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| Titel |
Soil radon as a possible earthquake precursor: Preliminary results from Ileia (Greece) |
| VerfasserIn |
Ermioni Petraki, Dimitrios Nikolopoulos, Anna Louizi, Athanasios Zisos |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250037560
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| Zusammenfassung |
| Radon (222Rn) is a naturally occurring radioactive gas which is directly produced by the
decay of the 238U series. It is significant for the studies of Earth, in hydrogeology and
atmosphere. Radon is used as a trace gas due to the long half-life (3.82-days) which allows
migration at long distances. In addition, it is an alpha emitter, fact which enables detection of
low levels of radon. Anomalies of radon impending earthquakes of a variety of magnitudes
have been observed in soil gas, ground- and thermal-waters and in underground tunnels.
Increasing is the scientific interest in this field during the last two years. However, the
majority of the published papers refer to data of rather long time intervals between sequential
measurements (~2-4 weeks).On the other hand, it is justified, both on laboratory
and geophysical scale, that when a heterogeneous material is strained acoustic and
electromagnetic (EM) emissions occur in a wide frequency spectrum, ranging from very low
to very high frequencies. These emissions are considered as precursors of general
fracture.
In the search of soil radon as a possible earthquake precursor, a station for quick and
continuous monitoring of soil radon has been installed in a very active tectonic site in Greece
(Ileia, Peloponnese, SW Greece). The monitoring site is Kardamas Ileias, located 3 km south
from Amaliada which is the second highly populated city. The instrumental and felt
seismicity of Ileia is dominated by extensional active seismicity structures (e.g. Alfeios,
Neda, Melpeia, Kiparissia-Aetos) and has shown more than 600 earthquakes of magnitude
greater than 4.0 R in the last 100 years. Two earthquakes were very destructive (5.8 R on
26/3/93 and 6.8 R on 8/6/08 respectively). The station consists of a high precision active
instrument (Alpha Guard-AG, Genitron Ltd.), equipped with an appropriate unit designed for
pumping and measurement of radon in soil gas (Soil gas Unit, Genitron Ltd.). Soil radon is
continuously pumped into AG at a rate of 1 L/min. Pumping is performed via a 1-m
soil probe to minimize meteorological influences and a 25-m radon proof 25-mm
tube to avoid simultaneous measurement of soil 220Rn. Proper dust and moisture
filters are employed. Radon is monitored every 10 minutes. This interval can be
reduced to 1 minute, however with lower accuracy and data storage capacity. For
comparison purposes, calibrated passive radon dosimeters based on CR-39 Solid State
Nuclear Track Detectors (SSNTD’s) were periodically installed and exposed to soil
radon in 50 cm holes were dug near the 1-m probe. The exposures lasted 1-2 weeks.
Afterwards, the SSNTD’s were removed, etched and measured via standard methods
(optical microscopy track counting). The period of comparison measurements was 6
months.
Continuous monitoring and passive measurements were cross-calibrated and found to
provide similar estimates of mean soil radon concentration. Active techniques are much more
precise and quick, however, they indicated the necessity of periodical checks for the pumping
and measurement status, especially after strong rainfalls.The mean soil radon concentration
was found fairly constant (~ 25-30 kBq m-3). Numerous soil radon concentration anomalies
were detected. These were arbitrarily corresponded in terms of magnitude and duration to
seismic events of the near area. All detected anomalies were sudden, significantly
(p |
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