Surface Rupture and Slip Distribution of the 1940 Imperial Valley Earthquake, Imperial Fault, Southern California: Implications for Rupture Segmentation and Dynamics

We analyzed high-resolution aerial photography taken soon after the 1940 Imperial Valley earthquake to provide a higher resolution distribution of displacement with which to test variation in lateral slip, to compare the difference in making slip measurements parallel to local or regional fault strike, and to compare to the seismological properties of the 1940 earthquake. We performed 648 new measurements of displacement along the 15 km section of rupture for which imagery exists. Nearly 7 m of maximum displacement occurred within 2 km of the border, which was higher than previous estimates, with an average slip of about 5.5 m for this section. There is considerable variation along the strike, on the order of 1 m of variability across hundreds of meters of lateral distance in the section of high slip; areas of larger offset tend to have greater variations in displacement. From these observations, we conclude that lateral variability of displacement typically varies by as much as 30% along a rupture section. The difference in measured displacement at a specific point along the fault varied considerably when using local fault strikes versus that of regional or average fault. However, the overall average difference between measurements for different azimuths at a specific point is only -0.01 +/- 0.19 m, indicating that this is not a major issue if one is consistent. Finally, abrupt changes in displacement along a strike are apparently related to segmentation and the rupture process, and mimic the subevents, as documented through seismological methods. The largest moment pulse corresponds to the section south of the border, whereas the greatest displacements occurred north of the border where the average moment release is inferred to be less. We explain this by inferring a strong, shallow asperity in the border region, consistent with 1979 rupture event beneath it.