At the Mendocino triple junction, located offshore from Northern California, three substantial tectonic plates converge. A recent investigation has illuminated that the subterranean geological configuration is considerably more intricate than previously understood by prevailing models.
Researchers from the United States have conducted a novel analysis of subtle, low-frequency seismic events. These events, captured by seismometers deployed across the Pacific Northwest, have revealed the presence of previously undetected fault lines.
The outcomes of their research indicate that the triple junction is not merely comprised of three distinct plates, but rather functions as an assembly of five actively moving segments.
Consequently, current predictive models for seismic activity may necessitate revisions to enhance the accuracy with which experts can forecast the timing of subsequent significant tremors. The investigators liken this discovery to the process of examining the submerged portion of an iceberg, observing that while the visible part offers clues, the underlying structure requires careful deduction.
“One can perceive a portion at the surface, yet the underlying arrangement must be ascertained,” states seismologist David Shelly, affiliated with the Geologic Hazards Center operated by the United States Geological Survey.
In addition to analyzing data from seismometers – instruments designed to detect minute ground vibrations from imperceptible seismic events – the research team corroborated their empirical field recordings by employing tidal-sensitivity models.
The diurnal ebb and flow of ocean tides impose incremental stresses upon the subterranean rock formations. By meticulously modeling these stresses to evaluate the rock’s response, scientists are able to validate their interpretations of low-frequency seismic phenomena, a verification that proved accurate in this instance.
The investigators have determined that a segment of the North American plate has fractured and is being drawn downward in conjunction with the Gorda plate (also known as the Juan de Fuca plate). Furthermore, the team confirmed the prior theoretical hypothesis regarding the existence of the Pioneer fragment, identified as an older rock section being dragged beneath the North American plate.
The convergence point known as the Mendocino triple junction is defined by the North American plate, the Gorda plate, and the Pacific plate. Within this configuration, the Gorda plate is undergoing subduction, whereby it is forced beneath the North American plate and subsequently assimilated into Earth’s mantle. Critically, the depth of this subducting boundary has been found to be less than previously estimated.
This investigative work prompts a reassessment of the most probable location for the plate boundary. The refined model is supported by empirical evidence from a 7.2-magnitude earthquake that occurred in California during 1992, which originated at a significantly shallower depth than contemporary geological models would have predicted.
“It was presumed that fault lines would align with the leading edge of the subducting slab; however, this particular incident diverges from that assumption,” explains tectonic geodesist Kathryn Materna, from the University of Colorado Boulder.
“The plate boundary appears to be situated elsewhere than our prior estimations suggested.”
Precision is paramount when forecasting seismic events, and it is in this domain that the present study is poised to offer the greatest utility. Both the San Andreas fault, marking the confluence of the North American and Pacific plates, and the Cascadia subduction zone, where the Gorda and North American plates interact, are capable of precipitating catastrophic earthquakes.
A multitude of dynamic factors must be considered concerning the tectonic fault systems and earthquake zones along California and the broader western seaboard of the United States. Scientific endeavors are continuously underway to develop the most comprehensive mapping of these geological features, thereby ensuring maximum preparedness for anticipated ground movements.
“Without a thorough understanding of the underlying tectonic processes, the accurate assessment of seismic hazards becomes exceedingly challenging,” comments geophysicist Amanda Thomas, affiliated with the University of California, Davis.
