# Volcano Watch — The year in earthquake review

Release Date:

As we approach the end of 2002, we pause to look back at the year. What can we say about earthquakes in 2002?

At the Hawaiian Volcano Observatory (HVO), part of our routine volcano and earthquake monitoring focuses on cataloging earthquakes and volcanic tremor. In other articles, we have described HVO's procedures for processing earthquake data in some detail. Depending on the context, we can report different earthquake numbers.

So far this year, we have cataloged and archived over 7,000 earthquakes. This number refers to events that our seismic data analysts have determined to be local earthquakes, obvious or sharp bursts of volcanic tremor, or signals from distant earthquakes. By the time we arrive at this number, we have culled electronic noise and other unnatural signals from the database.

The vast majority of these earthquakes are very small and are not felt by island residents. Such small earthquakes do not cause structural damage, but even the tiniest form part of the big volcanic puzzle that HVO is trying to piece together.

We apply additional cataloging steps to earthquakes that are large enough to register clearly on several of our stations. We gather earthquakes of magnitudes 1.5 and greater into a second catalog that we feel is more directly comparable to that of earlier periods, when the configuration of our field network, or our recording or processing capabilities, did not afford the same cataloging threshold for smaller magnitudes that we enjoy today.

Of the more than 7,000 earthquakes in 2002, we have thus far (through December 17, 2002) processed 3,015 earthquakes with magnitude of 1.5 or greater. The largest earthquake was a magnitude 4.1 earthquake on January 18, beneath the south flank of Kilauea volcano. This was our only magnitude 4 earthquake here this year. We have received reports of this and 61 other earthquakes being felt in Hawai'i (so far).

By way of comparison, in 2001 we cataloged 2,776 earthquakes of magnitude 1.5 or greater, out of a total of 5,622. Six were magnitude 4 or larger, and 52 earthquakes were felt. In 2000, we cataloged 3,173 earthquakes of magnitude 1.5 or greater, out of 5,556. That year had four magnitude 4s, and 41 earthquakes were felt. As these recent years bear out, the numbers of earthquakes from year to year will fluctuate.

Apart from larger earthquakes, both Kilauea and Mauna Loa provided us with other interesting signals. Our monitoring experiences shape some interpretive guidelines. But as we watch our volcanoes, we are reminded that there is much that we don't know or completely understand, such as what might control the details of earthquake occurrence or what physical significance we might attach to the varying earthquake statistics.

In early April, our networks of seismometers and tiltmeters on Kilauea followed a surge of magma that apparently passed from Kilauea's summit into the east rift zone. New procedures for locating volcanic tremor suggest that a deeper tremor source was activated beneath Kilauea caldera during this surge of magma, followed by a shallower source. This interpretation is not afforded by traditional earthquake data processing procedures, but it is consistent with our general understanding of magma movement through Kilauea.

At the end of April, we recorded a swarm of earthquakes deep beneath Mauna Loa. A notable increase in Mauna Loa earthquakes continued through May. As HVO reported in an earlier "Volcano Watch" and at public symposia in Hawai'i County, the deep Mauna Loa earthquake swarm was followed by a striking change in crustal deformation across Mauna Loa's summit. These observations are quite likely to be related to one another and may even be the earliest signs leading to Mauna Loa's next episode of volcanic unrest. At the very least, they effectively remind us that Mauna Loa remains active and demands our monitoring attention, along with Kilauea.

### Volcano Activity Update

HVO, in conjunction with Hawaii Volcanoes National Park, is sponsoring a public open house between 9 and 4 on Saturday, January 4. The open house is timed to the 20th birthday of the ongoing eruption, which started on January 3, 1983. Come meet our staff and learn about the eruptions of Hawaiian volcanoes, see demonstrations of equipment used to monitor volcanic activity, view dozens of displays about Kilauea and Mauna Loa volcanoes, take a walk on the wild side at Halemaumau, and much more. Check the web sites of HVO (http://hvo.wr.usgs.gov) and the park (http://www.nps.gov/havo/) for details.

Eruptive activity at the Puu Oo vent of Kilauea Volcano continued unabated during the past week. Lava flows through a tube system from the vent to the sea. Lava continues to enter the ocean and form lava deltas (benches) at Wilipea, West Highcastle, and between Highcastle and Lae'apuki. The east arm of the Mother's Day flow, which feeds the Highcastle-Laeapuki entry, has persistent breakouts between Paliuli and the coast. Another breakout, which lasted only 3-4 days, poured down Paliuli near the middle of the flow field. Other breakouts light the sky above Pulama pali, and two started down the uppermost part of the pali early in Christmas week. We have observed people on the West Highcastle lava delta during night hours of the past week. This is very dangerous, as the ocean entry areas can collapse at any time (as they did on December 15), 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.

Two earthquakes were felt during the week ending on December 26. One took place at 6:45 PM 1 mile west of Ookala, at a depth of 7 miles. The earthquake was felt in Papa`aloa, even though it had a magnitude of only 2.1. The largest felt earthquake occurred on Christmas Day at 2:53 PM. It was located 6 miles west-northwest of Pahala, had a magnitude of 3.8, and was 6.5 miles deep. This earthquake was felt across the island.

Mauna Loa is not erupting. The summit region continues to inflate. The earthquake activity is low, with only 4 earthquakes located in the summit area during the last seven days.