I have to admit from the beginning that I'm not a geologist, so I may well make a mistake or two in this section. I apologize in advance for any such problems you might find. If you do find one, please send me e-mail so that I can fix it.
Very little of what is now California was originally a part of North America back in the Precambrian Era; basically the Mojave Desert basement rocks were around then. Most of the rest of California is made up of large chunks of different kinds of rocks which were ``imported'' from other parts of the world through plate tectonic movements. These big chunks of rock are called displaced terranes.
California is an amazing mess of such terranes, and doing a good job of giving the full history of these terranes would certainly take a full book. So I'm not even going to try. Instead, I'll give a brief timeline in list form, with the main highlights of California's evolution outlined. At the end of this section, I'll try to list some other good references to look at for more detailed information. For reference, ``Ma'' means ``million years ago''.
Here's another list of odd or otherwise interesting facts I dug up in the process of making the list above:
Figure 1 is a nice picture from the USGS which shows the last 30 million years of California's evolution.
Here are a couple places to look for more information. Both are fairly technical.
OK, so we're now back to stuff I feel more comfortable with, like: What is a fault? What kinds are there? How can faults be recognized ``in the wild''? And where are the faults in southern California? All good questions.
A fault is a crack in the ground which divides a single block of rock into two blocks of rock. However, what makes a fault a fault (as opposed to a joint) is that there is motion between the two blocks of rock. So a fault is a crack in rock which has relative motion between the blocks of rock on either side of the crack.
There are three basic kinds of faults: strike-slip, normal, and reverse. Normal and reverse faults are also lumped together and called ``dip-slip'' faults. Figure 2 (borrowed from Lisa Tauxe's notes) gives a simplish diagram of each of these three kinds of faults.
Combinations of these basic kinds of faults are possible, making what are called oblique-slip faults.
Strike-slip faults come in two flavors as do dip-slip faults. There are right- and left-lateral strike-slip faults. Right-lateral faults are also known as dextral faults, and left-lateral faults as sinistral faults. Figures 3 and 4 show these kinds of faults.
Figure 3. Cartoon showing a right-lateral, or dextral, fault. You are at the spot marked with a circle, looking at the tree (the blobby thing) on the other side of the fault from you. The fault is represented by the fat black line. Since this is a right-lateral fault, the tree on the other side will appear to you to be moving to the right.
Figure 4. Cartoon showing a left-lateral, or sinistral, fault. You are at the spot marked with a circle, looking at the tree (the blobby thing) on the other side of the fault from you. The fault is represented by the fat black line. Since this is a left-lateral fault, the tree on the other side will appear to you to be moving to the left.
The San Andreas fault is a classic example of a right-lateral strike-slip fault. The Garlock fault, which runs along the northern edge of the Mojave Desert, is a left-lateral strike-slip fault. Big normal faults are found all over the Great Basin. Big reverse faults are found near subduction zones, such as in Alaska.
So how does one recognize a fault au naturel, as it were? If you are just walking around outside looking at the nice rocks and mountains and plants and rivers, how would you know you were looking at a fault?
It turns out you can use basically four methods: biology, geomorphology, geology, and geophysics. In this context, geology refers to looking at rocks, and in particular, noting rocks which are right next to each other and didn't form that way. Geomorphology is the study of the shape of the earth (``geo''=``earth'' and ``morphos''=``shape''), and it refers to using clues like creek beds, scarps, and sag ponds to find faults. Biology in this case means plants and where they grow. Geophysics can mean lots of things, primarily earthquakes, magnetics, and gravity.
One common denominator is that faults tend to be straight lines (more or less). Not much else in nature makes straight lines. If you find a straight line that is natural, it's a good bet you're looking at a fault. Nature abhors straight lines and vacuums...
You can use biology to find faults in certain situations, particularly in arid lands. Often, the rocks in a fault zone can become clayey, and act as a barrier to the flow of groundwater. When this happens, groundwater builds up on the uphill side of a fault, and the water table is raised relative to the other side of the fault. You can sometimes find a long, straight line of palm trees and other vegetation growing in an otherwise basically empty area. This is pretty suspicious and may represent a fault zone. You can find such vegetation lineaments in the desert near Indio, for example.
