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Carvajal, M, Araya-Cornejo C, Sepulveda I, Melnick D, Haase JS.  2019.  Nearly instantaneous tsunamis following the Mw 7.5 2018 Palu earthquake. Geophysical Research Letters. 46:5117-5126.   10.1029/2019gl082578   AbstractWebsite

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. Plain Laguage Summary Tsunami hazard assessment is routinely based on assessing the impacts of long-period waves generated by vertical seafloor motions reaching the coast tens of minutes after the earthquake in typical subduction-zone environments. This view is inadequate for assessing hazard associated with strike-slip earthquakes such as the magnitude 7.5 2018 Palu earthquake, which resulted in tsunami effects much larger than would normally be associated with horizontal fault motion. From an extraordinary collection of 38 amateur and closed circuit television videos we estimated tsunami arrival times, amplitudes, and wave periods at different locations around Palu Bay, where the most damaging waves were reported. We found that the Palu tsunamis devastated widely separated coastal areas within a few minutes after the mainshock and included unusually short wave periods, which cannot be explained by the earthquake fault slip alone. Post-tsunami surveys show changes in the coastline, and this combined with video footage provides potential locations of submarine landslides as tsunami sources that would match the arrival times of the waves. Our results emphasize the importance of estimating tsunami hazards along coastlines bordering strike-slip fault systems and have broad implications for considering shorter-period nearly instantaneous tsunamis in hazard mitigation and tsunami early warning systems.

Hauksson, E, Haase JS.  1997.  Three-dimensional V-P and V-P/V-S velocity models of the Los Angeles basin and central Transverse Ranges, California. Journal of Geophysical Research-Solid Earth. 102:5423-5453. AbstractWebsite

We use P and S arrival times from 5225 earthquakes and 53 explosions, recorded by the Southern California Seismographic Network, to invert for the three-dimensional P- velocity (V-p) and the P and S-velocity ratio (V-p/V-S) in the central Transverse Ranges and the Los Angeles basin. To model long-wavelength features of the velocity structure, we invert for the models by interpolating models determined successively from a sparse, medium, to a dense grid, with 40, 20, and 10 km spacing of horizontal grid nodes. Layers of grid nodes are placed at depths of 1, 4, 6, 8, 12, 16, and 20 km. The data variance decreased about 80% in the gradational inversion. Ample data from the 1994 Northridge and other earthquake sequences, the rich background seismicity, and the dense station distribution along with controlled sources made the model well resolved, except along the edges, to the southwest in the offshore region, and at depths greater than 20 km. The Vp model images the shape of the Los Angeles and east Ventura basins down to depths of 8 and 12 km. Three low-velocity areas at 1 km depth in the Los Angeles basin that coincide with recent sediment depositional areas are also imaged. The north edge of the Peninsular Ranges, the Santa Monica, and the San Gabriel Mountains, form discontinuous high-velocity ridges extending to depths df 20 km. The high V-P/V-S ratios in the near surface are consistent with high pore fluid pressures in the basin sediments. At depth beneath the east Ventura basin the high V-p/V-S and high. Vp suggest the presence of Ophiolitic assemblages or mid-Miocene volcanics. In contrast, a body with normal Vp and low V-P/V-S that is identified in the upper crust beneath Santa Monica Bay may be a fragment of quartz-rich continental crust. The Vp model of both the Ventura and the Los Angeles basins shows features that suggest deformation of the hanging wall or basin closure. At seismogenic depths of 16 km the hypocenters of moderate-sized and large earthquakes are located within or adjacent to high- Vp-velocity bodies. In most cases these high-velocity bodies form the upper block, consistent with shortening of the lower crust as described in thick-skinned tectonic interpretations.