The Bay Area newspapers have been oddly silent lately about the Hayward Fault, and the high probability of a major earthquake very soon. Tomorrow is the 140 year anniversary of the HUGE earthquake on that fault, named after the then tiny town of Hayward, which suffered the most. At the time that earthquake was dubbed “the Great San Francisco Earthquake“ because of the devastating effect it had on the City of San Francisco, but of course that was eclipsed 38 years later by the real “Great San Francisco Earthquake”.

The significance of 140 years is that paleo-seismological studies (those combining earthquake science, radiocarbon dating, and archeology) show that the AVERAGE INTERVAL ON THE HAYWARD FAULT BETWEEN THE PAST FIVE EARTHQUAKES IS 140 YEARS!!!

No, my caps lock didn’t get stuck. I just had to make that statement stand out in an attempt to get attention. I would have thought that this fact alone would have been enough to warrant a several day series in the San Francisco Chronicle on the increasing probability of a major earthquake on the Hayward Fault, what its effect will be, and what we can do about it.

But unless tomorrow morning‘s paper comes out with a major article headlined on the front page, it would seem that the ink devoted to the danger posed by this event has fallen way behind that allocated to effect of the economic earthquake that has hit our country in the last few weeks.

For Berkeley and points north it gets worse than that. The Hayward Fault earthquake of October 21, 1868, at 7:55 in the morning, caused a massive fracture of about six feet horizontally between the Pacific and the North American Plates on the portion of the fault south of Oakland, through San Leandro, Hayward, and Fremont. That means the tension between the two plates was relieved.

However, the northern part of the fault through north Oakland, Berkeley, and the cities to the north did not move. It is still stuck to this day, and the two plates have been stuck together for at least 250 years.

In the meantime, the two plates have been moving, deep down, past each other by at least 5 to 8 millimeters a year (minimum) throughout the whole length from Fremont to San Pablo Bay, and even further north through Napa County along the Rodgers Creek Fault. If you do the math, based on 250 years of lack of movement in the northern half of the fault, that means that about 4 to 7 or more feet of slippage has to happen soon, with an attendant massive release of energy, probably equivalent to about a 7 on the Richter Scale. And that’s a very conservative estimate.

The southern part of the fault from about the middle of Berkeley, through Oakland, and south to Fremont is characterized by continual slow slippage, as is evident by the cracking and bending of buildings like Memorial Stadium, and various points of evidence in the pavement, curbs, and walkways that have been well documented. However, the evidence of slippage through North Berkeley and points north to San Pablo Bay is much more sparse.

This could mean that, in addition to the 1868 Hayward Fault Earthquake, the tension between the plates in the southern portion has been relieved by aseismic creep (continual slippage not related to an earthquake), whereas in the northern portion it has not.

Overall, the United States Geological Survey estimates the probability of a 7.0 Richter Scale magnitude earthquake on the Hayward Fault at about 30% within the next 30 years. Forgive me if I seem a bit dense, but if the last earthquake occurred 140 years ago, and the average of the last five was 140 years, and the fault slippage was only on the southern part of the fault, and there has been no slippage on the northern part of the fault for at least 250 years, and in the interim the Pacific Plate has been sliding north-west against the North American plate at the same rate of 5-8 millimeters like it always has, THEN THE PROBABILITY OF A 7.0 RICHTER SCALE EARTHQUAKE TOMORROW MORNING SOMEWHERE ON THE HAYWARD FAULT APPROACHES 100%.

Sorry for the length of that sentence, and for my caps lock getting stuck again. Or am I missing something here?

When you fly over the Hayward Fault using the Google Earth Helicopter Tour, you will be struck by how busy and detailed the data points are in some areas. Conversely, in other areas there is a dearth of information, so much so that it raises the question: is it because there is little evidence to be found in those empty areas, or is it because more study has been given to certain areas where the evidence is more easily seen?

The sources and methodology for the original map by James J. Lienkaemper from which the Google Earth Helicopter Tour was derived are described in the documentation for the Digital Database of Recently Active Traces of the Hayward Fault. The fault location was determined and further refined by i) geomorphic expression, or the study of the shape of the earth’s surface, ii) fault creep, or aseismic fault slip, and iii) the study of trenches dug near or across the fault location.

Around the University, and south into Oakland there are so many places where fault creep is evident in the roads, curbs, and sidewalks, that if you don’t watch where you are walking you will trip over them. If you want to see the evidence on a big scale, just sit in Memorial Stadium and look around you.

