Volcano Watch — Isle may be zoned too low for level of seismic activity

The U.S. Geological Survey maintains a network of more than 50 seismic stations on Hawaii Island. The instruments and their distribution were originally designed to record and locate the many small earthquakesassociated with magma movement inside Kīlauea and Mauna Loa Volcanoes. Because most of the volcanic activity during the past 25 years has been at Kīlauea Volcano, we concentrated the stations there.

In the last few years, we have made a concerted effort to improve our ability to reliably record the larger earthquakes that occur here. Such improvements are of many types, but include new and different instruments installed in the field, better computer handling of the recorded seismic data, solar power for the field stations, and better backup systems so that the network will remain in operation during and after a large earthquakewhich may disrupt electrical power or physically damage the Observatory or its contents. Many of these changes have been completed, but the process continues.

We also have expanded the network on and around Mauna Loa Volcano so that we can better predict the next eruption there. Preceding the last two eruptions on Mauna Loa, in 1975 and 1984, there were year-long increases in seismic activity. Today, we have not seen such an increase in seismic activity, which suggests that the next eruption is a year or more away. However, magma continues to migrate up into the magma reservoirlocated beneath the summit region, as it has since the end of the eruption in 1984.

In 1970, Hawaii Island was assigned to seismic zone 3, the next to highest hazard zone, based on its history of large earthquakes. Within a few years, two additional large damaging earthquakes occurred here: the 1973 Honomu magnitude-6.2 earthquake and the 1975 Kalapana magnitude-7.2 earthquake. Since then, there have been additional large earthquakes in 1983 near the Volcano Golf Course (magnitude 6.6) and in 1989, again in Kalapana (magnitude 6.1). These earthquakes reinforced the idea that Hawaii Island was prone to frequent large earthquakes and warranted a high seismic zonation.

In 1970, the highest seismic zone (zone 4) was reserved for regions inside zone 3 that were in close proximity of known major fault systems. The main earthquake faults in Hawaii, unknown in 1970, are near-horizontal and located near the base of the volcanoes, and hence do not crop out on land. Starting with the 1988 Uniform Building Code, the definitions of the different seismic zones were changed and are now based on the 10% likelihood that certain peak ground accelerations will be exceeded within a 50 year period. Peak ground accelerations are expressed as a percentage of the acceleration due to gravity, where a vertical acceleration equivalent to gravity would momentarily make something weightless and an acceleration greater than gravity will cause items (such as your house) to be thrown off of the ground.

Hawaii County approved the 1991 version of the Uniform Building Code this past year, mainly in response to the extent of hurricane damage experienced on Kauai from Hurricane Iniki. A side-benefit of this newer code for residents of Hawaii Island is that it will also make homes constructed according to its guidelines more resistant to earthquakes. Adherence to the new building code will ensure that damage from the next large earthquake will be minimized.

On Friday and Saturday of this week, we displayed information about the seismic hazards on Hawaii at the Hawaii Island Contractors' Association Building Expo held in Hilo. The display included information about past earthquakes including maps showing peak ground accelerations experienced during those earthquakes. We also put on display a new interactive computer system that updates maps with the latest earthquakes on the Island. There are a series of animations that show seismic activity through time on the Island as well as some animations showing the sequence of events during the 1989 Loma Prieta earthquake in California. We have been developing this system as a means of teaching the public about seismicity in Hawaii and in the rest of the United States. It is intended as a teaching tool for the Visitors Center at the Jaggar Museum adjacent to the Hawaiian Volcano Observatory and for classroom use throughout the Island.

We have also been working with the Hawaii State Earthquake Advisory Board to develop an earthquake monitoring capability for the older islands because all of the larger earthquakes in the State do not occur beneath Hawaii. In order to correctly define the earthquake hazard, a record of past earthquakes and the ground accelerations they produced is required. For the older Hawaiian Islands, such a record is mainly provided by compiling damage reports.

A modern seismic network that records the smaller earthquakes that occur much more frequently than the large damaging events allows us to calculate the frequency of those infrequent larger earthquakes; in the State of Hawaii, only Hawaii Island has such a network. Such data help to constrain the earthquake hazard and ensure that the seismic zonation is neither too high nor too low. If the zoning is too low, or the latest version of the Uniform Building Code is not adopted or enforced, much more extensive damage and loss of life can occur during an earthquake (or hurricane). On the other hand, if the zonation is too high, buildings are built to withstand earthquakes or hurricanes that are unlikely to occur. Such overbuilding adds to the cost of the structure, but does not reduce losses since the damaging events do not occur.

Two different analyses of seismic hazard on Hawaii, recently published by the U.S. Geological Survey, suggest that Hawaii Island is presently zoned too low. One of these studies is based on the frequency of large and small instrumentally-recorded earthquakes and the peak ground accelerations measured here. The other is based on an analysis of all large historic earthquakes, many of which occurred before instrumental recording, and the distribution of damage and related earthquake phenomena. The results from the two studies are similar and document that Hawaii Island has had, and will have, frequent large earthquakes and strong ground accelerations. Adhere to the Uniform Building Code and you can minimize losses and damage to your home when the next large earthquake strikes Hawaii.