GEER findings – source description (very preliminary)

The faulting associated with this earthquake is complex and is not yet well understood. We hope to have a working hypothesis for the faulting mechanism by the time we publish our preliminary report in early May. The relevant data sources include the aftershock pattern and its variation with time, moment tensor solutions of the mainshock and aftershocks, and SAR data.

We do know that the faulting is shallow (around 10 km and less). The mainshock moment tensor indicates normal faulting on approximatley a 45 degree dip (in contrast to approximately 65 degree dips of well-expressed normal faults in the region). There was a large April 7 aftershock (Ml 5.3) SE of L’Aquila. For awhile we thought this might be a triggered event, but we now think it is probably an aftershock given significant aftershock activity in that area prior to the April 7 event. Whether the large April 9 event (Ml 5.2) was an aftershock or a triggered event remains under debate.

INGV’s working hypothesis is that the main shock occurred on a relatively shallow dipping blind normal fault. Under this hypothesis, the faults that are expressed at the surface (e.g., Pettino fault) are secondary features that terminate at depth on the main fault.