I find this: Since we’re all learning to read/interpret tremor graphs, here is an informative note from the PNSN (Pacific Northwest Seismic Network) regarding far-off explosions and how they were apparently detected by tremor graphs all through the Cascadia volcanic zone, which in turn caused some concern amongst seismologists/volcanologists as to what could have caused such a widely detected signal. The author, Ms. Holtvac, acknowledges that her interpretation of the data is still somewhat unconfirmed, as there is still some underlying question that some type of real tremor may have actually occurred (but unlikely). Also, as I look at the data, I see artifacts that resemble “drumbeats” in addition to the other prominent surges in “tremor”. I wonder if these “drumbeats” could be “echoes” of the priimary explosions?
[link to www.pnsn.org]