Collapsed building covers a car after the 2017 Puebla earthquake near Mexico City. Wikimedia Commons - CC-by-SA-4.0. This past month has seen Mexico suffer two major earthquakes. The latest earthquake destroyed multitudes of buildings in Mexico City and over 200 people died as a result of collapses and fires. As with any major natural disaster, a lot of misinformation or speculation gets thrown around and earthquakes tend to encourage a lot of the doomsayers. So, I thought it would be useful to try to set the record straight on what earthquakes can do, what they can't do and what may or may not cause earthquakes to happen.What earthquakes can do:
Shake the earth: Seismic waves are generated and they travel all the way through the planet. Now, by the time they reach the other side, they are only detectable by seismometers. Closer to the epicenter, seismic waves can cause major and minor shaking for hundreds or more kilometers.
Destroy buildings: This shaking can bring down buildings that aren't properly designed to withstand seismic waves. There can be two kinds of motion at the surface during an earthquake: side-to-side and rolling wave motion. Without reinforcement or specific design, buildings with weak plaster or concrete walls can easily collapse as we saw in Mexico City or Christchurch in 2011.
Liquefy the ground: Areas built on loose sediment can experience liquefaction, where sand and mud can go from "solid" to "liquid" (think quicksand), causing buildings that aren't built on bedrock to be heavily damaged or destroyed. The Tōhoku earthquake caused some remarkable liquification that was caught on video.
Cause giant waves: Earthquakes that occur under the sea (or near large bodies of water) can cause massive waves known as tsunamis. The most famous recent examples have been the 2011 Tōhoku tsunami in Japan and the 2004 Sumatra tsunami in Indonesia. In those cases, the tsunami was much worse for life and property than the large earthquake itself. The tsunamis are generated by either large landslides that move water or changes in the shape of the ocean floor.
Cause andslides: Big earthquakes can causes dozens or more landslides to occur, especially in areas with steep slopes. These landslides can be a major reason for loss of life and property, especially in earthquakes that occur in mountainous areas like Tibet.
Cause giant cracks to form: When faults move, they can cause cracks. Now, they don't open huge chasms like you see in disaster movies, but they can create fissures that you can follow for many kilometers.
Raise and lower the land surface: Just like creating cracks, earthquakes can make the land surface rise and fall depending on the motion on the fault. The M7.8 New Zealand earthquake in 2016 exposed the sea floor near the shore because the fault motion moved the land surface up meters.
Can cause more earthquakes nearby: Large earthquakes tend to have aftershocks that could be larger than the first earthquake. This is just the Earth responding to the changing stress released by the earthquake.
UPDATE: They can cause social change
: I've added this after reading this piece on how cities rebuild after disasters.
What earthquakes can't do:
Cause volcanoes to erupt: This is a little tricky. Very rarely, volcanoes that are already going to erupt might have a small eruption right after an earthquake. However, no earthquake is known to have instantly taken a dormant volcano and caused it to have an eruption. Now, some large earthquakes can be caused by magma moving under a volcano, but they are rarely (if ever) as big as the one's experienced in Mexico this month and seismologists can look at the waves generated by the earthquake to tell if it might have been caused by magma moving.
Trigger other large earthquakes far away: Another myth that inevitably comes up after a big earthquake is that it will cause more big earthquakes in distant places across the planet. There is little evidence that this happens. Geologists are looking closely at whether there is any real connection between large earthquakes, but clusters of earthquakes are more likely the result of the random nature of the distribution of earthquakes globally. Even big earthquakes that happen relatively closely - like this month's earthquakes in Mexico - aren't necessarily directly linked.
Swallow cities whole: Movies like to portray earthquakes as causing giant holes to open in the earth and people/buildings disappear into the abyss. Ask yourself: where did the hole come from? Earthquakes might cause small sinkholes to open, but the cavity exists beforehand -- and as people in Florida have seen, sinkholes definitely occur without earthquakes.
Cause animals to flee before the earthquake:Another big myth is that animals will run away or act crazy before an earthquake as if they know the earthquake will happen. There is no scientific evidence of this.
Create lights before all earthquakes: This is a little trickier. There have been occurrences of lights before and after large earthquakes, but not all the time. In fact, a recent study says it happens in less the 0.5% of all earthquakes.
What causes (and doesn't cause) earthquakes:
Stress in the crust and mantle, magma moving, mine explosions, landslides, pumping wastewater: These things can all cause earthquakes to occur and do every day.
Alignment of planets or the moon: This myth is propagated by charlatans who claim to predict earthquakes. We cannot predict earthquakes (at all) and this is doubly so by looking at the distance or phase of the moon. Triply so by using charts of planetary alignments. Quadruply so for asteroids that might pass near the Earth. Infinitely so for fake planets that are supposedly going to pass near Earth. All of this is pseudoscience peddled by people who want to sell you something.
Earthquake weather: There is no such thing. Earthquake weather is another form of pseudoscience.
Solar flares: Again, there is no evidence that solar flares or storms play any role in causing earthquakes.
Hopefully this helps you understand what is reasonable to connect with large earthquakes and what is merely fear-mongering and pseudoscience about something that happens dozens of times every day. Earthquakes aren't going to go away, but we can be better prepared in places where earthquakes are common.