## 4 Application to the L'Aquila 2009 Mw6.1 Seismic Sequence  
We tested the HYDE algorithm on a very well monitored real case: the seismogenic normal fault which caused the 2009 Mw6.1 L'Aquila earthquake in Central Italy.  
The L'Aquila earthquake sequence fits the seismotectonic context of the area well: seismologic, geodetic, DInSar and geological data clearly indicate that the Paganica-San Demetrio fault system (PSDFS) is the seismogenic fault of the Mw6.1 6 April 2009 main shock [14; 15; 16; 17; 18].  
The focal mechanism indicates a fault plane with an N140\({}^{\circ}\) direction and dipping about 45-50\({}^{\circ}\) to the SW [19], in agreement with the regional SW-NE extensional stress field active in the region. The seismogenic fault responsible for the 2009 l'Aquila main shock coincides with the surface expression of the PSDFS which bounds the northeast of the Aterno River valley (Figure 7). It shows complex surface expression, with several synthetic and antithetic plays affecting the Quaternary basin continental deposits [20; 21; 22; 23; 24].  
Before the 2009 earthquake, the geometry and activity of the PSDFS was only roughly known. It was mapped as an uncertain or buried fault [21; 22; 25; 26].  
During the 6 April main shock, this fault created a limitedoseismic surface displacement observed for a total length greater than 3 km, consisting of a complex set of small scarps (up to 0.10-0.15 m high) and open cracks with a direction in agreement with the focal mechanism of the main seismic events.  
Other discontinuous breaks occurred, with a trend ranging from N120\({}^{\circ}\) to N170\({}^{\circ}\), to the north and south of the main alignment of ruptures. In this case, the length of the surface faulting may exceed 6 km [18; 27].  
Figure 7: Location map of the 2009 L’Aquila main shocks: stars indicate M > 5.0. The red lines are the main active faults extracted from Galadini and Galli [28] and the Ithaca catalog [24; 29]. The bluebox is the projection to the surface of the \(\sim\)19 km long 2009 main shock causative fault according to several investigators ([30] and references therein). The red points are the 2009 l'Aquila seismic sequence relocated by [31]. The yellow dashed lines are the 2009 coseismic ruptures from [18].  
Other cracks are detected along the antithetic structures of Bazzano and Monticchio-Fossa. The seismicity distribution of the 2009 l'Aquila foreshock and aftershock sequence indicates that the epicentral area has a length of 50 km along the NW-SE direction defined by a right step en-echelon fault system, with earthquakes mostly confined below 2 km [14; 15; 31]. This suggests that only minor slips took place on the shallower portion of the fault, in agreement with surface observations.  
The 3D architecture of the Mw6.1 L'Aquila main shock causative fault has been accurately described by [31] by using a high-quality earthquake catalog composed of about 64,000 foreshocks and aftershocks that occurred within the first year, processed through automatic procedures. In particular, high-precision foreshock and aftershock locations delineated an about 16-18 km long fault (the Paganica fault), showing an almost planar geometry. The aftershock distribution around the main slipping plane varies from small (about 300 m) close to the main shock nucleation to large (about 1.5 km) at the south-eastern termination of the fault [32]. This along-strike change in fault zone thickness has been related to the heterogenous coseismic slip distribution along the fault and to an increase in fault geometrical complexity at the south-eastern fault termination [32].  
The heterogeneous 2009 coseismic rupture is well documented [33; 34]. The main shock nucleated at about an 8 km depth. At first, the rupture propagated up-dip with the coseismic slip located 2 km up-dip from the main shock hypocenter; then, it continued along the strike direction to the SE portion of the fault and at shallower depths [35].