Tilting of the slopes of a volcanic edifice is now a well-known indicator of changes in subsurface magma volume and therefore a key to predicting eruptions. Study of such tilt was pioneered at HVO (see Jaggar and Finch, 1929), and records and discussions of tilt at Kilauea appeared in the various HVO publications from their earliest years.
Seismologist Wood first noticed pronounced ground tilt soon after he set up the Bosch-Omori instruments in 1913. Systematic recording and study of daily and seasonal tilt changes and their relation to Kilauea and Mauna Loa began in 1917 and became important in volcanic research. Over HVO's pioneer period, there were three basic methods of measuring tilt: leveling, use of clinoscopes, and computation from seismograms. Leveling was expensive and time consuming but was performed intermittently with transits and spirit levels. Some precise runs were made up from sea level, and local level lines extended 300 m (1,000 it) or more.
Clinoscopes were in full use by 1932. But the standard was the reliable Bosch-Omori seismograms, which could be used to compute the tilt of the instrument's pendulums in relation to the floor of the Whitney cellar. Such computations were done almost daily from 1913 through 1963 (see Romberg, 1919). In 1940, Waesche believed that the Bosch-Omori piers were capable of detecting a 6-mm (0.25-in.) change in the end of a straight line 16 km (10 mi) long (0.3 microradian in modern terminology).
The clinoscope was a heavy, ring-shaped weight hung by a piano wire from a tripod 2.1 m (7 ft) high. The weight dipped into a bath of automobile oil for damping, and a boom came vertically upward from the space in the middle of the weight. At the top of the boom was a sharp point; this point nearly reached a horizontal circular card that was ruled in points of the compass and in concentric circles. The center of the card was placed precisely above the point at the start of each day, and the point's migration away from the center 24 hours later would indicate both direction and angular amount of change. HVO clinoscopes magnified the amount of change about 50 times.
By 1932, clinoscopes were installed at the corners of an equilateral triangle whose center was the center of Halemaumau. Two of these were in cellars, and the third was enclosed within a glass case in a hut. The frequent earthquakes beneath Kilauea, however, played havoc with the reliability of the readings, as did temperature changes. Staff at HVO hoped to redesign the clinoscopes to make them entirely earthquake proof and to allow readings to be made through a microscope. The HVO "wish list" included a ring of clinoscopes around the summit craters of both Mauna Loa and Kilauea and others at various distances away on the highways out from Kilauea.
Today, that wish has largely come true, but not with clinoscopes. Electronic tiltmeters, sensitive to 0.1 microradian, are installed at several locations on Kilauea and Mauna Loa; they radio their signals directly to recorders at HVO.
JAGGER AND THURSTON: BACKGROUND
BEGINNINGS OF THE OBSERVATORY
BUILDINGS AND FACILITIES
TECHNOLOGY STATION (1911-1918)
WHITNEY LABORATORY OF SEISMOLOGY (1912-PRESENT)
BUILDING 41 (1940-PRESENT)
BUILDING 131 AT UWEKAHUNA (1927-PRESENT)
NEW BUILDING AT UWEKAHUNA (1985-PRESENT)
PUBLICATIONS AND DOCUMENTS
ACCESS ROUTES AND FACILITIES
THE 1926 ERUPTION
CONTROLLING LAVA FLOWS
THE OHIKI AND OTHER EXPERIMENTS
TRAVEL TIMES OF EARTHQUAKE WAVES
SCALES OF EARTHQUAKE INTENSITY
SCIENCE AND THE PUBLIC