Modeling California Earthquakes and Earth Structures
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SPECIAL ISSUE MODELING CALIFORNIA EARTHQUAKES AND EARTH STRUCTURES Seismology has burgeoned into a modern science-force-fed by federal fund- ing to advance technology for detecting underground nuclear explosions and predicting earthquakes, and by industry to improve tools for gas and oil exploration. Computers, seismic instrument systems, telemetry, and data reduction have played key roles in this growth. MICHAEL R. RAUGH In conjunction with modern technology, the sophisti- FOREWARNED IS FOREARMED: A cated methods of modern seismology now enable earth HYPOTHETICAL CATASTROPHE IN CALIFORNIA scientists to peer into an opaque earth. The purpose of Many of California’s most attractive features are due to this article is to discuss some of those tools and the the same tectonic processes that we generally consider information they reveal about earth processes and destructive. A geological time-lapse film, beginning structures. The subject is broad; hence the focus is nar- around 140 million years ago, would show the Pacific rowed to several studies centered in three regions of Ocean lapping against the western slope of the ances- California, the best-studied sei.smically active state in tral High Sierras, a softly undulating range of low-lying the nation. mountains not at all like the lofty granite peaks of to- The article begins with a sobering discussion of the day. As millions of years roll by, leading edges of the potential impact of a major quake in California. Indeed, Farallon and Pacific plates would bump and jostle off- a disastrous shock with severe human and economic shore against the western edge of the North American consequences is expected within the next generation, plate, causing subduction of oceanic plates and crum- but good planning c:ould alleviate some of the hardship. pling and upward folding of the marginal terrain, grad- Next, for historical perspective, a few selected mile- ually extending the continental plate by aggregation of stones are sketched to show how rapidly seismology island arcs and new mountains arising out of the sea to has advanced in the past 25 years. In order to provide a the west. The sea would advance and retreat repeat- technical perspective, the two most extensive seismic edly, but ultimately retreat farther and farther to the instrumentation systems in California are described, il- west. We would see volcanoes exploding at various lustrating a complex system of instrumentation, teleme- places on land and offshore, and massive lava flows try, data reduction, and computers. Finally, a sampler rushing out over the ancestral Sierras. At stages, semi- of seismic studies that would have been inconceivable tropical forests would develop in central California before the present era of instrumentation and computer around the marshy edges of the Great Valley leaving systems is presented, centerin,g on three regions of Cali- coal deposits, then an ice age and glaciers would appear fornia (Figure 1): the Imperial Fault in the Salton scouring the High Sierras rising ever higher to the ac- Trough (site of the best-documented earthquake in his- companiment of violent earthquakes. We would see the tory), the Long Valley Caldera east of the High Sierras final draining of sea water from the Great Valley, the (where a potentially explosive volcano threatens to cut carving of the Golden Gate, and the encirclement of off Owens Valley water to Los Angeles), and the Big San Francisco Bay. What we would not see is the swal- Bend (a puzzling change in direction of the San An- lowing of the Farallon plate beneath the shores of dreas Fault through the Transverse Ranges of southern southern and central California, an invisible process California]. that continues today north of Cape Mendocino. But, ACM/IEEE-CS Joint Issue8 1985 ACM OCIO~-0782/65/1100-1130 750 when the present Pacific plate reached California, 1130 Communications of the ACM November 1985 Volume 28 Number II Special issue FIGURE1. Map of Californiaand SurroundingLandscape Showing SomeLocaffons and Geological FeaturesReferred to in the Text The light background north of the Transverse Ranges approximates the region instrumented by the USGS California Seismological Network (CALNET), and the darker back- ground to the south approximates the region instrumented by the Southern California Array, operated in cooperation by the USGS and Cal- tech. Owens Lake at the southern end of Owens Valley, designated in the figure as a drainage basin, was until early in the century a large body of alkaline water more caustic than Mono Lake; it has been reduced to an extensive alkali flat by diversion of the Owens River into the Los Ange- les Aqueduct. November 1985 Volume 28 Number 11 Communications of the ACM 1131 Special Issue shoving the Farallon plate beneath the continent some The most famous earthquake in American history 20 million years ago, the subduction of oceanic plates was that of 1906 in