Taiwan is tectonically situated in an oblique collision zone between the Philippine Sea Plate (PSP) and the Eurasian Plate (EP). Continuous observations of gamma rays at the Yangmingshan (YMSG) station and soil radon at the Tapingti (TPT) station were recorded in the volcanic area and around a major fault zone, respectively, in Taiwan for seismic studies. A number of anomalous high gamma ray counts and radon concentrations at certain times were found. It is noted that significant increases of soil radon concentrations were observed and followed by the increase in gamma rays a few days to a few weeks before earthquakes that occurred in northeastern Taiwan. Earthquakes such as these are usually related to the subduction of the PSP beneath the EP to the north along the subduction zone in northern Taiwan (e.g.,
The island of Taiwan is the result of an oblique collision zone, with a convergence rate of about 8 cm/yr in the direction of 306° [
The simplified sketch of the regional map and principal tectonic structures around Taiwan shows that the Philippine Sea Plate (PSP) is moving northwest at 8 cm/yr towards the Eurasia Plate (EP). The circle symbols represent the earthquake epicenter with the focal depths corresponding to different colors from July 1, 2014, to June 1, 2015. The open square represents the location of the YMSG gamma ray station and the TPT radon station. The red lines indicate the active fault proposed by the Central Geological Survey of Taiwan [
However, Yang et al. [
Radon (222Rn) is the major radioactive gas in this volcanic area. To investigate volcanic activity in the volcanic areas, the monitoring of radon level variations has been considered as a useful tool [
Radon is a naturally radioactive noble gas, widely distributed in various places throughout the Earth’s crust. Radon is generated in the uranium decay series. The emission of energy from radon can be in the form of alpha or beta particles and gamma rays. Thus, gamma radiation can be used for indirectly determining the radon concentration.
The YMSG monitoring station is equipped with a gamma ray spectrometer with a scintillation counter inside a container made of 7 cm thick lead, inserted in a High-Density Polyethylene (HDPE) tube at a depth of 2 meters (Figure
(a) Schematic of the gamma ray sensor installation in the YMSG monitoring station. (b) The block diagram and pictures of the gamma sensor assembly.
The methodology of the operation at the TPT radon station follows that as described in detail by Fu et al. [
The gamma ray results along with meteorological data recorded on an hourly basis at the YMSG station were presented in Figure
Temporal variations of gamma rays at the YMSG station (a) and (b) atmospheric data, including atmospheric pressure (green), humidity (blue), temperature (red), and hourly rainfall (black) (b) from July 1, 2014, to June 1, 2015.
The temporal variations of (a) radon and gamma rays from July 1, 2014, to June 1, 2015. Blue and red lines indicate the 24-hour running averages of radon and gamma ray data, respectively; grey lines represent the raw data. Horizontal dashed lines indicate the annual averages of the observed results, and the anomalous threshold value (±1
Some earlier investigations have revealed that changes in gamma rays may be affected by meteorological factors, such as atmospheric pressure, humidity, temperature, and precipitation [
Correlations between gamma ray variations and meteorological parameters, including atmospheric pressure (
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|
|
|
---|---|---|---|
2014/07 | 0.2306 |
0.0339 | 0.0064 |
2014/08 | 0.1056 | 0.0585 | 0.0504 |
2014/09 | 0.1105 | 0.0253 | 0.0091 |
2014/10 | 0.021 | 0.0095 | 0.0041 |
2014/11 | 0.0316 | 0.0124 | 0.0267 |
2014/12 | 0.249 | 0.0171 | 0.1124 |
2015/01 | 0.1932 | 0.0285 | 0.3428 |
2015/02 | 0.0082 | 0.0053 | 0.0405 |
2015/03 | 0.0004 | 0.1215 | 0.3099 |
2015/04 | 0.2202 | 0.0326 | 0.3606 |
2015/05 | 0.1838 | 0.02 | 0.3988 |
Fu et al. [
The temporary, abrupt increase in gamma ray was observed periodically, which may be related to heavy rainfall or precipitation accumulation. This sudden increase in the gamma dose was usually followed by a rapid return to background levels within a few hours after the rain stopped. These observations suggest that short-term (few hours) fluctuations in the rise of gamma rays were partially influenced by a heavy precipitation event, as has been similarly described before [
For the TPT station, Fu et al. [
The time series gamma ray data from YMSG was plotted with seismic events, rainfall records, and the radon concentrations of TPT for a period from June 1, 2014, to June 1, 2015, in Figure
The magnitude and depth of the earthquakes were plotted in Figures
Catalog of gamma ray and radon anomalies and related earthquakes occurring in Taiwan from July 1, 2014, to June 1, 2015.
