I'm not quite sure where you got your info - Here's Mt St Helen's eruptive history - nothing in 1857: http://vulcan.wr.usgs.gov/Volcanoes/MSH/EruptiveHistory/summary_msh_volcanic_history.html
No eruption showing in 1857.

The Fort Tejon quake was Jan 9th - the parkfield & fort tejon were all part of the same sequence of events... or parkfield was foreshocks - better explained here:
Accounts of the 1857 earthquake indicate that several shocks indicate that several small to moderate size California shocks preceded it by 1 to 9 hours. In particular, two foreshocks were widely felt. A study of felt areas and intensities of these two foreshocks by Professor Kerry Sieh of the California Institute of Technology indicates they were similar to the Parkfield main shocks of 1901, 1922, 1934, and 1966. Sieh concluded that the 1857 foreshocks were magnitude 5 to 6 earthquakes located within an area that includes the Parkfield section. Because foreshocks generally occur near the epicenter of the ensuing larger main shock. Sieh believed the slip in the great Fort Tejon earthquake actually began near Parkfield at the northwest end of the 1857 rupture zone and extended along the fault to the southeast through the Carrizo Plain to the vicinity of San Bernardino, east of Los Angeles. - taken from: http://quake.wr.usgs.gov/research/parkfield/history/1857.html

So, I guess, in answer to your question, I don't think history will repeat itself..

Canie