3000-YEAR PALEOSEISMOLOGICAL RECORD FROM THE REGION OF THE 1988 SAGUENAY, QUEBEC, EARTHQUAKE

The 1988, magnitude M-b 5.9 (m(bLg) 6.5) Saguenay, Quebec, earthquake occurred in a region considered to be aseismic, but the epicenter was less than 100 km northwest of the seismically active region of Charlev oix. Lake sediment cores representing some 3000 yr of sediment accumul ation contain abnormal silt layers attributed to seismic shaking event s prior to the 1988 earthquake. The layers are formed by the relativel y rapid settling of the silt portion of the seismically resuspended or ganic-rich sediment. Cores were obtained over a distance of 120 km, pe rpendicular to the Saguenay graben structure. Several silt layers are much thicker and more widely distributed than the effect of the 1988 e arthquake and are comparable to those observed at Charlevoix that were produced by magnitude 6 to 7 events. The silting events do not correl ate between the sampled lakes nor does the pattern match that at Charl evoix, so that the shaking events are interpreted to be of local origi n. The recurrence interval for magnitude greater than or equal to 6 ea rthquakes ranges from 350 to 1000 yr, in contrast to the roughly 75-yr historical recurrence of earthquakes of magnitude greater than 6 at C harlevoix.