Yuko Kondo

We have inverted a very large combination of delay times for P, pP, PcP, PKPab, PKPbc from the ISC-EHB catalog, augmented with hand-picked OBS and MERMAID onset data and cross-correlation delays from broadband stations to derive a new global model, UNICA25, of P-wave velocity variations in the mantle. This paper presents the model, summarizes the data processing and inversion, and shows resolution tests at different scales. After rejecting 109,917 outliers (1% of the total), the data set comprises 10,571,152 arrival times. The misfit of the observed delays, with respect to a background model consisting of the crust of LITHO1.0 on top of model AK135, is on average 2.14 standard errors (σ), which is reduced to 0.99σ by inverting for P-wave velocity variations using both ray theory and finite frequency theory, and 5000 iterations of the iterative solver LSQR. Extensive resolution tests show a resolution at 500 km scale or better in most of the lower mantle. An upper mantle resolution of the order of 300 km is obtained under densely instrumented continents, but upper mantle resolution remains deficient under much of the oceans except where covered by MERMAIDs. Linearized tomography is at least three orders of magnitude faster than recent full waveform inversions (FWI), and reaches much higher frequencies for cross-correlated delays. Model UNICA25 has a P-velocity resolution that rivals that of FWI and shows a superior performance in fitting delay times, notably the ISC-EHB P-delays.

We have inverted a very large combination of delay times for P, pP, PcP, PKPab, PKPbc from the ISC-EHB catalog, augmented with hand-picked OBS and MERMAID onset data and crosscorrelation delays from broadband stations to derive a new global model, UNICA25, of P-wave velocity variations in the mantle. This paper presents the model, summarizes the data processing and inversion, and shows resolution tests at different scales. After rejecting 109 917 outliers (1% of the total), the data set comprises 10 571 152 arrival times. The misfit of the observed delays, with respect to a background model consisting of the crust of LITHO1.0 on top of model AK135, is on average 2.14 standard errors (σ), which is reduced to 0.99σ by inverting for P-wave velocity variations using both ray theory and finite frequency theory, and 5000 iterations of the iterative solver LSQR. Extensive resolution tests show a resolution at 500 km scale or better in most of the lower mantle. An upper mantle resolution of the order of 300 km is obtained under densely instrumented continents, but upper mantle resolution remains deficient under much of the oceans except were covered by MERMAIDs. Linearized tomography is at least three orders of magnitude faster than recent full waveform inversions (FWI), and reaches much higher frequencies for cross-correlated delays. Model UNICA25 has a P-velocity resolution that rivals that of FWI and shows a superior performance in fitting delay times, notably the ISC-EHB P-delays.

This study presents new seismic imaging of the Andean subduction zone through P‐wave hybrid finite‐frequency and ray‐theoretical tomography. We measured both differential and absolute traveltimes using broadband seismic waveforms from stations in an array of ocean‐bottom seismographs near the Chile Triple Junction (CTJ) and stations within 30° of the array. These data were combined with the global traveltime data set to obtain a global P‐wave velocity structure with a focus on central to southern South America. The new tomographic image showed the Nazca slab geometry as a continuous fast anomaly, which is consistent with seismic activity and prior slab models. Furthermore, two notable structures were observed: a broad extension of the fast anomaly beneath the Nazca slab at 26–35°S and a slow anomaly east of the CTJ. The checkerboard resolution and recovery tests confirmed the reliability of these large‐scale features. The fast anomaly, isolated from the Nazca slab, was interpreted as a relic Nazca slab segment based on its strong amplitude and spatial coincidence with the current Pampean and past Payenia flat slab segments. The slow anomaly near the CTJ was consistent with the previously inferred extent of the Patagonian slab window. Moreover, the active adakitic volcanoes are aligned with the southern edge of the anomaly, and the plateau basalts are located within the anomaly. Our model showed that the slow anomaly extended to a depth of up to 250 km, suggesting a depth limit that the asthenospheric window can influence.

This study presents new seismic imaging of the Andean subduction zone through P-wave hybrid finite-frequency and ray-theoretical tomography. We measured both differential and absolute traveltimes using broadband seismic waveforms from stations in an array of ocean-bottom seismographs near the Chile Triple Junction (CTJ) and stations within 30° from the array. These data were combined with the global traveltime dataset to obtain a global P-wave velocity structure with a focus on central to southern South America. The new tomographic image showed the Nazca slab geometry as a continuous fast anomaly, which is consistent with seismic activity and prior slab models. Furthermore, two notable structures were observed: a broad extension of the fast anomaly beneath the Nazca slab at 26–35° S and a slow anomaly east of the CTJ. The checkerboard resolution and recovery tests confirmed the reliability of these large-scale features. The fast anomaly, isolated from the Nazca slab, was interpreted as a relic Nazca slab segment based on its strong amplitude and spatial coincidence with the current Pampean and past Payenia flat slab segments. The slow anomaly near the CTJ was consistent with the previously inferred extent of the Patagonian slab window. Moreover, the active adakitic volcanoes are aligned with the southern edge of the anomaly, and the plateau basalts are located within the anomaly. Our model showed that the slow anomaly extended to a depth of up to 250 km, suggesting a depth limit that the asthenospheric window can influence.