The beauty of garnet has been appreciated since the Bronze Age, as demonstrated by its use as a gemstone. But its utility extends far beyond the decorative: garnet is widely used to determine the temperatures and pressures a rock has been exposed to during metamorphism. However, there is yet more to learn from these remarkable minerals.

Credit: Figure reproduced from Viete et al. (Sci. Adv. http://doi.org/cmrt; 2018), AAAS.

Large, catastrophic earthquakes can occur in subduction zones. In this setting, the rocks undergo metamorphism at high pressures, but relatively low temperatures. As the seismic waves pass through, overpressure is released. After the earthquake, overpressure begins to develop again and garnet growth continues. These cycles of overpressure and dissipation can be recorded in garnets for individual earthquake events, note Daniel Viete and colleagues (Sci. Adv. http://doi.org/cmrt; 2018).

Akin to tree-rings as a store for a history of climate and environmental change, geochemical zonations in garnets lock in a record of earthquake-induced cycles of rising and falling pressure. Using a host of geochemical analyses and microspectroscopy, Viete and colleagues find that trapped within the major-element zoning of subduction-zone garnets from the Franciscan Complex, California, are geochemical indicators of four substantial pressure pulses, each of 100–350 MPa, within a geologically brief window of less than 300,000 years.

Our understanding of subduction-zone earthquake recurrence rates and magnitudes is currently somewhat limited, to the past 2.5 million years or so of Earth’s history, as we rely on the sedimentary record to identify the effects of subduction-zone seismicity. The chemical zoning in garnets, here dating back at least 145 million years, may therefore allow the paleoseismic record to be extended much further back in time. These gemstones may therefore help reconstruct the magnitude and reoccurrence rate of ancient megathrust earthquakes and the evolution of stress within the crust.