However, north of the University through North Berkeley and into Kensington and El Cerrito there is practically nothing. There used to be several places where the curbs were bent by fault creep around Marin Avenue and Spruce Street, but this whole area has been renovated recently with new curbs and concrete sidewalks, and the only pieces of evidence left are the symbols on the map that show where the creep evidence used to be.

There were only two trenches ever dug in Berkeley north of the University, and both were at the now-closed Hillside Elementary school. The evidence found was sufficiently alarming that the school was closed in very short order. The next trenches that were dug are about three miles to the north in the Mira Vista Golf Course in El Cerrito.

Interestingly enough, the archeological evidence discovered in these trenches shows that the northern half of the fault did not rupture in 1868 like the southern half did through Oakland, San Leandro, and Hayward. The best guess is that this part of the fault last ruptured some time between 250 and 400 years ago.

When you read the Lienkaemper documentation you will learn that the primary source for the study of geomorphic expression, or put simply, the rumpling of the earth caused by past surface rupture earthquakes, was a set of aerial photographs taken in 1939. At that time, the vegetation was less profuse than it is today, making it easier to see the evidence, but already at that time most of the evidence had been disturbed by the building of roads, schools, houses, parks, and gardens.

So we find ourselves with the situation that, through North Berkeley and beyond, not only is there very little evidence of fault creep, but few trenches have ever been dug to study the paleoseismological evidence, and the geomorphological evidence is pretty shaky due to extensive urbanization. Couple that with the fact that this part of the fault did not rupture in 1868, one can only conclude that this part of the fault is a disaster waiting to happen, any day.

Why? Because, as we said earlier, the Pacific Plate has been slowly moving in a north-west direction, deep underground, several millimeters per year, as is clearly evident at Memorial Stadium. That tension was last relieved in the southern part of the Hayward Fault by what was then called the “Great San Francisco Earthquake” of 1868, only to be supplanted by the real “Great San Francisco Earthquake” of 1906, on the San Andreas Fault.

However, on the northern part of the Hayward Fault through North Berkeley and beyond, the tension is unrelieved, either by a recent (within the last 250 years) earthquake, or by ongoing fault creep.

With a continuously moving strike-slip fault like the Hayward Fault, the longer we are away from the last earthquake, the more likely the next earthquake becomes as the years, the months, and the days go by.

I have spent the past few months studying this part of the Hayward Fault, walking its length and breadth, and documenting what I see. Recently I led a Hayward Fault Walk, which was attended by an enthusiastic crowd of about 60 people, most of whom were sufficiently interested to hang in for the whole 2 1/2 hours until the end. We only had enough time to scratch the surface of the evidence I have found.

The next installment in this journal will document where probable evidence of the fault can be seen in the various paths, steps, and other man-made artifacts in North Berkeley which criss-cross the fault location at just the right angle (90 degrees).

Finally, the Memorial Stadium Oak Grove has been cut down, the tree sitters are gone, and work is moving ahead on the strengthening of Memorial Stadium and the building of the new Sports Complex alongside. No longer do we have to read about the nonsense going on between protesters and politicians at the site.

But beware, there is other nonsense going on. Even more alarming nonsense. It would appear that either the University Engineering planners, or the San Francisco Chronicle, or both, don’t seem to know which way the Pacific Plate is moving in relation to the North American Plate.

To reiterate here, the Pacific Plate is moving in a North-West direction, and the North American Plate is moving, relatively speaking, in a South-East direction. All of the evidence points to this, and anybody who says otherwise is nuts.

In an article recently, the San Francisco Chronicle described how the west part of Memorial Stadium is sitting on the Pacific Plate, and moving south, and the east part is sitting on the North American Plate, and moving north. Really! I’m not kidding! Read all about it in this article.

As soon as I saw that I sent in a recommendation to the Chronicle for a correction. Thus far they have declined to print one, leading me to believe that the source documents from the University used by the Chronicle as a basis for their article are also wrong.

The article describes in great detail how the stadium will be retrofitted by slicing it into blocks and putting it on plastic sheets so that it will slide nicely and gently in the next earthquake. Honest! That’s what they are planning. But they don’t even understand in which direction way the ground will move in the coming earthquake!

