Pre-seismic slip on the 26 September 1997, Umbria-Marche earthquake fault? Unexpected clues from the analysis of 1951-1992 elevation changes

We studied 1951-1992 elevation changes recorded by a first order leveling line that intercepts the surface projection of the 26 Sep. 1997, M-W 6.0, Umbria-Marche earthquake causative fault. The line documents 1951-1992 localized subsidence along a 12 km section above the fault. We calculated the expected 1997 coseismic elevation changes along the line using standard dislocation modeling and found that their trend has an amplitude three times larger than the trend of the observed pre-1997 signal but with a similar shape. We suggest that this signal is the result of 10 cm of pre-1992 sip along the northernmost 5 km of the 1997 earthquake fault, where coseismic slip was found to be less than the average estimated for the entire fault. This result implies unusually fast slip along this section of the fault and may suggest slip acceleration in preparation for the impending failure.