Seismic monitoring in the oceans by autonomous floats

Abstract

Our understanding of the internal dynamics of the Earth is largely based on images of seismic velocity variations in the mantle obtained with global tomography. However, our ability to image the mantle is severely hampered by a lack of seismic data collected in marine areas. Here we report observations made under different noise conditions (in the Mediterranean Sea, the Indian and Pacific Oceans) by a submarine floating seismograph, and show that such floats are able to fill the oceanic data gap. Depending on the ambient noise level, the floats can record between 35 and 63% of distant earthquakes with a moment magnitude M≥6.5. Even magnitudes <6.0 can be successfully observed under favourable noise conditions. The serendipitous recording of an earthquake swarm near the Indian Ocean triple junction enabled us to establish a threshold magnitude between 2.7 and 3.4 for local earthquakes in the noisiest of the three environments.

Figure 1. Power spectral density of the ocean ambient noise estimated from the data sent by MERMAIDs.

Each curve represents the power spectral density estimated from the data sent by one MERMAID. For each curve, we specify the area, the average location and the deployment depth of the MERMAID during the data acquisition. The Indian Ocean: A (31.5° S, 80.2° E, 2,000 m), B (37.2° S, 70.5° E, 1,500 m), C (24.7° S, 69.7° E, 2,000 m) and D (21.6° S, 69.5° E, 1,500 m); the Mediterranean Sea: E (43.5° N, 07.9° E, 1,500 m); the Pacific Ocean: F (03.5° S, 93.0° W, 1,500 m).

Figure 2. Representative seismograms transmitted by MERMAIDs.

(a) Teleseismic events detected in the Mediterranean Sea. The amplitudes of all seismograms are normalized to one. Each panel specifies a seismic phase, an angular distance Δ between MERMAID and an event's hypocenter, magnitude and region. (b) Idem, in the Southern Indian Ocean. The Nicaragua signal was high-pass filtered at 0.1 Hz, whereas the last two seismograms were filtered with the pre-STA/LTA filter to amplify the P-wave onset. (c) Low-magnitude underwater earthquakes detected by the MERMAID during the swarm in the Indian Ocean. Each panel indicates the UTC time of the signal's trigger. All signals are filtered with the pre-STA/LTA filter described in the text. Amplitude scale (originally in counts) is scaled to represent the pressure in Pa at 2 Hz, which is the dominant frequency for the detected signals.

Figure 3. Gutenberg–Richter plot of the swarm events detected by one of the MERMAIDs in the Indian Ocean.

Logarithmic relation between the number N_M of swarm earthquakes with magnitude M. The slope of the graph (the b value) is indicated on the plot.

Figure 4. Resolution test.

At every depth, the Earth is filled with six chequerboard patterns. Each pattern is composed of regions in which P-wave velocity is by 5% either higher (blue colour) or lower (red colour) than the P-wave velocity given by the reference model IASP91 (ref. 21). The anomalies' lateral size varies from about 200 km in the lower mantle to 300 km at 768-km depth. Comparison of the patterns obtained when using (a) only the data provided by the ISC stations and (b) combined data of the ISC stations and the MERMAIDs shows a significant improvement of the resolution in the ocean basins at all depths.

Figure 5. Ray coverage.

(a) Red dots indicate both positions of the land stations, whose records comprise the ISC data base, and the simulated positions of the MERMAIDs. Note a virtually uniform distribution of the MERMAIDs thanks to their mobility. Green dots indicate epicentres of the earthquakes listed in the ISC catalogue for the period from 2004 to 2008. (b–d) The -norm of columns of the tomographic matrix. For any column, its -norm is proportional to the total length of rays crossing the volume cell corresponding to that column. Colour scale is in arbitrary units.