2004 Wave Amplitudes & Magnitudes

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This earthquake is the largest in our region since the 1998 Ohio-Pennsylvania EQ 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 & MichSeis stations.

This graph shows amplitudes for P, S, Lg, and LLg waves recorded by OhioSeis & MichSeis stations for the Ashtabula EQ. 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 -.9 for the MN formula.

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