Volcano Watch — Deep earthquakes and lithospheric flexure

Last Sunday evening, March 19, the Hawaiian Islands were shaken by a moderate earthquake located south of Kahoolawe and west of Hawaii. The earthquake occurred at 10:30 p.m., had a magnitude of 4.5, and was located about 30 miles deep. The earthquake was widely felt on Hawaii, Maui, and Oahu. These deep earthquakes are commonly felt far away because the seismic energy is efficiently transmitted through the Earth's upper mantle. A similar, but smaller, deep earthquake occurred at 12:10 p.m. on Friday, March 17. This earthquake had a magnitude of about 3.0, was located north of Hawaii and east of Maui, and was also deep, although the depth was poorly constrained.

These two earthquakes are part of a moderately large family of deep earthquakes that occur beneath and around the Hawaiian Islands. They are not limited to Hawaii Island, as are most of the shallower earthquakes. The largest of these to occur in the recent past was a magnitude 6.2 earthquake that occurred beneath Honomu, Hawaii on April 26, 1973. The Honomu earthquake occurred at a depth of about 25 miles and was strongly felt throughout the islands. It caused $5.6 million in damage and injured 11 people. In the more distant past, several earthquakes estimated to have magnitudes of around 7.0 occurred much farther west along the chain.

The earliest of these took place in 1871 and was located north of Lāna‘i. It caused damage in Honolulu. The second occurred north of Maui in 1938 and caused damage on Oahu and Maui. We are not sure that these large pre-instrumental earthquakes are of the same type as the two smaller ones that occurred in the past two weeks. However, it seems likely that they were.

What causes these deep earthquakes? Such earthquakes occur when strain is released in the Earth's outer nearly-rigid layer (the lithosphere) beneath and adjacent to the islands. This strain almost certainly is caused by bending of the lithosphere under the enormous load of the volcanoes. As the volcanoes grow, they add more and more weight on the lithosphere of the Earth, causing it to bend and flex downward. About 100 miles away from the islands, the lithosphere flexes upwards in response to the volcanic load. Such bending results in earthquakes that are most frequent beneath the actively growing volcanoes and less frequent beneath the older volcanoes. After a period of time, probably several millions of years, the flexing of the lithosphere stops, and the accumulated strain is released in earthquakes. The older parts of the chain are therefore seismically inactive.

Another effect of the flexing of the lithosphere is to cause the islands to sink or subside. This subsidence is most rapid at Hawaii and decreases to the northwest. The rate of subsidence is negligible at Oahu, in agreement with the rarity of earthquakes beneath Oahu.

At Hilo, the shoreline has sunk at a rate of about 2.2 millimeters per year over the last 47 years, which is added to the roughly 1.4 millimeter per year rise in global sea level to give a rate of submergence of about 3.6 millimeters per year. At Kahului, Maui, the rates are 0.8 millimeters per year of sinking added to the global rate of sea level rise to yield a submergence rate of 2.2 millimeters per year. Rates of sinking for Hawaii have averaged about 2.6 millimeters per year over the last 500,000 years. These rates may seem very small, but they add up surprisingly quickly. For example, in the last 50 years, Hilo has sunk relative to rising sea level by 18 centimeters (a little more than 7 inches), whereas Maui has submerged a little more than four inches, and Oahu has submerged a little less than three inches. Global sea level is rising due to a net decrease in the volume of water stored in the polar ice caps and a commensurate increase in the volume of sea water in the oceans.

These differences in submergence rates, caused by the differences in the rates at which the islands sink, cause large differences in the ability of coral reefs to grow around the shorelines of the different islands. At the rapid submergence rates at Hilo and around the entire Island of Hawaii, reefs cannot grow fast enough to keep pace with the changes in apparent sea level, whereas on Oahu and Maui, the reefs can flourish. The volcanic loading and bending of the lithosphere that cause frequent deep earthquakes beneath and around Hawaii are also the cause of the near absence of coral reefs and white sand beaches around Hawaii!