EARTHQUAKESImproving Bay Area Seismic Hazard Maps

Published 25 February 2022

Using the Santa Cruz Mountains as a natural laboratory, researchers have built a 3D tectonic model that clarifies the link between earthquakes and mountain building along the San Andreas fault for the first time. The findings may be used to improve seismic hazard maps of the Bay Area.

The Santa Cruz Mountains define the geography of the Bay Area south of San Francisco, protecting the peninsula from the Pacific Ocean’s cold marine layer and forming the region’s notorious microclimates. The range also represents the perils of living in Silicon Valley: earthquakes along the San Andreas fault.

In bursts that last seconds to minutes, earthquakes have moved the region’s surface meters at a time. But researchers have never been able to reconcile the quick release of the Earth’s stress and the bending of the Earth’s crust over years with the formation of mountain ranges over millions of years. Now, by combining geological, geophysical, geochemical and satellite data, geologists have created a 3D tectonic model that resolves these timescales.

The research, which appears in Science Advances Feb. 25, reveals that more mountain building happens in the period between large earthquakes along the San Andreas Fault, rather than during the quakes themselves. The findings may be used to improve local seismic hazard maps.

“This project focused on linking ground motions associated with earthquakes with the uplift of mountain ranges over millions of years to paint a full picture of what the hazard might actually look like in the Bay Area,” said lead study author Curtis Baden, a PhD student in geological sciences at Stanford University’s School of Earth, Energy & Environmental Sciences (Stanford Earth).

Bending and breaking
Geologists estimate the Santa Cruz Mountains started to uplift from sea level about four million years ago, forming as the result of compression around a bend in the San Andreas fault. The fault marks the boundary between the Pacific Plate and the North American Plate, which shift past each other horizontally in a strike-slip motion.

Measurements of deformation – changes in the shapes of the rocks – have shown that Earth’s surface warps and stretches around the San Andreas fault during and in between earthquakes, and behaves much like an elastic band over seconds, years and even decades. But that classic approach cannot align with geologic observational data because it doesn’t allow the rocks to yield or break from the stress of the warping and stretching, as they eventually would in nature – an effect that has been observed in Earth’s mountain ranges.