The worst case scenario is not what you think, warned Chris Goldfinger, a marine geologist at Oregon State University, when talking of the danger of two of the most notorious fault lines of North America. Throughout decades, the residents of the West Coast have been waiting anxiously the much-anticipated occurrence of the Big One that is along the San Andreas Fault or the Cascadia Subduction Zone. However, recent studies indicate that these seismic mega-thrusts might occasionally collide in wave a scenario that will saturate emergency services in California and British Columbia.
This disturbing connection came about through a long and intensive geological detective work that comprised the deep-ocean sediment cores, radiocarbon dating, and high-resolution seismic profiling. The results do not simply contribute to the scientific knowledge; they completely renegotiate the magnitude of possible disaster scenarios of millions of people who occupy the Pacific margin. The main scientific revelations relevant to this listicle are condensed, so the residents can know what is at stake and why preparedness needs to change.
1. Two Faults, One Catastrophe
The Mendocino Triple Junction off northern California is the intersection of the San Andreas Fault, a 750-mile strike-slip boundary and the Cascadia Subduction Zone where oceanic plates slide under North America. Scientists have discovered that such systems can occur in the form of major earthquakes lasting between a few hours to a few days apart. Goldfinger warned that a collision of the two would happen soon and cities such as San Francisco, Portland, Seattle, and Vancouver would be under simultaneous disaster by the time the rest of the country reacts, pushing disaster response boundaries.
2. Evidence There Hides in the Ocean Floor
This was finally broken when scientists in the year 1999 drilled sediment cores at the San Andreas zone as they sought Cascadia after a navigational error. The comparison of the cores showed that there were similar sequences of turbidite deposits of underwater landslides down both faults over the last 3,100 years. These sequences indicated some patterns of quakes in one system to cause quakes in the other system otherwise referred to as partial synchronization.
3. The Upside-Down Sediment Mystery
Ordinary turbidites consist of coarse sands on the bottom and fine silt on the top. However, at the Mendocino Triple Junction, inverted layers on cores, coarse sand, over fine silt, were used to indicate two different quakes, immediately following each other. In others, the upper layer consolidated and the bottom one was still moving, that is, the events were only separated by hours. This peculiar stratigraphy was one of the major identifiers of conjoined earthquakes.
4. Historical Doublets
Sediment record analysis showed that there were no less than eight big San Andreas earthquakes in the last 3,000 years, all in the decades after significant Cascadia earthquakes. It is interesting to note that the 1700 Cascadia quake, which had reached the magnitude of 9.0, and had caused tsunami waves to reach Japan, seems to have been closely followed by a rupture of the north San Andreas. These historical doublets contravene the belief that great earthquakes in other systems are independent.
5. Between Faults Stress Triggering
The rupture of one fault may also cause a stress to be passed on to other faults, which may lead to a second quake. Goldfinger compared it to the act of tuning an old radio: when these faults get into phase with each other, one fault may then tune the other and produce earthquakes in pairs. This process which is witnessed in smaller fault systems is now heavily evidenced on a continental scale.
6. Scientific Accuracy and Problems
The method of determining ancient quakes is based on radiocarbon datings of microfossils in sedimentary layers, but ocean mixing and erosion add decades-centuries ambiguities to dating. To deal with this, scientists employed age depth Bayesian models and cross-checked offshore turbidite data with onshore records (e.g. coastal subsidence and lake sediments). This multi-disciplinary method enhances correlations even with the existing dating difficulties.
7. The Turbidity of the Sea: Its use in Paleoseismology
Turbidites are used to record past seismic events, which is a physical record of underwater avalanches resulting because of a strong shaking. Though they may be also caused by floods or storms, the recurrence patterns over long distances, along with age similarities, both testify to the earthquake origins. Marine turbidite records in Cascadia tend to give more seismic-long term information than proxies onshore.
8. Not Always in Sync
Although the coupling is observed, the faults do not necessarily rip apart. Acts such as the San Francisco earthquake of 1906 were individual San Andreas faulting, not seen in the Cascadias. This fluctuation implies that the synchronization can occur, but it is not a given which complicates hazard prediction.
9. Preparedness Implications
New emergency planning is required by the potential to occur simultaneously, or nearly, near-simultaneously, with both megathrust and strike-slip earthquakes. The first disaster could use up the recovery resources leaving a small amount to the second disaster. As Goldfinger stressed, the results must encourage the leaders in the West Coast to prepare in the case of multi-event scenarios with geological information being incorporated into infrastructure resiliency and safety of population measures.
The new image of Cascadia and San Andreas as a possible and regularized system is scientifically intriguing and very disturbing. To the West Coast, it highlights that the earthquake preparedness cannot be specific to the threat posed by one fault. Knowledge of the geological interaction of the two giants is not an academic issue, but a question of survival.
