicted PGAs for Palu range from 1.1 g for GMICEs to 0.6 g for GMPEs.

Field Survey of Tsunami Heights and Runups Following the 22 December 2018 Anak Krakatau Volcano Tsunami, Indonesia

The 22 December 2018 Anak Karakatau tsunami in Indonesia was a rare event in that few instrumental records existed of tsunamis generated by volcanic sources before this event. The tsunami, which left a death toll of 437, is of global importance as it provides opportunities to develop knowledge on generation, propagation and coastal effects of volcanic tsunamis. Here, we report results of field surveys along the coast of the Sunda Strait, Indonesia to study tsunami wave heights and coastal damage. We surveyed 29 locations and measured ranges of tsunami runup from 0.9 to 5.2 m, tsunami heights from 1.4 to 6.3 m, flow depths from 0.2 m to 3.0 m and inundation distances from 18 to 212 m. The largest tsunami heights and concentration of damage and fatalities occurred on the western shore of Java from Tanjung Lesung to Sumur. The largest cluster of fatalities occurred at Tanjung Lesung, where more than 50 people died while attending an outdoor music being held at the shoreline. The tsunami runup and tsunami height in Tanjung Lesung were 4.0 and 2.9–3.8 m, respectively. We believe this tragedy could have been avoided if the event organizers were more aware of the hazard posed by the Anak Krakatau volcano, as it had been actively erupting for several months prior to the tsunami, and simply moved the concert stage 100 m inland. Many of the locations surveyed demonstrated a similar pattern where the majority of casualties and destruction occurred within 100 m of the coast; in several locations, lives were saved where buildings were located at least this distance inland. The significant damage and numerous deaths which occurred in Sumur, despite the moderate tsunami height of 2.3–2.5 m, can be attributed to the extremely low-lying coastal land there. Flow depth in Sumur was 0.9–2.0 m. During our field surveys, nearly one year after the event, we noted that some of the damaged buildings were being rebuilt in the same locations just 10–30 m from the shoreline. We question this practice since the new buildings could be at the same tsunami risk as those damaged in the 2018 event.

Field Survey of the 2018 Anak Krakatau Tsunami on the Islands in the Sunda Strait

<p>On December 22, 2018, an eruption and partial collapse of the Anak Krakatau volcano generated a tsunami in the Sunda Strait. The tsunami caused catastrophic damage and more than 400 deaths in coastal regions of the Sunda Strait in Lampung (Sumatra) and Banten (Java). An international tsunami survey team (ITST) was deployed 6 weeks after the event to document flow depths, runup heights, inundation distances, sediment deposition, impact on the natural environment and infrastructure. The 4 to 9 February 2019 ITST focused on islands in the Sunda Strait: Rakata, Panjang, Sertung, Sebesi and Panaitan. The survey team logged more than 500 km by small boat. The collected survey data includes almost 100 tsunami runup and flow depth measurements. The tsunami impact peaked along steep slopes facing Anak Krakatau with an 85 m runup on Rakata and an 83 m runup on Sertung. The extreme runup heights were within less than 5 km of Anak Krakatau. Flow depth reached more than 11 m above ground on Sertung where a boat landing was possible and trees remained standing. On Sebesi Island located 15 km northeast of the source tsunami runup heights remained below 10 m. In contrast, tsunami heights exceeding 10 m were observed in the Ujung Kulon National Park located 50 km southwest of Anak Krakatau. The runup distributions on the islands encircling Anak Krakatau highlight the directivity of the tsunami source with the Anak Krakatau collapse towards the southwest. Inundation and damage were mostly limited to within 400 m of the shoreline given the relatively short wavelengths of volcanic tsunamis. Significant variation in tsunami impact was observed along shorelines of the Sunda Strait with tsunami heights rapidly decreasing with distance from the point source. Field observations, drone videos, and satellite imagery are presented. The team interviewed numerous eyewitnesses based on established protocol and educated residents about tsunami hazards. The tsunami caught the locals off guard despite the history and a six-month long eruptive activity in the lead up. Community-based education and awareness programs are essential to save lives in locales at risk from locally generated tsunamis. The 500 m initial height difference between the 1883 Krakatau and 2018 Anak Krakatau collapses provides a perspective on these two tsunamis. Remaining and future tsunami hazards will be affected by volcanic edifice regrowth.</p>

A new predictor for tsunami runup

<p>We present a new Tsunami Runup Predictor (TRP). The TRP includes the length of the beach slope, the length of the accelerating phase of the wave plus the amplitude ratio for leading depression waves.</p><p>We use numerical and analytical tools to compute the runup for a dataset of 210 initial tsunami waveforms. In our tests, the slope angle of the beach varies between 1 and 5 degrees and the distance of the initial wave to the coast varies between 50 and 360 km. The results show a high correlation between the TRP and the dimensionless runup, enabling the definition of an empirical formula to predict the runup.</p><p>We further test the empirical formula using a set of past events with field data. The comparison of the empirical estimates with the runup measurements of post-tsunami surveys gives promising results.</p><p>The TRP allows estimating the tsunami runup in real-time once the offshore waveform is known.</p><p>The capacity to predict the maximum runup along the coast in real-time and include it in routine operations of Tsunami Early Warning Systems will constitute an enormous advance.</p><p><em><span>The authors would like to acknowledge the financial support  FCT through project</span></em><span> <strong>UIDB/50019/2020 – IDL.</strong></span></p>