Low-Frequency Earthquake Swarmsand Non-Volcanic Tremor

Under Shikoku

David R. ShellyStanford University

Collaborators:Gregory C. Beroza Stanford University

Satoshi Ide University of Tokyo

EpisodicTremorand Slip

Rogers and Dragert, 2003

Cascadia

Obara et al., 2004

Southwest Japan

Tremor Waveforms

1 hour

Low frequencyearthquakes(LFEs)

S-wave arrival

30 sec

Event Locations andInterpretation

Plate Interface

Stable Slip?

TransientSlip

Locked

Oceanic Moho

Island ArcMoho

High FluidPressure

Red=Low-Freq. EQsBlack=Regular EQs

Shelly et al., Nature, 2006

LFEs Generated by Shear Slip

1. Location on plate interface2. Temporal and spatial

correspondence observedbetween LFEs/tremor andslow slip

3. P-wave first motions (Ideet al., 2006)

Evidence:

High fluid pressure may enableslip under low shear stresses

But, are LFEs are representative of tremor as a whole?

Template Events

• Best-recorded oflocated LFEs

• Each with– 6+ stations– 18+ channels

• 677 LFEtemplate events

Template Event Waveforms

Template LFE Buried in Tremor?

Template LFE Continuous Tremor

Is it in here somewhere???

Summed correlation function

1 hour

Detection!

Putting detections from all 677template events together….

Strong Detection

Weaker Detection

Each frame =2s

Conclusions

1. LFEs are probably generated by fluid-enabledshear slip at plate interface asperities

2. Continuous tremor appears to be a swarm-like sequence of LFEs, demonstrating tremorand slow slip are simply differentmanifestations of a single process!