Abstract
This paper proposes an analysis method, using the National Oceanic and Atmospheric Administration satellite data, to trace variations in outgoing longwave radiation (OLR) for finding the precursors of earthquakes. The significance of these observations is investigated using data sets of recent M ≥6.0 earthquakes around the Taiwan area from 2009 to 2019. We suggest that the precursory signal could be an EIndex anomaly (EA) in the form of substantial thermal releases distributed near the epicenter. The consecutive appearances of OLR EAs are observed as precursors 2–15 days before significant earthquakes, and we refer to this as a pre-earthquake OLR EIndex anomaly (POEA). We interpret these thermal sources as possibly originating from electromagnetics together with gas emissions associated with pre-seismic processes. This study highlights the potential of OLR anomalous changes in earthquake precursor studies, at least in the Taiwan region.
Introduction
Outgoing long-wave radiation (OLR) is energy emitted from Earth in the form of electromagnetic radiation, which passes out of the atmosphere and into space in the form of thermal radiation. The flux of energy transport by OLR can be measured by a satellite. OLR is primarily sensitive to near-surface and atmospheric temperatures, the humidity of the air, and the presence of clouds, which are related to the intensity of convective activity as well as the latitude and altitude dependence of the variability (; ; ).
Analysis of the thermal anomaly phenomenon using satellite techniques for monitoring seismically active regions has been carried out since the 1980s (). Many studies have reported that thermal anomalous space-time transients were detected in the OLR or thermal infrared radiation (TIR) before and/or during the earthquake preparation stage (, ; , , , ; ; ; , ; ; , , , ; ; , ; ; , , ; ; ; ; ; , ). The main mechanisms proposed for the temperature increase are (1) the emission of diffuse gases (e.g., radon and CO2) leading to a local greenhouse effect or near-ground air ionization to release heat (; ; , ; ) and (2) positive hole charge or the excitation of an electric field from the stressed rock resulting in infrared emission and energy flow (; , ; ).
Taiwan is located on the Pacific seismic belt, where earthquake activity is intense. Many large earthquakes have occurred in Taiwan and caused seismic disasters in the past hundred years as a result of the continuous collision between the Philippine Sea Plate and the Eurasian Plate at a rate of 82 mm/yr to the north-west (). For example, the ML7.3 Chi-Chi earthquake in 1999 killed more than 2400 people and had a substantial impact on Taiwan.
Identifying earthquake precursors has often been considered a useful method to mitigate seismic disasters. In the past few decades, a wide variety of earthquake precursors have been investigated for Taiwan, including gas and fluid geochemical changes (; , ; ; , , , ,,; , ; , ; , ; ; ), gamma-ray monitoring (, ), groundwater levels (; ; ; , ), borehole strain changes (; , ; ), seismicity changes (, ; ; ; ; ; ), tidal variations (; ), GPS observations (, ), lightning activity (), geo-electromagnetic observations (, ; , ; , , , , ), and seismo-ionospheric anomalies (, , , , ; ).
Previous studies have applied remote-sensing techniques to survey thermal anomalies in Taiwan (; ; ; ; ; , ). It is worth mentioning that a significant sequence of TIR anomalies was observed 2 weeks before the Chi-Chi earthquake and close to its epicenter. Previous studies have also proposed that the observed TIR anomalies were related to earthquakes with M >4 that occurred around Taiwan from 1995 to 2002 ().
These studies provide the opportunity to explore OLR/TIR variations related to energy radiating from underground toward the atmosphere as a result of rock deformation and geodynamic processes in the Taiwan area. However, most of the above-referenced studies have been limited by a lack of sufficient earthquake events with OLR results (M ≥ 6) to further investigate the potential of detecting significant earthquakes in Taiwan. Nevertheless, these potential precursory phenomena for Taiwan may provide an opportunity to better understand the physical and chemical processes before earthquakes, which would be crucial for exploring the theoretical mechanism of lithosphere–atmosphere–ionosphere coupling (LAIC) (; , ).
This paper aims to describe the characteristics of the OLR monitoring system, process the data, and systematically assess the feasibility of using the OLR method to find precursors prior to the M ≥6 earthquakes that occurred over 2009–2019 around Taiwan (Figure 1). We also interpret the observed precursors of OLR and a possible pre-earthquake generation process by comparing the results with multiple observations.
