T165 Techniques and Results of Paleoseismological and Seismic Hazard Studies of the Stable Craton of Eastern North America

Wednesday, 28 October 2020
5:30 PM - 8:00 PM
Meeting Rooms

REGIONAL MAPPING OF THE FUNDAMENTAL SITE FREQUENCY IN THE CHARLEVOIX REGION, QUEBEC, CANADA: EARLY RESULTS

Paper number: T165-5
Program: Technical Sessions
Time: 6:30 PM – 6:45 PM

Authors

Abstract

One of the most significant factors affecting the level of earthquake ground shaking is the underlying geology or site conditions. The fundamental site frequency (f0) is an important site-specific parameter to establish the primary resonance mode of the soil column. This earthquake site effect is particularly important in Eastern Canada where a strong impedance contrast exists between the soft Quaternary deposits and underlying hard metamorphic and igneous rocks. Our objective of is to determine the spatial distribution of f0 inside the meteoritic impact crater of Charlevoix, Quebec, Canada, the most seismically active region in eastern North America. Here, most of the population lives within two buried valleys filled by up to 250 meters of glacial, glaciomarine and fluvial deposits with significant earthquake site effect potential. We use an innovative approach considering valley geometry and internal stratigraphy to model f0 from the thicknesses and representative shear wave velocity (vs) values determined for geological layers.

We collected microtremor-horizontal-to-vertical-spectral-ratio (MHVSR) and vs profiles, mostly in the two buried valleys. The MHVSR’s are used to obtain a preliminary spatial distribution of f0 in the main buried valleys. The f0 values vary between 0.5 to 20 Hz with significant differences between the valleys. The first valley is marked by low f0 between 0.5 to 1 Hz, with values increasing progressively from the valley center to its edges and decreasing towards its southern end. The second valley is marked by higher f0 between 1.4 to 2.5 Hz, with several areas of high f0 within the valley. The MHVSR f0 results suggest the first valley is deep with a simple valley profile, deepening towards the south. In contrast, the second valley is much shallower and has a more complex valley profile. Vs profiles were interpreted from dispersion curve estimates collected from multichannel analysis of surface wave (MASW) and ambient-vibration-array recordings. These vs profiles supplement available geologic and geophysical data to assign representative vs values for the glacial and glaciomarine sediments. Our 3D geophysical models of Charlevoix, innovatively developed using both in situ seismic measurements and geologic reconstruction, will impact future prediction of earthquake site response.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-354740

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