What appears to be such a pleasant meeting-place of the well-known rifts, in the Northern part of California, is proving to be something bigger and more dangerous than the maps indicate. Recent seismic observations suggest that the Mendocino Triple Junction, which has long been taught as a three plate intersection, has 5 moving pieces, 2 of which are not visible on the surface.
“If we don’t understand the underlying tectonic processes, it’s hard to predict the seismic hazard,” said co-author Amanda Thomas, professor of earth and planetary sciences at UC Davis.
The Mendocino Triple Junction is located out of the County of Humboldt where the system of San Andreas Faults terminates into the Cascadia subduction zone and the Mendocino Fault. At the junction, the Pacific and North American plates slide against each other in a transform boundary; to the north, oceanic crust slips under the continent in a system called subduction that is capable of creating the largest earthquakes on the planet. The area has never been silent but the new image makes it clear why: the line is not a straight scar. It is a stratified surface on which fragments slide, cling, and cut across each other in various directions and at any place, stress concentrates and rupture may commence.
Metaphor Lead author David Shelly, a geophysicist on the U.S. Geological Survey, provided a metaphor which was appropriate to the fog along the coastline and the concealed reefs. “You can see a bit at the surface, but you have to figure out what is the configuration underneath,” Shelly said.
It was a structure followed not by the shock of drama but by lividy swarms of signals that go unnoticed by the majority of the population. A system of seismometers installed in the Pacific Northwest monitored very small, “low-frequency” earthquakes deep underground – small, but very important events that show where the plates slide and creep. The model was compared to another natural metronome: the behavior of the plates to the tidal forces the slight pushes of gravity caused by the Sun and the Moon which can slightly increase the pace of microquakes when they approach a direction which the plate prefers to move.
There are two discoveries that one can make when travelling along the North Coast or a little further inland. The team discovered the evidence at the southern end of Cascadia that a piece has been dislodged off the North American plate and is being dragged along with the Gorda plate as it slides under the continent. South of the junction, in the meantime, the Pacific plate seems to be dragging away a remnant of an ancient Farallon system which was the Pioneer fragment–fault-material which makes it difficult to define because it can give rise to a nearly horizontal fault surface not visible on the landscape.
It is not just that the implications are academic redrawing of lines. The magnitude 7.2 Cape Mendocino earthquake happened in a shallower fault than many models predicted, a hint suggesting that the active interface is nearer to the surface than models had thought. “It had been assumed that faults follow the leading edge of the subducting slab, but this example deviates from that,” said tectonic geodesist Kathryn Materna of the University of Colorado Boulder. “The plate boundary seems not to be where we thought it was.”
To the residents, the take-home message is not a new prognosis, but a re-enactment of why preparedness advice is changing: hazards maps are based on the structure under the ground and the understructure is still being discovered-bit by bit, tremble by tremble.
