Various fault slip behaviors along the Sumatran subduction zone

• "Sibling" earthquakes within persistent rupture barriers

The Sunda megathrust has a history of generating great ruptures that are segmented by persistent rupture barriers. Although persistent rupture barriers arrest great ruptures (M≥8), they are home to smaller but still large ruptures (7≤M<8). For example, the persistent rupture barrier underneath central Simeulue hosted two “sibling” earthquakes that had similar magnitudes (M_w 7.3 and 7.4) and occurred only six years apart (2002 and 2008). The two events coincide with a locked asperity surrounded by the otherwise partially creeping Simeulue section. The asperity is perhaps structurally controlled by a broad morphological high on the Sunda megathrust. [Paper 21: Morgan et al. 2017]

• Rupture style sometimes stays the same, and sometimes changes over earthquake supercycles

Between the persistent barriers, some segments have a history of generating through-going ruptures of a similar style. Some other segments, however, change their rupture style over time. For example, the Mentawai segment ruptured in only two great earthquakes (M_w 8.6-8.8 in 1797 and M_w 8.8-8.9 in 1833) during the last earthquake cycle, while it broke in a sequence of partial ruptures during the uncompleted modern cycle, which started with the 2007 M_w 8.4 Bengkulu earthquake and its two aftershocks (M_w 7.9 and 7.0) [Paper 14: Tsang et al. 2016]. The 2008 M_w 7.2 North Pagai earthquake notably produced several meters of slip right in the middle of the unruptured section, but did not cascade into a great rupture [Paper 20: Salman et al. 2017].

• Tsunami earthquakes on the shallow portion of the megathrust

The interseismic nature of the shallow megathrust is poorly understood; however, we know that the shallow portion of the Sunda megathrust is capable of generating earthquakes as shown by the 1907 Nias [Martin et al. 2019] and 2010 Mentawai [Hill et al. 2012] M_w 7.8 tsunami earthquakes. The Mentawai event was potentially promoted by increased stresses and strain rates due to the cumulative contribution from the coseismic ruptures and afterslip of the 2007 Bengkulu sequence [Paper 14: Tsang et al. 2016]. On the other hand, the preceding cumulative slip from the 2007 sequence might have caused a ~0.1 MPa difference in pre-earthquake Coulomb stress between the southeastern and northwestern portions of the 2010 Mentawai afterslip region, which likely modified the pattern of afterslip [Paper 19: Feng et al. 2016].

• Postseismic deformation prevails after M≥7 Sumatran earthquakes

Both afterslip and viscoelastic relaxation are required to explain the complex spatiotemporal patterns of the postseismic deformation. For the 2005 Nias-Simeulue [Paper 27: Qiu et al. 2019], 2007 Bengkulu [Paper 14: Tsang et al. 2016], and 2008 North Pagai events [Paper 20: Salman et al. 2017], afterslip not only occurred updip and downdip of the coseismic rupture zone but also partially overlapped the coseismic rupture. Afterslip following the 2010 Mentawai event was concentrated downdip of the coseismic rupture, and was unusually large and rapid with a large amount occurring within the same day of the event [Paper 19: Feng et al. 2016].

• Slow slip events (SSEs)

No clear evidence for SSEs has been found in a decade of GNSS data [Paper 12: Feng et al. 2015], but corals recorded a 15-year SSE spanning the period from 1966 to 1981 [Tsang et al. 2015].

Other references:

Hill, E. M. et al., The 2010 M_w 7.8 Mentawai earthquake: Very shallow source of a rare tsunami earthquake determined from tsunami field survey and near-field GPS data, Journal of Geophysical Research: Solid Earth, 117, doi:10.1029/2012JB009159, 2012.

Martin, S. S., L. Li, E. A. Okal, J. Morin, A. E. G. Tetteroo, A. D. Switzer, and K. E. Sieh, Reassessment of the 1907 Sumatra “Tsunami Earthquake” based on macroseismic, seismological, and tsunami observations, and modeling, Pure and Applied Geophysics, 176(7), 2831–2868, doi:10.1007/s00024-019-02134-2, 2019.

Tsang, L. L. H., A. J. Meltzner, B. Philibosian, E. M. Hill, J. T. Freymueller, and K. Sieh, A 15 year slow-slip event on the Sunda megathrust offshore Sumatra, Geophysical Research Letters, 42(16), 6630–6638, doi:10.1002/2015GL064928, 2015.