FIGURE 1
Materials and Methods
The OLR data used in this study are daily measurements from the Advanced Very-High-Resolution Radiometer (AVHRR) polar orbiters NOAA-18, obtained from the NOAA Climate Prediction Center website.1 To compute the daily zonal OLR anomaly over the Taiwan area, we use a daily mean with a 1° × 1° spatial resolution in both longitude and latitude, as shown in Figure 2.
FIGURE 2

Flowchart of the methodology and procedure for extracting EIndex from OLR daytime data observed by the NASA-18 satellite during the period from Sept. 22 to Oct. 6, 2016: (A) the raw data of the OLR, (B) eddy, (C) OLR anomaly, and (D)EIndex. The epicenter of the M6.0 2016/10/06 earthquake is marked with a gray star. This shows an example of the abnormal EIndex anomaly (EA) within the epicenter’s vicinity with a red circle for September 23, 27, 29, and October 6 for Event 30 in southwest Taiwan.
Taiwan is located in the Pacific Ocean and has a subtropical climate, and it is frequently impacted by typhoons. Increasing sea surface temperatures can effectively increase the surface latent heat flux during the typhoon season, which means that the heavy precipitation and strong convective flow of a typhoon can induce a region of extreme negative OLR anomalies (
It is important to note that OLR raw data were acquired twice for each location and at the same time of day (daytime and nighttime) under the similar background conditions to reduce the variability of signals due to the daily variation of temperature and humidity and the vegetation and cloud coverage. The local grid value of the OLR is subtracted from the spatial variation of the 1-year mean value as the daily eddy. The current day value (the current daily eddy) is deducted from the average value of 16 days preceding the observation day as the OLR anomaly. A standardized anomaly, EIndex, can be determined, computed from the OLR anomaly divided by a standard deviation τi,j. In this study, when EIndex is observed to have consecutive abnormal changes in some specific area in a short period, it may then be considered as a possible precursor for identifying seismic events. Seismic data are acquired from the earthquake catalogs of the Central Weather Bureau and United States Geological Survey. There are 35 seismic events with magnitudes greater than 6.0 during 2009–2019 around the Taiwan area (Figure 1) and the variation in EIndex on the nearest grid-point adjacent to the epicenter of the earthquake was selected for further investigation.
The data processing method for extracting EIndex from daytime and nighttime OLR data connected with the earthquakes is summarized in Figure 2. The procedure of data analysis uses the following equations:
where Si,j is the OLR value of the target grid-point, as defined in Figure 2A, Wi,j is the number of the surrounding grid points around the target point, and is the yearly average. By subtracting the weighted yearly average from the four surrounding grid points for removing the regional mean, S∗(Xi,j,t) can be obtained as the daily eddy (shown in Figure 2B):
where S∗(xi,j,t) is the current daily eddy and is the 16-day average daily eddy fields excluding the current day. OLR(t)A indicates a residual value of the eddy as the OLR anomaly (Figure 2C) that can be determined as the difference between the current eddy and the average eddy of the 16 days preceding the observation day:
where EIndex is defined by the OLR anomaly divided by the standard deviation of τi,j. EIndex can then represent the maximum change in OLR values for specific spatial locations and predefined times as an indicator of abnormality (shown in Figure 2D). This methodology follows a similar analysis process to that described in detail by
Results
Figure 3 shows the daytime and nighttime variations of EIndex for each significant earthquake. The threshold values were determined to be when the EIndex value exceeded 2, which is called the EIndex anomaly (EA) to identify each seismic event’s precursor (Figure 1 and Table 1). The earthquake preparation is considered to be a continuous process, as shown in Figure 2D, and we propose that the consecutive appearances of EAs around the epicentral area are a precursor to an earthquake. Based on this scenario, the EQ-1 event in Figure 3 shows an example of the temporal variations of precursory EAs. Here, red and blue dots are the daily variations in the daytime and nighttime EAs, respectively. In this study, we propose that at least three appearances of EAs found within 4 days over the epicentral area before significant seismic events (M ≥ 6) be the criteria for earthquake precursory anomalies, and we refer to this as a pre-earthquake OLR EIndex anomaly (POEA). The EAs are observed on July 5, July 7, and July 8, 2009, in the daytime and nighttime results, respectively, which means there are a total of six EAs within 4 days (6, 7, and 9 days in the daytime and 6, 7, and 9 days in the nighttime before the 2009/07/14 M6.0 earthquake, respectively). Similarly, precursors in the OLR usually indicate significant and anomalous variability associated with significant earthquakes lasting more than 1 day and repeating at the same local time (
FIGURE 3

Time-series of daily OLR EIndex values associated with M ≥6 earthquakes over the epicenter area from 2009 to 2019 around Taiwan. The red and blue lines indicate the daily EIndex variations in the daytime and nighttime, respectively, within 25 days before and after the earthquake. The earthquake’s anomalous days are considered to be when the increasing EIndex value is greater than the threshold value (EIndex ≥ 2), as given by the red and blue dots. The occurrence of an earthquake is marked with the green dashed line. The shaded region indicates typhoon period. The relevant earthquakes are marked by the numbers shown in Figure 1 and listed in Table 1.
