Lately, there has been quite a bit of press about the recent “San Andreas” movie. To me, this movie sets back the general public’s knowledge and understanding about how earthquakes create damage in Southern California.
Some basic stereotypes exist in the movie, many of which are completely false. Starting with the magnitude of the earthquake in the movie – No fault line in Southern California is capable of anything larger than about an 8.2. The only one truly capable of such an event, the San Andreas Fault, is also many miles from Los Angeles and is mostly separated from the Los Angeles Basin by the San Gabriel Mountains. Anything larger than a 9.0 is in the domain of “megathrusts” or subduction zones. In California, the only subduction zone is the southern end of the Cascadia Subduction Zone, which ends at the “Mendocino Triple Junction” just offshore of Cape Mendocino. It last produced something close to a 9.2 or so in January 1700.
Tsunamis, especially ones of great height, are also not in the forecast for a large earthquake here in Southern California. Tsunamis are created by the large scale displacement of water, similar to sloshing in a bathtub. Move your hand below the water quickly, you create a wave on the surface. Usually, tsunamis that are related to earthquakes are caused by the movement of the fault itself, typically megathrust faults underneath the ocean. We just don’t have those in Southern California. Even the largest tsunami generated by such a fault may only be tens of feet high, certainly not hundreds of feet.
Big cracks just don’t open up in the land from earthquakes, certainly nothing like those represented in the movie. Fissures are created by earthquakes, however. These fissures are usually the result of settlement or fault movement. They aren’t that large either way.
Structural damage is also not going to be as great as represented. Mind you, a large magnitude earthquake centered in the Los Angeles Basin will do a great deal of damage. Water mains, sewer mains, gas lines, power lines, and other utilities will be compromised in many locations creating shortages and, in some cases, fires. Buildings may collapse or be damaged beyond repair. The underlying geology will determine some of the damage extent. The rest will be determined by building type and its susceptibility to seismic waves. Either way, the skyscrapers in Downtown Los Angeles won’t be toppling like trees anytime soon. I’d still stay away from the area after a major event though, as there would be an immense amount of glass and debris creating hazards for travel.
Keeping all this in mind, and also keeping with the theme that Southern California officials have been doing, use this opportunity to prepare yourself for a major earthquake. They can strike at any time and will create problems for all of us that live, work, and visit this region. The best way to survive a major catastrophe is to be prepared. Part of that preparation is to know your region, know the routes, and know where the problem areas may be following a major earthquake.
For further information, I highly recommend contacting your local Emergency Services agency in your city and county. They have a great deal of resources to help you prepare for an event like a major earthquake.
My friend Jake and I left San Diego rather early, about 4:30 am. Our goal was to get up to the San Luis Obispo area as early as we could, so that we could take our time north of there to the Gilroy / Hollister area. It seemed to have worked. As we were making pretty good time, we decided to check out some old alignments of US 101 a bit earlier. We followed some alignments near Gaviota and Buellton, both of which had good sections of old concrete. After that, we didn’t make any major stops until north of San Luis Obispo.
At Bradley, we stopped at the large bridge over the Salinas River at the north end of town. It was built in 1931, replacing a multiple through-truss span built in the 1910’s. Much to our surprise and enjoyment, we got a much needed break from the rain here. The break was just long enough to walk the span and take many photos. I hadn’t had the opportunity to really view the bridge, so this was quite welcome. Once we left the bridge, we took the old alignment of 101 south back to the freeway, taking a slight detour onto an even older alignment, bypassed in 1939. Most of it was still fairly well paved, complete with sections of double white striping. One spot, however, proved to be a bit trickier. Some years in the past, a small landslide took out a portion of the roadway. A short dirt bypass was made, which was rather muddy after all the rains. I got of the car to scout the roadway ahead, to see if it was passable. I decided it was, and the car was able to make it. It did slide a bit down the slope, but not to worry, it wasn’t a problem. By the time we got back to the main road, the tires had acquired a thick ring of mud, almost a new tire in itself. Hey, it’s a rental car right?
