Get ready to shake: Earthquake 101

October 20 is World Shaking Day in 2022. Have you ever wondered what causes earthquakes? Or about the techniques used to predict their frequency? How about how to protect yourself during an earthquake?

Civil and Environmental Engineering Professor Jonathan Stewart of UCSD’s Samueli School of Engineering answered these questions and more in the latest episode of the “I Don’t Know That With Jim Jeffreys” podcast.

So you feel that initial vibration, what you have to do is ‘drop, cover, and hold,’ said Jonathan Stewart.

Stewart’s research interests include geotechnical engineering, earthquake engineering, and earthquake engineering. He has studied what happens to infrastructure during earthquakes around the world and is co-director of the Center for Interdisciplinary Natural Hazard and Resilience Research and universities and a member of the University of California Seismic Advisory Board. In 2020, Stewart was awarded an Erskine Fellowship from the University of Canterbury in Christchurch, New Zealand, where he collaborated with researchers on studies of the ground motion of earthquakes and soil liquefaction.

Below are excerpts from Stewart’s interview on the show, with edits for brevity and clarity.

Q: Why do earthquakes happen A: Tectonic plates are the underlying cause of earthquakes. Essentially, Earth is still a relatively young planet, and it’s hot inside and that heat dissipates. There is a process called convection where the heat from the inside comes out – the surface of the planet – and then begins to travel horizontally. This hot material cools as it pulls the Earth’s surface (referred to as the “plate”) horizontally. These plates move in different directions on the Earth’s surface and when you have two plates moving relative to each other, that’s the plate boundary and you get seismic concentrations at that boundary.

Q: What should you do to protect yourself in an earthquake? “Drop” means that you are bent on your knees. “Cover” means that you take one hand and put it behind your head to sort of protect yourself if something falls, and you injure your arm, not your head. The “hold” is where you take your other hand and, if you can, grab a table leg, or something like that, to keep yourself steady.

Q: Can earthquakes be predicted accurately? A: When people say “expected”, sometimes what they ask for is a forecast like what can be provided for the weather. We can’t do that. It is not possible to predict “there will be an earthquake on Friday” or something like that. What we can do is identify fault locations that are likely to cause earthquakes, evaluate approximate rates and magnitudes of future earthquakes, and predict ground shaking and the effects of those events. These forecasts are for long horizons, and are expressed as the probability of a certain event or level of vibration being exceeded in a time frame such as 50 years or so.

Q: What does land liquefaction mean? A: Liquefaction is when soil loses its strength during an earthquake. It occurs when sandy soils with shallow groundwater are subjected to strong vibration. The pore pressure in the water goes up and the strength goes down, hence the ground can be subjected to significant deformations and the ground-based structures can overturn due to the loss of bearing capacity. The most famous recent example is Christchurch, New Zealand in 2011, which was severely damaged by widespread liquefaction.

Q: What is the “Ring of Fire” A: The “Ring of Fire” is around the circumference of the Pacific Plate where the relative motions of the plate occur. California is part of this ocean, with Los Angeles actually on the Pacific side of the plate boundary (the North American plate is to the east). In California, the Pacific plate is gradually moving to the northwest, and, as it happens, strains accumulate along the plate boundaries, which are released in the form of earthquakes. In other parts of the Ring of Fire, volcanic eruptions and large earthquakes occur as a result of the Pacific plate slipping under the continent, for example in the Pacific Northwest, Alaska, Japan, etc.

Q: Do any of the effects of climate change affect earthquakes the way hurricanes, droughts, etc. do? A: Climate change does not affect the generation of earthquakes. It affects many other types of natural disasters, but not earthquakes.

Riley de Young contributed to this story.

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