عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Since the beginning of seismology, a hundred years ago, seismologists have always been hopeful to be able to predict earthquakes in order to help populations across the globe avoid destruction and casualties. However, earthquakes continue to occur without any warning.
Soil and groundwater gas variations due to the change in stress related to seismotectonic activity are well documented and used extensively in seismotectonic studies, including fault tracing and seismic surveillance as a precursor. Radon has a more sensitivity than other precursory gases and is considered as the most appropriate gas form precursor.
The first continuous radon monitoring station in Iran, for assessment of radon as an earthquake precursor, was established in September 2007, in Jowshan hotspring (N 30°09'38.7" – E 57°35'57.5"), Kerman Province, SE of Iran. Jowshan complex is composed of six springs, which outflow through fractured limestone, along the Gowk fault. This fault is stretched from the southwest of Bam Plain to the west of Shahdad Town which is well-known for its reservoir-triggered seismicity and has an active history. Occurrence of more than 20 earthquakes M > 5 in the 20th century (According to IIEES earthquake catalogue). As an example, Bam earthquake with M 6.6 (According to USGS data) and more than 25000 victims on December 26, 2003, represents this active history. In this study, in order to measure radon concentration in the spring water, an Alpha Guard PQ 2000 PRO (Genitron Instruments) detector was used. This instrument with a resolution of 1 Bq/m3 is able to measure 222Rn concentration from 2 to 2000000 Bq/m3. Detection of Alpha (α) particles produced by radon decaying in the ionization chamber depends on the instrument. This detector, like the most of the radon detectors, has measured the concentration of radon in the gas phase - not in the liquid phase. Therefore, at the beginning, the radon must be transformed from liquid phase to gas phase. The detector was setup in the Jowshan hotspring outlet as shown in Figure 1, schematically. In this system, water enters into an exchange unit which acts as a flow stabilizer. Gaseous radon that leaves the spring water by diffusion is pumped into the detector. This instrument has humid sensitivity. Therefore, the air contains radon cross from humidity absorption chamber (Silica Gel). In this study, a sodium chloride crystal was used as a humidity absorber. Radon was detected every 10 minutes by mentioned instrument and the results were saved into the internal memory of detector.
There are different experimental equations concerning the relation between earthquake magnitude and its effective range on geo-gas variations. According to Dobrovolsky’s empirical relation (D=100.43M) and the stress-strain theory, strain distribution radii (D), depend on the earthquake magnitude (ML). Therefore, anomalies due to earthquakes can indicate whether precursor monitoring station lies in the stress - strain field. Three spike like radon anomalies have been observed several days prior the M 4.9 Shahdad and M 4.3 Sirch earthquakes that occurred on May 11 and 12, 2009, respectively. Location of Jowshan hot spring in the stress - strain field of 11/05/2009 and 12/05/2009 earthquakes in Shahdad and Sirch respectively, can be ascribed to the radon concentration anomalies on 05/05/ 2009 and 09/05/2009.