A massive arc of seismic activity—stretching from Australia to Japan, up toward Alaska, and down along the west coast of the Americas—is known among geologists as the Ring of Fire. Roughly 90% of the world’s earthquakes occur there.
In April 2017, a swift but powerful magnitude 7 earthquake hit off the coast of Valparaíso, Chile, near the base of the Ring of Fire. About 2 days before it began, scientists detected intense seismic activity in the region. In a new study, Ruiz et al. zeroed in on this earthquake to better understand the formation, or nucleation, of earthquakes, as scientists do not know exactly why or how earthquakes start, become larger, and stop.
Using data collected by a large network of GPS and broadband stations (suites of instruments taking seismic measurements at a broad range of frequencies) along the coast of Chile, the researchers studied the nucleation of the Valparaíso quake in great detail, as well as its rupture dynamic: the movement, deformation, and breakage of rock as an earthquake develops.
The researchers analyzed seismic data from the days leading up to the most intense point of the earthquake, or main shock. They also used a series of GPS readings from before, during, and after the main shock to determine that the earthquake was triggered by a slow-slip event, which is characterized by a slower velocity rupture in comparison with a regular earthquake. Essentially, the team created a detailed map of the Valparaíso seismic sequence, charting the landscape of seismic movement—from the first foreshocks to the main shock to the final aftershocks.
This study provides the first clear picture of the dynamics of a slow-slip earthquake in the central Chile zone, where the last tsunamigenic megathrust earthquake occurred in 1730. It is also one of the best records in existence of the nucleation phase of an earthquake. This is an important step toward understanding how earthquakes develop and could help scientists predict seismic events with more accuracy. (Geophysical Research Letters, https://doi.org/10.1002/2017GL075675, 2017)
—Sarah Witman, Freelance Writer