Seismogenic characteristics of the Northern Mariana shallow thrust zone from local array data

The Northern Mariana seismogenic zone has no shallow thrust earthquakes larger than Ms 7.4 in the historical seismological record and is traditionally considered 'decoupled' or 'aseismic'. During the 2003-2004 Mariana Subduction Factory Imaging Experiment, we recorded local shallow earthquakes throughout the central and northern regions of the Mariana forearc using an array of terrestrial broadband and ocean bottom seismographs. Accurate locations for both the 2003-2004 local seismicity as well as earthquakes with Global Centroid Moment Tensor (GCMT) solutions from 1976 to 2008 were obtained using the hypocentroidal decomposition relocation method and a local velocity model. Additionally, focal mechanisms for the largest 2003-2004 earthquakes were determined using regional waveform inversion. Thrust faulting earthquakes occur along the Mariana megathrust between depths of 20-60 km, showing that the lack of great shallow thrust earthquakes does not result from a narrow seismogenic zone and that most seismicity occurs where the downgoing plate contacts the overriding mantle wedge. Clusters of small plate interface earthquakes with Ml 1.6-4.7 occur within patches 100-120 km west of the trench at depths of 30-45 km. Furthermore, the larger GCMT earthquakes (Mw 4.9-5.8) occur mostly updip and downdip of the patches of smaller earthquakes recorded by our local array and is suggestive of changes in the fault properties with depth. Clusters of small, forearc earthquakes occur discontinuously along the length of the Mariana subduction zone, showing that Northern Mariana is variable both along the strike of the margin and with depth along the seismogenic zone. We propose that the lack of great (Mw > 8) thrust faulting earthquakes is due in part to the variable frictional heterogeneity along the megathrust.