Research helps paint finer picture of massive 1700 earthquake
Though it was powerful enough to cause a tsunami as far as Japan, a lack of local documentation has made studying this historic event challenging.
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“Geologist [I'm Climate Change]”
Since: Mar 07
#1 May 15, 2013
The variable nature of local subsidence following Rebound/long duration Mercalli X (r/lx) events @ cascadia has been documented in palaeotsunami studies in Cascadia over the last 8 years.
The problem comes in the reportage which was definitely NOT available to the wider geoscience community during the Bush regime years (in large part because publishing authors tended to get as buried as the work they reported on).
This has resulted in few works coming to press outside the USA on easy access. The result of summing up 14C dates for all the basins however did result in a series of several entire Cascadia crossing events (of which the 1700AD event was the last), and with variable runout the, tsunami runup and also the subsidence of the various coastal sectors did not reveal the size of the runout of each crustal relaxation event.
The research also on summing up did however indicate several smaller events nucleated in the S. of cascadia (Nor Cal) and extending partway up the Cascadia zone resulting from spreading into the Basin & range of the Gulf of California rift.
The result has been a variable interval time between quakes with the shortest interval time in the S. and longest in the N. with a typical runout vector being either NW. or WNW. and the fastest arc convergence also in the S. part of Cascadia nearest to the gorda arc (Mendocino region) junction box near Mt. Lassen. The 1700AD r/lx may actually have had a W. vector, but crustal relaxation results post 1700 indicate WNW directed backarc crustal relaxation through the colorado hotspot & the New madrid region & eventually the Charleston rift (these latter 2 events historically documented).
The longest interval time appears to be in the far NW at the junction box with the garibaldi arc, with thus far the forearc near Silverthrone volcano (BC Canada) being r/lx free throughout the bulk of the late Holocene (about 5000Bp to present with the sea level @ or near the current interglacial highstand), and the forearc block [the area of W. Vancouver island] being swept by teletsunami from Cascadia and Alaska rather than being hit by a local tsunami from a home grown Garibaldi arc r/lx. Cascadia r/lx covering the whole subduction zone has occasionally hit the E. part of vancouver island forearc block as in the 1700AD event.
The interesting bit to come appears to be the squeeze out of magma from the Chilcotin hotspot (Wells Grey-Clearwater volcanic field),as a result of recompression of the Chilcotin backarc basin to the magma filled fractures near the backfault controlling hotspot, indicating a possible near future (a few centuries), r/lx beginning @ the garibaldi arc near silverthrone volcano & extending all the way to California via Cascadia with a SW. vector relaxing the Chilcotin backarc basin & the bulk of the Cascadia frontal part of the basin & range causing a SW. lurch of the N. American plate.
That one would be bigger than average and likely followed in relatively short term by an r/lx nucleated in N.California and crossing a small sector of the S. part Cascadia likely terminating in Oregon.
Being a junction box of hotspot controlled backarc basins spreading @ variable rates the date of the next r/lx & its vector will be as difficult to pin down as the size of the preceeding events. Interval time in reportage so far has been anywhere from 280 years to 1200 years in the listed holocene record in the available reportage. The variation in vectors is the root cause of the heterogeneous results of tsunami size and associated coastal forearc subsidendce.
Have a nice day: Ag
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