Volcano Watch — What does the Hawaiian Volcano Observatory do during an earthquake?

Volcanic eruptions and underground magma movements are almost always accompanied by lots of earthquakes. Typically, the earthquakes that occur during magma intrusion are small in magnitude, shallow in depth, but plentiful in number. Like flocks of geese and pods of whales, volcano earthquakes occur in flurries and swarms. An earthquake swarm can consist of hundreds of earthquakes per hour. While impressive in number, these earthquakes are rarely damaging.

Magma injection into a volcano over time pushes the sides of a volcano apart. Hawai`i is an island that has grown on top of much older oceanic crust. If a volcano is pushed apart long enough, its base will slide along the buried ocean floor, producing earthquakes that are infrequent but powerful. These earthquake depths cluster around 8 km (5 miles). The largest in recent memory was the magnitude-7.2 Kalapana earthquake of 1975 that also generated a tsunami that killed two campers.

And then there are the large earthquakes that aren't as closely related to volcanic activity like last Sunday's magnitude-6.7 and magnitude-6.0 shakes. These earthquakes occur under the volcano, under the oceanic crust, and in the thick-molasses-like upper mantle.

The crust beneath the ocean floor supported only the Pacific Ocean for tens of millions of years before the Hawaiian Islands started building on top of it. Imagine the weight of a 10-mile-high by 100-mile-diameter volcanic island. Last Sunday's earthquakes are probably the result of ongoing bending of the oceanic crust under our volcanoes. The most recent earthquake of this type was the magnitude-6.2 Honomu earthquake that occurred in 1973 off the east coast of the island at a depth of about 39 km (24 miles).

So what do volcanologists do during a large earthquake like last Sunday's? We quickly determine where and how big the earthquake was, then immediately check other instrumental data for effects on nearby active volcanoes. On Hawai`i, that includes Kīlauea, Mauna Loa, Hualālai, and Mauna Kea. Then we alert Hawai`i County Civil Defense to report these preliminary findings and dispense information to the media and through our various Web pages.

The U.S. Geological Survey has the responsibility of monitoring both volcanoes and earthquakes. On the mainland, there are different USGS teams that specialize in monitoring one or the other. The Hawaiian Volcano Observatory is somewhat unique, as we bear the responsibility of monitoring, reporting, and responding to both.

Unlike the Pacific Tsunami Warning Center, however, HVO is not staffed 24/7. On Sunday, HVO staff came in from as far away as Papa`aloa to verify and improve our initial automatic earthquake determinations, assess damage to our monitoring networks, and answer phones. About a dozen staff were here within one hour of the first earthquake.

Helicopter overflights of Kīlauea's eruption site a few hours after the quake and of Hualālai and the north flank of Mauna Loa verified what our instruments had already indicated - that the earthquake had no visible effect on Kīlauea's ongoing eruption or other volcanoes.

The USGS's National Earthquake Information Center (NEIC) is also monitoring earthquakes in Hawai`i, although HVO normally has primary authority. Sunday's quakes were so large and so close that nearly all of our instruments "clipped" - the instruments experienced vibrations greater in amplitude than they were designed to record reliably. We could not retrieve accurate amplitudes of the recorded vibrations for the first two quakes, which made it difficult to determine their magnitudes.

Fortunately, NEIC is a 24/7 operation, and they receive seismic signals from all over the world. Our quakes were large enough to be recorded on the mainland, and they were able to get more accurate estimates of the magnitude than HVO. The reported magnitudes were revised to magnitude-6.7 and magnitude-6.0 based on their work.

HVO staff continue to monitor the earthquake and volcano activity in Hawai`i, to answer phone and email questions from media and the public, and to repair and maintain in good working order all the instruments we have installed for just such an event. Please check our Web site for the latest information and go to the "Did You Feel It?" Web page to report earthquake effects.

————————————————————————————————————————————————————————————————

Volcano Activity Update

This past week, activity levels at the summit of Kīlauea Volcano have remained at background levels. The number of earthquakes located in the summit area has slightly increased (usually less than 10 per day that are large enough to locate), with the largest number located south and west of Halema`uma`u. Widening of the summit caldera, indicating inflation, continues.

Eruptive activity at Pu`u `O`o continues. On clear nights, glow is visible from several vents within the crater. Lava is fed through the PKK lava tube from its source on the southwest flank of Pu`u `O`o to the ocean. About 1 kilometer (1 miles) south of Pu`u `O`o, the Campout flow branches off from the PKK tube. The PKK and Campout tubes feed two widely separated ocean entries, at East Lae`apuki and East Ka`ili`ili, respectively. Both entries are located inside Hawai`i Volcanoes National Park.

In the last week, intermittent breakouts have occurred from the PKK tube just inland from the sea cliff at East Lae`apuki. Surface flows also have been active at the 200-400-ft elevation of the Campout flow.

Access to the sea cliff near the ocean entries is closed, due to significant hazards. The surrounding area, however, is open. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.

There were lots of earthquakes beneath Hawai`i Island reported felt within the past week. A magnitude-6.7 earthquake occurred at 7:07 a.m. H.s.t. on Sunday, October 15 and was located 20 km (13 miles) southwest of Kawaihae at a depth of 39 km (24 miles). Seven minutes later, a magnitude-6.0 earthquake occurred and was located 19 km (12 miles) northwest of Kawaihae at a depth of 19 km (12 miles). Fifteen aftershocks were reported felt between Kailua-Kona and Hawi, ranging in magnitude between 2.1 and 4.4, and occurring through Thursday. Three of these were located at depths shallower than 15 km (9 miles); the rest were located at depths between 25 and 45 km (15 to 28 miles).

In addition, there were four earthquakes beneath the lower east rift zone of Kīlauea reported felt Monday through Wednesday. These earthquakes had magnitudes ranging from 0.0 to 2.4 and were located east of Leilani Estates at depths less than 3 km (2 miles).

Mauna Loa is not erupting. During the past week, earthquake activity remained low beneath the volcano's summit (one earthquake was located). Extension of distances between locations spanning the summit, indicating inflation, continues at slow rates.