Focal depths and mechanism of Mid‐Atlantic Ridge earthquakes from body waveform inversion

We have determined the source mechanisms (double-couple orientation, moment, centroid depth, source time function) of 14 earthquakes on the northern Mid-Atlantic Ridge (0°–72°N) from an inversion of long-period P and SH waveforms. The earthquakes are all characterized by nearly pure normal faulting on fault planes that dip at about 45° and strike parallel to the local trend of the ridge axis. Moments range from 3 to 15×1024 dyn cm, and the source time functions are all of simple form. The P and S waveforms for all earthquakes can be well matched using conventional values for t* (1 and 4 s, respectively). These earthquakes are all very shallow; centroid depths range between 1.2 and 3.1 km beneath the seafloor. The P waves from these earthquakes show strong water column reverberations, suggesting that fault rupture extended to the seafloor. The predominant period of these reverberations constrains the water depth in the epicentral region. On the basis of estimated water depth and epicentral location, all of these earthquakes can be shown to have occurred beneath the inner floor of the median valley. The centroid depths do not show a correlation with either spreading rate or seismic moment. Under the assumption that the centroid depth marks the mean depth of fault slip, earthquake faulting extended to depths of 2–6 km for these events.