THE USE OF RADIOMETRIC TECHNIQUES TO ESTIMATE THE TIME OF PAST EARTHQUAKES: A CASE STUDY AT THE SPARTA FAULT, PELOPONNESUS, GREECE

• D. Papanastassiou (Institute of Geodynamics, National Observatory of Athens, P.O. Box 20048, GR-11810 Athens)

Destructive earthquakes are produced by the rupture of big faults, after long periods of inactivity that may last even thousands of years. Through the use of suitable quantitative geomorphologic analysis, such faults can be identified and characterized, while by using different dating techniques the time of occurrence of previous events could be determined. The combined use of these techniques provide information on repeated seismic cycles occurred over a time interval of several thousand years whereas historical and instrumental seismicity usually span a period that is only some centuries.

The reconstruction of the seismic history of individual fault it is a significant supplement to the record of seismicity, is used to forecast the future seismic activity of the fault and becomes an essential tool in modern Seismic Hazard Assessment studies.

The eastern front of Taygetos mountain is an impressive landform, delineated by an active normal fault composed of several segments. The southern one, having a length of 20Km is known as the Sparta fault and is located just 5Km west of the famous city of Sparta. Detailed morphotectonic observations on this suggest that it should be active at least during Quaternary. This area has been characterized by low seismicity for the last 25 centuries. However during the 6th and 5th BC centuries, several destructive earthquakes have been reported. That of 464BC was the most destructive and devastated the whole city of Sparta.

In order to determine the time of past earthquakes, a palaeoseismological investigation of the Sparta fault was conducted. This work included the following steps:
1. trenching along the southern part of the fault, studying and dating with ¹⁴C the sedimentary layers exposed at the trench walls.
2. dating with ¹⁴C the destruction layers exposed in archaeological excavations inside the modern city of Sparta,
3. assessing the earthquake slip history at the central and northern part of the Sparta fault, by determining ³⁶Cl exposure ages as a function of height on the scarp. 36Cl is produced primarily through interactions of cosmic ray and secondary by neutrons and muons with Ca in the scarp limestone. Since the upper parts of the scarp have been exposed longest they have the highest ³⁶Cl concentrations, while the concentrations decrease toward the base.

Our ³⁶Cl results indicate that during Holocene, the central part of Sparta fault ruptured at 4.5 ka, 5.9 ka, 8.4ka and 12.9ka ago, while the northern part at 2.8ka, 4ka, 4.5ka and 5.9 ka. The age of the most recent earthquake is that of 2.8±0.3Ka, consistent with the 464BC date of the destructive Sparta earthquake. The results obtained from ¹⁴C dating show that the southern part of the fault reactivated during the 464BC event and about 990-1165AD, while the destruction layer inside the archaeological excavations indicates that a damaging event occurred at Sparta sometime in the 6th - 7th century AD.

The obtained dates of the palaeoearthquakes expanded our seismic record for strong events prior the 25^th BP century and recognized medium size events during the historical times.