abstract

The Maniwaki earthquake of July 12, 1975 occurred in a diffuse, poorly understood zone of significant seismicity in western Québec. A field survey detected 14 aftershocks ranging from ML -2 to ML 1, in a 4-day period following the main shock. Main shock and aftershock activity occurred within an active volume of about 1 km diameter located at 46° 2712′N, 76° 17′W, depth 17 km, with an estimated uncertainty of 2 km on all three hypocentral parameters.

Travel-time curves for Pn, P1, Sn, and Lg phases yielded apparent velocities of 8.26 ± 0.02 km/sec (226 to 1165 km), 6.19 ± 0.09 km/sec (116 to 480 km), 4.72 ± 0.02 km/sec (364 to 1165 km), and 3.61 ± 0.01 km/sec (125 to 1165 km), respectively. P travel times to stations on the Canadian Shield at epicentral distances less than about 2200 km were consistently earlier than predicted by the P curve of Herrin et al. (1968), and suggested a P curve for the Canadian Shield by Hashizume (1974a) might be more appropriate.

Body-wave magnitude mb was calculated to be 4.2 ± 0.2 from stations for which Δ > 2400 km. In good agreement was Nuttli's (1973) mb (Lg) magnitude, calculated to be 4.1 ± 0.2. The coefficient of anelastic attenuation of vertical-component Lg waves of about 1-sec period was calculated to be 0.06 ± 0.02 deg-1 and again demonstrated the applicability of Nuttli's formula for earthquakes in eastern Canada.

The P-nodal solution indicated predominantly thrust motion on a plane striking N64°W and dipping 65°SW or on a poorly defined plane confined between N34°W and N120°W, dipping 25° to 40° to the north. The deviatoric pressure axis was nearly horizontal in a SW to S direction.

Seismic moment, average fault dislocation and stress drop in the main shock were calculated to be 1.7 × 1022 dyne-cm, 6 cm and 50 bars, respectively. The small source dimensions, small average dislocation and mid-crustal focal depth may explain the absence of active surface structures.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.