SCEC INSTANeT News
	A Trio of October Earthquake Anniversaries

By John Marquis The halfway point in the month of October is a good time to reflect upon the lessons learned from previous California earthquakes; the anniversaries of three recent large earthquakes fall on consecutive days this week. October 15 marks the anniversary of the 1979 Imperial Valley earthquake, October 16 is the one-year anniversary of the Hector Mine earthquake, and October 17 is the eleventh anniversary of the Loma Prieta earthquake. What did each of these three earthquakes teach us? The Mw 7.1 Hector Mine Earthquake: October 16, 1999 Residents of southern California who were here one year ago may remember being awakened in the early morning of October 16, 1999. Just before 2:47 a.m. that morning, an unnamed fault zone in the Mojave Desert gave way, breaking under the load of massive tectonic stresses that had been building up in the area for thousands of years. Along this fault zone, the rupture spread simultaneously north and south from its hypocenter, the starting point of the earthquake, 47 miles (75 kilometers) east-southeast of the town of Barstow. When the fault rupture ended roughly 10 seconds later, it had cut a scar over 25 miles (41 kilometers) long across the surface of the Mojave, creating right-lateral offsets of up to 17 feet (5.2 meters)! For southern Californians, the story was just beginning as the fault rupture died out. Radiating outward from the rupture like shock waves from an explosion, seismic waves, generated by the shifting of immense slabs of the Earth's crust, began to pass beneath the homes and businesses of southern California, southern Nevada and western Arizona. As these seismic waves passed the instruments of the TriNet seismic network, the ground motions they generated were recorded and relayed to the Caltech Seismological Laboratory. There, the automated computer systems began to calculate the location and magnitude of the earthquake. Within 90 seconds of the beginning of the rupture, a preliminary epicenter had been determined, and the magnitude estimated at 6.6. After 30 more seconds of analysis, this value was revised to Mw7.0. As this was occurring, people throughout the region were using their computers to log on to the Internet and find out just what had occurred. We have no way of knowing just how many people felt this earthquake, though the number is surely in the hundreds of thousands, and probably well into the millions. Sleepers were roused from their rest, workers on the late-shift took cover, and even gamers in Las Vegas paused and hoped for an extra bit of luck to see them safely through the shaking. Luck, it turned out, was on the side of almost everyone that morning. Despite the extent of the surface rupture, the location of the earthquake was so remote that not a single paved road or power line was cut. An overpass on Interstate 40 sustained damage but did not fail. An Amtrak passenger train in the same area derailed but did not overturn. Even the damage at Ludlow, the town nearest the epicenter, was fairly limited. Injuries were relatively few, and no deaths were reported. Ironically, one of the few hard-luck stories from this earthquake was that of seismologists preparing for LARSE II, the Los Angeles Regional Seismic Experiment, which had been planned for that very same week. The rush to study the earthquake funneled some manpower and resources away from LARSE II, but the coincidental earthquake increased public awareness of the earthquake hazard and served as a reminder for why the project was necessary. Hector Mine ShakeMap. Click image to view larger version. (Map from TriNet ShakeMap) Back at the Caltech Seismological Laboratory, news crews were arriving and calls were coming in from media organizations eager to learn the details about the earthquake that had just occurred. Similarly, Internet access to computer servers was exceptionally heavy; this is considered the first major "online earthquake." The initial TriNet ShakeMaps were created within four minutes and were in great demand. The Community Internet Intensity Maps were accessed almost immediately. Within six hours, almost 5000 responses had been received and processed. Once seismologists found time to review the preliminary location of the earthquake, they had to choose a name for it. Naming earthquakes is somewhat subjective, but generally follows a few simple rules. A town name is typically assigned to an earthquake if the epicenter or the area of strongest shaking falls within city limits. For this earthquake, that was not the case, so a different rule was used, and the name of the nearest mapped feature was assigned to the event. In this case, that feature was the Hector Mine. Thus was the earthquake dubbed the "Hector Mine earthquake."   