Monday, 10 March 2025/ Edition 49

The scenic Cascade Range, spanning from northern California through Oregon and Washington into British Columbia, is a natural laboratory for subduction-related volcanic and seismic activities. Let’s take a peek!

Rasoul Sorkhabi

Editor, Core Elements

Analyzing Shaking Intensity of Past Cascade Quakes

Wikimedia Commons

Geologist Bates McKee coined the name “Cascades” (from previous terms “Cascadia rapids and waterfalls”) in his 1972 geology textbook Cascadia to describe the orogenic province of the Pacific Northwest.

The Cascadia subduction zone is a roughly 1,100-kilometer-long plate boundary between the Juan de Fuca oceanic plate and the North American continental plate. The zone is highly seismic. The last earthquake that occurred there was a 9.0-magnitude quake on 26 January 1700.

New research: A new study in Geophysical Research Letters offers a novel method to quantify the shaking intensity of past earthquakes by looking at landslides along the Oregon and Washington coasts.

What they did:

• The researchers mapped the rotational landslide headscarps within uplifted marine terraces along the entire Pacific Coastline of Washington and Oregon.

• The mapping came from lidar topographic data with a sub-meter resolution.

• The height of the failed terrace of the landslide and landward extent of the landslide slip surface were measured.

• These two measurements were used in 3D slope stability modeling.

• The model results were filtered for co-seismic landslides.

What they found:

• Model results show distributions of possible landslide extents in marine terraces under different triggering conditions.

• A total of 618 landslide headscarps were mapped to estimate ground shaking intensity.

• The heights ranged from 12.5 to 122 meters with a median value of 43 m. Landward extents ranged from 0 to 285 meters with a median value of 37 meters.

• Of these, 154 landslides required strong ground motions.

• Minimum peak ground acceleration that triggers landslides range from 0.4 to 0.8, which is consistent with model predictions of magnitude 9 earthquakes.

A shortcoming of the study is that there is no geochronological data for the landslides.

Go deeper: Read the full study in Geophysical Research Letters here.

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Turbidite Correlation for Paleo-Seismology

Blair Davidson/ Shutterstock.com

In another study published in the GSA Bulletin, researchers used the geologic records of marine turbidity currents to infer rupture extent and recurrence of earthquakes along the Cascadia subduction zone.

Why it matters: One challenge in turbidite paleo-seismology is the presumption of synchronous triggering of turbidity currents. The new study offers a methodology to circumvent this problem.

Methods and software: The researchers used photographs, X-ray computed tomography, and high-resolution (centimeter-scale) magnetic susceptibility (MS) logs from nine cores. The data were collected during the Oregon State University RV Melville cruise.

• Additional data from previously published literature and cores at the Oregon State University Marine and Geology Repository were included in the new analysis.

• For correlation of MS logs, the researchers used the dynamic time wrapping (DTW) algorithm of the Librosa Python package.

What they found: Study results suggest that turbidites of nearby cores, less than 100 meters apart, can be correlated in a statistically significant way.

Go deeper: Read the study in GSA Bulletin here.

Monitoring Mt. St. Helens' Lava Dome

Nadia Young/ Shutterstock.com

Mt. St. Helens' eruption in 1980 was the deadliest and most economically destructive volcanic catastrophe on record in U.S. history. Fifty-seven people died, including U.S. Geological Survey geologist David Johnston, who was on duty. St. Helens had another eruptive cycle from 2004–2008.

Recent monitoring: In a new study in the Journal of Volcanology and Geothermal Research, researchers assessed the long-term development of a new lava dome in the crater of Mount St. Helens after the 2004–2008 eruptive cycle.

• Using LiDAR data, researchers found that the lava dome has decreased in elevation by more than 35 meters from 2009 to 2019.

• They also found that heat output from the lava dome has decreased constantly. Fumarole temperatures at the dome summit decreased from 380° C in 2014 to 60° C in 2024.

Hidden magma chambers: Continental arc volcanic eruptions like Cascadia are pumped from deep crustal magma systems; however, the imaging and characterization of these magma chambers is challenging, and many questions around their generation, size, depth, and time span, remain unanswered.

In a recent study published in Nature Geoscience, researchers have used seismic evidence to image and characterize magma chambers beneath the Cascade Volcanic Arc.

What they did: By converting P- and S-wave scattering beneath seismometers in the region, the researchers constructed 3D subsurface images beneath six volcano summits.

What they found: The researchers found the following top-bottom depths and volumes for magma chambers beneath the six volcano summits:

• Mount Rainier in Washington State: 3–7 km deep, 39–534 cubic kilometers

• Mount St. Helens in Washington State: 5–10 km deep, 65–534 cubic kilometers

• Mount Hood in Oregon: 10–15 km deep, 262–534 cubic kilometers

• Newberry Volcano in Oregon: 3–7 km deep, 39–157 cubic kilometers

• Crater Lake in Oregon: 4–10 km deep, 157–785 cubic kilometers

• Lassen Peak in California: 4–9 km deep, 261–785 cubic kilometers

The study shows the presence of long-lived, shallow, melt-dominated chambers across the Cascadia revealed by low-seismic velocity data.

Go deeper: Read the full study in Nature Geoscience here.

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✉️ To get in touch with Rasoul, send an email to editorial@aapg.org.