Volcano Watch: Volcanic tremor or distant earthquake? Distinguishing seismic signals

“Volcano Watch” is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates.

The Island of Hawaiʻi is one of the most seismically active areas in the world, producing thousands of earthquakes every year. It also commonly produces a signal called tremor that can originate from the volcanic system.

Tremor is characterized by slowly emerging seismic waves that can last for minutes to many days when there are ongoing eruptions. It is important to identify tremor because this signal can be associated with fluid movement, which can sometimes indicate an increased chance of eruption.

However, tremor is often difficult to locate because the waves slowly emerge from the background noise and can persist for minutes to weeks or even years — shallow tremor at Kīlauea or Mauna Loa is often directly associated with ongoing eruptive activity.

In contrast, earthquakes usually have sharp onsets that are easy to see.

The time an earthquake signal appears at the many stations that make up the Hawaiian Volcano Observatory seismic network can be used to find the earthquake’s location.

Instead, the location for tremor bursts is often estimated by looking at the signal size (or seismic amplitude) at multiple seismic stations and then determining the approximate position and depth.

As an example, shallow tremor bursts at Mauna Loa summit might have strong seismic amplitudes at stations near the summit, but very weak signals by the time the signal arrives at the Kīlauea seismic stations.

By contrast, a tremor burst deep below Pāhala might have almost the same seismic amplitudes on several stations near Pāhala only to become weak at seismic stations in Kona or Hilo.

It is important to note that not all tremor-like signals are related to volcanic activity. In some cases, it might not even be from the Island of Hawaiʻi.

A large magnitude-7.4 earthquake at 3:50 p.m. July 19 deep beneath Chile produced strong ground shaking there and was also recorded on the Hawaiian Volcano Observatory seismic network.

This earthquake also produced a strong hydroacoustic signal (a sound wave traveling through water) — called the T-phase — that was recorded throughout the Pacific basin.

T-phases are very different from a tsunami wave and are not hazardous to people. They move at a speed of almost about 3,355 mph and would take just fewer than 2 hours to get from Chile to Hawai‘i.

In this case, the wave is transmitted through the ocean predominantly within a horizonal layer called the Sound Fixing and Ranging (SOFAR) channel, which tends to efficiently trap or guide hydroacoustic sound waves.

When the T-phase reaches the Hawai‘i coast, its energy is converted back into a seismic signal that looks remarkably like local volcanic tremor on the Hawaiian Volcano Observatory seismic network.

Because the observatory is always vigilantly monitoring seismic signals, its scientists often need to distinguish between local volcanic tremor and the T-phase from a distant earthquake.

The figure here shows an example of local tremor burst deep beneath Pāhala at 4:13 p.m. July 30.

By comparing this local tremor burst to the earlier T-phase event from July 19 in Chile, we see that the T-phase event has arrivals spread out in time across the Hawaiian Volcano Observatory seismic network — a result of the slower speed of the acoustic waves in the water.

By contrast, the deep volcanic tremor beneath Pāhala arrives at the different observatory stations at about the same time — a result of the local source and faster wave speeds found for basaltic rocks.

Observatory scientists can distinguish local tremors from distant earthquakes by assessing arrival times for signals and evaluating their amplitudes. They can further confirm that the observed signal is a T-phase by looking for large events in the global earthquake catalog.

This seismic sleuthing is one of the many facets of volcano monitoring in Hawai‘i.

Volcano Activity Updates

Kīlauea is not erupting. Its Volcano Alert level is Advisory.

Seismic activity and ground deformation continues at Kīlauea’s summit and upper and middle East Rift Zones, with another intrusive event near Pauahi Crater from Aug. 19-20.

During the past week, about 800 earthquakes were detected beneath Kīlauea’s upper East Rift Zone and about 100 events were detected between Maunaulu and Puʻuʻōʻō in the middle portion of the East Rift Zone within Hawaiʻi Volcanoes National Park.

Kīlauea’s summit and middle East Rift Zone showed slow inflation followed by slow deflation during the intrusive event. Since then, there has been relatively little ground deformation recorded in these regions.

Unrest might continue to wax and wane with changes to the input of magma in these areas. Changes can occur quickly, as can the potential for eruption.

Mauna Loa is not erupting. Its Volcano Alert Level is at Normal.

Fourteen earthquakes were reported felt in the Hawaiian Islands during the past week. Only those above magnitude of 3 are listed here:

• Magnitude-4.7 earthquake 9 miles south of Fern Forest at 4 miles depth at 12:52 a.m. Aug. 22.
• Magnitude-3.4 earthquake 3 miles south of Volcano 0 miles depth at 9:22 a.m. Aug. 20.
• Magnitude-3.2 earthquake 4 miles south of Volcano at 0 miles depth at 7:22 a.m. Aug. 20.
• Magnitude-3.8 earthquake 6 miles northeast of Pāhala at 19 miles depth at 5:32 a.m. Aug. 20.
• Magnitude-3.9 earthquake 18 miles west-northwest of Ocean View at 7 miles depth at 3:52 a.m. Aug. 20.
• Magnitude-3.2 earthquake 3 miles south of Volcano at 0 miles depth at 2:46 a.m. Aug. 20.
• The Hawaiian Volcano Observatory continues to closely monitor Kīlauea and Mauna Loa.

Visit the observatory’s website for past “Volcano Watch” articles, Kīlauea and Mauna Loa updates, volcano photos, maps, recent earthquake information and more. Email questions to askHVO@usgs.gov.