Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Jake Lowenstern, Chief of the USGS Volcano Disaster Assistance Program and former Scientist-in-Charge of the Yellowstone Volcano Observatory.

How do you describe the state of activity of a volcano to the public, or even to other scientists? Those of you who track volcanoes around the world might notice that different countries take different approaches, and that their descriptions often differ based on the kind of volcano, its level of activity, and even the number of monitoring instruments. 

To start, most of you are aware that volcano scientists often use alert levels or color codes to communicate the amount of activity at a volcano.  Yellowstone caldera has been at “Normal” since YVO was founded in 2001, consistent with the observation that even when there are changes in seismicity, deformation, and geyser activity, Yellowstone’s behavior is well within background levels observed over the ~150 years of the historical record.

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Alert levels are accompanied by updates that provide details and justify the current alert level. In their updates, volcano observatories use a wide range of indicators to track volcanic activity and the potential for volcanic eruptions.  Examples include gas discharge, heat flow, ground deformation, infrasound, and the most common indicator, earthquake activity.

In its monthly updates, YVO always provides a discussion of earthquakes.  Because earthquakes occur frequently, and all over Yellowstone National Park, and because the Yellowstone Seismic Network contains dozens of sensors, YVO has the ability to precisely characterize an earthquake’s size, location, and depth.  The earthquake information is provided in near-real-time on the YVO and UUSS website, and monthly updates describe the number and locations of earthquake swarms (groups of earthquakes with similar location and timing).  At Yellowstone, virtually all earthquakes are brittle-failure events due to cracking rock, so there is rarely need for discussion of other sources of seismicity.

However, other situations frequently exist.  For example, this summer, the beautiful cone-shaped Mayon Volcano in the Philippines has been erupting.  The Mayon seismic network is sufficient to calculate earthquake locations, but because all activity is near the summit, where lava is erupting, the volcano scientists of PHIVOLCS choose to focus on other information.  For example, they can detect rockfalls and pyroclastic bursts (landslides of searing ash) with their seismometers, as well as more traditional earthquakes.  They prioritize discussing the numbers of those varied events at the summit, rather than the actual locations of earthquakes in their daily volcano updates. They then describe visual observations on the lava effusion, ash plume height, thermal measurements, and estimates of sulfur dioxide gas discharge from the summit.

The staff at CVGHM in Indonesia oversees over 70 active volcanoes, many of which have only one or two seismic stations. Accurate locations are impossible with so few seismometers, so instead the CVGHM staff discuss their observations in a different way.  They describe 11 different kinds of seismic events, including rockfalls, tremor, gas release, distant tectonic, nearby tectonic, etc., all of which can be recognized by their patterns on the seismic record.  Moreover, for earthquakes, CVGHM often lists the amplitude (in millimeters) of an earthquake’s maximum waveforms on an old-fashioned seismic drum recorder.  The amplitude will vary depending on gain and other settings for the instrument, but the reading is still useful if you are comparing earthquakes at different times with the same instrument and settings.  In essence, because they cannot provide accurate locations, depths, and magnitudes, they instead focus on other ways to track changes and forecast future events. For volcanoes that are already erupting, this is actually more informative because seismometers can detect more than just earthquakes.

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Another common method to describe seismic activity is RSAM (Real-Time Seismic Amplitude Measurement), which is akin to the overall energy recorded at the seismometer. Though RSAM can be affected by wind or cultural/industrial/traffic noise, it often is a useful indicator for dormant volcanoes that “perk up” and move towards an eruptive state.  And because it can be calculated quickly and with a single seismometer, it is especially useful for volcanoes where few seismic stations are deployed. The Alaska Volcano Observatory posts RSAM for its active volcanoes, like Shishaldin. But AVO also locates earthquakes and provides the public with a summary of the general trend of earthquakes per hour or day.

Finally, it’s worth noting that different countries and regions have different policies towards releasing seismic data to the public, which then impacts how language, plots, and descriptions are released on public websites.  In the U.S. and Europe, the raw data are usually archived and freely available and viewable with special software. Images of the data are plotted as helicorders and spectrograms.  Most countries around the world are slowly trying to match this level of data availability as money, equipment, and staffing allow.  Some countries still lack any equipment for volcano surveillance.  The January 2022 eruption of the Hunga Volcano in Tonga was the largest in the world since 1991, but it was tracked only with satellites in space and seismometers located over 500 miles away from the eruption site (fortunately, Tonga has since acquired monitoring equipment).

In the future, we expect that the equipment, methods, and terminology for volcano monitoring will become more equalized, but important differences may still remain, depending on what information is most critical to relate to the public during periods of volcanic unrest.