by John Dvorak
But the historic and prehistoric earthquakes Sieh identified at Pallett Creek represented only one point along the hundreds of miles of the San Andreas Fault. Certainly, there must be similar records at other places. And indeed there are.
In the decades since Sieh first pushed a shovel into the soil at Pallett Creek, 22 sites have been uncovered where paleoseismic work has been done. They extend from the northernmost site at Shelter Cove just south of Cape Mendocino to Salt Creek on the east edge of the Salton Sea. But the record at Pallett Creek is one of the best because it has the second longest record, exceeded only by a slightly longer one at nearby Wrightwood, located halfway between Pallett Creek and Cajon Pass. Here the record of earthquakes along the San Andreas Fault goes back an additional 200 years, the earliest event occurring in about 530 a.d.
Not surprisingly, when the records of different sites are compared, the effect of the same earthquake is seen at more than one site. In the last 1,500 years, there have been at least three earthquakes that broke the same section of the San Andreas as happened in 1857. These three earlier events occurred in about 1360, 965, and 655 a.d. We also know that there was a flurry of four events that broke through Wrightwood near Cajon Pass between 600 and 800 a.d. In fact, when looking at records from all 22 sites, it seems that after three or four centuries of quiescence, seismic activity resumes and a series of major earthquakes occurs along the entire length of the fault. It took a record that extended back more than 1,000 years into prehistory to give us a long enough sequence of earthquakes to make these correlations; relying on a history of barely 200 years was not taking into account enough time to understand how activity shifts along the San Andreas Fault.
In short, thanks to Sieh’s findings, it was revealed that major earthquakes along the San Andreas Fault are not random but occur as clusters—an important point that will be returned to later in this book.
The success of paleoseismology in California, pioneered by Sieh, set off a revolution in seismological research. In Italy, for example, more than 100 trenches have been dug across dozens of active faults, revealing evidence for scores of previously unknown individual earthquakes dating back as far as 6,000 years. So far, the longest and most complete paleoseismic record comes from a region of the Dead Sea in the Middle East where thinly bedded sediments laid down by a former, much larger sea contain evidence of individual earthquakes going back 50,000 years.
A study of those sediments confirms that a major earthquake happened in the Dead Sea region in 31 b.c., an earthquake that, according to historical documents, damaged the Second Temple in Jerusalem and Herod’s Winter Palace in Jericho. Archaeological evidence suggests it also damaged the city of Qumran on the West Bank, disrupting the city’s water supply and causing people to leave. It has also been suggested that the disruption recorded in the bedded sediments of the Dead Sea in 31 b.c. might have been associated with shaking so strong that it closed off a cave near Qumran, sealing for nearly two millennia an archaeological treasure known as the Dead Sea Scrolls.
Paleoseismology has solved a host of other problems. For example, in Greece there has been a long-running controversy about the sanctuary of Delphi, where in ancient times a series of specially trained women sat inside a temple and answered questions with prophecies. According to Plutarch, who ran the sanctuary for many years, a priestess uttered an oracle after inhaling vapors that rose from a chasm in the earth beneath the spot where she sat. For years, the existence of the chasm was dismissed because archaeologists could not find any trace of it. But paleoseismologists, who examined the site with differently trained eyes, have identified an active fault that passes directly under the temple. Groundwater is easily accessible near the fault because the fault gouge is impermeable. Furthermore, they have also found a large body of limestone with bitumin, a petrochemical that, when dissolved by groundwater, might have released ethylene. Inhaling ethylene produces euphoria—which is why it was once used as an anesthetic—and that could explain the trance-like state of the Delphic oracles.
Another mystery that paleoseismology has shed light on is the apparition of Archangel Michael at Monte Sant’Angelo on the Adriatic coast of Italy in 493 a.d. According to the one person who claimed to have seen the archangel, the first Bishop of Siponto, Michael’s appearance coincided with immenso termore, an immense earthquake. Paleoseismology has confirmed that a large earthquake did occur near Monte Sant’Angelo in about that year; in fact, it was the largest earthquake to strike that part of Italy in the last few thousand years. The tools of paleoseismology can also explain the material evidence left of Michael’s apparition as described by the Bishop of Siponto: the appearance of a giant footprint at the entrance of a sacred cave (where, later, the Footprints Altar was built). Trenching shows that the “giant footprint” was a local subsidence of the land related to the surface rupture that caused the earthquake.
