Supershear shock front contribution to the tsunami from the 2018 $M_w$ 7.5 Palu, Indonesia earthquake
(1) Department of Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, USA, (2) Laboratoire de Géologie, École Normale Supérieure, CNRS-UMR 8538, PSL Research University, Paris, France, (3) Faculty of Aerospace Engineering, Delft University of Technology, Delft, Netherlands, (4) Graduate Aerospace Laboratories, California Institute of Technology, Pasadena, California, USA, (5) BIG (Badan Informasi Geospasial / Geospatial Information Agency of Indonesia), Java, Indonesia, (6) Department of Civil and Environmental Engineering, University of Illinois at Urbana Champaign, USA, (7) Department of Geodesy and Geomatics Engineering, Institute of Technology Bandung, Indonesia
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Hazardous tsunamis are known to be generated predominantly at subduction zones. However, the 2018 $M_w$ 7.5 Palu (Indonesia) earthquake on a strike-slip fault generated a tsunami that devastated the city of Palu. The mechanism by which this tsunami originated from such an earthquake is being debated. Here we present near-field ground motion (GPS) data confirming that the earthquake attained supershear speed, i.e., a rupture speed greater than the shear wave speed of the host medium. We subsequently study the effect of this supershear rupture on tsunami generation by coupling the ground motion to a one-dimensional non-linear shallow-water wave model accounting for both time-dependent bathymetric displacement and velocity. With the local bathymetric profile of Palu bay around a tidal station, our simulations reproduce the tsunami arrival and motions observed by CCTV cameras. We conclude that Mach (shock) fronts, generated by the supershear speed, interacted with the bathymetry and contributed to the tsunami. This suggests that rupture speed should be considered in tsunami hazard assessments.
1. The Earthquake
Causing wide spread destruction and loss of lives$^2$, the magnitude ($M_w$) 7.5 Palu earthquake that struck on 28 September 2018 occurred on a strike (lateral)-slip fault in Sulawesi (Indonesia). This earthquake also generated a tsunami that devastated the city of Palu. Since such strike-slip earthquakes do not involve large vertical movements (cf. Tohoku, Japan during the 2011 earthquake), the origins of this mysterious tsunami are still being debated.
However, this was a "Supershear" Earthquake
2. Supershear Earthquakes
As an earthquake starts unzipping a fault, the front of the earthquake
(the zipper) constantly emits Pressure-waves and Shear-waves into the
medium. P-waves travel at a speed of about 5 km/s (18000 km/hr) and
S-waves at about 3.5 km/s (12600 km/hr).
For typical earthquakes the
speed of the front is slower than the S-waves. They are called subshear
earthquakes.
For Supershear earthquakes, on the other hand, the earthquake front
travels faster than the Shear-waves. As the S-wave speed barrier is
broken linear shock fronts manifest themselves. These are exactly akin
to the sonic boom we hear from supersonic aircrafts.
3. The Evidence
Supershear earthquakes have unique ground motion signature. The way
the ground moves parallel and perpendicular to the fault tells us
about how fast the earthquake front moved. We showed that to best
explain the recorded ground motion at the PALP GPS station the
earthquake had to go supershear. This is an indubitable proof of the
speed of the earthquake.
Incidentally, this is the first time a Supershear earthquake was
detected using a GPS station.
4. Classical Tsunami Generation
Classical Tsunamis are generated by the large vertical displacement of the sea floor around the fault. Subduction Zone earthquakes are typical sources of such tsunamis. Strike-Slip earthquakes, on the other hand, do not displace the ocean-bottom so much.
Except when they are "Supershear" Earthquakes
5. Tsunami Generation by Supershear Earthquakes
Supershear ruptures manifest shock fronts. These shock fronts carry energy from the fault to large distances without much loss. In this work here we show that even if the displacements are small, the very fact that the shock fronts affect a large region is sufficient to generate a tsunami. In Palu bay, the bathtub like bathymetry further helped this tsunami become quite large.