Black dashed contours show depth of plate interface. Red dots are low-frequency earthquake (tremor) locations. Red and orange "beach balls" are focal mechanisms of LFEs and VLFs. Green rectangles and beach balls show extent and mechanism of slow slip events. Pink shows 1946 M 8.0 Nankai earthquake with mainshock slip contours in meters. All mechanisms are consistent with plate boundary slip and slow earthquakes outline slip in the 1946 mainshock.
Past megathrust and recent slow earthquakes of various types in southwest Japan after Ide et al. (2007). Black dashed contours show the depth of the Philippine Sea Plate and large arrow shows direction and rate of plate motion. Red dots show low-frequency earthquake (tremor) locations as determined by JMA. Red and orange "beach balls" indication focal mechanisms of LFEs and longer-period VLFs. Green rectangles and beach balls show extent and mechanisms of slow slip events. Pink mechanism shows mechanism for 1946 M 8.0 Nankai earthquake. Corresponding contours show slip in that earthquake in meters. All focal mechanisms are consistent with shear slip accommodating relative plate motion. Note the complementary spatial relationship between slow earthquakes and the 2 m contour of 1946 mainshock slip.
Deep tectonic tremor is a weak shaking of the ground that lasts for minutes to as long as weeks. It was discovered in southwest Japan in 2001, and there has been a great deal of work on this phenomenon since then.
Our research group pioneered new approaches to characterizing tremor and we demonstrated that it occurs as a swarm of small, low-frequency earthquakes on the deep extension of the plate interface in subduction zones. We have also shown that tremor is one of a family of slow earthquakes that grow linearly with time, as opposed to ordinary earthquakes which grow in size as the cube of time. We continue to study deep tectonic tremor through precise waveform measurements in geographically diverse regions, including: Southwest Japan, Cascadia, Costa Rica, Chile, and California.
Tremor has only been detected in only a fraction of the world's tectonic environments, but this is certainly due to the very limited seismic instrumentation in most parts of the world. We are working to expand the geographical frontiers of tremor. By systematically determining where it does, and doesn't, occur, we hope to understand the conditions that control its occurrence. We are currently analyzing tremor from the Alaska Peninsula across the Aleutian Arc because the age and temperature of the subducting plate increase steadiliy from east to west.
We are interested in the relationship between tremor and slip in large mainshocks. Our results in Shikoku suggest that tremor limns the down-dip extent of the locked zone. If true, tremor could provide valuable constraints on the down-dip extent of rupture in future large megathrust earthquakes for subduction zones where that boundary is currently uncertain. Cascadia is one example, but considerable uncertainty exists on the down-dip extent of locking in most subduction zones. By searching for tremor in areas that have suffered large earthquakes recently, we hope to test this correspondence. We are also working on methods to distinguish tremor from noise through statistical characterizations of waveform similarity through time.