The July 14 earthquake and the July 26 earthquake was caused by the subduction of the Solomon Sea Plate beneath the Pacific Plate and the Bismark Sea Plate, respectively.
The Cascadia subduction zone (also referred to as the Cascadia fault) is a subduction zone, a type of convergent plate boundary that stretches from northern Vancouver Island to northern California.
Mélange, a geological breccia above a subduction zone environment
These volcanoes are commonly referred to as Cascade volcanoes because they were formed by Cascadia subduction.
In Cretaceous time, masses of molten rock that originated from subduction rose underneath what is now Whitney and solidified underground to form large expanses of granite.
Also this subduction zone has created arguably the world's largest active volcano Lake Toba which produced the largest volcanic eruption in human history erupting just over 2,800 km³ of magma, a VEI of 8.