TABLE 1
| No.a | Date | Long. | Lat. | Mag. | Depth | Daytime EAb | Nighttime EAb | Pc |
| (°E) | (°N) | (ML) | (km) | (Days before the EQ) | ||||
| 1d | 2009/07/14 02:05 | 122.22 | 24.02 | 6.0 | 18.1 | 6, 7, 9 | 6, 7, 9 | Y |
| 2d | 2009/08/17 08:05 | 123.88 | 23.37 | 6.8 | 43.3 | N | ||
| 3d | 2009/10/04 01:36 | 121.58 | 23.65 | 6.1 | 29.2 | 9 | 9 | N |
| 4 | 2009/11/05 17:32 | 120.72 | 23.79 | 6.2 | 24.1 | 8, 9, 10 | 6, 7, 8, 9, 10 | Y |
| 5 | 2009/12/19 21:02 | 121.66 | 23.79 | 6.9 | 43.8 | 5, 6, 7, 8, 9, 10 | 6, 7, 9, 10 | Y |
| 6 | 2010/02/07 14:10 | 123.77 | 23.33 | 6.6 | 88.0 | 5, 9, 14 | 9 | N |
| 7 | 2010/03/04 08:18 | 120.71 | 22.97 | 6.4 | 22.6 | 5, 6, 7, 8, 9, 10 | 5, 6, 7, 8, 9, 10 | Y |
| 8 | 2010/04/26 10:59 | 123.74 | 22.15 | 6.8 | 73.4 | 6, 8 | 11, 13 | N |
| 9d | 2010/10/04 21:28 | 125.51 | 23.97 | 6.6 | 35.0 | 6 | 6, 11 | N |
| 10 | 2010/11/21 20:31 | 121.69 | 23.85 | 6.1 | 46.9 | 5, 6 | 2, 7, 24 | Y |
| 11 | 2011/10/30 11:23 | 123.16 | 25.31 | 6.3 | 215.8 | 5, 7, 8, 9, 10 | 9 | Y |
| 12 | 2012/02/26 10:35 | 120.75 | 22.75 | 6.4 | 26.3 | 9, 10 | 10 | Y |
| 13 | 2012/06/10 05:00 | 122.31 | 24.46 | 6.6 | 69.9 | 7, 9 | 9 | Y |
| 14 | 2013/03/27 10:03 | 121.05 | 23.90 | 6.2 | 19.4 | 5, 6, 8, 9, 10 | 4, 5, 6, 7, 9, 10 | Y |
| 15 | 2013/04/11 04:20 | 122.22 | 20.65 | 6.1 | 22.1 | 6, 7, 9, 10, 14 | Y | |
| 16 | 2013/05/21 16:25 | 123.69 | 23.33 | 6.1 | 53.1 | 7, 8 | 3, 7, 11 | Y |
| 17 | 2013/06/02 13:43 | 120.97 | 23.86 | 6.5 | 14.5 | 6, 7, 9 | Y | |
| 18 | 2013/06/08 00:38 | 122.65 | 23.98 | 6.2 | 35.3 | 6, 7 | 6, 7 | Y |
| 19d | 2013/09/06 19:33 | 122.28 | 20.05 | 6.8 | 206.2 | 3, 5 | 11, 13 | N |
| 20 | 2013/10/31 20:02 | 121.35 | 23.57 | 6.4 | 15.0 | 5, 6, 8, 9, 10 | 5, 6, 7, 9 | Y |
| 21 | 2014/12/11 05:03 | 122.61 | 25.45 | 6.7 | 268.6 | 3, 13 | 3, 13, 20 | N |
| 22 | 2015/02/14 04:06 | 121.40 | 22.66 | 6.3 | 27.8 | 5, 8, 10 | 3, 9 | Y |
| 23 | 2015/03/23 18:13 | 121.67 | 23.73 | 6.2 | 38.4 | 5, 7, 9, 12 | 2, 3, 5, 9 | Y |
| 24 | 2015/04/20 09:42 | 122.44 | 24.02 | 6.4 | 30.6 | 4, 9, 10, 11 | Y | |
| 25 | 2016/02/02 22:19 | 123.54 | 25.42 | 6.7 | 203.7 | N | ||
| 26 | 2016/02/06 03:57 | 120.54 | 22.92 | 6.6 | 14.6 | 9, 25 | 11, 12 | Y |
| 27 | 2016/05/12 11:17 | 121.98 | 24.69 | 6.1 | 8.9 | 8, 11 | 12 | Y |
| 28 | 2016/05/31 13:23 | 122.68 | 25.49 | 6.9 | 256.9 | 12 | 21 | N |
| 29 | 2016/06/24 05:05 | 123.42 | 23.53 | 6.1 | 47.0 | 10, 14, 15 | Y | |
| 30 | 2016/10/06 23:52 | 121.34 | 22.63 | 6.2 | 23.7 | 7, 9, 13 | 8 | Y |
