Unravelling the intraplate 2019 Broome earthquake in the North West Shelf, Australia, through 3D CMT analysis

The July 14, 2019 Broome earthquake, with a magnitude of 6.6, was a significant seismic event in the North West Shelf (NWS) region of Western Australia where the dynamics of earthquakes are not well-understood. This study examines the focal mechanisms, centroid time, and locations of the Broome earthquake and its aftershocks using Centroid Moment Tensor (CMT) analysis.

The NWS is located in an intraplate tectonic setting where the dynamics of earthquakes are not well-understood. The region has a complex history of fault activity, and it transitions from an active collisional plate boundary in the north to a passive continental margin in the west. These tectonic regimes have generated seismic zones along the plate interface and reactivated older faults.

The Broome earthquake, a strike-slip event, exhibited a dominant NNE-SSW maximum horizontal stress orientation, prevalent across the NWS and responsible for numerous regional earthquakes. Despite the NWS's seismic activity, focal mechanism studies have been limited due to sparse seismic stations, significant azimuthal gaps, and considerable distances between stations and earthquake locations.

We utilized CMT inversion with a high-resolution 3D AusREM velocity model of the region. This method offers enhanced accuracy in determining earthquake source parameters at frequency up to 0.1 Hz, which allows us to study smaller magnitude events, which pose challenges for traditional 1D models.  

The results reveal primarily strike-slip faulting with considerable non-double-couple components, indicating complex rupture processes beyond planar assumptions. The 25-58% double-couple percentages highlight significant deviation from standard seismic models, underlining the NWS's geological complexity.