The October 23, 2001 Calhoun Co., Michigan earthquake
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 earthquake and propagates to BGSO at 140 km and located on the structural arch that separates 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 and LNSM records do not show such a spectacular near-source LLg (i.e., as the LCCO and 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 and stations.
This graph shows amplitudes for P, S, Lg, and LLg waves recorded by OhioSeis and MichSeis stations for the Calhoun County, MI earthquake. Amplitudes are plotted in "digital units". The microseism noise amplitudes for this day are around 40 du, hence most waves are less than the 6-sec microseism amplitude. The SeismoView filter can extract these small amplitude waves since they have larger amplitude at periods less than 6 sec. Notice that LLg amplitude is greater than S amplitude for distances less than 100 km, and for some OhioSeis stations at greater distances. The distinct trend of amplitudes increasing with distance around 200 km is also correlated with station azimuth, thus might be a focal mechanism effect. For reference, a dash-dot line is drawn for log slope -.90 and at a level appropriate for MN=2.5 for the ground velocities listed along right-axis. These ground velocities are converted from du at a period of 1.5 sec, the typical period of the LLg phase. The best-fit line for a power law between distance and LLg amplitude is plotted as the solid line. Notice that its log slope is -1.1, a slightly stronger spatial decay than the log slope of -.90 for the MN formula.
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