No.1 | Date | Long. (°E) | Lat. (°N) | Mag. ( |
Depth (km) | Distance (km) |
|
|
Type |
---|---|---|---|---|---|---|---|---|---|
1 | 2014/7/8 07:06 | 122.41 | 23.29 | 5.2 | 38.8 | 224 | None | None | — |
2 | 2014/7/17 14:11 | 121.36 | 22.25 | 5.3 | 19.1 | 322 | None | None | — |
3 | 2014/8/6 11:46 | 121.46 | 22.23 | 5.1 | 84.2 | 324 | None | None | — |
4 | 2014/9/21 05:14 | 121.54 | 23.59 | 5 | 31.5 | 173 | 21 | None | C |
5 | 2014/9/25 18:35 | 121.3 | 22.74 | 5.3 | 18 | 269 | None | None | — |
6 | 2014/10/8 02:08 | 121.56 | 23.65 | 5.2 | 33.4 | 167 | 15 | 10 | C |
7 | 2014/10/26 12:47 | 122.23 | 24.75 | 5 | 86.0 | 81 | 8 | 3 | W |
8 | 2014/11/20 01:46 | 122.02 | 24.89 | 5.2 | 13.9 | 55 | 17 | 17 | O |
9 | 2014/11/21 11:29 | 120.13 | 20.65 | 5.8 | 71.3 | 520 | None | None | — |
10 | 2014/12/11 05:03 | 122.17 | 25.7 | 6.7 | 268.6 | 86 | 23 | 14 | W |
11 | 2014/12/31 11:06 | 121.79 | 24.89 | 5.1 | 68.9 | 37 | None | 20 | W |
12 | 2014/12/31 15:54 | 122.6 | 24.55 | 5.6 | 96.1 | 125 | None | 20 | W |
13 | 2015/1/5 13:53 | 122.04 | 24.74 | 5.1 | 73.8 | 67 | None | 20 | L |
14 | 2015/1/7 12:48 | 121.7 | 24.26 | 5.5 | 30.4 | 100 | None | Unknown | C |
15 | 2015/1/19 11:48 | 121.57 | 22.76 | 5 | 117.9 | 265 | None | None | — |
16 | 2015/2/14 04:06 | 121.46 | 22.66 | 6.3 | 27.8 | 276 | None | None | — |
17 | 2015/2/27 00:50 | 122.29 | 24.68 | 5.3 | 97.6 | 90 | 13 | 11 | L |
18 | 2015/3/23 18:13 | 121.76 | 23.7 | 6.2 | 38.4 | 162 | 9 | None | C |
19 | 2015/4/6 19:37 | 122.75 | 24.4 | 5.4 | 79.4 | 146 | None | None | — |
20 | 2015/4/20 09:42 | 122.37 | 24.05 | 6.4 | 30.6 | 147 | 21 | 12 | I |
21 | 2015/4/20 09:49 | 122.49 | 24.02 | 5 | 30.7 | 157 | 21 | 12 | I |
22 | 2015/4/20 19:20 | 121.75 | 23.63 | 5.5 | 38.3 | 170 | Unknown | Unknown | C |
23 | 2015/4/20 19:45 | 122.41 | 24.12 | 6.2 | 33.9 | 143 | 21 | 12 | I |
24 | 2015/4/20 19:59 | 122.39 | 24.01 | 6 | 29.4 | 152 | 21 | 12 | I |
25 | 2015/4/26 04:01 | 122.44 | 24.03 | 5.7 | 34.8 | 153 | Unknown | Unknown | — |
26 | 2015/5/3 21:09 | 122.42 | 23.94 | 5 | 24.8 | 160 | None | None | — |
27 | 2015/5/26 08:56 | 122.49 | 23.87 | 5 | 28.9 | 171 | 7 | 6 | I |
a | 2014/7/26 00:43 | 122.11 | 24.92 | 4 | 101 | 61 | None | 4 | — |
b | 2014/8/1 12:38 | 121.6 | 24.58 | 4 | 6.1 | 64 | None | 3 | — |
c | 2014/8/5 08:34 | 121.67 | 24.4 | 3.6 | 38 | 84 | None | 3 | — |
d | 2014/8/16 02:51 | 121.53 | 24.92 | 3.8 | 84 | 26 | None | 7 | — |
e | 2014/8/17 06:52 | 121.04 | 24.72 | 2.3 | 6.2 | 71 | 3 | None | — |
f | 2014/8/24 20:39 | 122.09 | 24.3 | 4 | 57.5 | 109 | None | 6 | — |
g | 2014/10/19 17:50 | 121.1 | 24.68 | 2.9 | 6 | 70 | 4 | None | — |
h | 2014/11/18 07:22 | 121.56 | 25.16 | 2.8 | 2.5 | 1 | None | 2 | — |
