To what extent do slow slip events impact megathrust earthquakes at subduction zones?
Research and case studies in favour
Sources
Research and case studies not in favour
Japan Trench (Tohoku-Oki) 2011
Ito, Yoshihiro, et al. “Episodic Slow Slip Events in the Japan Subduction Zone before the 2011 Tohoku-Oki Earthquake.” Tectonophysics, vol. 600, 16 Aug. 2012, pp. 14–26., doi:10.1016/j.tecto.2012.08.022.
Michel, Sylvain, et al. “Similar Scaling Laws for Earthquakes and Cascadia Slow-Slip Events.” Nature, vol. 574, no. 7779, 2019, pp. 522–526., doi:10.1038/s41586-019-1673-6.
Ikari, Matt J. “Laboratory Slow Slip Events in Natural Geological Materials.” Geophysical Journal International, vol. 218, no. 1, 2019, pp. 354–387., doi:10.1093/gji/ggz143.
Voss, N., et al. “Do Slow Slip Events Trigger Large and Great Megathrust Earthquakes?” Science Advances, vol. 4, no. 10, Oct. 2018, doi:10.1126/sciadv.aat8472.
Episodic slow-slip events occured a month prior to the M7.3 Earthquake (Foreshock)
These slow slip events continued until March 9th, 2011 (the day of the largest foreshock of the Tohoku-Oki earthquake)
The fault of the slow-slip EQ was located within the cosesimic slip area of the Tohoku-Oki EQ
Possible Model:
Slow-slip event started far from the trench (~40-70km)
The episodic events continued, eventually causing the rupture of the 2011 T-O EQ
^These episodes also triggered the largest foreshock (M7.3) in addition to accompanying moderate earthquakes
Key Terms
Coseismic Slip
Slow slip events
the slow release of energy over a fault line (when the plates @ a subduction zone become "unstuck")
Subduction Zone
Fault/Fault line
Focus
Hypocenter
Rupture
(*)Other prominent fault zones (Costa Rica, Cascadia, New Zealand, Barbados, San Andreas)
ruled out aseismic slip events (slip events that happen in the absence of seismic activity) due to the episodic seismic activity occuring for months before T-O EQ
Most SSEs occur offshore, where data is sparse and poorly recorded
SSE repeat times are short (<10 years), compared to earthquake recurrence timespans (30 - 500 years)
Geodetic network
Geodetic network design and strategies followed for drilling a 25 km tunnel for high speed railway in Spain - Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/Schematic-view-of-the-geodetic-networks_fig2_242600210 [accessed 21 Nov, 2020]
Earthquake nucleation
The process between the near-static and the dynamic rupture propagation of an earthquake fault.
(*) That study is based on slip/frictional instabilities. These include effects due to SSEs, aseismic slips, etc., whose 'enhancements', in many cases, are relatively small
Coulomb failure stress (CFS)
A study done on these areas showed that SSEs behave similarly to 'regular' earthquakes (in terms of peak slip velocity, magnitude, and nucleation).
SSEs near natural fault zones create stress/store elastic potential energy along fault lines
Summary, Conclusion, and Limitations
In many cases (such as Japan 2011), the episodic SSEs did foreshadow & magnify the earthquake that followed.
Mehek Mathur & Chucks Iluyemi
Group 17
On the other hand, many of the studies suffer from a lack of conclusive evidence as well as narrowing down the cause of megathrust earthquake
Therefore, from the evidence we gathered, we can say that slow slip events have a fairly large impact on megathrust earthquakes at subduction zones.
^This is not to say that SSEs always impact an earthquake. At times, they are coincidental/too far from the fault to affect the EQ.
We cannot be certain of our conclusions above, since we have had access to a limited perspective and cannot draw conclusions about an entire phenomenon from a few papers.
Moreover, owing to confirmation bias, we cannot say for sure whether the perspective we took with respect to this project was inclusive of all viewpoints.
Megathrust earthquake
Nicoya interseismic SSEs may allow fluid migration which can trigger an earthquake if the seismogenic zone is close to failure.
Research from Costa Rica Nicoya peninsula shows that spatiotemporal patterns of seismicity do not always track SSE behaviour.
Epicenter
Definitions will be explained during the presentation!
Earthquake forecasting from SSEs and foreshocks is difficult and not completely reliable.
Megathrust earthquake nucleation doesn't require SSEs - better measurements required.