Page 48 - Summer 2021

MOBILE EARTHQUAKE RECORDING
Figure 7. Seismograms reported by MERMAIDs in the Pacific. Graphs correspond to two different earthquakes, identified by the titles. Mw is a measure of earthquake magnitude. The time (in seconds), is since the earthquakes’ origin time. The source-receiver, or epicentral, distance, is measured in decimal degrees. Every degree is about 111 km measured along the surface of the Earth. The seismograms are drawn centered at the distance between the earthquakes’ epicenter and the individual MERMAIDs, which are distinguished by arbitrary colors. The acoustic pressure records are scaled to show only relative differences in amplitude. Slanted lines: predictions (p or P) for when the seismoacoustic compressional waves from the earthquakes should arrive, made in simple reference Earth models. Lowercase p waves travel up from the earthquake; uppercase P waves propagate down into the Earth before curving back to the surface. When the seismic waves arrive later than predicted, as they do in this region, the mantle through which they propagate before conversion to acoustic pressure at the ocean bottom is slow and, by inference, hot, which is in line with the idea that deep zones of buoyant material feed the abundant volcanoes in this area of the Pacific.
Figure 8. Distribution of energy per frequency interval, or spectral density, of acoustic noise at a 1,500 m depth in the Pacific as captured by MERMAID’s hydrophone. In the months shown, the instrument was turned on for close to 85% of the time (uptime); 22% and 39%, respectively, (signal) of the record contained earthquakes and other “signals” that were removed so as to leave only the ambient “noise” of the ocean itself. The stored 1-year record was divided in 100-second-long segments whose spectral densities were calculated, and their collective distribution is rendered as a color density map. Red curves, median spectral densities for each month, and their white envelope marks the 5th and 95th percentiles. Vertical lines, frequency intervals of interest for earthquake (green curve) and hydroacoustic (black line at bottom) observations, respectively. Seasonality is most apparent in the frequency range of 0.08-0.8 Hz and can be clearly linked to a mechanism whereby wind-generated surface water waves couple to acoustic waves at double the driving frequency. Newly modified MERMAIDs will specifically target and report these sources of ambient noise for studies of the marine environment.
48 Acoustics Today • Summer 2021