Volcano watch: What is this (seismic) noise?

The US Geological Survey’s Hawaiian Volcano Observatory uses dozens of seismometers to locate individual earthquake sites and identify signals associated with fault and lava movement within volcanoes.

Seismographs also record vibrations caused by a variety of other sources. Signals produced by seismic noise vary widely in intensity, duration, and source type. Some are easily identifiable while others remain a mystery. Every region of the world has a different set of seismic noise depending on its geological environment, cultural activities, and weather.

In today’s article, we’ll describe some of the more interesting sources of seismic noise that HVO seismologists see on a fairly regular basis. The following figures are called spectrograms. These diagrams can be a useful addition to the sinuous lines (or wave shapes) that one commonly associates with earthquakes because they allow the observer to easily identify complex or even multiple signals. The time is displayed on the horizontal axis, the frequency of the signal is displayed on the vertical axis, and the signal intensity is shown in color. The warmer the color, the stronger the signal at that specific time and frequency.

Unsurprisingly, one of the common sources of noise in the HVO seismic record is bad weather. This is especially true along the Kilauea East Rift Region. Noise generated by wind and rain is characterized by its spreading content from medium to high frequencies. In this specific example, the station starts recording the approaching rainstorm just after 12:40 PM, since heavy rains and winds can last for days at a time, these signals tend to be very steady.

If an analyst has any doubts about whether the signals he’s monitoring are actually the weather, take a quick look at one of the webcams (https://www.usgs.gov/volcanoes/kilauea/webcams) overlooking Ahu’aila ‘au, Pu’u’ O’o, or Halema’uma’u will quickly confirm their suspicions.

This spectrogram does not show one signal, but rather two common signals. Most noticeable is the set of bar-like lines across the top of the spectrum plot. This high-frequency undulating feature is a helicopter flying near the seismic station, likely carrying the HVO crew to monitor the ongoing eruption at Halema’uma’u at the summit of Kilauea. Seismologists can sometimes determine the path of a helicopter by tracking it as it passes over multiple stations.

Speaking of the recent eruption, the steady, low-frequency signal seen at the bottom of this spectrum as a yellow-orange stripe is the volcanic tremor that began shortly after the lava broke off the surface at Halema’uma’u on the night of December 20. Since then, nearly all stations near the newly formed lava lake at Kilauea summit have been recording this continuous signal.

Telesis is earthquakes observed from at least 620 miles away. By the time telematics reach very distant stations, all but the lowest frequencies have been lost. The low-frequency signal starting around 11:19 p.m. in this March 19 spectrogram is teles from the 7.0 magnitude earthquake that struck near Ichinomaki, Japan last month (https://earthquake.usgs.gov/earthquakes/eventpage) / us7000dl6y / executive). For comparison, the wide-frequency “spikes” that appear as lighter-colored vertical lines appearing throughout this spectrogram are small local earthquakes.

HVO seismologists use the recent earthquake list released by the National Center for Earthquake Information (https://www.usgs.gov/natural-hazards/earthquake-hazards/earthquakes) along with travel time curves (https: //www.usgs. gov / natural-hazards / earthquake hazards / science / earthquakes-travel times? qt-science_center_objects = 0 # qt-science_center_objects) to help distinguish between telesiology and local low-frequency events such as tremor that may indicate a change or increase in volcanic activity.

Rockfalls have a wide frequency content and a gradual start. These types of events can last for minutes at a time. One trick that seismologists use to identify avalanches is to look for a slight decrease in the low-frequency content as the event progresses. This feature appears as a shallow slope in the March 25 spectral plot starting at 2:59 am. The majority of recent rockslides observed by HVO seismologists were in Pu’u’o’o, preceded by helicopters flying near (or perhaps even!) A carbuncle cone.

Around the world, seismometers have been used to document events such as impending tornadoes, whale songs, fans celebrating during big football matches and even nuclear tests. Here in Hawaii, weather, local air traffic, volcanic tremors, and avalanches are just a few of the interesting seismic signals that HVO seismologists can see while monitoring earthquake activity in active volcanoes.

April is Tsunami Awareness Month in Hawaii. For information on tsunami preparedness and safety, please visit these websites: TsunamiZone, International Tsunami Information Center, National Weather Service TsunamiReady, and Hawaii En Emergency Management Agency.

Visit https://www.usgs.gov/observatories/hawaiian-volcano-observatory for past articles on Volcano Watch, Kilauea and Mauna Loa updates, volcano photos, maps, recent earthquake information, and more. Email questions to [email protected].

Volcano Watch is a weekly article and activity update written by the scientists and affiliates of the Hawaiian Volcano Observatory of the US Geological Survey.

Volcano activity updates

Kilauea volcano erupting. USGS Volcano Alert Level at WATCH (https://www.usgs.gov/natural-hazards/volcano-hazards/about-alert-levels). Kilauea updates are released daily.

Lava activity is limited to Halema’uma’u with lava erupting from a ventilation hatch on the northwest side of the crater. Laser rangefinder measurements this morning, April 8, indicate that the lava in the western (active) part of the lake is 738 feet deep, with the eastern part of the lava lake hardening at the surface. The top tilt gauges have recorded little change in the past 24 hours. The sulfur dioxide emission rates measured on April 7 were 900 tons / day. Earthquakes remain stable, with higher tremor. For the most recent information on the eruption, see https://www.usgs.gov/volcanoes/kilauea/current-eruption.

Mauna Loa does not erupt and is still at Volcano Alert Level. This alert level does not imply that a volcano eruption is imminent or that progression to an eruption from the current level of turbulence is a certainty. Mauna Loa updates are released weekly.

Last week, around 134 small-scale earthquakes were recorded under the upper elevations of Mauna Loa; Most of these occurred at depths of less than about 5 miles. Recent GPS measurements showed a change in shrinkage at the summit which is most likely due to the shrinkage of the shallow summit reservoir. Gas concentrations and fumarol temperatures both at the summit and in the sulfur cone in the southwest rift region remain stable. Web cameras do not show any changes to the landscape. For more information on the current monitoring of Mauna Loa volcano, see: https://www.usgs.gov/volcanoes/mauna-loa/monitoring.

There have been four events with three or more reports on the islands over the past week: a 1.9-magnitude earthquake about a mile east-northeast from Pahala on April 3 at 11:19 a.m., and an earthquake measuring 4.3 on the Richter scale some distance away. 3 miles from north to northwest. Pahala on April 3 at 11:15 a.m., an earthquake measuring 3.9 on the Richter scale 3 miles northwest of Pahala on April 3 at 11:01 a.m., and an earthquake measuring 3.2 on the Richter scale 4 miles west of Pahala on April 1 at 7: 53 AM

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