# Volcano Watch - Monitoring volcanic activity with electronc tiltmeters

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Volcanic activity often results in various forms of ground motion. The scale of this motion, or deformation, to use the scientific term, ranges from the imperceptible bulging of a magma chamber to the meters-tall fault scarps formed during large earthquakes. The focus of today's article is at the lower end of this range-deformation that is too small for human perception.

HVO has a variety of instruments designed to detect these very small ground motions. One of the most important and useful of these instruments is the electronic tiltmeter. To understand what a tiltmeter measures, consider what happens to the ground above a pressurizing magma chamber. During pressurization, a bell-shaped zone of uplift forms, with the maximum uplift over the center of the chamber. Of course, the bell in question is rather flat, since the maximum uplift from even a large event is often only a few centimeters (an inch or two). A tiltmeter located in the uplifting zone detects the deformation by sensing the minute change in slope caused by the uplift.

Slopes are measured in degrees or radians (1 radian = 57.3 degrees), but the slope changes caused by volcanic deformation are about a million times smaller than a single radian. Hence, scientists often use units of microradians (1,000,000 microradians = 1 radian) to describe tilt measurements. To put this scale in perspective, imagine a long plank extending from one end of a football field to the other. Placing a penny under one end of the plank would create a tilt of 50 microradians, an amount of tilt not seen at Kilauea's summit since the onset of the current eruption in January 1983. A modern tiltmeter can detect tilt changes 500 times smaller than this.

Last Mother's Day (05/12/2002) the scientists at HVO woke up to an email warning of a 2.5 microradian tilt at Puu Oo. The direction of the tilt was toward the cone, indicating subsidence caused by rapid magma withdrawal. The 2.5 microradians was a good-sized signal, but not big enough to warrant a formal alarm. A few minutes later however, things really got moving-the tiltmeter on the north side of Puu Oo cone started to measure a large tilt that quickly exceeded 10 microradians. This triggered the main alarm, and HVO suddenly became very busy for a Sunday morning. By the end of the day, more than 17 microradians of tilt had been recorded, and it was clear that a major flow was issuing from the southwest flank of Puu Oo. Thus, the tiltmeter was recording the deflation of Puu Oo as part of its magma system drained away.

HVO maintains a network of more than a dozen electronic tiltmeters. Three instruments monitor Kilauea's summit, two monitor Puu Oo cone, and four more are distributed along the length of the east rift zone. The remaining instruments are at Mauna Loa's summit waiting for the day when that volcano finally wakes from its almost two-decade slumber. Along with the seismic network, the electronic tiltmeters represent the first line of defense against any volcanic surprises. So, it is crucial to place these instruments in areas where potential volcanic activity could occur. The current network provides good coverage for the populated regions along the lower east rift zone, but coverage of Mauna Loa's rift zones is spotty at best.

Every year, HVO enlarges the electronic tilt network. Starting in 2003, we plan to place several tiltmeters on Mauna Loa's southwest rift to provide some level of forewarning to the communities downslope from this area. Expanding the tiltmeter network to include Mauna Loa's rift zones will be very expensive. Tiltmeter installation involves drilling a small but deep hole, and drilling in the high-altitude wilderness requires many back-and-forth helicopter trips. Unfortunately, these are lean times for the Observatory, and funds for this project may simply not be available. So, the tiltmeter network may not expand next year into Mauna Loa's rift zones after all. But, if not in 2003, there's always another year.

### Volcano Activity Update

Eruptive activity of Kilauea Volcano continued unabated at the Puu Oo vent during the past week. The three flows reported last week are still active. The "Mother's Day" flow is now cascading down Pulama pali and providing visitors a gorgeous, even awesome, display. The distal end of the multi-lobed flow is 2.64 km (1.6 mi) above the Chain of Craters road. The two older flows emanating from the "rootless" shields are slowly moving forward. The lower flow along the National Park-Royal Gardens boundary is on the coastal flats and creeping toward the ocean. The higher flow entered the top of Royal Gardens and destroyed a house on May 26 as it spread in the upper section of the subdivision. There are no ocean entries.

One earthquake was reported felt during the week ending on May 30. Residents of Waimea and Kawaihae felt an earthquake at 4:56 a.m. on May 27. The magnitude-2.3 earthquake was located 13 km (7.8 mi) northwest of Kawaihae at a depth of 47.2 km (28.3 mi).