From The Landslide blog:

In reviewing seismic data with two distinct events, it’s not clear if a top portion slipped first, causing the bottom to give, or vice versa:

Our author:

‘To my untrained eye the initial seismic event shows a slow increase in energy release rather than an abrupt peak.  Note also that the second event appears to be much smaller than the first.  Thus, in this interpretation, the initial failure was, I think, a small, slow failure in the lower slope that destabilised the upper block of the Oso landslide, which slid onto the mass below.’

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