In September 2018 and April 2019, earthquakes of magnitude 4 and 4, 1 were recorded in the south-central part of Utah, respectively. Analysis of the data for each showed that they are radically different from the earthquakes usual for this area. And, in fact, they represent the "grumble in the stomach" of an ancient volcano, which last erupted more than nine thousand years ago.
A report by American scientists on the observations made during this period and conclusions from their analysis are published in the peer-reviewed journal Geophysical Research Letters. Among other things, the authors of the work explained why those earthquakes are so interesting and whether the residents of the state should worry. The source of seismic waves in both cases was located at a depth of only one and a half miles below the surface (slightly less than 2.5 kilometers). Which is strange, since the earthquakes that occur in the nearby Wasatch Rift occur almost ten kilometers below the surface (about six miles). But the most interesting characteristic of the tremors in September 2018 and April 2019 under the Black Rock Desert volcanic field was the absence of S-waves.
Thanks to the presence of two seismometer networks at once, the scientists managed to collect a large amount of data on these series of shocks. In fact, according to lead author Maria Mesimeri of the University of Utah, it was two sets of instruments that helped determine the nature of anomalous earthquakes. The propagation of waves was simultaneously recorded by seismometers from the Utah Regional Seismic Network and a distributed network of sensors from the FORGE project, installed by order of the US Department of Energy (DOE) to monitor sources of geothermal energy.
Interestingly, the DOE network turned out to be more sensitive than the university network - it detected a total of 35 shocks for both earthquakes against 19. And another interesting fact: additional information about the shocks was also obtained using Earth remote sensing satellites. This is usually impossible for such weak earthquakes, but in the described case the hypocenter was very close to the surface, and the orbiters were able to record its movement.
By the nature of seismic waves and the place of origin, scientists concluded that their source was the movement of a certain liquid medium. There are only two options: either water, under the influence of high temperature and pressure, was pushed through the rocks, or their displacement provoked the movement of magma. In the latter case, as Mesimeri notes, it is too early to be afraid of an eruption - nothing indicates such a possibility. But scientists will now be watching the region much more closely.
The ancient Black Rock Desert volcanic field is not to be confused with the Black Rock Desert, which is famous for its Burning Man festival, speed records and amateur rocket tests. There are about 600 kilometers between them and they are located in different states. But Black Rock Desert is located on an area of the earth's crust, which is thinning due to the constant stretching of tectonic plates moving relative to each other - the Pacific, North American and the small Cocos plate. The last major eruptions in the region occurred between nine and 12 thousand years ago, with the last noticeable activity about 660 years ago.