World earthquakes

RMW Musson (UK)

For millions of people in the western part of Sichuan province in China, the morning of 12 May 2008 started out as a day like any other. People left their homes for work as usual, saying goodbye to family members without any thought that they would never see them again. Children packed into their school classrooms, their minds on lessons and games. It seemed like just another busy day.

At 28 minutes past two in the afternoon, catastrophe struck. There was no warning – just a sudden terrific roaring sound, as buildings bucked violently, sending down clouds of plaster dust, and then giving way completely, as the heavy ceilings came crashing down. People outside were the lucky ones. For them, the ground itself rocked violently like the deck of a boat in a storm. All around, clouds of dust arose from collapsing buildings, amid the sounds of roaring, of crashing masonry and of screaming people fleeing into the streets.

These scenes were repeated in the same instant across a huge area, nearly 300km long, at the edge of the Sichuan plain where the mountains of Longmen Shan rise up abruptly from the flat farmland. In the narrow mountain valleys, steep, rocky slopes came tumbling down, as massive rockslides added to the devastation and blocked the roads. In a matter of a couple of minutes, over 70,000 people were killed and another 370,000 injured. Around five million people were left homeless. It was the worst earthquake disaster in China in over 30 years.

An earthquake is a calamity like no other. Most natural disasters can be seen coming. When Hurricane Katrina struck New Orleans in 2005, meteorologists had been tracking it for days. Everyone knew in advance when it would hit, where it would hit and how hard it would hit. No-one could claim to be surprised by it. Volcanoes are harder to predict in advance, but an eruption usually starts off with minor activity that shows something is going on, well before the volcano blows its top. The biggest killer amongst natural disasters is probably drought – and that kills slowly and gradually (through famine) over a period of years.

Only an earthquake has the capability to kill so many people, so quickly, over such a large area and so suddenly. And, therein, lies some of the terrible fascination of earthquakes. They are always there, lying in wait.

So what exactly is an earthquake and why do they occur? Well, it may sound surprising, but unlike most other types of disaster, there is a good side to earthquakes. In fact, a world without earthquakes would be good news for fish, but not for us. This is because the land on which we live is perpetually being eroded by the forces of wind and water. Mountains are worn down into soft silt that is washed away by rivers and dumped on the ocean floor. Furthermore, this process has been going on for millions of years – so why isn’t the Earth one big shallow sea by now?

The answer is that the downward processes of erosion are balanced by upward processes of mountain-building; and earthquakes are a necessary part of the huge upheavals needed to push up great chains of mountains like the Alps or Himalayas.

The ultimate cause of all these movements lies deep within the Earth, in the region known as the mantle. Here, heat from the planet’s molten core creates vast, slow convection currents turning over the mantle material, similar to what you see in a saucepan of soup heating on a stove. The Earth’s outer layer, on which we live, is a thin crust of lighter rocks, not much more than 30km thick in most places that floats on top of the mantle. The currents in the mantle beneath this crust have long since broken it up into chunks (called plates), which now move independently of one another, sometimes moving apart and, at other times, scraping past each other or colliding. And, where they collide, rocks are scrunched up under pressure, broken, folded over and pushed upwards into mountain chains.

Since rock is basically a hard, brittle material, this cannot happen smoothly in the way two rugs might ruck up if pushed into each other. Instead, the rocks break into huge blocks along the line of least resistance. Each time the rock breaks this way, it creates an earthquake.

The line along which rocks have broken is what we call a fault. After it has been created the first time, it continues to be a line of weakness controlling the movement of the rocks either side of it under the pressure of the forces exerted on them. However, since the surface of the fault is usually very rough, it doesn’t move constantly. Usually, it is locked shut by sheer friction. That is, until the stresses acting on it build up to the point when they are stronger than the frictional force resisting movement can withstand. Then, the fault fails again – the rocks on one side jerk forward suddenly a few metres, and a new earthquake has occurred.

