Source Seismic Parameters of the New Seismic Zone of Porto dos Gauchos - Amazon Craton Brazil

Porto dos Gaúchos seismic zone (PGSZ) is already a well-known seismogenic zone in terms of stress regime, fault location, physical dimension and structural characteristics. It is located in a Phanerozoic sedimentary basin; such 500 m thickness sediments overlie the crystalline basement of the Amazon Craton - Brazil. Recently we detected 15km away from the PGSZ area, another seismic activity that seems to be an extension of the PG fault.

The largest earthquake ever observed in the stable continental interior of the South American plate occurred on January 31, 1955 with magnitude 6.2 m_b,100 km to SW of PGSZ (in Serra do Tombador – ST). Since then no other earthquake has been located near the ST epicenter. However, in PG a recurrent seismicity has been observed since 1959. Two magnitudes 5 earthquakes occurred in 1998 and 2005 with intensities up to VI and V, respectively. These two main shocks were followed by aftershock sequences, both studied by local seismic networks lasting up today. Both sequences occurred in the same WSW-ENE oriented fault zone with right-lateral strike-slip mechanisms. The epicentral zone is near the northern border of Parecis basin, where there are buried grabens, generally trending WNW-ESE, such as the deep Mesoproterozoic Caiabis graben.

Not considering the possibility of miss location in the ST earthquake, its isolated occurrence - from the perspective of new studies on intraplate seismicity - lead us to think that the PGSZ was activated by stresses released by the earthquake of 1955 and that the seismogenic fault of ST would have closed a cycle of activity. This would explain its seismic quiescence.

The new seismic zone, with magnitudes up to 4, presents a relevant information about the power of earthquakes in PGSZ. The old fault of PGSZ, responsible for earthquake up to 5.2, has a length of 5km, and the new zone is located 15 away from PGSZ. If the new seismic zone is a prolongment of the PGSZ, this fault as a whole can produce bigger earthquakes that we are waiting for the region. So, the seismic hazard studies should take into account this new reality. Studies on seismic sources parameters that are been done has an important hole for the seismic risk analysis.