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This graph shows the number of earthquakes that occurred beneath Mount Spurr every day between February 1 and June 30, 1992. The red line indicates the time of the June 27 eruption. Though the earthquake activity showed the volcano was indeed restless, there was no significant change in the activity in the days before the eruption. Scientists issued general warnings about the potential for an eruption at the volcano in the Alaska Volcano Updates on June 8 and 26 (see below), but on the basis of the earthquake activity, they were not able to issue a specific short- term warning that an eruption was likely in a few days.
The Alaska Volcano Observatory provides information about Alaskan volcanoes through written updates (weekly and as needed), a level of concern color code, emergency notification by telephone, and direct personal communication with the public and individuals working in private industry, Federal, State, and local government agencies, and the news media. The written updates that provided key information about Mount Spurr during this activity.
August 30, 1991
5:00 p.m. ADT
"Forty small, shallow (approx. 3 km beneath crater) earthquakes have been detected beneath Mt. Spurr in the last 2 weeks. Mt Spurr is located approx. 130 km (80 miles) west of Anchorage. Most of the events were located under Crater Peak, a basaltic-andesite cone of the Spurr Volcanic Complex. The largest event was a MI = 1.4 on August 26th. The largest number of events within one day occurred on August 22nd with 11 volcano-tectonic events. Since August 22nd, activity has slowed to 0-4 events per day."
"A small SO2 signal was detected at Crater peak on August 29th, whether this is significant or coincidental is not known. Another flight is planned for Tuesday."
February 7, 1992
"Since late August AVO has recorded intermittent swarms of shallow earthquakes beneath Crater Peak, a small basaltic-andesite cone located on the south slope of Mt. Spurr. The most recent of these swarms began on February 5. Earthquake magnitudes are generally less than 1.0, and hypocenters range in depth from 0 to 3 km. This swarm activity is slightly higher than normal, but is interpreted as simply a fluctuation in background level. AVO will continue to monitor this seismic activity closely."
On March 13, 1992, AVO began to provide information about the seismic activity at Mount Spurr in the weekly updates, which are issued every Friday.
June 5, 1992
10:00 a.m. ADT
"The level of seismic activity at Mt. Spurr has remained well above normal. Since late August of 1991, AVO has detected an above average number of small earthquakes beneath Mt. Spurr, the hypocenters of which are generally between 0 and 5 km depth. Magnitudes of most of the events are between 0 and 1.0, too small to be felt. Most of the earthquakes form clusters beneath the summit and beneath Crater Peak, on the south flank of the volcano. Earthquake swarms were recorded at Mt. Spurr in 1982 and 1989. This abnormal seismic activity may represent some form of magmatic activity at depth, possibly an intrusion. Similar seismic activity has preceded eruptions at some volcanoes yet, at others, seismicity has died off without an eruption. No seismic activity thought to be typical of imminent eruption, such as long-period events or tremor, have been recognized. AVO is in the process of reviewing the nature and character of the activity beneath Spurr. The most recent eruption of Mt. Spurr occurred in 1953."
June 8, 1992
"Over the past several months, the level of micro-earthquake activity at Mt. Spurr volcano has increased significantly (i.e., an order of magnitude) above that of the past 10 years. This increased seismicity could represent early forerunner activity to an eruption. However, it is at least as likely that the seismicity could remain at this level indefinitely, or even decrease, without an eruption. An overflight today suggested some changes in the thermal area at Crater Peak, the site of the 1953 eruption. Our concern over the state of the volcano is sufficiently high, therefore, that through this Update we are sharing our interpretation of these recent changes at Spurr with interested parties and reviewing AVO response plans."
"Mount Spurr volcano is the nearest active volcano to Anchorage located 80 miles west of the city. The snow- and ice-mantled volcano was most recently active in July 1953 when its only historic eruption resulted in a day of explosive activity which deposited about 0.25 inches of ash on Anchorage. The actual site of the 1953 eruption was not at the 10,000-foot-high summit of Spurr itself but rather at a subsidiary cone, Crater Peak, located on the south flank of the volcano at about 7,000 feet. This cone has been the site of about 15-ash-producing eruptions in the past 1,000 years. Crater Peak contains a small turquoise colored hot-water lake with water temperatures of about 130o F (55o C) and a small fumarole (steam vent) field."
"Most of the earthquakes being recorded are considered to be volcano-tectonic in origin ÷ that is, caused by fracture of brittle rock. Much of the elevated seismicity since last August has been beneath the main summit of Spurr and, more recently, Crater Peak. None of the earthquakes has exceeded magnitude 1.7. We have seen no evidence to suggest that the increase in seismicity is related to regional earthquake activity rather than a localized volcanic process."
