Monday, March 21, 2011

A Time to Be Informed about Potential Disasters


Given the number of disasters in the world today we thought we would share some information from our book Supervolcano: The Catastrophic Event That Changed the Course of Human History (Could Yellowstone Be Next?) from authors Marie D. Jones and John Savino. Being informed and prepared about the possibilities that exist in this world are important lessons that everyone should learn. This excerpted section is from Chapter 9: The Lurking Mega-Disaster.

In the case of Yellowstone, there are typically 1,000 to 3,000 earthquakes that occur each year within the park and its immediate surroundings. Although most are too small to be felt, these quakes, recorded by seismograph systems located in and near the Park, reflect the fact that the Yellowstone caldera is an active volcano, and one of the most seismically active areas in the United States. Each year, several quakes of magnitude 3 to 4 are felt by people in the park. Although some quakes are caused by rising magma and hot-ground-water movement, many emanate from regional faults related to crustal stretching and mountain building. The most notable earthquake in Yellowstone’s recent history occurred in 1959. Centered near Hebgen Lake, just west of the park, the earthquake had a magnitude of 7.5 and killed 28 people, most of them in a landslide that was triggered by the quake. Geologists believe that large earthquakes such as the Hebgen Lake event are unlikely within the Yellowstone Caldera itself, because subsurface temperatures there are high, weakening the bedrock and making it less able to build up sufficient strain to result in a rupture. However, quakes within the caldera can be as large as magnitude 6.5. A quake of about this size occurred in 1975 near Norris Geyser Basin, and was felt throughout the region.

Map of the Yellowstone Park area showing the locations of major earthquakes occurring in the past 48 years, and the caldera rim associated with the supereruption 640,000 years ago.

As for deformation of the caldera, geologists most familiar with the Yellowstone region point out that while the caldera has been relatively dormant in terms of major volcanic activity for more than 70,000 years, it has been rising and falling for at least the past 15,000 years, at times more than 10 feet. In more recent times, between 1923 and 1975, the entire Yellowstone caldera rose about 3 to 4 feet. Suddenly in 1985, it reversed and started subsiding; in 1990 it reversed again and started to inflate.

Between 1997 and 2003, the northern part of the Yellowstone caldera began to bulge. The bulge, measuring about 25 miles across, rose approximately 5 inches. Simultaneously, there was a sudden rise in temperatures, new steam vents, and the awakening of the geysers in the area. Steamboat Geyser, dormant for nine years, erupted in May 2000, and then several times between 2002 and 2003. The nearby Porkchop Geyser awoke after 14 years of dormancy. Unusual thermal phenomena at the nearby Norris Geyser Basin resulted in such high ground temperatures in 2003 that Yellowstone officials decided to close some boardwalks out of fear that visitors might be burned.

A LONG VALLEY CALDERA

We now turn to the Long Valley caldera. In 1978, a Richter magnitude (M) 5.4 earthquake struck 6 miles southeast of the caldera. This event ended two decades of relatively low earthquake activity in the area, and was followed in May 1980 by an earthquake swarm that included four strong M 6 earthquakes. These events struck the Mammoth Lakes area on the southern margin of the Long Valley caldera. The largest of the M 6 earthquakes occurred one week after the Mount St. Helens eruption of May 18. A report in the Mammoth Times newspaper dated September 7, 2000, recalled how the southern California news media in late May 1980, through implicit association of the Mount St. Helens eruption and the strong earthquakes near Mammoth Lakes, had Mammoth ready to blow. The Mammoth Times report went on to note that, “local governments worked to create an all-encompassing emergency plan. Mammoth got an escape route, known euphemistically as the Scenic Loop.”

In response to continuing seismic activity and uplift observed within the central portion of the Long Valley caldera, the U.S. Geological Survey issued a “Notice of Potential Volcanic Hazard” in 1982. The notice was blamed for causing a severe drop in tourism, and also a downturn in what had been a boom in the Mammoth Lakes housing market. When, after a period of time, a volcanic eruption didn’t happen, the residents of Mammoth Lakes began referring to the USGS as the “U.S. Guessing Society,” among other names.

