Quote:
Originally Posted by iheartthed
Actually... it wasn't understood that the Cascadia fault produced seismic activity until the 1980s.
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The last big "megaquake" (9.0 or larger) on the Cascadia subduction fault was in the year 1700. They even know the month--January 1700. Plenty of geologic and historic evidence. Google: Cascadia 1700 megaquake. Wikipedia has an article on it. The coast was hit by huge tsunamis from N. California up to BC, and some traveled over to Japan and caused damage. Large parts of the coast subsided and forests were flooded. The quake was probably as large as that which hit Japan in 2011 and Indonesia in 2004.
The last big "megaquake" on the southern San Andreas fault (Salton Sea to Riverside/San Bernardino segment) was also about 300 years ago. So that is probably overdue as well. However, the magnitude would probably be 8 or less. Subduction quakes tend to be very large. The SA segment north of Cajon Pass to Palmdale probably moved in 1812, causing the collapse of the "Great Stone Church" at Mission San Juan Capistrano. The segment from Palmdale north to Parkfield broke in 1857, producing the big Ft. Tejon quake that year. So the the southern segment of the San Andreas is probably most overdue.
Finally, some geologists speculate ("geopoetry") that a fault zone in the Mojave north into Nevada ("The Walker Lane" etc.) may be increasingly taking up the plate movement from the San Andreas, which is impeded by the transverse ranges (San Gabriels etc.). In the far future, the "Walker Lane" may become the new plate boundary, and the Gulf of California may move north into Nevada if the zone starts rifting open (sea-floor spreading). Don't buy beachfront land yet, this will take millions of years if it happens at all. Sizable quakes like Landers and Hector Mine (>7) were on the inland "Walker Lane", as was the more recent Ridgecrest quake. Some of the fairly recent volcanic activity along the Walker Lane (southern Owens Valley, Mammoth area etc.) may be related to this incipient rifting. Plate boundaries are not fixed. They evolve and shift.