The dynamic motion of an earthquake begins abruptly, but is often initiated by a short interval of weak motion called the seismic nucleation phase (SNP). Ellsworth and Beroza [1995, 1996] concluded that the SNP was detectable in near-source records of 48 earthquakes with moment magnitude (Mw), ranging from 1.1 to 8.1. They found that the SNP accounted for approximately 0.5% of the total moment and 1/6 of the duration of the earthquake. Ji et al [2010] investigated the SNP of 19 earthquakes with Mw greater than 8.0 using teleseismic broadband data. This study concluded that roughly half of the earthquakes had detectable SNPs, inconsistent with the findings of Ellsworth and Beroza [1995]. Here 69 earthquakes of Mw 7.5-8.0 from 1994 to 2011 are further examined. The SNP is clearly detectable using teleseismic data in 32 events, with 35 events showing no nucleation phase, and 2 events had insufficient data to perform stacking, consistent with the previous analysis.

Our study also reveals that the percentage of the SNP events is correlated with the focal mechanism and hypocenter depths. Strike-slip earthquakes are more likely to exhibit a clear SNP than normal or thrust earthquakes. Eleven of 14 strike-slip earthquakes (78.6%) have detectable NSPs. In contrast, only 16 of 40 (40%) thrust earthquakes have detectable SNPs. This percentage also became smaller for deep events (33% for events with hypocenter depth>250 km). To understand why certain thrust earthquakes have a visible SNP, we examined the sediment thickness, age, and angle of the subducting plate of all thrust earthquakes in the study. We found that thrust events with shallow (600 m) on the subducting plate tend to have clear SNPs. If the SNP can be better understood in the future, it may help seismologists better understand the rupture dynamics of large earthquakes.

Potential applications of this work could attempt to predict the magnitude of an earthquake seconds before it begins by measuring the SNP, vastly improving earthquake Early Warning Systems for populated areas.