Volcano Watch — Strong earthquakes more frequent than thought

A magnitude 3.0 earthquake was widely felt in the Hilo area on Thursday afternoon at 1:45 p.m. The earthquake was about 25 miles deep and was located offshore about 10 miles east of Hilo. This type of earthquake is not caused by volcanic activity or by instability of the flanks of the active volcanoes, but by flexure of the lithosphere (the 60 mile outer layer of the Earth) due to the great weight of the volcanoes. 

An example of flexure from everyday life would be the bending of a board suspended between two scaffolds when a person stands near the middle of the board. The lithosphere bends downwards due to the weight of the island in much the same way that the board bends downwards due to the weight of the person. Such bending of the brittle lithosphere causes earthquakes. 

This type of earthquake can be quite large on Hawai`i and can also occur beneath or adjacent to the older islands. They occur at irregular intervals and are therefore difficult to predict. This is the type of earthquake that poses a seismic hazard to Maui and O`ahu. On April 9 at 5:10 a.m., a magnitude 3.5 earthquake of this type rattled Lana`i. The epicenter was just west of Lana`i. At 8:37 a.m. on April 30, another reminder of the potential for earthquakes occurred when a magnitude 3.3 earthquake rattled Moloka`i and was felt on West Maui, as well. The epicenter of this earthquake was just a few miles north of Moloka`i.

The most recent large, damaging earthquake of this type occurred on April 26, 1973, just north of Hilo. It was a magnitude 6.2 and caused $5.6 million worth of damage in the Hilo and Hamakua regions. Like the earthquake on Thursday afternoon, it was 25 miles deep.

On January 22, 1938, such an earthquake with an estimated magnitude of 6.7 occurred north of Maui. Parts of Maui experienced strong shaking of Mercali Intensity VIII. Mercali Intensities are another way of estimating the size of earthquakes based on the types of damage caused. Mercali Intensity VIII is defined as shaking severe enough to affect the steering of automobiles, damage unreinforced masonry, cause some damage to reinforced masonry, result in twisting and falling of chimneys, towers, and elevated tanks, move frame houses off foundations if they are not bolted down, break branches from trees, change the flow of wells and springs, and cause cracks in wet ground and on steep slopes.

A similar earthquake also occurred near Moloka`i on February 19, 1871. It had a similar estimated magnitude, and Mercali Intensity VII was recorded on O`ahu, Moloka`i, Lana`i, and West Maui. Mercali Intensity VII is characterized by difficulty in standing; is noticed by drivers of automobiles; causes hanging objects to quiver, large bells to ring, weak chimneys to break at the roofline, and plaster, loose bricks, stones, and tiles to fall; damages unreinforced masonry and concrete irrigation ditches; and creates waves on ponds and small landslides in sand or gravel banks. 

From these lists of the types of damage experienced during the earthquakes near Maui and Moloka`i in 1938 and 1871, it is obvious that these were strong earthquakes. If earthquakes of comparable magnitudes were to recur near Maui or O`ahu today, they would cause extensive damage. Such earthquakes are more common in that region than most people realize with strong earthquakes in 1870 near Moloka`i, in 1871 offshore Maui (discussed above), in 1881 near Maui, in 1887 near O`ahu, in 1938 north of Maui (discussed above), in 1940 off eastern Maui, and in 1948 near O`ahu. The expected maximum magnitudes (high 6's) and frequency of earthquakes in this region indicate that the seismic hazard is moderate compared to the high hazard on Hawai`i, where larger and more frequent earthquakes occur.