ESTIMATION OF LARGE PREHISTORIC EARTHQUAKE INTENSITIES AND MAGNITUDES FROM LARGE SUBMARINE LANDSLIDES IN PUGET SOUND

Submarine landslides, with volumes ranging from 10⁵ to 10⁹ m³, are used to determine prehistoric earthquake magnitudes and shaking intensities on the three main surface fault zones in Puget Sound, WA, USA. Highest concentrations of submarine landslides occur along the fault zones that cut across the Sound; the Seattle fault zone (Alki Point), the Tacoma fault zone (Maury Island) , and the South Whidbey Island fault zone (Possession Point, Mukilteo, Edgewater). The areal distribution of landslides around these fault zones allows for the determination of prehistoric earthquake magnitudes of 7-7.5 on all three faults. Using magnitude-intensity relationships developed by Bakun et al. (2002), the minimum Modified Mercalli Intensity, for an earthquake of magnitude 7 on all three fault zones is approximately intensity VIII with intensity IX also possible, depending on the location of the hypocenter. Independent of magnitude-intensity relationships, the INQUA Earthquake Environmental Effects intensity scale is also used to determine shaking intensity. In the INQUA EEE scale, frequent, large (>10⁵-10⁶ m³) submarine landslides occur at intensity IX, while frequent very large (>10⁶ m³) submarine landslides occur at intensity XII. Submarine landslides are initially present at intensity V in the scale; however, these consist of smaller landslides, and are not representative of the landslides identified in this study. The landslides for which volumes are calculated are all >10⁶ m³ while many of the other slides identified are likely >10⁵ m³. The presence of large and very large landslides in Puget Sound suggests very strong ground shaking throughout the region with INQUA EEE intensities of at least IX and potentially as high as XII with intensity IX to X favored. Submarine landslides representative of intensities greater than X in Puget Sound are likely the result of individual site effects. Based on the landslide distribution patterns and volumes, major earthquakes of Ms 7.0 to 7.5 have occurred on the three surface fault zones and, along with great earthquakes on the Cascadia Subduction zone, have led to very strong shaking with INQUA EEE intensities of at least IX and potentially as high as XII experienced in the region.