American Scientist

I have spent my entire career studying earthquakes, most of that time living in—and traveling around—California. I thus regard my home state as both a gigantic geologic jigsaw puzzle to be investigated and a vast outdoor playground to be enjoyed.

Sometimes the different facets of my appreciation come together, as they did recently when I set out to write a tour guide to California's myriad geological faults. Along the way I found myself following an idea that sprang into my head seemingly from nowhere but that, once it entered my mind, sparkled and danced and grew. It started with a simple realization: that California's most spectacular petroglyph sites tend to coincide with the state's most spectacular earthquake and volcano sites.

Such a connection is scarcely without precedent. Even before geologists had come to appreciate fully the earthquake potential of the Pacific Northwest, legends and oral histories of native tribes there were known to be suggestive of earthquake and tsunami activity. Ruth Ludwin, a seismologist at the University of Washington, has studied these stories at great length, in some cases using them to corroborate recent geologic inferences about great earthquakes that took place before written records began to be kept.

 In contrast, earthquake motifs do not figure prominently in the lore of Californian tribes. This absence might seem odd to modern residents, who can't help but talk about earthquakes that have just happened. Not surprisingly, Californians these days do much of that talking over the Internet.

I Felt It Too

On the 18th of June, 2002, my workday was interrupted by a familiar rattle, rattle, rattle, bam. I went immediately to the U.S. Geological Survey's "Did You Feel It?" Web page to fill in the online questionnaire. Within hours several hundred others had done the same—we all dutifully reported our accounts of what turned out to be a humble magnitude-2.8 earthquake.

The "Did You Feel It?" page has succeeded beyond the wildest dreams of its developer, David Wald, a colleague as well as a good friend of mine at the U.S. Geological Survey. More than 25,000 reports came in for the magnitude-7.1 Hector Mine earthquake in 1999. Perhaps more impressively, some 15,000 people checked in following a modest (magnitude 4.9) temblor that originated near Gilroy in May of 2002.

My scholarly studies of large historic earthquakes have made me aware of another common response, recognized by historians: to renew the connection with one's spiritual beliefs. For example, after a series of large earthquakes struck the central United States in 1811 and 1812, church membership in the affected regions rose significantly, earning some newfound worshippers the name "Earthquake Christians." When terra firma ceases to be firm, many people turn to religion for a sense of stability.

So what about the people who lived in California for thousands of years before the arrival of the Spanish in the late 18th century? These earliest Californians left behind no conventional written history, which is to say they committed no words to paper. They did, however, leave their mark on the land itself: intriguing and sometimes intricate art painted or etched onto rock.

Archaeologists have identified, investigated and endeavored to protect California's surviving petroglyphs. Their meaning and purpose remain mostly unknown, but there are hints. It's agreed, for example, that zigzag lines, one of the most common patterns, were often imbued with a specific significance related to snakes and serpents. These creatures, in turn, are commonly associated in Amerindian legends (from various locales) with unrest within the Earth. Ruth Ludwin has, for instance, suggested that tales of sea serpents in Puget Sound may be telling the story of a large earthquake on the Seattle fault around 900 A.D.

In arid desert regions of the Southwest, surviving legends—and rock art—invariably portray drought as the natural hazard of prime concern. But a geologist considering a map of Californian petroglyph sites cannot help but be struck by another thought: Many of these places are located in seismically active parts of the state. And so, as a seismologist, I set out to pursue an idea: Is it possible that these ancient petroglyphs document California's earliest historic earthquakes?

This question cannot be answered the way that most seismological questions can: with quantitative, testable models. As is often the case in archaeology, the arguments are qualitative, the inferences indirect. This is a different kind of science from what we seismologists are normally used to doing—but a kind of science that is nevertheless required for some types of investigation.

And so I proceeded with cautious enthusiasm. But I had first to concede something to the skeptical side of my brain: The fact that rock art is found in proximity to faults is not in itself surprising. To a fair approximation, rocks in California are found in proximity to faults. Away from the state's fault-controlled mountains, one typically finds valleys and basins filled with sediments that have washed down over the ages from higher elevation. Where there's no rock, there can be no rock art.

Still, I suspected there was more to it than this. Consider the zigzag lines found at one site within the Mojave desert—for all intents and purposes, the middle of nowhere. Yet this site is just a stone's throw from the location of the fault that ruptured in 1992 producing a whopping magnitude-7.3 earthquake. The age of the last big earthquake in this region is very close to the age of the oldest petroglyphs at the site. Or take what is perhaps the best known petroglyph locale in California, the Coso geothermal region, which earth scientists recognize for its rich volcanic history in recent geologic times and for its substantial earthquake activity in modern times. Indeed, Coso experiences one of the highest rates of perceptible (magnitude 3+) earthquakes in California. During the 1990s, for example, nearly 300 earthquakes with magnitudes greater than 3.0 took place near Coso. Six of them were pretty big, with magnitudes between 5.0 and 5.8.

