Volcano Watch — Most of Hawaii is vulnerable to big LA-style earthquakes

A magnitude-6.6 earthquake devastated the Northridge area of Los Angeles on Monday, January 17, at 4:31 a.m. The extent and type of damage in the Los Angeles region caused by an earthquake of this magnitude has been shocking. The many freeway collapses, building failures, ruptured water and gas lines have severely disrupted the infrastructure of the region. Much of this damage will take a year or longer to repair, particularly the damaged and destroyed freeeway sections. 

This was the third large earthquake in California in the last three-and-a-half years. The October 1989 Loma Prieta magnitude-7.1 earthquake caused less damage to the San Francisco bay area, and the June 1992 Landers magnitude-7.4 earthquake caused relatively little damage, despite their larger magnitudes. The main reason for these differences in damage is the proximity of the epicenter to densely populated areas. The epicenter of the Loma Prieta earthquake was in the lightly populated Santa Cruz mountains about 50 miles south of San Francisco, and the Landers earthquake was located in the Mojave Desert about 50 miles east of Los Angeles. In contrast, the Northridge earthquake was centered in the midst of the densely populated San Fernando Valley just north of Los Angeles. 

There has been some local news coverage following the Northridge earthquake to the effect that Hawai‘i has a relatively small earthquake hazard. This is simply not the case. Hawai‘i County, in particular, is seismically active and has had about 14 earthquakes with magnitudes larger than 6.0 in the last 125 years. The two largest, an estimated magnitude-7.9 one in 1868 and the magnitude-7.2 Kalapana earthquake in 1975, caused widespread damage in Hawai‘i, as did several smaller ones, such as the magnitude-6.6 Ka‘oiki earthquake in 1983. Large earthquakes beneath Hawai‘i are often caused by motion of the south flank of Kīlauea as it slips towards the sea. 

However, the other islands are far from immune to earthquakes, as seen in two earthquakes that occurred near Maui, one west of Lana‘i in 1871 and the other, north of Maui in 1938. Both had estimated magnitudes greater than 6.5 and caused damage on Maui and O‘ahu. These earthquakes belong to a family of earthquakes that occur beneath all the islands at depths as great as 30 miles. We know little about these earthquakes, but they are thought to be caused by stresses related to bending of the rigid outer layer of the Earth under the load of the great weight of the volcanoes.

Hawai‘i has many more smaller earthquakes that do not cause damage. On Hawai‘i, we average several such felt earthquakes each week. They serve as a gentle but persistent reminder that the Island of Hawai‘i is the most active seismic zone in the United States. The map shows the locations and gives the dates and magnitudes of the 14 larger earthquakes that occurred here in historical times. Future earthquakes of similar magnitudes pose greater risk because of the ever-increasing population of the island. 

We should not feel helpless about earthquakes because there are many things we can do to prepare for the next one. In particular, there are specific simple and inexpensive steps that can be taken to strengthen your house. Some of these steps will be outlined in a guest-authored "Volcano Watch" column by members of the American Institute of Architects that will appear in two weeks. We can also prepare ourselves for the next large earthquake by maintaining a home survival kit and following some simple precautions around our homes. 

The column next week, to be authored by the staff of the County of Hawai‘i Civil Defense Agency, will outline some specific steps you can take to make your home safer during an earthquake or other disaster, as well as present a list of supplies you should have available for emergencies.