i | 2015/5/7 04:04 | 121.33 | 24.1 | 4.4 | 68 | 119 | None | 4 | — |
j | 2015/5/9 20:32 | 121.23 | 24.66 | 3.5 | 10 | 64 | 1 | None | — |
1The label of relevant earthquakes marked in Figures
The temporal variation of gamma rays and radon shows similar patterns, whereas some high gamma ray and radon concentration peaks in the entire spectrum can also be observed. In addition, it is noted that the increase of soil radon concentrations usually occurred before the change in gamma ray counts. For calculating the anomalies, the anomalous thresholds are found using the average value plus one to three standard deviations [
A comparison of gamma ray data and earthquakes detected by the CWB over the monitoring period indicates that the presence of gamma ray anomalies was quite often accompanied by seismic activity, except for fourteen of twenty-seven marked earthquakes with
Similarly, the presence of radon anomalies at the TPT station could also be connected with marked seismic events with a
Each gamma ray and radon anomaly is assigned to a definite earthquake and could only be correlated with the precursory anomalous increase a few days to a few weeks before the event occurrence. Each precursory time was illustrated in Figure
Variations of gamma ray and radon data during the period from March 29 to April 22, 2015. The preearthquake anomalous time from the gamma ray and radon results is represented by
Figure
Most large events could be linked to anomalous changes in gamma rays and radon at both stations (Table
Group A included the PE with an epicenter distance of <30 km and showed anomalous gamma ray changes over a period of a few days (e.g., Event h). The PE with epicenter distances of 30 to 180 km showed anomalies in gamma rays over several days to weeks (e.g., Events 10, 17, and 20) which was identified as Group B. It was noted that no anomaly was found when the distance of the earthquake epicenter was farther than 180 km, such as Events 1, 2, 3, 5, 9, 15, and 16.
A diagram showing the relationship between the depth of the PE and latitude location and the seismic velocity structure along the a-a
The seismic velocity structure transecting northern Taiwan along the a-a
For the TPT, Fu et al. [
Some fault plane solution diagrams of the studied earthquakes were plotted in Figure
The dependent relationship between the epicenter distance to the YMSG site and the magnitude of the PE was shown in Figure
Variation diagrams showing the relationship between earthquake magnitude and (a) epicenter distance to the YMSG monitoring site and (b) distribution of the precursory time. Fitted curves are shown as solid lines. Dashed lines indicate the 95% confidence interval of the data set.
