Volcano Monitoring – Volcanic shaking or distant earthquake? Identifying seismic signals

Volcano Watch is a weekly article and activity update written by scientists and affiliates at the U.S. Geological Survey’s Hawaiian Volcano Observatory.

Tremors are characterized by slow-emerging seismic waves that can last from minutes to days while a volcano is erupting. Identifying tremors is important because the signal can be associated with fluid movement and can sometimes indicate an increased likelihood of an eruption. However, tremors are often difficult to locate because the waves emerge slowly from background noise and can last for minutes, weeks or even years – shallow tremors at Kilauea or Mauna Loa are often directly correlated with ongoing eruptive activity.

In contrast, earthquakes usually have a distinct starting point that is easy to see. The time at which the seismic signal appears at the numerous stations in the USGS Hawaiian Volcano Observatory (HVO) seismic network can be used to find the location of the earthquake. In contrast, the location of an earthquake outbreak is usually estimated by looking at the signal size (or earthquake amplitude) at multiple seismic stations and then determining the approximate location and depth.

For example, a shallow earthquake eruption at Mauna Loa’s summit may produce strong earthquake amplitudes at stations near Mauna Loa’s summit, but the signal is very weak when it reaches the Kilauea seismic station. In contrast, an earthquake eruption deep at Pahala Volcano may produce nearly identical earthquake amplitudes at several stations near Pahala, but become weak at seismic stations in Kona or Hilo.

It is important to note that not all tremor-like signals are associated with volcanic activity, and in some cases may not be coming from the Hawaiian island. July 19 at 3:50 pm (Hawaii Standard Time) It caused strong ground shaking in Chile and was also recorded by the HVO seismic network.

The earthquake also generated a strong hydroacoustic signal (a type of sound wave that travels through water), called a T phase, which was recorded throughout the Pacific Basin. T phases are very different from tsunami waves and are harmless to people. They move at speeds of nearly 1.5 kilometers per second (about 3,355 miles per hour), so they would travel from Chile to Hawaii in less than 2 hours.

In this case, the waves propagate through the ocean primarily in horizontal layers called the Sound Field Positioning and Ranging (SOFAR) channel, which tends to effectively capture or direct hydroacoustic sound waves. When the T phase reaches the Hawaiian coast, its energy is converted back into a seismic signal that looks a lot like local volcanic shaking on the HVO seismic network.

Because HVO is always closely monitoring seismic signals, HVO scientists often need to distinguish between local volcanic shaking and the T-phase of distant earthquakes. The diagram here shows a localized shaking that erupted deep below Pāhala on July 30, 2024 at 4:13 pm HST. By comparing this localized shaking to an earlier T-phase event in Chile on July 19, we see that the arrival times of the T-phase event were spread out across the HVO seismic network—due to the slower speed of sound waves in water. In contrast, the deep volcanic shaking beneath Pāhala arrived at different HVO stations at approximately the same time—due to the localized source and faster wave speeds in the basalt.

HVO scientists can distinguish between local and distant earthquakes by evaluating the arrival time and amplitude of the signals. They can further confirm that the observed signal is a T phase by looking for large events in the global earthquake catalog. This earthquake detection is one of many aspects of Hawaiian volcano monitoring.

Volcano Activity Update

Kilauea volcano is not erupting. The U.S. Geological Survey volcano alert level is “warning”.

Seismic activity and ground deformation Continue to the summit of Kilauea volcano and the upper east-central rift zone, Another intrusion occurred near Pauahi Crater on August 19-20Over the past week, about 800 earthquakes were detected beneath the upper East Rift Zone at Kīlauea, and about 100 earthquakes were detected between Maunaulu and Pu’u’ō’ō in the central part of the East Rift Zone within Hawaii Volcanoes National Park. Over the past week, the summit of Kīlauea and the central East Rift Zone experienced slow expansion during the intrusive event, followed by slow contraction. Since then, relatively little ground deformation has been recorded in these areas. Unrest is likely to continue to ebb and flow as magma input to these areas changes; changes could occur quickly, and so could the potential for an eruption.

Mauna Loa is not erupting. The U.S. Geological Survey volcano alert level is normal.

A total of 14 earthquakes were reported in the Hawaiian Islands over the past week; only earthquakes with a magnitude of 3 or greater are listed below: At 12:52 a.m. Hawaii Standard Time on August 22, a magnitude 4.7 earthquake occurred 15 kilometers (9 miles) south of Fern Forest at a focal depth of 6 kilometers (4 miles); at 9:22 a.m. Hawaii Standard Time on August 20, a magnitude 3.4 earthquake occurred 6 kilometers (3 miles) south of the volcano at a focal depth of 0 kilometers (0 miles); at 7:22 a.m. Hawaii Standard Time on August 20, a magnitude 3.2 earthquake occurred 8 kilometers (4 miles) south of the volcano at a focal depth of 0 kilometers (0 miles); at 5:32 a.m. Hawaii Standard Time on August 20, a magnitude 3.8 earthquake occurred 11 kilometers (6 miles) northeast of Pahala at a focal depth of 31 kilometers (19 miles); at 3:52 a.m. Hawaii Standard Time on August 20, a magnitude 3.8 earthquake occurred 30 kilometers (18 miles) west-northwest of Ocean View, Hawaii A magnitude 3.9 earthquake occurred at a depth of 11 km (7 miles) at 2:46 a.m. (Hawaii Standard Time) on August 20, at a depth of 1 km (0 miles) on the south side of the volcano.

HVO continues to closely monitor Kilauea and Mauna Loa.

Visit the HVO website for past volcano monitoring articles, the latest news on Kilauea and Mauna Loa, volcano photos, maps, recent earthquake information, and more. Please email questions to askHVO@usgs.gov.