There are more than 1000 active volcanoes on the planet that are not locally monitored, and some of them are very near potentially impacted populations. One such location is the Tongan capital of Nukualofa, where the only warnings before the nearby (70 km south; 44 mi) Hunga Tonga-Hunga Haapai (HTHH) volcano violently erupted on January 15, 2022, were some observed low-energy eruptions starting December 20, 2021, on the islands that form the northwest rim of the submerged caldera.
Without any local seismometers, seismologists at the National Earthquake Information Center (NEIC) had to rely solely on data recorded on the distant seismometers. Although not as straightforward as using nearby recordings, the scientists were able to glean important information about the activity going on beneath and around the caldera before, during, and after the main eruption. The methods they developed could be used for other volcanoes that do not have local monitoring in the event they become more active and threaten a local population.