Pre-Earthquake Early Warning: Monitoring Transient Acceleration of Subducting Plates

Slow earthquakes as a transient acceleration of plate motion are now being identified as ubiquitous phenomena in subduction zones. They distribute around the strong interplate coupling area or near the asperity of megathrust earthquakes in several subduction margins, such as the Casicadia, Alaska, Middle American, Hikurangi and Nankai subduction margins.

Recent seismic and geodetic facilities have successfully captured some slow-earthquake activity leading up to large earthquakes, especially megathrust events in subduction zones. For instance, they observed slow earthquakes by the trench near the foci of the 2011 Tohoku-Oki earthquake before the mainshock occurred. They also observed similar slow earthquakes prior to the magnitude-6 class earthquake in 2008 at the same region as that of the 2011 slow earthquakes. These slow earthquakes on the plate interface increase stress on the foci of the large earthquakes, ultimately triggering the largest interplate earthquakes.

The difference between the 2008 and 2011 earthquakes is whether they ceased or continued before they triggered the large interplate earthquakes. Consequently, the faults of the slow earthquakes were included in extremely the huge coseismic-slip region exceeding 30 m slip during the megathrust event. This suggests that slow earthquakes facilitate huge coseismic slips on its own fault if coseismic ruptures reach the ongoing slow-earthquake’s fault. A recent laboratory rock experiment on the frictional properties of slow earthquakes shows that a rise in sliding velocity on the slow-earthquake’s fault could induce frictional weakening behavior, specifically of slip-weakening. This suggests that a fault experiencing slow earthquakes may be induced more easily to slip coseismically if a dynamic rupture from a large earthquake propagates onto the fault.

Slow earthquakes both trigger ruptures at asperities of large earthquakes and facilitate coseismic slips on ongoing slow-earthquake faults. The latter eventually causes the large coseismic slip near the trench and the extremely huge tsunami height on the 2011 Tohoku-Oki earthquake. Recent progress in ocean bottom seismic and geodetic networks hopefully contribute to real-time monitoring of earthquake activity and early warning to the public. Evaluating slow earthquake activity will be useful for forecasting large earthquake/tsunami disasters and for issuing early warning if we can monitor them in real time.