| 31 | 2018/02/06 23:50 | 121.73 | 24.10 | 6.2 | 6.3 | 14 | 15 | N |
| 32 | 2018/10/23 12:34 | 122.62 | 24.00 | 6.1 | 31.2 | 11 | 9, 10 | Y |
| 33 | 2018/11/26 07:57 | 118.52 | 23.38 | 6.2 | 12.3 | 8, 10 | N | |
| 34 | 2019/04/18 13:01 | 121.56 | 24.05 | 6.3 | 20.3 | 4, 5, 6 | 2, 20 | Y |
| 35 | 2019/08/08 05:28 | 121.91 | 24.44 | 6.2 | 24.2 | 7 | 7, 8 | Y |
Catalog of OLR EIndex anomalies (EA) and related earthquakes occurring around Taiwan from 2009 to 2019.
aNumbers of the relevant earthquakes marked in Figures 1, 3, 5. bAn EIndex value higher than 2 can be called the EIndex anomaly (EA), and only EAs observed before the EQ are listed. C“P” denotes “Precursor.” dThe number of the relevant earthquake denotes a typhoon occurring during 25 days before and after the EQ.
Discussion
Figure 3 presents the temporal evolution of the propagation characteristics of the EA to the earthquake, which may be related to the POEA. Comparing the rates of anomaly occurrences in the daytime and nighttime and 35 earthquakes with M ≥6 during 2009–2019 shows that the POEA quite often occurred ∼2–15 days before the earthquakes, except for 11 of the 35 selected seismic events (EQ-2, EQ-3, EQ-6, EQ-8, EQ-9, EQ-19, EQ-21, EQ-25, EQ-28, EQ-31, and EQ-33 in Figure 3 and Table 1). In other words, the progressive POEA accompanying the relevant earthquake can be termed as the precursory event (PE). It is worth noting that, for all of the cases, significant EAs are much weaker and fewer after the earthquake occurrence (Figure 3).
The Influence of Typhoons
The disturbance to OLR EIndex variations seems to be caused by the occurrence of a typhoon before but not after the earthquake. There is no frequent appearance of EA before the EQ-2, EQ-3, EQ-9, and EQ-19 events during the occurrence of a typhoon before the earthquake. Similarly,
Influence of the Focal Depth of the Earthquake
An earthquake’s focal depth appears to be a significant parameter that affects EIndex variations. Earthquakes with a hypocenter depth greater than 70 km, such as EQ-6, EQ-8, EQ-19, EQ-21, EQ-25, and EQ-28, have no precursors of the abnormal EIndex anomaly, except for the EQ-11 event. It is worth noting that the POEA can be observed for EQ-11, but not for EQ-21, EQ-25, and EQ-28 in similar tectonic settings (Figure 1), which might be attributed to the different focal mechanism. However, no further discussion can be made as only one case of this has been observed.
