Understanding Extreme Geohazards: The Science of the Disaster Risk Management Cycle

European Science Foundation Conference
November 28 to December 1, 2011, Sant Feliu de Guixols, Spain

Paleoseismology: reading the geological book in search for earthquakes and tsunamis of the past

Daniela Pantosti
Italian INGV, Seismology and Tectonophysics, Rome, Italy, daniela.pantosti@ingv.it

Extreme geohazards are the cause of major disasters; to face them and plan prevention/recovery actions, we are expected to know what we are really facing (size, extent, expected effects, frequency and probability, etc.). To get and use these information as a key for forecasting future occurrences, the approach starts from the characterization of the natural events of the past. For events occurring frequently (years or decades) the observation of the past few hundred years may be enough to produce a reference model of the threatening hazards, however, for events that are more rare (hundreds to thousands of years) we should explore the past with other tools. This is the case for earthquakes and tsunamis. The historical records of seismicity and tsunamis are traditional inputs to the description seismicity of the past. Although for some regions these records are longer than a millennium, they may not be sufficiently long to provide a full description, in fact, the seismic cycle is generally of the same length of the historical record or longer. To overcome this limitation, during the past decades a new discipline developed: paleoseismology. This is based on the principle that we can decipher and date the record of earthquakes of the past in the recent geology and geomorphology. The fundaments of paleoseimology sit on the fact that earthquakes leave permanent signatures in the landscape and geology (fault scarps, warps, drainage diversion, uplift, subsidence, liquefactions, tsunami deposits). As earthquake repeats these signatures accumulates and become the geological records of seismicity; for example, a buried fault scarp formed before the most recent one is the evidence for the previous earthquake on that same fault. Following the paleoseismological approach, integrated with the historical and instrumental data, the records of large earthquakes/tsunamis can be extended back in time through most of the Holocene. Paleoseismology is based on the integration of several disciplines (es. quaternary geology, tectonics, sedimentology, geochronology, etc.) and has lately increased its effectiveness because it benefited of the technological advancements in these fields, particularly in geochronology. The term paleoseismology is generally referred to inland studies. However, most part of our planet is covered by water and a lot of tectonic information is there. Because of this, in recent times, this discipline is looking also at the offshore for reconstructing both earthquake and tsunami records.