EARTHQUAKES are happening constantly around the world. The seismic network that measures earthquakes in Southern California, where I live and spent my career as a seismologist, has an alarm built into it that goes off if no earthquake has been recorded for 12 hours – because that must mean there’s a malfunction in the recording system. Since the network was put into effect in the 1990s, Southern California has never gone more than 12 hours without an earthquake.

The smallest earthquakes are the most common. Magnitude 2s are so small they are felt only if someone is very near their epicentre, and one happens somewhere in the world every minute. Magnitude 5s are big enough to throw objects off shelves and damage some buildings, most days a few of these strike somewhere. Magnitude 7s, which can destroy a city, occur more than once a month on average, but luckily for humanity, most take place underwater, and even those on land are often far from people. But for more than 300 years none of these, not even the tiniest, has occurred on the southernmost part of the San Andreas Fault. Some day that will change. Big earthquakes have happened on the southern San Andreas in the past. Plate tectonics hasn’t suddenly stopped. It is still pushing Los Angeles toward San Francisco at the same rate your fingernails grow – almost two inches each year.

Even though the two cities are in the same state and on the same continent, they are on different tectonic plates. Los Angeles is on the Pacific plate, the largest of the world’s tectonic plates, stretching from California to Japan, from Alaska to New Zealand. San Francisco is on the North American plate, which extends east to the Mid- Atlantic Ridge and Iceland. The boundary between them is the San Andreas Fault. It is there that the two plates get carried slowly past each other – their motion cannot be stopped any more than we could turn off the sun.

In a strange paradox, the San Andreas produces only big earthquakes because it is what seismologists consider a “weak” fault. It has been ground so smooth, across millions of years of earthquakes, that it no longer has rough spots to stop a rupture from continuing to slip.

The distance the rupture front travels is one of the chief determinants of an earthquake’s size. If it moves a yard and stops, it is a magnitude 1.5 earthquake, too small to be felt. If it goes for a mile down the fault and stops, it’s a magnitude 5, causing a little damage nearby. If it goes on for 100 miles it is now a magnitude 7.5, causing widespread disruption.

The San Andreas Fault has been smoothed to such a degree that now, when an earthquake begins, there is nothing left to keep it small. The ripple will continue to move down the fault, radiating energy from each spot it crosses, creating an earthquake that lasts for a minute or more and a magnitude that grows to 7 or even 8. Only after such an earthquake has broken the fault and made new jagged edges can it begin to produce smaller, less damaging earthquakes.

So we wait for that big earthquake. And wait.

The southernmost part of the fault had its last earthquake some time around 1680. We know this because it offset the edges of Lake Cahuilla, a prehistoric lake in much of what is now the Coachella Valley, filling with water the flats where the Coachella music festival meets each year. It left behind geologic markers, as did previous earthquakes, so we know that there were six earthquakes between AD 800 and 1700. That means the 330 years since the last earthquake on this part of the San Andreas is about twice the average time between its previous earthquakes.

We don’t know why we are seeing such a long interval. We just know that plate tectonics keeps on its slow, steady grind, accumulating energy, waiting to be released in one great jolt. One day, perhaps tomorrow, maybe in a decade, some point on the fault will lose its frictional grip and start to move. Once it does, the weak fault, with all that stored energy, will have no way of holding it back. The rupture will run down the fault at two miles per second, its passage creating seismic waves that will pass through the earth to shake the megalopolis that is Southern California.

Maybe we will be lucky and the fault will hit something that can stop it after only 100 miles or so – a magnitude 7.5. Given how much energy is already stored, however, many seismologists think it will go at least 200 miles, and thus register 7.8, or even 350 miles and reach 8.2.

In 2007-8, as Science Advisor for Risk Reduction at the US Geological Survey, I led a team of more than 300 experts in a project we called ShakeOut, to anticipate just what such an earthquake will be like. We created a model of an earthquake that moves across the southernmost 200 miles of the San Andreas, extending from near the Mexican border to the mountains north of Los Angeles. In the earthquake we modelled, we found that Los Angeles would experience intense shaking for fifty seconds (compare this to the seven seconds of the Northridge earthquake in 1994, which caused $40 billion of damage). A hundred other neighbouring cities would as well. Thousands of landslides would cascade down the mountains, blocking our roads, burying houses and lifelines.

In our model, 1500 buildings collapsed and 300,000 were severely damaged. We know which ones. They are the types of buildings that have collapsed in other earthquakes in other locations, and which we no longer allow to be built. But we have not forced existing buildings to be retrofitted to accommodate what we know. We might see some high-rise buildings collapse. The 1994 earthquake in Los Angeles and the 1995 earthquake in Kobe, Japan, exposed a flaw in how steel buildings had been constructed, causing cracks in their frames. Buildings of that type are still standing in downtown Los Angeles.

