Ring-Shaped Seismicity Structures in the Cascadia Subduction Zone: Possible Upcoming Large Earthquakes

Abstract

Seismicity characteristics in the Cascadia subduction zone located west of North America are considered. It has been established that, since 1964, shallow (h = 0–33 km) ring-shaped seismicity structures with threshold magnitudes M_t of 4.4 and 4.6, respectively, have been formed prior to two large earthquakes (on August 17, 1991, M_w = 7.1 and April 25, 1992, M_w = 7.2). Note that, unlike many other subduction zones, deep ring-shaped structures (h = 34–70 km) have not been observed here. At the same time, deep seismicity strips are sometimes formed in this zone. Two new ring-shaped structures with M_t equal to 5.0 and 5.2, which began to form here since 1973, are distinguished. There are grounds to suppose that large earthquakes can be expected in the regions of these structures. We estimate the magnitudes of possible large earthquakes: M_w = 7.8 and 7.9 on the basis of the correlation dependences of the seismicity ring sizes and M_t values on the magnitudes of major events obtained earlier for the East Pacific. These data allowed us to conclude that processes of preparation for a great earthquake with M_w ~ 9.0, similar to the 1700 event, did not begin in the subduction zone. At the same time, a large earthquake (M_w = 7.9 ± 0.1) is possible in a few years in the region of Vancouver Island. We suppose that the formation of shallow rings and deep strips of seismicity is related to deep fluid migration, as occurs in other regions.