Laurie Dengler | Are there earthquake-free places? – Standard Times

Not many people like earthquakes. I have many friends who really despise them. The M6.4 Ferndale earthquake of December 20, 2022 was horrific and every smaller earthquake since then has inserted another needle into the wound of fear. This fear has become so strong that some of my friends are considering leaving the area.

Is it possible to find a place where there are no earthquakes? If you look at it long enough, the answer is no. Every part of the earth’s surface has been shaken at some point in the past and will happen again in the future. But some places are definitely more shaky than others.

Why do earthquakes happen? Most fifth graders will quickly say plate tectonics. I am glad that earth science is taught in elementary school and plate tectonics is involved but their answer is not entirely correct. The best answer is heat.

It is true that the majority of earthquakes on our planet are concentrated in relatively narrow areas. In the 1960s, geoscientists used this and other data to develop a theory of plate tectonics. The Earth’s surface is organized into a number of relatively stable or “rigid” regions that move relative to each other. Stress and seismicity are concentrated in the boundary regions between the plates.

Heat and gravity drive the formation and movement of plates. The interior of the Earth is hot, and heat is always directed toward the cooler surface. The most efficient way to get rid of heat on a planetary scale is convection. Convection allows heat to move quickly, like a pot of boiling water. It’s hard to think of “boiling” hard ground, but it does albeit very slowly, like cold molasses with plenty of time to slowly ooze.

Tectonic plates have been operating on Earth for billions of years, constantly rearranging the planet’s surface. The heat causes the continents to split and spread, creating new oceans. Seafloor rocks cool, and eventually gravity pulls them down, creating new subduction zones. The process is repeated over and over. Continents are capable of sinking, but they are constantly being rearranged – sometimes spreading out wide and other times coalescing into huge supercontinents and developing geological scars in the process.

The result is very different from the simple painting picture learned in elementary schools. Plate boundaries are not the simple lines shown in textbooks and can include areas hundreds or thousands of miles across. Stresses left over from faults and collisions in the distant geological past still lurk in the depths and cause earthquakes millions of years later.

The global seismic hazard maps (https://www.gfz-potsdam.de/en/GSHAP%20-%20Global%20Seimic%20Hazard%20Map) show a relative risk of strong shaking anywhere on the planet. They are based on historical earthquake patterns, active faults, and stress calculations. Not surprisingly, the Pacific Rim emerges as one of the highest seismic regions because it is almost entirely surrounded by subduction zones. The continent with the least risk is Antarctica, which isn’t too surprising since it is surrounded by divergent plate boundaries averaging over 1,000 miles from the coast. But low risk does not mean no risk. Four earthquakes of magnitude 7.5 on the Richter scale and larger have occurred near Antarctica in the past 50 years.

Antarctica is not a viable moving option. What about the United States? The USGS is mandated to compile and update earthquake hazard maps at least every five years (https://www.usgs.gov/programs/earthquake-hazards/hazards). Official maps are presented from where engineers use them to design buildings and plan infrastructure and most people quickly get lost in the technical language of maximum ground speed and probabilities.

A simplified picture of relative hazards shows the general vibration potential. For engineering purposes, the probability of shaking with enough force to damage structures within a 50-year time window is what is usually determined. Fifty years is a standard lifespan for structures and is incorporated into building codes.

The latest US 50 Years map was compiled in 2018 – a new map will be released later this year. A fifth-graders’ map of plate tectonics will only show plate boundaries along the western edge of the lower 48 states, so it’s not surprising that the west coast appears in bright colors indicating the greatest danger. But the map is more complicated. There is an evenly bright area roughly parallel to the California-Nevada border and the colors continue as far as the Rocky Mountains reflecting that some Pacific-North American plate motion is being absorbed in this entire region.

Even more surprising are the bulls of the Midwest and eastern part of the country. They reflect the sequences of two major earthquakes that occurred in the nineteenth century – the New Madrid earthquakes of 1811 and 1812 in Missouri and Arkansas and the 1886 Charleston, South Carolina earthquakes. There is also a belt of seismic activity that runs roughly the length of the Appalachian Mountains and extends into Canada.

Destructive earthquakes are much rarer than real earthquakes. I recently found a 2018 USGS map that estimated the chances of experiencing an Intensity III or stronger shake (noticeable by most people indoors) over the next year. There are some similarities between this map and the 50 Year Strong Vibration Map. California still appears as an above average place to feel an earthquake.

But there are surprises. The place most likely to experience modest shaking in the 2018 map was Oklahoma. And the map was almost correct. Between 2014 and 2017, Oklahoma experienced more earthquakes than anywhere else in the continental United States. The peak point was in 2015 when Oklahoma had earthquakes 3 times as powerful and larger than California.

These 1-year maps are strongly influenced by two factors that do not appear in the 50-year Strong Vibration Maps: induced earthquakes and shock sequences. Oklahoma bullseye occurs by injecting drilling tailings fluids into deep wells. The most dramatic example of a sudden increase in earthquakes that began in 2010 was Oklahoma. Most of the earthquakes were small but four were in the 5 magnitude range and caused some damage.

The USGS discontinued the one-year maps after 2018. A similar map today would look very different. Oklahoma began regulating injections in 2015 and seismic activity has dropped from over 800 magnitude 3 and larger earthquakes in 2015 to just 12 last year. The new aiming point will be in West Texas. We will also see higher areas where larger earthquakes have occurred recently including the Mendocino Tri-Link Area.

The message you take home? There are places with fewer earthquakes. North Dakota and Florida have had the fewest earthquakes in the past hundreds of years. But there is no place in the United States where the earthquake will never be felt. And every state has other threats, too — from winter storms to floods and hurricanes. Every state has experienced at least one federally declared natural disaster in the past five years. Given all the risk factors, I will continue to take my chances on the North Coast.

Laurie Dengler is Professor Emeritus of Geology at Cal Poly Humboldt, and an expert on tsunami and earthquake risk. Questions or comments about this column, or want a free copy of Living on Shaky Ground readiness magazine? Leave a message at 707-826-6019 or send an email to [email protected].

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