Possible relationship between nonvolcanic tremor and the 1998-2001 slow slip event, south central Alaska

We document the occurrence of nonvolcanic tremor (NVT) in south central Alaska for 3 months of each year between 1999 and 2001 to provide a first-order attempt to understand the relationship between NVT and a large slow slip event that occurred over a 2 to 3 year period between 1998 and 2001. The majority of the NVT signals observed after the onset of the slow slip event are bursts lasting between 10 and 15 min with frequencies ranging from 1 to 6 Hz. By the summer of 2001 the quantity of NVT has decreased dramatically from the previous summers, and episodes rarely last up to 10 min. The decrease in NVT activity in the summer of 2001 is coincident with the end of the slow slip event. There is a clear spatial correlation between the observed NVT and the slow slip event, with most of the NVT events located on the downdip edge of the region of highest slip. Depth estimates range widely over the region, with approximately half concentrated near the interpreted North America-Yakutat/Pacific plate interface and the remainder located above the plate interface. However, owing to inadequate station coverage and poor knowledge of the local structure, determining accurate depths is difficult and thus the relationship between the NVT and the plate interface is problematic. This is the first study giving evidence that the relationship between slow slip events and NVT seen in Cascadia and southwest Japan may also exist in Alaska.