Super Shear Earthquakes – Deadlier Than Deadly
One of the enduring mysteries in the investigation of Earthquakes is why the damage done over a century ago by a 7.8 magnitude Earthquake in San Francisco far exceeded what would be expected by an Earthquake of that magnitude. It was, in fact, what launched the intensive study of the San Andreas fault.
It has been known for some time that there are waves that are associated with ruptures in crustal fault systems. To review, there are the primary or P waves which are compressional waves that travel the fastest of all the known wave types and are the wave that reaches a seismic station first. There are the secondary or S waves which are transverse or shear waves–slower than the P wave but higher in amplitude and with either a lateral, shaking movement or an up and down, rolling type movement, depending on the waves orientation with respect to the surface. It is because of their greater amplitude, as well as their shearing action, that makes the S waves the most destructive of the two types. But in recent years, a revolutionary breakthrough has been made in understanding a new a kind of wave that is exclusive to strike-slip (lateral) faults. This new type of shock wave is what is known as super shear.
So what exactly is super shear? Super shear, which was first hypothesized and experimentally verified by Professor Ares J. Rosakis (pictured above), the Theodore von Kármán Professor of Aeronautics and Professor of Mechanical Engineering at CalTech, is a type of sonic boom of S waves that occurs only in relatively straight sections of a strike-slip (lateral) fault. Usually, in a strike-slip (lateral) fault, the focus of an Earthquake is at one end or the other of the fault line. With crustal Earthquakes that result from a normal dip-slip fault or a reverse thrust dip-slip fault, the seismic waves propagate in a more or less uniform, omnidirectional fashion. But in the case of a strike-slip (lateral) fault, the waves will travel in a unidirectional fashion following the fault line rupture. And that’s where the trouble begins, as Professor Rosakis learned from his experiments with 5mm thick transparent blocks of photoelastic polymer with a hairline fracture in each of them (pictured below) containing a thin exploding wire that is transformed into plasma once triggered, thus simulating the San Andreas fault, i.e., a strike-slip (right-lateral) fault, during an Earthquake.
So what is actually happening here? Well actually it’s all rather quite simple. Since the S wave is traveling along a line of weakness, namely the fault line itself, a sonic boom is being created as a rupture tip is formed in the propagating S waves. As the rupture overtakes the previously propagated S waves, the more recently created S waves overtake the previously created S waves, much the same way a supersonic aircraft overtakes the sound waves that it generates once its velocity exceeds the speed of sound, thus creating a Mach cone or shock wave.
It is the overlapping of S waves in the Mach cone that gives the S waves so much more power than they would normally have in a normal rupture. And thus the century-old mystery of the devastation of San Francisco has finally been solved.
But, 200 miles to the south of Los Angeles, along another straight line in the San Andreas fault there is another section of the fault in which the surface rocks are made of the tough, but brittle intrusive igneous rock, granite. Now, the last major Earthquake to have occurred in Los Angeles was 300 years ago. On average their is a major Earthquake in the Los Angeles area every 200 years, making a major Earthquake in the Los Angeles area statistically overdue by 100 years. But, these aren’t the kind of statistics that can be manipulated by any good statistician. These are statistics that are planted in (pardon the pun) rock-solid scientific data. The underlying plate has been steadily moving one inch per year towards the north for the last 300 years, as it has been doing along the entire stretch of the San Andreas fault, a strike-slip (right lateral) fault.
Let me spell it out for you. The Pacific plate in this region has moved 300 inches or 25 feet in the last 300 years. Whereas, the overlying granitic rock, and this is according to the type of GPS survey that Bente Lilja Bye, Research Director for the Norwegian Mapping Authority talks about in her article, The Haiti Earthquake: Science, Early Warning And Mitigation, quite literally hasn’t moved an inch. So when that overlying granite does eventually snap, it’s going to be displaced by 25 feet within an instant over a distance of roughly 200 miles. And between this place in southern California and 200 miles north in Los Angeles there is a straight stretch of the San Andreas fault that is perfect for producing one of these deadlier than deadly super shear Earthquakes.
So, not even structures that have been reinforced specifically for the expected “Big One” could withstand the force of a super shear Earthquake like the one that’s coming–not with a 25 foot displacement all at once! I don’t even want to think about the fatalities, injuries and utter destruction that will occur in Los Angeles when this finally happens. And it’s not a question of if, but when. It is going to happen!
A highly recommended site to visit:
USGS – Earthquake Science Center Seminars – a video of an in-depth lecture given by Professor Ares J. Rosakis with an accompanying slide show about his experiments creating laboratory Earthquakes and super shear.
Copyright © 2010 Eric F. Diaz