September 20, 2001
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
How our capabilities have improved with time
Forty years ago, at 12:36 p.m. on September 21, a swarm of large, shallow earthquakes accompanied by strong harmonic tremor began to emanate from the vicinity of Napau Crater on the east rift zone of Kilauea Volcano. A rapid deflation of the summit of Kilauea occurred in conjunction with the earthquake swarm.
These events were precursors to an eruption and were caused by magma draining from the summit storage system and forcefully intruding the rift zone. Personnel at the Hawaiian Volcano Observatory were uncertain about the location of the possible eruption because of the scant monitoring network in place at that time.
HVO directly recorded the signal from only four seismic stations on Kilauea. Three more stations, one each in Pahoa, Hilo, and Kealakekua, were recorded on site, and their records had to be transported to HVO. One tiltmeter that had to be manually read was the only daily source of information about surface deformation.
The first observation of a possible eruption was a glow over the east rift zone forest at 4:30 a.m. on September 22. At 8:00 a.m., a tourist flight confirmed a breakout near Kalalua Crater. With no immediate access to a helicopter or airplane, HVO personnel had to wait until noon before they were able to take an overflight of the eruption area with the Hawai`i National Guard.
The observers found a series of discontinuous fissures stretching 15 km (9 mi) from Napau Crater downrift to Kalalua Crater (the initial phase of the current Pu`u `O`o eruption had nearly an identical breakout). Only copious steam and volcanic fumes were being emitted from the cracks by the time HVO personnel flew over the area. Fresh lava pads surrounded three of the fissures, but no fountaining was observed.
The island was rocked at 5:02 p.m. on September 22 by a large earthquake from the south flank of Kilauea. The earthquake apparently allowed the eruptive dike to move farther down the rift zone. At about 10:00 a.m. on September 23, a new 4-km-long (2.5-mi-long) fissure broke out about 8.3 km (5 mi) east of the previous day's activity.
The discontinuous curtain of fountains lasted only a few minutes and consolidated to a 200-m-long (700-ft-long) vent located northwest of Heiheiahulu Crater. Activity at this vent produced flows that threatened the Kalapana road, and the Hawai`i County Civil Defense Agency ordered the evacuation of homes from `Opihikao to Kapa`ahu. Most of the lava, however, poured down large cracks, and no extensive flow resulted. This eruptive phase lasted 15 hours and produced three distinctive flow fields. The easternmost flow field, Jonika, was named for the rancher who allowed HVO personnel to pass through his property and guided us to the eruption site.
Eruptive activity resumed at 2:15 p.m. on September 24 from a 150-m-long (500-ft-long) fissure located between Kalalua and Heiheiahulu craters. The fountains were low-level and ended by early the next morning. Two small elongated cones resulted from this activity.
During the evening of September 24, a swarm of earthquakes from the south flank shook the area. Three earthquakes were felt throughout the island, with the largest, at 7:30 p.m., having a magnitude of 5.5. This earthquake nearly caused a newly arrived HVO staff member to go back to O`ahu, but he stayed on and worked in seismology for another 28 years before retiring from HVO.
The 1961 flank eruption ushered in 40 years of great activity along Kilauea's middle and upper east rift zone. Since then, HVO has gradually increased our real-time monitoring capabilities to cope with all this activity and that of Mauna Loa. Now the seismic network consists of more than 60 stations on this island and three on Maui, and all of their signals are telemetered and recorded at HVO. A dozen electronic tilt stations, four borehole strainmeters, and nearly two dozen GPS receivers send their signals to HVO. We also have ready access to a number of helicopters. When the first eruptive fissure of the current eruption broke the surface near Napau Crater on January 3, 1983, HVO personnel were standing only a few meters (yards) away.
Eruptive activity of Kilauea Volcano continued unabated at the Pu`u `O`o vent during the past week. Lava moves away from the vent toward the ocean in a network of tubes and descends Pulama pali in three separate tubes. Two surface flows from breakouts of the tube system continue to stream down the pali, and many surface flows are active in the coastal flats. Lava enters the ocean in the area east of Kupapa`u and provides visitors to the viewing area a great show.
The public is reminded that the bench of the ocean entry is very hazardous, with possible collapses of the new land. The steam cloud is extremely hot, highly acidic, and laced with glass particles. Swimming at the black sand beach of the bench can be a blistering or even deadly venture.
There were no earthquakes reported felt during the week ending on September 20. The swarm of earthquakes from Lo`ihi Volcano has subsided. HVO located a total of 57 earthquakes from Lo`ihi since September 10.
Updated: October 10, 2001 (pnf)