Local rift and intraplate seismicity reveal shallow crustal fluid-related activity and sub-crustal faulting

Abstract

Seismicity delineating the mid-ocean ridge system in the Panama Basin is mostly concentrated along transform offsets. Most intraplate areas show little-to-no seismic activity. Here we analyze passive recordings from a short-term deployment of 75 ocean-bottom seismographs (OBS) at the Costa Rica Rift (CRR) and across its southern flank along a flowline to Ocean Drilling Program (ODP) Hole 504B. The OBS array recorded 1061 events along the CRR and the Ecuador Fracture Zone, and 127 intraplate events around ODP Hole 504B. The seismic activity along the CRR occurred in clusters, with some events associated with an axial magma lens (AML) located at 3–3.5 km depth below seafloor (bsf), and four events exhibiting normal and reverse faulting focal mechanisms. These deep events are followed by the largest event cluster whose epicenters connect the AML to the seafloor, at a location where a hydrothermal plume was previously reported. During the same period, another event cluster occurs close to the seabed. This spatio-temporal pattern suggests that deeper events close to the AML, which might be related to thermal stresses or stress perturbations due to a volume change in the AML, trigger fluid-related seismicity within the shallower hydrothermal system. The easternmost extent of the seismicity along the CRR corresponds to an overlapping spreading center (OSC). At depth, the seismicity around the OSC is focused beneath one limb, but is spread over a larger area closer to the surface. The focal mechanisms calculated for OSC events show both normal and reverse motions, and might reflect complex stress or fault orientations. Intraplate seismicity around ODP Hole 504B shows both isolated and clustered events from the seafloor to ∼15–25 km bsf. Some of this seismicity is likely associated with the reactivation of east-west trending normal faults under high pore pressure and thermoelastic stresses conditions. Deep seismicity (i.e. between 15 and 20 km bsf) occurs in small, well-defined clusters close to ODP Hole 504B, at the theoretical depth of the brittle-to-plastic transition for a ∼6.9 Ma oceanic lithosphere. Our results show the importance of short-term OBS deployments for improving our understanding of mid-ocean ridge seismicity and hydrothermal processes, as well as intraplate seismicity and deformation mechanisms.