On September 9, 1349, a strong earthquake shook much of the Italian peninsula—an account of which was written by the poet Petrarca—causing many major buildings to collapse, including the outer south wall of one of the iconic monuments of the ancient world, the Colosseum in Rome, giving the monument its famous asymmetric look. But where did that earthquake originate?
Paleoseismological sleuthing, which involves trenching as well as—in this case—the examination of aerial photographs and the reading of numerous Annals and Chronicles from the Middle Ages, reveals that a 15-mile-long rupture formed at the base of Matese Massif in the southern Apennine Mountains about 50 miles southeast of Rome in 1349. But why did the south wall of the Colosseum collapse, and not the north wall?
Trenching into the base of the Colosseum has shown that the northern half of the monument lies on firm ground, while the southern half was built on loose sediments that filled a former tributary of the Tiber River. The ground shaking in 1349 was amplified by the sediments, which, because they were not compacted, behaved like quivering jelly, causing the outer south wall of the Colosseum, having no firm base, to collapse. It is also recorded that the failure of the south wall set off a quarrel between Pope Clement VI and some powerful Roman citizens over who had the right to collect the fallen stones and use them in construction projects.
The use of paleoseismology to solve archaeological problems and theological quandaries shows that the techniques of the new science are not limited to just an examination of individual geologic layers and how they may be disrupted. It is much broader than that. A case in point is how a 3,000-year history of major earthquakes was determined for the offshore region of northern California and the Pacific Northwest—and how those earthquakes relate to the San Andreas Fault.
This is the region of the Cascadia subduction zone where two remnants of the Farallon plate—the Juan de Fuca and the Gorda plates—are sliding under the North American plate. The sliding does produce large earthquakes, the latest on January 26, 1700. That earthquake and its predecessors certainly disrupted geologic layers, but these layers lie under the sea and are not easy to study. Thankfully, there is another record of these earthquakes.
Strong seismic shaking can cause the release of submarine landslides in the form of turbidity currents. These are rapidly moving, sediment-laden slurries that slide down steep undersea canyon walls, pulled downward through the water by gravity. The evidence for scores of such turbidity currents have been found by retrieving samples from the ocean floor off the coasts of northern California, Oregon, and Washington. The timing of these currents—and the earthquakes they imply—shows a remarkable correlation with the timing of major earthquakes along the northern segment of the San Andreas Fault.
In the last 3,000 years, 14 of the 15 major earthquakes that occurred along the northern segment were preceded by a major earthquake along the Cascadia subduction zone. The average time interval between a Cascadia event and the subsequent San Andreas earthquake is 40 years, yet some of these paired events might have been simultaneous. The only major earthquake not
preceded by a Cascadia event was the 1906 earthquake—which adds to the intrigue as to exactly why the 1906 earthquake was different.
But the important point is that the timing of major earthquakes along the San Andreas Fault seems to be influenced, at least in part, by factors that extend far beyond the main strand of the fault, over a region that extends north into the Cascadia subduction zone. In fact, to get a proper understanding of what causes earthquakes in California and how the San Andreas Fault is evolving, it is necessary to look not only across all of California, but across the entire American West.
Chapter 11
Disassembling California
The whole place was shaking like crazy.
—A guest on the 18th floor of the Mirage Hotel,
Las Vegas, October 16, 1999
Two subtle clues to the history of the San Andreas Fault can be found at Weston Beach near Point Lobos south of Monterey Bay. One of the clues is the existence of distinct purple rocks with pink flecks that can be found embedded in a conglomerate at the south end of the beach.
