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GEOPHYSICS DIVISION GEOLOGICAL SURVEY OF CANADA, OTTAWA, Canada
Abstract
The magnitude 6.0 Saguenay earthquake of 25 November 1988, in Quebec, Canada, was one of the largest recorded in eastern North America (ENA) during the past 50 years, and it has provided important high-quality data for estimation of earthquake hazard in ENA. Ground motions for the event recorded by both Eastern Canadian Telemetered, and Strong Motion, Networks were anomalously large and unexpectedly rich in high-frequency energy, as compared with most other events having the same mb or moment magnitudes. Displacement waveforms derived from the eastern Canadian strong-motion acceleration data have been modeled in detail in an attempt to explain these anomalies. The modeling has provided strong evidence that the surface on which the faulting occurred is relatively long and narrow, with its long axis nearly horizontal and situated immediately above an internal crustal discontinuity that lies at a depth of about 30 km; the faulting process was essentially antiplane and unilateral, with the crack starting at the northwest end of the long axis and propagating in the negative strike direction towards the southeast; and slip durations at individual points on the fault were short (of the order of 0.2 sec) compared with the time taken for the crack to propagate from its starting point to the far end of the fault (about 2.5 sec in the model). The inferred short slip durations are consistent with the inferred fault geometry, and both are consistent with "normal" static and dynamic stress drops (i.e., within a factor of 2 of 100 bars). The narrow elongate fault geometry and associated short slip durations are responsible for the enhanced high-frequency radiation (as compared with "normal" events having the same seismic moment release). The associated spectra differ greatly from that of the Brune spectral model, with the former having at least two corner frequencies, whereas the latter has only one.
The unilateral fault propagation resulted in a strong azimuthal variation of waveform characteristics, with the largest amplitudes and shortest durations concentrated towards the south and southeast and the smallest amplitudes and longest durations in the opposite directions. As the majority of strong-motion instruments and closer ECTN stations were located within the former azimuthal range, the anomalous high-frequency content and large observed amplitudes are accentuated by the station distribution with respect to the directivity characteristics of the source. Another significant factor likely to have contributed to the observed peak amplitudes, at least in some instances, is an indicated superposition of direct waves from the source with reflections from the inferred mid-crustal discontinuity. Elongate source geometry and associated effects such as those described above may not be uncommon for large thrust intraplate earthquakes and may have significant design implications for critical structures such as nuclear power plants.
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