An ominous sign of unrest beneath the Long Valley caldera occurred in 1989 when magma intruded beneath Mammoth Mountain, a volcano located on the western rim of the caldera, and the site of a major ski resort. While magma did not erupt, large volumes of carbon dioxide were discovered seeping into the soil in an area known as Horseshoe Lake. In a paper published in Nature in 2002, scientists reported the results of a soil-gas survey begun in 1994 where they observed carbon dioxide concentrations of 30–96 percent in a 75 acre region of dead trees. Based on their study, they concluded that, although the tree kill coincided with the episode of shallow dike (magma) intrusion, the magnitude and duration of the carbon dioxide flux indicated that a larger, deeper magma source and/or a large reservoir of high-pressure gas was being tapped.

In 2003, researchers reported the results of geodetic and gravity surveys conducted in the Long Valley caldera. They concluded that the results of their surveys did not support hydrothermal fluid intrusion as the primary cause of unrest, instead confirming the intrusion of silicic magma beneath the caldera. Various signs of unrest continue at the present time, and should serve as a reminder of the boiling cauldron that lurks below the surface.

On November 3, 2002, the largest inland earthquake in North America in almost 150 years struck Alaska, about 85 miles south of Fairbanks. This M 7.9 event, known as the Denali fault earthquake, ranks among the largest strike-slip ruptures of the past two centuries. Its length and slip magnitudes are comparable with those of the great California earthquakes of 1857 and 1906. Horizontal displacements of up to 26 feet were measured along sections of the roughly 200-mile-long fault. Large-amplitude surface waves from the Denali Fault earthquake triggered a series of earthquake swarms and changes in geyser activity at Yellowstone, along with strain offsets and microseismicity under Mammoth Mountain on the rim of Long Valley caldera. The amazing thing is that the Denali fault earthquake was about 1,940 and 2,160 miles distant from Yellowstone and Long Valley, respectively. This leads to the question of what impact a very large (M > 7.75), but closer, earthquake might have on the calderas. For instance, could a large earthquake in southern California trigger an eruption at either Long Valley or Yellowstone? Major earthquakes in southern California in 1992 and 1999 suggest a possible answer to this question.

A EARTHQUAKES AS A TRIGGER?

On June 28, 1992, a M 7.3 earthquake occurred near the town of Landers, approximately 100 miles east of Los Angeles in the Mojave Desert of southern California. Even though the earthquake was more than 770 miles (1,250 km) southwest of Yellowstone, the large-amplitude surface waves from this event changed the relatively periodic eruption rate of one of the geysers in the park from approximately 56 minutes to an erratic rate of about 34 hours, and triggered a swarm of small earthquakes beneath the caldera.

In their Summary of Long Valley Caldera Activity for 1992, the USGS scientists commented:

Certainly the most remarkable and energetic event in the caldera during 1992 was the abrupt surge in local seismicity that began immediately following the June 28, M=7.3 Landers earthquake, which was located in southern California some 400 km south of the caldera. The surge in seismicity triggered by the Landers earthquake was the strongest “swarm” in the caldera during 1992. It included two M>3 earthquakes and over 250 smaller events in the first six days after the Landers earthquake.

Signals associated with transient periods of deformation near the western boundary, and within the caldera, also accompanied the triggered seismicity.

In the October 29, 1998, issue of Nature, two scientists from the Carnegie Institution of Washington reported results of an analysis of the historical record of earthquakes and volcanic eruptions conducted to see if there are significantly more eruptions immediately following large earthquakes. They noted that their study was, in part, motivated by the triggering of seismicity and deformation at the Long Valley caldera by the 1992 Landers earthquake. They found that, “within a day or two of large earthquakes there are many more eruptions within a range of 460 miles (750 km) than would otherwise be expected. Additionally, it is well known that volcanoes separated by hundreds of kilometers frequently erupt in unison; the characteristics of such eruption pairs are also consistent with the hypothesis that the second eruption is triggered by earthquakes associated with the first.”

On October 16, 1999, a year after the Nature paper appeared, another major earthquake occurred in the Mojave Desert in southern California. This event, referred to as the Hector Mine earthquake, was slightly smaller than the Landers earthquake, weighing in at M 7.1. The epicentral distances from the Hector Mine earthquake to the Yellowstone and Long Valley calderas were comparable to distances from the Landers event. While we are not aware of any reported triggering of activity at Yellowstone from the Hector Mine earthquake, deformation associated with the earthquake was observed at Long Valley, and was followed 20–30 minutes later by a swarm of small earthquakes localized beneath the north flank of Mammoth Mountain.

