The Cascadia Subduction Zone, which runs along Washington and Oregon, has highly active tectonics and is associated with mega-earthquakes. It has definitely proven active this past month!
In AGU Advances, Biemiller and colleagues present a study of splay fault ruptures and shallow earthquakes in the Cascadia Subduction Zone offshore Washington using 3D numerical simulations.
What they found: There are tradeoffs between megathrust slip and splay faults:
Greater slips on splay faults result in less frequent shallow earthquakes on the megathrust.
Gently dipping, seaward-vergent splay faults host more earthquakes than steeper, landward-vergent splay faults.
In other words, splay faults aligned with the main plate boundary tend to slip more than those faults oriented opposite or at high angles to the megathrust.
Deep marine turbidities provide significant records of subduction zone earthquakes; however, correlating turbidites over long distances and linking them to synchronous earthquakes is challenging.
A new study in Science Advances takes on this challenge in a southern section of the Cascadia Subduction Zone.
What they did: Jenna Hill and colleagues conducted an integrated study using:
High-resolution bathymetry
Subbottom profiles
Sediment cores from a key paleo-seismic site
Radiocarbon dating
What they found:
The study offers empirical data for earthquake-generated landslides on the lower slope that result in mass transport deposits and grade offshore into abyssal turbidites.
The researchers present a geologic model for seismo-turbidite generation, in which abyssal planar strata are recycled through uplift, oversteepening, and failure across the lower slope with each earthquake cycle.
Seafloor deformation and steepening are more important factors in slope destabilization than sediment compaction and dewatering.
The study offers a framework for marine turbidite paleo-seismology in other subduction zones.
About every 14 months, a magnitude 6 to 7 earthquake occurs at depths of about 35 kilometers in the Cascadia Subduction Zone beneath southern Vancouver Island. How is this episodic tremor and slip generated? Researchers don’t fully know.
A recent study published in JGR Solid Earth tackles this problem.
What they did:
Sammis and Bostock constructed a two-layer model in which stick-slip events occur at the interface between an aseismic layer and an elastic layer.
They fitted the model to a wide range of structural and seismic data collected from the study region.
What they found:
The total slip during a 14-month seismic event is between two and four centimeters, which is less than the 4.4 centimeters of plate convergence over the same period.
The researchers propose that the large but slow slip occurs within a narrow shear zone of less than 500 meters in the upper layers of the subducting oceanic slab.
The missing slip of 1–2 centimeters is accommodated by aseismic creep in an overlying layer during the 14-month period between events.
The creep is responsible for the formation of parallel seismic reflectors that overlie the elastic layer.
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A paper in Science Advances presents an interesting study of how transform faults impact the tearing of a subducting slab, and even termination of segmented subduction.
Something about subduction:
Ocean subduction is an important feature of plate tectonics that generates explosive volcanoes, earthquakes of magnitude 8.0 and higher, and devastating tsunamis. It creates tsunamis by displacing the ocean floor and overlying water column.
Slab pull is a major driving force in subduction zones, but the limit of this force is less investigated.
New study: Brandon Shuck and colleagues used seismic images from northern Cascadia, where an important triple junction brings together:
Mid-ocean ridges of Explorer and Juan de Fuca
The Cascadian subduction trench
The Queen Charlotte Fault, which accommodates strike-slip between the North American plate and the Pacific plate
What they found:
About 4 million years ago, a broad shear zone was formed by exploiting the ridge-parallel structural fabric of nascent oceanic crust.
The shear zone progressively developed into a mature, trench-perpendicular transform boundary.
This process severed an oceanic microplate and diminished its subduction relative to the adjacent subducting lithosphere.
Deep seismic interpretation reveals trench-parallel slab tears intersected by the transform fault, thus enabling separation of the ocean microplate.
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Tectonic Linkage Between the Cascadia and San Andreas Fault
Plate boundary observation station near Mt. Lassen in northern California. Sundry Photography/Shutterstock.com
Are there tectonic connections between Cascadia earthquakes and those along the San Andreas Fault in California? A study published in Geosphere suggests there may be.
New study: Chris Goldfinger and colleagues have studied the Holocene-age turbidite paleo-seismology in Cascadia and California. Their study reveals that:
During the past 3,100 years, 18 likely earthquake-generated turbidite beds were deposited in the southern Cascadia subduction zone, and there were 19 such deposits in the Noyo Channel along the northern San Andreas Fault.
Of these, eight turbidite beds were not only coeval based on carbon depth-profile dating, but also unusually thick doublets with a lower silty unit directly overlain by a sandy unit along an erosional unconformity.
Why it matters:
The southern Cascadia Subduction Zone and the northern San Andreas Fault meet at the Mendocino triple junction offshore northern California.
Stress transfer and simultaneous fault rupture on these two plate boundaries can trigger two big earthquakes.
The new study is consistent with turbidite records of the 1906 San Francisco earthquake found in both regions.
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