You can use geomorphology to find faults as well. Since faults tend to be straight lines, you look for linear features in the shape of the ground, or features which change abruptly at some straight line. For example, streams which cross active faults can be offset by fault motion over time, and eventually come to have sharp jogs in their courses across the ground. Figure 5 shows a good example of this. Also, linear ponds called ``sag ponds'' form along fault zones, and a chain of these is a good hint of a fault's position. San Andreas Lake (the fault was named for the lake, not vice versa) near San Mateo is a large sag pond dammed to form a reservoir (see Figure 6). Other features which give a clue to the existence of a fault are scarps (see Figure 7), springs, and long, linear valleys.
Figure 5. Wallace Creek on the Carrizo Plain, in central California. Movement along the San Andreas Fault in several earthquakes has offset the two large streams in the center of this image by as much as 130 meters. The San Andreas Fault runs from lower right to upper left through this picture. Image by Fairchild Aerial Photography.
Figure 6. San Andreas Lake, along the San Francisco Peninsula. This is a large sag pond which was dammed to make a reservoir. The fault runs from lower right to upper left in this image. Fairchild Aerial Photography Collection Photo C-6660 31 Dated 23 March 1941.
Figure 7. A classic picture of a scarp formed by the 16 December 1954 Dixie Valley earthquake (magnitude 6.8) in Nevada. The miner's shack next to the scarp was relatively undamaged, despite being only a few meters from the two- to three-meter offset. Image by Karl V. Steinbrugge.
Geology can also be used to find faults in the wild. By mapping the location of various rock units, and by noting their characteristic geologies, you may find very similar rock units which are hundreds of miles apart. And you may find rock units which are right next to each other and clearly could not have formed so near to one another. A good example of nearly identical rock units which are widely separated is the Pinnacles Volcanic Formation, which is separated by 315 km along the San Andreas Fault from its ``mate'', the Neenach Volcanic Formation in the Mojave Desert. The Pinnacles are now SE of Salinas in central California, but were once a single unit with the Neenach Formation; these blocks have been separated by movement along the San Andreas fault.
Geophysics can help locate faults as well. In particular, one can plot the locations of earthquakes in a given region over a long period of time and find the currently active faults (since earthquakes happen along active faults). To find faults which only rarely have earthquakes or are no longer active, one can use gravimetry and geomagnetic measurements to find linear features in the gravitational and magnetic fields of the Earth. Figure 8 shows a nice image of the anomalies in the Earth's magnetic field caused by a series of faults near Portland, Oregon.
Figure 8. Map of the intensity of the geomagnetic field near downtown Portland, Oregon. The colors represent the relative intensity of the magnetic field. The dashed lines were drawn to emphasize faults known from previous geologic mapping. Note that there is a strongly linear feature in the geomagnetic anomalies parallel to both the Portland Hills fault and the East Bank fault. Image by Richard J. Blakely, US Geological Survey.
So, to sum up, you find faults in a number of ways, but mostly, you look for straight lines which you can't explain through human intervention.
OK, so all that said, where are the faults in southern California? Basically, the shortest possible answer is: damn near anywhere you look. It will take much to long to explain all of it, and besides, the Southern California Earthquake Center has done a much better job than I ever could on their Faults of Southern California page. Take a gander over there when you can.
First off, let's make sure we get terms right. An earthquake forecast is not an earthquake prediction. An earthquake forecast is a long-term (years to decades) statement of the probabilities for a large earthquake on a given fault or in a given region. Such forecasts are now made by responsible scientific and governmental organizations, such as the United States Geological Survey. An earthquake prediction is a short-term (hours to days) statement that an earthquake will happen in a given region. Such predictions are made by fools or charlatans or, I must admit, by a few people who sincerely believe that they have found a way to make valid short-term predictions. More on this in Section 4.
The obvious question is, if earthquake forecasts are in some sense ``approved'' and are based on science, how do they work? The answer is that it's complicated, and I'm not an expert in this, so I will try to simplify things and again give you a list of suggested places you could turn to for more information.
In order to understand the discussion below, you need to understand two other concepts: slip rate and the characteristic earthquake model. The slip rate is the rate at which the two blocks of rock on either side of the fault would move if no friction held them together. In some places of California, for example along the San Andreas fault near Hollister, the blocks of rock do move relative to one another at the slip rate -- a phenomenon called aseismic creep. But in most places, the rocks are stuck, but ``want'' to move at the slip rate.