Hector Mine fault rupture (Photo: Katherine Kendrick, USGS) As data continued to stream in and be subjected to further analysis, details about the Hector Mine earthquake were refined. Records from distant seismic stations suggested that the local network had underestimated the magnitude of the earthquake; it was soon revised to Mw7.1. Surface mapping along the rupture trace began as soon as permission was received from the Marine Corps, as the rupture lay entirely within the bounds of the Twentynine Palms Air Ground Combat Center. This mapping revealed a wealth of information. The fault zone that initiated the earthquake was indeed unnamed, but not entirely unmapped as some had originally thought. In addition, part of the rupture occurred along the previously mapped (and named) Bullion fault zone. The unnamed part of the trace was soon dubbed the Lavic Lake fault zone, after the dry lakebed (or playa) cut by the rupture. Hector Mine Epicenter and fault rupture. (Photo: USMC, Twentynine Palms. Graphics: USGS) Because of its remote location and the relative lack of damage and human drama it caused, the Hector Mine earthquake dropped fairly quickly from the headlines. In seismological terms, however, its legacy lingers to this day. As is typical for an earthquake this large, aftershocks began to occur immediately after the mainshock. (The threat of aftershocks "washing out" their experimental data was another concern of those leading LARSE II in the wake of this earthquake.) Though the number of aftershocks decays exponentially with time, an earthquake of this size can have an aftershock sequence that lasts for years. In the year since the Hector Mine mainshock, seismic analysts have reviewed over 11,000 aftershocks. At least 400 of those were greater than magnitude 3, easily large enough to feel near the epicenter. Those numbers are minimum estimates; so much seismic data came in during October and November 1999 that records from those months still await review. Even now, aftershocks continue to shake the region around the Lavic Lake fault zone. In the last thirty days alone, roughly 500 aftershocks of the Hector Mine earthquake were recorded and analyzed. The Hector Mine earthquake has left other legacies in the scientific community. The Lavic Lake fault was largely unmapped prior to the earthquake because it appeared to have last ruptured more than 10,000 years ago. According to the definitions of "active" and "inactive" faults as established for zoning purposes, that defined it as "inactive," making its study a low priority. These definitions clearly need review and revision. Hector Mine earthquake sequence (yellow circles) and Landers earthquake sequence (clear circles). Click map to view larger version. (Map: Egill Hauksson, Caltech Seismological Laboratory) Like the 1992 Landers earthquake, the Hector Mine earthquake seemed to "trigger" earthquakes well outside of its aftershock zone. It was also noted that the earthquake was preceded by a small series of foreshocks, and that these foreshocks occurred in an area that saw increased seismicity in the wake of the Landers earthquake. Did the Landers earthquake influence the onset of the Hector Mine earthquake? Most scientists accept the idea of some sort of connection between these earthquakes, since their proximity within the Eastern California Shear Zone (ECSZ) and the coincidence of their timing -- only seven years apart on faults that do not rupture but once every several thousand years -- seems too great to ascribe to pure chance. The mechanism responsible for such a connection and the possibility that this might lead to more large earthquakes in the ECSZ in the near future are just a few of the unresolved issues brought to light by the occurrence of the Hector Mine earthquake. The greatest legacy of this earthquake may be the quantity and quality of data recorded by instruments across southern California. This information may help us gain insight into the mysteries of fault mechanics, stress transfer and other contentious issues involving the physics of earthquakes. It may also provide career foundations for budding geoscientists- this summer three SCEC Undergraduate interns (Marie Ammerman, Teresa Baker, and Allison Jacobs) focused their research projects on the Hector Mine earthquake.   The Ms 7.1 Loma Prieta Earthquake: October 17, 1989 The start of the 2000 World Series a few days from now may bring back, for some, the memories of one of the most remarkable events in the history of baseball, television and seismology. It happened on October 17, 1989, at 5:04 p.m., just minutes before Game 3 of the 1989 World Series was to begin. Television Graphic just prior to the Loma Prieta earthquake (KGO-TV News/ABC-7, San Francisco) Fans across America were gearing up at home for the televised coverage of the game between the Oakland Athletics and the San Francisco Giants at Candlestick Park. Suddenly, the television feed began to cut out, finally going completely black just as an announcer shouted out, "We're having an earthquake!" The