Multiple branching rupture of the 2009 Tonga‐Samoa earthquake

Several source models have been proposed to explain the enigmatic 2009 Tonga‐Samoa earthquake. The long‐period data require a composite source model and can be fit with a normal‐faulting subevent followed by one or more reverse‐faulting subevents. The short‐period data, in contrast, indicate a more compact rupture pattern around the epicenter. The lack of a unified source model reflects the complexity of the event. We analyze the spatiotemporal evolution of this earthquake with P wave back‐projection from globally distributed stations in different frequency bands (low frequency: 0.05–0.2 Hz, high frequency: 0.2–2 Hz) and a multiple moment tensor inversion. The rupture propagation revealed by back‐projection exhibits frequency‐dependent behavior, with two branches of high‐frequency‐enriched bilateral rupture around the epicenter and a high‐frequency‐deficient rupture branch at the subduction interface. A composite source model with one M w 8.0 normal‐faulting earthquake east of the trench axis (seaward) followed by one M w 8.1 reverse‐faulting earthquake along the subduction interface west of the trench axis (landward) can explain the very long period data (200∼500 s). Combined with high‐resolution swath bathymetry data, the back‐projection images show that the azimuth of rupture branches east of the trench axis were controlled by the geometry of bending‐related faults on the Pacific plate and that the rupture branch west of the trench axis may correlate with the along‐strike fore‐arc segmentation. The rupture along the subduction interface was triggered by the seaward rupture and a partially subducted normal fault may have played a key role in facilitating the triggering. The apparent normal‐reverse faulting interactions pose a higher seismic risk to this region than their individual strands at the northernmost corner of the Tonga subduction zone.