Of course, to argue for a causal relation one would want to build a case on more than just simple correspondence. But the physical evidence is what it is. The zigzag lines are suggestive but certainly not ironclad. And it's unlikely that anyone will find a Rosetta Stone and decipher the rock art beyond all reasonable doubt. What else can one do to test the hypothesis?  Perhaps one can listen to the words of modern tribe members for whom the old legends still actively echo—not in the Mojave or in Coso, but elsewhere in California.

Voices of the Earth

The roots of the Cahuilla Indians trace back to the Palm Springs area—and to the nearby canyons where the searing desert heat is buffered. The native legends describe Tahquitz Canyon, just south of Mount San Jacinto, as the home of Tahquitz, an immortal and malevolent being who steals the souls of young women who dare venture too far into the canyon at night. His growlings account for the rumblings sometimes heard emanating from the mouth of the canyon. One version of the "Tahquitz legend," published in 1950, mentions an earthquake explicitly:

No sooner had her lips sounded the last syllable of the feared name (Tahquitz), than there was an explosive-like rumble from the direction of Tahquitz Canyon. It grew steadily louder until it became a deafening roar. The earth shook and quivered underfoot. Rocks tumbled loose and started landslides. Clouds of dust curled into the overhead darkness.

Almost all of the petroglyphs within Tahquitz Canyon have been erased by vandalism, but surviving remnants of one prominent image near the mouth of the canyon appear to depict a large shaman and, tellingly, a snake.

Could a giant earthquake have taken place recently enough to have shaped the oral traditions of the Cahuilla Indians? Probably. Many small quakes take place in the area. Two journalists who live near Tahquitz Canyon reported to me that rumblings from the canyon frequently precede ground shaking from earthquakes. And various geologists, including Tom Fumal of the U.S. Geological Survey and Kerry Sieh from Caltech, have found evidence that four or five large earthquakes have taken place along the San Andreas fault at Thousand Palms Oasis, near Palm Springs, in the last 1,200 years.

The most recent large earthquake there appears to have been in the late 17th century, with a preferred date of 1676 ± 35 years. Interestingly, this is close to the inferred date of the last great earthquake in the Pacific Northwest, which Kenji Satake, now a seismologist with the Geological Survey of Japan, and his colleagues were able to fix precisely to 9 p.m. on January 26, 1700. They used ancient Japanese tide-gauge records of the resultant tsunami, which, after several hours' delay, struck on the other side of the Pacific. It only makes sense, then, that an earthquake might figure as prominently in the stories of the present-day Cahuilla as does the 1700 event in legends of the tribes of the Pacific Northwest.

Similar but somewhat less colorful legends are told by the Southern Diegueno tribe, for whom the Superstition Mountains featured prominently in their oral traditions, as Carl Strand pointed out in 1980, when he was a geology student at San Diego State University. The very name of the mountains reportedly resulted from an earthquake that occurred during a tribal ceremony. Here also, other legends describe moans and terrible sounds emanating from caverns, which were believed to be the home of a great and evil serpent that changed the very face of the mountains with his writhings.

These highlands abound with petroglyph sites, many featuring zigzag lines. Rock art sites are so abundant in the Superstition Wilderness area that some scholars once proposed that the natives must have had a great deal of free time on their hands. Although this explanation is discredited by serious archaeologists today, it does testify to the prevalence of the petroglyphs.

To earth scientists, the legends about this area and about Tahquitz Canyon immediately ring true, to the extent that we know that both areas experience frequent earthquakes. The San Andreas fault runs through the heart of the Coachella Valley, in which Palm Springs is located, and just a stone's throw (so to speak) from Tahquitz Canyon. And the Superstition Mountains are surrounded by active faults, including the highly active San Jacinto fault system to the east.

California's earliest residents would have almost certainly experienced earthquakes that were similar in overall rate to those seen in recent times. This fact, combined with surviving lore, the locations of petroglyph sites and the nature of the drawings themselves, starts to make the conclusion almost self-evident. It would be more surprising if earthquakes, and other attendant phenomena, did not figure prominently in the ancient depictions.

And so I found myself with an indirect but intriguing inference—one that speaks to the complex interplay between the natural world and the spiritual beliefs of earlier cultures. I also found myself with an argument that didn't seem to fit any scientific journal that I could find. I talked with a couple of archaeologists, who didn't seem particularly receptive to the idle musings of a seismologist with no expertise in the subject matter. Within my own community, I suspected there would be interest but also skepticism. My ideas are not what a seismologist would consider "science."

That the natural environment shapes culture is well accepted. And although scientists recognize such things and often champion the need for "interdisciplinary investigations," we still find ourselves lacking the scholarly infrastructure—and sometimes maybe the right mindsets—to pursue them. So instead of publishing peer-reviewed papers on such topics, people like me satisfy the urge to broadcast our offbeat ideas in other ways. We give talks, offer bits of the argument in books or, as I'm doing here, share our observations in the popular press. Perhaps it is fitting that we are, in a sense, emulating earlier peoples who carved their messages into the very walls of California. One scarcely dares to hope that our stories will last as long.