San Andreas Fault Theories Shaken Up by New Studies
For years, geological evidence has suggested that large earthquake systems, such as California’s San Andreas Fault, are driven by powerful forces that are held in check only by friction along the fault that is nearly as strong as the forces themselves.
Textbooks used to train generations of earth scientists have stressed that concept, and most geologists today have had little reason to doubt it.
But now it appears that the textbooks probably are wrong.
According to new scientific studies, the friction that holds the mighty San Andreas Fault in check is so weak that it would take only the geophysical equivalent of a gentle nudge--not powerful forces--to send the fault rumbling on its way.
But the research also suggests--albeit tentatively--that large segments of the San Andreas are not on the verge of breaking loose. Studies of strain along the fault system reveal that the forces that drive the fault are, in addition to being weak, pointed in the wrong direction in many areas to trigger an earthquake.
None of that necessarily means California is either better or worse off, because no one knows whether the new evidence means the San Andreas is more or less likely to rupture in the near future.
The startling conclusions explain some geological conditions that have long baffled scientists, but they also pose profound new questions about the dynamics of the legendary San Andreas system.
What the conclusions suggest is that major reorientations of stress patterns along the San Andreas would have to take place before a cataclysmic earthquake could erupt on the fault, according to scientists involved in the study.
Dramatic changes in those stress patterns in the years ahead could be extremely important in determining whether a dormant segment of the fault is “ripe for an earthquake,” said Mark Zoback, a geophysicist with Stanford University.
Ironically, one of the main tools that led to those conclusions--a deep research well on the Cajon Pass, just 2 miles from the fault--will be shut down within the next few days because of a lack of funding. That will leave scientists uneasy with the data they have collected because drilling will be halted just a few thousand feet above the fault itself, thereby leaving some important data incomplete.
The National Science Foundation has cut the funding from $6 million to $4.8 million because anticipated increases in the foundation’s budget were denied by Congress.
The well, now about 11,000 feet deep, has revealed that “stress is almost 90 degrees (in direction) from what we expected,” according to Zoback, chief scientist on the project. “That would tend to push the fault the wrong way. So in a way it’s like a negative signal. We’re seeing no stress driving the fault” in the direction that it should be moving.
That is but one piece in a complex puzzle emerging from the research well, which will be halted about 4,000 feet short of its target, and it has left scientists such as Lee Silver of Caltech extremely agitated over having to stop drilling at this critical point. “We live with this monster,” he said recently as he drove along the fault. “We need to understand it.”
The San Andreas is the dominant active geological feature of the convergence of the Pacific and North American plates, the two giant chunks of the Earth’s crust that support the Pacific Ocean and the North American continent. Like about a dozen or so other plates, these two slabs drift slowly, driven by powerful and poorly understood forces deep within the Earth.
There is no doubt that the deep forces that drive the plates are powerful, indeed.
“If you just go and look at a mountain range and make some calculations, you know that the very existence of a mountain range implies high stresses in the Earth,” said Tom Heaton, chief of the U.S. Geological Survey’s Pasadena office.
Same Forces?
For many years geologists thought that those same powerful forces drove such surface features as the San Andreas Fault.
Rocks analyzed in laboratories tended to support that concept. Using high-temperature, high-pressure devices in the laboratory, scientists rubbed rocks together to try to simulate faulting. The results were strikingly similar to what seemed to take place during actual earthquakes.
That data made scientists such as Zoback “a high stresser,” he said during an interview at Stanford.
But although they tried, Zoback and others could not resolve one crucial problem:
Rocks subjected to intense stress should release heat as they grind slowly against each other. Instruments in the field thus should be able to detect heat flowing through the rocks, but that did not turn out to be the case.
“All the work we had done suggested that the laboratory data were valid,” Zoback said. “But I couldn’t explain away the heat flow. There were more than 100 heat flow measurements (in oil wells along the San Andreas) and not one of them showed high heat flow.”
Drilling for Answers
Other scientists had suggested that the heat generated by the stresses was carried away by ground water moving through the area, but there had been no way to measure that in a controlled environment--until the Cajon Pass project.
The results there have shown that there is no heat, even below the depth of the ground water, and the rock is not fractured enough to allow fluids to move through the area in sufficient quantities to account for the loss of heat, Zoback said.
That, he added, is the most conclusive evidence to date that the San Andreas is a “weak” fault in that it requires only weak forces to send it rumbling on its way. Thus even weak stresses cannot be ignored.
The mighty San Andreas, according to Caltech geologist Eric James, may be like “two pieces of Teflon pressed together,” just waiting for a nudge in the right direction.
Stress Patterns
If that is the case, then the orientation of stress patterns, no matter how weak, could be critically important in understanding the dynamics of the San Andreas. With that in mind, Zoback and a host of other scientists set out to determine stress patterns along most of the fault.
Their primary tool was data from scores of oil wells drilled near the fault.
Oil wells, even in solid rock, do not leave perfectly round holes because rock is somewhat elastic. Stresses compress the hole into an elliptical shape, and small chunks of rock tend to break off from the sides of the well hole at right angles to the direction of maximum stress.
Colleen Barton, one of Zoback’s researchers, has developed a computer program that allows scientists to reconstruct the entire topography of a bore hole.
“Stresses virtually squish the bore hole,” she said. “That causes compressional failure, generating the breakouts.”
The location of the breakouts, she said, “gives us the orientation of the stresses. The breakout width gives us an indication of stress magnitude.”
