This week an entire community in northern Washington was buried under a massive landslide. The human part of this tragedy is still playing out as people search for loved ones, pets, and the remnants of their lives. As in most disasters, people are asking why. The geologist in me knows that this is really a rhetorical or spiritual question, but wants to answer it in tangible terms anyway.
Landslides happen when the downward forces on a slope exceed the strength of the material to hold it in place. Steep slopes, undercutting of the leading edge, and lack of vegetation are all contributing factors, but in this case,as with many others, it was rain that tipped the balance. Like when building a sandcastle, some water helps a slope stabilize up to a point; too much water though increases the weight of a slope and its likelihood to flow. Also, areas that have already failed are more prone to secondary events as the slide itself exposes subsurface materials, removes stabilizing vegetation, and over-steepens the slope. The Washington community buried this week had seen another large slide event in the last ten years.
So there it is. Rain. Steep slopes. Previous slides. Undercutting. Why. And not at all satisfying to know. Look around at the world. Understand how it works. Make good choices. Hope for the best.