(Guest post by Shinji Toda)
To estimate local stress field, geologists collect on-fault slip direction (striation) and fault geometry at outcrops, whereas seismologists compile focal mechanisms of earthquakes occurred at depth. Stress tensor inversion, which is a pervasive methodology in both geology and geophysics communities, enables us to quantitatively estimate orientations of the principal stress axes and their relative magnitudes. But in reality we cannot seek local and regional stress fields straightforwardly. It is rather heterogeneous in space and scale dependent. During the inversion process, sampling areas (size) and their extents collecting striation data or fault plane solutions influence the result.
A typical situation showing scale-dependent stress heterogeneity is “slip partitioning.” It is used to describe oblique motion along a fault system that is accommodated on two or more faults with different mechanisms. Slip partitioning is observed at a variety of scales. Probably the largest case is accompanied with an oblique subduction (Figure 1). For example, the Philippine Sea plate underneath southwest Japan is accommodated not only by the megathrust earthquakes along the Nankai Trough but also by right-lateral slip along the Median Tectonic Line active fault system. Partitioned slip separated by several-tens-of-kilometer distance is also seen in many places (e.g., San Andreas fault and transverse range thrusts in southern California). But none of these cases did or would rupture simultaneously.
Toda et al. (2016) argue that a-few-km scale slip-partitioning occurred at the Kumamoto earthquake. It is indeed “coseismic slip partitioning.” About 10-km-long segmented fresh normal fault scarps (Figure 2), dipping to the north-west, mostly along the previously mapped Idenokuchi fault, emerged 1.2 to 2.0 km south of and subparallel to the right-lateral Futagawa fault. The maximum amount of coseismic throw on the Idenokuchi fault is ~2 m, which is nearly equivalent to the maximum slip on the Futagawa fault rupture. Interferogram fringe offsets in InSAR images also manifest the locations and slip motions of the 2016 rupture. Together with geodetic and seismic inversions of subsurface fault slip, Toda et al. illustrate a schematic structural model where oblique motion occurred on a north-west-dipping subsurface fault and the slip is partitioned at the surface into strike-slip and normal fault scarps. Simultaneous movements of strike-slip and dip-slip faults at a single earthquake were firstly reported at the Mw 7.8 2001 Kokoxili earthquake along the Kunlun fault. The Kumamoto case would be the second significant case around the globe but it is validated by rich geophysical data sets that are not available in the Kunlun case.
Again, coseismic slip partitioning during the Kumamoto earthquake provides us with a caveat against stress tensor inversion using local geological data. The coordinates of the principal stress axes near the Idenokuchi fault, which are favorable for normal faulting, are different from the ones along the Futagawa fault. A large group of samples from the entire 2016 rupture zone would reproduce a representative stress field with a mix of two types of fault slip. It demonstrates a good example of scale and depth dependency of the stress heterogeneity.
Another implication from the Kumamoto coseismic slip partitioning is how to properly evaluate a future large earthquake from grouping several active faults located closer each other. Probably co-existence of different type faults within several kilometers cannot simply exclude them being individual seismic sources. Instead, multiple fault strands, even different slip sense, may be joined together as a single seismogenic source at depth. For example, there are many such cases in Japan (Figure 3). So far we do not know if these possible simultaneous ruptures change the estimated size of a future earthquake. But it would broaden the areas affected by strong ground motion and damage zones by fault displacement at one earthquake.