Triggered aseismic slip adjacent to the 6 February 2013 Mw 8.0 Santa Cruz Islands megathrust earthquake

- We model the source processes of the 2013-02-06 Mw 8.0 Santa Cruz earthquake sequence.
- We infer the occurrence of a large (Mw∼7.6) aseismic slip event on the adjacent thrust interface.
- The aseismic event occurred within the seismogenic zone, based on stress and aftershock modeling.
- This plate boundary can support both seismic and aseismic deformation.
- These megathrust slip events released most of the stored strain along this plate boundary segment.

Aseismic or slow slip events have been observed in many subduction zones, but whether they affect the occurrence of earthquakes or result from stress changes caused by nearby events is unclear. In an area lacking direct geodetic observations, inferences can be made from seismological studies of co-seismic slip, associated stress changes and the spatiotemporal nature of aftershocks. These observations indicate that the February 2013 Mw 8.0 Santa Cruz Islands earthquake may have triggered slow or aseismic slip on an adjacent section of the subduction thrust over the following hours to days. This aseismic event was equivalent to Mw 7.6, significantly larger than any earthquakes in the aftershock sequence. The aseismic slip was situated within the seismogenic portion of the subduction interface, and must have occurred to the south of the main seismic slip and most aftershocks in order to promote right-lateral faulting in the upper plate, the dominant deformation style of the aftershock sequence. This plate boundary segment can support either stable sliding (aseismic) or stick-slip (seismic) deformation in response to different driving conditions. The complete lack of aftershocks on the thrust interface implies this pair of megathrust slip episodes (seismic and aseismic) released a substantial portion of the stored strain on the northernmost section of the Vanuatu subduction zone.