While the evidence for remote triggering of unrest at volcanic calderas by large earthquakes is reasonably convincing, what’s the possibility for an earthquake occurring in southern California with a significantly larger magnitude than the 1992 Landers event? Two studies that are relevant to this question were reported in 2005 and 2006. These studies consider the potential for a large earthquake along the southern section of the San Andreas Fault, which has been ominously quiet for way too long.

Map of the San Andreas Fault (dark line). South of San Bernardino, the fault is better referred to as a “system” where it’s likely that the San Andreas and the roughly parallel San Jacinto fault zone to the west (not indicated on this map) accommodate the bulk of the relative motion between the North American and Pacific tectonic plates. The 1857 M 8 Fort Tejon earthquake referred to in the text is thought to have originated near Parkfield, in the north and extended southeast for about 220 miles (360 km) to the San Bernardino area.

THE SAN ANDREAS FAULT

The San Andreas Fault is a continental transform fault that accommodates the relative right-lateral motion of the Pacific and North American tectonic plates. During a major strike-slip earthquake a person standing on the western, or Pacific, side of the fault would see someone on the eastern, or North American, side move to the right. On June 22, 2006, Yuri Fialko, an associate professor of geophysics at the Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California at San Diego in La Jolla, California, reported in the journal Nature that the southernmost segment of the San Andreas fault running from San Bernardino in the north to the Mexican border in the south is primed for a major earthquake with between 20 and 30 feet of displacement possible, most likely in a right-lateral strike-slip sense. This section of the San Andreas has not experienced a major earthquake for more than 300 years. The last major earthquake, an M 7.7, occurred in 1690 and ruptured about a 140-mile-long segment of the fault. Although a major event, it went largely unnoticed because hardly anyone lived there at the time. Professor Fialko’s study, which was based on both satellite and ground-based measurements of large-scale deformation, indicates that stress has been building up since then, and that this southern segment of the fault may be approaching the end of the interseismic phase of the earthquake cycle. He is quoted by the Scripps Howard News Service as saying, “All these data suggest that the fault is ready for the next big earthquake, but exactly when the triggering will happen and when the earthquake will occur we cannot tell. It could be tomorrow or it could be 10 years or more from now.” An earthquake in southern California would only be, on average, about 280 and 770 miles (450 and 1250 km) from the Long Valley and Yellowstone calderas, respectively, and could possibly trigger significant activity at either, or both, of those sites.

In the May 13, 2005 issue of Science, four scientists reported results of an investigation of spatial and temporal characteristics of large earthquakes that occurred since 1200 A.D. along the southern 340 miles (550 km) of the San Andreas Fault system. This section of the fault includes the 1857 M 8 Fort Tejon earthquake, which ruptured between 180 and 220 miles (300 and 350 km) of the northern portion of the section studied. They identified geologic records for up to 56 earthquakes, although some of the records may pertain to the same event. The lead author of the Science paper noted that most of the fault ruptures every 200 years, but because of uncertainty in dating the individual ruptures, they were not able to tell whether it was one earthquake or a number of closely timed earthquakes. The scientists determined the probability of the current lull in activity ending in the next 30 years to be 20 percent, 40 percent, and 70 percent, depending on how the earthquake ruptures were modeled in space and time. The principal conclusion of their study was that the next rupture may be one great earthquake of M 8 or greater, or a series of large earthquakes all smaller than M 8. In either case, the southern section of the San Andreas is locked and loaded.

The lack of experience that scientists have when it comes to recognizing the signs or signals coming from a volcanic system or caldera on the verge of a supereruption poses a major challenge to successful forecasting. Recall that the last supereruption, Taupo, occurred about 26,000 years ago. In addition, the last VEI 7 eruption, Tambora, occurred in 1815, well before instrumental records became available. Thus, volcanologists will be hard pressed to not only identify a signal from an eruption that could be imminent, but recognize a bona fide signal (for example, volcanic tremor) for a VEI 7 or 8 eruption, as opposed to a 4 or 5.

In a recent paper published in the Philosophical Transactions of the Royal Society, scientists from the USGS and the University of Utah made the following concluding remarks:

With limited experience monitoring and responding to large-scale volcanic crises, society cannot expect a 100 percent success rate at avoiding future volcanic catastrophes. We can, however, make sure that we learn from the next VEI 6 or 7 eruption, by recording a full spectrum of signals emitted prior to eruption. At present, only a small fraction of Earth’s high-threat volcanoes is monitored in a manner that would provide a useful history of the run-up to a volcanic disaster. If we are to reduce the risk from future large eruptions, we will need to do better.


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