Back-Projection Imaging: From Complex Earthquake Ruptures to Volcano-Induced Landslides

Abstract

Following the 26 December 2004 Sumatra-Andaman (Mw 9.3) earthquake, back-projection became a widely used technique for remote imaging of large earthquake ruptures. We introduce a new teleseismic back-projection method that uses multiple seismic arrays and combines both P and pP seismic phases. For earthquakes deeper than 40 km, we incorporate pP backprojections, particularly when the pP amplitude is at least 40% of the P wave amplitude. The contribution of each array to the rupture image is controlled by its azimuthal distribution. This approach allows us to algorithmically determine key rupture parameters, including rupture length, directivity, speed, and aspect ratio.

Multi-array and multi-phase back-projection enhances resolution, facilitating the tracking and analysis of short-period earthquake rupture complexities. Early developments and applications include imaging the 23 January 2018 Gulf of Alaska (Mw 7.9) intraplate rupture, the 24 January 2020 Doğanyol-Sivrice (Mw 6.7) earthquake (Türkiye), and the 30 October 2020 Néon-Karlovásion (Mw 7.0) earthquake (Greece). The finalized method was also used to characterize the 12 August 2021 South Sandwich tsunamigenic earthquake (Mw > 8.2; South Atlantic) and the 6 February 2023 Türkiye seismic sequence (Mw 7.7 and 7.6).

We applied the newly developed back-projection method to characterize all large earthquakes with magnitudes Mw ≥ 7.5 and depths less than 200 km that occurred between 01/2010 and 12/2022 (56 events). For subduction megathrust earthquakes, we observed complex short-period ruptures (0.5-2.0 Hz) outlining megathrust asperities. Our results confirmed the prevalence of short-period radiation from the central and down-dip parts of the megathrust. Notably, we found that up-dip emissions from the main asperity are more common than previously reported. We also evaluated the prevalence of supershear ruptures and established new magnitude-rupture length scaling relationships for thrust, normal, and strike-slip earthquakes, consistent with previously published relationships based on aftershocks and total slip estimates.

We observed asperity encircling short-period ruptures of the 14 November 2007 Tocopilla (Mw 7.7) earthquake in Northern Chile using teleseismic and local strong-motion backprojections. The complex rupture was attributed to several factors: 1) the high-stress gradient caused by a kink in the slab interface, which had previously been proposed as the main mechanism arresting the trenchward rupture propagation; 2) the down-dip limit of the rupture, coinciding with the depth of the Continental Moho; and 3) the high-stress gradient surrounding the asperities.

Finally, we explore back-projection applications for locating volcano-induced landslides and assessing tsunami-warning strategies in Indonesia, focusing on the 22 December 2018 flank collapse of Anak Krakatau. Using long-period back-projection (40-70 s) of surface wave envelopes from the Indonesian seismic network, we demonstrated the flank collapse localization with two minutes of data after its initiation. Spectral analysis of the first 100 s of seismic data can distinguish the flank collapse from typical tectonic earthquakes, using stations at epicentral distances of 1.0°-2.5°. The results showed that massive landslides can be distinguished from earthquakes using a simple frequency ratio. We conclude by discussing some practical aspects for tsunami early warning systems, particularly for detecting and locating volcanic collapses and landslide-triggered tsunamis using real-time seismic data.