How small are the smallest earthquakes in this sequence that we can see in the record? For most of the events, standard magnitudes cannot be determined: the events are too small to be recorded at other stations and, strictly speaking, the local magnitude scale is not defined for events so close to the recording station. To determine the magnitude threshold for the events, I calibrated a manual magnitude scale using events from the catalog. Following the definition of local magnitude (Richter, 1935), I measured the peak-to-peak amplitudes in the instrument-corrected velocity seismograms of the two horizontal components of the borehole velocity sensor and multiplied by the period and by the magnification of a Wood-Anderson instrument at that period. As the distances of these events from the station BRIB are all nearly the same, no distance correction is necessary. Figure 13.14 shows a comparison between the catalog and manual magnitudes (gray dots). The line shows the least-squares regression between the catalog and manual magnitudes and allows the manual magnitudes to be projected to corresponding catalog magnitudes (black dots). The smallest events analysed correspond to -1.5; however, there are still smaller events in the seismograms from the borehole geophone. No manual magnitudes have been determined for these tiny earthquakes yet, but the smallest events recorded at station BRIB are more than five units of magnitude smaller than the mainshock.
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