The Jan. 26, 2001 Ashtabula, Ohio Earthquake
This earthquake is the largest in our region since the 1998 Ohio-Pennsylvania earthquake and the establishment of the state-wide OhioSeis network. It was well-recorded by all stations in operation that day and thus provides excellent amplitude data.
Here are a few example seismograms that show the waveform character at distances less than 100 km where a remarkable "Local Lg" wave is the largest arrival from in frequency pass band of the S102/OhioSeis system.
The LLg amplitude is much larger than the S wave for these seismograms. Is that true for the other OhioSeis stations? Below, I plot the amplitudes for the P, S, Lg (where distinct at larger distances), and LLg phases on OhioSeis and MichSeis stations.
This graph shows amplitudes for P, S, Lg, and LLg waves recorded by OhioSeis and MichSeis stations for the Ashtabula earthquake. Amplitudes are plotted in "digital units". The microseism noise amplitudes for this day range from 120 to 230 du, hence comparable to P wave amplitudes. Notice that LLg amplitude is greater than S amplitude for distances less than 100 km, but that S amplitudes are about a factor of 1.5 larger at greater distances. For reference, a dash-dot line is drawn for log slope -.90 and at a level appropriate for MN=4.0 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 -1.57, a much stronger spatial decay than the log slope of -.90 for the MN formula.
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