The Submarine Paleoseismic Recording Threshold of Shallow Cascadia Earthquakes Under a Wide Variety of Site Conditions

Abstract:

The triggering, recording, and preservation of seismoturbidites depends on thresholds for each step that involve slope stability, shear strength, sediment supply, bioturbation and other factors. An important factor is the triggering distances for earthquakes of varying magnitudes from a given site. Data from paleoseismic studies globally (Japan, Taiwan, Sumatra, Black Sea, Sea of Marmara, Portugal, Haiti, California Borderlands, Monterey Bay, alpine lakes in Chile, Lake Washington, and southern Cascadia lakes) were compiled as a basis for comparison between sites. A limited assessment of site conditions such as across strike vs. along strike position of core sites, fault type, directivity, and Q value was included. Global averages suggest an MMI of 5.5 and distance of ~ 50-100 km is most common. Maximum triggering distances were estimated as 60-120 km for the CSZ, and 40-75 km for the NSAF based on existing data and comparisons to global values. The region between Eel Canyon and Mendocino Channel (CSZ) and Noyo Canyon (NSAF) has a higher likelihood of recording at least a portion of past seismicity from both the NSAF and CSZ, though Noyo Canyon is likely at the limit of possible influence from the CSZ. The high seismoturbidite frequency (240 yrs. vs. 500 yrs. in Washington) of southern Cascadia (excluding the Eel system) shows clear continuity along strike nearly bed for bed over 170 km (cores, CHIRP profiles), crossing four canyon systems. The record shows a weak relationship to sediment supply from the canyons, and no relationship to slope angle, recently suggested as an explanation. The average axial slope is 3-6 ^o for these systems, with a few short pitches of 12-20 ^o. By comparison, the lower frequency record at Hydrate Ridge (~ 350 yrs.) has a steeper average source slope of 10^o, steep 20-30 ^o pitches and no modern sediment supply. The turbidite frequency at the base of the upper reach of Quinault Canyon is lower still (~ 500 years), but the source slope angles average ~ 7 ^o, with the upper pitches ~ 13-16 ^o and abundant modern recharge. The record in the relict JDF system (avg. 0.48 ^o profile), with no modern sediment supply, is identical. These observations demonstrate that the variable turbidite frequency in the CSZ is not an artifact of either sediment supply or slope angle, and is most likely the result of segmented ruptures.