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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.

Emore, GL, Haase JS, Choi K, Larson KA, Yamagiwa A.  2007.  Recovering seismic displacements through combined use of 1-Hz GPS and strong-motion accelerometers. Bulletin of the Seismological Society of America. 97:357-378.   10.1785/0120060153   AbstractWebsite

Retrieving displacement from seismic acceleration records is often difficult because unknown small baseline offsets in the acceleration time series will contaminate the doubly integrated record with large quadratic errors. One-hertz Global Positioning System (GPS) position estimates and collocated seismic data are available from the 2003 M-W 8 Tokachi-Oki (Hokkaido) earthquake. After a process of correcting for possible misorientation of the seismic sensors, an inversion method is used to simultaneously solve for ground displacement with both data sets as input constraints. This inversion method takes into account the presence of unknown offsets in the acceleration record, and the relatively large uncertainties in the estimated 1-Hz GPS positions. In this study, 117 channels of seismic data were analyzed. Only 5% of the time does the static displacement retrieved from traditional baseline correction processing without GPS information agree with the absolute displacement measured with 1-Hz GPS to within the errors of the GPS data. In solving simultaneously for constrained displacements that agree with both the seismic and GPS data sets, an optimal solution was found that included only one- or two-step functions in the acceleration records. Potential explanations for the offsets are analyzed in terms of tilt of the sensor or electronic noise. For nine stations, clear misorientations of the seismic sensors of more than 20 deg from the reported orientation were found. For this size event, the 30-sec sampled GPS solutions were also a sufficient constraint for establishing the offset errors and recovering reliable displacements. The results significantly extend the frequency band over which accelerometer data are reliable for source inversion studies.