Mantle structure and tectonic history of SE Asia

Seismic travel-time tomography of the mantle under SE Asia reveals patterns of subduction-related seismic P-wave velocity anomalies that are of great value in helping to understand the region's tectonic development. We discuss tomography and tectonic interpretations of an area centred on Indonesia and including Malaysia, parts of the Philippines, New Guinea and northern Australia. We begin with an explanation of seismic tomography and causes of velocity anomalies in the mantle, and discuss assessment of model quality for tomographic models created from P-wave travel times. We then introduce the global P-wave velocity anomaly model UU-P07 and the tectonic model used in this paper and give an overview of previous interpretations of mantle structure. The slab-related velocity anomalies we identify in the upper and lower mantle based on the UU-P07 model are interpreted in terms of the tectonic model and illustrated with figures and movies. Finally, we discuss where tomographic and tectonic models for SE Asia converge or diverge, and identify the most important conclusions concerning the history of the region. The tomographic images of the mantle record subduction beneath the SE Asian region to depths of approximately 1600. km. In the upper mantle anomalies mainly record subduction during the last 10 to 25. Ma, depending on the region considered. We interpret a vertical slab tear crossing the entire upper mantle north of west Sumatra where there is a strong lateral kink in slab morphology, slab holes between c.200-400. km below East Java and Sumbawa, and offer a new three-slab explanation for subduction in the North Sulawesi region. There is a different structure in the lower mantle compared to the upper mantle and the deep structure changes from west to east. What was imaged in earlier models as a broad and deep anomaly below SE Asia has a clear internal structure and we argue that many features can be identified as older subduction zones. We identify remnants of slabs that detached in the Early Miocene such as the Sula slab, now found in the lower mantle north of Lombok, and the Proto-South China Sea slab now at depths below 700. km curving from northern Borneo to the Philippines. Based on our tectonic model we interpret virtually all features seen in upper mantle and lower mantle to depths of at least 1200. km to be the result of Cenozoic subduction

A reassessment of outer-rise seismicity and its implications for the mechanics of oceanic lithosphere

We use body-waveform modelling to constrain the source parameters of earthquakes occurring globally in oceanic lithosphere beneath the subduction zone outer rise and outer trench slope. These data are then used to map the stress state in the lithosphere of the downgoing plate as it bends into the subduction zone. Our results provide new constraints on the faulting of oceanic lithosphere at the outer rise, which is important for understanding the transmission of plate-driving forces through the subduction system. In all cases, shallow normal-faulting earthquakes are observed at the top of the plate, and are separated in depth from any deeper thrust-faulting earthquakes. No temporal variation associated with large thrust-faulting earthquakes on the subduction interface is seen in the depth extent of each type of faulting at the outer rise. The transition depth from trench-normal extension to compression is found to vary in agreement with models in which deformation is driven by the combination of in-plane stresses and bending stresses, resulting principally from slab pull. Combining the seismologically derived constraints on the thickness of the elastic core of the plate with estimates of the plate curvature, we place upper bounds on the strength of the lithosphere at the outer rise, which is required to be ≲300 MPa for a constant yield stress model, or governed by an effective coefficient of friction of ≲0.3

Prevalence of Seismic Rate Anomalies Preceding Volcanic Eruptions in Alaska

Seismic rate increases often precede eruptions at volcanoes worldwide. However, many eruptions occur without such precursors. Additionally, identifying seismic rate increases near volcanoes with high levels of background seismicity is non-trivial and many periods of elevated seismicity occur without ensuing eruptions, limiting their usefulness for forecasting in some cases. Although these issues are commonly known, efforts to quantify them are limited. In this study, we consistently apply a common statistical tool, the β-statistic, to seismically monitored eruptions in Alaska of various styles to determine the overall prevalence of seismic rate anomalies immediately preceding eruptions. We find that 6 out of 20 (30%) eruptions have statistically significant precursory seismic rate increases. Of these 6 eruptions, 3 of them occur at volcanoes with relatively felsic compositions, repose periods >15 years, and VEI ≥ 3. Overall, our results confirm that seismic rate increases are common prior to larger eruptions at long dormant, “closed-system” volcanoes, but uncommon preceding smaller eruptions at more frequently active, “open-system” volcanoes with more mafic magmas. We also explore the rate of other anomalies not precursory to eruptions and investigate their origins. Some of these non-eruptive anomalies can be explained by aftershocks of regional seismic events, magmatic activity that did not lead to eruption, or unrest at other nearby volcanoes. Some open-system volcanoes have high non-eruptive anomaly rates and low pre-eruptive anomaly rates and are thus not amenable to forecasting based on earthquake catalogs. In this study, we find that 31% of anomalies lead to eruption. With continued calibration at more volcanoes, the β-statistic that we apply may be used more broadly to analyze future periods of seismic unrest at other volcanoes, properly placing such episodes into the context of the long-term background rate. These results may be useful for informing future eruption forecasts around the world, and the statistical tool may aid volcano observatories in identifying future seismic rate anomalies under changing network conditions