(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

At the U.S. Geological Survey’s (USGS) Hawaiian Volcano Observatory (HVO), we focus on keeping up with the flow of data coming into our systems and servers from HVO’s monitoring networks and instruments. But we also take time each November to recognize the anniversaries of two significant Hawaiian earthquakes that are both important to remember, even as we pore over new data.

Seismology and Seismometry. Ka‘oiki aftershocks recorded on a rotating drum seismograph at Desert station located within a few miles of the epicentral area of the Hawaii earthquake of November 16, 1983. USGS Photo

The first of these is the thirtieth anniversary of the magnitude-6.6 (M6.6) Ka‘ōiki earthquake that struck on November 16, 1983. The earthquake was named for the system of faults that extends up the eastern slope of Mauna Loa Volcano between the summits of Kīlauea and Mauna Loa.

The 1983 Ka‘ōiki earthquake was the most recent of at least three, and possibly five, moderate earthquakes, all greater than M5.5 and centered in the same region. This sequence includes earthquakes in November 1983, November 1974 (M5.5), and June 1962 (M6.1). Earthquakes in September 1951 and September 1941 may also have occurred in the same region but, prior to 1962, HVO was not able to precisely estimate earthquake locations and magnitudes.

In addition to the apparent 10-year intervals separating these earthquakes, the earthquakes in 1941, 1974, and 1983 all preceded Mauna Loa eruptions in April 1942, July 1975, and March 1984, respectively. It would require considerable effort to try to understand the detailed behaviors associated with these observations—for example, to understand why the 1983 earthquake, at M6.6, was significantly larger than the others in the series. We now look to future moderate-magnitude Ka‘ōiki earthquakes as possible indications of imminent Mauna Loa eruptions.

Hawaii Earthquake November 1975. Surf surges through a palm grove near Halape as a result of the 7.2 magnitude earthquake. Photo by B. Morrison, November 1975. Photo via USGS

A more significant November earthquake anniversary is that of the November 29, 1975, earthquake that was centered near Kalapana. This earthquake was considerably larger than the 1983 Ka‘ōiki earthquake. It resulted in widespread damage due to the earthquake shaking and also generated a tsunami that inundated the southeast coast of the island of Hawai‘i and resulted in two deaths.

In 1975, it was seismological practice to describe magnitudes of large earthquakes in terms of MS, or surface wave magnitude, derived from teleseismic measurements of seismic waves with 20-second periods of oscillation. For the 1975 Kalapana earthquake, MS was determined to be 7.2, which we continue to use when we mention this earthquake in presentations or other communications.

Seismologists have since recognized that the 20-second oscillatory period would not be fully representative of the rupture process of very large earthquakes. The MS estimate of magnitude was known to underestimate the energy released by very large earthquakes.

Hawaii Volcanoes National Park. 1975 earthquake damage to Crater Rim Drive. November 25, 1975. Photo courtesy of USGS

In 1979, the moment magnitude scale was proposed, based on the concept of seismic moment that can be related to the area of rupture and the average amount of slip. Seismic moment is now routinely computed from seismic records of large earthquakes, and use of moment magnitude MW—also simply labeled M—is now common when describing large earthquakes greater than M5. The concept is tremendously insightful and valuable, as it connects seismically derived parameters to fault properties, through physics and models of earthquake rupture.

In 2004, Drs. Meredith Nettles and Goran Ekstrom at Harvard—and now associated with the Global Centroid Moment Tensor (CMT) project supported by the National Science Foundation (www.globalcmt.org)—revisited analyses of the 1975 Kalapana earthquake. Several aspects—ambiguity of the mechanism of this earthquake, its tsunamigenic aspect, and recognition that the widely used MS might underrepresent the energy released by the earthquake—motivated their study.

Their detailed investigation accounted for a number of observations, including the tsunami. They showed that the earthquake mechanism is consistent with seaward fault movement along the décollement, or detachment surface, separating the volcanic edifice and the underlying ancient oceanic crust.

Along with these interpretations, Nettles and Ekstrom computed the moment magnitude MW to be 7.7. This is not to say that 7.2 was incorrect. Rather, it simply means that surface wave magnitude (MS), determined from the 20-second energy content, is an underestimate of the size of the earthquake.

HVO and the USGS continually strive to describe and catalog earthquake processes with increasing accuracy and interpretive content. In keeping with this, we need to embrace changes like referring to the 1975 Kalapana in terms of moment magnitude MW7.7 as we approach the earthquake’s 40th anniversary.