Volcanic transcrustal magmatic systems imaged with teleseismic converted phases

The Aleutian arc in Alaska consists of dozens of active volcanoes. Their eruptions imperil local communities as well as domestic and international air traffic. Such arcs are also of scientific interest because they are the geological setting in which new continental crust forms.

Our understanding of the subterranean plumbing of volcanoes has recently evolved. From the simple picture of a single magma chamber, the image of a complex system of interconnected chambers at all levels in the Earth’s crust has emerged. Investigating these deep structures in detail is critical to better understand volcanoes and their associated hazards.

However, such investigation has been challenging because of technical limitations. Here the researchers use a new method. In collaboration with the Alaska Volcano Observatory, they use seismic records of earthquakes occurring around the globe.

By analyzing the whole shape of recorded waveforms – instead of just the arrival times - they image the 3D magmatic structure underneath volcanoes. They reveal important features, e.g., the orientation of crystals in solidified magma and of melt-filled cracks in the adjacent rocks. New observations show complex subterranean structures that appear concentric around volcanoes, detected at all depth levels of the crust.

The project helps improving hazard assessment for local communities, infrastructures as well as air traffic routes. It fosters outreach to K-12 science teachers, and to the public through the Museum of the Aleutians in Unalaska with, notably, a local television broadcast. It also promotes training for undergraduate students via programs geared towards groups underrepresented in Earth Sciences.

Island arcs are of interest in tectonics as sites of continental crust production. Volcanoes in the Aleutians and Alaska offer an excellent cross section of types of arc volcanism. This project leverages recent updates to existing monitoring networks at Alaskan volcanoes.

It tests a new approach for passive source imaging of magmatic systems under arc volcanoes through all crustal depths. The proposed work is to develop a new waveform-based method for detailed imaging and characterization of volcanic structures into the deep crust. It uses sparse numbers of broadband stations, complementing more expensive active source experiments and dense deployments.

The approach also uses large arrivals in teleseismic receiver functions from the vicinity of arc volcanoes that are dominated by a periodic backazimuthal component (polarity reversals with azimuth of incidence). The positions of the polarity reversals fall on azimuths that are concentric to the volcanic edifice. The observed arrivals imply dipping contrasts between isotropic layers and/or anisotropic fabric centered on the volcano to at least midcrustal depths.

Recent station upgrades to monitoring networks by the Alaska Volcano Observatory more than double the number of volcanoes available for study. A modeling effort will help distinguish between several endmember models of transcrustal magmatic structure that have been proposed to explain results from other methods such as petrology, tomography, geodesy, magnetotellurics, and seismicity relocations.

These endmember models include dipping interfaces to high- or low-velocity crustal bodies, dipping faults above magma chambers, and dip in dike and sill complexes. The added constraints offered by the proposed method will allow higher resolution magmatic structures throughout the crust than currently possible.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.