Volcano Watch — Are earthquakes on the rise?

Small earthquakes that would normally never make their way into the news suddenly receive national attention. This often leads to the question, "Are we having more earthquakes these days?"

On a global scale, the U.S. Geological Survey's National Earthquake Information Center (NEIC) located over 23,000 earthquakes last year, their highest annual total ever. Prior to 1993, the NEIC never catalogued more than 20,000 earthquakes in one year.

Further study of the annual statistics shows that the apparent rise in earthquakes worldwide disappears above the magnitude 5 level. Since 1900, the average number of major and great earthquakes (those with magnitudes 7 or greater) was approximately 19 per year worldwide. In 2001, only 15 such earthquakes were catalogued by the NEIC. Thus, we attribute the rising earthquake totals to an increase in the number of smaller earthquakes that we are able to study because seismological tools have rapidly increased and improved over time.

A major factor allowing us to study more small earthquakes has been a more than ten fold increase in the number of seismographic stations reporting data to the NEIC, from 350 to over 4,000 in the last 70 years. Computerization and automation have further aided NEIC's earthquake cataloging capabilities, allowing for larger volumes of data to be processed faster and with less human intervention.

At the Hawaiian Volcano Observatory (HVO), our modern earthquake catalog extends back to 1960. From the end of 1960 to 2000, the seismographic network grew from 11 to 50 sites. By the late 1960s, radio telemetry had begun to replace direct cable connections, which facilitated new site installations and broader network coverage. Monitoring earthquakes associated with frequent eruptions also motivated significant expansion of our seismic network.

As at NEIC, HVO's evolving seismic recording and analysis tools, and the growth of our network, have allowed us to catalog greater numbers of small earthquakes. We have complemented the familiar rotating drum recorders with our third generation of computer-based data acquisition and processing systems. In the 1960s, earthquakes below magnitude 1.5 comprised only 2 percent of the catalog. Today, about 30 percent of all earthquakes we locate are below magnitude 1.5.

For geologic reasons, the number of earthquakes in Hawai`i have fluctuated widely. Earthquake swarms related to underground magma movement might occur. Large numbers of aftershocks following large earthquakes, like the November 1975 M7.2 Kalapana event, inflate catalog totals for many months afterward. To this point, the 1970s was the decade with the largest number of earthquakes, averaging 4,100 per year.

Through the 1990s, our numbers suggest that activity has declined. We averaged 3,300 earthquakes per year in the 1980s and 2,300 per year in the 1990s. Since 1989, we have had no large earthquakes above magnitude 6. Possibly, as a result of the extended and continuing Pu`u `O`o eruption, we have also seen fewer earthquake swarms.

With improved technology, we can locate, with greater precision, more of the smaller earthquakes occurring beneath Hawai`i. While we are enjoying a relatively quiet period of late, without large or damaging earthquakes, we hope to utilize the data from small earthquakes to further our understanding of the island's volcanoes and faults.

Volcano Activity Update

Eruptive activity of Kilauea Volcano continued unabated at the Pu`u `O`o vent during the past week and occasionally lit the night sky. Bright glow persists over the "rootless" shield area, where short flows emanate from overflows of the perched ponds and from the base of the shields. Flows on Pulama pali are mainly crusted over. Surface flows appear on the fan at the base of the pali and extend onto the coastal flats. The distal end of the flows is nearly three kilometers (1.8 miles) from the seashore.

There were no earthquakes reported felt during the week ending on March 14.