"The recent increase of seismicity beneath Spurr may be the result of some form of magmatic activity in the volcano's plumbing system, possibly an ongoing episode of magma intrusion at depth. To date no long-period events or volcanic tremor (two types of seismic activity that usually mean an eruption is imminent) have been recognized. However, over the past 72 hours, an increase in very localized seismic activity has occurred immediately beneath Crater Peak; this may be related to a change in the near-surface thermal activity. Our principal concern at Spurr at the present time is monitor the activity in case it develops into a short term (such as a 24-72 hour) pre-eruption crisis."
"The seismicity level at Spurr can no longer be considered normal in that it differs markedly from previous seismicity at Spurr, and also differs from the character of normal background earthquake activity observed at other Cook Inlet volcanoes. Increased seismic activity of the type seen at Spurr has on occasion preceded eruptions from volcanoes elsewhere by about two months; on the other hand, similar earthquake swarms have been recorded beneath volcanoes that were not followed by eruptions."
"An increase in the amount of sulfur dioxide being emitted from volcanoes is another sign of possible eruptive activity. An airborne gas sampling flight today indicated no increase over that measured in October, 1991; however, a change in the color of the hot lake inside the crater from turquoise-blue to black was noted. This type of change has been noted in other volcano lakes owing to an increase in iron sulfide and may reflect temperature changes in the near-surface thermal areas."
"To reiterate, we have no reason to believe an eruption at Spurr is imminent in the next few days. Because of the anomalous seismic activity and change in the appearance of the lake, however, we are responding as follows:
1. By copy of this Update, we are notifying interested parties to the ongoing situation at Spurr.
2. Although we consider the volcano to be adequately instrumented with the eight seismographs now in place, we are increasing the sensitivity of the 24-hour alarm at the AVO Seismological Lab in Fairbanks to respond to the seismicity at Spurr. The seismic records are also being analyzed more frequently.
3. Routine airborne gas sampling and observation overflights will be increased if significant premonitory changes in the seismicity occur. Sampling of the fumaroles gases inside Crater Peak will be done in late June, weather permitting.
4. Given the present lack of immediate precursor seismic activity at Spurr, we believe it premature to implement a hazard color code similar to that used the during the Redoubt crisis.
5. We will continue to issue Updates on Spurr activity as the situation warrants."
June 26, 1992
10:30 a.m. ADT
"The level of seismic activity at Mt. Spurr has remained well above normal this week and has increased slightly. The rate of volcano-tectonic earthquakes--those caused by the fracture of brittle rock--has fluctuated somewhat, and remains at elevated levels that have persisted since August 1991. These events occur at depths of 0 to 5 km beneath the summit region, and have magnitudes between 0 and 1.0, too small to be felt. In addition, very localized seismic activity continues to occur immediately beneath Crater Peak. This activity is generally characterized by bursts of 1 to 10 minutes duration that show up most strongly at the seismic stations closest to Crater Peak. Based on comparison with similar events at other volcanoes, we are calling these events tremor bursts. The sizes of the tremor bursts are small, comparable to those that generally accompany hydrothermal activity. Their rate of occurrence has fluctuated somewhat, and remains at elevated levels. Two longer tremor bursts, each about 2 hours long, occurred on June 24-25."
"The abnormal seismic activity may represent some form of magmatic activity at depth, possibly an intrusion related to the increase in geothermal activity. Similar seismic activity has preceded eruptions at some volcanoes yet, at others, seismicity has died off without an eruption. No seismic activity though to be typical of imminent eruption, such as swarms of long-period events, has been recognized. AVO is continually reviewing the nature and character of the activity beneath Mt. Spurr. The most recent eruption of Mt. Spurr occurred in 1953."
June 26, 1992
4:30 p.m. ADT
"Since today's earlier Update, AVO scientists have observed an increase level of seismic activity at Mt. Spurr located 75 miles west of Anchorage. The character of seismic activity indicates that the state of unrest in increasing, however, it does not suggest that an eruption is imminent within the next few days. Long-period seismic events, which are common in the days and hours before large volcanic eruptions, have not yet been recognized."
"The largest episode of continuous seismic tremor seen to date began at 12:04 p.m. ADT and is continuing at the time of this release. The amplitude of the tremor is twice as large as that for any of the previous episodes recorded at Mt. Spurr since bursts of tremor began on June 6. The duration of this episode greatly exceeds that of all previous episodes, including the two 2.5 hour- long bursts on June 24 and 25 that were reported in today's previous Update."
"An AVO geologist flew past Mt. Spurr at about 11:00 a.m. this morning, and reported a decreased level in the lake compared to June 11 observations. The lowering of the lake may indicate increased heating at the volcano."
