The solid-state nuclear track detectors have long been applied to assess the activity of tectonic faults. We measured the Radon (R n) concentration in soil gas on 09 profiles with a total of 245 measurement points in Thua Thien Hue area (Vietnam). Measurement results show that the Rn concentration in soil gas ranges from ~10 Bq/m 3 to 144,570 Bq/m 3. There is a clear difference in the Rn concentration on the granite rock of Dai Loc complex compared to the remaining rocks. The calculated background value and anomaly threshold of Rn concentration on the granite rock of Dai Loc complex and on the remaining rocks are 33,488 Bq/m 3 , 82,839 Bq/m 3 and 5313 Bq/m 3 , 24,850 Bq/m 3 respectively. Spatial distribution of Rn concentration in soil gas reflects the existence of faults since Rn concentration increases at the points above or near fault lines. The level of expression of fault activity is assessed by radon activity index (K Rn), which is calculated by the ratio of anomaly value to background value of Rn concentration. According to K Rn , in Thua Thien Hue area, only the Dakrong-Hue fault shows a strong expression of activity with high and very high K Rn (the maximum K Rn = 27.21). The Truong Son, Huu Trach River, Ta Trach River and Bo River fault zones do not show clear activity with low and medium K Rn. There is no Rn anomaly value recorded on the Rao Trang fault, demonstrating that it is likely to be inactive at present. The study results indicate that the Rn concentration in soil gas is a strong evidence of the existence of faults or tectonic fracture zones as the Rn concentration always increases at the points above or near the fault line.

This paper presents the results of soil radon gas measurement in three areas, including Thac Ba and Song Tranh 2 hydropower plants, and the planned locations of the nuclear power plants Ninh Thuan 1&2 using solid-state nuclear track detectors (SSNTD) with the aim of clarifying the activity of tectonic faults in these areas. The activity of tectonic faults was assessed through radon activity index KRn (the ratio between anomaly and threshold), which was divided into 5 levels as follows ultra-high (K Rn > 10), high (10≥K Rn > 5), high (5≥K Rn > 3), medium (3≥K Rn > 2) and low (K Rn ≤2). Soil radon gas measurement results showed that in the radon gas concentrations in the Thac Ba hydropower plant area ranged from 72 Bq/m 3 to 273.133 Bq/m 3 and maximum radon activity index KRn reached 9.75 (high level). High K Rn indexes show Chay River fault active in recent time and the sub-meridian distribution of Rn anomalies suggested a right-slip motion of the fault. Rn concentrations in the Ninh Thuan 1&2 areas ranged from 6 Bq/m 3 to 52.627 Bq/m 3 , however, the K Rn indexes were mostly low (K Rn ≤3) and the highest value was only 3.42, suggesting that expression of activity of the tectonic faults in this region is not clear, even no expression of fault activity. In the Song Tranh 2 hydropower plant and adjacent areas, radon concentrations ranged from 29 Bq/m 3 to 77.729 Bq/m 3 and maximum K Rn index was 20.16 (ultra-high level). The faults having clearer activity expression are Hung Nhuong -Ta Vy, Song Tra Bong and some high order faults, especially the northwest -southeast segments of these faults or their intersections with the northwest -southeast faults. In addition, the high values K Rn in the mentioned intersections can be evidenced for the activeness of northwest -southeast faults at the present time. The studies on active faults using soil radon gas method were performed in areas with very different geological and structural features, but the results are well consistent with the results of previous investigations obtained by other methods. It confirmed the effectiveness and capability of soil radon gas geochemistry applying to study active tectonic faults.

Using cornbinarion of geomorphologic, topographic, remote sensed images, structural features and tectonophysic analysis we have deterrnined 4 principle tectonic phases in Red River Cenozoic Basin and adjacent area  as follow (in the order from ancient to recent). The first tectonic phase was characterized by east - west extension. It can be remarked in many places of Ninh  Binh  province, in particular within Dong Giao limestone forrnation (T2 a dg), with numerous of large  sub-longitudinal extensional cracks, filled up by calcite up to meters in thickness. The event can be related to  one of the dextral strike slip of Red River fault zone and had participated in the earliest rift - forming process of region. The second tectonic phase had rnainly strike slip nature, with east-west cornpression, that marked as well by  cessation of regional rift - forming process in the region (the unconformity of 32-30 M.y.BP), as by onset of Cenozoic oceanic crust spreding of Eastern Vietnam Sea and by sinistral strike - slip of Red River Fault. The third tectonic phase, with maximum NE - SW compression, has caused some significant thrust activities along coastal zone and has stimulated an inverse uplift - shortening in NW part of Red River Basin in the Early - Midle Miocene, with the unconfornities 15.5 and 5.5 M.y.BP. The most recent tectonic phase that is revealed in study area by very recent NE - SW extensions, that creates  a numerous of narrow valleys of NW - SE orientation and causes a dextral and a sinistral movements of sub-latitudinal and sub-longitudinal accordingly. This extension can be considered as accompany of intensive  extension-subsidence in Red River basin, that contents of 4 km Pliocene - Quaternary sediment in the center.

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