Abstract
In order to determine whether slip during an earthquake on the 26th September 1997 propagated to the surface, structural data have been collected along a bedrock fault scarp in Umbria, Italy. These collected data are used to investigate the relationship between the throw associated with a debated surface rupture (observed as a pale unweathered stripe at the base of the bedrock fault scarp) and the strike, dip and slip-vector. Previous studies have suggested that the surface rupture was produced either by primary surface slip or secondary compaction of hangingwall sediments. Some authors favour the latter because sparse surface fault dip measurements do not match nodal plane dips at depth. It is demonstrated herein that the strike, dip and height of the surface rupture, represented by a pale unweathered stripe at the base of the bedrock scarp, shows a systematic relationship with respect to the geometry and kinematics of faulting in the bedrock. The strike and dip co-vary and the throw is greatest where the strike is oblique to the slip-vector azimuth where the highest dip values are recorded. This implies that the throw values vary to accommodate spatial variation in the strike and dip of the fault across fault plane corrugations, a feature that is predicted by theory describing conservation of strain along faults, but not by compaction. Furthermore, published earthquake locations and reported fault dips are consistent with the analysed surface scarps when natural variation for surface dips and uncertainty for nodal plane dips at depth are taken into account. This implies that the fresh stripe is indeed a primary coseismic surface rupture whose slip is connected to the seismogenic fault at depth. We discuss how this knowledge of the locations and geometry of the active faults can be used as an input for seismic hazard assessment.
Highlights
For seismic hazard assessment it is important to know the locations and geometries of active faults, as the proximity of a location to an active fault is a key factor that determines the predicted degree of shaking (e.g. Roberts et al, 2004)
There are some examples of seismic hazard assessment in different tectonic settings that use active fault traces as an input for probabilistic seismic hazard assessment (PSHA), notably the Uniform California Earthquake Rupture Forecast (UCERF, Field et al, 2009), as well as PSHA for Taiwan (Cheng et al, 2007) and New Zealand (Stirling et al, 2002)
The results show that several of the hypocentral locations fall within the range of the down-dip projection of the surface trace of the fault from our structural measurements and postulated dips overlap within error (Fig. 7)
Summary
For seismic hazard assessment it is important to know the locations and geometries of active faults, as the proximity of a location to an active fault is a key factor that determines the predicted degree of shaking (e.g. Roberts et al, 2004). Surface effects of the three mainshocks were widely recorded in the epicentral region immediately after the earthquakes occurred (Basili et al, 1998; Cello et al, 1998; Vittori et al, 2000). Such effects included cracking of road and the ground surface, open fissures, alluvial scarps, landslides, and the appearance of a brown, soilcovered, stripe at the base of the bedrock scarp. Other authors argue that all surface effects are secondary (i.e. non-tectonic). Cinti et al, 2000 argue for broad NW-SE zones of deformation, comprising of surface breaks and landslides, these zones partially coincide with the active fault traces in Fig. 1. Basili et al (1998) argue that the stripe described formed due to compaction of debris and lower slope deposits, because their observations suggested that the direction of movement was parallel to the maximum slope direction
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