Exploring the Fault

When I was much younger, our cousin Pascal would regularly tease me and my sister with stories about demonic “earth rays”. While the scientific evidence he provided was a bit lacking, it definitely instilled an interest in everything that happens beneath. In 2008, when I first moved to Seattle, I quickly discovered that the Pacific Northwest was a very unique place, right on the edge of a continent. Within a four hour drive, there were no less than six active volcanoes, one of which had actually violently erupted in the last 30 years.


On my first work day at Microsoft, I also found a document hanging from my brand new office door, warning me on what to do in case of an earthquake. I was a bit surprised, and started doing some research. Turns out that Seattle actually has a number of fault lines, where earthquakes can occur. Right through downtown Seattle runs the Seattle Fault, and not too far out the Pacific coastline, the massive Cascadia Subduction zone threatens the entire Pacific Northwest with earthquakes of up to magnitude 9. The latter is an especially interesting fault, as it’s a place where the oceanic crust sinks below the North America plate, over an extremely long distance- from Vancouver Island to Northern California.


A rupture taking place over the entire area could cause such massive earthquakes. The last time such an event happened in the Pacific Northwest was in 1700. Though there are no written records in America about this, folk tales of Indian tribes are still passed along, and Japan has written records of a Tsunami, which showed up without an earthquake. Ever since moving to San Francisco, I have really missed the interesting geology of the Northwest. The closest worthwhile stratovolcano, Mt Shasta, is a 4 hour highway cruise away. While there are some other volcanic areas closer by, none of them has the appeal of a Mount Rainier in the Northwest. However, there’s still a lot to see nearby. Today, I took a trip down the 101 corridor to find artefacts of the San Andreas Fault, California’s “big one”.
The San Andreas fault runs some 800 miles North-South through California. It is the boundary between the Pacific and North American plates. Both major earthquakes which San Francisco has known, the 1906 San Francisco earthquake, magnitude 7.8, and the 1989 Loma Prieta earthquake, a 6.9, had their epicenter along the San Andreas fault. In the San Francisco area, the fault runs right below Daly City, and then disappears into the Santa Cruz mountains, where it follows a mountainous path down to the Los Angeles area.
In order to get to know the fault at its best, I drove down a small road which left US-101, the main highway between San Francisco and Los Angeles, from San Miguel CA. There the road splits- allowing you to drive through the hills to Pinnacles National Monument (in a previous post, I called this “Indian Valley Road” a candidate for the title of loneliest road in America), or to a small town called Parkfield. Parkfield is a town of 18 inhabitants. It’s very significant, though, because it is probably the best monitored town from a seismic perspective anywhere in the world. It is right on top of a very active part of the San Andreas fault. The town has been close to the epicenter of consecutive magnitude 6 earthquakes between 1857 and 1966, and had another one in 2004.
Earthquakes happen when friction exists between two tectonic plates as they attempt to move, pressure builds up, and then suddenly the plates dislodge. They make up their intended amount of travel in a very short period of time, and unleash a tremendous amount of power. The amount of friction that exists depends on quite a few things, such as the type of rock and the heat of the underground material.

It’s unclear why Parkfield is exactly an earthquake hotspot. Scientists believe it may be related to the fact that just south, the fault is “locked”, meaning that both plates are not slipping, because the resistance between them is larger than the pressure on them to move. Gradually, pressure builds, and then has to release. Just north, the fault is “creeping”- meaning it is moving along gradually without earthquakes. The combination of these two appears to make the Parkfield area particularly prone to pressure releases.

Due to its regular earthquakes, Parkfield is a hotspot for seismological research. It was the place where the first earthquake prediction was attempted (and failed), and is heavily monitored. Today, it also houses the San Andreas Fault Observatory at Depth (SAFOD) initative, an attempt to deploy instruments at 2-3 kilometers of depth, right on the fault line. This will allow the US Geological Survey, and the National Science Foundation to determine what exactly happens during an earthquake, much deeper than is the case today.
Here you can actually see the fault. To your left is the North American Plate, to your right, the Pacific Plate, which goes all the way to Japan, to connect with the Eurasian plate.
Another view of the fault. North American Plate on your right, Pacific Plate on your left.
Nice seismological measurement equipment on show in the local pub.
Here you can see a bridge which is specially constructed across the fault. In this area, the two plates move 1.4mm each year, so the bridge is constructed on large pillars which allow both parts to move.
You can see at the end that the bridge railing is not entirely straight, and is bending over time. This bridge was only a few years old.
A seismometer dug in a few yards away from the center of the fault, on the Pacific Plate side.
My car Suzy, parked a few hundred yards from the center of the fault, while I went and looked for a seismometer which turned out to have been removed.
Another, different view of a more pronounced part of the fault.
It was quite the adventure getting back to a highway. There was a small road running across a mountain, to I-5, on the map. It turned out to be an unpaved, single lane road making tight turns across the mountain. It took over an hour longer than expected to get back, and I was quite happy to make it across before sunset. This way, I was still greeted with this beautiful sight of a higher altitude pond in the coastal mountain range.

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