Eruptions create new features in the Yellowstone Geysir Mound

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators at the Yellowstone Volcano Observatory. This week’s contribution is from David Shelley, a USGS seismologist.

Methods for routinely detecting and locating earthquakes are simple and powerful. With the help of computer algorithms, seismologists determine when P (and sometimes S) waves appear at seismic stations across the network. Then, using a model of seismic wave velocities in different parts of the crust, the best suitable source location is determined. Because of the uncertainties in the seismic velocity model and in the P and S wave onset times, the estimated location is never accurate.

This “independent” earthquake site works fine for public use. But in some cases, particularly when earthquakes are concentrated in space and time during an earthquake swarm or aftershock sequence, we can greatly improve results by locating a large group of nearby earthquakes simultaneously and relative to each other.

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To get a sense of relative earthquake location versus independent earthquake location, consider trying to meet a friend in a crowd near an Old Faithful geyser. If you tell your friend that you will meet him 1,000 paces in a certain direction from where you both stand, you are giving him a position that points directly to your starting point. However, your friend may have a hard time finding you because his stride length may not exactly match your stride, and his direction may be a little off. This is analogous to providing an independent earthquake location, where the estimate of the location may differ from the real location because we do not know the exact speed at which seismic waves travel in each part of the Earth’s crust.

In contrast, if you have some familiarity with the Old Faithful area, you may be able to describe a place to meet your friend based on their relative location with the known landmarks closest to your destination. For example, you could decide to meet “just outside the visitor center’s north entrance” or “halfway between the visitor center and benches towards Old Faithful.” This is similar to earthquake relative location, where earthquakes are located relative to other nearby earthquakes. This approach removes most of the effects of the non-model seismic velocity structure (think small differences in stride length), such that small differences in the arrival timing of the P and S waves of two earthquakes through the seismic network can very accurately be attributed to differences in earthquake locations. And when many earthquakes occur simultaneously, we can compute an optimal “network” of relative locations between many different earthquakes, which stabilizes the locations and helps reduce the impact of errors in determining the arrival times of the P or S wave.

In ideal cases, this measure can reduce absolute position uncertainties of 1 kilometer (0.6 miles) or more to relative uncertainties closer to 10 meters (about 33 feet). That is, even if the absolute location of the entire earthquake ensemble is not completely known, we can accurately determine the relative positions and determine the structure within the ensemble locations.

Precisely located earthquakes can produce exciting scientific insights. For example, precise locations sometimes reveal the systematic migration of an earthquake swarm over time. They can also show whether earthquakes are caused by a single fault, a group of faults, or they activate a wide volume of crust. These features can help scientists identify the physical process(s) underlying seismic activity, such as stimulation by stress transfer, water pressure, or magma movement. This provides valuable information about dynamic processes deep in the Earth’s interior, which is critical to understanding the seismic and volcanic effects of regions such as Yellowstone.

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