Weak Near‐Field Behavior of a Tsunami Earthquake: Toward Real‐Time Identification for Local Warning

• Published in: Geophysical research letters Vol. 46; no. 16; pp. 9519 - 9528
• Main Authors: Sahakian, V.J., Melgar, D., Muzli, M.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 28-08-2019
• Subjects: Algorithms; Earth; Earthquakes; Emergency warning programs; Global navigation satellite system; Global positioning systems; GPS; Navigation; Navigation satellites; Navigation systems; Navigational satellites; Ocean floor; P-waves; Seismic activity; Seismic data; Seismic waves; Seismographs; Seismological data; Seismometers; Shaking; Spatial data; Subduction; Subduction (geology); Subduction zones; Tsunamis
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2019GL083989

Tsunami earthquakes produce some of the most devastating tsunamis. These rare events have comparatively modest magnitudes but rupture the shallowest portion of a subduction zone megathrust with exceptionally large seafloor displacements. Previous teleseismic observations found that they radiate seismic waves weakly. They should therefore not be strongly felt in the near field, but to date no near‐source seismic recordings of these events exist that confirm this. Here we analyze near‐field records of a tsunami earthquake, the 2010 M7.7 Mentawai, Indonesia event, which show remarkably weak shaking. This is strong evidence that this earthquake does indeed have a weakly radiating or inefficient source process, in spite of its large slip. Finally, we find that, when combined with near‐source Global Navigation Satellite System displacement recordings it is possible to correctly characterize tsunami earthquakes in real‐time and to provide local tsunami warning which is currently out of reach today for monitoring agencies.
Plain Language Summary
This study looked at seismometer and GPS data near a tsunami earthquake—a moderate‐sized earthquake that produced a very large tsunami for its size. We found that the seismic data infers the earthquake is small, but the GPS data are more in line with the size that the earthquake actually is. This fits with our hypothesis that tsunami earthquakes happen in the really shallow Earth near the seafloor, and break soft rock—so they do not create strong shaking (which is why the seismic data think the earthquake is small), but they do still deform the nearby coastline (why the GPS think the earthquake is more normal). The fact that it breaks the seafloor is why it produces such a large tsunami. This means that we can use “near‐field” seismic and GPS data together to discriminate events as a tsunami earthquake or not by the time they have ruptured, thus allowing us to issue local tsunami warnings for the earthquake—something current algorithms cannot do.
Key Points
Near‐field seismic data show that tsunami earthquakes radiate energy inefficiently, producing low accelerations
Near‐field geodetic data indicate that these earthquakes produce displacements approximately equal to similarly sized events
These data can be combined as a proxy radiated energy to moment ratio, to discriminate the events in real time for local tsunami warning