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Earthquake Early Warning and the Physics of Earthquake Rupture


One of the great debates in seismology today revolves around the question of whether earthquake ruptures are self-similar, cascading failures, or whether their size is somehow predetermined at the start of the rupture. If earthquakes are self-similar there is theoretically no way to determine the magnitude of an event until the rupture has completely terminated, while if it is deterministic the magnitude should be immediately discernible. Recent advances in Earthquake Early Warning methodologies provide new insight into the fundamental physics of earthquake rupture and highlight the importance of understanding the answer to this question.

Observations of the amplitude and frequency content of early P-wave arrivals suggest that some information about the final size of an earthquake is already present within a few seconds of the initiation of rupture, in agreement with a host of other observations that show a degree of scaling between large and small earthquakes. While this suggests that earthquakes are deterministic, there is likewise a large body of work, both observational and model-based, that indicates that this is not true and earthquakes are self-similar.

This work documents the process of calibrating and testing the ElarmS Earthquake Early Warning methodology in northern California on the Northern California and Berkeley Digital Seismic Networks. In the process the work adds to the body of observations which show a dependency on event magnitude of P-wave frequency content and amplitude. These observations are corroborated with a new set of independent observations of kinematic slip distributions. These new observations indicate that the early slip on a fault also scales with magnitude and suggest again that earthquakes are not entirely self-similar cascading events.

In an effort to assign a physical mechanism to the observations of scaling, both in P-waves and in kinematic slip inversions, a hypothetical model is tested wherein the intensity of the early rupture imparts more or less energy to the rupture front and affects the likelihood of the rupture continuing or dying out in the face of unfavorable conditions further along the fault plane. The results of testing this hypothesis are somewhat equivocal, but they are suggestive of the likely truth, that earthquakes exhibit aspects of both deterministic and cascading rupture to some degree. Understanding the details of the interplay between these two aspects is crucial to the successful application of Earthquake Early Warning systems, especially in rare large earthquakes for which there is little empirical data on the performance of these systems.

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