How do earthquakes start? A seismological investigation into earthquake nucleation and determinism

Thesis

Scientists continue to debate many questions about the beginnings of earthquakes. At the single event scale, what processes go on in the earliest stages of rupture, and do they depend on magnitude? At a large scale, what processes cause earthquakes to occur when they do, and can foreshocks give insights into earthquake nucleation?

I begin by using short windows of seismic waveforms to examine the earliest parts of earthquakes around the world. I find that the predominant period, average period, and peak ground displacement scale with final magnitude even when calculated in windows just one-third of the earthquake duration. Meanwhile, the integral of the velocity squared scales with magnitude when calculated in windows just one-tenth of the earthquake duration. I propose physical origins for these relationships. These results suggest that small and large earthquakes begin differently and that earthquakes are deterministic.

Next, I investigate the differences between small and large earthquakes using source time functions (STFs), which show the time evolution of moment release. I find that larger earthquakes release more moment even in very short, early windows of time. I also find that STFs are generally complex with multiple peaks and have several clear phases of rupture. These STF properties have important implications for earthquake source modelling.

Finally, I investigate the extent to which earthquake-earthquake triggering can explain foreshock occurrence on the Hikurangi subduction zone, New Zealand. I compare new observations of seismicity to what is expected from an epidemic-type aftershock sequence (ETAS) model. I find that ETAS models explain the foreshock-to-aftershock ratio observed, but that the model underestimates the absolute number of foreshocks and aftershocks we observe and does not reproduce the power-law increase and decrease in seismicity we observe. Therefore, earthquake nucleation must be more complex, with stochastic triggering mediated by fluid flow or aseismic slip.

Authors: Colquhoun, RL

• DOI: 10.5287/ora-qmnn4b9zg
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: earthquakes; source time functions; earthquake mechanics; earthquake determinism; geophysics; seismology
• Subjects: Geophysics; Earthquakes; Seismology