Seismicity and thermal structure of the St. Paul Transform System, equatorial Atlantic: insights from focal depth analysis

Abstract. The slow-spreading Mid-Atlantic Ridge is segmented and offset by transform faults. Among these, the St. Paul Transform System (SPTS), located in the equatorial Mid-Atlantic Ridge, is a complex multi-fault system with four transform faults (Transform A, B, C, and D) and three intra-transform spreading segments with a cumulaTed offset of 630 km. The SPTS is seismically active, with strong strike-slip earthquakes reaching moment magnitudes of 6.9. In this study, we have re-analyzed the focal depth of 35 earthquakes of the Global Centroid Moment Tensor (GCMT) catalog with 5.3 ≤Mw≤ 6.9 occurring at Transform A, B, and C since 2004, using regional surface waveform modelling. In addition, we derived a 3-dimensional half-space cooling thermal structure along the transform system that relates the focal depths to the temperature structure. Our results of focal depths indicated that the seismogenic zone of both Transform A, B, and C reaches from 5 to 18 km below the seafloor, with the deepest earthquakes located at the center area of the strike-slip segments, and shallower earthquakes occur towards the ridge-transform intersections. The comparison with the half-space cooling indicates that the deepest earthquakes are located in a cooler lithosphere located near the center of the transform segments, with their deformations reaching 600–900 °C. These observations challenge previous viscoplastic mantle geodynamic models that indicated a warm temperature toward the center of a transform fault. The main features found at the SPTS are supported by previous studies of focal depth in other transform faults in the Atlantic Ocean, supporting a global pattern and the occurrence of a cooler mantle at the center of the oceanic transform faults.