# Volcano Watch — Sleeping giants Hualālai and Haleakalā require prudent monitoring

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In some regards, monitoring an active volcano is easy; the constant bustle keeps a watcher on his or her toes. But what if a volcano hasn't erupted in 200 years?

In some regards, monitoring an active volcano is easy; the constant bustle keeps a watcher on his or her toes. But what if a volcano hasn't erupted in 200 years?

Quiescent, potentially active volcanoes require less intense monitoring. A 200-year dormancy since their last eruption characterizes Hualālai on the Big Island and Haleakalā on East Maui. The challenge is to use techniques that are economically prudent while providing sufficient information to recognize potentially disruptive changes at these volcanoes.

Monitoring earthquakes remains a key to forecasting changes at volcanoes. Along the Hawaiian Island chain, earthquakes are monitored constantly by seismometers. Hualālai has one seismometer at its summit and several surrounding its base from Captain Cook to Waimea. We can, therefore, locate even small Hualālai earthquakes relatively precisely.

Ground deformation monitoring at Hualālai is undertaken every few years, using a combination of conventional surveying and Global Positioning System (GPS) receivers. Swelling or deformation of the volcano, although miniscule, can be measured with these highly precise instruments. Given Hualālai's inactivity, this level of monitoring is appropriate and economical.

Our basic data for Haleakalā are much poorer. One seismometer has been on Maui since 1957, and two seismometers are now being maintained there by the Pacific Tsunami Warning Center. This sparse coverage augments the regional network; but compared to the dense network on the Big Island, the Maui seismometers may not be as reliable in locating earthquakes smaller than about magnitude 4 that originate in the volcano or beneath the ocean floor.

The U.S. Geological Survey is collecting baseline data to better understand the earthquakes and ground deformation near Haleakalā. In the past few years we have established and measured a network of GPS stations to monitor ground deformation. We plan surveys every few years, unless we detect high rates of movement or unless there are other signs of volcanic unrest.

Our plans also call for establishing a network of several seismometers to improve the routine monitoring of small earthquakes in East Maui. These earthquakes, too small to be felt by Maui residents, result from the load imposed on the ocean floor by Haleakalā volcano. We need to know the depth, arrangement, and frequency of these "tectonic" earthquakes in order to distinguish them from "volcanic" earthquakes that result from movement of magma.

Geologic mapping and dating of young lava flows is also ongoing at Haleakalā and its rift zones. We know surprisingly little about the frequency of eruption, although the abundance of young lava flows shows that the volcano has erupted numerous times in the past 10,000 years. Are the eruptions episodic, with many vents active over a short period of time? Or do the eruptions occur on a more regular basis but involve less extensive activity? These questions need to be answered before a comprehensive hazard analysis can be offered.

The lengthy eruptive history and relative recency of eruption—200 years—at both Hualālai and Haleakalā indicate that these volcanoes will erupt again. In the meantime, HVO will be keeping a watchful eye on them in order to recognize the premonitory activity as early and as economically as possible.

### Volcano Activity Update

This past weekend saw a short-lived small disturbance in the east rift zone eruption. On Saturday, magmasupply to the eruption site was obstructed. Consequently, the tubes drained of their lava and the plumes at the coastal entry dissipated. Coincident with the drainout, the summit area of Kīlauea inflated gradually, gaining about 3 microradians of tilt. A 4.1-magnitude earthquake from the upper east rift zone early Sunday morning marked the end of the summit inflation. Magma stored temporarily then moved down the east rift zone in an amount sufficient to overwhelm the capacity of the tube system. New lava flows issued from skylights at the top of Pulama pali and were active until about noon Sunday. Since then, the tube system has returned to normal, and lava is again reentering the sea at the Wahaula and Kamokuna sites. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying frequent collapse of the the lava delta. The steam cloud is highly acidic and laced with glass particles.

Three earthquakes were felt this past week. The 4.1-magnitude earthquake on Sunday morning (12:43 a.m.) was felt widely in the upper Puna district. On Tuesday, April 7, a magnitude 2.6 earthquake struck at 8:31 p.m. near Puulena Crater, at the east end of Leilani Estates. Early Thursday, April 9, a magnitude 3.9 earthquake propagated from great depth (38 km) at 12:52 a.m. Epicenter for this temblor was 2 miles west of Pahala.