The October 23, 2001 Calhoun Co., Michigan EQ
This event occurred in the Michigan Basin, to the west of the Grenville Front. Thus the details of the geologic setting differ from the eastern Ohio location. On the other hand, the geology is still characterized by a Paleozoic basin on pre-Cambrian basement -- is "LLg" still generated and propagated?
The BGSO seismogram is quite interesting. First of all -- YES -- a large LLg phase is generated by the Michigan EQ and propagates to BGSO at 140 km and located on the structural arch that seperates the Michigan and Appalachian basins. Why is LLg so large?
A very nice seismogram at BHSO in western Ohio. The distance is now greater than 200 km, and similar to the behavior observed for the Ashtabula and Alliance events, the LLg phase becomes a well-dispersed arrival with amplitude comparable to the S wave for distances beyond 200 km.
Thus, we see that the "LLg" phase is also generated by Michigan earthquakes. One difference is that the AAMC & LNSM records do not show such a spectacular near-source LLg (i.e., as the LCCO & CLEO records for Ashtabula). Nonetheless, the similar character for distances greater than 200 km implies that a regional coherent propagation character does exist.
Lets plot the amplitudes for all phases & stations.