What appears to be three-way clash of blame somewhere off Northern California ends up being a crowded intersection with blank spots. The Mendocino Triple Junction has long been instructed as the intersection of three tectonic systems of the San Andreas Fault, the Cascadia subduction zone, and the Mendocino Fault, in remote north coast California. It already is among the most seismically active locations in the United States and it produced a magnitude-7.2 earthquake in Cape Mendocino in 1992. This is an uncomfortable addition made by a Science study: so much more like a stacked, moving puzzle, five sliding moving plates, two pieces invisible on the surface, but making the junction.
It is important since earthquake hazard models rely on the actual contact points of the plates, scraping, locking, and breaking. According to UC Davis geophysicist Amanda Thomas, when we do not know the processes underlying the tectonics, it is difficult to foresee the seismic hazard. According to the new work, some of the most significant boundaries are more superficial and elsewhere than previously thought.
The research team did the diagnosis of the problem just like engineers do when they are trying to diagnose a sealed machine, by listening to the slight internal vibration. They employed a dense network of regional seismometers to trace low-frequency earthquakes and tremor deep in the crust, or in other words, too small to be felt at the surface but in any case, abnormally informative of slow slip and frictional contact on the large faults. The study enhanced the depth and geometry of active structures by piling up a lot of small events, and subsequently verified the interpretation by tidal-sensitivity modeling which analyses whether the tremor is in response to the minor fluctuations of stress due to ocean tides.
Among the main attractions is the Pioneer fragment which is a fragment of the old Farallon plate system that used to ride along the California margin. This fragment is, the study suggests, being stretched down the North American plate at the southern end of Cascadia, and providing a fault surface that is nearly horizontal as shown in most standard hazard illustrations. The lead author, David Shelly, likened the scenario to the phenomenon of incomplete visibility on the surface: You can see a little on the surface, but you have to work out what is the configuration underneath.
The second player in the shadows is the country of North America. The researcher determines a fragment of the North American plate that seems to have fractured and is now being dragged under the sinking Gorda plate. Practically, that provides another interface through which stress may build and discharge and it alters the range of depth in which harmful rupture can take place. It is based on the same geometry that the origin of the 1992 event was so shallow – approximately 10 kilometers – when older ideas had the corresponding subducting slab deepening down to many kilometers.
The higher context, as far as hazard planning is concerned, is the fact that the Mendocino region is not an island; it is situated at the southern rim of a much greater engine. The Cascadia Subduction Zone extends between the Northern part of California and British Columbia and it can very much produce earthquakes. The Oregon emergency management guidance outlines a planning baseline whereby the coastal residents are designed to last at least two weeks following a massive Cascadia occurrence, that is likely to result in infrastructure disruption and delayed external response.
To that end, the new plate-fragment picture is not quite a one-off frightening finding, but a prompt: the most significant details of earthquakes are sometimes found under the clean lines of classroom maps. Most scientists and engineers have been given the task to update where the stress is held, how the rupture may cross-boundaries among the structures, and which communities are on top of the most shallow-rooted (and hence most destructive) of the system when the plate boundary is no longer where we thought it was, as geodesist Kathryn Materna said.
