Volcano Watch — Volcano Watch (no. 5)

Kīlauea Volcano continues to erupt from both the Kupaianaha vent on the East Rift Zone and from the new episode 49 fissure located between Pu`u `O`o and Kupaianaha. The output of lava from each vent continues to decline, with the output from Kupaianaha nearly stopped. All flow activity is restricted to above the 1,900-foot elevation. The distribution of lava is unchanged from the map in last week's column, so we elected to use this lull in activity to describe the earthquake hazards present on the Big Island.

During a typical week, 300 to 400 located earthquakes occur beneath the Big Island, as recorded on the U.S. Geological Survey's seismic network. However, only a few have magnitudes greater than 3.0, roughly the threshold for felt earthquakes. This past week, the only such earthquake, which occurred at 7:50 a.m. on November 23, had magnitude 3.3 and occurred beneath the Ka`u district. Recent earthquake activity has been limited to many small events; the largest, occurring on November 1, had a magnitude of 4.3 and was located beneath Kīlauea's south flank. This earthquake was felt over much of the southern part of the Big Island, but no damage was reported.

In the past the Big Island has experienced numerous large damaging or potentially damaging earthquakes. The map shows the locations of earthquakes of magnitudes greater than 5 which have been recorded since 1962, when the seismographic network operated by the Hawaiian Volcano Observatory was greatly improved. Earthquakes at this magnitude are potentially damaging, whereas those of magnitude 7 or greater occurring near populated areas will cause substantial property damage. The most recent major earthquakes beneath Hawai`i are the April 26, 1973, magnitude-6.2 earthquake centered 25 miles beneath Honomu; the November 29, 1975, magnitude-7.2 earthquake centered near Kalapana; the November 16, 1983, magnitude-6.6 earthquake in the Ka`oiki region between Kīlauea and Mauna Loa Volcanoes, and the June 25, 1989, magnitude-6.2 earthquake centered near Kalapana. As a comparison, the deadly and destructive October 17, 1989, Loma Prieta (Oakland) earthquake in central California was magnitude 7.1. 

The 1975 Kalapana earthquake resulted in 11 feet of coastal subsidence, generated a tsunami that took two lives, destroyed homes in Punalu`u, sank boats in Keauhou Bay, damaged boats and piers in Hilo, and was felt along the island chain. Financial losses associated with the 1983 Ka`oiki earthquake were estimated at $7 million, and structural damage occurred from the North Hilo to Ka`u districts. The 1973 Honomu earthquake caused over $5 million of damage. The 1989 Kalapana earthquake caused an estimated $1 million in property damage in the Puna and Hilo areas.

The largest Hawaiian earthquake in recorded history occurred beneath the Ka`u district in 1868. This event is thought to be the equivalent of up to a magnitude-8 earthquake. The earthquake caused damage over the entire Big Island and was felt as far away as Kaua`i. It triggered a landslide and generated a tsunami that took lives and destroyed several villages. However, because there were no instrumental records of this earthquake, both the location and magnitude can only be roughly estimated from the historic accounts of its effects, but it was an event comparable in magnitude to the 1906 San Francisco earthquake.

Large earthquakes have occurred in the past and will occur in the future here on the Big Island. Some useful information on tsunami escape routes, how to prepare for an emergency, and what to do in the event of a large earthquake can be found in the Civil Defense pages in your phone directory; more detailed brochures on earthquake preparedness can be obtained at the County Civil Defense office in Hilo.