In light of last week’s massive 9.0 magnitude earthquake off the northeast coast of the island of Honshu in Japan, I thought it was time to examine the literature from various sources that discuss the probability of an earthquake on the eastern side of the Ring of Fire, more specifically, the west coast of the United States that is adjacent to the well-known San Andreas Fault. Here is a map showing the San Andreas Fault:
The San Andreas Fault behaves differently than the fault that caused the earthquake in Japan on March 11, 2011. The San Andreas Fault is termed a right lateral strike-slip fault by geologists. In these faults, the fault surface itself dips very steeply into the earth’s surface. There is very little vertical (up and down) motion along these faults, rather, the motion is offsetting from one side of the fault to the other. In this case, if a line were painted on the surface of the ground across the fault, after an earthquake, the line would be offset where it crossed the fault line. In the case of Japan’s most recent massive earthquake, the fault surface dipped relatively steeply (but not vertically) into the ground at the margins of a crustal plate boundary as one plate was riding under the other. In the case of California, the San Andreas fault is found at the boundary between the Pacific Plate and the North American Plate but, because the motion is strike-slip, the land on the Pacific Plate (west side of the fault) is moving slowly to the northwest and the land on the North American Plate (east side of the fault) is moving to slowly to the southwest. The amount of motion on the fault is roughly 1.3 to 1.5 inches per year. In other words, the east side of California is headed south and the west side is headed north.
Here is a block diagram showing the mechanics behind the movement of the earth's crust along the San Andreas Fault:
Here is a photograph of the fault:
Earthquakes occur along strike-slip faults when the forces that are pushing the sides of the fault in the opposite direction overcome the friction along the surface of the fault plane. The sudden release of energy when the two sides "unlock" is what we feel as ground shaking.
Despite advances in the tools available to them, geoscientists find it very difficult to predict when an earthquake will take place and what its magnitude will be when it does occur. Part of the problem is that most of the very large earthquakes in the world, especially those in California, took place well before the advent of seismometers that are used to measure the magnitude of ground motion.
Using advanced (and advancing) technology, geoscientists have recently discovered that geological records indicate that very large earthquakes took place along the San Andreas Fault in 1417, 1462, 1565, 1614 and 1713, roughly every 50 to 100 years and averaging every 88 years (plus or minus 44 years). This was far more frequent than earlier interpretations that had stated that major earthquakes occurred every 250 to 450 years. The last major earthquake along the fault, a magnitude 7.9 quake, took place in 1857, nearly 155 years ago. By extrapolation from historical records, with earthquakes having taken place every 50 to 100 years for the past 600 years, it appears that the San Andreas Fault is well overdue for a major release of stored energy.
An important issue that faces the inhabitants of southern California is the composition of the sediment underlying major cities like Los Angeles and San Bernadino. The material is relatively soft and unconsolidated because it has been eroded from nearby mountains and accumulated in basins covering large areas overlying the San Andreas and its related faults. This material is structurally unstable and when long amplitude earthquake waves are injected into the material, it loses its stability and liquefies and acts in a similar fashion to jelly. This liquifaction was seen in the February 2011 earthquake that impacted Christchurch, New Zealand where large areas of the city were flooded by a mud and water mixture as the fluids trapped within the sediment caused it to become unstable when it was shaken.
Several scientific organizations in California study and track the occurrence of earthquakes in an attempt to gain a better understanding of both timing and magnitude. The Southern California Earthquake Data Centre tracks earthquakes in, of all places, California (and Nevada). From their website, here is a map showing earthquakes over the past week, day and hour. Note that the small brown lines on the map are faults. Most of the recent earthquakes are quite small with the largest earthquake over the past week having a magnitude of 3.4:
Here’s a map showing some of the most significant and damaging earthquakes in Southern California since 1812:
There have been several massive earthquakes but only one with a magnitude of 8.0 or greater. This earthquake with an approximate epicenter northeast of San Luis Obispo, is last major earthquake on the San Andreas Fault as mentioned above. It took place on January 9, 1857 and had an estimated magnitude of 7.9. The surface of the earth ruptured for over 350 kilometres, yet only two people were killed because the area was very sparsely populated at the time. The fault’s average slip was 4.5 metres or 15 feet and its maximum slip was about 9 metres or 30 feet (meaning that one side of the fault moved laterally up to 30 feet with respect to the other side). Here is a map showing the area of the fault that moved in red and the approximate epicenter:
Geoscientists feel that the area where the 1857 earthquake took place is the most likely location for the next major earthquake, particularly because it has been locked in place for far longer than normal. Unfortunately, the area is far more densely populated now than it was in 1857. Geoscientists are constantly monitoring the seismic activity along the San Andreas Fault in an attempt to approximate when a major earthquake will hit. Before the next earthquake, geoscientists expect that there will be an increase in small earthquakes,. As well, there should be a change in the measured distance and elevation profiles of survey lines that cross the fault.
While science cannot predict the timing or magnitude of the next major earthquake along the San Andreas Fault, geoscientists can say with confidence that the next major event will happen sooner rather than later. The Uniform California Earthquake Rupture Forecast (UNCERF) report released by the 2007 Working Group on California Earthquake Probabilities (WGCEP – don’t you love those acronymns?) used improvements in earth sciences to predict that California has a 99.7 percent chance of having a magnitude 6.7 or greater earthquake in the next 30 years and a 46 percent chance of having a magnitude 7.5 or greater earthquake in the next 30 years. Here is a map showing the probability of various magnitude earthquakes over the next 30 years for the entire State of California:
Over the next 30 years, there is a 59 percent chance (the highest risk fault in the state) that there will be an earthquake with a magnitude of greater than 6.7 along the San Andreas Fault as shown here:
One of UNCERF’s particularly unsettling predictions is their prediction for the Cascadia Subduction Zone that runs in a north – south direction through the northern part of California and the states of Oregon and Washington and north into British Columbia, Canada. Here is a map showing the location of the Cascadia Subduction Zone:
An earthquake of this type would be formed in a very similar manner to the March 11th, 2011 earthquake in Japan. UNCERF predicts that there is a 10 percent chance of a magnitude 8 to 9 earthquake over the next 30 years along the Cascadia Subduction Zone, a very similar magnitude when compared to the earthquake just experienced in Japan.
In closing, the population of Seattle, Washington was 608,660 in 2010 and the greater Seattle – Tacoma – Bellevue metropolitan region has a population of 3.34 million, Portland, Oregon has a population of 545,140 and the population of San Luis Obispo Country was estimated to be 267,000 in 2009. Let’s hope that the UNCERF study is wrong….but I don’t think that’s likely.