crowd of 62,000 at Candlestick Park was overcome with fear and confusion as the stadium rocked back and forth. The shaking ended about 20 seconds later, but the damage had already been done. Direct losses incurred during that brief but terrifying period are estimated at over $6 billion. When the television feed from San Francisco was reconnected, viewers at home found themselves watching live disaster footage instead of a ballgame. Fires had broken out and were raging across the Marina District of San Francisco. A single section of the Bay Bridge had collapsed, as had much of the double-decker Cypress Street section of Interstate 880 in Oakland. In all, at least 62 people were killed, more than 3,700 were reported injured, and over 12,000 people were displaced from their homes. Cars damaged in the collapse of the Bay bridge. (Photo: Steve Ringman, San Francisco Chronicle) What many found truly disturbing about this event was the fact that, despite the concentration of damage around the San Francisco Bay area, the epicenter of the earthquake was located in the Santa Cruz Mountains near a peak named Loma Prieta, some 60 miles (96 kilometers) south-southeast of San Francisco! A 28-mile (45-kilometer) section of the San Andreas fault zone ruptured to produce this earthquake, measured at Ms7.1 (Mw6.9). The Loma Prieta earthquake, while frightening and destructive, was not the "Big One" -- the inevitable repeat of the Great San Francisco Earthquake of 1906. But perhaps because of the lessons learned from the Loma Prieta earthquake, the San Francisco Bay area may fare better than expected when the "Big One" does strike. The Loma Prieta earthquake showed in dramatic fashion what a difference soil conditions can make in determining the amount of damage a structure will sustain in an earthquake. It is hoped that this knowledge, though gained at an unfortunate cost, will at least help planners and builders prevent future tragedies like those of October 17, 1989. For more detailed information on the Loma Prieta earthquake, please see the following reports from the USGS and EQE: http://quake.wr.usgs.gov/study/LomaPrietaAnniversary/ http://www.eqe.com/publications/lomaprie/lomaprie.htm   The Mw 6.4 Imperial Valley Earthquake: October 15, 1979 Strike-slip offset of 57 centimeters on a canal on the Imperial Fault. The concrete patch over the broken area has already been cracked by post-earthquake slip. (Photo: Robert E. Wallace) This past Sunday, October 15, was the twenty-first anniversary of the Imperial Valley earthquake of 1979. While not nearly as large as either the Loma Prieta or Hector Mine earthquakes, this rupture along the Imperial and Brawley fault zones, southern extensions of the San Andreas fault system, caused $30 million worth of property damage and 91 injuries. Loss resulted primarily from shaking and surface rupture damage to the extensive agricultural infrastructure -- canals and irrigation systems -- of the Imperial Valley, an important farming area of southern California. Why should you remember this earthquake? The reason is simple: another one like it will probably occur there in the next few decades. Five large earthquakes have ruptured this fault zone in the past 100 years. In 1906, an earthquake of magnitude 6 struck the area. Just nine years later, a pair of magnitude 6 earthquakes separated by about 58 minutes shook the region and led to six deaths. {} In 1940, a magnitude 6.9 earthquake rocked the region violently, killing eight people and causing millions of damage in lost crops and ruined infrastructure. Together with the earthquake of 1979, these events seem to indicate that at least one section of the Imperial fault zone will rupture in a magnitude 6 or greater earthquake, on average, every 30 to 40 years. The high frequency of damaging earthquakes in the Imperial Valley, combined with sprawling agricultural infrastructure and a fairly low population density, gives the region the dubious distinction of being a national leader in seismic risk per capita. A recent report on the estimated annualized earthquake losses for the United States published by the Federal Emergency Management Agency (FEMA) found that Imperial County (home to the fault zone of the same name) was among the top counties in the nation when expected annual losses are averaged out on a per-capita basis. That same report found that Los Angeles County alone accounts for one-quarter of the nation's total annualized earthquake loss, with the rest of southern California shouldering another quarter of that burden (see SCEC's response to this report) These numbers, like these earthquake anniversaries, are a reminder to all of us in "earthquake country" that we need to do our part to be prepared for the inevitable. Through preparation, we can minimize damage and increase our chances of safely riding out the next major earthquake.

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