Seismic waves amplify inside sedimentary basins, creating dangerous conditions for cities built on flat terrain far from earthquake epicenters. Researchers have identified the specific mechanism behind this phenomenon, explaining why distant tremors pose genuine threats to basin cities.
When earthquakes rupture, they release seismic waves that travel outward. Most wave energy dissipates as it spreads across distance. But basins, which consist of softer sediment layered over harder bedrock, act as wave traps. The softer material slows waves down and reflects them upward, while the harder bedrock below bounces waves back up again. Waves bounce repeatedly between these layers, becoming trapped in what scientists call basin resonance.
This trapping effect amplifies ground shaking. Waves that would be minor vibrations in solid rock become powerful motions in basin sediment. The amplification occurs at specific frequencies determined by the basin's depth and sediment composition. When incoming seismic waves match these natural frequencies, resonance intensifies the shaking further.
Cities built on basins have learned this lesson repeatedly. Mexico City experienced severe damage from the 1985 earthquake centered 350 kilometers away because the city sits on a drained lakebed basin. Salt Lake City, built on ancient lake sediments, faces elevated risk from Wasatch Range earthquakes. Los Angeles, constructed partly on sedimentary basins, remains vulnerable to distant San Andreas Fault ruptures.
Understanding basin amplification has become essential for earthquake preparedness. Building codes now account for basin effects. Seismic monitoring networks map basin boundaries and measure sediment properties. Engineers design structures to withstand amplified shaking.
The research demonstrates that earthquake risk depends on geology as much as distance. A magnitude 6 earthquake hundreds of kilometers away can produce stronger ground motion in a basin city than a magnitude 5 nearby.