The characteristic earthquake model is pretty simple really. Basically, it says that a given fault, with a given set of geologic conditions (strength of the rock, length of the fault, rate of slip along the fault, etc.) ``wants'' to break in a particular characteristic fashion. It ``wants'' to break repeatedly over time when a similar, characteristic set of conditions is reached; this generates a similar, characteristic-sized earthquake. This concept is fundamental to earthquake forecasts, because it says that, while the timing of earthquakes is not necessarily predictable, the past behavior of the fault can be a useful guide to its future behavior.
You need several ingredients for an earthquake forecast. These include:
Question 1 is pretty obvious. If you don't have faults, you can't have tectonic earthquakes, and where there are faults, there is the potential for earthquakes on them.
Question 2 is not so obvious. Even very large faults such as the San Andreas are not one long continuous crack in the ground. Instead, a fault is usually made up of a series of smaller segments which either meet end-to-end or form a zone over which movement takes place. On the San Andreas, the longest segment is only 18 km long, and an estimate of the average segment length is something like 3-5 km. How a fault is segmented is important because the longer a segment is, the larger an earthquake it is capable of generating, on average. Segmentation is usually decided by detailed geologic mapping.
Questions 3 and 4 go together. It's important to know how often a given segment ruptures in earthquakes, and how long it has been since the last one, because those two things together can give you some idea of how long it will be until the next one. Keep in mind that ``some idea'' here is often measured in decades, with error bars of decades or even centuries! This is definitely not a completely exact part of science; but don't get me wrong -- it often works well. Clearly, a fault which has earthquakes frequently, and which has not had one in a while, is likely to be more of a risk than one which does not have large earthquakes very often and just had one. But keep in mind that none of this is exact; we are discussing probabilities.
Questions 5, 6 and 7 also go together and link also to Question 4. If you know about how much displacement, or relative movement between the two rock bodies on either side of the fault, happens in the characteristic earthquake on a given fault, and you know about how fast the two sides of the fault are moving relative to one another, both over geologic time scales and currently, you can get an estimate of how much time usually separates characteristic quakes. And if you can compare this to how long it has been since the last one, you can develop an idea of about how close the rock is to snapping. For example, if you have a fault slipping 30 mm/year, and during the characteristic quake on a given segment of that fault about 3 meters of relative motion happens, you can divide 3 meters by 30 mm/year and come up with a time of 100 years between repetitions of a characteristic earthquake. This time is called, not too surprisingly, the repeat time.
You can also apply this knowledge in a somewhat different way to estimate how big a particular earthquake might get on a given fault segment. If you know the last earthquake on a given segment was, say, 300 years ago, and you know that the slip rate along that fault is, say 30 mm/year, you can multiply those two numbers together to decide how much stored-up motion there is along that fault, in this case 9000 mm, or 9 meters. You can then make an estimate of how big an earthquake would occur if all 9 meters of stored-up slip were to be released in one earthquake. This stored-up slip is called a slip deficit.
OK, so how are some of these questions answered? Well, as I said, Questions 1 and 2 are usually answered through detailed geologic mapping, to find where the surface manifestations of various faults and fault segments are. Question 6 is also usually answered through mapping and studying geomorphology.
Questions 3, 4, and 5 are now usually answered by digging a network of trenches in three dimensions across a section of a fault. My friend Dr. Tom Rockwell at San Diego State has dug trenches of this sort across the Rose Canyon fault (among many others) to look at its behavior, for example, and he very kindly gave me Figures 9, 10, and 11 as examples of what these trenches are like (Thanks, Tom!). Other people also do this sort of work; the other true expert of the whole field (besides Tom Rockwell), still extremely active in it, is Dr. Kerry Sieh at Caltech. He did the pioneering work of this kind at a place called Pallett Creek along the San Andreas Fault (and has some great stories to tell, believe me!).
Figure 9. Overhead view of a trench across the Rose Canyon fault near the Pacific Beach In-N-Out Burger restaurant. The fault runs left-to-right through the center of the picture. The trench is roughly 2-4 meters deep in places, and about 30 meters long by 15 meters wide. It took five people working 6 days a week, 10 hours a day over 7 weeks to do the trench by hand. Image by Tom Rockwell, San Diego State University.