Here’s the paragraph from the Chronicle, nonsense in italics, lifted verbatim from the article:

“The eastern half is built into the hillside and does not need to be retrofitted. But the western half, with its Beaux Arts flourishes and spectacular views of the hills and bay, rests precariously on landfill over a creekbed. Its concrete walls are cracked and strained, as the Pacific Plate - which is under Sections M through XX – inches south and the North American Plate - under Sections MM through X – creeps north.”

Scary, truly scary.

There’s a very good recent article about Memorial Stadium and its retrofit in Andrew Alden’s Oakland Geology journal. In the comments below that article Andrew mentions the San Francisco Chronicle article, but if he spotted that error, he doesn’t mention it.

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.

If you click on the picture of the plaque above and read it, you will see that the women of the Hillside Club had all the best intentions when they succeeded in implementing a design for development of Daley’s Scenic Park Tract that took into account the rolling, twisting nature of the topography of the Berkeley Hills. Little did they know, and nor did the City of Berkeley and the architects Maybeck, Coxhead, and Bolton, that the area would become one of the most dangerous places in North America in which to live.

The very topography, and the beauty of the landscape, all the way along the lower part of the Berkeley Hills, was formed by thousands of years of ripping and wrenching of the land by the inexorable movement of the chunk of land known as the Pacific Plate, sliding in a north-west direction past the North American plate. Every 100 to 200 years or so, a massive earthquake of magnitude of about 7 or 8 on the Richter Scale would rip open the hillside with a ten to twenty foot horizontal jump, all in the space of a minute or two, sending shock waves hundreds of miles across the State of California.

Of course, it didn’t matter when nobody lived there. Even when the only inhabitants were Native Americans, they were so spread out, and their homes so flimsy, that the chances of getting hurt in an earthquake were slim. The last big earthquake on the Hayward Fault, with a magnitude of about 7 on the Richter Scale, occurred on October 21, 1868, and was but a distant memory when the Hillside Club ladies planned their paradise of winding streets, retaining walls, and stairways. That earthquake left no trace in Berkeley, because only the southern portion of the fault ripped open from San Leandro south through Hayward and Fremont towards San Jose.

Even the Great San Francisco Earthquake of 1906, which happened when they were in the middle of their design, apparently did not affect their planning, probably because all the evidence in that event pointed towards a different fault ripping open near Olema in Marin County. They probably thought they were building in an area of stability, far away from the constant dangers of the San Andreas fault.

The fact is, they just didn’t know. There was no way they could know. The tools they needed to determine that they were building right on top of a fault even more dangerous than the San Andreas, just hadn’t been invented yet, and it would be another fifty years before science advanced enough for us to realize that the area is nothing more than a ticking time bomb. It was the same up and down the Hayward Fault. The area was urbanized, and houses, schools, roads, offices, pipelines, and a great university, were built right on top of the fault before anybody knew how vulnerable it is.

Two facts have conspired to make Daley’s Scenic Park Tract a very, very dangerous place in which to live. Both are related to the passage of time.

Firstly, the northern section of the Hayward Fault from Berkeley north to San Pablo Bay has not broken for at least 240 years, and all of the scientific evidence points towards the fact that a massive rupture in this area is long overdue.

Secondly, since the 1923 Berkeley Hills Fire destroyed many of the homes in the area, the vegetation surrounding the wood shingle homes has grown so thick,

and so far out of control in the intervening 85 years, that a massive firestorm of the magnitude of the 1991 Oakland Hills Fire is almost inevitable. And a combination event is highly likely. When the earthquake occurs, a firestorm will certainly result.

When the ladies of the Hillside Club started their planning for the area in 1903, the state of the science of geology was quite primitive. Even though a good foundation had been laid by Charles Lyell in the early 1800’s, which described how most geologic processes were very gradual and occured at much the same rate through time, nothing in the science yet addressed the very sudden alteration of the landscape by means of earthquakes and volcanoes. Geological exploration was done with hand tools, measuring instruments were primitive, and the start-of-the-art technology used for getting around was the horse.

