# Crack measurement-old fashioned but good

May 1, 2003

Most monitoring done by HVO is designed to interpret changes taking place below the surface of the earth.

In general, the deeper the process, the more widely spaced must be the monitoring devices. Consequently, seismometers and GPS receivers are generally kilometers apart, and leveling benchmarks, a few hundred meters. To study shallow features, more closely spaced monitoring is needed. An inclusive monitoring network has some stations located far apart and others close together.

Such a monitoring network is required to study the Koae fault system, which connects the east and southwest rift zones of Kīlauea. We really don't yet know how deeply the faults in the Koae extend, nor do we know whether they remain nearly vertical or flatten at some depth below the earth's surface.

Continuously recording GPS stations and seismometers around the Koae give information of possible deep processes. A network of survey lines about 1 km long spider-webs its way across the Koae and provides information about horizontal movements. Level lines across the Koae have benchmarks spaced from about 1 km (0.6 miles) to about 90 m (300 feet) apart to tell us about vertical changes. But this type of monitoring says little about individual cracks and faults, and the Koae has thousands of them. How can they be monitored?

Very simply--with a tape measure. We use a time-honored method first used at Kīlauea by Thomas Jaggar when HVO was established in 1912. Permanent markers, generally stainless steel carriage bolts, are cemented into the ground on either side of a crack, spaced closely enough so that a 30-m-long (100-feet-long) tape measure can reach between them. At least three such markers are established, two on one side of the crack to form a baseline, and a third across the crack. The first taping of the distances between the markers provides a basis for comparison with future measurements. Subsequent tapings will tell if the crack has remained stable or has opened more.

This type of station, termed a crack station, can also provide information about vertical changes, provided that the markers are surveyed with a level. A complete crack station contains information about both horizontal and vertical changes.

In practice, most crack stations are more than simple triangles. The additional complexity is necessary when the cracks and faults themselves are complicated. There may be a family of cracks spanning a width of several tens of meters, so that a station may need as many as 10 different markers for complete coverage. Some crack stations consist only of a long line of markers rather than connected triangles or other configurations.

Another complexity is the topography. To tape accurately between markers, there can be no obstacle. An intermediate marker may have to be emplaced to get around or over the obstacle.

This kind of monitoring is done on the cheap. No fancy equipment here. But we can't realistically have many crack stations, because it takes a couple of hours to measure a complex one. Currently we have some 20 stations spaced throughout the Koae.

Measurement of crack stations shows that some cracks creep open slowly and then jump during earthquakes or intrusions of magma into the nearby east rift zone.

Two creeping cracks are located at the intersection of the Chain of Craters and Hilina Pali Roads. Since September 20, 1999, these cracks have gradually widened 1-1.5 cm (0.4-0.6 inches). Each probably opened rapidly during an intrusion on September 11 between Mauna Ulu and the Escape Road, for each widened by 1 to 1.5 cm (0.4-0.6 inches) in the one year ending on September 20. These two crack stations, the granddaddies at Kīlauea, were first measured in 1966; since then, one has opened 30 cm (12 inches) and the other 60 cm (24 inches).

Measurement of cracks is not glamorous, but a few can be measured from time to time to provide important information on how the surface of Kīlauea breaks up.

### Volcano Activity Update

Eruptive activity at the Puu Oo vent of Kīlauea Volcano continued unabated during the past week. A string of incandescence stretches from top to bottom of both Pulama pali and Paliuli, and slight glow comes from the base of Paliuli, showing the presence of surface lava there. Surface breakouts are also found along the east edge of the Kohola flow, and the National Park Service has marked a trail out to the closest activity. Ocean entry activity at the two sites on the West Highcastle delta waxed and waned during the week.

The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The steam clouds are highly acidic and laced with glass particles. The National Park Service has erected a rope barricade to delineate the edge of the restricted area. Do not venture beyond this rope boundary and onto the lava deltas and benches.

No earthquakes were reported felt during the past week.

Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity remains low with no earthquakes located in the summit area during the last seven days.