Figure 10. View of the wall of the trench in Figure 9. The active strand of the Rose Canyon Fault is shown by the small red squares (the heads of marking pins). The dark grey rock layer on the left side of the fault represents the youngest rock material. The layer below the grey material is the same as the layer to the right of the grey material, but has been dropped as much as 50 cm relative to the right side. Image by Tom Rockwell, San Diego State University.
Figure 11. Diagram of the layers in one wall of the trench. Red lines are fault strands, while layers A, B, C1, C2, C3, and D are different rock layers. Notice the layers are broken by the various strands of the fault. The other symbols mark spots at which 14C dating samples have been taken. Image by Tom Rockwell, San Diego State University.
What you do is dig a trench across the fault and work to identify different rock layers. After that, you find where the breaks in the layers are and marks those as faults. You make measurements of the amount of relative movement between the same rock layers on different sides of the fault, which tell you how much motion there was in the earthquakes which broke the rock. If you can dig out samples of wood, bone, or other organic materials that have 14C in them, you can then date these samples, and by keeping track of which layers they were in, you can begin to develop ages for the layers, and thus ages for the earthquakes which broke those layers. With much painstaking work, you can develop a history of the fault motions at that point along the fault. By making many trenches and repeating this kind of work, you can begin to match ruptures in time, and thus map out the extent of motion in old earthquakes. This is extremely important information for figuring out the characteristic earthquakes on your fault segment.
Question 7 is answered by making extremely precise measurements of the positions of various points on the earth's surface, called benchmarks. If you go out and repeatedly measure the positions of the same set of benchmarks over time, and your measurements are accurate enough, you will observe changes in the relative positions of these marks. If you manipulate these positions and the changes in them, you can work out how the shape of the ground in your study area is changing over time. From that, you can try to make models of the faults in the region and see how they are slipping.
There are a number of techniques which people use to make these sorts of measurements. They are lumped together under the term geodesy. Among the techniques used are
OK, so all that said, what about southern California? Well, the overall earthquake forecast for southern California is that there is about an 85% chance of at least one magnitude 7 earthquake in southern California between 1994 and 2024, not including the Northridge earthquake. That's a very high probability of having a destructive earthquake. It is not, however, the same over all areas of southern California. Areas near major faults, such as the San Andreas, San Jacinto, Newport-Inglewood, etc. have a higher chance of a large earthquake than other areas. Also, please note that the statement above does not mean that you can relax, because we won't have an earthquake until 2024. There is an 85% chance of at least one between now and then -- one might not happen for ten years, but one could also happen in the next minute. Also, please remember that damage from an earthquake depends strongly on local soil and rock conditions, and on the condition of your building. And finally, you must keep in mind that this forecast was based on the best information available when it was made. Things do change in this field, and new forecasts will certainly be made -- keep yourself up to date. Figure 12 shows the latest forecast for southern California in a nice map.
Figure 12. Latest forcast for strong ground shaking in southern California. The color shows the number of times per century that a given area might encounter ground shaking strong enough to cause major damage. Note that this map assumes that all the ground is solid rock --- there will be big differences if you take into account the variations in rock type. Also note that the most recent data on the Rose Canyon fault (among others) is not included in this map. Keep in mind that things will change! Map by Southern California Earthquake Center.
There are numerous web sites devoted to earthquake forecasting. Here are a couple good ones:
Please keep the most important part of this in mind: earthquake forecasts are not earthquake predictions. Earthquake forecasts are an emerging, tested, scientifically sound way of making long-term statements about the seismic hazard in a given region. Earthquake predictions, on the other hand, are usually attempts by fools or charlatans to make short-term predictions. Only one prediction has ever been proven to have been valid, and it was over 20 years ago. Section 4 discusses earthquake prediction in more detail.
Several methods have been tested in the effort to learn how to predict earthquakes. Among the more serious methods which have been examined are seismicity changes, changes in seismic wave speed, electrical changes, and groundwater changes.
``Seismicity changes'' is really a fancy way of saying ``foreshocks''. A foreshock is an earthquake which precedes a larger earthquake and is ``near'' the epicenter of the larger earthquake. The problem for earthquake prediction is that not all earthquakes have foreshocks, and we don't yet understand what to look at to figure out if a given small earthquake will become a foreshock. This is an active area of research.
