New earthquake category Nature 447, (3 May 2007) | doi: /nature05780; Received 8 December 2006; Accepted 26 March A scaling law for slow earthquakes See more references here: Add ambient noise seismology

Episodic slow-slip and tremor activity

Dragert et al., Science, 2001

Contineous GPS sites in southwestern British Columbia and northwestern Washington state have been moving landward as a result of the locked state of Cascadia subduction fault offshore. In the Summer of 1999, a cluster of 7 sites briefly reversed their direction of motion. No seismicity was associated with that event.

Dragert et al., Science, 2001

The sudden displacements are best explained by ~2 centimeters of aseismic slip over a 50- kilometer-by-300-kilometer area on the subduction interface downdip from the seismogenic zone. Such a rupture is equivalent to an earthquake of moment magnitude 6.7. This provides evidence that the seismogenic locked part of the subduction is loaded in episodic pulses.

Obara, Science, 2002

Hi-net is a newly established seismic network, with each station consists of 3- component velocity seismometer, installed at the bottom of a m deep borehole and whose sampling rate is 100 Hz.

Obara, Science, 2002 Small amplitude tremors that lasted from a few minutes to a few days. The predominant frequency of tremors ranged from 1 to 10 Hz, and was lower than that of ordinary earthquakes of similar size.

Obara, Science, 2002 Raw seismograms were transferred to RMS amplitude. Note that the envelop shapes of the tremor were very similar at different stations. In contrast to ‘normal earthquakes’, the tremor envelopes had gradual rise times.

Obara, Science, 2002 The center of the distribution of tremors determined for 1 hour is shown. The tremors were distributed along the strike of the subducting Philippine Sea plate over a length of 600 km. The epicentral distribution of the tremors corresponded to the seismicity with the depth range from km. No tremors have been detected around the Kii channel, nor in the Eastern Shikoku Island.

Obara, Science, 2002 Frequency of tremors in 1hr windows. Usually the activity continued for several days and sometimes for a few weeks. Some tremors seem to be triggered by a nearby earthquake. Activity sometimes ended right after a nearby earthquake.

Obara, Science, 2002 Tremor episodes did not always remain in one region, but migrated.

Rogers and Dragert, Science, 2003

Slip events occur down-dip from the currently locked, seismogenic portion of the subduction zone with a repeat time of mounts. Were not accompanied by earthquakes and were thought to be seismically silent.

Rogers and Dragert, Science, 2003 A tremor on an individual seismograph is unremarkable and does not appear different from transient noise due to wind or cultural sources. It is only when a number of seismograph signals are viewed together that the similarity in the envelope of the seismic signals become apparent.

Rogers and Dragert, Science, 2003 If the one-to-one correlation between transient slip and seismic signatures proves to be robust, then the tremorlike signal can provide a real-time indicator for the occurrence of slip. Because slip events on the deep slab interface increase the stress across the locked plate interface, it is conceivable that a slip event could trigger a large subduction thrust event.

Obara et al., GRL, 2004

Four episodes of step-like tilt can be identified that coincide with the time of tremor activity in that region. A tilt step of up to 0.1 micro radian is too small to be recognized as a meaningful signal by using only tilt data. The consistency between the tilt change and the tremor activity indicates that the two are coupled/related geophysical phenomena.

Obara et al., GRL, 2004 The tilt steps are not instantaneous, but change gradually over an interval of several days. The four episodes can be classified into summer and winter groups. Winter: N-S component started 3 days before the E-W component. Summer: the two components started simultaneously and the N-S component continued 2 days after the E-W component stopped.

Obara et al., GRL, 2004 Tilt vectors for the first and second stages are plotted on the bottom panels. Winter: South down changes to southeast down Summer: Southeast down changes to south down.

Obara et al., GRL, 2004 Additional tilt-meter data are available for the Aug event. The crustal tilt deformations were not detected by the GPS monitoring system.

Obara et al., GRL, 2004 The source area of the slow slip seems to migrate gradually with the propagation of tremors. The observed data can be explained by slow slips with dislocations of 3 and 0.7 cm on two successive reverse faults. The total corresponding moment magnitude is of 6.0 The faults are located just above the dipping seismic zone of the subducting slab. The up-dip end of the slip episode corresponds to the source area of the tremors.

Nadeau and Guilhem, Science, 2009

Thomas et al., Nature, 2009

Non-volcanic tremor represent a different class of earthquakes? Tremor seems to represent shear failure on a critically stressed fault. Thomas et al., Nature, 2009