Seismogeodetic P‐wave Amplitude: No Evidence for Strong Determinism

Whether the final properties of large earthquakes can be inferred from initial observations of rupture (deterministic rupture) is valuable for understanding earthquake source processes and is critical for operational earthquake and tsunami early warning. Initial (P‐wave) characteristics of small to moderate earthquakes scale with magnitude, yet observations of large to great earthquakes saturate, resulting in magnitude underestimation. Whether saturation is inherent to earthquake dynamics or rather is due to unreliable observation of long‐period signals with inertial seismic instrumentation is unclear. Seismogeodetic methods are better suited for broadband observation of large events in the near‐field. In this study, we investigate the deterministic potential of seismogeodetically derived P‐wave amplitude using a dataset of 14 medium‐to‐great earthquakes around Japan. Our results indicate that seismogeodetic P‐wave amplitude is not a reliable predictor of magnitude, opposing the notion of strong determinism in the first few seconds of rupture. Plain Language Summary Rapid estimation of an earthquake's magnitude is key for the effectiveness of earthquake early warning. Large magnitude earthquakes can take minutes to fully evolve; whether information from the first few seconds of a seismogram is indicative of the final magnitude is therefore of significant interest for early warning operations as well as general understanding of seismic source processes. This study investigates these early rupture observations using a combination of seismic and GPS instrumentation. We find that observation of the first few seconds of rupture is not sufficient to reliably estimate the magnitude of an earthquake greater than magnitude 7.5. Key Points Seismogeodesy provides a more accurate P‐wave amplitude than high‐pass filtered seismic‐only methods due to its broadband sensitivity. Magnitude saturation from P‐wave characteristics is inherent to large earthquake dynamics and not due to instrumental limitations. P‐wave amplitude measurements are not predictive of final magnitude for medium‐to‐large magnitude earthquakes.