In the 1970s in the Soviet Union, several seismologists noted changes in the speeds of seismic waves in regions where later there were earthquakes. They noted that first the speeds dropped by a small amount up to a year ahead, and then a few weeks to months prior to the earthquake, the speeds began to increase again. Unfortunately, while this topic caused a lot of excitement in seismology 20 years ago, it has not lived up to the hype.
Changes in the Earth's electrical conductivity and electric and magnetic fields have sometimes been noted in areas near the future epicenter of a large earthquake. These changes could be brought on by changes in deep fluids along a fault plane or by stresses in the rocks along a fault. For example, just prior to the 1989 Loma Prieta earthquake, a group of scientists from Stanford University detected a massive increase in the noise level on one of their electric field recorders located in the Santa Cruz Mountains only a few kilometers from the San Andreas Fault. This got lots of people excited, and several experiments are in progress, but for now, the predictive value of these changes has not yet been demonstrated.
Changes in groundwater are also sometimes noted before earthquakes. These changes can include flow rates (sometimes wells dry up or dry wells become flowing again), taste or smell changes, and changes in chemistry, particularly in the concentration of the radioactive gas radon. All these have been noted in some earthquakes, but again, they are not universal features.
Are you beginning to see a pattern here? The real problem with earthquake prediction is that nobody has yet found a single factor or set of factors that always behaves in the same fashion before every earthquake of a given size. Until such a set of factors is found, we won't be able to make successful short-term earthquake predictions. In fact, some seismologists are now coming to believe (after 20-30 years and millions of dollars) that earthquakes may be fundamentally unpredictable -- that earthquake prediction is, in fact, completely impossible.
OK, so by now you are probably wondering if there has ever been a successful earthquake prediction anywhere in the world. The answer, surprisingly enough, is yes -- one. In February, 1975, Chinese seismologists predicted that a large earthquake would hit the Haicheng area within 72 hours. They made this prediction based on a long-term observation of increased seismicity in the region, coupled with short-term changes in groundwater, and widespread observations by farmers, fishermen, and the like that animals were behaving very strangely (the key thing here is that it was widespread). Despite the fact that it was February in northern China and bloody cold, the populace was evacuated to the city squares in the area. 65 hours later, a magnitude 7.3 earthquake hit the area and destroyed thousands of buildings. Due to the massive evacuation, fewer than 30 people lost their lives -- the toll could have been in the hundreds of thousands (including surrounding towns) otherwise.
The problem is that the very next year, in July 1976, the very same Chinese seismologists neglected to predict the devastating Tangshan earthquake. The city of Tangshan was nearly completely leveled. The official death toll stands at about 225,000. However, American seismologists who visited the area shortly after the earthquake have gone on record stating that more than 650,000 people were actually killed -- because the earthquake had not been predicted.
So, yes, there has been one successful prediction. No, there have been no more since then.
Just out of fairness, I should say that the American seismological community has had no better luck with earthquake prediction. Some of you may have heard of the Parkfield prediction. Parkfield is a tiny town (population 33, I think) in central California which seems to have magnitude 6 earthquakes at regular intervals, about every 22 years. Since the last one was in 1966, a prediction was made in 1984 that there was a 90% chance of another magnitude 6 earthquake in Parkfield by 1993.
It hasn't happened. And many millions of dollars worth of geophysical equipment -- everything from seismometers to tiltmeters to strainmeters to radon detectors to laser distance measuring systems to electric and magnetic field records -- sits in the hills there waiting for the next quake. When it comes, we should get loads of potentially very valuable data on what happens near the epicenter of a large earthquake just before that earthquake happens. But it must be said that the prediction itself has failed -- the earthquake did not come in the specified window of time.
You may detect a note of frustration on my part with earthquake prediction. While I believe that it would be useful to be able to make short-term earthquake predictions, I personally doubt we will ever get there. My frustration comes from having dealt with many amateur predictors who simply don't understand that they need to prove they can predict earthquakes, rather than just say they can. OK, you ask, but how would you go about proving that you can predict earthquakes?