The violence of this sudden movement releases a lot of energy – some of it in the form of heat, but most in the form of shockwaves. These radiate outwards rapidly. The fastest waves are back-and-forward waves like sound waves that travel at around 6 to 7km/sec. When they reach the Earth’s surface, because they are like sound waves, some of them cross over into the air and become sound waves – hence the roaring or rumbling noise that often accompanies earthquakes. We call these primary waves or p-waves.

Next come the secondary or s-waves, which move with a side-to-side motion like a snake. These are also called shear waves. Finally and slowest of the lot, come the surface waves. These radiate out from the surface projection of the fault, and are rolling waves like the waves of the sea. These are the ones that cause the strongest shaking, the biggest movements – and the most damage.

As the waves travel outwards from the breaking fault, three things happen:

• Firstly, the waves get weaker simply because of spreading out, just as the rubber of a balloon gets thinner as you inflate it.
• Secondly, some of the shaking energy just gets absorbed by the rocks it travels through.
• Thirdly, the waves get spread out in time, as the p-waves run ahead of the slower waves travelling on behind, like runners in a race.

Some distance from the fault, you might feel the impact of the p-waves or s-waves quite distinctly, a few seconds before the surface waves hit. At the epicentre of the earthquake, everything will hit you at once. But also, because of the spreading and the absorption, the further you are from the fault, the weaker the shaking will be. Far enough away, only a seismologist’s sensitive recording equipment will detect the waves as they go by. A giant earthquake of Boxing Day 2004 was so strong that the waves from it could still be detected by monitoring equipment after going around the entire planet three times.

So, this is what an earthquake is like. What it is not like is the sort of thing that so often passes for an earthquake according to Hollywood – a huge chasm opening up in the ground, inevitably with glowing lava at the bottom, about 10m down. In legends and fairy stories (and, sadly, also the Bible: Numbers 16, 31-32), one reads of earthquakes swallowing up people and even swallowing up towns and cities. However, the occurrence of people being swallowed by the ground is extremely rare. Sometimes, under strong shaking, soft sand can turn literally into quicksand, with unfortunate consequence for anyone or anything standing at the surface. Fissures can also be produced on river banks and sloping unstable ground – but these are shallow secondary features. They can look spectacular, but they aren’t particularly dangerous.

So, why are earthquakes so deadly? A comparison of two events of similar size and very different outcomes will help to explain this. On 7 December 1988, an earthquake around 7 in magnitude struck Armenia. The death toll is uncertain, but certainly exceeded 25,000. Less than a year later, a similar-sized event struck California. The death toll was 62. What was the difference? Buildings. It is possible to be killed directly by an earthquake (usually through heart failure) but, most often, fatalities result from some secondary effect. People can be drowned by a tsunami or buried by a rockslide, but the overwhelmingly greatest cause of earthquake deaths is building collapse.

Imagine a very simple building consisting of three slabs of stone. Two are balanced on their ends (the walls) and the third is placed across the top (the roof). This arrangement is perfectly stable under normal conditions, because the only force being exerted on it is the vertical force of gravity. So long as the centre of gravity of each wall is over the wall’s base, it will stay standing indefinitely. But along come some surface waves from an earthquake, and now a sideways (lateral) force is applied to the walls. Once the centre of gravity of a wall is displaced so that it is no longer vertically aligned with the base, down it comes, bringing the roof down with it.

Now we can compare California with Armenia. In California, most houses are built of wood. Timber frames are very resistant to earthquake shaking – they are flexible – they bend and then bend back again. Some of the worst-damaged houses in the 1989 Loma Prieta, CA, earthquake were thrown completely off their foundations. Yet, they retained their basic structural integrity – the roofs were still in the right place relative to the walls. Even the non-wooden buildings were mostly prepared for earthquakes, with reinforcement specifically intended to give buildings lateral as well as vertical strength.

