TRIGGERING OF GRINDING TREMOR BY THE STRONG SHAKING OF THE 2002 DENALI EARTHQUAKE

During the strongest shaking in the Mw7.8 2002 Denali, Alaska earthquake we identify bursts of high-frequency energy being radiated from part of the Cascadia subduction zone off the coast of Northern Vancouver Island. These bursts of energy are similar to nonvolcanic tremor in that they have no clear onset, an extended duration, and to the resolution of our location techniques they appear to emanate from the same place. Unlike nonvolcanic tremor they have high-frequency spectra similar to earthquakes. Both the Love waves and the Rayleigh waves from the Denali earthquake triggered these bursts. These high-frequency bursts of energy are triggered when the Love wave motion is to the southwest (the direction of slip of the overriding plate on the megathrust) and shut off when the motion switches to a northeast polarity, suggesting that these bursts are related to slip on the subduction interface. This observation of shear inducing tremor-like grinding demonstrates, in contrast with previous observations, that tremor can be induced by fluctuations of both shear and normal stress. The on/off modulation of the grinding tremor by the surface waves of the Denali earthquake suggests that there is plastic deformation on local faults. In all likelihood, this is even more prevalent nearer the mainshock. This observation provides one of the first in situ measures of their plastic rheology.