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.

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