A conglomerate, as every introductory geology textbook explains, is an easily recognized sedimentary deposit comprised of rounded boulders, cobbles, and pebbles set in a matrix of fine sand and silt. This particular conglomerate, part of the Carmelo Formation—named and first described by Lawson in 1892—formed as a result of an undersea landslide that slid down the side of a steep submarine canyon. It is the product of a turbidity current. Later, as the Coast Ranges were formed, the conglomerate was lifted up to sea level. How it was lifted does not concern us at the moment. Instead, focus on the purple rock.
I draw attention to this particular component of the conglomerate because it is easy to identify. About one out of every ten boulders, cobbles, or pebbles in the conglomerate is this purple rock peppered with pink flecks of feldspar crystals, which adds to its attractiveness and ease of identification. It is a volcanic rock—actually, part of a volcanic tuff, a thick ash deposit—exploded 150 million years ago from a volcano far to the south that has long been extinct and now likely eroded away. Keep in mind the existence of this purple rock at Weston Beach.
At the north end of Weston Beach is a light-gray sandstone with an abundance of fossil imprints. The imprints include mud cracks, animal burrows, and trails of animal tracks. All are easy to find and quite easy to see, especially when the angle of the sun is low and shadows are long in early morning or late afternoon. There is a peculiar imprint that looks like it was made by a feathered serpent, or perhaps a feather boa. Even experienced geologists have commonly mistaken it for some type of ancient seaweed, but it was part of an animal: It is the imprint of an inhalant siphon of the bivalve mollusk Hillichnus lobosensis.
The imprints of Hillichnus lobosensis in the sandstone at Weston Beach are one to two inches wide and as much as five feet long. They often occur in clusters, each individual imprint forming an arc. What makes it all the more intriguing is that Hillichnus lobosensis is a rare fossil, found at only two places along the California coast—at Weston Beach and at Point Reyes, 100 miles to the north.
At Point Reyes, there is also a light-gray sandstone that contains imprints of animal burrows and animal tracks and of Hillichnus lobosensis. There is also a conglomerate at Point Reyes with boulders, cobbles, and pebbles of a purple rock with pink flecks, best found just before making the descent to the lighthouse, hanging near the skull of the female gray whale that was put on display many years ago.
Hillichnus lobosensis and the purple rock are just two indicators that the rock sequences at Point Lobos and Point Reyes are similar; there are more. In fact, every characteristic that has been studied—detailed structures in the sandstone that reveal the direction and speed of ancient ocean currents, microfossils in the conglomerate, other rock types in the conglomerate, as well as a white granite exposed as basement rock beneath the conglomerate at both Point Lobos and Point Reyes—points to the same conclusion: The rocks at Point Reyes and at Point Lobos were once adjacent. But how did they become separated by 100 miles?
The obvious answer is that they were slid apart by the San Andreas Fault, but an alert reader who knows California geography will realize that this is impossible because both Point Lobos and Weston Beach and Point Reyes and its lighthouse lie on the same side of the San Andreas Fault—on the west side.
So how do we account for this phenomenon? Perhaps there is another San Andreas–type fault west of the actual one, and it was movement along this fault that separated the rocks that are now at Point Reyes from those at Point Lobos. That, indeed, is what happened.
The San Gregorio-Hosgri Fault begins just south of Point Reyes and runs for more than 200 miles southward off the coast of central California, coming onshore at just a few places: at Bolinas Lagoon near Point Reyes; a short segment on the San Francisco Peninsula north of Santa Cruz; at Big Sur, where movement along this fault is responsible for the spectacular sea cliffs; and at San Simeon near Moonstone Beach not far from the famous Hearst Castle.
In every regard, the San Gregorio-Hosgri Fault can be considered an early version of the San Andreas Fault. It has slid blocks horizontally at least 100 miles; it was active earlier than the current San Andreas Fault; and it remains active today, though at a much reduced rate—the most recent earthquake of note was a magnitude-4.6 earthquake in 1963 in the Santa Cruz Mountains.
Today, as the Pacific and North American plates slide past each other in California, the motion is taken up mostly along the San Andreas Fault. Earlier, it was along the San Gregorio-Hosgri Fault. It is one of the obvious clues that there has been an evolution of the Pacific–North American plate boundary, and the boundary continues to evolve, which explains why earthquakes occur across almost the entire state and why, according to the California Geological Survey, there are more than 700 different faults scattered across California that have ruptured in the last 10,000 years. California is, indeed, earthquake country.
