The tsunami observations produced by the 2018 magnitude 7.5 Palu strike‐slip earthquake challenged the traditional basis underlying tsunami hazard assessments and early warning systems. We analyzed an extraordinary collection of 38 amateur and closed circuit television videos to show that the Palu tsunamis devastated widely separated coastal areas around Palu Bay within a few minutes after the mainshock and included wave periods shorter than 100 s missed by the local tide station. Although rupture models based on teleseismic and geodetic data predict up to 5‐m tsunami runups, they cannot explain the higher surveyed runups nor the tsunami waveforms reconstructed from video footage, suggesting either these underestimate actual seafloor deformation and/or that non‐tectonic sources were involved. Post‐tsunami coastline surveys combined with video evidence and modeled tsunami travel times suggest that submarine landslides contributed to tsunami generation. The video‐based observations have broad implications for tsunami hazard assessments, early warning systems, and risk‐reduction planning.

Distinguishing intraplate from megathrust earthquakes using lacustrine turbidites M. Van Daele et al. METHODS Sedimentology Cores were retrieved during three field seasons (2011, 2012 and 2015) (Table DR1), and were opened, macro-and microscopically described, imaged and logged with a Geotek multi-sensor core logger (MSCL) at a down-core step-size of 2 mm (Bartington MS2E point sensor for magnetic susceptibility (MS) and Konica Minolta spectrophotometer for reflectance spectroscopy) at Ghent University. For each data point, the visible-light (360-740 nm) reflectance spectrum was obtained at 10 nm intervals with a half bandwidth of ~10 nm. The R560/R665 reflectance ratio was calculated by dividing the average of R550, R560 and R570 by that of R660 and R670, in order to mimic the bandwidths obtained from the satellite reflectance data (see further), and thereby obtain comparable R560/R665 reflectance ratios. Most (13) cores were scanned with a Siemens Somatom Definition Flash medical X-ray computed tomography (CT) scanner at the Ghent University Hospital, which was employed at 120 kV, an effective mAs of 200 and a pitch of 0.45. Reconstructed volumes have a voxel size of 0.15x0.15x0.6 mm (2011 and 2012) or 0.15x0.15x0.3 mm (2015). In the master core (i.e. ENC12-03), the sampling procedure for laser-diffraction grain-size analysis of the clastic fraction was as follows: turbidites were contiguously sampled at 3 mm intervals and in laminated hemipelagic mud 3 mm wide samples were retrieved at ~1 cm intervals. Samples were treated with boiling H202 and HCl for removal of organic matter and carbonates, respectively, before being analyzed with a Malvern Mastersizer 3000 (see also Van Daele et al., 2016). Throughout the master core, organic-matter content was estimated for 35 samples using loss-on-ignition (Heiri et al., 2001). Core ENC12-04 (i.e. a replica of master core ENC12-03) was scanned with a Itrax XRF core scanner at a resolution of 1 mm at the University of Innsbruck. The XRF scanner was employed at 30kV and 45 mA, and using a 5 s exposure time. Reproducibility was shown by executing duplicate tests.

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We present evidence that suggests a new risk scenario for the Valdivia basin in south Chile, located in the area of the magnitude 9.5 1960 earthquake. In 1960, three mass movements, triggered by the earthquake shaking, dammed the upper course of the San Pedro River and threatened Valdivia City until it was opened in a controlled manner by its inhabitants. Published historical accounts indicate that the 1575 earthquake, predecessor of the 1960 event, also triggered a mass movement that dammed the upper course of the river. However, here we reinterpret the published account and present new historical records, which we combined with satellite imagery and field surveys to show that the volume of the landslide in 1575 was smaller than the smallest of those of 1960, yet its outburst flood killed thousands of natives located downstream. Additionally, we characterized different mass movement deposits in the upper course of the San Pedro River, including both ancient and those formed in 1960, and we evaluated the mechanisms that could contribute to their generation at present (e.g. land use). Our results suggest that in the present-day conditions a moderately-sized (Mw ~8) earthquake can be sufficient to cause damming the San Pedro River, which challenges the previous assumption that such phenomena are exclusively related to giant 1960-like earthquakes.