What are the Three Plates that Make up the Mendocino Triple Junction?

Caitlin Dempsey


The Mendocino Triple Junction is located in the Pacific Ocean near Cape Mendocino in northern California.  This area is the most seismically active active regions of the San Andreas transform system [1].  

What is a triple junction?

A triple junction is a point on the Earth’s surface where the boundaries of three tectonic plates meet. At a triple junction, three plate boundaries come together to form a node, where three plate boundary types can coexist such as divergent, transform or convergent.

Plates are either ridges (R), trenches (T), or transform faults (F).

What is the Mendocino Triple Junction?

The Mendocino Triple Junction is the point where the Gorda Plate, the Pacific Plate, and the North American Plate meet.

This area is active tectonically, and the junction is the site of frequent earthquakes as well as active seafloor spreading. It is also the location of the Mendocino Fracture Zone, which is a significant transform fault that accommodates much of the relative motion between these three tectonic plates.

Seismologists have measured over 80 earthquakes each year since 1983 that are magnitude 3.0 or greater.  This seismic activity is due to the plate motions between the three plates of the lithosphere that form the Mendocino Triple Junction.

The Three Plates of the Mendocino Triple Junction

The concentration of earthquakes at Cape Mendocino is known as the Mendocino Triple Junction.  This junction is where the Gorda plate (a south section of the  Juan de Fuca Plate), the North American plate, and the Pacific plate meet.

This map shows the triple junction (red arrow) off the coast of California in the United States.

Map showing the Mendocino Triple Junction off the Pacific Coast of California.
Map showing the Mendocino Triple Junction. Map: Plate GIS data from An updated digital model of plate boundaries by Peter Bird, 2003. Base layers from Natural Earth. Map: Caitlin Dempsey.

Access GIS tectonic plates and boundaries data

The GIS tectonic plates and boundaries data used to create the map showing the three plates that form the Mendocino Triple Junction was accessed from Github and is a conversion of the dataset originally published in the paper An updated digital model of plate boundaries by Peter Bird (Geochemistry Geophysics Geosystems, 4(3), 1027, doi:10.1029/2001GC000252, 2003).


[1] Oppenheimer, D. (2014).  Mendocino Triple Junction Offshore Northern California.  Retrieved from https://woodshole.er.usgs.gov/operations/obs/rmobs_pub/html/mendocino.html


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About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.

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