ESTIMATED EFFECTS OF EPISODIC SLOW SLIP ON THE OCCURRENCE AND PROBABILITY OF GREAT EARTHQUAKES IN CASCADIA

Previous estimates of the effect of episodic slow slip on the probability of great earthquakes in Cascadia [Mazzotti and Adams, 2004] find enhanced probability of 30 to 100 times during the approximately two week duration of slow slip events beneath northern Washington and southern British Columbia. Despite the enhancement, the absolute probability of a great Cascadia earthquake during a slow slip event remains very low at ~0.03% per week. In short, while it’s much more likely that a great earthquake will occur during a slow slip event, a great earthquake is unlikely to occur during any particular slow event.

This previous probability estimate implicitly used a failure model in which great earthquakes initiate instantaneously at a stress threshold. In the present study, I refine the estimated probability change assuming laboratory-observed slip initiation applies to natural events. Lab tests show that failure of intact rock in shear and the onset of rapid slip on preexisting faults do not occur at a well-defined threshold stress, are severely damped to stress change and have a characteristic delay time that depends on the loading rate and on the effective normal stress. These related effects can be accounted for using existing lab-based constitutive relations in dimensioned fault simulations or analogue models, or using seismicity rate equations derived from these equations [e.g. Dieterich, 1994]. I apply these to estimate the probability of a great earthquake in Cascadia during slow slip events of two weeks duration. The probability enhancement during the slow slip is negligible for effective normal stress of 10 MPa or more and only doubles for an effective normal stress as low as 1 MPa.