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November 24, 2004

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Deep Mauna Loa Seismicity Continues

While breakouts in the coastal area below Pu`u `O`o, attract renewed attention from scientists and visitors alike, seismologists at the U.S. Geological Survey's Hawaiian Volcano Observatory (HVO) have been vigilantly recording and cataloging what has been termed "unprecedented" deep seismicity under the summit region of Mauna Loa. In the over 40-year span covered by HVO's modern earthquake catalog, no other period has included such large numbers of Mauna Loa earthquakes at the depths now being observed.

In October 2002, a "Volcano Watch" article introduced a scale for reporting Mauna Loa seismicity to the public. Based on the history of modern instrumental recording and processing of earthquakes at HVO and, accounting for recent improvements in instrumentation and processing capabilities, seismic activity was rated from "high" to "low." A "high" level was declared when the number of catalogued earthquakes exceeded 60 and lower thresholds were set for "low," "moderate," and "moderately high" levels of activity. These limits were set on the basis of the catalogued background seismicity and earthquake increases preceding the 1975 and 1984 eruptions.

Under the scale described above, seismicity has remained moderate to high beneath Mauna Loa for the past 17 weeks. Over 1,300 earthquakes have now been catalogued in the summit region since early July 2004. As discussed in September 2004, the vast majority of these earthquakes are of the long-period (3-5 Hz) type, originating from a depth of between 35 and 50 km (22 and 31 miles), in the upper mantle well beneath the island.

Does this sustained high level of seismicity mean a Mauna Loa eruption is close at hand? While the current seismic activity is likely related to magma movement far beneath Mauna Loa, remember that clusters of earthquakes have not been seen at this depth before. The last two eruptions were preceded by different but clear changes in the nature of the seismicity and rapid changes in ground deformation. We expect these changes to occur again before the next eruption.

What sort of changes in seismicity do we expect? Significant earthquake activity occurred northwest of Moku`aweoweo at intermediate depths between 5 and 13 km (3 and 8 miles) prior to both 1975 and 1984 eruptions; this region has yet to show signs of reawakening. In fact, earthquakes shallower than 30 km (19 miles) have been conspicuously absent. We can expect a significant increase in shallow earthquakes and, likely, volcanic tremor as an eruption nears. The migration of earthquake locations will enable tracking of magma as it nears the surface.

The present activity may lead to changes in seismicity and, ultimately, an eruption. Regardless, the earthquakes now being catalogued will be an invaluable contribution to the understanding of Mauna Loa's historical seismicity. These deep-focus earthquakes will likely cause us to rethink the categorical scheme now used to characterize the level of activity there. As events unfold, HVO will provide updates of monitoring efforts through the Mauna Loa current activity web page at

Activity update

Eruptive activity at Pu`u `O`o continues. The PKK flow continues to host scattered breakouts from near the top of Pulama pali to the coastal plain. The new ocean entry, at Lae`apuki, 3.6 km (2.2 mi) from the end of the pavement on Chain of Craters Road in Hawai`i Volcanoes National Park, remains active. Expect a 2-hour walk each way and remember to bring lots of water. The new lava bench is about 260 m (855 ft) long (parallel to shore) and 90 m (295 ft) wide. Stay well back from the sea cliff, heed the National Park warning signs, and don't even think about descending onto the growing bench. The eruptive activity in the crater of Pu`u `O`o remains weak, with several spatter cones glowing but not doing much else.

No earthquakes were reported felt on the island during the week ending November 24.

Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity is elevated with over 1300 earthquakes recorded since July 2004. 105 earthquakes were recorded beneath the summit area during the week ending November 23. Nearly all are 40 km (23 miles) or more deep and are the long-period type with magnitudes less than 3.

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Updated: November 30, 2004 (pnf)