Bridging the gap between seismically and geodetically detected slow earthquakes

Recently observed unusual seismic events: deep low frequency earthquakes and tremor, very low frequency earthquakes, and slow slip events form a family of slow earthquakes that are governed by a scaling law different from ordinary earthquakes. Guided by this scaling law, we have observed previously unknown events, with source duration of 20 - 200 s and moment magnitude of 3 - 4, under the Kii Peninsula in western Japan. These events radiate seismic energy in the 2 - 8 Hz band in direct proportion to their seismic moment-rate, though the constant of proportionality is 4 - 5 orders of magnitude smaller than for ordinary earthquakes. Slow earthquakes in this region may be comprised of numerous shear slip events, which in aggregate are manifest seismically as tremor or isolated longer events up to 200 s and geodetically as slow slip events.