# Volcano Watch - Mauna Loa stirring

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We have a number of ways to monitor a volcano, each tuned to different sets of measurements or observations and telling us different things that we hope to be able to integrate into coherent interpretations and an improved understanding of how volcanoes work.

At the U. S. Geological Survey's Hawaiʻian Volcano Observatory (HVO), for example, our core monitoring activities use tools from the disciplines of geology, chemistry, geodesy, geophysics, and seismology.

Much of our current thinking suggests that, of the different things that we can observe, our earliest clues of a volcano's restless intent will be seismic. That is, we will see increased earthquake activity or episodes of volcanic tremor at a volcano as the time of eruption nears. A lot of our seismological research is directed at looking very closely at the earthquake and tremor signatures and developing physically reasonable models of what causes them.

For an active, but resting, volcano like Mauna Loa, it is obvious that the longer the time since its most recent eruption, the closer we are to its next one. In such a case, our challenge is to recognize and understand any clues regarding what the volcano will do next.

Since early July 2004, HVO's seismographic network has recorded an increasing number of earthquakes from beneath Mauna Loa. From week to week, the numbers fluctuate. Through the first week of September, we recorded and logged over 350 earthquakes centered beneath Mauna Loa's summit caldera and the highest elevations of its southwest rift zone.

These earthquakes are, for the most part, quite deep - located anywhere from 35 to 50 km below the ground surface. From their appearances on our seismographs, we also describe them as "long-period" or LP earthquakes, which means that their signals gradually arise out of the ambient seismic background. Such a concentrated number of deep LP earthquakes from this part of Mauna Loa is unprecedented, at least in our modern earthquake catalog dating back to the 1960's. Their locations - 35 km from the closest recording station - and their LP character present challenges to our ability to interpret this activity. Because they are so deep, we have not yet recognized any non-seismic expressions of anything clearly related to them. Because they are LP, they are also very difficult for our data analysts to locate using our routine earthquake location algorithms. We depend on an earthquake's location to provide basic clues to the earthquake's cause, so we are obliged to refine these locations as much as we can.

LP earthquakes beneath Kīlauea and other volcanoes have been linked with magma movement and associated with magma bodies. It is not unreasonable to think that the current earthquakes are also reflecting magma movement deep within Mauna Loa.

Fundamental questions remain in trying to relate these earthquakes to future eruptions. At this time, an eruption does not appear imminent. We would look for earthquake activity to become progressively more shallow and more intense, and we would also expect to see other expressions of magma preparing to erupt, such as increased gas emissions or accelerated ground deformation, before we might declare Mauna Loa's next eruption to be much closer at hand.

### Volcano Activity Update

Eruptive activity at Puu Oo continues. Lava in the Banana flow, which breaks out of the Mother's Day lava tube a short distance above Pulama pali, has been visible on the pali for the past several weeks. The activity is lessening, however. Small breakouts are active along the PKK flow east of the Banana flow, but they are not visible from the Chain of Craters Road. The eruptive activity in Puu Oo's crater is weak, with sporadic minor spattering.

Three earthquakes were reported felt on the island during the week ending September 8.A magnitude 3.3 earthquake shook Hawaiʻian Ocean View Estates at 6:57 p.m. on September 3. It was located 11 km (7 miles) west-northwest of Mauna Loa's summit at a depth of 13 km (8 miles). At 7:12 a.m. September 6, many residents of the southern two-thirds of the island felt a magnitude 3.5 shake located 14 km (8 miles) southeast of Mauna Loa's summit at a depth of 13 km (8 miles). Residents of LauPāhoehoe and Papaaloa reported feeling a magnitude 3.5 earthquake at 7:20 p.m. September 8. It was centered 6 km (4 miles) north-northwest of Keaau at a depth of 38 km (24 miles).

Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity was notably high for the seventh week in a row, with 72 small earthquakes recorded in the summit area. As reported above, most of the earthquakes of this ongoing activity are of long-period type and deep, 40 km (23 miles) or more.