Consider the following:

• The science of seismology began to be developed in the 1880’s, and began to provide useful information in the wake of the 1906 San Francisco earthquake, too late to be of much help before the plans for development on the Hayward Fault through Berkeley were well underway.
• The seismograph, or seismometer, was not invented until 1880, and began to be used extensively to monitor earthquakes in the early 1900’s.
• Continental drift was first postulated by Alfred Wegener in 1912, but nobody believed him. It was not until the 1960’s that Tuzo Wilson developed the first serious scientific proof of what came to be known as plate tectonics , which was able to be applied to explain earthquakes like those which occur on the San Andreas and Hayward faults.
• Although Geochronology, the science of dating rocks and archaeological evidence began to be developed by Arthur Holmes around 1911, radiocarbon dating was not developed until the late 1940’s, and radiometric dating was not developed until the 1950’s.
• The Richter Scale, a standard for measuring the magnitude of earthquakes, was not invented until 1935.
• Aerial photography and photogrammetry, which allows the accurate mapping of both planimetric and contour features from photographs taken with an aerial camera, was not invented until World War II, and began to be used commercially during the 1950’s.
• Satellite imagery, while used by the military beginning around the 1960’s, was not opened up and made available to the public at large until the 1980’s.
• The science of paleoseismology, the study of historical earthquake evidence, did not emerge as a discipline until the 1990’s.

It’s no wonder that, with the absence of physical evidence of the Hayward Fault on the ground, the lack of knowledge about what really causes earthquakes, and the absence of tools and scientific methods to accurately locate faults, the entire Hayward Fault was built upon, mostly during the first half of the last century, long before it became common knowledge that the makings of a disaster was in the works.

For example, here is the former Hillside Elementary School at 1581 Le Roy Avenue,

designed in 1925 by Walter H. Ratcliff, Jr. Would anybody in their right mind build a school for children right on top of an earthquake fault? The fact is, nobody knew.

Take a look at this satellite view of the Hillside Elementary School from the Google Earth Hayward Fault Helicopter Tour. The red line is the Hayward Fault, running right underneath the classrooms at the rear of the school. No wonder the School Board closed and abandoned it in 1983!

It has taken more than fifty years of scientific development and study, using the disciplines of plate tectonics, remote sensing, paleoseismology, geochronology, and a host of other disciplines related to seismology to determine this, culminating in the publication of these images by the USGS over the past couple of years.

The ladies of the Hillside Club, the architects, and all involved, would have been shocked, absolutely shocked, if they had been able to find out what we know about the Hayward Fault today.

My last post, in which I mentioned the subject of the Pacific Plate twisting slowly to the north-west, while the North American Plate slides in the other direction, generated the question ”What makes the plates move anyway?” Not only is Berkeley slowly moving along as part of the edge of the Pacific Plate, but two areas of Berkeley are moving at different speeds. And it has nothing to do with politics.

Would you believe it if I told you that the part of Berkeley at the top of these beautiful La Loma Steps is moving in a different direction relative to the part where the cameraman is standing? Right now, you can see that there is absolutely no evidence of that fact. But there will be very soon, and when that happens, it will be frightening.

All of the tectonic plates which make up the surface of the earth, including the ocean floors, are moving around relative to each other like slabs of congealed scum on a slowly simmering pot of soup. Berkeley is merely a couple of insignificant pieces of congealed scum sliding around right on the edge of the Pacific Plate.

Calling Berkeley a couple of insignificant pieces of congealed scum sounds like it came right out of the mouth of extreme right wing talking head Bill O’Reilly from the Fox Television Network News, but in fact that’s a good analogy. For many years, scientists have used the picture of a pot of simmering soup to illustrate the movement of the earth’s tectonic plates.

In some cases, two pieces of scum slowly crash into one another and pile the earth’s crust high up into the air, like the Himalayas, in what is known as a convergent boundary; in some cases they split apart leaving a void, as is the case underneath the center of the Atlantic Ocean, giving rise to islands like Iceland and the Azores, in what is known as a divergent boundary; and in some cases two pieces slide slowly past one another in what is called a transform boundary, resulting in a fault system like what we have here in the Bay Area.

Atomic and residual energy from the center of the earth is the source for the heat for the simmering soup with its convection currents that cause the earth’s crustal plates to move relative to one another, and the Berkeley flatlands to slide north-west a little bit faster than the Berkeley Hills. See, it’s not politics at all!

Where the Pacific Plate and the North American plate meet is at the complex series of faults known as the San Andreas Fault, the Hayward/Rodgers Creek Fault, the Calaveras Fault, and other minor faults, some yet to be discovered. It’s a mess, it really is. It wouldn’t matter if it were just a bowl of soup covered in cracking pieces of scum, but it’s the place where we live!

