Reconstructing the damage field of the 1855 earthquake in Switzerland:: historical investigations on a well-documented event

The 1855 earthquake at Visp remains the strongest in the last 300 yr in Switzerland. It caused heavy damage in the region of the middle Valais. We reconstructed the damage field of this event studying also site effects and building vulnerability. The key factors in such a study are the availability and accessibility of sufficient historical data. Given the existence of a complete contemporary damage assessment and the availability of early statistics, our investigation drew upon an excellent pool of data. These sources enabled us to quantitatively analyse the damage field on the village level. Other historical sources described some of the losses with more detail but not across the whole area. These data were qualitatively analysed and then used to calibrate and verify the outcomes of the quantitative analysis. When it was possible to describe the damage fields of single villages, as with Visp, a high percentage of damaged buildings could be located so exactly as to foster further investigation. In addition we identified several secondary effects such as rock fall, landslides and changes in the subsurface. The observed damages support the thesis of a normal fault striking along the Valley of Visp (north-south) and dipping to the west.