# Volcano Watch — Hazard and risk: What's the difference?

September 10, 1993

The Center for the Study of Active Volcanoes at the University of Hawaii at Hilo and the U.S. Geological Survey's Hawaiian Volcano Observatory sponsored a public symposium on the prediction and mitigation of volcanic eruptions and earthquakes about a month ago.

The Center for the Study of Active Volcanoes at the University of Hawaii at Hilo and the U.S. Geological Survey's Hawaiian Volcano Observatory sponsored a public symposium on the prediction and mitigation of volcanic eruptions and earthquakes about a month ago. The differences between predictions and forecasts of eruptions and earthquakes and how they apply to Hawaii were discussed at the symposium and in last week's "Volcano Watch" column. This week we will discuss the difference between hazard and risk and some means to mitigate geologic risks.

Geologic hazards in Hawaii include earthquakes, landslides, and volcanic eruptions. Some secondary effects of these hazards, such as tsunami, may threaten an even greater region than that affected by the initial event. A hazard can exist without posing any risk to people if there are none living within the region affected by the hazard.

Risk is the likelihood that an event will occur in a certain time period, multiplied by the number of people who can be killed. In other words, risk involves people, buildings, and man-made infrastructure that can be damaged or destroyed by an event. The more people living in a region prone to hazards, the greater the risk. The more likely a certain hazard is to occur, the greater the risk to the people who live in that area.

There is virtually nothing that scientists or policy makers can do to reduce geologic hazards, as these events are beyond our control. Earthquakes, landslides, eruptions, and tsunami (not to mention weather-related hazards, such as hurricanes, tornadoes, winter snow storms, and floods) will occur, and there is nothing anyone can do about it. Risk, however, can be dramatically reduced by selecting where we will live and how well we construct our buildings.

In Hilo, a land-use decision was made, following the devastation wrought by the tsunami in 1946 and again in 1960, that the downtown area would not be re-built so close to the bay. The parks and playing fields along the bayfront occupy the former downtown area. This is an example of wise long-range planning that will dramatically reduce the loss of life and property when the next tsunami strikes.

Lava flows also pose a risk to people and property in Hawaii. The risk varies with the density of population and the likelihood that lava flows will cover an area within a certain time period. Since half of the risk calculation depends on the likelihood of a hazard occurring, the U.S. Geological Survey has determined lava flow hazards for all of Hawaii, based on frequency of eruptions and coverage by historic and prehistoric flows. The main mitigation against lava flow inundation is in land-use planning for very low population densities in Lava Hazard Zones 1 and 2. There is little else, short of building a house which can be readily moved, that can be done to reduce the risk posed by lava flows. The Lava Flow Hazard map is a tool available for the State and County to use in land-use planning. It also serves to educate people about the risks they assume when they chose to live in certain areas of Hawaii.

Hazard Zone 1 consists of the summit calderas and active rift zones on Kīlauea and Mauna Loa Volcanoes. We have now used digital mapping techniques to determine that 29.3% of Hazard Zone 1 on Kīlauea Volcano has been covered by flows in the last 150 years and that 23.5% has been covered in the last 50 years. The percentages covered on Mauna Loa are only completed for the eastern half of the volcano, but yield 57.3% covered in the last 150 years and 25.0% covered in the last 50 years.

Hazard Zone 2 consists of those areas immediately downslope from the active rift zones. On Kīlauea, 26.4% has been covered in the last 150 years; however, all of that was covered in the last 50 years. The large percentages covered in the last 50 years reflect the change from dominantly summit eruptions in the 1800s and early 1900s to dominantly rift eruptions since 1955. On Mauna Loa's eastern half, 33.1% has been covered in the last 150 years, but only 4.4% has been covered in the last 50 years. The low percentage area covered in the last 50 years reflects a dramatic decrease in eruption frequency on Mauna Loa after the 1950 eruption.

Hazard Zone 3 includes areas on the two volcanoes farther from eruptive vents that are covered less frequently. On Kīlauea, Hazard Zone 3 has seen only 2.1% covered in the last 150 years, although nearly all was covered in 50 years. On Mauna Loa's eastern half, only 1.6% has been covered in 150 years, and none of that was covered in the last 50 years. These low lava coverage rates in Zone 3 are the basis for distinguishing Hazard Zones 2 and 3.

At the symposium, Dr. Carl Johnson from the Geology Department at the University of Hawaii at Hilo made front-page news by stating that much of Hawaii should be zoned in Earthquake Zone 4 rather than the current Zone 3. He presented data that demonstrate that the earthquake hazard on Hawaii has been underestimated, thereby resulting in an inappropriately low seismic zonation. He noted that large earthquakes occur far too frequently here, compared to other areas in Zone 3, and that the most severe earthquakes here are far stronger than should be expected in Zone 3.

The Hawaii State Earthquake Advisory Board, on which Dr. Johnson sits, is evaluating the scientific evidence to support such a zonation change. Following their evaluation, the Board can recommend that the change be adopted. Such a change will be important in reducing the seismic risk on Hawaii because the Unified Building Code for Zone 4 has stricter requirements that will result in fewer people injured or killed and fewer homes damaged or destroyed when the next large earthquake strikes. Many of the requirements for buildings in Earthquake Zone 4 will also result in structures that can withstand hurricanes, as the lateral stresses caused by earthquake shaking and strong winds are similar.

In summary, man can do little to reduce or eliminate hazards, but he has a great opportunity to reduce the risk posed by natural hazards. The ways to reduce risk include proper land-use planning, adoption of appropriate building codes, and education so that people know what to expect during and after a disaster strikes. Because the population here continues to grow, the sooner such risk mitigation efforts are adopted, the more prepared we will be to face the next hazard.

For those of you who missed the symposium but wish you had come, Jones Space Link will give you another opportunity to see each of the four panel presentations and the discussion that followed. If interested, mark your calendar for 7:30 p.m. on October 10, 11, 13, and 16.