Foreshocks, mainshocks, and aftershocks - oh, my!

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Many of us in Hawai`i received a rude awakening on October 15, 2006, as the first of two earthquakes struck near Kiholo Bay on the west side of the Big Island. 

Foreshocks, mainshocks, and aftershocks - oh, my!...

The graph shows the daily number of aftershocks (as of November 12) for the October 15, 2006 M6.7 Kiholo Bay earthquake. Note the exponential decay in activity.

(Public domain.)

Since then, numerous additional earthquakes have occurred in the same area, including a magnitude (M)5.0 on Thanksgiving morning. This elevated earthquake activity raises a number of questions. Are all of these earthquakes related? Has the seismic activity in Hawai`i increased? Is a bigger earthquake on the way?

These are important questions to people who are worried about the likelihood of future damaging earthquakes. Perhaps it will bring some reassurance that, thus far, Kiholo Bay earthquakes since October 15 mirror trends observed after countless other quakes around the world.

Immediately after the October 15 earthquakes, rumors were flying that a larger quake was imminent. These rumors implied that the October 15 events were foreshocks. While foreshocks are real and have been recorded before some large quakes, they are usually part of a build-up. Over the course of a few days, the number of earthquakes increases, culminating in a larger event, called a mainshock. However, following October 15, the number of earthquakes has been decreasing, indicating that they are aftershocks, and not foreshocks.

In fact, the decreasing rate of Kiholo Bay earthquakes since October 15 has followed a relationship first worked out in 1894 by the Japanese geophysicist Fusakichi Omori. Omori found that the number of aftershocks following a major earthquake decayed exponentially over time. With only small deviations, this model has described the aftershock occurrence after most major earthquakes worldwide, including the October 15 Hawai`i quakes. For example, on October 15, there were about 74 significant aftershocks to the M6.7 earthquake. This number decreased to about 11 the following day, and to 8 on October 18. Now, seven weeks later, we are seeing only 1-3 earthquakes per day in the region.

Aftershocks occur geographically near the mainshock, usually on the same, or on a nearby, fault. The smaller earthquakes typically represent minor readjustments as stresses caused by the earthquake relax along the part of a fault that slipped during the mainshock.

The total number of aftershocks generally scales with the magnitude of the mainshock. In other words, bigger mainshocks are followed by more aftershocks. Aftershock sequences can last for many years following a very large earthquake, and aftershock durations vary in different parts of the world.

For example, current projections show that aftershocks following the 1995 Kobe, Japan, earthquake, which was similar in magnitude to the M6.7 October 15 earthquake, will probably continue until about the year 2018. Aftershocks from the December 26, 2004, M9.3 Sumatra earthquake may occur for several decades. Closer to home in Hawai`i, aftershock durations following the 1975 M7.5 Kalapana and 1983 M6.7 Ka`oiki earthquakes are roughly 10 and 1-2 years, respectively.

Interestingly, although the number of aftershocks decreases over time, there is no way to predict their magnitudes. Aftershocks can be felt for months after a major earthquake, with the smaller events lasting years or even decades. The largest aftershock is usually about a full magnitude less than the mainshock.

Let's take the 1983 M6.5 Coalinga, California, earthquake, as an example. After the mainshock in May of that year, six aftershocks with magnitudes between M5 and M6 occurred in the following four months. With the M6.7 and M6.0 events of October 15, we should expect one or more aftershocks in the M5 to M6 range. The M5.0 Kiholo Bay aftershock of Thanksgiving morning is therefore not unexpected, and quakes of similar magnitude may occur in future months.

The take-home message is that the earthquakes recorded thus far in west Hawai`i following the October 15 activity are typical of a mainshock-aftershock sequence. There is no evidence suggesting that a larger earthquake is on the way, and the current seismicity, though disconcerting, is a normal byproduct of large earthquakes.

So what can we learn from this? The recent earthquake activity serves as a poignant reminder that Hawai`i is one of the most seismically active area in the United States and is frequently exposed to large, damaging quakes. Earthquakes cannot be predicted or prevented, so it is important that we be prepared for such natural events. Information and links about earthquake preparedness can be found at the Hawaiian Volcano Observatory website.

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Volcano Activity Update

This past week, activity levels at the summit of Kīlauea Volcano have remained at background levels. The number of earthquakes located in the summit area is low (usually less than 10 per day that are large enough to locate). Widening of the summit caldera, indicating inflation, stopped in early October.

Eruptive activity at Pu`u `O`o continues. On clear nights, glow is visible from several vents within the crater. Lava is fed through the PKK lava tube from its source on the southwest flank of Pu`u `O`o to the ocean. About 1 km south of Pu`u `O`o, the Campout flow branches off from the PKK tube. The PKK and Campout tubes feed two widely separated ocean entries, at East Lae`apuki and East Ka`ili`ili, respectively. Both entries are located inside Hawai`i Volcanoes National Park.

In the last week, intermittent breakouts from the Campout tube have occurred near the base of Pulama pali and on the coastal plain.

Access to the sea cliff near the ocean entries is closed, due to significant hazards. The surrounding area, however, is open. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.

There were four earthquakes beneath Hawai`i Island reported felt within the past week. Only one of these quakes was an aftershock of the October 15 magnitude-6.7 earthquake. A magnitude-5.0 aftershock occurred at 9:20 a.m. H.s.t. on Thursday, November 23, and was located 21 km (13 miles) southwest of Waikoloa Village at a depth of 38 km (23 miles). A magnitude-1.9 earthquake occurred at 10:47 p.m. the same day, and was located 3 km (2 miles) south of Laupahoehoe at a depth of 12 km (8 miles). A magnitude-1.7 earthquake occurred at 6:39 a.m. on Friday, November 24, and was located 7 km (4 miles) west of Kailua at a depth of 32 km (20 miles). A magnitude-3.2 earthquake occurred at 5:49 a.m. on Saturday, November 25, and was located 10 km (6 miles) northeast of Kukuihaele at a depth of 14 km (8 miles).

Mauna Loa is not erupting. During the past week, earthquake activity remained low beneath the volcano's summit (one earthquake was located). Extension of distances between locations spanning the summit, indicating inflation, continues at slow rates.