Two of Hawaii’s largest historical earthquakes occurred during an intense swarm of more than 6,200 earthquakes beneath Hualalai volcano between mid-September and early November 1929.
Two of Hawaii’s largest historical earthquakes occurred during an intense swarm of more than 6,200 earthquakes beneath Hualalai volcano between mid-September and early November 1929.
During the early part of the swarm, a magnitude-6.1 earthquake on Sept. 25 was followed by a magnitude-6.5 earthquake on Oct. 5. Both events generated strong ground shaking on the Island of Hawaii, causing significant damage, especially in Kona, to houses, masonry, water tanks, pipes, stone fences and roads.
Both earthquakes were felt as far away as Oahu and were followed by strong aftershocks that added to the damage.
As the anniversaries of these earthquakes approach, a quick review of the 1929 earthquake sequence provides a vivid reminder of the hazards associated with strong earthquakes in Hawaii. It also reminds us that Hualalai is an active volcano.
At the time, the seismic network in Hawaii was extremely limited, by today’s standards, so the estimated locations and magnitudes of the earthquakes are not well-known.
When the swarm began, the nearest seismometers were located at Kealakekua, Hilo, and the Hawaiian Volcano Observatory. A temporary station was installed by Sept. 21 at the Pu‘u Wa‘awa‘a Ranch near the center of activity.
Based on the pattern of ground-shaking, as reported by island residents, the Sept. 25 earthquake is estimated to have occurred at a depth of less than 10 miles. The Oct. 5 event is characterized as being deeper than 15 kilometers.
The swarm of earthquakes began on the afternoon of Sept. 19 and was first felt by people living near Pu‘u Wa‘awa‘a Ranch, 7 miles north of Hualalai’s summit. During the next few days, the earthquakes increased in number and intensity.
Frank Greenwell kept count of the earthquakes felt at his residence 6 miles southwest of Hualalai’s summit and reported them to the observatory. He recorded 444 felt quakes over a three-day period, averaging six felt quakes per hour. There were even more earthquakes that were not felt. During the first five days of the swarm, a seismic station at Kealakekua recorded up to 280 “disturbances” per day.
The temporary station at Pu‘u Wa‘awa‘a Ranch recorded the highest number, with a peak of 599 earthquakes on Sept. 26.
During the first week of the seismic crisis, Dr. Thomas Jaggar, director of the Hawaiian Volcano Observatory (1912-1940), and his colleagues could not be certain whether the earthquakes were associated with Mauna Loa or Hualalai, or both. With the quakes seeming to be located between Mauna Loa and Hualalai, observatory scientists expressed early concern about the potential eruption of the more active Mauna Loa.
Jaggar visited the summit of Hualalai on Sept. 22 and found no fresh cracks or signs of heat or new steaming vents. A flight over the island with the Naval Air Service on Oct. 3 revealed nothing unusual on the volcano and no lava outbreaks.
After the Sept. 25 magnitude-6.1 earthquake, however, the Honolulu Advertiser quoted Jaggar as saying “We may expect an eruption, possibly from the northwest flank of Hualalai within a month, if the earthquakes which have shaken the island of Hawaii for the past five days, particularly in the Kona district, continue.”
The earthquakes signaled to Jaggar that magma was moving upward in the direction of Hualalai.
By the middle of October, the frequency and intensity of earthquakes had diminished substantially, although a few events were still felt in Kona. The seismic crisis — which caused significant earthquake damage but resulted in no eruption — was essentially over by early November.
The 1929 seismic event is a good reminder that Hawaii residents have, and will again, experience destructive earthquakes and should be prepared for the next “big one.” To that end, we encourage you to participate in the upcoming Great Hawaii ShakeOut earthquake drill. More information is posted at http://shakeout.org/hawaii/.
Kilauea
activity update
A lava lake within the Halema‘uma‘u Overlook vent produced nighttime glow that was visible from the Jaggar Museum overlook and via HVO’s webcam during the past week. A deflation-inflation cycle (DI event) early in the week caused the summit lava lake level to fluctuate sympathetically.
On Kilauea’s East Rift Zone, small active breakouts from the Peace Day tube are scattered widely across the coastal plain, but lava is no longer entering the ocean. Above the pali, the Kahauale‘a 2 flow, fed from a spatter cone on the northeast edge of the Pu‘u ‘O‘o crater, continues to burn forest north of Pu‘u ‘O‘o.
One earthquake was reported felt on the Island of Hawaii in the past week. A magnitude-3.9 earthquake occurred Monday at 11 a.m. and was located 60 miles northeast of Laupahoehoe at a depth of 23 miles.
Visit the HVO website (http://hvo.wr.usgs.gov) for Volcano Awareness Month details and Kilauea, Mauna Loa and Hualalai activity updates, recent volcano photos, recent earthquakes, and more; call 967-8862 for a Kilauea summary; email questions to askHVO@usgs.gov.
Volcano Watch (http://hvo.wr.usgs.gov/volcanowatch/) is a weekly article and activity update written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.