Baffling Results
By sampling scores of wells, Zoback and his team were able to determine stress patterns along most of the San Andreas. The results have baffled many geologists.
The San Andreas is known as a “right-lateral” fault. That means that if you were facing the fault when a quake struck, the other side of the fault would move laterally to your right. That type of fault is common throughout the world, because it is caused by two tectonic plates sliding past each other as they move in opposite directions.
The Pacific plate moves in a northerly direction, so it had been assumed that the orientation of stresses generated by the Pacific plate would point toward the north. Likewise, the North American plate is moving south, so the stresses from that plate should also be pushing southward.
Thus scientists had expected stress vectors from the San Andreas to look like the needle on a compass, pointing almost due north from the Pacific plate and south from the North American plate.
But according to the data collected by Zoback and his team, which was published late last year in the journal Science, that did not turn out to be the case.
Running Perpendicular
In some areas along the fault, such as Parkfield, where scientists are expecting a moderate earthquake at any time now, the stresses do seem to be aligned as expected. But over vast regions, the stress patterns run perpendicular to the fault, thus seeming to deprive the San Andreas of the forces it would need to move in an earthquake direction.
In extreme cases, like that found at the Cajon Pass well, the stresses are almost in the opposite direction from what had been expected.
“That’s pretty hard to understand,” Caltech’s Silver said.
He and others have concluded that the stress patterns add enormous weight to the concept of the San Andreas as a weak fault.
The stresses would have to be oriented predominantly across the fault, rather than in the direction the plates are moving, if the sides of the San Andreas are to be viewed as coated with Teflon.
“It’s just like the processes of lubrication,” Zoback said. “If you have a heavy box on the floor, it’s hard to push. If there’s oil on the floor, it’s easy to push. What we are finding is that there is oil on the floor.”
Carrying the analogy a step further, Zoback said that if stress is applied to the box on the oily floor, it must be applied straight down.
“Otherwise, the box would slide,” he said.
Must Stay Perpendicular
Thus if the analogy holds for the San Andreas, stress patterns must remain perpendicular to the fault for it to remain peaceful. A slight change in direction could send the oily box sliding on its way.
That understanding of the San Andreas could help explain some of the mysteries that have baffled geologists for years. Why, for instance, do some earthquakes near the San Andreas rupture in the opposite direction, moving contrary to the direction of the plates? Why are there regions near the fault that have been lifted up, if the primary energy is released through lateral slippage?
Zoback believes that the weak nature of the San Andreas has combined with deep forces within the Earth to reorient surface stresses in many mysterious ways. The result is that as the tectonic plates push laterally past each other, stresses pushing toward the fault fold the countryside into hills and valleys, pushing even giant mountain ranges east or west, and sending some faults off in the wrong direction.
Valuable Minerals
That, he said, has “profound” implications, even suggesting that valuable natural resources may have been overlooked because they are covered with rock formations that historically have not been associated with exploitable resources.
“Lee (Silver) is finding evidence of over-thrust of older rock over younger rock,” Zoback said, meaning that some sedimentary basins that contain oil and other minerals may be hidden beneath older structures that have been thought to be unpromising. “That has enormous resource potential.”
Surely one of the most puzzling pieces of the puzzle concerns the Cajon Pass well. The well is just north of the southernmost leg of the San Andreas, a segment of the giant fault that many experts believe may be due for a massive quake. Such a quake could devastate huge areas of Southern California, particularly in the Riverside-San Bernardino area.
“By any stretch of the imagination we would have always thought that this was a very dangerous part of the San Andreas Fault,” Zoback said. “Except this data is telling us there is no right lateral sheer stress.”
One possibility, which Zoback described as “a long shot,” is that the last time that a segment of the fault ruptured--and no one is sure when that was--”it took out so much of the stress that in fact this is not a very dangerous section.”
Different Viewpoint
Other scientists suggested that the Cajon Pass well is in the wrong place to truly reflect the stresses on the southern San Andreas. The well is just south of where the 1857 rupture ended, on the border between two segments of the fault. It may be that the situation there is quite different from what would be found if similar wells were drilled farther south, a goal Zoback would love to reach.
“A measurement in a hole is a terribly localized measurement,” said the Geological Survey’s Heaton. He said other nearby faults, and even the mountain ranges that tower over the well, could have some effect on stress orientations.
“It may be that it (Cajon Pass) is not typical” of the southern San Andreas, Heaton added.
Cajon Pass was picked partly because an oil well on the site was available, reducing the drilling needed to reach the fault by one third.
But it also was acceptable to the scientists because it could tell much about such things as heat and ground water movement near the San Andreas.
Zoback believes that it has virtually resolved those two issues: There is no significant heat, so the San Andreas must be a weak fault, with all that that implies.
STRESSES ON THE SAN ANDREAS FAULT Scientists have been studying stress measurements taken in oil well bore holes in hopes that they may be one way of determining when a fault is ripe for an earthquake. Since the Pacific plate moves north, and the North American plate moves south, scientists had expected stresses along the San Andreas to be predominantly in a north-south direction. That turns out to be true in some cases. Stresses along much of the San Andreas have been found to be perpendicular to the direction of the fault, so there is no stress driving the fault in those areas at this time. Scientists say that suggests there is little friction in the system and a quake on the San Andreas can be triggered by a relatively weak geological force. In some areas, including the Cajon Pass research well, stress is oriented the opposite of what had been expected. This suggests that major reorientations of stress patterns along the San Andreas Fault would have to take place before a cataclysmic earthquake could erupt.
