Hydroacoustic observations of a seismic cluster at Melville Fracture Zone along the Southwest Indian Ridge in 2016-17

Seismic clusters of volcanic and tectonic events along mid-oceanic ridges are inherent to seafloor spreading. Due to the rapid attenuation of seismic waves in the solid Earth, land-based seismic networks lack the low-level seismicity associated with such clusters. However, regional studies using autonomous underwater hydrophones overcome this difficulty due to their sensitivity to low-frequency hydroacoustic waves, known as T-waves, that travel in the SOund Fixing And Ranging (SOFAR) channel over very long distances with little attenuation. Using hydroacoustic records from the temporary OHASISBIO network and permanent stations of the CTBT Organization, we have examined a seismic cluster near the Melville Fracture Zone (FZ) at 61°E along the ultraslow spreading Southwest Indian Ridge (spreading rate: 14-15 mm/yr).

Near 61°E, 259 events were reported in the International Seismological Center (ISC) catalogue between 9^th June 2016 and 25^th March 2017 in the region of 3 x 3 degrees in latitude and longitude around Melville Transform. Out of them, 17 events display normal faulting mechanisms parallel to the ridge axis (Global Centroid Moment Tensor (GCMT) solutions).

In the preliminary analysis, we have detected 4273 hydroacoustic events between 9^th June and 11^th July 2016, vs 28 events in the ISC catalogue, so with ~150-fold increase in the event detections. These events are mostly aligned parallelly to the ridge axis near its intersection with the Melville FZ. The event median uncertainties are ~4.7 km in latitude and longitude, and ~1.4 s in origin time. Their median acoustic magnitude or Source Level (SL) is 225.26 dB.

This seismic cluster includes several highly energetic and short duration (~10 s) impulsive events, located on the slopes of seamounts near the FZ at 61.2°E. These events are interpreted as thermal explosions resulting from direct magma supplies on the seafloor. Also, most of the hydroacoustic events are clustered around the same seamounts. There is no evidence for long mainshock-aftershock sequence at the onset of this seismic cluster. These observations point to a magmatic origin for this seismic cluster with an active source located near a chain of seamounts in the vicinity of Melville FZ.