No. 111: May-Jun 1997
Not all terrestrial tremors emanate from buckling crust and slipping tectonic plates. The crashing of large waves on a seashore sends "microseisms" to sensitive instruments hundreds of kilometers away. The bubblings of Old Faithful geyser give rise to an enduring and engaging "harmonic tremor."
As for the more "exotic" sources of seismic signals, they had an entire session devoted to them at the Fall 1996 meeting of the American Geophysical Union. Here are two of the unusual seismic events presented there.
July 10, 1996. Yosemite National Park.
"At 6:52 pm PDT Wednesday, July 10, 1996, a large block of granite, with an estimated mass of 80,000 to 184,000 tons, detached from the cliff between Washburn Point and Glacier Point, in Yosemite Valley. The rock mass subsequently launched from the cliff and free-fell ballistically an estimated 550 m before impacting approximately 30 meters from the base of the cliff in the Happy Isles area of the valley floor in Yosemite National Park...This rock fall was well recorded by 3 UC Berkeley (BDSN) and Caltech (TERRAscope) broadband seismographic stations and 15 shortperiod seismographic stations (operated by the USGS in Menlo Park and the University of Nevada, Reno). In fact, it is the largest vertical rock free-fall ever recorded seismically and it registered on seismographs up to 200 km distant."
(Uhrhammer, R.A.; "Seismic Analysis of the Yosemite Rock Fall of July 10, 1996," Eos, 77:508, 1996.)
December 9, 1995. Southern Ecuador.
"On the afternoon of December 9, 1995, a bolide exploded in the atmosphere over the Andes in southern Ecuador. Many people in nearby towns witnessed the event. They reported seeing a streaking meteor, which terminated in a loud and brilliant explosion. In some locales, the flash was noticeable even through cloud cover. The burst of light was observed by satellite optical sensors used to detect atmospheric nuclear tests. Three local seismic stations also recorded signals from the explosion...This bolide appears to be unique in that it was observed by eyewitnesses and located by both satellite and ground-based sensors."
The sound of the meteor's explosion was heard at least 56 kilometers away.
(Chael, Eric P.; "Seismic Signals from a Bolide in Ecuador," Eos, 77:508, 1996.)