Non-volcanic Tremors in Subduction zones: Physical Basis and Seismic Implications



The discovery of Episodic non-volcanic Tremor and Slip (ETS) events in multiple subduction zones is arguably the most intriguing new phenomenon in earthquake seismology in the past decade.  ETS consists of non-impulsive seismic radiation (“tremor”) with characteristic frequencies of 1-10 Hz and exceptionally long durations of minutes to days, coupled with geodetically observed slow slip events (SSE) with duration of order weeks.  Both tremor and slip episodes occur down-dip from the seismogenic zones, posing significant questions as to their origin, and also relative to existing concepts of interseismic loading.  For example, what are the causes of the spatiotemporal coincidence between tremor and slip in subduction zones?  Why do low-frequency earthquakes, part of the tremor signal, last much longer than earthquakes of similar magnitudes?  Why is there a “gap” between the ETS region and the full interseismic locking depth in northern Cascadia, while the down-dip ends of the 1944 Tonankai and 1946 Nankai megathrust ruptures seem to coincide with the onset of ETS in SW Japan?  And what are their implications for megathrust earthquakes and seismic hazard assessment?

Physical mechanism studies using numerical models in the framework of rate and state-dependent friction have so far only focused on explaining the “slip” part of ETS.  Furthermore, such numerical studies have all assumed time-independent fluid pressure, although fluid movement has been suggested to play an important role in tremor generation and ETS migration.  Building upon our experience in slow slip events modeling, we seek DOEI support to start constructing a more realistic subduction fault model that encapsulates the essential physics of non-volcanic tremors, in a context consistent with the occurrence of SSEs, and to study their relation to megathrust earthquakes.  In the proposed work, we plan to develop a more complete physical description of fluid transport, pore dilation and pressurization coupled with rate and state friction evolution.  Within this framework, we will specifically investigate

  • the spontaneous generation of LFE/tremor sources on small heterogeneities under the influence of dilatancy-strengthening and shear heating induced fluid pressurization,
  • physical conditions for tremor to be triggered by small cyclic stress perturbations from ocean or solid-earth tides, or dynamic stresses by teleseismic surface waves,
  • how frictionally unstable fault can slip aseismically and properties that affect the relative depth ranges of earthquake rupture and ETS events.

The largest and most devastating earthquakes occur in shallow subduction zones.  The discovery of ETS events in these areas is a major development, and improved understanding of their physical basis may increase our knowledge for earthquake forecast and seismic hazard assessment.  Progress in this project will contribute the Institute’s theme on “Dynamic processes at the seafloor”.  Results will be valuable for a larger NSF project, which studies coupled ETS events and megathrust earthquakes, with constitutive properties constrained by laboratory friction experiments and geodetic observations.