In contrast, in Armenia, many blocks of flats were constructed out of pre-formed concrete panels that were not much better than my three-slab example above. Under the impact of the earthquake, these collapsed totally. Anyone inside had little chance of survival. A bitter joke was told afterwards, that Mr Brick, Mr Mortar and Mr Steel Reinforcement were arrested and tried for manslaughter. Mr Brick and Mr Mortar both pleaded guilty, and confessed tearfully that they had not done their job the way they should have. Mr Steel Reinforcement, on the other hand, coolly pleaded not guilty. “Not guilty?” roared the judge, “How can you say you are not guilty?” “Simple, your honour,” came the reply, “I wasn’t there.”

The story repeated itself in Sichuan, twenty years later. This time, it was schools that turned out to be particularly badly built and, even away from the zone of total devastation, there were cases where schools collapsed partially or completely, even when neighbouring buildings were relatively undamaged. Again, these buildings were of heavy concrete construction. When they came down, the occupants were buried. If they weren’t crushed to death, they risked suffocation from pulverised concrete dust.

The key to protecting society against earthquakes lies in safe construction. One cannot prevent buildings from being damaged in earthquakes. One can design them in such a way that the damage will be controlled and will not lead to total collapse. Collapse is what kills people: and it can be prevented. This is the first principle of earthquake engineering.

The response of the Chinese authorities to the Sichuan earthquake was exemplary. They knew what had to be done, and the whole state mobilised to support the disaster relief, from the Premier himself who left his office for the disaster zone within 30 minutes and immediately set about superintending the rescue work – to the local taxi drivers, who set about driving in volunteer rescue workers from the surrounding area. Now will come a phase of reconstruction and, this time, one can hope that lessons of safe earthquake construction will have been well learnt.

Aftershocks

After an earthquake has occurred, the first thing people usually want to know is, “Is it over now?” The answer is usually “No”. Any large earthquake is normally followed by a whole sequence of smaller shocks called aftershocks. These can go on for some time – the 2008 Sichuan earthquake was notable for the occurrence of quite strong aftershocks for several weeks after the first event, which is called the mainshock. The aftershock sequence was spread out, right the way along the fault that broke (for a distance of about 300km north-east of the epicentre). In fact, if one plots a map of the aftershocks of the Sichuan earthquake, what you get is virtually a map of the fault-break of the mainshock, all 300km of it. On the morning of Boxing Day 2004, it was immediately apparent that the great Sumatran earthquake was huge on an almost unparalleled scale, just from looking at the enormous area where the aftershocks were occurring.

Compared to other aspects of seismology, aftershocks are relatively predictable in that, in general, the length of the sequence and the size of the largest aftershock are generally related to the magnitude of the mainshock, and the frequency and size of aftershocks both decline as a function of time after the mainshock. The largest aftershock is usually not larger than one magnitude unit less than the mainshock, and most commonly occurs in the first 48 hours. That said, in any individual sequence, one can get a lot of variation.

Since aftershocks are mostly much smaller than the mainshock, they are not usually so dangerous or damaging. The problem is that many buildings, which were weakened by the mainshock but did not collapse, may be in such a perilous state that, even a minor earthquake can finish them off. Therefore, aftershocks can be disproportionately damaging for their size. Fortunately, survivors of the earthquake will generally be camping out in tents and will have the sense to stay away from dangerous buildings.

One can imagine a sequence something like this. Before an earthquake, the fault is locked shut. Friction prevents it from moving and it holds the same position for decades or even centuries. Finally, the accumulated stress is too much and it jerks forward into a new position. This new position is what results after the fault movement grinds to a halt, but it may not be anything like as stable as the previous position. Consequently, it will continue to adjust its position in further little movements, until everything is locked solidly once more.

This is an oversimplification. If it were exactly like this, all aftershocks would be perfectly positioned on the fault plane, which is not the case. It seems that the force of an earthquake actually damages the rocks on either side of the fault plane, producing a zone of minor faulting, all of which has to settle down before activity completely stops.

Earthquake prediction

“Why can’t you predict earthquakes?” is one of the most common questions seismologists get asked. In the 1950s, it was confidently expected that, by the end of the century, earthquake prediction would be routine, and next week’s major earthquakes would be printed in your newspaper. Despite vast efforts, it did not turn out like that. In fact, we are not really much closer to the goal than we were 50 years ago. Some seismologists now believe that prediction was always a false hope: that earthquakes are totally chaotic and, therefore, unpredictable.

