Volcano Watch — Geodetic monitoring of the submarine south flank of Kīlauea Volcano

Last week we wrote about the large Kalapana earthquake of 1975. In the article we discussed the importance of repeated geodetic observations that monitored the accumulation of strain before the earthquake and allowed HVO researchers to forecast the event. Geodetic observations after the earthquake established the amount of surface displacements caused by the earthquake. All of the measurements were made at subaerial or land-based stations, and no measurements were made on the submarine flank of the volcano. The mystery of what happens beneath the water will soon be solved.

For the past three weeks, researchers from the University of California, San Diego, Scripps Institution of Oceanography (SIO) have been deploying and measuring an array of 10 transponders on the submarine section of the Hilina fault system. The measurements employ an acoustical technique that was first used to measure seafloor spreading across the Juan de Fuca ridge off the coast of Oregon. The precision of the technique is about 1 cm (.4 in).

The changing locations of the ship, the RV Revelle, down to the pitch and roll of its deck, are determined by three onboard GPS receivers collecting data every second. These GPS data together with the GPS data from HVO's permanent network continuously fix the differential location of the ship relative to the stations on land.

Low-frequency acoustical signals from the ship turn on the power supply of the deployed ocean-bottom transponders. High-frequency acoustical signals from the transponders determine range distances between adjacent transponders and between individual transponders and the ship. Each transponder is also equipped with pressure and tilt sensors to detect and correct localized settling of the instrument. The power supply of the transponder is turned off between measurements and is designed to last 10 years.

The high precision of the range measurements is attained by correcting the travel-time of the acoustical signal. The speed of sound energy varies with the density of the medium, so corrections for the temperature, pressure, and salinity of the seawater along the signal path are applied. The distance is also corrected for the roll and pitch of the ship when the measurement is taken. With distances between the ship and individual transponders measured from at least four known locations of the ship, the location of the individual transponders can be accurately determined.

Thus, the location of each transponder in the array is determined relative to the land-based GPS stations. The HVO GPS measurements indicate that the south flank of Kīlauea is creeping up to 8 cm/yr (3 in/yr) to the southeast (seaward). The SIO ship is scheduled to return in 2002 to relocate the underwater array. If the submarine flank is moving along with the subaerial flank, the change in position of the transponders should be well above the detection level of the acoustical measurements. The results will be conclusive.

If a large earthquake should occur along the south flank, we would be able to determine the submarine displacements of the volcano when the transponders are relocated. With this additional information, we can develop a better model of the earthquake fault and structure of the south flank.

With all the geodetic measurements being taken on the south flank of Kīlauea, we should learn a lot when the next large earthquake strikes. However, no matter how much we learn, we will not be able to predict an earthquake and issue a warning. The best way that you can mitigate this hazard and protect your family is to build or reinforce your home according to building code standards. You can save your life by immediately rushing to high ground if you are near the ocean when a major earthquake occurs.

Volcano Activity Update

Eruptive activity of Kīlauea Volcano continued unabated during the past week. Lava is erupting from Pu`u `O`o and flowing southeast through a tube system down to the flats below Pulama pali and beyond to the ocean. Lava is entering the ocean at Kamokuna located 1.6 km (1 mi) west-southwest of Waha`ula. A small surface flow was observed at the base of Pulama pali.

The public is reminded that the ocean-entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The active lava flows are hot and have places with very thin crust. The steam clouds are highly acidic and laced with glass particles.

There were no earthquakes reported felt during the week ending on November 30, 2000.