Under forests and streams of California are mountains, and under the mountains are rocks. In the landscapes where I have lived most of my life, the rocks are old and quite sedentary. The bedrock of the central US hasn’t moved since it was deposited in shallow oceans several hundred million years ago. The rocks of western Pennsylvania are only slightly less stagnant, as they gently moved upward a few thousand feet during the Allegheny orogeny, some 300 million years ago, but the sedimentary beds remained nearly flat, and they haven’t moved much since. Thus, the landscapes of the central portion of the continent are shaped almost completely by erosion working downward through rocks that haven’t moved in inch in a million years.
In California, the rocks, like everything else, are different. The really surprising thing about the rocks of coastal California is that they move. By themselves, each and every day. And really fast – for rocks. Let me provide two examples, one vertical and one horizontal.
The dominant topographic feature of my neighborhood is the Berkeley Hills. I live on Virginia St. and the walk, toward the hills, is a moderate and continuous uphill climb for 3/4 of a mile, passing through neighborhoods of closely packed homes on the edge on campus. After 3/4 of a mile, I am standing at 400′ above sea level, and the straight road abruptly ends. The reason is clear, as the gradient abruptly changes to a much steeper one, and the only roads going up the steep hill are very narrow and snake back and forth. Looking at a map, one can see that beginning at this discontinuity, the land surface rises more than 1000′ in less than a mile. Grizzly Peak is just a bit over one mile away from the end of Virginia St., but with the summit at 1754′ msl (feet above sea level), getting there would involve climbing 1400 feet. These are very steep slopes, 25% gradient and even more over smaller scales. Being California, they actually build houses, and even a University, on these slopes.
View from near Grizzly peak, looking down Strawberry Canyon onto Berkeley and San Francisco Bay. Cal Stadium is visible at the bottom of Strawberry Canyon. Buildings in the foreground, built on the slope, include the Lawrence Berkeley National Laboratory.
The important point here is that the steep sides result not from the repose of rocks as wind and water erode down through them, as in the central US, but instead are the result of moving rocks. Large blocks of rock, separated by faults, move past each other, and the differential motion has created an impressive topography. The rocks are moving because they are being pushed around by the collision of the Pacific oceanic plate with the North American continental plate. The collision has generated many ancillary fractures, one of which is the Heyward Fault.
Google Earth depiction of Heyward Fault. Note Cal Stadium at center.
The 75-mile long Heyward Fault parallels the more famous San Andreas fault, and generally runs north-south at the very foot of the rising hills. The elongate crustal block west of the fault sits under San Francisco, the Bay, and Berkeley. The entire block is twisting, with the west edge lifting up to form the hills of San Francisco, the Marin Headlands, and Mt. Tamalpais. The east edge has slipped down to form the bay and the flat lands abutting the Berkeley Hills. The block east of the fault is similarly being tilted, with the west edge uplifted, and the result is a dramatic escarpment that began forming only about one million years ago and continues to develop. Note that it is a bit misleading to say that the hills have risen in that time – the escarpment is formed as much by the Bay plate slipping down as by the uplift of the Berkeley Hills plate. After all, what is now San Francisco bay was just 10 million years ago a modest mountain range with active volcanoes. Those rocks have in the relatively short time since literally slipped below the waves.
Today, most of the motion associated with the fault is horizontal – it is a right-hand slip-stike fault. Some faults move only during earthquakes, but the Heyward Fault is unique in that portions of the fault show substantial aseismic creeping movement, and the examples can be surprising the those of us not from California. The builders of Cal Stadium knew full well that they placed their stadium on top of a stream, and accommodated it with a culvert buried under the 50-yard line, but they did not appreciate the athleticism of the Heyward Fault. Here is the result – a couple of photos I took of the south end of the stadium, showing displacement along an expansion joint.
The fault bisects the stadium, endzone to endzone. Fault creep in the southern portion of the fault is about 20 inches each 100 years, which, I don’t need to emphasize, is a lot in a city filled with object that are supposed to be immoveable. The neighborhoods here are filled with examples of the moving earth – stream channels that jog to the right as they cross the fault, displaced curbstones, displaced sidewalks. Here is a link to a KQED story with local examples of the moving rocks. And here is a USGS website with cool Google Earth layers. It is a very odd thing to realize that the earth is rupturing beneath your feet, and everything is being torn apart with it.
Finally, some faults creep, some erupt, and some do both. The infamous 1906 earthquake that destroyed San Francisco produced an earth rupture of 20 feet displacement at Point Reyes. Here is a photo of a fence after the earthquake (the fence has been recreated, based on photographs from this site). Joshua is standing on the Pacific Plate, Caleb, a couple of feet further away, on the North American plate. The lateral movement took place in seconds.
San Andreas Fault at Point Reyes National Seashore
The Heyward Fault creeps, but like the San Andreas fault, it also erupts. The last earthquake associated with the Heyward Fault occurred in 1868 and left 30 people dead in the sparsely populated bay region. It was known at the “Great San Francisco Earthquake” until the 1906 quake eclipsed it. Studies show that the Heyward Fault has given rise to a major earthquake about every 140 years.
Let’s do a few calculations and look at the results. 1868 plus 140 years = 2008. Now, add in several million people living close to the fault, often on steep slopes, in a region subject to both wildfires and landslides, depending on the season. Also add a transportation infrastructure with a few critical bottlenecks. What do we have here? Me hoping to make a safe return to a more seismically sedate portion of the world where the rocks don’t move so damn much!
Expeditions 