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Earthquakes are only part of the conversation
USGS: Ninety seconds of raw seismic data from four seismic monitoring stations around the Colawea Summit. The dates and times, below the graph, are in Coordinated Universal Time (UTC). The stations picked up two types of seismic signals that we typically observe in volcanic environments: (left) a typical tectonic earthquake signal, which results from rocks liberating and sliding into each other; and (right) an earthquake signal associated with magma, which typically results from magma recoil. The difference in vibration frequency is starkly noticeable because the magma-related signals (right) have divergent peaks and troughs when compared to tectonic earthquake signals (left).”
(BIVN) – Neither Kilauea nor Mauna Loa volcanoes have erupted. Both are at advisory alert level, with earthquakes generally stable.
In this week’s Volcano Watch article, USGS Hawaiian Volcanic Observatory geophysicist Jefferson Chang writes about the art of monitoring seismic activity, and what it can reveal about magmatic activity below the surface. From the article:
The journey from volcanic stillness to volcanic eruption is like a mystery movie that keeps you on the edge of your seat. The story is full of twists and turns that often lead to a dead end.
Earthquakes are an early warning system for volcanoes. The first page of work in our story is page turning. It’s very difficult to move molten magma through solid rock without making the kind of “noise” or vibrations that seismographs don’t pick up. So we often “hear” magma coming before it erupts as lava at the surface.
Earthquakes by themselves are not enough. Everyone at the US Geological Survey’s Hawaiian Volcano Observatory (HVO) plays an essential role in studying, monitoring, and responding to volcanic hazards on the island of Hawaii. We’re all following the same thing – to uncover evidence of volcanic unrest so that no one is surprised when the plot turns toward the inevitable eruption.
Basically, we use notes based on the five senses: sight, smell, touch, taste, and sound. But enhanced with modern devices, for continuous and more accurate monitoring. Each of these technologies requires different sets of skills and background, so we work together to give the audience the best information available.
For sight, rather than eyes, we keep watching with cameras scattered in various places around the island that are likely to give us our first hints of volcanic activity. We also monitor how the Earth is moving up, down, or sideways, using highly accurate tilt gauges and GPS instruments that measure changes to less than millimeters, or tiny fractions of an inch.
As for smell, instead of the nose, we have gas sensors tuned to detect volcanic gases. These include carbon dioxide, sulfur dioxide, hydrogen sulfide, and others, which let us know how deep or shallow the magma is, depending on how much of these gases seep to the surface.
To touch, rather than the skin, we monitor temperatures with thermal cameras or temperature sensors. Obviously, the higher the temperature, the more likely the magma is to come close to the surface, but at times of eruption, the temperature of the lava can also tell us where the flow is more active, and therefore more dangerous.
For tasting, which is basically a chemical experiment involving the tongue (something no one should try with lava), we sample rocks and lava in the field to analyze their chemical composition.
As for sound, rather than with the ears, we listen to the ground with seismometers that can detect vibrations, which volcanoes emit when a fault slides off during an earthquake or as magma moves underground. This is where I play my role at HVO as part of a team that monitors seismic activity under the island of Hawaii.
Part of monitoring seismic activity is locating and quantifying earthquakes: the increase in the number of detected events is a soliloquy of volcanic disturbances; Big strong earthquakes are those indelible moments that people quote with big eruptions. For those playing a whodunit, these earthquake stats seem like a straightforward way to follow the plot.
The lesser known art of observing seismic activity deals with petrographic signals. These signals often appear as noise with scattered tectonic events that may help decipher what the magma is actually doing. These events are usually not detected by automated computer algorithms (created for tectonically active regions such as California) and their sources are known to be difficult to quantify, which means they are not reported in seismic indexes.
This is where the human ability to recognize patterns becomes indispensable. HVO seismic analysts are looking at waveform data day in and day out (literally), looking for the seemingly chaotic middle patterns of the oscillations to make sense of it all.
Volcanic seismology is more than just copying earthquakes into a census or almanac. Seismic investigators need to pay attention to the entire soundtrack – this includes the magma-related earthquakes that often go unnoticed but are important in determining momentum and mood, and ultimately deciphering the story of the eruption as it unfolds.
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