| Subduction zones produce a wide array of seismicity including the planet's largest earthquakes. Recently, modern geodetic instruments have illuminated transient slip events in subduction zones (and elsewhere) with durations ranging from days to years. These events appear to be relatively common on the portion of the plate interface downdip of the region that generates earthquakes. Such events are of particular interest because they likely increase the stress on the shallower more brittle portion of the fault and therefore could trigger a large earthquake. Although these slow events do not generate strong shaking, they are often accompanied by a weak semi-continuous seismic signal that has been termed non-volcanic tremor. A variety of mechanisms have been proposed to explain this tremor and its association with slow slip, often involving fluid flow. By utilizing seismic data from a dense borehole network in southwest Japan, I demonstrate that tremor is instead generated by a sequence of small shear failures on the plate interface. These failures likely occur under very low shear stresses and may be enabled by high pore fluid pressures interpreted to exist in this zone. Added together, these shear events form the semi-continuous tremor signal, usually during much larger-scale slow slip events. In addition, I present precise locations of tremor, obtained using a matched filter technique with previously recorded "template events," which allow slip to be tracked with unprecedented resolution. These locations reveal a complex evolution of slow slip, with smaller, shorter duration subevents exhibiting along-dip migration rates of 20-150 km/hour in repeated rupture episodes over the several-day course of an event. Tremor may be generated primarily at plate interface heterogeneities that stick and slip while driven to repeated failure by slip on neighboring portions of the fault. |
