Famous earthquakes of the future: Rupturing of the San Andreas Fault
The largest and most devastating earthquakes occur at subduction zones, where one tectonic plate is forced to descend into the mantle beneath an adjacent plate in order to accommodate convergence between them. If we inspect a list of the largest earthquakes ever recorded, it is the sudden release of accumulated strain energy from locked subduction zones which produced the top nine. This includes the magnitude 9.5 Chilean earthquake of 1960 (the largest earthquake ever recorded) and the recent magnitude 9.0 Tohoku earthquake of 2011, which resulted in a much larger proportion of fatalities from the ensuing tsunami than the earthquake itself.
Despite the devastating nature of subduction zone earthquakes, it is actually a different class of earthquake that does a far better job of capturing the popular imagination. These are earthquakes generated by continental transform faults, which are very long, near vertical faults which separate two continental plates, and are often clearly expressed at the surface. Unlike subduction zones, there is either no or little plate convergence, and strain energy is accumulated when the plates become locked together instead of sliding past each other. Well known examples include the North Anatolian Fault in Turkey and the Alpine fault in New Zealand. However, by far the most famous is the San Andreas Fault, which runs approximately parallel to and inboard of the west coast of California, and is dangerously close to many large population centres including San Francisco and San Jose. Apart from towns and cities, San Andreas’ sphere of influence also includes places like Hollywood and Silicon Valley, both of which have a major social and economic impact on the world at large. Indeed, Hollywood and the film industry can be thanked for helping to elevate our awareness of the devastating consequences of large earthquakes in California with movies like San Andreas, San Francisco, Earthquake and the TV mini-series 10.5. Misinformation is unfortunately rife; in the case of the film San Andreas, a magnitude 9.1 earthquake in LA is followed by a magnitude 9.6 in San Francisco, which also produces a massive tsunami. But we know that continental transform faults like the San Andreas Fault are unlikely to produce an earthquake in excess of a magnitude 8 (and remembering that with a logarithmic scale, the energy released by a magnitude 9 earthquake is over 30 times that of a magnitude 8), and only subduction zone earthquakes are capable of producing tsunamis of any significance (e.g. 2004 Sumatera Earthquake, 2011 Tohoku earthquake). A magnitude 10.5, as suggested by the TV miniseries of the same name, is practically impossible.
With this backdrop, it is therefore no wonder that when Thomas Jordan, Director of the Southern California Earthquake Center and Professor of Earth Sciences at the University of Southern California, stated at a conference that the southern section of the San Andreas Fault is “locked, loaded and ready to roll” it rapidly made headlines around the world. This was no glib warning, but a statement of fact based on the historic earthquake record, our knowledge of earthquake source processes and the relative motion that has occurred between adjacent plates on either side of the fault since the most recent earthquake in southern California more than 150 years ago. It is worth remembering that California is probably the most well studied region in the world when it comes to earthquakes, and that many of the tools employed in modern seismology were developed by scientists working on data recorded by the various seismic networks that span this part of the US. In fact, the so-called elastic rebound theory, which explains the earthquake cycle, was conceived as a direct result of the great 1906 San Francisco Earthquake. The basic idea behind this theory is that ongoing application of stress on either side of a fault results in elastic deformation and the gradual accumulation of strain energy (much like stretching a spring or elastic band), which is eventually released when the frictional resistance of the fault is overcome, resulting in a sudden displacement and the generation of seismic waves.
So how worried should we be that the southern part of the San Andreas Fault is “locked, loaded and ready to roll”? We know that the relative motion of the tectonic plates on either side of the fault is of the order of several cm per year, and that the total slip deficit may be as great as 10m, which means that a huge amount of strain energy has been accumulated. It must eventually be released, and with the historic return period for large earthquakes in this region estimated to be around 150 years, it would be reasonable to conclude that one is certainly “due”. However, large fault systems like San Andreas are incredibly complex, and our understanding of the genesis of large earthquakes is still limited, so nothing is absolutely certain. For example, stored energy could be released via a series of smaller earthquakes or via slow slip (which does not produce a definable earthquake). However, our best evidence suggests that a large earthquake is on the horizon, and while it comes as no surprise, the warning carried by the news headlines is timely. It is warnings of this kind which motivate the public to take earthquake preparedness seriously, and help lubricate decision making processes within government directed at improving the safety of our built environment. For example, in October last year, the city of Los Angeles required as many as 15,000 buildings to be reinforced, an unpopular move due to the high cost. However, improving the resilience of homes, schools, office buildings, factories and infrastructure is our best defence against the destructive power of earthquakes.
We can probably thank Hollywood, the global social and economic influence of Californian culture and industry, and cutting edge earthquake science, for making the San Andreas Fault the site of the most anticipated earthquake of the future. But while we’re on the subject, there are other places in the world where earthquakes are imminent and the consequences likely much worse. For example, seismic gaps – where there is an absence of earthquakes – have been identified along the Sunda Arc, which marks the boundary between the Australian and Eurasian plate. In 2004, the western portion of this subduction zone ruptured, producing a devastating earthquake and tsunami which killed over two hundred thousand people (bear in mind that the total number of recorded deaths from all earthquakes in California since 1800 is less than 3500). Istanbul, a city of approximately 14 million people, is at high risk of a large earthquake; over the last 60 years, the North Anatolian Fault has ruptured via a series of large earthquakes from east to west, which has lead to predictions of a destructive earthquake striking the city, replete with many unprotected buildings, in the next few decades. So let’s not just focus on the San Andreas Fault, tempting though it is when movies like San Andreas depict such catastrophic (and ludicrous) consequences.