ABUHRC ALERT 5: Paper highlights seismic threat to cities from low slip-rate faults
The Acropolis and Parthenon dominate the Athens skyline at night. Courtesy: Thermos.
A paper presented at the 14th World Conference on Earthquake Engineering, by ABUHRC researcher, Ioannis Papanikolaou, and co-workers, highlights the risk posed to cities in seismically-active areas, by low slip-rate faults. The slip-rate of a fault is a fundamental parameter that governs earthquake occurrence and which provides a vital clue to the seismic hazard in an area. For example, as slip rates decrease, so average recurrence intervals tend to increase. Due to their long recurrence intervals, therefore, earthquakes on low slip-rate faults are often absent from the historical catalogues and thus may be excluded from the standard seismic hazard assessment process. Where earthquake recurrence intervals on specific faults are on the order of several hundred to several thousand years, the length of the historical record for the area is often shorter. In such cases, geological data are vital in order to extend the history of the fault back in time, unravel the long-term slip history, and constrain the recurrence interval. This is particularly critical where the fault in question is close to a large urban centre.
Such a situation is notably applicable to the Athens region of Greece, where the low slip-rate Afidnai Fault borders the northern edge of the Athens Plain. Here, offset Pleistocene terraces have been used by the authors to calculate a long-term throw rate (amount of vertical deformation) of 0.1 to 0.3 mm a year on the Afidnai Fault. Based upon a worst-case scenario, the 14 km long fault can generate earthquakes up to magnitude 6.4 about every 2,000 years. As the completeness of the historical record in Greece, for such events, only extends back about 200 years, it is clear that this only represents a small fraction of the mean earthquake recurrence interval of the Afidnai Fault.
Given that the moderate (Surface Magnitude 5.9) 1999 Athens earthquake resulted from the rupture of an unknown fault, killing 143 people and causing 3.5 billion Euros of economic loss (~ 3 percent of GDP), the authors highlight the importance of geological studies to better constrain the true threat to Athens from low slip-rate faults, and to urban centres in other parts of the world where circumstances are comparable. Worryingly, they also note that the fact that faults, such as the Afidnai, have not been recently activated, suggests that they may be getting towards the end of the seismic cycle, so that a rupture could have a relatively high probability.
Papanikolaou, I. D., Papanikolaou, D. I. and Lekkas, E. L. 2008 Low slip-rate faults around big cities: a challenging threat. The Afidnai Fault as a case-study for the city of Athens. Proceedings of the 14th World Conference on Earthquake Engineering. October 12 – 17, 2008. Beijing, China. (download here)
For further information contact Ioannis Papanikolaou at: