Volcano Watch — The 28th anniversary of a very damaging earthquake

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In last week's article we mentioned that April is tsunami awareness month in Hawai`i and earthquakeawareness month in California.

This past Wednesday, two large earthquakes rocked the Kīlauea Volcano summit area to again remind us that we live in one of the most seismically active areas in the United States, and that residents of Hawai`i Island should be made aware of the hazard and take steps to mitigate it.

This advice could have been very helpful 28 years ago when one of the most destructive earthquakes struck the island. On April 26, 1973 at 10:26 a.m., a magnitude-6.2 earthquake was felt throughout the entire State of Hawaii. The epicenter of the earthquake was 48 km (28.8 mi) beneath the town of Honomu, north of Hilo. The deeper-than-usual location of the earthquake allowed energy to spread over a broader area. Over 5.6 million dollars of damage was attributed to the Honomu earthquake. And those are 1973 dollars!

Eleven people in Hilo were injured, four seriously enough to be hospitalized. An additional undetermined number were hurt in Waimea, where four students sustained injuries from falling fixtures at Waimea School. A man was pinned under the roof of a building that partially collapsed on Waianuenue Avenue.

Seven major slides occurred in the Hamakua district and blocked State Highway 19 for at least seven hours. Rockslides sealed off the three major gulches of Maulua, Laupahoehoe, and Ka`awali`i. Tourists trapped between slides were rescued by the Hawai`i County Fire Department helicopter and transported to their hotels or the airport.

Hawai`i County suffered extensive road damage in the Wainaku and Kaiwiki areas. Water pipes were cracked from Hilo to Papa`ikou, and service was not restored for several days. Electrical lines were down between Hilo and Hakalau, and power was turned off between Pepe`ekeo and Hakalau to prevent anyone from being electrocuted. Gravestones in Alae Cemetery were toppled, as was the rock chimney of the Waimea Steak House restaurant.

Downtown Hilo was extensively damaged. Many storefront windows were cracked, and nearly all unsupported overhangs fell. Several buildings on Waianuenue Avenue collapsed nearly completely, as did some homes in Pu`ueo, Wainaku and Amau`ulu. In all, 355 homes and 72 business establishments were significantly damaged.

Island schools were dismissed early, and all districts, except Ka`u, Kona, and North Kohala, were declared disaster areas. Access to downtown Hilo was by permit only to discourage looting.

The 1973 Honomu earthquake is one in a family of deep earthquakes that occur beneath and around the southern Hawaiian Islands. The weight of the volcanoes causes the rigid lithospheric plate on which the islands ride to bend and flex downward. This bowing of the lithosphere often exceeds the elasticity of the plate, and earthquakes result from brittle failure. There are no surface manifestations of this earthquake zone. These earthquakes can occur anywhere from the island of Hawai`i north to Moloka`i.

Wednesday evening's earthquakes were not of the lithospheric flexure variety. They were shallow and most probably the result of the stresses caused by the seaward movement of Kīlauea Volcano. The first earthquake occurred at 5:37 p.m. and was felt in Ka`u, Puna, Hilo, and Hamakua. The magnitude-4.4 earthquake was located 1 km (0.6 mi) southwest of the summit of Kīlauea Volcano at a depth of 5.2 km (3.1 mi).

The second felt earthquake occurred at 6:19 p.m. and was an aftershock with nearly the same location as the first one. The magnitude-4.0 event was reported felt in the same districts. Civil Defense authorities received no reports of damage or injuries from either earthquake. These earthquakes serve to remind us of the certainty that at some time we will have larger, even devastating earthquakes. Now is the time when we must prepare for them.

Information on ways to mitigate the earthquake hazard can be obtained from the Hawai`i County Civil Defense office (935-0031) or from the Center for the Study of Active Volcanoes (CSAV) at the University of Hawai`i at Hilo (974-7631). CSAV also has a web site with the necessary information (https://hilo.hawaii.edu/csav/).

Volcano Activity Update

Eruptive activity of Kīlauea Volcano continued unabated at the Pu`u `O`o vent during the past week. Breakouts from the tube system occur above Pulama pali and feed multiple pahoehoe and `a`a flows cascading down Pulama pali to the delight of television film crews from CNBC and The Weather Channel. Active surface flows are seen throughout the coastal flats. Lava was first observed entering the ocean in an area east of Kupapa`u beyond the eastern boundary of Hawai`i Volcanoes National Park at 3:30 p.m. on Wednesday, April 25. A small lava delta about 50 m (165 ft) long was formed by Thursday afternoon.

The two Wednesday evening earthquakes were the only events reported felt during the week ending on April 26.