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Caldera records: How earthquakes occur in the Yellowstone region | regional news
Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators at the Yellowstone Volcano Observatory. This week’s contribution is from Jamie Farrell, associate research professor at University of Utah seismic stations and chief seismologist at Yellowstone Volcano Observatory.
Over the past decade, the University of Utah Seismological Stations (UUSS) have identified an average of about 1,570 earthquakes in the Yellowstone region each year—an average of several earthquakes per day.
The largest number in any year during that period was 3,427 in 2017, and the lowest was 708 in 2011. Seismic rates fluctuate naturally, but most of the variance in Yellowstone depends on the number and size of earthquake swarms that occur during the year. For example, in 2017, more than 2,400 of the earthquakes that occurred that year were part of a single swarm – the Maple Creek earthquake swarm.
When an earthquake occurs and you see vibrations on the seismogram, the first question to answer is: “Where is the earthquake?” This is important for a number of reasons. First, the site can find out what fault caused the earthquake. Second, and most importantly, in the event of a major and potentially devastating earthquake, its location could be used to help guide first responders.
How do earthquakes occur in the Yellowstone region? Before we talk about it, let’s correct some terminology. Sometimes people talk about epicenter or low epicenter, and these two terms can be confusing. The hypocenter is the point in the Earth where the earthquake occurs and includes longitude, latitude, and depth (X, Y, Z). The epicenter is the point on the Earth’s surface directly above the hypocenter and includes only the longitude and latitude (X,Y).
When an earthquake causes a fault at depth, it generates seismic waves that radiate in all directions from the lower center. Depending on the type of wave and the characteristics of the rock it travels through, waves travel at different speeds. These waves are then recorded on a network of seismometers in the Yellowstone region. The two waves that are most important for locating an earthquake are the initial wave (P wave) and the shear wave (S wave). On average, the speed of the P wave is about 5-8 kilometers per second, and the speed of the S wave is about 3-4.5 km/s through solid rock. Because of the different speeds, the P wave always arrives first, but due to the nature of waves, the S wave that comes later is stronger.
All seismic data is wirelessly transmitted to UUSS in real time and recorded and analyzed on a sophisticated seismic computer processing software that automatically picks the arrival times of P waves at as many seismic stations as possible, then calculates the location and magnitude of the earthquake. Depending on the earthquake’s magnitude and location, the system sends alerts to several UUSS employees, including a duty seismologist — a rotating position held by a seismologist that responds to earthquake alarms 24 hours a day, seven days a week.
The seismologist on duty reviews the earthquake to ensure that it is a true local earthquake and not an out-of-area misleading event, sonic boom, or other event, and then evaluates the computer-specified location and magnitude for accuracy. Computer calculations of location and magnitude are usually close, but always require some optimization by a trained analyst (eg selection of S-waves for better depth estimates) – which is why earthquake magnitude and exact location sometimes change after the initial report. With more data carefully analyzed by an experienced seismologist, the magnitude and location can be determined with greater accuracy.
Determining the location of the hypocenter is more complicated than simply choosing the arrival times of the P and S waves on multiple stations. Once these arrival times are determined, the UUSS uses the Yellowstone region seismic velocity model to simulate the propagation of seismic waves given an initial epicenter. The velocity model describes the speed at which seismic waves travel through the Earth at a given depth. The computer algorithm then compares the arrival times at each station of the simulated run against the actual arrival times chosen by the analysts. The differences between the two are called “incompetent”.
Then the computer program chooses a different core and does the simulation again and calculates a new error. It does this over and over until it finds the smallest error between the simulation data and the actual data. This becomes the final site of the earthquake. It can take up to several hours to zero in on the earthquake source, depending on the magnitude of the earthquake and the number of stations recording the vibration.
In general, to find the epicenter of a hypocenter, you need to monitor the earthquake at at least three seismic stations. If you also want to calculate the time when the earthquake occurred, you need at least four observations. However, to get the best possible hypocenter solution, you would ideally have several stations located throughout the earthquake and at least one station close to the event.
The more stations that record the earthquake, the more accurate the location. On the contrary, if the earthquake is recorded on only one or two stations, its location cannot be determined. This is why seismic reports tend to say “how many earthquakes there are” and not “how many earthquakes have occurred” when giving earthquake statistics for Yellowstone on a particular day, month, or year.
Of course, this procedure only covers locating the earthquake. But what about size? For this story, check out a previous Caldera Chronicles article on the topic at https://www.usgs.gov/center-news/how-big-was-earthquake.
Determining the location and magnitude of an earthquake requires a modern seismic network and a team of trained analysts who are available to examine the data 24/7. Yellowstone is fortunate to have both, thanks to seismologists at the University of Utah seismic stations. For the latest information on earthquakes in the Yellowstone region, visit the UUSS website at https://quake.utah.edu.
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Sources 2/ https://www.kpvi.com/news/regional_news/caldera-chronicles-how-earthquakes-are-located-in-the-yellowstone-region/article_a5d98712-fee7-5c03-9e62-70d0ed920b27.html The mention sources can contact us to remove/changing this article |
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