Seismic events at the San Andreas Fault could be started by activity at the Cascadia subduction zone, according to new investigations.(Image credit: Cavan Images / Peter Essick via Getty Images)
A substantial earthquake within the Cascadia subduction zone, located in the Pacific Northwest, may spark an equally critical earthquake within the San Andreas Fault located in California, recent investigations indicate.
The conclusions are founded on sediments acquired from the ocean bottom off the shore of Cape Mendocino, California, including offshore Oregon. California’s well-known San Andreas fault finishes and the Cascadia subduction zone initiates at Cape Mendocino.
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However, if the two fault systems are indeed interconnected, it could pose a significant problem for providing aid after disasters, clarified Chris Goldfinger, a paleoseismologist, emeritus professor at Oregon State University, and the study’s primary author.
“Dealing with these events simultaneously would present a major challenge,” Goldfinger mentioned to Live Science. “There may not be adequate resources available to effectively respond to even a single one of these, and managing two would be extremely challenging.”
Earthquake potential
Cascadia is capable of producing very intense seismic events. For instance, the area famously saw an earthquake of an estimated magnitude between 8.7 and 9.2 during the year 1700, resulting in destructive tsunami waves that affected even Japan. Such earthquakes result from the movement of three oceanic plates—the Explorer, the Juan de Fuca, and the Gorda—as they slip under the North American continent.
Conversely, the San Andreas Fault is regarded as a strike-slip fault, denoting rock masses move horizontally past one another on opposite sides of the fault. The San Francisco earthquake of 1906, which registered at nearly magnitude 7.9, remains the most significant known earthquake on the northern stretch of the San Andreas. Because of the fault’s presence through densely inhabited areas, it has the capacity to inflict considerable devastation, similar to the Loma Prieta earthquake during 1989 that caused 63 fatalities.
The two fault frameworks join off the coastline of Mendocino within a location recognized as the “triple junction.” During a research expedition within 1999, Goldfinger in conjunction with his team were engaged in drilling core samples originating from the ocean’s floor inside Cascadia, aiming to locate indications of ancient earthquakes. When extensive earthquakes manifest inland, they are capable of activating underwater movements of sand in addition to sediment, commonly referred to as turbidites. Turbidites follow a distinctive pattern where coarser sediment precipitates from the water initially, thereby creating a layer. Progressively finer sands in addition to silt subsequently trail, leading to the creation of another distinct layer.
Nevertheless, a miscommunication throughout that specific excursion caused the vessel to deviate by a span of 60 miles from its intended location. The team of scientists, while intermittently resting among duties, solely came to the realization of this oversight upon the ship’s eventual arrival.
“I was really thinking, ‘Oh no, we’re practically midway toward San Francisco,'” Goldfinger recollects.
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Nevertheless, he chose to procure a core sample from that spot in any case. Upon later assessing the sample, the team understood that it held a puzzling aspect. Unlike the expected coarse layer at the base and a finer layer at the summit in typical turbidites, those contained in this specific sample exhibited an anomaly.
“This initial San Andreas core showcased deposits exhibiting an apparently inverted arrangement,” Goldfinger noted. “The sand was positioned at the highest point.”
Inverted Evidence
The scientists initially lacked a justification intended for this flipped arrangement. Furthermore, they formerly lacked a rationale pertaining to another puzzling facet from these offshore procured samples: cores obtained southward of the triple junction, within the jurisdiction related to the northern San Andreas, seemingly showed seismic events that closely matched the timings exhibited with seismic events noticed toward the northern section of the triple junction located inside Cascadia. Records spanning the last 1,300 years disclosed the finding involving approximately 18 turbidites suspected of having resulted from earthquake activity existing inside Cascadia in conjunction with 19 additional samples gathered away from the northern portion linked to the San Andreas. Furthermore, it turned out that ten of those showed up being positioned anywhere between 50 up to 100 years within each other.
Core samples indicate that at least three earthquakes at the Cascadia subduction zone were followed by events at the San Andreas Fault within just hours or days.
Furthermore, on three events, the heavy sand comprising the elevated stratum tied to those inverted turbidites mixed among the fine-grained sand from within the lower stratum, implying the upper stratum resolved while the lower stratum remained subjected to activity. Which implies the two strata formed merely hours or a few days from one another. Included among these three incidents were the 1700 Cascadia quake in addition to quakes occurring around 1,200 years prior including the event around 1,500 years earlier.
Many years involving additional radiocarbon dating passed, corresponding seismic records accumulated originating from diverse sources like lakebeds across California, and the significance associated with all of these bizarre “doublet” turbidites got considered at great length. However, Goldfiner eventually said he came to realize that these San Andreas turbidites might signify two separate seismic events: one initiated originating from the far-off Cascadia area, solely stirring up the light-weight silt coupled with sand, coupled with the next emerging following a shortly-following San Andreas seismic event displaying greater local intensity that resulted in being qualified to shift material exhibiting coarser qualities.
“That particular detail may clarify it,” Goldfinger said. “It explains the correlation involving ages… what ultimately lifted the barrier.”
In their new report released on Sept. 29 appearing inside the journal known as Geosphere, the researchers contend that large seismic activities within Cascadia have got the aptitude to transmit stress toward its neighboring San Andreas, thus creating a San Andreas seismic event shortly afterwards.
Harold Tobin, a seismologist belonging to the University of Washington who possessed zero participation related to the investigation specified that the mechanism associated with earthquakes initiating other earthquakes exists as not entirely uncommon. Nonetheless, those cases mostly transpire inside identical fault boundaries.
“Genuine examples wherein disparate kinds consisting of plate-boundary faults get so closely interconnected to the point demonstrated throughout this current document, seem generally absent,” Tobin stated during his conversation with Live Science. He followed by adding that although the task was diligently executed, “for my assessment, it could demonstrate far too premature toward conclusively determining whether alternative justifications appropriate to such sedimentary deposits get lacking or not.”
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Tobin stated the Cascadia along with its northern San Andreas region reveal elevated seismic activities, whereby abundant additional faults hold capacity to trigger seismic events. Decoding sedimentary deposits can be complicated while uncertainties exist that often come associated with carrying out radiocarbon dating.
“A considerable range of rationales could certainly describe what constitutes a complex system,” Tobin said. “Even though this consists as an interesting collection exhibiting specific observations, more comprehensive analyses may become essential prior to corroborating all findings.”
Goldfinger expressed his intention for this task toward stimulating Cascadia geologists alongside San Andreas geologists toward synergistically improving collaboration aimed to perform such thorough analyses.
“Everyone exhibits ample learning opportunities with each other,” he expressed. “I am inclined towards elevating the level involving scientific methodology originating across both ends.”
Stephanie PappasSocial Links NavigationLive Science Contributor
Stephanie Pappas contributes for Live Science, covering a variety of topics ranging from geoscience to archaeology, not forgetting the human brain and behavior. Although previously functioning as a senior writer on behalf of Live Science, she presently operates at Denver, Colorado, working freelance and consistently contributing to Scientific American coupled with The Monitor, in addition to the American Psychological Association’s monthly magazine. Stephanie secured a bachelor’s certificate spanning across psychology originating from within the University relating to South Carolina, including procuring a graduate-level certification about the skill of science communication when situated within the University relating to California, Santa Cruz.
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