Volcano Watch is a weekly article and activity update written by scientists and partners at the U.S. Geological Survey’s Hawaiian Volcano Observatory.
The island of Hawaii is one of the most seismically active areas in the world. Thousands of earthquakes occur there every year and a signal called a tremor is often generated there, which can originate from the volcanic system.
Tremor is characterized by slowly occurring seismic waves that can last from minutes to several days during sustained eruptions. It is important to recognize tremor because this signal can be associated with fluid movement, which can sometimes indicate an increased likelihood of eruption.
However, earthquakes are often difficult to locate because the waves slowly emerge from the background noise and can last from minutes to weeks or even years. Shallow earthquakes at Kīlauea or Mauna Loa are often directly related to ongoing eruptive activity.
In contrast, earthquakes usually have a sharp onset that is easy to detect.
The time at which an earthquake signal appears at the many stations that make up the Hawaiian Volcano Observatory seismic network can be used to determine the location of the earthquake.
Instead, the location of earthquake eruptions is often estimated by looking at the signal magnitude (or seismic amplitude) at several seismic stations and then determining the approximate position and depth.
For example, shallow earthquakes at the summit of Mauna Loa may produce strong seismic amplitudes at stations near the summit, but only very weak signals when they reach the seismic stations on Kīlauea.
In contrast, an earthquake deep beneath Pāhala might have nearly the same seismic amplitudes at several stations near Pāhala, but be weak at seismic stations in Kona or Hilo.
It is important to note that not all earthquake-like signals are related to volcanic activity. In some cases, they may not even originate from the island of Hawaii.
A large magnitude 7.4 earthquake at 3:50 p.m. on July 19 deep beneath Chile caused strong ground shaking there and was also recorded by the Hawaiian Volcano Observatory seismic network.
This earthquake also generated a strong hydroacoustic signal (a sound wave traveling through water) – the so-called T-phase – that was recorded throughout the Pacific Basin.
T-phases are very different from a tsunami wave and pose no danger to humans. They move at a speed of about 5,447 km/h and would take just under 2 hours to travel from Chile to Hawaii.
In this case, the wave is transmitted predominantly within a horizontal layer called the Sound Fixing and Ranging (SOFAR) channel through the ocean, which tends to efficiently capture or guide hydroacoustic sound waves.
When the T-phase reaches the coast of Hawaii, its energy is converted back into a seismic signal that looks remarkably similar to a local volcanic earthquake in the Hawaiian Volcano Observatory seismic network.
Because the observatory constantly monitors seismic signals, scientists there often have to distinguish between local volcanic quakes and the T-phase of a distant earthquake.
The image here shows an example of a local earthquake deep beneath Pāhala that occurred on July 30 at 4:13 p.m.
By comparing this local earthquake wave with the earlier T-phase event on July 19 in Chile, we see that the arrival of the T-phase event is distributed in time in the Hawaiian Volcano Observatory seismic network – a consequence of the lower speed of sound waves in water.
In contrast, the deep volcanic quake beneath Pāhala reaches the various observation stations at about the same time – a consequence of the local source and the higher wave speeds observed in basalt rocks.
Scientists at observatories can distinguish local quakes from those that occur far away by measuring the arrival times of the signals and evaluating their amplitudes. They can also confirm that the observed signal is a T-phase signal by searching the global earthquake catalog for large events.
This seismic detective work is one of the many aspects of volcano monitoring in Hawaii.
Updates on volcanic activity
Kīlauea is not erupting. The volcano alert level is “advisory.”
Seismic activity and ground deformation continue at Kīlauea’s summit and in the upper and middle eastern rift zones, with another intrusion event occurring near Pauahi Crater on August 19-20.
During the past week, approximately 800 earthquakes were recorded beneath the upper East Rift Zone of Kīlauea, and approximately 100 events were recorded between Maunaulu and Puʻuʻōʻō in the central part of the East Rift Zone in Hawaiʻi Volcanoes National Park.
The summit of Kīlauea and the Middle East rift zone experienced slow inflation followed by slow deflation during the intrusion event. Since then, relatively little ground deformation has been recorded in these regions.
Unrest could continue to wax and wane as magma supply changes in these areas. Changes can occur quickly, as can the eruption potential.
Mauna Loa is not erupting. The volcano alert level is “Normal.”
Last week, 14 earthquakes were reported in the Hawaiian Islands. Only earthquakes with a magnitude of over 3 are listed here:
- Magnitude 4.7 earthquake 9 miles south of Fern Forest at a depth of 4 miles on August 22 at 00:52.
- Magnitude 3.4 earthquake, 3 miles south of the volcano, 0 miles depth, at 9:22 am on August 20.
- Magnitude 3.2 earthquake, 4 miles south of the volcano at 0 miles depth, on August 20 at 7:22 am.
- Magnitude 3.8 earthquake 6 miles northeast of Pāhala at a depth of 19 miles at 5:32 am on August 20.
- Magnitude 3.9 earthquake 18 miles west-northwest of Ocean View at a depth of 7 miles on August 20 at 3:52 am.
- Magnitude 3.2 earthquake 3 miles south of the volcano at 0 miles depth at 2:46 am on August 20.
- The Hawaiian Volcano Observatory continues to closely monitor Kilauea and Mauna Loa.
Visit the observatory’s website for past Volcano Watch articles, updates on Kīlauea and Mauna Loa, volcano photos, maps, information on recent earthquakes, and more. Email questions to [email protected].
