Stress drop variations among small earthquakes in the Tohoki-oki region, Japan, and implication for the 2011 Tohoku-oki earthquake

Abstract:

It is important to assess the likely rupture characteristics of future megathrust earthquakes. One approach is to study the spatio-temporal variation of geophysical properties in active subduction zones. We explore this idea by examining stress drops of 1563 small earthquakes (Mw 3.0 – 4.5) shallower than 80 km in the Tohoku-oki region before the 2011 Tohoku-oki earthquake. Although individual stress drop estimates exhibit considerable scatter, we find a strong increase in stress drop with depth between 30 km and 60 km, whereas stress drops for shallower and deeper events, respectively, are nearly constant. We also identify lateral variations in stress drop along strike. Higher-than-average stress drops are found in East Aomori-oki and Miyagi-oki, whereas Sanriku-oki is a moderate stress-drop area. The high stress-drop zone in Miyagi-oki is located just south of the large slip area of the 2011 Tohoku-oki earthquake, and possibly acted as a barrier to further rupture propagation during the event. In addition, the frequency dependence of the seismic radiation observed during the mainshock, with proportionally higher frequencies coming from the deeper parts of the fault, mimics the depth dependence we see in small earthquakes in the same region. These results imply that smaller pre-mainshock earthquakes can provide insight into the fault properties and consequent rupture processes of future megathrust earthquakes.