Last February, a 7.8 -magnitude earthquake shook the Turkish -Syrian border, followed by an earthquake with the same power after approximately 9 hours. The shallow rust, which is located at a depth of less than 29 km below the surface, exploded and caused violent focused tremors that led to the settlement of thousands of buildings on the ground and killed tens of thousands.
Similar shallow rusts exploded about 1,000 years ago in the Boujuj Lolands region in West Washington, and according to a new research conducted by the University of Arizona, the tree rings helped to determine that the seismic event occurred in late 923 AD or early 924 AD.
The results that the research team – which was published in the journal Sains Advanz ‘means that a similar event can shake the region that now includes more than 4 million people again, including Seattle, Takoma and Olympia.
The ancient earthquakes were either the result of all the shallow faults in the area bursting together to produce a 7.8-magnitude earthquake, or, as happened in Turkey and Syria, two back-to-back twin quakes with estimated magnitudes of 7.5 and 7.3. Shallow faults usually cause more violent and concentrated shaking than earthquakes caused by other geological formations.
While earthquakes are not new to the Pacific Northwest, the study determined that events on these shallow faults are linked to each other in some way, either by underground communications or by one fault transferring stress to another fault, and regional hazard models, used to develop Structural engineering designs do not currently reflect this possibility, but they should.
Scientists have been exploring shallow faults in the area since the 1960s, with the Seattle Fault being discovered first, followed by the Saddle Mountain Fault, which runs along the eastern slopes of the Olympic Mountains, and the Tacoma and Olympia Fault.
These four shallow faults had ruptured nearly 1,000 years ago in a group of earthquakes called the ‘Millennium Group,’ causing local tsunamis and landslides that stripped the mountainsides of entire forests and dumped them into nearby Lakes Washington and Sammamish.
Until now, scientists were not sure when and how these rifts last appeared. They may have occurred at the same time, or hours or centuries apart. Scientists were not sure, so the research team led by Brian Black turned to the trees.
With the passage of every year, the trees add a ring around their trunks. The display of the episode is determined according to the climate you live. Facant conditions mean wider rings and unfavorable conditions means thinner rings. Given the different climate from year to year, it creates specific time patterns such as the tape code (or barcode) in the growth of trees inside the region.
Researchers in the science of tree ages can match the time specified growth patterns in dead trees with patterns of living trees. If there is an overlap with living trees, the exact dates in which the trees lived can be determined before they die. This was the approach used to determine when the trees died by the earthquake in the Boujih area.
In 2021, Black traveled to the northwestern mountains of the Pacific Ocean to participate in harvesting the trunks of trees that died when the Rift of Jabal Sadeer besieged a waterway flooding the forest, where the lake in which the trunks of these sunken trees are still standing until today, where divers jumped into the water to cut Samples of trees that were killed when the lake was formed from the Millennium Earthquake Group.
Black and his team also had parts obtained from the nearby trees that were killed almost at the same time during a rock ice collapse that resulted in the detention of a waterway immersed a nearby river.
They also obtained parts of the trees that were collected more than 30 years ago, which were drowned in landslides in Lake Washington and Lake Samamish during a major earthquake in Seattle.
When Black compared the patterns of growth, he noticed that the trees died exactly in the same year with the crack of Jabal Sad and Sayyel. To determine an accurate evaluation year for death, the team built a 1300 -year -old chronology of live trees, but very old – which when they match with the trees that the earthquake killed – showed that the inactive death season was in the late 923 m to the early 924 m.
“Our team was also lucky with a huge solar storm between 774 AD and 775 m, which caused a sudden global rise in radioactive carbon,” said Charlotte Pearson, a assistant professor in the history of trees history and the author participating in the study.
Where radioactive carbon fluctuations, such as climate, can be used to determine the history of tree rings. “We measured radioactive carbon in the tree rings killed by the earthquake to show that this height happened where we thought it should happen ‘, and this independently confirmed the date of the earthquake.
Narrow time interval
“The evidence combined showed us that these trees from all parts of the region died together, and this was in fact a associated event,” Black said. As Black confirms, the study of his team demonstrated the occurrence of the two earthquakes with a narrow time separation on these two righteousness.
As he says, it is different if we have earthquakes on these two righteousness, separating them 100 years or separating them only 100 hours. Hence, the study confirms that shallow rifts can explode simultaneously or in a very rapid sequence ‘and that this changes the known concept of rifts in the region.
Nevertheless, current risk models do not recognize the possibility of synchronous or consecutive earthquakes in a short separation as a result of shallow cracks, and fortunately – according to his saying – that the greater and more severe earthquake, the less frequency, so, while the earthquakes of this size may be Best to the region, but it is relatively uncommon.