The primary constraint on the recurrence of Cascadia
earthquakes is that great earthquakes occur on average every
500 years beneath all sites on the coast of the Cascadia subduction
zone, based on paleoseismic studies of coastal subsidence
and tsunami deposits (for example, Atwater and Hemphill-
Haley, 1997). We considered two sets of rupture scenarios
for these events: (1) M9.0±0.2 events that rupture the entire
Cascadia subduction zone every 500 years on average and
(2) M8.0–8.7 events with rupture zones that fill the entire zone
over a period of about 500 years. Each of these large to great
earthquakes is expected to rupture the entire seismogenic area.
Ruptures in the latter set of scenarios float along the Cascadia
subduction zone.
For these maps, we assign a probability of 0.67 to the
M8.8–9.2 scenario and a probability of 0.33 for the set of
M8.0–8.7 scenarios with floating ruptures (fig. 20). In the
2002 maps, M9.0 and M8.3 scenarios were given equal
probabilities. The higher probability of the M8.8–9.2 rupture
scenario in this update reflects consensus of scientists and
others at the March 28–29, 2006, Pacific Northwest NSHMP
workshop. In that meeting it was concluded that the M9.0± 0.2
scenario was more likely than more frequent, floating smaller
earthquakes.
Figure 17. Logic tree for fault sources in the Intermountain West (IMW). Parameters in this table include some aleatory
variability as well as depicted epistemic uncertainty. Additional aleatory variability may be associated with all models
depicted. We treat aleatory variability in ground motion in the hazard code. Most faults in the Intermountain West have
assigned characteristic magnitudes (see first panel) based on surface-rupture length except where historical earthquakes
serve as analogs and where the characteristic magnitude is constrained to M7.5 (shown in table G-3 in Appendix G). Short
faults (less than 17 kilometers) in the region with characteristic magnitude less than 6.5 are treated like the upper branch
but with full weight. NGA, Next Generation Attenuation; gnd is the logarithm of median spectral acceleration or peak ground
acceleration; dgnd is uncertainty in median spectral acceleration or peak ground acceleration at a given distance (R) and
magnitude (M). See table 6.