San Francisco. Two other California changed to horizontal slip south of Cape Mendocino, earthquakes at least as large also occurred within his- creating the highly visible San Andreas fault system. In torical times, but are less well known because less the final sequence, we would see the edge of California damage was associated with them. The great quake of west of the San Andreas gliding briskly northward (see 1857, sometimes referred to as the Fort Tejon earth- Figure 2). The film would conclude with dramatic quake, produced approximately 350 km of faulting all views of the major landmarks of California, a region the way from Parkfield in Monterey county southeast justly famous for its natural resources: mountains, through Fort Tejon in the Tehachapis to Cajon Pass in beaches, deserts, lakes and rivers, abundance of min- San Bernardino county. In fact, some regions of the erals, and fertile plains. California’s beauty and wealth Tejon fault slipped as much as 11 m, compared to 7 m are due to tectonic processes. Yet even today these for the earthquake of 1906. The Owens Valley earth- same relentless processes also cause the catastrophes quake of 1872, centered near Lone Pine, may have been associated with earthquakes and volcanoes. the largest of all. In this century, there have been Generation of Generation of Ridge axis Volcanic (source) oceanic crust oceanic crust chain and lithosphere / - and lithosphere \ According to plate tectonics, the eisrth’s lithosphere dstrong dgntinent, creating an ocean trench associated with the line outer layer) is divided into about a do?en large oontiguous where Su~udtiotif3agins. As the ocean plate descends into plates, each of which is in slow motii corresponding to thehot &ihtlebf t#e earth, ii partly mefts, and tighter por- convection occurring within the mantle beneath We plates tir$rs float back uptoward the surface, causing volcanic flowing plastically at a rate measured in centimeters per year. eruptiOns and the uplift of mountains. It is a matter of debate Earthquakes occur when one plate scrapes against enother. whether convection drives the plates, or gravitational forces In special areas of the earth where plates diverge, c@led dsSvethe conveotion by pulling down upon the uplifted ocean midocean ridges, molten material (magma) rises conve&yely tic&es @k& push*) and pulling the descending slabs down from the earth’s mantle and fills the gap between the diverg- (aslabpull”). Although easy to grasp in principle, the modem ing plates, creating tractions that pull the ocean ridge apart. theoty of plate teqtonics was not easy to perceive and verffy. Fresh magma flows into the cracks, where it adheres and GaologikM processes are notoriously slow, and the global solidifies within the ridge. When an oceanic and conlinental s patterns in which they operate become apparent only when plate converge (as at the western boundary of South Amer- abseWed systematically on a, worldwide basis. (After Dewey, ica), the denser ocean floor is driven (subducted) beneath the J.P. Plaita tectonics- Sci. Am. (1972).) FIGURE2. plate Tectonics 1132 Communications of the ACM November 1985 Volume 28 Number 11 Special Issue smaller destructive earthquakes in Santa Barbara, Long capability may be incorporated as they become avail- Beach, Tehachapi, San Fernando, Imperial Valley, and able, without adverse affect on the functioning of the Coalinga. overall system. Based on the observed regularity of earthquakes and Initial scenarios are planned for developed regions in other structural factors of known California fault sys- California where high probabilities exist for large-to- tems, earth scientists expect an earthquake comparable great-magnitude earthquakes in the near future. Pre- to that of 1906 to strike somewhere, probably in south- liminary studies have yielded ominous results. ern California, within the next 30 years. A major earth- Steinbrugge and others (1981) found that direct prop- quake near Los Angeles or Silicon Valley, on the scale erty losses for a magnitude 7+ shock centered in highly of 1906, would have a terrible impact on the economy developed regions of Los Angeles or in the San Fran- of California and on the nation. Such an earthquake is cisco Bay region would be in the $40-60 billion (1980) inevitable; the only question is when. Although earth- range, an order of magnitude more damage than the quakes cannot be prevented, they can be anticipated worst hurricane in U.S. history (Hurricane Agnes, and planned for. 1969). But that may be just the tip of the iceberg. What The Federal Emergency Management Agency has not been included are estimates of the indirect (FEMA) is responsible for developing contingency plans losses, for example, unemployment and loss of supplies for responding to and planning the orderly recovery and services due to shutdown of factories, disruption of from an earthquake disaster as well as creating emer- lifelines such as highways, water, and electric-power gency policies in anticipation of such a disaster.