What exactly causes the seismic signals? In other words, what is a glacial earthquake?
We had suspected that the seismic signal came from the main chunk of the glacier lurching forward very abruptly. It does do that. But now what we’re seeing is that no, the seismic signal comes from the collapse of this large mass of ice off of the end of the glacier. It’s only in the last year or 18 months that it has become clear that for these big outlet glaciers, the biggest control on changes in their speed comes from what’s happening at the calving front. You lose this ice off the front of the glacier and it looks like that reduces resistance to the glacier flow, so the whole glacier speeds up. I’ve seen glaciers calving into the sea, but you’re clearly talking about something far larger. Have you witnessed a glacial quake up close? We saw a couple last summer from our helicopter, near the calving front. We were at the outlet to the Helheim glacier, in a system of fjords with sheer rock walls that are 500 meters [more than 1600 ft] tall. Typically, you start to see a rift open up
