Using geoid height changes to study the thermal evolution of a sedimentary basin and possible relation with earthquake occurrence in the region

Geoid height values obtained in 1984, 1996 and 2008 were used to study the thermal evolution (considering density variations caused by temperature alterations) of a sedimentary basin located in the central eastern part of the Atlantic Ocean region. Historical and registered earthquakes have been detected in this basin.

The results obtained show a heterogeneous basin with increases/decreases in density (temperature) values occurring in the time interval between measurements and points with identical values in different years, separating regions of warming from regions of cooling. It is also observed that the maximum values obtained increase from 1984 to 1996 and 2008, occurring at different latitudes.

The minimum values obtained in 1984 are clearly higher than values obtained in 1996 and 2008 at same latitudes. The minimum values obtained in 2008 are higher than those of 1996 in latitudes between 35.8 and 36.2 N and also for latitudes equal to or greater than 36.6 N. At intermediate latitudes, the values obtained in 2008 are lower than those obtained in 1996.

Climate data presented on IPMA website show high values of precipitation data occurring in 1996 in months with lowest temperature values in Mainland Portugal, suggesting that the low values of temperature found may be related with infiltration of cold water and to an increase of water pressure in depth.

In the present work, special attention is given to the western boundary of the basin, where it is possible to observe high temperature values associated with lateral cooling of seamounts linked to cooling in the sedimentary basin, and a consequent increase in temperature in the inner part of the seamount. The location of 3 earthquakes recorded in May, July, and August 2005 showed  that they occurred near points without changes, separating a warming area (on the West side) from a cooling area (on the East side). The earthquakes are located in the warming area.

The analyzed data show that the region under study experienced warming in the past and is now in a heterogeneous cooling phase. Areas in a warming phase can be identified with the 2008 geoid height values, after been cooled in 1996.

Climate data was used to identify temporal relationships between geoid height values and precipitation and temperature values in mainland Portugal.

Earthquakes with magnitude greater than 4.0 were identified in the region in 2005. They are located close to the crossing points of geoid height values between 1996 and 2008, which separate areas under heating from areas under cooling, giving rise to different horizontal thermal and pressure gradients in the western and eastern side of the point with no changes in density (temperature) and possible contribution to the occurrence of earthquakes.