Posted by: walkingthefault | June 25, 2008

How Wide Is the Hayward Fault Anyway?

It has come to my attention that I may have not given sufficient emphasis to the fact that the location of the red line on the Google Earth Helicopter Tour, on which I base quite a bit of my location information on this site, is not a hard and fast location, and could be off by a couple of hundred feet in places. In fact, when the northern part of the Hayward Fault next breaks open in a surface rupture, it might be even farther away than that.

By my comments about the location of the Hayward Fault scattered throughout these pages, I was sure I had conveyed this fact, but some readers may not have understood this important point. But before I go into an explanation of why it is not an exact location, I want to stress that IT PROBABLY DOES NOT MATTER ONE BIT!

The tension on the northern part of the Hayward Fault must have by now built up to the point that when the earthquake occurs, which will probably be in the range of 7.2 to 7.5 on the Richter Scale, it will not matter much whether your house is on the fault, 100 feet from it, 500 feet from it, or even ten miles from it. The only thing that really matters is whether you have done the needed structural improvements to your dwelling, and whether you and your family are prepared for the aftermath.

Maps of the location of the Hayward Fault, and the Google Earth Helicopter Flyover, show the Hayward Fault as a precise line on the surface of the earth. However, that should actually be interpreted as a fault zone of about 100 to 200 feet in width. The legend on the left side of the Google Earth Helicopter Flyover makes this very clear, and you can turn on or off the various accuracy levels to see the red line appear or disappear depending on its relative accuracy.

There are several reasons for this. One is that the visible evidence of the surface ruptures which have occurred over the past few thousand years, where those ruptures can clearly be seen, is actually 50 to 100 feet wide or even more, and the USGS scientists and photo-interpreters were guessing in places.

Because of urbanization, that is the building of dwellings, offices and roads, and extensive landscaping, most of the evidence of past surface ruptures on the fault in Berkeley has been wiped out. Surprisingly, even in this age of high-resolution satellite photography, one of the best sources of geomorphological evidence for the fault is a set of aerial photographs taken in 1939, when there had been less building, and the vegetation was less dense.

Another reason is because of inherent inaccuracies in the maps which were used to compile the fault location. Depending on the scale of the map, any point could be up to 100 feet off from its true location, just due to the inaccuracies inherent in map compilation.

So if you take all of the sources of error into account, you are looking at 100 to 200 feet of possible error. So, for example, if we are walking around the fault location, and I tell you that right now you are standing on the fault, and this is where the surface rupture will occur in the next earthquake, don’t be disappointed if it actually happens under those trees over there.

Some geologists say that past ruptures of an earthquake fault are not necessarily a predictor of where future ruptures would occur. I do not believe that. If, as in the case of the Hayward Fault through Berkeley, all of the evidence is concentrated in a fairly narrow band about one to two hundred feet wide, I think that is precisely where the next earthquake will break through. The earth’s surface is already weakened at that point from thousands of years of ruptures.

The actual location of the fault was kept in the dark for many years, because of pressure from special interests like the real estate companies. When I moved here in 1986 and bought a house in El Sobrante, I remember looking at some maps in a back room on the wall of the local realtor’s office where the location fault had been marked on it with a felt pen, and I was quickly told to leave the room.

The State Law known as the Alquist Priolo Act of 1972 mandated that the California Geological Survey map all of the earthquake faults in California, and that property owners and real estate agents disclose whether the buildings are within the fault zone. New construction within these zones was prohibited, unless a geologic investigation shows that the fault is not a hazard at that location.

However, it took another twenty years before all of the faults in California were fully mapped, and it wasn’t until the late 1980’s and 1990’s that purchasers of property could obtain full knowledge as to whether the property was on or near an active fault or not. That full knowledge is now available to everybody on the Internet via Web sites like the USGS and the Google Earth Helicopter Tour, and is not restricted as to whether or not you are interested in buying a particular dwelling.

The fact that your home may be located on or alongside the fault is no secret any more, and whether you or anybody can determine that fact from Google Earth, the USGS, or this Web site, is immaterial.

It is interesting that right now, arguments costing hundreds of thousands of dollars in fees for lawyers, seismologists, and structural engineers, over whether the proposed Sports Center alongside Memorial Stadium is precisely on the fault or not, and whether it is in fact part of the stadium or not, and how much the stadium is worth, are being made by the very same professionals who say that the fault location is approximate, and “subject to interpretation”.

There is a wonderful and informative Web site I have found out about called Oakland Geology, that has some very good and interesting content about the Hayward Fault in Oakland. Its author, Andrew Alden, a science writer, photographer, editor and blogger with a lifelong passion for rocks, minerals, fossils and the planets they come from, has raised some very good and valid points about this Walking the Fault journal. I encourage you to visit that site, read the points that its author has made, and post your comments here.



  1. Thanks for your kind words about my blog, and I look forward to your next installments of the Berkeley fault walk.

    The Alquist-Priolo law is a funny law, as many laws are, but what makes it funny to me is that it’s aimed only at the risk of fault movement (coseismic offset) pulling apart a structure. As you say, living in earthquake country is just as dangerous 10 miles away from the fault, but there the danger is from the shaking (and fire, of course). It’s as if earthquakes, in the eyes of the law, didn’t even shake but were merely slow, gentle motions of the Earth’s crust akin to foundation settling that buyers ought to be warned about. So the Alquist-Priolo warning zone is something like 50 feet on either side of the officially mapped fault trace, but in terms of practical earthquake safety it’s almost meaningless.

    Another thing that’s funny is that Alquist-Priolo actually makes sense in the few rare faults–the most important of which just happens to be the Hayward–where creep is important. Fault creep really can gently break your water lines, given a few decades, without any shaking. But even though Alquist-Priolo is a funny law that unexpectedly makes sense in Berkeley, a major university full of smart people can carry on in the notorious folly of its fault-straddling football stadium for eighty years with straight faces. You should see the innards of that structure some time.

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