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3d ago

Tiny Quakes Expose Five-Piece Tectonic Model at Mendocino Triple Junction

Researchers mapped concealed plate fragments with tidal-sensitive microearthquakes to clarify the shallow source of the 1992 Cape Mendocino quake.

Firefighters respond after a home caught on fire in Rio Dell, Calif., on Wednesday, Dec. 21, 2022. A 6.4 magnitude earthquake struck the coastline near Eureka early the prior morning. 
A crack in the seabed caused by tectonic plates, not associated with this study. 
A 1992 photograph of a car crushed by falling bricks from a house, triggered by the magnitude 7.2 Cape Mendocino earthquake. A hidden tectonic plate boundary may have been the source.
Image

Overview

  • The Science study from USGS, UC Davis and CU Boulder proposes five moving pieces that replace the long-held three-plate picture at the junction.
  • South of the junction, the Pacific plate is dragging the Pioneer fragment beneath North America along a nearly horizontal boundary invisible at the surface.
  • At the southern end of Cascadia, a piece of the North American plate has broken off and is being pulled down with the sinking Gorda plate.
  • The newly imaged geometry offers an explanation for the unusually shallow rupture of the 1992 magnitude 7.2 Cape Mendocino earthquake.
  • The authors and outside seismologists say the hidden structures could represent unaccounted earthquake sources and may raise the odds of Cascadia–San Andreas interaction.