Seismic wave radiation energy of deep low-frequency tremor in the Nankai subduction zone

We propose a method for estimating seismic wave energy radiated from deep low-frequency tremor in the Nankai subduction zone, southwestern Japan. Tremor is commonly detected associated with slow slip event on the subducting plate interface at the downdip part of the megathrust seismogenic zone. Spatio-temporal distribution of tremor has been well investigated to get a general feature of tremor activity. However, more detailed characteristics may be revealed by estimating tremor energy through densely observed seismic traces, which considerably contribute for understandings of megathrust earthquakes.

To estimate total tremor energy as accurate as possible, we first measured time duration segment of tremor by systematic multi-station analysis. The tremor element was located every one minute by using the envelope correlation method and the duration segment was searched around each origin time of the tremor element. The duration with amplitude simultaneously higher than the noise level at some stations was extracted. If the duration continued longer than two hours, it was separated. Then one or two source location was determined by applying clustering process to each duration segment using hypocenters of tremor elements. Finally, we estimated tremor energy for each duration segment.

By analyzing continuous seismograms at western Shikoku area in southwestern Japan for 1 year in 2012, we obtained characteristic spatial distribution of tremor energy. Larger energy was concentrated with a length of about 20km at the updip part of tremor zone. It has been already known that there are two peaks of the number of tremor along the dip direction. The spatial distribution of energy of tremor is different with that of the number of tremor. Concentrated energy at the updip part suggests that the updip part is relatively more brittle in the brittle-ductile transition zone.