Ground Motions From Tsunami Earthquakes: An Example From Indonesia and Implications for Hazard and Warning

Date: 4/25/2019

Time: 11:30 AM

Room: Cascade II

Tsunami earthquakes are highly destructive because the runup produced by their associated tsunami is significantly higher than expected for most megathrust events of the same magnitude. These events rupture close to the trench with slow rupture velocities and large slip. At most subduction zones, tsunami first arrivals are expected within the first 5 - 15 minutes after origin time. Reliable and unsaturated estimates of the moment magnitude are not generally available in this time frame. While new advances in Global Navigation Satellite System (GNSS) algorithms significantly speed up magnitude estimation, magnitude alone is not an indication of a tsunami earthquake, thus these events continue to be very challenging for warning systems.

In this study, we propose a new method to distinguish tsunami earthquakes in real-time using both near-field high-rate (HR) GNSS data and strong-motion data. We focus on the 2010 M7.8 Mentawai earthquake which produced runup to ~15m. This is the only tsunami earthquake to date with near-field seismic and geodetic data. We combine rapid magnitude estimation from the HR GNSS data from peak ground displacement (PGD), with strong ground-motion engineering intensity measures. Our key observation is that at low frequency and from GNSS data, tsunami earthquakes are similar to other megathrust events. However, at short period strong-motion observations, these events are indeed fundamentally different from other earthquakes of the same magnitude. We show that rapid discrimination is possible within seconds of PGD magnitude being obtained.

Finally, we study source parameters of similarly-sized events and explain why the Mentawai event displays this distinct behavior. Our findings imply that because other tsunami earthquakes are inferred to have similar source properties, our methodology should be applicable to other events. Local tsunami early warning is possible for these challenging earthquakes but only through combination of geophysical sensors.

Presenting Author: Valerie J. Sahakian

Authors