Characterizing large earthquakes before rupture is complete

Whether large and very large earthquakes are distinguishable from each other early on in the rupture process has been a subject often debated over the past several decades. Studies have shown that the frequency content of radiated seismic energy in the first few seconds of an earthquake scales with the final magnitude of the event, implying determinism. Other studies have shown that the recordings of ground displacement from small-to-moderate sized earthquakes are indistinguishable, and thus earthquakes share a universal early rupture process. Regardless of how earthquakes start, however, at some point in the rupture process events of different sizes must be distinguishable from one another. If that difference occurs early - i.e., before the rupture duration of the smaller event - this implies that earthquakes demonstrate some level of determinism. Here we show through analysis of a large database of source time functions and near-source displacement records that after an initiation phase, ruptures of M7-9 earthquakes organize into a slip pulse, the kinematic properties of which scale with magnitude. As such, early in the rupture process - after about 10s - large and very large earthquakes demonstrate different properties and can thus be distinguished.