 The region near the triple junction experiences high seismicity , with more than 60 earthquakes of intensity VI ( Strong ) or greater or magnitudes ≥ 5 @.@ 5 since 1853 . The mainshock in the sequence ( 11 : 06 a.m. Pacific Daylight Time ) occurred onshore , 4 kilometers ( 2 @.@ 5 mi ) west of Petrolia at a depth of 10 @.@ 5 kilometers , and was among an infrequent number of earthquakes with fault @-@ plane solutions that conveyed evidence of slip at the Cascadia subduction zone . While the focal mechanism indicated slip on a thrust fault striking <unk> ° W with a shallow dip of 13 ° to the east @-@ northeast , the rupture most likely propagated to the west , based on the mainshock location at the southeastern boundary of the aftershock zone . Investigation of more than 1 @,@ 200 surveys from the North Coast area led to the assignment of an intensity rating of IX ( Violent ) on the Modified Mercalli scale for the region near Petrolia . 
