Abstract - It is well established that earthquake size (magnitude, seismic moment) scales with the dimensions of the rupture. Using a database of historical surface rupturing earthquakes from Greece and the USA, we suggest that the elastic strains associated with earthquakes exercise a critical control on earthquake scaling. As a consequence, large earthquakes and associated elastic strains cannot be accommodated in isolation on small faults or short fault segments, and must therefore be transferred onto nearby faults or segments, thereby providing a larger rupture length. This model is tested by comparing the coseismic slip/length ratios of earthquake ruptures and those of their component segments. Segments on which the strain was too high to be accommodated in isolation will have s/l ratios much greater than those of the entire rupture and of individual, non-segmented, ruptures. Of the data studied, the s/l ratios of individual segments are significantly higher than the s/l ratios of earthquake ruptures; more than 50% of the segments have higher s/l ratios than the highest values observed for earthquake ruptures worldwide. This is consistent with the premise that large slip events cannot be accommodated on short fault segments and also supports the notion that, even on earthquake timescales, individual fault segments are components of kinematically coherent arrays.
The database also highlights an unexpected characteristic of the scaling of active segmented arrays. Earthquake ruptures are often observed to terminate at structural discontinuities (e.g. relays, cross-faults, bends, gaps) within fault systems, structures that are commonly cited as being responsible for impeding or terminating earthquake ruptures. In agreement with previous studies (Wesnousky 1988, 1994; Zhang et al. 1999), our data indicate broad positive correlations between the width (i.e. measured perpendicular to strike) of discontinuities, on the one hand, and earthquake surface rupture length, displacement and magnitude, on the other. Surface ruptures are generally observed to stop at the largest discontinuities in the fault system, whilst often breaking through, or transferring between, smaller discontinuities (e.g. relays). Positive correlations are observed for two subsets of the data: discontinuities that lie at the ends of ruptures and those that lie within ruptures. There is remarkably little overlap of these subsets, implying that the maximum size of a discontinuity within a fault system that can be ruptured during an earthquake scales with earthquake size (rupture length, displacement, magnitude). There therefore appears to be a well-defined scale of sustainable discontinuity on active segmented arrays. We speculate that this scale could be linked to the combined effects of smaller-scale relay breaching and a limiting elevated elastic strain associated with active relays?
Abstract of talk given to:
Tectonic Studies Group Annual Meeting, Durham, January 2004