Seismic echoes in sedimentary basins can amplify earthquake shaking

The Hidden Danger Beneath Our Cities: How Sedimentary Basins Amplify Earthquakes

Sedimentary basins are depressions in Earth's crust caused by tectonic activity. Their often-flat surfaces make them preferred locations for building cities. However, during earthquakes, seismic waves can become trapped in these basins and reflect between sides, creating what scientists call "seismic echoes."

The shape and depth of a basin directly influence the amplification of these echoes, which can be highly destructive.

Wellington's Unexpected Shaking

Wellington, New Zealand, is built on a sedimentary basin. During the 2016 magnitude 7.8 Kaikōura earthquake, shaking in the central business district exceeded design predictions, causing severe damage to infrastructure.

Archival records reveal that during the 1942 magnitude 7.3 Wairarapa earthquake, approximately 10,000 chimneys were destroyed in Wellington.

New research provides an updated model for the central Wellington basin, finding it is almost twice as deep (about 500 meters) than previously thought, with a significantly different shape. These differences may explain the stronger-than-expected shaking.

A Deadly Historical Precedent: Mexico City, 1985

The 1985 Mexico City earthquake killed 8,000 people and destroyed high-rise buildings.

The epicenter was 350 kilometers west of the city, but moderate-amplitude waves became trapped in the basin's low-wave-speed sediments, becoming amplified.

The trapped waves created a standing wave, resulting in specific narrow zones of extreme destruction.

The Science of Amplification

Seismic waves amplify when moving from fast wave-speed solid basement rocks to low wave-speed sedimentary rocks. This is similar to a tsunami slowing and increasing in amplitude as it approaches shore.

Resonance occurs when the wavelengths of incoming seismic waves match the vertical and horizontal dimensions of the basin. Additionally, steep basin sides can create an edge effect, causing strong amplification near the basin edge.

New Insights into the Wellington Basin

The effective western edge of the Wellington basin is not the Wellington Fault, as previously assumed, but follows the line of the Terrace and Lambton faults.

Computer simulations using a 3D model of the basin for shaking at 0.7 Hertz predicted horizontal ground motion amplifications of 2.5-3 times background level adjacent to the western edge.

There was some correlation between predicted amplified shaking zones and locations of damaged buildings during the Kaikōura earthquake, though caution is needed as other factors may contribute.

What This Means for Urban Safety

Simple geophysical methods can map basin depth and shape in urban areas, allowing computer simulations to predict amplified shaking locations. This can lead to more granular zoning for vulnerable parts of cities.

There is a clear need for higher awareness of risk for cities on sedimentary basins from both local and distant earthquakes.