Volcano Watch — Tsunami threat real in Hawaii (submitted title: The 1960 tsunami, Hilo)

Thirty-four years ago, on May 23, 1960, a tsunami destroyed much of downtown Hilo. Tsunami, or seismic sea waves, are generated in several ways, including by large submarine explosive eruptions, by landslides where rock slides into or beneath the sea surface, and by large earthquakes that displace rocks below sea level. The waves generated spread outward in all directions and travel across the oceans at speeds between 425 and 500 miles per hour. Most tsunami that cause widespread damage are produced by large earthquakes that cause fault movements of the sea floor, including the one in 1960. These giant waves wreak their havoc first near to, and then far from, the site of the original earthquake.

The earthquake that caused the 1960 tsunami occurred off the west coast of South America and had a magnitude between 8.25 and 8.5. The waves reached the Hawaiian Islands in about 15 hours. This tsunami caused little damage elsewhere in the islands, but the Hilo Bay area was hard hit. Sixty-one people lost their lives and about 540 homes and businesses were destroyed or severely damaged. The wave heights in Hilo Bay reached 35 feet compared to only 3-17 feet elsewhere. The water washed as far inland as Kīlauea Avenue/Keawe Street through the entire present downtown area and to Kekuanaoa Street near Kīlauea Avenue.

Each tsunami consists of a succession of waves that arrive from 12 to 20 minutes apart. The 1960 tsunami had eight separate waves that crested between 4 and 14 feet above sea level at the Wailuku River bridge. The first wave is not neccessarily the largest, and each wave may crest higher at different locations. Each tsunami may also have its maximum crest at a different location. This fact contributed to many deaths in Hilo in 1960, because people remained in the Waiakea peninsula area, which had minimal damage during the even larger 1946 tsunami.

It is easy, but dangerous, to forget about tsunami because it has been so long since the last one occurred, but damaging tsunami caused by distant earthquakes have occurred with startling frequency in Hawaii: 1837, 1841, 1868, 1869, 1877, 1883, 1906, 1918, 1923, 1933, 1946, 1957, and 1960, in addition to locally generated damaging tsunami in 1868, 1872, and 1975. Other tsunami have been generated by distant earthquakes in 1896, 1901, 1906, 1919, 1922, 1923, two in 1927, 1928, 1929, 1931, 1938, 1944, 1952, and 1964, but the waves were too small to cause damage here. In other words, we have experienced about one damaging tsunami every 12 years for the last 157 years, but none in the last 34 years.

Tsunami that have caused damage in Hawaii have been generated by large, distant earthquakes and by local earthquakes. Today, tsunami generated by distant earthquakes are tracked by the National Oceanic and Atmospheric Administration's (NOAA) Pacific Tsunami Warning Center in Ewa Beach on Oahu. The minimum elapsed time between the earthquake and the arrival of the tsunami in Hawaii is about 4.5 hours for earthquakes in the central Aleutian Islands. Tsunami generated by earthquakes elsewhere around the rim of the Pacific Ocean have elapsed times of as long as 15 hours (like the one in 1960 from near South America). These times are adequate to issue warnings and evacuate low-lying areas on the islands. The size and danger posed by tsunami are not straightforward to predict, and occasionally false alarms may occur. In these instances, the earthquake occurs, but for reasons that are not clear, no tsunami or only a small tsunami may be produced.

On the other hand, tsunami generated by local earthquakes may have extremely short time periods between the earthquake and the tsunami. Although only a few local earthquakes have been large enough to generate tsunami during historical times, those in 1868 (magnitude 7.9) and in 1975 (magnitude 7.2) produced tsunami that were large enough to kill people. With the increasing population along the coastlines of Hawaii, any future locally-generated tsunami pose an even greater threat to life and property. It is unlikely that adequate warnings or orderly evacuations can occur because of the short time period (as little as a few minutes if you are near the earthquake epicenter) between a local earthquake and a tsunami it could generate. Your best precaution if you feel a strong earthquake is to immediately move to higher ground if you are near the coast at a low elevation.

The Civil Defense pages in the front section of the phone book (pages 45-53) include maps of coastal areas around the island showing the areas where tsunami inundation can occur and the evacuation routes to use. You should heed all warnings. To do otherwise is, literally, to gamble with your life.