# Volcano Watch - Inflation of Mauna Loa Volcano slows

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The inflation of Mauna Loa Volcano that started last year appears to be slowing down. From May 2002 until very recently, continuously recording instruments spanning Mokuaweoweo, Mauna Loa's caldera, showed that the summit area was extending and rising, indicating that magma was pressurizing a reservoir beneath the surface.

The rate of filling has slowed down over the last couple of months.

Such changes in magma pressure have been observed before. We've been monitoring Mauna Loa's changing ground surface since the mid 1960's. Over much of this time, we've measured inflation of the summit reservoir, which started immediately after the latest two eruptions in 1975 and 1984. However, the inflation was not steady. There were several intervals in which it apparently slowed or stopped, only to resume again. The longest of these periods was from about 1994 to 2002.

Up until quite recently, we have had to rely on infrequent measurements, so the time scale over which we could see changes was usually on the order of a year. It is only recently that we have the capability to observe these sorts of changes in near-real-time and with unprecedented precision. Thus, we can't really put this recent slowing-down in a historical context. We can only wait and see how long it lasts.

During the inflation last year, the pattern of swelling was very similar to that observed before the latest two eruptions, though the rate was somewhat higher than we'd observed in the past. These ground surface changes could be explained by magma filling a reservoir 4-5 km (about 2 miles) beneath the southeastern part of the summit caldera.

This model is remarkably similar to what we observe on Kilauea Volcano as well - a summit reservoir 3-4 km beneath the southern part of the caldera. There is no doubt that Mauna Loa and Kilauea each have their own, distinct shallow storage reservoirs, but whether there is any relationship between them, or whether they are completely independent systems, has been debated in scientific papers for the last century.

Some early observers considered Kilauea simply a vent on Mauna Loa. That view became hard to support when it was found that their lavas had distinctly different chemical compositions. The chemical differences imply that they must be tapping into different parts of the Hawaiian hot spot, currently centered very deep beneath the Big Island. Other investigators have noted that there appears to be an inverse relationship in the eruptive behavior of the two volcanoes. That is, when Kilauea is in a period of intense volcanic activity, Mauna Loa tends to be less active, and vice versa.

A recent observation revives the questions about the relationship, if any, between the two volcanoes. Continuously recording monitoring instruments allowed us to measure last year's onset of inflation on Mauna Loa at an unprecedented level of detail. We observed that the inflation most probably started on May 12 - the same date that a high-volume lava flow started from a new vent on the flank of Puu Oo cone on Kilauea (the "Mother's Day" flow). If not purely coincidental, the temporal correlation between these events suggests some interaction between the magma systems of Mauna Loa and Kilauea.

In the months leading up to the event, Kilauea's shallow magma system had been slowly inflating. This may have resulted from decreased lava transport capacity at the old vent, as it occurred during a period of decreasing lava flow activity. A pulse of magma introduced into Mauna Loa's plumbing system, perhaps during a swarm of deep earthquakes detected in late April, could have increased the pressure in Kilauea's adjacent and already overpressurized shallow magma system. In other words, a pulse of magma delivered to Mauna Loa could have "squeezed" Kilauea just enough to trigger the surge in lava production at Puu Oo.

We continue to monitor the activity underneath Mauna Loa closely and will report any changes both through the media and on our web site (hvo.wr.usgs.gov). Updates of deformation and seismic monitoring data are posted daily to the Mauna Loa current activity page on the web site.

### Volcano Activity Update

Eruptive activity at the Puu Oo vent of Kilauea Volcano continued unabated during the past week. A familiar pattern of tilt changes in the summit region of Kilauea Volcano and at the Puu O`o cone caused predictable consequences in eruptive activity. The tilt pattern of slow, moderate deflation, followed by rapid, large inflation, then deflation resulted in large breakouts from the lava tube system near the top of the Mother's Day flow. This sequence of events, seen many times in the past, started at 5:10 p.m. on Monday, January 20, and the first breakout was observed at 8:35 p.m. on Tuesday night.

Large surface flows above Pulama pali contain most of the erupted lava with only a small volume flowing through the tubes down to the coast at the West Highcastle entry. The public should be aware that the ocean entry areas could collapse at any time, potentially generating large explosions in the process. The steam clouds rising from the entry areas are highly acidic and laced with glass particles. The National Park Service has erected a rope barricade to delineate the edge of the restricted area. Do not venture beyond this rope boundary onto the lava deltas and benches. Even the intervening beaches are susceptible to large waves suddenly generated during delta collapse; these beaches should be avoided.

One earthquake was reported felt during the past week. Residents of Volcano, Kurtistown, and Glenwood felt an earthquake at 1:55 a.m. on January 23. The magnitude-3.4 earthquake was located 5 km (3 mi) south of Volcano at a depth of 4 km (2.4 mi).

Mauna Loa is not erupting. As mentioned in the article above, the summit region continues to inflate, though the rate of inflation has slowed gradually during the past month or two. The earthquake activity is low, with only 2 earthquakes located in the summit area during the last seven days.