EXTension on the ENDeavour Segment (EXTEND): Illuminating the seafloor spreading cycle

Seafloor spreading centers are plate tectonic boundaries where submarine volcanoes generate new oceanic crust. They account for about 80% of the Earth’s volcanism. During a volcanic spreading event, the tectonic plates move apart along a section of the plate boundary and magma rises into an approximately 1-m-wide vertical crack where it can feed volcanic eruptions.

When the magma in the crack solidifies, it forms a geological feature known as a dike. Many small precursory earthquakes occur before a diking event as stress builds up in the Earth’s crust and the rupturing that occurs during the diking event is accompanied by a large swarm of earthquakes. At any one location on the plate boundary, diking events occur at intervals of a decade or more and because the precursory earthquakes are too small to detect from land, it is difficult to anticipate when they will occur.

This project takes advantage of a cabled observatory on Endeavour Segment of the Juan de Fuca Ridge that is in a location where the spreading center volcano supports several large hydrothermal vent fields. A small network of seismometers on the cabled observatory has detected the precursory seismicity of an impending diking event. This project will deploy autonomous seafloor seismometers to enhance the cabled seismic network so that the earthquakes associated with the anticipated diking event are well recorded.

The earthquake locations and characteristics will be used to investigate what triggers diking events, determine whether some extension occurs by creep on faults, and improve the understanding of how hydrothermal systems are supported by submarine volcanos. The earthquake catalog will be of value to scientists using the cabled observatory to investigate the impact of diking events on black smoker hydrothermal vent fields and the biological communities they support.

This project will train both undergraduate and graduate students in seagoing research and in modern computational techniques to analyze earthquake recordings.

Seafloor spreading through diking is a fundamental geological process that regenerates 60% of the Earth's surface on timescales of 100 Myr, but it is hard to capture with autonomous ocean bottom seismometers alone because the decadal timescale of spreading cycles is an order of magnitude greater than the recording duration of an autonomous instruments. The Endeavour segment of the Juan de Fuca Ridge is an intermediate rate spreading center that last ruptured in a sequence of diking events from 1999-2005 and has been monitored for approaching a decade by a small network of 4-5 cabled OBSs on the Ocean Networks Canada NEPTUNE cabled observatory.

Based on observations with this network of accelerating rates of microseismicity, another spreading event is expected soon. This project is the US component of a US-Canadian collaboration to take advantage of this rare opportunity for a preemptive seismic response. The cabled seismic network will be augmented for 5-years by the addition of 5 Canadian National Facility for Seismological Investigations autonomous broadband ocean bottom seismometers.

This will expand the seismic network to a size that allows the determination of focal mechanisms and depths near the center of the segment and the triangulation of epicenters on the southern part of the segment. In 2025-6, the network will be further expanded to include 20 ocean bottom seismometers. This larger network will fully cover the footprint of off-axis seismicity near the center of the segment that is related to two propagating rifts. The experiment is designed to address the following questions:

(1) Are diking events at the Endeavour triggered by extensional stresses, or magmatic pressures from a centralized source?

(2) Do temporal changes in seismic velocity show evidence for precursory aseismic strain or ridge extension events that are not linked to dikes? (3) Is carbon dioxide exsolution linked to off-axis seismicity at the Endeavour? (4) Is the permeability beneath the Endeavour hydrothermal vent fields enhanced by the nearby propagating rifts?

To analyze the data, an artificial intelligence-aided earthquake catalog building procedure will be implemented to detect, pick and associate volcanic-tectonic earthquakes, hybrid/long period earthquakes, and notably, seasonal fin whale calls which confuse the current conventional earthquake catalog building software. Earthquakes will be located using existing 3-D models of P- and S-wave velocity structure, the double-difference technique will be applied to improve relative earthquake locations and focal mechanisms determined from P-wave first motions and P- to S-wave amplitude ratios.

To detect velocity changes linked to strain, time-lapse imaging of structure using 4-D travel time tomography will be explored and ambient noise techniques applied to both single stations and pairs of stations. Families of multiplet earthquakes will also be investigated. The improved real-time earthquake catalog for the cabled network will benefit scientists seeking to respond to seismic events that might perturb the Endeavour hydrothermal systems and the retrospective catalog that add the autonomous data will contribute to the interpretation of the hydrothermal response to diking events.

The project will train graduate and undergraduate students in seagoing marine geophysics and seismic analysis.

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.