What Mexico’s earthquake means for California

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SAN DIEGO — As we wake up to footage of the devastation in Mexico in the aftermath of the second earthquake that country has suffered this month, people in California might well be wondering how well-prepared their local communities are.

But earthquakes in California and Mexico differ in ways that are important to understand — if we are to learn lessons that allow us to better prepare for them in the future.

First, the tectonic activity in each area is different: California sits at the boundary between two plates that rub each other horizontally. The plates off the shore of Mexico and the rest of the Pacific “ring of fire,” an area of intense seismic activity, rub against each other vertically.

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As a result, Mexico has bigger earthquakes. The biggest Mexican earthquakes happen offshore and create tsunamis, but the earthquakes themselves are far from Mexico City. The biggest Californian earthquakes happen inland on the San Andreas Fault, and as a result generate no tsunami. Though they are smaller than earthquakes like the one we are seeing cause such devastation in Mexico, they occur dangerously close to Los Angeles.

Because of this difference of distance to the major fault, it is more challenging to build an earthquake early warning system for Los Angeles than for Mexico City, which means that the time available to declare a warning is much shorter in California.

The second major difference is the soil upon which both cities are built. Mexico City was built on a former lake, which means that its ground is soft and wet. It behaves like a bowl of jelly when it shakes. It renders ground motion stronger and longer in duration. For this reason, the impact of distant earthquakes is more dramatic in Mexico City than in California.

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Finally, houses in California and Mexico are built differently. The typical one-story wooden house in California is light and does not collapse during earthquakes. The biggest danger is caused by objects flying around due to strong shaking, and the general advice in California is to drop, cover and hold on.

The safest reaction to have during an earthquake in Mexico, especially in weak constructions that are likely to collapse, can be completely different.

The biggest dangers in the immediate aftermath of an earthquake are also different in both cities. For example, in California, wooden houses with public gas system have a more severe risk of fire.

We should be careful to understand the different weaknesses of the built environment around us when traveling and migrating. Being aware is the best way to ensure safety during earthquakes in different countries. Wise advice that has worked well in Mexico may fail miserably in California, and vice-versa.

When the damage caused by a large earthquake exceeds the capacity of official emergency responders, citizens become their own first responders.

In the aftermath of the Mexico earthquake, we are seeing inspiring images of people helping each other, going to the rescue of their neighbors and coworkers. The people of Mexico City gave the world a lesson of citizen solidarity and self-organization after the 1985 earthquake, and we’ve seen similar images from the areas hit by recent hurricanes in the US.

Here is an important lesson: We can be prepared for earthquakes as individuals, and as families, but if we’re not prepared as a community, we all sink together. And if California falls, the whole of the US can fall.

Preparedness is a community responsibility. We can prepare together by Community Emergency Response Team (CERT) training, by identifying the resources in our neighborhood like knowing who is a medical doctor, who has heavy-duty tools, and by being aware of the most vulnerable people around us.