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 l ine that intercepts the surface projection of the 26 Sep. 1997, M-W 6.0, Um bria-Marche earthquake causative fault. The line documents 1951-1992 locali zed subsidence along a 12 km section above the fault. We calculated the exp ected 1997 coseismic elevation changes along the line using standard disloc ation modeling and found that their trend has an amplitude three times larg er 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 t he 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 res ult implies unusually fast slip along this section of the fault and may sug gest slip acceleration in preparation for the impending failure.