We lived in Los Angeles for a couple of years, a decade before the devastating Northridge earthquakes rolled out from a huge slip on a hitherto undetected fault.
Our first home was in the foothills of the fault-bounded San Gabriel Mountains. The office was close to the Raymond Fault. Despite such proximity to future earthquake epicentres, we never experienced any severe ground shakes though the potential for them lurked large in our subconscious.
Crumpled, folded and faulted rocks are a mark of a tectonic power that is more monstrous than any nuclear device yet created. One place such power is expressed is in a road cut on Interstate 5 at the top of Tejon Pass through the Transverse Ranges of Southern California. We had relocated to LA from Dallas by car and one of the highlights of the trip was the twenty seconds it took to cross over the San Andreas Fault.
The dark symmetry in two events that last twenty seconds wasn’t lost on us. Our time spent passing across the fault could one day be the duration of a cataclysmic earthquake somewhere on the fault, perhaps somewhere we’d become involved in it.
The sight of the fault excited the geologist that lurks within me. We almost missed it while travelling at 55 mph in a small car complete with a disgruntled six month old in the back. Out to our right, we saw two clearly identifiable rock formations, one layer significantly folded and crumpled lying at an angle on the other layer which was masked in scree and vegetation. It was a tiny section of the rare surface expression along the 1200 kilometre length of the San Andreas Fault. We were crossing from the North American Plate onto the Pacific Plate and for no important reason I can think of, I was surprised that one side was brown and the other greyish. Twenty seconds in the passing and perhaps an ominous sign we were leaving the very ancient and warmly coloured rocks of the North American Plate for the dull greys of a relatively youthful, 145 million year old Pacific Plate?
It’s hard to conceive that these two layers of rock were sliding past each other as quickly as my finger nails grow. Yet this transform slippage is unlike my finger nails in that the movement is not continuous. It happens as ruptures that occur at huge and semi-random intervals of as many as thirty or forty or hundreds if not thousands of years at a time. I happen to have reason to know that a lost finger nail takes about six moths to regrow which is about 4 cm per annum, give or take. Imagine that an earthquake happens when a fault ruptures all at once after one year of inactivity. You might notice a tremble from 4 cm of displacement. Make that interval a century and the stored stresses will be released as perhaps 4 metre displacement. One massive, jarring rupture from which the energy propagates to bring devastating topographic changes and in the case of populated areas, destruction and death on a huge scale.
A few months into our LA experience, I read articles in the local newspapers about how 1981 earthquake ordnances had mandated brick buildings be retrofitted or demolished. This was in 1982. That article set-off an alarm in my inner geophysicist. It was clear that LA was playing catch-up and meantime, it was unprepared for a major earthquake. We had already learned that there were nuclear fallout shelters against the improbable yet admittedly catastrophic consequences of any detonation of nuclear missiles from foe or local military accident. Then again, from a military perspective, the region around LA merited defence as it was also the largest defence-related aerospace complex in the world. But surely a place where two continental plates were sliding alongside each other merited more serious disaster preparations?
The world’s oldest wooden building is Horyuji Temple, in Ikaruga in the Nara Prefecture, Japan. It’s a Buddhist pagoda that survives in the one of the most earthquake prone parts of the world. It’s built around a tree that was felled in 594 which is presumably when the pagoda construction started. The pole at the core is called a shinbashira. It’s just like a wooden telegraph pole, flexible like the tree that it once was. The pagoda has many layered roofs and they act to dampen the movement as the shinbashira flexes. The dynamic balance in the construction engineering that keeps the pagodas standing has evolved from centuries of trial and error. Modern computer simulations can do no better than verify its sublime efficacy.
The rocks hold stress in such a way that geophysicists can read the change in stress and potentially predict the likelihood of an earthquake. Unfortunately, geoscientists can’t hang around for hundreds if not millions of years to check their predictions. This time gap is why one or more generations become comfortable living on top of ticking time-bombs like volcanos and major faults. You might say the earth’s metabolic rate is different to ours. So, being a clever species, we build numerical models to accurately simulate earth’s processes. These models are computationally intensive and subject to improvement as we learn more about initial and boundary conditions that come from continual study of the rocks.
We are using computer simulations to keep us safe but they are blunt tools as we know well in Ireland from weather forecasting. As sophisticated as we considerer our computers to be in 2020, they only serve to increase our understanding of the fundamental processes and deliver a range of outcomes. The models for weather and earthquakes are not able to forecast details.
And yet, so often reactivity is our legislative response rather than proactivity. The fuzzy predictions do not satisfy the administrative need for budget certainty.
Constant earthquakes in Japan and New Zealand, for example, have changed their administrative outlook. We were in Japan a couple of years ago and saw how the urban areas are readied for reconstruction by leaving unsightly infrastructure above ground; the wires and pipes are in part isolated from earthquakes by being above ground and such exposure renders them all the more accessible for repair. Building codes for skyscrapers and bridges in New Zealand include lead-rubber bearings to achieve ‘base isolation’. The piers slide laterally on flexible mounts. The Kiwi invention was in use in one of ten hospital buildings around Northridge in 1994, the only one that remained functional. Every bridge and building using the Kiwi system in Kobe survived the 1995 earthquake.
And looking around the world, the one city that worries me the most is Istanbul. Turkey’s largest city has been bracing for a major earthquake from the very active North Anatolian Fault, a strike-slip fault very much like the San Andreas system. Despite huge ‘quakes in 1063, 1509 and 1766, Istanbul has become a populous city with many high rise building, very lax building codes and even laxer enforcement.
Having been in a hotel in earthquakes in both Corfu and Wellington, let me say that I’ve chosen to live with the uncertainty of an Irish weather forecast rather than worry about poor earthquake predictions.
And as a postscript, as a geophysicist I was using the accelerometer in my 2009 iPhone as a party trick, showing people how tables or floors vibrate when you thump them. The engineering and science behind this is very serious and MyShake (U Berkley) and ShakeAlert (USGS) apps are downloadable for use as early earthquake warning tools. However, these are not a panacea. While the shear waves generated by the ruptures along faults vary in power in in proportion to the length of the rupture itself, what remains roughly constant is that they travel along the ground at about three kilometres per second. If you are 10 km from an epicentre, you might get a 30 second warning. That’s only enough time to shout out a warning to anyone socially distant to you, put down the kettle of boiling water and lie flat.
And finally, the Ridgecrest M7.1 earthquake in July, 2019 took place in a low-population region. earthsky.org reports that ‘It was felt across much of Southern California, parts of Arizona and Nevada, as far north as the San Francisco Bay Area, and as far south as Baja California, Mexico. An estimated 20 million people experienced the foreshock and 30 million people experienced the earthquake.’ The real question is what other faults were put under greater stress by this ‘quake and there’s a body of opinion that this makes a major ‘quake close to LA an event that may occur sooner rather than later.