"Explosive ejection of the crater lake or melting of fresh snow by hot eruption products could likely cause localized debris flows. These flows could become larger as they move downstream into the valley of Chakachatna River and could present a hazard to persons near the stream channel."
"Current wind directions for the next 24 hours indicate trajectories to the north and northeast. The nearest towns are Skwentna approximately 60 miles NE and Shirleyville 35 miles to ESE."
"A color code specific to Spurr volcano accompanies this update. As of this update our present condition is yellow. This condition will persist until Monday afternoon or until an additional update is issued."
"In view of these new observations, AVO scientists are intensifying their watch of Mt. Spurr commensurate with their increased level of concern. AVO maintains a 24-hour computerized alarm system which will alert observatory seismologists should unusual seismic activity occur at any of the Cook Inlet volcanoes."
On June 27, 1992, at 7:16 a.m. PDT, AVO initiated notification by phone that an eruption began at 7:04 a.m. and changed the color code to ORANGE.
June 27, 1992
8:00 a.m. ADT
"A moderate explosive eruption occurred at Mt. Spurr at 7:04 a.m. today. Pilots reported an ash plume which reached an altitude of 26,000 feet. Accordingly, AVO has raised the level of concern color code to ORANGE. The explosion was preceded by volcanic tremor which began at 12:04 p.m. on June 26, and a swarm of volcano-tectonic earthquakes--those caused by the brittle fracture of rock--which began at roughly 3:00 a.m. These changes in seismicity suggest the movement of magma at shallow depth."
"Based on the rapid and continuing changes in seismic activity beneath Crater Peak there is an increased likelihood of continued eruptive activity. We will remain at color code ORANGE until further notice."
"Explosive ejection of the crater lake or melting of fresh snow by hot eruption products could likely cause localized debris flows. These flows could become larger as they move downstream into the Chakachatna River and could present a hazard to persons near the stream channel."
"Current wind directions for the next 24 hours indicate trajectories to the N and NW. If an eruption occurs similar to that in 1953, the most recent eruption, ash plumes to more than 40,000 feet may be expected. The nearest towns are Skwentna approximately 60 miles NE and Shirleyville 35 miles to ESE."
"AVO scientists are monitoring the situation on a 24 hour a day basis, and will update the level of concern color code as appropriate."
June 27, 1992
9:15 a.m., ADT
"Eruptive activity at Mt. Spurr is continuing, additional seismic signals associated with eruptive activity occurred at 8:14 and 9:04 a.m. Pilots have reported ash plumes reaching elevations of 30,000 feet. The initial eruptive event occurred at 7:04 a.m. today. Accordingly, AVO has raised the level of concern color code to RED."
"Based on the rapid and continuing changes in seismic activity beneath Crater Peak there is an increased likelihood of continued eruptive activity. We will remain at color code RED until further notice."
"Current wind conditions for the next 24 hours indicate trajectories to the N and NW. If an eruption occurs similar to that in 1953, the most recent eruption, ash plumes to more than 40,000 feet may be expected. The nearest towns are Skwentna approximately 60 miles NE and Shireyville 35 miles to ESE."
"AVO scientists are monitoring the situation on a 24 hour a day basis, and will update the level of concern color code as appropriate."
June 27, 1992
9:00 p.m. ADT
"Seismic activity at Mt. Spurr remains at high levels, and has fluctuated somewhat over the past 8 hours. Based on the character of the most recent seismicity the level of concern color code will remain at RED. As long as the level of seismicity remains elevated the possibility of additional eruptive events exists. These events may occur with little or no advance warning."
"An overflight of the volcano conducted today, under conditions of poor visibility, revealed a small debris flow in drainages on the west side of Crater Peak. The flow caused minor short- lived flooding in the Chakachatna River. Additional overflights and gas measurements are planned for tomorrow, weather permitting."
"Eruptive events occurred at 7:04, 8:14, and 9:04 a.m. today. Ashfall from these eruptions has been reported at Rainy Pass, Kantishna, and Wonder Lake. Wind directions for the next 24 hours indicate ash trajectories to the N and NW."
June 28, 1992
"Seismic activity at Mt. Spurr remains at elevated levels, but has declined somewhat since the end of yesterday's eruptive events. The character of recent seismicity indicates the volcano remains restless and additional eruptive events are possible over the next few days or weeks. Based on the decline in seismic activity the level of concern color code has been reduced to YELLOW."
"Overflights of the volcano and gas measurements are planned for today, weather permitting."
"Wind directions for the next 24 hours indicate ash trajectories to the NW."
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URL of this document: http://volcanoes.usgs.gov/About/What/Erupt/SpurrChange.html
Last modified: December 17, 1998