How well do surface slip measurements track slip at depth in large strike-slip earthquakes? The importance of fault structural maturity in controlling on-fault slip versus off-fault surface deformation

- Surface slip localization in large earthquakes depends on fault structural maturity.
- Surface slip measurements underestimate slip at depth in a predictable manner.
- Slip rates from immature faults may underestimate the true rate by as much as 30-40%.

Comparisons of observed surface displacements with geodetically inferred slip at depth in six large (Mw⩾7.1) strike-slip earthquakes reveal a correlation between the structural maturity of the fault and the ratio of deep slip to surface slip that occurs on localized zones of surface rupture. Specifically, structurally immature faults (⩽25 km of total displacement) manifest only ∼50-60% of total slip on narrow fault surface traces versus ∼85-95% for structurally mature faults (⩾85 km total slip). The same pattern holds when structurally simple parts of a surface rupture are analyzed separately from parts that exhibit obvious structural complexity (e.g., discontinuous or multiple traces or significant dip slip). These results imply that geologic measurements of surface slip along structurally immature faults are likely to significantly underestimate the true slip at depth in large earthquakes. This observation has implications for a number of important problems, including determination of fault slip rates, which are based on surface offsets; earthquake probability assessments, which are based on geologic fault slip rates; comparisons of geologic and geodetic rate data in the search for strain transients; the structural evolution of fault zones; estimation of paleo-earthquake magnitudes based on geomorphic offsets; analyses of the relative importance of faulting vs. distributed deformation in accommodating relative plate motions in continental crust; the design and construction of infrastructure built near active faults; and possibly for the prediction of strong ground motions, which may at least partially depend on the degree of slip localization to the surface.