Do earthquakes start with a precursory phase of slow slip? To investigate this question, we compiled all the GPS data recorded before large (Magnitude ≥ 7.0) earthquakes, projected them in the direction of expected displacements, and "stacked" (i.e. summed) everything. The result is a time series showing that on average the consistency with precursory slow slip increases in the 2 hours preceding the ruptures (Bletery and Nocquet, Science, 2023). This cannot be used to predict earthquakes but suggests that earthquakes do start with a precursory phase of slow slip on the fault. This signal is rather subtile though and this result will need to be confirmed by future studies.
A. Stack of 3,026 GPS time series recorded before 90 (Magnitude ≥ 7.0) earthquakes after projection in the direction of expected displacement in the 48 hours preceding each event (with a 5 min sampling). B. Moving average applied to the stack (each point of the curve is the average of the 11 points before and 11 after) reducing the noise level and showing a maximum in the last point (about twice as high as the maximum in the first 46 hours). C. Exponential fit to the stack, giving a 1.3 h time constance. D. Sinusoidal fit to the stack, giving a period of 12.9 h.
Some outreach articles: Science (en) The Scientific American (en), France Inter (fr), El País (es),