Triggering of slow slip and tremor by small earthquakes at the Nankai subduction zone

Abstract:

Nearby earthquakes and slow slip could potentially trigger each other. This correlation, however, has not been clearly quantified. Previous studies of the triggering between nearby earthquakes and slow slip (as delineated by tremor) are mostly based on a single slow slip event or big earthquake, and have been quantified only for stresses from distant earthquakes and tides.

We investigate 12-year earthquake and tremor catalogs for Southwest Japan, and find that local intraslab earthquakes (M1-4) are weakly correlated with subsequent tremor. Earthquakes bigger than M2 tend to be followed by tremor more than expected at random; smaller earthquakes show less triggering. The number of correlated tremor before earthquakes is not as significant, although there are marginally more than expected.

To understand the possible underlying mechanisms for triggering of tremor from earthquakes, we evaluate both the static and the dynamic stress changes associated with the earthquakes. The triggering mechanism is most likely to be the dynamic rather than the much smaller static stress changes. We find that dynamic shear stress changes of several tens of kPa due to incoming S-waves from nearby earthquakes increase the tremor rate by a factor of about 2 to 5. This rate is similar to although perhaps a factor of a few lower than rates of tremor triggering observed for similar stress changes by the much lower-frequency teleseismic surface waves and tides, suggesting that the duration of an applied stress perturbation affects triggering efficiency much less than the amplitude affects it.