Positive hole charge carriers are proposed as a physical mechanism for the buildup of surface charges over stressed rock to transport energy from the Earth’s crust to the surface. This hypothesis is used to explain the pre-earthquake thermal anomalies, by generating electric fields strong enough to ionize the air (
Earthquakes occurring at a focal depth of greater than 70 km are classified as intermediate-focus earthquakes, suggesting that they are located lower than the crust–mantle boundary of Taiwan (
Relationship Between OLR EIndex Variations and Seismicity
Presented here are a few examples of the abnormal POEA, along with different types of observations in Taiwan. The 2010/03/04 M6.4 Jiashian Earthquake (EQ-7) occurred in southern Taiwan. Significant changes in soil radon concentrations along the fault zones over the epicenter were observed ∼14 days before EQ-7, followed by the appearance of the POEA from satellite data and the crust with surface movement based on GPS residual displacement nearly 10 days before the event (
Most precursory phenomena in previous studies of earthquake precursors in Taiwan have been fragmentary, especially for atmospheric OLR investigations using satellite techniques. The mechanism of the earthquake generation process and the reasons for the OLR anomalies are not well-understood. According to the mechanisms and previous results described above, we propose a possible mechanism based on the previous observations together with the results of this study (e.g., multi-precursors before EQ-7, EQ-14, and EQ-26). Gases spread out along the fault zones together with the positive hole charge carriers, generated by the amount of gases degassed from compressed rock and the magnetic enhancements of high-conductivity materials around the earthquake preparation zone emanating to the surface, which is caused by changes in rock permeability and conductivities due to pre-earthquake stress accumulation. The ionization of the near-surface air and latent heat exchange is related to gas emissions and the upward flow of current with the positive hole charge carriers, respectively, leading to an increase in air temperature and intensive outgoing longwave radiation. The time sequence of anomalous changes between the lithospheric and atmospheric observations explain the appearances of the precursory OLR anomalies in the epicentral areas before earthquakes with M ≥6 around the Taiwan area. Then, the influence on the total electron content (TEC) leads to substantial ionospheric perturbations, followed by the contributions of the lithosphere-atmosphere before the impending earthquake. Hence, the significant relationship between different types of observations between the lithosphere, atmosphere, and ionosphere can be validated in Taiwan. Similar models have also been proposed by
When the EA occurrence is present within a specific day in the daytime and the nighttime before and after an earthquake, we count one for a certain day to the relevant PE. Figure 4 shows all PEs to display the proportion of the number of total EAs during the observation period. It can be seen that the occurrence of EAs mainly appears from 5 to 10 days before the earthquake, regardless of being in the daytime or the nighttime. This provides a useful precursory index for OLR monitoring of the impending significant seismic activity, at least in the Taiwan area. The dependent relationship between the number of total precursory EAs and the magnitude of each PE is shown in Figure 5. A weak correlation (R2 = 0.05) is obtained between these variables, suggesting that the number of precursory EAs does not reflect the magnitude of the earthquake. The amounts of EAs may not be used to estimate the magnitude of an earthquake for neither inland nor offshore earthquakes.
FIGURE 4

Temporal profile of daily OLR EIndex anomalies (EA) associated with each precursory event within 25 days before and after the earthquake. The black and gray bars denote the EAs in the daytime and the nighttime, respectively.
FIGURE 5

Variation diagrams showing the relationship between earthquake magnitude and distribution of the number of total precursory EIndex anomalies. Solid and open symbols denote the inland and offshore earthquakes with the associated anomalous OLR EIndex. Dashed lines indicate the 95% confidence interval of the data set.
A confutation test was performed to verify the appearance or absence of the POEA before or after significant earthquakes over the same area (
FIGURE 6

Time-series of daily OLR EIndex anomalies (EA) associated with the M ≥6 earthquakes from 2009 to 2019 around Taiwan for a confutation test over the same area. The red and blue squares indicate the daily EAs in the daytime and the nighttime, respectively, within 90 days before and 25 days after the earthquake. When at least three EAs are found within 4 days, this is referred to as a POEA (marked by the yellow highlighting). The occurrence of the strong earthquakes (M ≥ 6) and the 2013/03/07 M5.9 earthquake are marked with the green and gray dashed lines, respectively. The occurrence of the earthquakes with the magnitude between 5–6 and 4–5 are denoted by light-blue line and light-green line, respectively. The unperturbed period without the strong earthquakes during 90–25 days before the event is marked by the gray dashed rectangle.
