How does lithospheric viscosity control the temporal patterns of seismicity during an earthquake cycle?

Abstract

The temporal patterns of regional seismicity, such as acceleration and deceleration patterns of seismic moment release, Benioff strain release, or number of earthquakes over time before a big earthquake or between two big earthquakes, have been observed in various regions. However, the mechanisms or controlling factors to cause these temporal patterns of seismicity have not been fully clarified. In this study, we collect, summarize, and analyze temporal patterns of regional seismicity (acceleration and deceleration patterns) in the world and observe a statistical relation between them and regional lithospheric viscosity values. We also develop and use a three-dimensional viscoelastoplastic finite-element model and an analytical model, to investigate and explain the importance of lithospheric viscosity on temporal patterns of seismicity during an earthquake cycle. We find that viscosity in lower crust and upper mantle can control seismicity acceleration and deceleration patterns during an earthquake cycle: low viscosity tends to cause deceleration pattern because fast postseismic stress accumulation within upper crust due to fast stress relaxation in viscoelastic layers following an earthquake induces frequent occurrence of regional earthquakes at early stage of an earthquake cycle, but on the contrary, high viscosity encourages acceleration pattern with frequent occurrence of regional earthquakes at late stage of an earthquake cycle. Between the low and high viscosity, a transitional viscosity zone roughly ranging from ∼1 × 1019 to ∼1 × 1020 Pa s exists. When average viscosity in lower crust and upper mantle stays in this range, regional seismicity acceleration and deceleration patterns can coexist at similar percentages and it is statistically hard to distinguish which pattern dominates in the region. Regional tectonic loading velocity also has an impact on temporal patterns of seismicity during an earthquake cycle: slow tectonic loading velocity tends to induce seismicity deceleration pattern, but fast tectonic loading velocity encourages seismicity acceleration pattern.