Earthquake Focal Mechanism – Life is like a beach (ball)

The Yellowstone Caldera Chronicle is a weekly column written by Yellowstone Volcano Observatory scientists and collaborators. This week’s contributor is David Shelly, a seismologist with the U.S. Geological Survey.

When an earthquake occurs, the first characteristic usually reported is the earthquake’s Place and MagnitudeBut with a little more analysis, we can often use earthquakes to gain information about the orientation of the fault that caused the earthquake and the direction of slip on that fault.

The orientation of the faults and the direction of slip of an earthquake is described by what is called the focal mechanism. Focal mechanisms are often visualized using so-called “beach ball” diagrams. These diagrams are circles with various shades that form arcs on the circles – hence the name beach ball.

Beach ball diagrams can be confusing, but with a little explanation and experience, anyone can use them to better understand the characteristics of an earthquake. The diagrams depict an earthquake “source ball” — a sphere containing the faults that slip in an earthquake. Depending on the direction the earthquake waves leave the source, the initial motion of the waves may be compressional (usually shown as black or gray quadrants in the beach ball) or tensile (usually shown as white in the beach ball). And, depending on where seismic stations are located relative to the earthquake’s surface, they receive waves from different parts of that source ball. When the first part of the earthquake wave hits, some stations feel a push (compression), while others feel a pull (tension).

Depending on the size of the earthquake, seismologists use different methods to determine the focal mechanism. For moderate to large earthquakes (typically magnitude 3.5-4.0 or larger, depending on seismic network quality and noise levels), seismologists can model the earthquake source and fit long-period (~10 second period or longer) ground motion records recorded at multiple locations. This is called the moment tensor.

But smaller earthquakes don’t produce enough low-frequency energy to use this technique effectively, and we generally don’t know the Earth’s structure in enough detail to calculate the moment tensor using high-frequency seismic waves. Therefore, the focal mechanism of a small earthquake is usually calculated primarily from the direction of motion of the observed first-arriving seismic wave. As mentioned earlier, this motion can be compressional (upward or away from the source) or tensional (downward or back toward the source). If we have a dense network of local earthquakes, these observations can be combined to determine the focal mechanism. But if there aren’t enough seismic stations, or if the earthquakes are too small to be clearly recorded at many stations, the focal mechanism may not be determined.

Once we calculate the focal mechanism, we know the exact orientation of the fault plane, right? Unfortunately, in a cruel twist of physics, each focal mechanism has two possible fault plane orientations. One of them is the fault plane, and the other is perpendicular to the fault plane, known as the “auxiliary plane.” Earthquakes in either orientation will produce nearly identical seismic waves, so to distinguish between the fault plane and the auxiliary plane, we need more information. Sometimes, we use the alignment of multiple earthquake locations or analysis of rupture directions to distinguish between the fault plane and the auxiliary plane, or we can determine the fault plane based on our knowledge of the stress field in the area. Likewise, the stress field can be estimated by analyzing many focal mechanisms (but that’s another topic).

Our discussion so far has assumed that earthquakes occur on a single planar fault. This is called a “double-coupled” event. But is nature always that simple? Of course not! While this is a reasonable approximation for most earthquakes, we know that some earthquakes are more complicated. They can occur on non-planar faults, they can be generated by slip on multiple faults, or, especially on volcanoes, they can sometimes be generated by other processes, such as the growth and collapse of bubbles. In these cases, the earthquakes may be identified as “non-double-coupled” events, and their beach ball representations may become a little less beach ball-like!

The main point is that deriving beach ball plots from earthquake data can be a challenge, but they also provide important information about how and why faults slip during earthquakes.

So what about the focal mechanism of Yellowstone? Although there are some exceptions, most Yellowstone earthquakes fit this “two-couple” model. They are mostly caused by “normal” faults (caused by extension, including a vertical component, common in the western United States), with some “strike-slip” (horizontal motion) events. In fact, the two largest earthquakes recorded in the area, the 1959 Hebgen Lake M7.3 earthquake and 1975 Norris Junction M6 earthquake They are mainly normal fault events.

So, the next time you’re at the beach discussing earthquakes (most people talk about earthquakes when they’re at the beach, right?), you can impress your friends by grabbing the nearest inflatable beach ball and teaching them about focal mechanisms and earthquake faults!