Conclusions and Recommendations
The main conclusions are:
- The "LLg" phase gives the maximum wave amplitude in OhioSeis seismograms at distances less than a hundred kilometers.
- The "LLg" phase appears to be a surface wave trapped in the Paleozoic sediments above the pre-Cambrian basement. It is easily identified by its very slow group velocity of about 2 km/sec.
- Out to distances of several hundred kilometers, LLg has an amplitude comparable to Sg or Lg.
- LLg is robust in that it is generated and propagated by earthquakes at various locations in eastern Ohio, and even by the earthquake in Michigan.
- While our use of LLg and the Hermann & Kijko mbLg formula has produced reasonable magnitude estimates, the observed spatial amplitude decay slope appears to be steeper than the slopes assumed in the mbLg or mN formulas.
The main recommendations are:
- We cannot ignore LLg, we need it to detect, locate, and assign magnitudes to small Ohio events.
- 2004 Update:
Additional EQs (i.e. 2003.06.30 & 2004.06.30) in NE Ohio futher corroborate all the above statments. In particular, the observed steeper spatial amplitude decay slope for LLg at distances less than 200 km is confirmed. We should begin to develop and experiment with a modified magntiude formula.