Investigation of Earthquake Rupture Dimension Through Seismic Wave Interferometry

Abstract:

Earthquake source dimension is one of the most fundamental source parameters but remains difficult to be directly constrained. Many of the key questions in earthquake physics such as earthquake self-similarity are subject to heavy debate due to lack of reliable observations of earthquake source size. Taking advantage of the recent developments of large-scale regional seismic arrays (e.g. USArray) and seismic waveform interferometry, we propose to examine earthquake source dimensions through data-mining the waveform coherency as a function of inter-station distances. Systematic analysis of deep earthquakes show that relatively small earthquakes (M~6) are highly coherent across the USArray over inter-station distances >10 wavelengths and up to 4 Hz, indicating a minimal 3D structural effect on the waveform coherency. However, the inter-station coherence of M>7 earthquakes falls off with faster decay rates for larger magnitudes. For the same earthquake, the coherence pattern depends on the orientation of station pairs that can be explained by the directivity effect. We hypothesize that these patterns are governed by a finite source effect. We verified this hypothesis by establishing the analytical solution of inter-station coherency of a 1D rupture embedded in a 2D medium. We derived a multi-variable relationship to systematically measure the earthquake source dimension based on the coherency function. Rupture dimension, determined through back-projection studies, validates our case study for deep earthquakes in the sea of Okhotsk.. The coherency pattern revealed an elongated rupture on sub-horizontal fault plane of 87 km × 26km on the sub-horizontal fault plane of the for 2008 M7.8 earthquake, while a rupture of 10 km × 60 km occurred in the sub- vertical fault plane for of the 2013 M7.3 event. Future studies will explore the interstation coherency as a function of angular separation between stations. Establishment of a 2D coherency pattern may potentially further constrain other source properties, such as aspect ratio and rupture speed that remain difficult to determine by conventional approaches.