“Segment D” ruptures! In Chile.
The September 16 8.3 earthquake in Chile is a good example of what we suspect the southern most Cascadia ruptures have been like in the past, and will be in the future. From the offshore paleoseismic record, these events extend from about Cape Blanco Oregon, to either the Mendocino Fault (southern terminus if Cascadia), or possibly a bit shorter, it’s not presently possible to tell. The Chile version of Segment D generated a ~ 4.7 m tsunami, similar to our models for Cascadia. Previous studies onshore for the most part have little evidence of Segment D ruptures; they appear to be below the threshold required for generation and preservation of a tsunami or land subsidence record in the areas and environments studied so far. Bradley Lake for example contains one of the best Cascadia paleostunami records. It’s a coastal lake with a 5.5 m berm separating it from the sea. At an average tide, a tsunami must overtop the berm with enough vigor to leave a deposit in the lake. The barrier largely prevents the recording tsunami from “smaller” earthquakes such as the Segment D ruptures, which also appear to terminate south of Bradley Lake, further decreasing the chance of recording them in the lake. Of the events in Bradley Lake with potential time correlatives offshore during the past ~ 4500 years (when Bradley was a good recording site), 5 of 7 of them are Segment C, larger events that extend much farther north, and two appear to be segment D events, the rest (~7) are absent. On the other hand, other southern Cascadia lakes further inland do seem to record these smaller events as turbidites from internal lake sidewall failures, something we published in Morey et al. (2013); ongoing work on this is coming soon.
The main value for us is to look at the Chile earthquake as a good example of what is typical of roughly half of the Cascadia earthquakes of the past are like. They are not all “The Big One”. Still these are big, damaging earthquakes locally.