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Watching the Volcano: Semi-Earthquakes – Earthquakes are only part of the conversation
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 USGS Hawaiian Volcano Observatory plays an essential role in studying, monitoring, and responding to volcanic hazards on the Big Island. 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 Big Island.
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.
Visit https://www.usgs.gov/observatories/hawaiian-volcano-observatory for past articles on the Volcano Watch, Kilauea and Mauna Loa updates, volcano images and maps, recent earthquake information and more. Email questions to [email protected].
Volcano Watch is a weekly article and activity update written by USGS Hawaiian Volcano Observatory scientists and affiliates.
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Volcano activity updates
Kilauea volcano does not erupt. USGS Volcano Alert Level in ADVISORY (https://www.usgs.gov/natural-hazards/volcano-hazards/about-alert-levels). Kilauea updates are released weekly.
The supply of lava to the lava lake Halema’uma’u has ceased and SO2 emissions have decreased to approximately pre-eruptive background levels. The peak tilt gauges have recorded one cycle of deflation and inflation over the past week. Earthquakes have remained generally stable, with nearly 450 over the past week. There are currently no indications that a resumption of volcanic activity is imminent. Kilauea is still an active volcano and future eruptions may occur at the summit or elsewhere on the volcano. For more information on current monitoring of Kilauea volcano, see https://www.usgs.gov/volcanoes/kilauea/monitoring.
Mauna Loa is not erupting and is still at volcano alert level. This alert level does not mean that a volcano eruption is imminent or that progression to an eruption from the current level of disturbance is certain. Mauna Loa updates are released weekly.
Last week, about 166 small earthquakes on the Richter scale were recorded under Mauna Loa. GPS measurements show low rates of distortion in the summit region over the past week. Gas concentrations and fumarole temperatures at both the summit and in the sulfur cone of the Southwest Rift Zone remain stable. Webcams do not show any changes to the landscape. For more information on current monitoring of Mauna Loa, see: https://www.usgs.gov/volcanoes/mauna-loa/monitoring.
There have been six events with three or more reports that were felt in the Hawaiian Islands over the past week: a 0.6-magnitude earthquake 9 miles east-southeast of Nalihu on June 23 at 8:22 a.m., a 3.3-magnitude earthquake a few kilometers away 16 miles west. NW Poaco on June 20 9:36 p.m. Richter-scale 3.5 earthquake 6 miles east-northeast of Pahala June 17 5 p.m. Magnitude 2.1 earthquake 4 miles east-northeast of Pahala June 17 4: 40 p.m., a 4.5-magnitude earthquake 4 miles east-northeast of Pahala on June 17 at 4:32 p.m., and a 2.5-magnitude earthquake 9 miles south of Fern Forest on June 17 12:31 p.m.
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