This also provides a view as to the future of the state. California is not going to fall catastrophically into the ocean, as some doomsday predictors profess, but it is being sliced and slid apart incrementally, most of the sliding occurring during the occasional large earthquake.
This realization is not some inconsequential fact. Because earthquakes cannot be predicted—at least, not at present—it is not possible to pinpoint the time and place and exact severity of the next major one. But it is possible to know where the next potentially damaging earthquakes are most likely to occur. And this rests on knowing not only how the state of California is being torn apart, but how the various terranes that are California were assembled.
The East and the West Coasts of the United States are profoundly different socially, historically, and geologically. The East Coast is stable and urbane, while the West Coast is free-wheeling and sprawling and the people are highly mobile. And then there are the differences in geologic histories.
Long ago the East Coast was dominated by episodes of mountain building, called orogenies, interrupted by prolonged periods of extension and basin formation. The first such mountain-building episode, the Taconic Orogeny, occurred about 500 million years ago and was caused by a collision of an island arc, probably similar to the one formed by the Aleutian Islands today, against what was the nascent east continent of North America. The line along which the collision took place and where the island arc is held fixed today against the continent, a line appropriately known as a “suture,” can be followed from Newfoundland to New Jersey. It passes through Maine, Massachusetts, and Connecticut. It runs along Long Island Sound and the Harlem River, then the East River, and crosses New York Harbor to Staten Island. If one cares to stand along it, an accessible spot can be found in the Bronx at Tremont Park, where a baseball diamond straddles it. The rocks exposed along the right-field line are those of the old continent—part of the Manhattan Schist—while those on the opposite, left-field side are the younger arrivals—deep
-water shales deposited on oceanic crust—that once rode atop a tectonic plate and smashed against North America.
Another collision, the Arcadian Orogeny, occurred about 100,000,000 years later and involved the collision of two entire continents, forming the northern Appalachian Mountains. Then after a period when the two continents might have pulled apart, they were pushed together again, this time at a slightly different angle, and the Alleghenian Orogeny was the result, forming the southern Appalachian Mountains from Alabama to New Jersey.
Then the motion of the continents reversed and the continents were pulled apart again, but this time the pulling continued until a great rift formed a mid-ocean ridge and a new ocean crust was created. This was the breakup that formed the Atlantic Ocean. Since then, the East Coast of North America has no longer been a plate boundary where continents are shoved together and mountains created. Instead, it has been a passive margin and those mountains formed by the Acadian and Allegheian Orogenies, which were once as high as the Alps and the Rocky Mountains, have been eroded down to the sequence of rhythmic hills seen today.
Meanwhile, 500 million years ago when the Taconic Orogeny was happening on the East Coast, the West Coast of North America was a passive margin located about where the Nevada-Utah border is today. To the west was a vast ocean. Then about 350 million years ago, just after the Arcadian Orogeny, the West Coast changed in character—drastically.
Exactly how it happened is still debated, but the basic timeline is this: A mountain-building episode—this one known as the Antler Orogeny, perhaps involving a collision with an island arc—extended the West Coast as far as central Nevada. Then 50 million years later, a huge block of continental material of unknown origin—known as the Sonomia Block—collided with the West Coast and extended the coastline westward hundreds of miles into what is now eastern California. That was followed by the subduction of a series of oceanic plates. One of those oceanic plates carried a continental block—known as the Smartville Block—which arrived about 150 million years ago and pushed up next to the western edge of the Sonomia Block.* Oceanic plates continued to subduct under the continent of North America—the last one being the Farallon plate—and as that happened, parts of the ocean crust were scraped off to form the suite of rocks known as the Franciscan, such as the red chert visible on the north side of the Golden Gate, and, ultimately, to be the source of most of the colorful pebbles at Moonstone Beach. Thus, in piecemeal fashion, was the state of California assembled.