The good correlation between the magnitude of PE and precursory time (
The YMSG station is located at the western end of the subduction system in the northern Taiwan area, where the PSP is subducting northward beneath the EP. Furthermore, this region is still seismically active due to continuous convergence and volcanic activity, with changes in high-pressure fluid affected by a deeper pump system [
It may be suggested that due to continuous convergence, the preseismic slow slip may occur around the subduction zone before the event. The volumetric expansions of the EP around northern Taiwan then released the additional radiogenic gas from the crustal component. The increase in gamma rays can be attributed to the expansion around the YMSG that produced new fractures for gas and fluid migrations, potentially associated with the slow slip (e.g., Events 6, 7, 8, 10, 17, 20, 21, 23, 24, and 27). A similar explanation of the preseismic slow slip before the Tohoku earthquake was illustrated by Orihara et al. [
Hence, based upon the above-mentioned points, the preliminary outline of anomalous data regarding relevant earthquakes can be concluded, which may be associated with the dynamics of the subduction process.
The geodynamic setting of the arc-continent collision in Taiwan is well defined by the oblique collision between the PSP and the EP, with the former plate moving about 8 cm/year [
(a) A schematic diagram of the proposed physical mechanism based on the gamma ray anomaly of YMSG and radon anomaly at TPT. Stages 1 (b) and 2 (c) are considered to have occurred in order before the significant earthquakes. The schematic diagram showing a crustal-scale cross-section along the geophysical transect (AA
Based on GPS observations, Rau et al. [
Therefore, anomalous signals from gamma rays and radon that are attributed to changes in pore pressure by the Coulomb stress changes may be attributed to near/far-field earthquakes or strain-induced changes in permeability within the preparation zone of the earthquake [
The major findings of this study are listed below:
Gamma ray variations are monitored at the YMSG monitoring station, located in the volcanic area of north Taiwan. Results of the temporal gamma ray variations are compared with meteorological records. The marginal effect of atmospheric parameters is observed in gamma ray measurements at the YMSG station Under continuous monitoring, many anomalously high radon values at TPT and gamma rays at YMSG were observed a few days to a few weeks before the seismic events Two groups of earthquakes can be identified at the YMSG station based on relevant earthquakes: earthquakes with shallower hypocenters and that are concentrated within 30 km of the monitoring station with a smaller magnitude and the earthquakes with deeper hypocenters and that are distributed at greater distances (~30 to 180 km) from the monitoring site The precursory changes in soil radon at the TPT station and gamma rays at the YMSG station may represent the preparation stage of an earthquake. When the observed precursory signals from these two stations with time delay occur, the possible impending large earthquake around the plate boundary in northeastern Taiwan can then be expected, which is located approximately between latitudes 23.9°N and 26.7°N and longitudes 121.3°E and 122.6°E Both radon and gamma ray measurements provide a useful tool and act as a good indicator for exploring earthquake precursors, especially in north and northeastern Taiwan
The data used to support the findings of this study are available from the corresponding author upon request.
A preliminary study has been presented as a conference abstract in the 2005 AGU-Fall Meeting.
The authors declare that they have no conflicts of interest.
We dedicate this paper to the late Prof. Tsanyao Frank Yang (TFY), who passed away on the 12th of March 2015. He had worked as a geochemist at the Department of Geosciences, National Taiwan University (NTU), Taiwan, for more than 20 years. His passion for research included setting new standards for relevant measurements, which enables us to present our work here. We thank Messrs. Shin-Jung Lin and Kou-Wei Wu for helping in the operation and maintenance of the monitoring station. The research is supported by grants MOST 107-2116-M-002-019 through the Ministry of Science and Technology (MOST) and MOTC-CWB-106-E-01 from the Central Weather Bureau (CWB), Taiwan. The TEC contribution number for this article is 00151. We would like to thank Uni-edit (