Holocene Activity of the Porto Alto Fault, Portugal

The Lower Tagus Valley area (LTV), where Lisbon is located has been struck by several destructive earthquakes whose sources remain still to be determined. The identification of surface ruptures in the area is a challenging task that requires the use of geophysical techniques. This paper focuses on the identification, for seismic hazard purposes, of faults segments of the Porto Alto fault, one of the most important structures in the LTV. High-resolution P-wave seismic reflection data was acquired to confirm the fault activity in the Holocene but the lack of resolution to detect a fault of maximum 2m anticipated vertical throw in the 50m thick alluvium column was evident. We revisited the site to acquire high-resolution S-wave seismic and GPR data. The seismic profile shows reflectors interruptions in the stacked section, changes in amplitude/shape of the reflection hyperbolae in the shot gathers and coincident low velocity anomalies, indicating the presence of a fault segment at the anticipated location below 25m depth. The GPR profile reaches a maximum depth of about 15m and does not show the presence of any fault. Other geophysical techniques and trenching investigation are under consideration for corroborating the activity of the Porto Alto fault.