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

The Pacific is home to dozens of active volcanic systems, including the immense Hawaiian shield volcanoes KÄ«lauea and Mauna Loa. Most of the basaltic shield volcanoes in the Pacific are linked to the hot spots that created the Hawaiian Islands and many Polynesian and Micronesian island chains. These enormous hotspot shield volcanoes are built largely by eruptions of lava that are periodically interrupted by cycles of explosive activity. There are other large shield volcanoes found along subduction zones bordering the Pacific Ocean, but they can behave very differently from KÄ«lauea and Mauna Loa.

Ambae Island, Vanuatu, is a large basalt shield volcano located along the subduction zone between Fiji and Papua New Guinea in the South Pacific. Since 1995, Ambae has experienced explosive episodes once or more per decade. In physical form, Ambae resembles a smaller version of Mauna Loa. The island is 14 km wide and 39 km wide (see figure) with gentle slopes and dense vegetation. Like Mauna Loa, the summit has more than one large crater.

The Ambae craters are filled with colorful lake water which is testament to a deep system of heated, sulfur-rich groundwater below the summit. At Ambae, these large crater lakes and associated groundwater contribute to a specific style of activity called phreatic or phreatomagmatic eruptions.

Ambae experienced two strong episodes of moderate to large explosive eruptions in 2017-18 after mostly minor activity in the previous decade. The first episode happened in October 2017 and blanketed the island in ash, gas and acid rain causing crop damage, fouling water and respiratory problems. These impacts, compounded by the lack of new precipitation to replace the affected drinking water, forced the evacuation of around 11,000 residents from late 2017. Eruptive activity declined soon after, prompting locals begin to return to the island around the start of the New Year.

Eruptions and their impacts in Vanuatu are monitored by the Vanuatu Meteorology and Geohazards Department (VMGD) using transmitted seismographic data (ground shaking) and periodic visits to island sites by volcanic specialists. After the first episode, low-level volcanic activity continued with minor releases of gas and ash from the volcano. The remobilized ash also turned into mudslide lahars throughout the rainy season, from October to April.

In July and August 2018, VMGD and a New Zealand-based research team arrived at Ambae to collect ash and water samples, acquire seismographic and acoustic (sound) data, and document the impacts of the eruptions. Coincidentally, field teams and local residents were faced with more and more explosive eruptions. The largest of these eruptions produced ash plumes over 9,100 meters (30,000 feet) above sea level, which affected South Pacific air traffic.

The newly deployed seismic-acoustic network captured most of the second phase of the eruption and this data was then analyzed by researchers in New Zealand, Vanuatu and the United States. The data showed in detail the timing and size of the explosions on the volcano. An example of time recording for one of the eruptions is shown in the figure which documents a previously unrecognized event. The data not only shows the ground shaking from the eruption, but also the sounds of the volcano. Currently, monitoring in Vanuatu is skillfully conducted by real-time observation of seismic data transmitted to the remote monitoring center in the capital Port Vila. Acoustic observation of volcanoes in Vanuatu is still in its infancy, but the temporary deployment illustrates the value of this data for monitoring purposes.

The new eruption phase eventually forced Ambae’s second full evacuation in August 2018. Interestingly, while the 2018 eruption on the island of Hawaii gained worldwide attention, the eruption of ‘Ambae had a bigger global impact due to the huge amount of gas released. Fortunately, the eruption ended later in the year and local residents of Ambae were able to return home safely. At the end of September 2019, scientists returned to Ambae to remove the temporary seismic and acoustic stations. Local farmers, who had returned earlier in the year, reported bountiful harvests, possibly due to the newly rejuvenated ash-rich soils.

Updates of volcanic activity

KÄ«lauea is not erupting. Its USGS Volcano Alert Level is at ADVISORY (https://www.usgs.gov/natural-hazards/volcano-hazards/about-alert-levels). KÄ«lauea updates are released weekly.

The KÄ«lauea volcano is not erupting. Following the recent intrusion of sub-surface magma in the area south of the KÄ«lauea caldera, which slowed significantly on August 30, earthquake rates and soil deformation in this area have remained close to levels of before the intrusion. Other monitoring data streams, including sulfur dioxide emission rates and webcam views, show no significant change. For more information on the current monitoring of KÄ«lauea, see https://www.usgs.gov/volcanoes/kilauea/monitoring.

Mauna Loa is not erupting and remains at the ADVISORY volcanic alert level. This alert level does not mean that a rash is imminent or that progression to a rash from the current level of unrest is certain. Mauna Loa updates are released weekly.

Last week, around 88 low-magnitude earthquakes were recorded below the summit and upper elevation flanks of Mauna Loa – the majority of them occurred at depths below 8 kilometers (5 miles). Global Positioning System (GPS) measurements show no major distortions over the past week. The gas concentrations and the temperatures of the fumaroles at the summit and at the sulfur cone in the southwest rift zone remain stable. The webcams do not show any change in the landscape. For more information on the current surveillance of Mauna Loa, see: https://www.usgs.gov/volcanoes/mauna-loa/monitoring.

There have been 2 events with 3 or more felt reports in the Hawaiian Islands in the past week: an M3.3 earthquake at 1 km (0 mi) SSW from Pāhala at 34 km (21 mi) deep September 21 at 2:11 am. HST and an M3.6 earthquake at 9 km (5 mi) ENE of Pāhala at 31 km (19 mi) deep on September 20 at 12:49 HST.

The HVO continues to closely monitor KÄ«lauea and Mauna Loa for any signs of increased activity.

Visit the HVO website for old Volcano Watch articles, Kīlauea and Mauna Loa updates, photos of volcanoes, maps, recent earthquake information, and more. Email your questions to [email protected].