It will be tragic, and sometimes people should take notice of their surroundings.

A quick look at the geologic map you linked to scared the hell out of me. The first thing I see is the Qls, as you noted. This unit is all over the valley. Examining the outcrop pattern compared to the topographic contours, I can see how the landslides fill the valley, flank the lower slopes of the hills, and run up the upper slopes to their source.

Near the slide area, the map has provided cross-sections that give us even more information. The trace of the cross-sections are shown as a line on the map with labels like “A” and “A’”. Look at the bottom of the map and you’ll see the cross section “B-B’”. The elevation scale of the vertically exaggerated section indicates that the Qls is more than fifty feet thick! Yikes.

Now look at the bedrock units. No wonder the valley is full of landslide debris. the hills are composed of unconsolidated silt, sand and gravel, all outwash deposits of the last glaciation. And nearly at the angle of repose.

Did the small temblor trigger the slide? Possibly. But it wasn’t needed. Did the heavy rain cause the slide? Probably. Increasing the water within the spaces between the various cobbles and sand grains (termed “clasts”) reduces the stress needed to allow this material to fail under its own weight. But even without the rain, this slope was doomed to slide, given time. Gravity never stops.

As a final note, damn that’s a beautiful geologic map! The geologic map is the perfect example of an information dense infographic and this one is wonderful. Detailed without clutter, with an encyclopedic explanation of symbols and rock unit descriptions, and cross-sections with seismic data. Damn. Haven’t ever seen that on a map like this before. That’s the art of science right there in one pdf.

(And it was free, thanks to the Washington State Department of Natural Resources.)

Yes, Qal is the symbol for Quaternary landslide deposits. So it has happened before.