Getting back to current US 101, we continued north to San Ardo, where the old highway crossed the river again. The bridge here is very similar to the North Bradley Bridge, built in 1929, but without the old railing. In 2001, the span was seismically retrofitted and the old railing was removed. One thing that was not removed, and only discovered by us on this trip, was a 1929 FAP sign, complete with a CSAA yellow diamond. These are rare to find out in the field, so it was quite something to see. This section of the road, from San Ardo up to San Lucas, was the last section of two-lane highway on US 101 from San Francisco to Los Angeles. It was bypassed in 1971. It still makes a nice detour from the main highway, with little traffic to deal with. At the north end in San Lucas, there is another old bridge nearby, crossing the Salinas River on Lockwood-San Lucas Road. That bridge, from 1915, is a through-truss span. Some of the girders had Carnegie Steel stamped on them.
Still heading north, we arrived at Gonzales, where we diverged from US 101 to see another old bridge over the Salinas River. I first found this bridge in 2001 on my first bicycle tour. I wasn’t planning to take that route at the time, but had to make the detour as there was no other route available. The bridge, a 1930 through-girder span, reminded me of another bridge that was near where I used to live. Through-girder spans are rare, usually used by railroads not highways. Instead of returning to US 101, we decided to change course, and follow the River Road on the west side of the valley. I hadn’t been there before, and it took us north. So, why not take a different path?
We crossed back over the river near Chualar, and went back to US 101. We stayed on it until the south end of Salinas, where we exited onto Abbot St, the old alignment and business route. The southern end of Salinas had the distinction of still having old US highway shields in place until the early 2000’s. The signs are gone, but the porcelain business banners remain. We found two of them, one northbound and one southbound. Heading through Salinas started off alright, but as we got closer to the center of town, things went downhill – fast (or was it slow). Traffic increased, roads turned in confusing directions and we didn’t have a map. Still, we managed to get through town and onto the road we wanted – San Juan Grade.
San Juan Grade Road is the original alignment of US 101 from Salinas to San Juan Bautista. Having been bypassed in 1932, the roadway still retains most of its old feel. It wasn’t widened, and much of the original 15’ concrete is still visible. A few of the curves were straightened, but even that appears to have been done long ago. A lot of the old wooden guard railing is still in place, mostly on the north side of the summit. South of the summit, a few bridges with pipe railing are still intact. Rain and fog prevented a lot of good photos from being taken, but we still got a few. On the north side, in San Benito County, more of the old concrete was preserved. It was good to see such an old highway in as good a shape as this was. At the north end, where it met old SR-156, there was an old white directional sign from 1959 which was on our list of things to see. Having taken care of that, we headed east to Hollister. It was time for some geological tourism.
Hollister, not the town from which the clothing comes from, is a growing suburb in the southern Bay Area. It still has a lot of farms surrounding it, and still retains a lot of its old downtown. It also is well known to geologists. The Calaveras Fault, a branch of the San Andreas Fault, runs right through town. This particular segment of the fault creeps. Most fault move in large jolts, known as earthquakes. This moves along like a conveyor belt, but much slower. The results can still be quite dramatic. Along many streets in the northwestern end of Hollister, there are offset curbs and sidewalks. A portion of an old fault scarp is also visible, usually as an abrupt rise in the street. All the roadways crossing the fault have fractures, bumps, and lots of patching. The earth waits for no one, and is more than willing to show its power. Sometimes structures lie atop the fault, and they too have been made a bit askew. Some are just cracked, while others are twisted out of shape. I noticed one such structure, on Suiter St, that was so badly cracked, I wondered how much longer it would stay up. It was certainly proof positive why the Alquist-Priolo Zoning Act was created.
After seeing all we could see of the Calaveras Fault, we headed out of town, first following the new alignment of Hwy 25. The new bypass runs to the east of town, connecting two places that 25 used to turn at. From there, it was on to Gilroy, where we stayed for the evening. Another adventure awaited us the next day – the Big Sur Coast!