Roads and San Andreas Fault

[lamsalfl]

I'm from New Orleans, so I'm not an earthquake expert.  This may sound as a stupid question, but if the two plates along the San Andreas Fault are moving at 1.5 inches a year, what happens to the roads as they cross the fault line?  Are they constantly in a state of (dis)repair to keep them safe?  Does the asphalt continuously need to be updated?  I realize 1.5 inches is not a lot, but what happens every 4 years when there is a SIX INCH difference? 

[Occidental Tourist]

The reason we have earthquakes is that the plates do not move linearly.  Some areas along the fault move and others get hung up on the opposing plate.  In the areas along the fault that don't move, or don't move as fast as other areas, stress builds up at those points. When the stress at those points that aren't moving overcomes the resistance from the opposing plate, the fault slips, and an earthquake results from the sudden movement. So it is more common to see surface along faultlines stay undisturbed for five to ten years, and then move six inches to a foot during a moderate to large earthquake.

There are areas where the movement is more linear, like the Parkfield area.  It has lots of little earthquakes, and you do see more gradual movement like you envisioned.  The surface doesn't usually move in exact proportion with the fault below, so on roads crossing or atop the fault, you'll see pavement undulations develop over a series of years that will either turn into pavement splits or sinkholes that need to be repaved.

[Brandon]

^^ IIRC, in and around Parkfield, there are examples of curbs that have been offset by the fault line.

[The High Plains Traveler]

^^ IIRC, in and around Parkfield, there are examples of curbs that have been offset by the fault line.

Parkfield is one place where it is possible to see a bridge offset by the fault. The bridge was constructed so as to allow movement of the piers without significant damage to the deck. At this point, the fault is contained in a creekbed, and the movement from one side to the other is very evident. I don't know if this is a creeping segment or if the offset was generated by the one or two M6 events that have occurred since the bridge was built.

To answer the OP, though, you generally wouldn't see movement until a significant quake, in which that annual average fault movement is made up in one event. There are no cracks in I-5 at Gorman, for instance, though the fault trace is fairly evident through there. Also, not all plate movement is accommodated by the San Andreas alone. In southern California, the San Jacinto and Elsinore fault systems carry some of the movement.

Year before last, I made an effort to follow the northern part of the San Andreas system from Cape Mendocino down to Tomales Bay. There is a good exhibit at Point Reyes National Seashore.

[Alps]

So wait... that means 100 years ago, roughly when grids were laid out, you now have 150 inches difference, or 12.5 feet. Shouldn't there be some evidence of roads moving over an entire lane?

[The High Plains Traveler]

So wait... that means 100 years ago, roughly when grids were laid out, you now have 150 inches difference, or 12.5 feet. Shouldn't there be some evidence of roads moving over an entire lane?

The last big quakes along the San Andreas were 1857 near where I-5 crosses the fault between Gorman and Lebec; 1906, when the northern part of the fault, north of San Francisco moved; and 1812, when the southern part of the fault near San Bernardino shifted. In the interim, things are pretty stable. What you're referring to is aseismic creep, which does occur along the San Andreas near Gilroy or along the Hayward Fault in Oakland. There, you will see the aforementioned offset curbs that move a few centimeters a decade. I also mentioned my own visit to Parkfield, where there is visible evidence (that bridge) that the fault moves, but less than the 5-10 meter offsets seen in large quakes, in roughly 22 year intervals. Otherwise, you will have to wait for the next Big One to see dramatic offset.

My money is on the southernmost segment, between the Salton Sea and San Bernardino.

[Urban Prairie Schooner]

Would be curious to know how seismic activity affects property lines, section/township/range lines, etc. that cross the fault(s). Wonder how much resurveying has to be done over time.

[SimMoonXP]

The reference link is for maps of Marin to Santa Clara Counties area in 1960s. You can see Point Reyes to Southeast Santa Clara County area.

https://www.aaroads.com/forum/index.php?topic=4456.msg202592#msg202592

[NE2]

The reference link is for maps of Marin to Santa Clara Counties area in 1960s. You can see Point Reyes to Southeast Santa Clara County area.

https://www.aaroads.com/forum/index.php?topic=4456.msg202592#msg202592

OK...

[sdmichael]

State Highway 14 (Antelope Valley Freeway) shows some cracking and offset at its crossing of the SAF. It used to be a distinct bump until a more recent resurfacing.

[brownpelican]

The last big quakes along the San Andreas were 1857 near where I-5 crosses the fault between Gorman and Lebec; 1906, when the northern part of the fault, north of San Francisco moved; and 1812, when the southern part of the fault near San Bernardino shifted. In the interim, things are pretty stable. What you're referring to is aseismic creep, which does occur along the San Andreas near Gilroy or along the Hayward Fault in Oakland. There, you will see the aforementioned offset curbs that move a few centimeters a decade. I also mentioned my own visit to Parkfield, where there is visible evidence (that bridge) that the fault moves, but less than the 5-10 meter offsets seen in large quakes, in roughly 22 year intervals. Otherwise, you will have to wait for the next Big One to see dramatic offset.

See also the football stadium at Cal-Berkeley. The Hayward Fault goes right through that stadium.

[Road Hog]

The places where the fault is sliding merrily along seem like the safest places in the state to live. Fixing the roads here seem to be a happy problem.

[Desert Man]

From my observations of Palm Drive in Desert Hot Springs (north of Palm Springs), the road appears "crooked" to bend toward the right by 15 degrees as you look at DHS uphill on the northern horizon. The San Andreas fault crosses the Coachella valley twice with Dillon road east of Indio/Coachella bending abit to the left in the northern horizon. Palm Drive is a straight road, while Dillon Road turns north-northwest as it meets Palm Drive near DHS.

[Interstatefan78]

Just also look at CA-46 at the San Luis Obispo/Kern County line or the CA-41/CA-46 intersection the pavement seem to rise 1 inch, but US-101 at the San Andreas fault line north of the CA-156 East exit by the San Benito/ Monterrey County line it seems that US-101 climbs some short hills and grades created by the fault itself, and that stretch of the El Camino real has a 55 mph limit due to the gradients created by the fault.