Volcano Watch — Most active seismic zone in the nation here

Nearly everyone on the island of Hawai`i should have felt an earthquake this past week. As the accompanying map shows, Hawai`i was rattled by 11 earthqukes large enough to be felt in less than four days. 

The largest of these earthquakes, with a magnitude determined to be 5.0, was the largest in Hawai`i since mid-1990. The earthquakes began with a swarm just outside Kīlauea caldera and near the Volcano Golf course. The swarm began last Sunday evening shortly after 10:00 p.m. The second large quake registered 4.5 and got the attention of most residents between Pahala and Mountain View. The swarm consisted of about 750 earthquakes, but only eight of them had magnitudes greater than 3.0 and were felt beyond the Volcano area. All of the located earthquakes in this swarm are at shallow depths (from 2.5 miles to near the surface). This swarm does not appear to be directly related to migration of magma inside Kīlauea Volcano, but may be related to subsidence of the summit caldera as a result of 10 years of continuous withdrawal of magma from Kīlauea's summit.

On Monday afternoon, a magnitude 3.6 earthquake rattled the Hamakua coastal region. This earthquake was about 24 miles deep, much deeper than those near Kīlauea's summit the previous night and morning. This and other deep earthquakes, like the one beneath Honomu on April 26, 1973, were caused by bending of the rigid outer part of the Earth under the weight of the volcanoes that make up the islands.

On Tuesday morning, many people were awakened by a magnitude 5.0 earthquake that occurred at 5:24 a.m. This earthquake was widely felt from the Kona coast to Hilo. This earthquake has had a sequence of roughly 350 aftershocks, but none have been large enough to be felt beyond the Pahala area. This earthquake was located near a moderate-sized earthquake that occurred on January 21, 1982. More importantly, the location is also near that estimated for the great earthquake of April 2, 1868. That earthquake was the largest in historical times and had an estimated magnitude of about 8.0. In an earlier column, I included some of the colorful, but frightening, eyewitness accounts of that earthquake. Earthquakes in this area are caused by seaward movement of the south flank of the island along a nearly horizontal fault plane that separates the overlying volcanic pile from the underlying ocean crust.

As if the week had not had enough shaking, another earthquake occurred on Thursday afternoon. This one was located beneath the south flank of Kīlauea Volcano near the East Rift Zone. It had a magnitude of 3.0 and was located about 6 miles beneath the surface. Like the earthquake located near Pahala a few days earlier, this one was caused by seaward movement of the south flank of the island.

The earthquakes that occurred this past week caused little damage beyond having a few books fall to the floor. However, they serve as a gentle but persistent reminder that the Island of Hawai`i is the most active seismic zone in the United States. The second map shows the locations and gives the dates and magnitudes of larger earthquakes that occurred here in historical times. These 14 earthquakes, with magnitudes greater than 6.0, have occurred in the last 125 years; most of them caused damage. Future earthquakes of similar magnitudes pose greater risk because of the ever-increasing population of the island. We can prepare ourselves for the next large earthquake by maintaining a home survival kit and following some simple precautions that can be found in the pages on Civil Defense instructions of the telephone book.

None of these earthquakes affected the ongoing eruption on Kīlauea's East Rift Zone. The lava flows continue to enter the sea near Kamoamoa.