The question of which city is most likely to experience a strong earthquake is a matter of calculating probability, not making a specific forecast. A “strong” earthquake is defined as one with a Moment Magnitude of 6.0 or greater, capable of causing significant damage in populated areas. Seismologists assess seismic hazard, which is the likelihood of intense ground shaking occurring in a given time period, rather than predicting the exact timing of a future event. This assessment combines geological history, fault mechanics, and resulting ground motion effects to determine the probability of a damaging quake.
Geological Factors That Determine Earthquake Likelihood
The primary determinant of earthquake likelihood is a city’s proximity to a tectonic plate boundary, where the Earth’s lithospheric plates interact. Most major earthquakes occur along these boundaries, which can be convergent (colliding), divergent (separating), or transform (sliding horizontally past one another). Transform boundaries, such as the one that hosts the San Andreas Fault, are especially prone to frequent, shallow, and highly damaging strike-slip events.
The risk assessment of a specific fault system depends on its historical behavior, including the slip rate and the recurrence interval. Slip rate is the speed at which the two sides of a fault move past each other, dictating how quickly stress builds up in the crust. The recurrence interval is the average time between major earthquakes on that particular fault, determined by paleoseismology, the study of prehistoric earthquakes.
A location’s geology also plays a role in determining how much damage a city might sustain. Loose sediments and soft soils can significantly amplify seismic waves, causing much more intense ground shaking than areas built on solid bedrock. This amplification effect can turn a moderate earthquake into a highly destructive event for a city built on ancient lakebeds or artificial fill.
North American Cities Facing the Highest Seismic Risk
The highest probability for a strong earthquake in North America is concentrated in three distinct geological settings. The Pacific Coast, particularly the urban centers of Southern California and the San Francisco Bay Area, faces a threat from the San Andreas Fault system. The southern segment of the San Andreas Fault is capable of generating a great earthquake, potentially reaching magnitude 8.0, posing a risk to the greater Los Angeles area. The Bay Area is also threatened by several parallel active faults, including the Hayward and Calaveras faults, which are accumulating stress as the Pacific Plate grinds past the North American Plate.
Further north, the cities of Seattle and Portland are threatened by the Cascadia Subduction Zone, a fault stretching from northern California to British Columbia. This zone is capable of producing a megathrust earthquake, potentially exceeding magnitude 9.0, which would be accompanied by a destructive tsunami. Geological evidence suggests this fault system has a recurrence interval of several hundred years for great quakes, with the last major event occurring in 1700. Current models estimate the probability of a magnitude 8.0 or greater event in the next 50 years to be 10 to 15 percent.
The New Madrid Seismic Zone (NMSZ) affects cities like Memphis and St. Louis in the central United States. This is an intraplate zone, far from a tectonic plate boundary. The zone was the site of a series of powerful earthquakes in 1811 and 1812, estimated up to magnitude 7.7, which caused the Mississippi River to temporarily flow backward. While rare, the potential for a strong NMSZ quake remains, and the deep, soft sediments of the Mississippi River valley would amplify shaking over a vast region.
Comparative Global Risk: The Most Threatened Urban Centers
The risk profile for a city is highest when intense seismic hazard overlaps with high population density and vulnerable infrastructure. Several global urban centers face a higher combined risk than any in North America, largely due to their position on the Pacific Ring of Fire or major plate boundaries. Tokyo, Japan, is situated near the complex intersection of four tectonic plates. Despite having some of the most stringent building codes in the world, the concentration of population and economic assets makes the potential for loss immense.
Istanbul, Turkey, faces a threat from the North Anatolian Fault, a major strike-slip fault that runs just south of the city. Seismologists have noted that seismic activity is progressing westward along this fault, increasing the probability of a major event striking the city’s densely populated, older districts. Similarly, Mexico City is built upon the soft sediments of a dried lakebed, which amplifies seismic waves traveling from subduction zone earthquakes off the Pacific coast.
Other cities situated on active fault systems and with vulnerable infrastructure include Tehran, Iran, which sits atop several major fault lines, and Kathmandu, Nepal, which lies in the Himalayan seismic zone. These global centers often face compounded risk due to rapid, unregulated urban development and the presence of many structures not built to modern seismic standards.