The story gets worse. If Berkeley were, in fact, moving slowly, we might not get devastating earthquakes every hundred or so years, but unfortunately it’s all jammed up. The two pieces of real estate on the edge of the Pacific Plate are stuck fast against each other at a line a little way up the Berkeley Hills. The piece of crust between the Hayward Fault in Berkeley and the San Andreas Fault is struggling to move north-west, but cannot right now because it is jammed, and the piece of crust between the Hayward Fault in Berkeley and the Calaveras Fault in Concord is struggling in the same way, but at a slower speed, because it is stuck where the Calaveras Fault meets the North American Plate.

Let’s take a look again at the Bay Area fault diagram (courtesy of USGS).

Now think once more about the scum on the soup analogy, and you may understand why two pieces of Berkeley are moving, or would be, if they weren’t stuck together while the simmering soup underneath inexorably increases the pressure. One day soon, it’s got to break.  

A few people have given me some feedback and comments on my recent posts, which I have paraphrased and then addressed, as follows:

You talk about the possibility of the Hayward Fault through Berkeley “ripping open like a burst zipper”. Is that where the epicenter will be?

The short answer is, not necessarily. In a big earthquake there are three potential problem areas where the most damage can occur, and they may not all be in the same place.

1. At or around the epicenter, which is the point on the earth’s surface above the hypocenter where the earthquake originates.
2. Where the fault actually rips open (the zipper effect).
3. Wherever there is extreme vulnerability due to ground conditions and/or poor construction.

So, where do you want to be when the earthquake happens? Preferably as far away as possible from all of the above.

In the 1989 Loma Prieta Earthquake, the epicenter occurred in the Santa Cruz Mountains, far from inhabited areas. That’s near where the fault ripped open too. But the greatest damage occurred in San Francisco’s Marina District, 60 miles away, due to the liquefaction of the soil and construction deficiencies, and on Oakland’s Cypress Structure, 65 miles away, due to inadequate freeway construction.

In the Great 1906 San Francisco Earthquake, the epicenter occurred two miles offshore near Mussell Rock. Where the fault ripped open though was near Olema, forty miles to the north, “where the fault zone is half a mile wide, and filled with soft soils, crumbling limestones, rotting granites, and generally pulverized rocks” (Simon Winchester). But as we well know, the greatest damage occurred in San Francisco itself, due mainly to the ensuing fires and lack of water

In your Scary Picture chapter you show that the fault line passes right under the Tots’ Playground in Codornices Park. How do you know it will be exactly at that spot? Couldn’t it be a hundred feet to the east or west?

Yes, absolutely it could. But really, what difference does it make? The Hayward Fault is actually a “complex zone of deformation which can span hundreds of feet in width”, the leftover evidence of many earthquakes over thousands of years. Various university teams, together with the USGS have dug trenches at various places across the fault in order to pinpoint its location.

If the experts say the red line is in that spot, I am inclined to believe them. However, I don’t believe that even they know if the red line is exactly where the zipper will necessarily rip.

Another issue may be with the differing relative accuracies of the source material for the fault line before it was superimposed on the Google Earth imagery. I have seen many places where, it the line were moved a bit, it would fit the topography better.

For example, look at the location of the fault line here at the back of the Claremont Hotel. The location of that trench (in blue) is too close to the back of the hotel. In fact, it’s almost under it.

If you take a look at this photo of the same scene as taken from the stick pin vantage point. the fault line is likely a bit to the left, closer to the bank. 

That’s little consolation for the guests though if the fault rips open there. The hotel’s going to fall down anyway, and then catch fire.

We already had the Loma Prieta Earthquake in 1989. Didn’t that relieve the tension, making another big earthquake unlikely for a very long while?

Not on the Hayward Fault it didn’t! The branch of the San Andreas Fault in the Santa Cruz mountains is about eighty miles from the Hayward Fault in Berkeley. In fact, there is recently discovered evidence that a slippage on one fault may actually INCREASE the strain on another one parallel to it.

For example, scientists postulate that the 5.6 earthquake on the Calaveras Fault last October near San Jose may have increased the strain on the Hayward Fault farther north.

Take a look at this map and try to imagine the Pacific Plate twisting slowly around anti-clockwise in a north-west direction. Think what happens each time one of those faults breaks. You can easily see that the pressure on the next fault over could just as easily increase as decrease. Map courtesy of USGS.