That's a good question. Here are the five crucial features that any good earthquake prediction must have:
OK, so you've listened patiently to me spouting off about one of my personal crusades, and now you're wondering -- what about San Diego?!
The short answer is that, while San Diego is relative safe (seismically, anyway) compared to San Francisco or Los Angeles, it is still in California. And keep in mind that being relatively safe in California is still pretty risky, seismically at least. So while we are safer than much of the rest of the state, we are still at risk for large earthquakes.
Some of the faults nearby which might have earthquakes which cause damage in San Diego County are:
And I have deliberately left out the biggest single threat to San Diego itself: the Rose Canyon Fault Zone (RCFZ). The RCFZ is active and runs right through downtown, under Lindbergh Field, through Old Town, across the east side of Mission Bay and Pacific Beach, up along Interstate 5, over Ardath Road, and out into La Jolla Bay near the Scripps Institution of Oceanography. The latest information on the fault is that it can generate earthquakes as big as magnitude 7 -- more than big enough to cause severe damage to San Diego proper.
Here's what we now know about the RCFZ, thanks mostly to trenching work done by Dr. Tom Rockwell and his students at San Diego State University (see Figures 9, 10, and 11):
Figure 13 is a nice map I borrowed from the Southern California Earthquake Center. It shows the faults in and near San Diego, and should emphasize further in your mind that San Diego is definitely earthquake country. You can see a nice version of this map, complete with clickable faults which (when you click one) bring up a bit of information on the fault you chose.
Figure 13. Map of San Diego County and surround area, with major known faults shown. Also shown for reference are the major freeways and elevation shading. Map by Southern California Earthquake Center.
San Diego County has had damaging earthquakes in the past, though not as many as some other areas of the state. The biggest earthquakes that we are sure were nearby were in 1862 and 1892, with another big one possible in 1800. The 23 February 1892 earthquake was probably on the Laguna Salada fault, about 125 kilometers east of San Diego, but still caused much damage in San Diego (big cracks in the walls of some buildings, etc.). San Diego also suffered some damage from the 28 June 1992 Landers earthquake, mostly in the form of broken dishes and such and mostly along the beach, where the loose ground amplified the shaking.
There have also been smaller, more recent earthquakes which caused some concern and damage. In 1986, there was a sequence of magnitude 3 and 4 earthquakes just off the coast of Coronado which stirred up a lot of interest and caused a bit of damage. And later in 1986, there was a magnitude 5 earthquake just off Oceanside which caused some damage and was widely felt.
The point of all this is that San Diego has an active earthquake history and is certainly going to have more earthquakes in the future. Obviously, the biggest single risk is the RCFZ, but there are others as well. Further research, including trenching by Tom Rockwell and others and GPS studies by my friend Hadley Johnson and others, will help place more and useful constraints on the dangers facing San Diego.
You might be interested in knowing where you can go to see faults in San Diego. It turns out that there are a couple of nice spots to go to see the Rose Canyon Fault. One of the best places to go is Tecolote Canyon Natural Park, east of Mission Bay. The visitor's center there can direct you to the best exposure of the Rose Canyon fault in the park (it turns out to be just behind the ball fields!). The park has a web page.
Another good spot is the Bayside Trail out on Point Loma. Go to the Cabrillo National Monument Visitor's Center on a nice clear Sunday afternoon. It will cost $4 for a daily permit for one car. The Monument is open from 9 am to 5:15 pm. Bring a picnic lunch -- there are some nice views of San Diego Bay from the trail. You can get a map of the Bayside Trail from the Visitor's Center, and set off on your hike. About a half-mile or so into the hike, you will go around a point and turn left into a canyon. On the far side canyon wall, you should be able to make out a clear fault trace, with offset rock layers. There's also a small cave (really nothing more than an alcove in the wall...) you can poke your head into. Anyway, the trail's nice and a good place for a light hike. I think the total hike is about two miles.
The Cabrillo Monument has a web page.
Lecture 21 - Southern California Hodgepodge
This document was generated using the LaTeX2HTML translator Version 96.1 (Feb 5, 1996) Copyright © 1993, 1994, 1995, 1996, Nikos Drakos, Computer Based Learning Unit, University of Leeds.
The command line arguments were:
The translation was initiated by Greg Anderson on Wed Mar 19 00:10:13 PST 1997