The diagram for Reason 1 compares seismograms recorded on the surface and at 250 m immediately below the surface site. The surface seismograph’s signals are dominated by wind and cultural noise, so much so that the small M~1 microearthquake is only known from the downhole data. How many such events have been missed by the local seismic network, based on whose data the surrounding area has been reported to be aseismic.
The diagram for Reason 2 shows the dramatic effects of strong near surface attenuation and scattering. The seismograms on the top and bottom of this Figure are for the same 0.5 microearthquake, the differences in waveforms results from loss of high frequency signal by both energy damping and scattering. Ultimately, as illustrated in the next Figure, the loss is such that the signals of many small earthquakes never make it to surface stations, leading to a cut off in event detection and location. In many places this cut off is higher than M~1 to 2 events, suggesting that many 10s of events are going unnoticed.
A microearthquake swarm captured by the LVEW borehole seismograph that went unnoticed by the local surface seismic network of 25 stations. Because the surface net’s detection/location threshold is for events bigger than M~0.7 only the M~1 was included in the catalogue for this period. The borehole sensor is a 3 component 4.5 Hz seismograph located at 2.4 km depth, in 115oC water.
A microearthquake swarm captured by the LVEW borehole seismograph that went unnoticed by the local surface seismic network of 25 stations. Because the surface net’s detection/location threshold is for events bigger than M~0.7 only the M~1 was included in the catalogue for this period. The borehole sensor is a 3 component 4.5 Hz seismograph located at 2.4 km depth, in 115oC water.