Twelve years of geophysical research in the Charlevoix seismic zone

Abstract

Various and extensive geophysical monitoring began in 1974 in the Charlevoix Seismic Zone (CSZ). The historical record and a major earthquake in 1925 characterize this zone as the likely site of a future damaging earthquake (i.e. M = 5.5 or larger) and the aim of the monitoring was to develop the capability of predicting such earthquakes. The parameters studied included: microseismicity, seismic travel times, electrical impedance, vertical movement, horizontal movement, tilt, gravity change, and strain through water well level changes. Although no clear precursor was detected in the months before the largest seismic event that occurred during the observation period ( M = 5.0), the multiparameter experiment provided new insights into the structure and the mechanics of this active region. Observations of several hundred well-located microearthquakes have permitted the delineation of ancient rift faults that are now being activated by high horizontal compression. The geographical extent of the zone is sharply defined by the microseismicity which occurs in an area of 35 km × 80 km straddling the St. Lawrence River. Observation of P-waves from accurately-timed explosions and S-waves from earthquakes support the hypothesis of Extensive Dilatancy Anisotropy (EDA) involving saturated parallel cracks in the crust. The data have permitted an estimate of both crack density and the direction for the principal compressive stress in the region (N 30° E). Changes in electrical parameters with periods of months to years were detected at five of the six magnetotelluric stations installed in the region. Changes in the polarization angle of the earth's electrical (telluric) field correlated well (for seven years) with calculated groundwater accumulation levels at one site. Electric impedance changes showed no clear correlation from station to station, but changes at one station appear to correlate with a seismic P-wave travel time change anomaly and with the disappearance of a negative gravity anomaly at the time of an unusual number of earthquakes in the range M = 2 to 3 from mid 1976 to mid 1977. It is suggested that these correlated changes could be related to aseismic stress redistribution and fluid flow along one of the NE striking rift faults about two years before the M = 5.0 earthquake in 1979. Continuous monitoring of tilt (surface and borehole) and well-water levels was carried out at a single site (Charlevoix Geophysical Observatory). Although there were unexplained near-surface tilts of the order of the earth tide amplitude within two or three days before some earthquakes, they were either not spatially coherent or were related to small unexplained temperature changes in the tiltmeter vault. No intermediate-term (one week to one year) precursors were seen against a noise level of 1–3 μrad (RMS) for near-surface tilt observations and a noise level of 0.2–0.3 μrad (RMS) for borehole tilt measurements at a depth of 50–100 m. The threshold of detectable tidal tilt admittance changes for the borehole tiltmeters was lowered to 2% but no significant changes were seen. Four coseismic changes in the levels of water in boreholes were observed (all were drops in water level) which were much larger than expected on the basis of elastic dislocation theory. It is suggested that these could be explained either by the collapse of a dilatant zone around the respective hypocentres or by triggered aseisnuc movement on a nearby active fault. Borehole water level tides observed from 1980 to 1986 exhibit significant variations in amplitude and phase which are thought to be the result of time-varying elastic properties in a nearby structure. Some constraints can now be placed on the long-term deformation of the CSZ as a whole from tiltmeter, levelling and horizontal surveys. Data from a levelling array observed from 1976 to 1986 puts a limit of 0.54 μ rad/yr on the secular tilt rate at the observatory: a further limit of 0.1 μ rad/yr is provided by borehole tilt measurements over the period 1983–1986. A limit of 2 cm is placed on vertical movements northwest of the St. Lawrence River by levelling surveys in 1965, 1978, 1980 and 1982. No significant (90% confidence) deviatoric strain has been detected across the CSZ from horizontal control surveys carried out in 1912–1919, 1965, 1978 and 1983: the maximum allowable long-term strain rate across the CSZ from 1919 to 1983 is 0.16 μ strain/yr.