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The problem is even more difficult in making regional predictions of debris flow activity. Five geologists and engineers were assigned the task of designing a method to make such regional predictions. The clues for solving the problem came from understanding the behavior of debris flows that occurred during the past spring. The volume of the landslide material that mobilized or could be mobilized, the average gradient of the stream channel, and the presence or absence of historic and prehistoric debris flows in a canyon system seemed to be the most important factors, apart from the water needed to mobilize the material. This information was determined for 24 canyons from Salt Lake City north to Willard, and a map was made to show the relative potential for debris
flows reaching the mouth of each canyon. For those canyons with very high potential, one of the engineers made recommendations for methods of mitigating the debris flows after they emerged from the canyon mouths and extended into communities below.
The causes and mechanism of the unprecedented landslide activity are being investigated by the Geological Survey. The problem of high ground water triggering reactivation of old landslides is being studied at two landslides on the Wasatch Plateau. The team of scientists is also analyzing the runout patterns of debris flows in areas where flows have not been interupted or influenced by manmade structures to formulate a method to predict runout distances.
Eruption of Kilauea Volcano, Hawaii
Kilauea Volcano began a major eruption on January 3, 1983. It was not unexpected; the weekly forecast of eruption probability issued on December 30, 1982, based on the state of strain of Kilauea's summit area, indicated that the likelihood of an eruption during the period of December 30 to January 5 was nearly double the average long-term probability.
Just past midnight on the morning of January 2, a major earthquake swarm and harmonic tremor set off the seismic alarm that sent the staff of the U.S. Geological Survey's Hawaiian Volcano Observatory rushing to the observatory to see what the monitoring instruments were showing. Three to five small, generally unfelt earthquakes were occurring every minute at shallow depths beneath the upper east flank of Kilauea along a zone of weakness known as the east rift. The earthquakes were accompanied by harmonic tremor on the summit and east rift seismographs, indicating that magma (molten rock) was on the move through underground fractures. After 1:00 a.m. the earthquake swarm increased, and several quakes of magnitude 2.5 to 3.0 were felt by local residents.
Another clue to what was happening underground became evident just after 2:00 a.m. as recording tiltmeters began to show rapid subsidence of the summit of Kilauea, amounting to 7 inches in the next 24 hours. A tiltmeter is an instrument that can measure changes in inclination as small as one part per million. All signs indicated that a major new fracture was forming beneath the surface on the east flank of Kilauea and that magma from beneath the summit of Kilauea was moving into this newly forming, but still hidden, crack. During the day of January 2, the earthquake swarm beneath the flank of Kilauea slowly migrated about 5 miles farther down the east rift zone indicating that the underground fracture was still extending eastward. Geologists in the field moved down the surface of the rift zone hoping to be at the location of any breakout of the fracture to the surface.
They were not disappointed; at 12:31 a.m. on January 3, 24 hours after the underground movement of magma began,
lava burst to the surface in a remote area on the flank of Kilauea Volcano near an old crater on the rift zone. The erupting fissures rapidly extended until they were about 5 miles long and produced a line of incandescent lava fountains called a curtain of fire.
The eruption continued intermittently through mid-January with high rates of lava emission on January 7 that fed a major lava flow. This flow moved 4 miles in 5 hours towards the relatively undeveloped Royal Gardens subdivision of house lots. This first threat to Royal Gardens was perhaps a warning of destruction yet to come.
The eruption paused until late February as the magma reservoir beneath the summit of Kilauea slowly refilled. Evidence for this recharge came from the recording tiltmeters which showed the summit to be slowly swelling upward after its rapid subsidence during January's emission of lava. When enough pressure was regained in the molten rock beneath the summit, the flank eruption was renewed. No new swarms of earthquakes occurred, indicating that the same fracture system was acting as the magma conduit for the eruption.
Five more major eruptive phases have occurred to date (July 27, 1983) late February to early March, late March to early April, mid-June, late June to early July, and late July. Each of these phases has erupted from vents on the fracture system in the roadless and trailless fern forest of the middle east rift zone of Kilauea. They have produced spectacular lava fountains, and four phases also have fed 5- to 6-milelong lava flows that have invaded the Royal Gardens subdivision. These thick, relatively slow-moving lava flows move through the forest-covered slopes of the south flank of Kilauea at speeds that average 3 feet per minute with surges in speed up to 100 feet per minute. They utterly destroy everything in their path and, so far, have consumed 16 dwellings and covered 330 houselots in the subdivision.
This eruption is already one of the longest lasting rift-zone eruptions in Kilauea's recorded history, and most indications suggest that it is not over.
The 1983 Earthquake Sequence Near Coalinga, California
Figure 1. — Damage to older unreinforced masonry building, Coalinga.
On May 2, 1983, the town of Coalinga, California, was heavily damaged by an earthquake that was centered about 8 miles northeast of town. The earthquake measured 6.5 on the Richter scale and was felt as far away as Las Vegas, Carson City, Los Angeles, and Sacramento. Estimates by the Office of Emergency Services indicate that shaking from the earthquake caused approximately $31 million damage in Coalinga, making this the most damaging earthquake in California since the San Fernando earthquake of 1971. Damage was most severe to older unreinforced masonry buildings (fig. 1) and some older wood framed residences. Newer buildings of nearly all construction types, both within and outside of the downtown area, suffered much less damage. The extensive damage generated by the earthquake in Coalinga reemphasized the seismic vulnerability of older unreinforced masonry structures with the sobering realization that most older California communities contain
many such structures. In the event of
larger earthquakes along other portions of the San Andreas fault system, the amounts of damage to life and property could approach catastrophic proportions.
Seismic data continuously transmitted via telephone and radio from the Survey's California Seismographic Network together with automatic computer analyses of the incoming data provided accurate locations within minutes of the occurrence of the mainshock and its more than 6,000 aftershocks. Information collected from this network suggests that the Coalinga earthquake sequence was the result of seismogenic failure in a portion of the Earth's crust outlined by previous earthquake sequences. The seismic gap filled in by the Coalinga earthquake sequence and the previous earthquake sequences is illustrated in figure 2.
Precise elevation measurements and analyses of seismic data suggest that the Coalinga mainshock occurred along the
Anticline Ridge fault with the result that after the earthquake, the region northeast of the fault was elevated by about 16 inches and the area to the southwest lowered by about 6 inches.
Of the more than 6,000 aftershocks which have occurred in the Coalinga region, six were larger than magnitude 5.0, with five of these on the nearby Nunez fault. Surface rupture along the Nunez fault began on June 11 with a magnitude 5.2 aftershock. Subsequent aftershocks lengthened and enlarged the rupture so that it now extends for about 2 miles and has a maximum vertical displacement of over 23 inches.
Strong-motion instrumentation operated by Survey as part of the U.S. National Strong-Motion Network has provided numerous recordings of the damaging levels of ground shaking generated by the mainshock and subsequent aftershocks. This information, crucial for earthquakeresistant design analyses and description of the nature of seismogenic crustal failure in the region, was recorded on conventional photographic film recorders as well as newly developed computerized recording systems which provide data of exceptional quality, such as the General Earthquake Observation System (GEOS). Such recordings show significant geographic