Based on the observations from the present study, the consecutive POEAs generally appear several days over the epicenter before a strong earthquake. This can be utilized to trace the appearance of EAs around Taiwan using the spatial-temporal monitoring of OLR observations. Currently, an automatic-process monitoring system has been established in Taiwan to routinely monitor variations in OLR. As mentioned above, substantial POEAs can be observed, followed by an increase in radon concentrations, before significant seismic activities in Taiwan. The appearance of the significant POEAs could be further compared with soil radon observations along fault zones around Taiwan (
Conclusion
In the present study, we applied a satellite technique to outgoing longwave radiation (OLR) variations in order to search for the precursors of M ≥6.0 earthquakes over 2009–2019. The major findings of this study are listed below:
- (1)
The results showed that most precursors, as shown by the pre-earthquake OLR EIndex anomaly, mainly appeared 5–10 days before significant earthquakes.
- (2)
There is a weak correlation between the number of EIndex anomalies and the earthquake’s magnitude, indicating that the increasing intensity of OLR anomalies is not associated with increased magnitude.
- (3)
The effects of the focal depth and typhoon events are proposed as critical factors that affect OLR variations for earthquake precursor studies, as when an earthquake has a hypocenter depth greater than 70 km and a typhoon occurs before the earthquake, consecutive EIndex anomalies are rarely observed.
- (4)
A high percentage of earthquakes (∼77%) with pre-earthquake OLR anomalies are consistently observed when the possible factor of a typhoon’s occurrence is removed.
- (5)
We proposed a potential mechanism to interpret the pre-earthquake generation process, suggesting the existence of multiple responses triggered by coupling processes between the lithosphere, atmosphere, and ionosphere through OLR anomalies by combining different types of observations.
- (6)
Therefore, our studies provide a useful indicator for exploring earthquake precursors, especially in the Taiwan region.
Statements
Data availability statement
The datasets generated for this study are available on request from the corresponding author and from the NOAA Climate Prediction Center website (http://www.cdc.noaa.gov/).
Author contributions
C-CF organized and wrote the manuscript. L-CL provided the concepts of the manuscript. DO and J-CJ assisted in improving the data processing. All authors contributed to the article and approved the submitted version.
Funding
The research was supported by Grants MOST 108-2116-M-002-025 and MOST 108-2116-M-001-024 through the Ministry of Science and Technology (MOST) and MOTC-CWB-108-E-01 from the Central Weather Bureau of Taiwan.
Acknowledgments
We thank Mr. Peng-Kang Wang and Kuo-Hang Chen for help with data collection and preliminary analysis. We would like to thank Uni-edit (www.uni-edit.net) for editing and proofreading this manuscript. The TEC contribution number for this article is 00163. We are also thankful to the editor and reviewers and the referee for their valuable comments and suggestions, which helped us to improve the manuscript.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Footnotes
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Summary
Keywords
National Oceanic and Atmospheric Administration, outgoing longwave radiation, pre-earthquake OLR EIndex anomaly, earthquake precursor, Taiwan
Citation
Fu C-C, Lee L-C, Ouzounov D and Jan J-C (2020) Earth’s Outgoing Longwave Radiation Variability Prior to M ≥6.0 Earthquakes in the Taiwan Area During 2009–2019. Front. Earth Sci. 8:364. doi: 10.3389/feart.2020.00364
Received
07 June 2020
Accepted
07 August 2020
Published
02 September 2020
Volume
8 - 2020
Edited by
Giovanni Martinelli, National Institute of Geophysics and Volcanology, Italy
Reviewed by
Nicola Genzano, University of Basilicata, Italy; Marzieh Khalili, Shiraz University, Iran
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© 2020 Fu, Lee, Ouzounov and Jan.
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*Correspondence: Ching-Chou Fu, ccfu@earth.sinica.edu.tw
This article was submitted to Solid Earth Geophysics, a section of the journal Frontiers in Earth Science
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