# Volcano Watch - The strong, silent type

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In early November 2000, the largest earthquake to strike the Big Island in more than a decade occurred, yet no one, not even seismologists, noticed it. This earthquake was not located a great distance offshore; indeed, part of the fault that slipped lies beneath the Chain of Craters Road where it approaches the currently active lava flow. So why was this quake not felt?

Like many riddles, the answer involves a play on words. The phenomenon that occurred in early November 2000 was, in almost every respect, identical to a typical earthquake: the island moved in response to sudden slip on a fault. The only difference was that instead of lasting just a few seconds, the November 2000 event lasted for nearly 36 hours.

Slip this slow does not excite seismic waves, and without seismic waves there is no shaking. Unobtrusive earthquakes of this type are called "silent" earthquakes, a moniker that admittedly is very nearly self-contradictory. We ask, however, that geologists be granted a little poetic license, since the alternative is the very un-poetic "aseismic slip event."

Why faults sometimes slip silently and why they sometimes produce violent earthquakes is not well understood. In fact, as far as complexity goes, the silent vs. the violent behavior of faults is just the tip of the iceberg. Fault slip at Kilauea is known to occur in at least three different modes. The first is the typical earthquake, consisting of nearly instantaneous slip and depending on the size of the fault considerable shaking.

The second is the persistent and very slow slip observed year after year by instruments like Global Positioning System (GPS) receivers. The persistent slip is, at maximum, about 7 cm/yr (3 inches/yr), though in past decades it may have been much higher. To put things in perspective the persistent slip is about one trillion times slower than the slip occurring in earthquakes.

The third mode of fault slip on Kilauea is the silent earthquake. Though a billion times slower than normal earthquakes, silent slip is still hundreds of times faster than the persistent rates.

A number of factors control when, where, and how faults slip. Fundamentally, slip occurs when the forces encouraging slip exceed those that resist it. The resistive forces arise from friction on the fault's surface combined with the pressure of the overlying rock. In this part of the world, the forces that encourage fault slip come mainly from volcanic processes; however, the slow, gravity-driven collapse of the island can also contribute.

With the exception of sudden volcanic events like eruptions or large underground magma movements, most of the forces encouraging fault slip are very steady over time. Yet, as discussed above, fault slip on Kilauea exhibits at least two additional modes beyond the steady persistent slip: normal and silent earthquakes. One way of explaining these departures from steady state is to invoke the perturbing forces of volcanic events. These events explain a good-sized fraction of earthquakes, but not all of them. The other culprit is probably friction, which, on deeply buried faults, is thought to be quite a dynamic quantity.

Understanding the nature of friction on faults is one of the most important areas of current geophysical research. In many ways, Kilauea Volcano is a great natural laboratory for such research. The faults here show a diverse range of slip behavior, and they tend to slip rather frequently, as long-time island residents will attest.

Faults on this island are at least as active as their counterparts in California. Indeed, in many respects, they are more active. Processes that might take decades to study elsewhere in the world can evolve in just a few years here. So, along with being home to one of the world's most active volcanoes, Hawaii is also home to one of the world's most active centers of fault slip research. The ultimate goal is to understand (and dare we say predict?) when and how faults will slip, and to know whether they will slip silently as in November 2000 or violently as in November 1975.

### Volcano Activity Update

Eruptive activity of Kilauea Volcano continued unabated at the Puu Oo vent during the past week. The "Mother's Day" flow is inflating and moving across the coastal flats beyond the base of Paliuli. The leading edge of the flow is approaching the ocean near the old "High Castle" ocean entry, and at 1:30 p.m. on July 18, it was within 25 m (80 ft.) of the old seacliff and 85 m (280 ft.) from the ocean. Lava last entered the ocean in this area during Spring of 1995 and will probably be flowing into the sea again by the time that this article is printed. The lava viewing is spectacular, and the National Park Service allows visitors to hike out and get up close to the active flows.

The eastern Boundary flow emanating from the "rootless" shields displayed resurgent activity on Pulama pali this past week. Lava was observed streaming down the pali and forming a fan at the base of the pali. A breakout from the fan fed a flow heading west.

Three earthquakes were reported felt during the week ending on July 18. Two earthquakes were felt by residents of Leilani Estates on July 12. The shallow earthquakes, located 6 km (3.6 mi) east of Pahoa, occurred at 2:37 p.m. and 3:35 p.m. with magnitudes of 2.2 and 2.3, respectively. Residents of Papaikou, Papaaloa, and Kapulena felt an earthquake at 6:15 a.m. on July 13. The magnitude-2.6 earthquake was located 11 km (6.6 mi) north of Honomu at a depth of 25.8 km (15.5 mi).