Monday, July 11, 2011

Nisqually Rock Avalanches - Frequency and Size

For those of you still watching these large rockslides from the Nisqually Cleaver, here's a little more information for you. I called Kate Allstadt of the Earth and Space Sciences Department at the University of Washington and asked her for some seismic data that may show a better picture of the frequency of rock avalanche events.

She was able to graph some data that shows the distribution of the major rock avalanche events over the period from June 24th to July 7th. You can see in the graph that there is a trend decreasing in frequency and size. So it appears that the danger could be abating - but only gradually.

I must say that this correlates with direct observation, as there have not been any major rock flows down the mountain in the last week or more. The lowest extent of the largest debris path is to an elevation of about 8200 feet. Here is a graph Kate produced that shows the data from the seismic sensors installed on Mt. Rainier. The horizontal axis represents the date. The vertical axis represents the number of events per hour. The top row identifies single and large events.