"We found this very interesting feature in the data," she told Life Science: Signal Patterns of Crustal Expansion and Contraction. Now, in a study describing this discovery in the Journal of Geophysical Research, Kraner and her colleagues also explored whether the seasonal fluctuation of local aquifers can explain this cycle of expansion and contraction, which itself may cause earthquakes. [Seven Changes of the Earth in the Blink of an Eye]
When an earthquake occurs, slowly moving tectonic plates suddenly slide against each other, usually due to the accumulation of stress, which makes them unstable. "It's like breaking a stick," kleiner said. If you pull it and pull it again, it will eventually reach the breaking point.
However, the crustal fluctuation observed in kleiner indicates that there may be an additional seasonal factor that triggers this process. She determined this model by collecting data from Earthscope, a high-precision GPS sensor network all over the earthquake-prone areas in the western United States. Since 2005, these sensors have been recording the millimeter displacement of the earth's crust and established a huge and detailed data set. Using this information, Kraner can accurately describe the expansion and contraction of the crust around the earthquake zone.
The crust is in a state of extension in late summer and contraction in winter. "You see this happening every year in that place," kleiner said. "Our theory is that this seasonal factor provides the last straw for earthquakes," Kraner told Life Science.
But what is the reason behind this unusual seasonal cycle and its relationship with earthquakes? After the team ruled out several other factors, "the only thing we can think of is that it is related to some local aquifer systems," Clana said.
To verify this idea, Kraner used satellite data of Napasonoma Valley, which showed that the ground elevation of aquifer basin had obvious seasonal variation. [Photo Gallery. The devastating earthquake of the Millennium]
As we all know, the rise and fall of groundwater level will lead to the rise and fall of surface elevation. Kleiner didn't measure the amount of water in these basins, but she speculated that if the groundwater level did drop in summer due to water shortage, the land above would sink. When this happens, it will pull the crust, make it shrink horizontally, and "stretch" or "expand" the land around the aquifer. According to this logic,
If the aquifer basins on both sides of the fault line retreat, it will also lead to horizontal contraction of the land on both sides of the fault. (In fact, the GPS records in kleiner data show that in summer, the distribution range of GPS on the surface is 3 mm) Just like loosening the clamp, this will release the stable pressure on the fault, making it easier for the plates to slide against each other and cause earthquakes. Kleiner explained: "The earthquake occurred in the expansion/extension area between these two aquifers."
Does it mean that human-driven aquifer depletion will start this process and trigger an earthquake? Kleiner warned that the study could not answer this question. Even so, the characteristic of this area is the large water consumption of vineyards, which may lead to the surface shrinkage of dspe in this area.
Above the aquifer, California does have wineries. They are pumping water. We don't know how many. In dry summer and during drought, the dependence on groundwater may increase. But Kraner soon noticed that her research did not measure the water quantity or pumping rate before and after the earthquake.
She said that due to rainfall, evaporation and other natural phenomena, the groundwater level will naturally fluctuate.
Kleiner's research shows that seasonal stress in the earth's crust may be one of the many factors leading to earthquakes. She said that understanding this seasonal factor may one day help scientists make richer and more accurate predictions about these phenomena.
"You increase the time dependence of earthquake prediction." This is very important to understand how earthquakes are triggered.
Editor's Note: This article is updated as a statement, not an earthquake record. As mentioned above, scientists have checked the high-precision GPS time series.
Original articles on life sciences.