We are going to see many brand-new buildings “red- tagged” as too dangerous to enter and in need of major repairs or demolition. Our building codes do not require developers to make buildings that can be used after a major earthquake, only buildings that don’t kill you.

If the code works as it is supposed to, about 10 per cent of the new buildings constructed to the latest code will be red-tagged. Maybe one percent will have partial collapse. A 99 per cent chance of not collapsing is great for one building, but accepting the collapse of one per cent of the buildings in a city with a million buildings is a different matter. The earthquake will probably not kill you, but it will likely make it impossible for you to get to work – for a very long time.

Of the results we projected, one of the most frightening was the impact of fires triggered by the earthquake. Earthquakes damage gas lines, break electrical items and throw them onto flammable fabrics, spill dangerous chemical – and generally have many, many ways of starting fires. Two of the biggest urban earthquakes of the 20th century were the 1906 San Francisco and 1923 Tokyo earthquakes. Both set off fires that turned into firestorms and burned down much of those cities.

Some people think that modern technology has solved much of the fire problem because the two big California earthquakes of the late 20th century, the 1989 Loma Prieta earthquake in San Francisco and the 1994 Northridge earthquake in Los Angeles, did not lead to devastating fires. This is a mistake. Not because technology hasn’t changed, but because, in the eyes of seismologists, Loma Prieta and Northridge were not big earthquakes.

Those who lived through them may disagree and the damage they inflicted on those cities is undeniable. But these people simply don’t know what a really big earthquake will be like. What seismologists call “great” earthquakes (magnitude 7.8 and larger) are not just about stronger shaking – they are also about much larger areas. Loma Prieta and Northridge caused their strongest shaking near their epicentres, neither of which was in an urban core.

Loma Prieta’s was in the Santa Cruz Mountains, the strongest shaking of Northridge was felt in the Santa Susana Mountains. Even so, more than 100 significant fires broke out in each of those earthquakes. They were fought through mutual aid. San Francisco and Los Angeles put out calls for help, and firefighters from other jurisdictions poured in to help. City-wide fires were averted because of the amazing, courageous work of firefighters from across the region.

When an earthquake like the one we modelled happens, every city of Southern California will have fires that need to be fought. Calls for help will be answered with desperate pleas for help in return. Aid will have to come from Northern California, Arizona, and Nevada. Those people will have to come to Southern California from the other side of the San Andreas Fault, which will have moved 20 to 30 feet, offsetting all the highways into the region. Those responders will struggle, maybe for days, to bring equipment across broken roads. The firefighters who are here will be sent to fight fires in places where the pipes feeding the fire hydrants have broken and gone dry.

Our analysis, reviewed by the fire chiefs who had led the firefighting in Northridge and Loma Prieta, concluded that the fires would double the losses of the earthquake, in terms of both economic impact and casualties. Around 1600 fires could break out, 1200 growing large enough to require more than one fire company. We don’t have that many fire companies in all of Southern California.

As bad as this picture looks, it could be worse. In ShakeOut, I got to specify the weather. I made it a cool, calm day. Unfortunately, I don’t get to do this for the real thing. If the earthquake happens during the infamous Santa Ana winds, which have spread great Southern California wildfires and caused billions of dollars in losses, the fires that get started may be unstoppable.

Most of us will survive. Our estimate was that 1800 people will die and 53,000 will need emergency medical care. A significant number of hospital beds will be out of commission as hospitals suffer their own damage. And it will be very difficult to get to them. Bridges will be impassable, collapsed buildings will leave rubble in the street, and power will be knocked out, darkening traffic lights. Many people will be trapped in buildings, first responders will be overwhelmed. Most victims will be rescued by their neighbours. Losses will exceed $200bn. Life will not return to any semblance of normality for quite some time for the residents of Southern California.

In the following months, tens of thousands of aftershocks will occur, some of which will be damaging earthquakes in their own right. The systems that maintain urban life – electricity, gas, communication, water, and sewers – will all be broken. The transport systems that bring food, water, and energy into the region all cross the San Andreas and will be cut. In a simpler world, when you lose your sewer system, you build a temporary outhouse in the backyard. In the dense urban environment of a modern city, a lack of sewers is a potentially catastrophic public health crisis.

Cities are possible because of the complex engineering systems that support life. Those will be lost in such an earthquake. Half of the total financial losses in our model were from lost business. A beauty salon cannot reopen without water. Offices cannot function without electricity. Tech workers cannot telecommute without internet capabilities. Retail stores struggle if their clerks and customers don’t have the means of transportation to get there. Gas stations cannot pump gas without electricity and cannot take your credit card if they’re not online. And how many of us will want to stay in Los Angeles, much less go to work, when none of us have had a shower in a month?

Here we reach the limit of our technical analysis. Our scientists and engineers and public health experts can estimate buildings down, pipes damaged, legs broken, transportation disrupted. But the future of Southern California is the future of communities. We know what will happen to its physical structure, but what will happen to its spirit?