A document by Guilherme Weber Sampaio de Melo. Click on the document to view its contents.

Four transform faults and three intra-transform segments located at Equatorial Atlantic form the Saint Paul Transform System (SPTS), with a long-offset of 630 km. In the northern transform, the 200 km long and 30 km wide Atoba Ridge is a major topographic feature that reaches the sea level at the St. Peter and St. Paul Archipelago island (SPSPA, 00º 55 ‘0’ ‘N and 29º 20 ”43”W). The islets have an average uplift rate of approximately 1.5 mm/year. The southern and northern flanks of the Atobá Ridge are marked by a series of large thrust faults visible in the bathymetry and clearly imaged through seismics and correspond to an exceptionally serpentinized mantle. We have determined the hypocentral location of 62 minor-moderate earthquakes of SPTS, with magnitudes 1.9 ≥ M ≤ 5.3. The earthquakes occurred in 2013 and were recorded by a seismometer installed in SPSPA and three autonomous hydrophones deployed during the COLMEIA cruise. The HYPOCENTER software and Seismic Analysis Code (SAC) were used for data analysis and hypocenter location. The depth range is from 0.2 to 17.5 km and are concentrated in three different zones: the East Shear Zone (ESZ), the Atobá Ridge Zone (ARZ) and the Central Fracture Zone (CFZ). A seismogenic zone with a deep britle-ductile transition was identified in SPTS, with hypocenters reaching 18 km beneath the seafloor. We observed that this lithospheric structure presents relation with the offset age and controls the maximum hypocentral depths of oceanic transform faults. Besides, the earthquakes indicated the existence of a broad serpentinization depth reaching 18 km beneath the ARZ. This was interpreted as the effect of deep water percolation into the mantle in the SPTS, which caused a fluid-mantelic rocks interaction and allowed the expansion of faults into the mantle. Some hypocenters were located in the central fracture zone (CFZ) segment of SPTS and their depths reached 8.8 km beneath the seafloor. We interpreted this seismicity as reactivation of a weakness zone existent in CFZ due to the transpressive load-induced stress originated in Atobá Ridge.

The equatorial region of the slow-spreading Mid-Atlantic Ridge is characterized by several major transform faults, which are some of the longest on Earth. Among them, the St. Paul Transform system (SPTS) is a complex group of four transform faults, bounding three short intra-transform segments with total offset of 630 km. The northernmost transform is the 200 km-long, 30 km-wide Atoba Ridge, which represents a major topographic feature that rises above sea level at the St. Peter and St. Paul islands (SPSPA). This push-up ridge formed from transpressive stresses along several transform fault step-overs and restraining bends, uplifting mantle rocks at a rate of ~1.5 mm/yr. Moderate-sized earthquakes (>4.0 Mw) have been located by global teleseismic networks along the SPTS and near region. These earthquakes are recorded at large epicentral distances, and include raypaths that travel within the upper mantle (Pn and Sn phases). Pn velocity estimates can help to understand the dynamics of upper mantle structure around of the transform faults. Here, waveforms recorded over ~6 months of 2012 by two autonomous hydrophones moored north and south of SPTS (EA-2 and EA-8), and a seismographic station installed on SPSPA island (ASPSP station) are examined. These data allow us to make Pn velocity estimates from 32 earthquakes that occurred in the SPTS region from 1.5º S to 4.5º N. Pn wave velocities are typically thought to be 8.0–8.2 km/s in upper mantle, however we identify Pn velocities ranging from 7.5 to 9.0 km/s. The slower velocities (7.5-8.0 km/s) are from ray paths oriented parallel to the ridge axis and could be explained by elevated mantle potential temperature and the presence of melt. Ray paths passing through the transform fault system have Pn velocities from 8.1 to 9.0 km/s, indicating that upper mantle conditions are strongly affected by the presence of the crustal fault system. We will also compare our velocity estimates to global shear-wave tomographic models of the upper mantle. Hence it is our goal to show that the availability of autonomous hydrophones and a single island seismic station can be used to make rare estimates of Pn velocities, as well as provide insights into upper mantle structure, in this remote part of the Atlantic Ocean.

Tectonic information available from remote seismometers is compromised by uncertainties in epicenter and depth, which often prevent earthquakes being associated with faults observable in morphologic data. Here we show that careful study of regional recordings can achieve reasonable position and, for the larger events, also depth estimates. We illustrate this with swarms totallying 90 earthquakes of 3.6 mb to 5.5 Mw recorded at coastal stations of the Brazilian Seismographic Network, GSN, GEOSCOPE and Cape Verdes. Seismicity occured in 4 swarms, firstly in 2012 with 7 events in the valley floor and eastern wall near the ridge-transform intersection (4.05º-4.2ºN). Next, five events occurred in 2014, with three in the inner floor (4.8º-4.9ºN) and two under the valley wall (4.7º-4.8ºN). A 70-earthquake swarm in 2016 occurred in the 4.4º-4.8ºN valley floor, in both walls and floor, involving slip on different faults. The last 8 events in 2019 were located outside the median valley near a volcanic seamount. We estimated focal depths of the strongest events with Mw > 5.4 using waveform modeling and focal mechanims reported by the Global Centroid-Moment-Tensor Project. Our best estimated hypocenters lie 5-8 km beneath the seafloor, in keeping with maximum depths typically found with OBSs on the MAR. In contrast, deeper events (10 km) have been found in an OBS experiment around an active detachment fault elsewhere. Multibeam sonar data from the area do reveal a detachment fault surface (“megamullion”), but it ends 10-15 km from the median valley floor, suggesting that it is probably inactive. Although our uncertainties still do not allow the event depths to be discriminated from 10 km, our best estimates are more compatible with shallower normal faulting. Over time, repeating this exercise with many such datasets and comparing with morphologic data should help to resolve the incidence of deeper ruptures associated with detachment faults.