Approaches to earthquake prediction follow three main trends:

1. Looking for measurable changes in the properties of the rocks around a fault that might occur in the state just before it fails.
2. Looking for signals that might be given off by rocks in an impending earthquake zone, like gas emissions or electrical discharges.
3. Looking for recognisable patterns in earthquake catalogues.

Some successes have been claimed with all three of these approaches, but demonstrably, none of them work consistently. So, it looks like the best one can hope for is a system that works only some of the time.

Earthquake prediction is a subject that notoriously attracts cranks and, unfortunately, it is very easy to cheat at earthquake prediction. Here is an illustration of how to do it. First, make a prediction on your web site that there will be an earthquake in California next Tuesday. This is a dead cert, there’s always some sort of earthquake in California. When Tuesday comes, assuming, as is likely, that the biggest event is a magnitude 2 somewhere in the hills north of Doohickeysville, count this quietly as a success that you add to your “score”.

Repeat. And repeat. And repeat.

Eventually, there will come something larger, let’s say a magnitude 6 in Santa Barbara. Now, you hide all evidence of all your previous predictions, so that your web site shows only the one satisfied by this big event. Rush to the Los Angeles Times and show them that you certifiably predicted the 2008 Santa Barbara earthquake! Admittedly, you didn’t specify Santa Barbara, and you didn’t give any magnitude, but people who don’t know any better will still be impressed, and it should be good enough for a few invites to appear on talk shows.

The moral is that, unless a prediction specifies the date, the place and the magnitude of an earthquake within narrow bounds (“California” and “greater than 2” are not good enough), then it’s worthless.

Glossary

Aseismic creep: If a fault is smooth enough, it may just slide slowly without producing earthquakes at all.

Asperity: A rough patch on a fault where it sticks. Eventually, it will get broken when the stress builds up enough to produce the next earthquake.

Depth: How far below the surface of the earth the hypocentre is. Deep earthquakes tend to be less dangerous since, if the shock waves have to travel 60km just to get to the epicentre, some of the energy will have been absorbed by the rocks they travel through.

Epicentre: Everyone knows this word, or thinks they do. When a fault breaks, the energy radiates out from the whole length of the break (or rupture). The point on the Earth’s surface directly above the spot where the initial fault break begins is the epicentre. For large earthquakes, this has no practical significance except to seismologists. Newspaper diagrams showing energy radiating out in a series of circles from a point labelled “epicentre” are nonsense, except for small earthquakes where the length of the rupture can be discounted.

Hypocentre: The actual point on the fault plane, at depth, where the rupture begins. Also known as the focus.

Intensity: How strong the earthquake shaking is at a particular place, as expressed by the effects observed. It works like the Beaufort Scale of wind strength. Therefore, rattling windows is intensity 4, falling ornaments is intensity 5, slight damage is intensity 6, moderate damage is intensity 7 and so on. Intensity will be strongest near the fault rupture and progressively less with increasing distance. Never confuse intensity with magnitude!

Magnitude: How big an earthquake is. It is often asked why there are no units for this. The answer is that modern magnitude is related to moment (the amount of work done) and it is easier to say “magnitude 7.0” than “moment of 3.9 x 1026 dyne/cm”. Note that the phrase “Richter Scale” is journalese and not normally employed by seismologists unless forced to. Magnitude is computed as the mean of a number of readings from different stations, which is why the magnitude of an earthquake that has just occurred tends to get revised within a few hours – the more data, the better the result.

Seismometer: A device for recording earthquakes, generally based in some way on a pendulum. In modern instruments, this is usually a mass suspended on a spring, held in place by electromagnets. The amount of electricity needed by the magnets to stop the mass from moving relative to the case is an analogue of the ground motion. The word seismograph is also used. However, technically, a seismograph only records the time of occurrence of an earthquake, whereas a seismometer records the exact ground movement.

About the author

RMW Musson is part of the (UK, British Geological Survey.