Earthquake Reaearch in China  2017, Vol. 31 Issue (3): 422-430
The Change in Outgoing Long-wave Radiation before the Ludian MS6.5 Earthquake Based on Tidal Force Niche Cycles
Zhang Yan, Kang Chunli, Ma Weiyu, Yao Qi     
China Earthquake Networks Center, Beijing 100045, China
Abstract: The cycle process of the tidal force niche for the Ludian MS6.5 earthquake occurring in Ludian County, Yunnan Province, China on August 3, 2014 was calculated. The earthquake occurred near the middle point phase. It indicates that the type of seismogenic fault that the tide force acted on belongs to the thrust fault. According to the tidal niche cycle, the abnormal OLR (Outgoing Long-wave Radiation) change was analyzed based on NOAA satellite data around the whole land area of China before and after the earthquake. The result shows that the OLR changed evidently with tide force change. Temporally, the change went through the evolution process of initial OLR rise→ strengthening→ abnormal peaking→ attenuation→ returning to normal; and spatially, the abnormal area wound its way along the Zhaotong-Ludian fault and went through a scattered→ conversion→ scattered process. This process is similar to the change process of rock breaking under stress loading. The results indicate that the tidal force of a celestial body could trigger an earthquake when the tectonic stress reaches its critical breaking point and the OLR anomaly is proportional to the seismic tectonic stress change. It is of great use to combine OLR and tidal force in earthquake precursory observation.
Key words: The tidal force niche     Thermal infrared     OLR     The Ludian MS6.5 earthquake     NOAA satellite    

INTRODUCTION

At present, the main methods using satellite thermal infrared information to monitor and identify thermal anomalies before earthquakes include image interpretation (Gorny V.I. et al., 1988; Ma Jin et al., 2006; Ma Xiaojing et al., 2009; Lü Qiyi et al., 2000), differential analysis (Ouzounov D. et al., 2004), computer-assisted statistical techniques (Robust Satellite Technique, RST) (Filizzola C. et al., 2004; Li Jinping et al., 2008), comparison of abnormal brightness temperature increase (Kang Chunli et al., 2009), vorticity calculation based on the digital image enhancement technique (Kang Chunli et al., 2009) and the wavelet power spectrum method based on time and frequency domain transformation (Zhang Yuansheng et al., 2011; Ye Xiuwei et al., 2010). All of these research methods are based on average processing of remote sensing data over many years. Due to meteorological fluctuations, the multi-year average algorithm often covers up thermal radiation fluctuations caused by the rapid change of tectonics in the short term. Experimental results of remote sensing rock mechanics show that in the process of rock breaking under stress loading, the outgoing radiation change along fracture plane follows the basic pattern of enhancement-abnormal peaking-attenuation-returning to normal (Cui Chengyu et al., 1993; Wu Lixin et al., 2004; Ma Jin et al., 2012). An earthquake is a mechanical process from the interior of the earth, and the development of imminent radiation anomaly research with mechanical significance has a certain value for distinguishing the radiation enhancement caused by seismic activity from that and by non-seismic activity. Currently, as we are unable to obtain in-situ stress changes from direct observation, its worth exploring the relationship between mechanical causes that induce imminent radiation change and the possibility of making use of external mechanical factors influencing earth movement.

Other studies have shown that tidal force is an important external mechanical factor triggering earthquakes (Heaton T.H. et al., 1975; Li Yanxing et al., 2001; Chen Ronghua et al., 2006; Zhao Xiaomao et al., 2010). Therefore, the periodic change of tidal force niche at the epicenter location of the Ludian earthquake is firstly calculated in this article, and the calculated cycle is used as time deixis for selection of the outgoing long-wave radiation background (each high point of tidal force niche cycle serves as a temporal setting for this earthquake) to carry out research on imminent outgoing long-wave radiation.

1 CALCULATION OF TIDAL FORCE NICHE

The tidal force niche changes periodically and continuously. When triggering an earthquake, the tidal force niche changes with various tectonic settings (Ma W.Y. et al., 2012). Earth tide has rich tidal components (Chen Ying et al., 2012), thus in order to be consistent with the daily observation frequency value of remote sensing observations of outgoing long-wave radiation, theoretical values of daily tidal force niche changes from June 20 to September 30, 2014 are selected for analysis.

According to the calculation method of Calvin, tidal force niche Wi(p) generated by any celestial body at any point P in the interior of the earth (Wu Qingpeng, 2001) is as follows:

$ {W_i}\left(p \right) = k\frac{M}{{{r_{\rm{m}}}}}\sum\limits_{n = 2}^\infty {{{\left({\frac{r}{{{r_{\rm{m}}}}}} \right)}^n}{P_n}\left({{\rm{cos}}{Z_{\rm{m}}}} \right)} $ (1)

Where, Pn(cosZm) is the Legendre polynomial of cos Zm. Zm is the zenith distance of celestial body; M denotes quality of the moon and the Earth; k is gravitation constant, r distance between epicenter and the earths core, rm distance between the center of the moon and the earths core. For the moon, we take n=2 and n=3 under current precision level, then we get respectively;

$ {W_{{\rm{m}}2}}\left(P \right) = \frac{3}{4}k\frac{{{M_{\rm{m}}}}}{{{r_{\rm{m}}}}}{\left({\frac{r}{{{r_{\rm{m}}}}}} \right)^2}\left\{ \begin{array}{l} (1 - 3{\rm{si}}{{\rm{n}}^2}\varphi)(\frac{1}{3} - {\rm{si}}{{\rm{n}}^2}{\delta _{\rm{m}}}) + \\ {\rm{sin}}2\varphi {\rm{sin}}2{\delta _{\rm{m}}}{\rm{cos}}{H_{\rm{m}}} + {\rm{co}}{{\rm{s}}^2}\varphi {\rm{co}}{{\rm{s}}^2}{\delta _{\rm{m}}}{\rm{cos}}2{H_{\rm{m}}} \end{array} \right\} $ (2)
$ {W_{{\rm{m}}3}}\left(P \right) = \frac{3}{4}k\frac{M}{{{r_{\rm{m}}}}}{\left({\frac{r}{{{r_{\rm{m}}}}}} \right)^3}\left\{ \begin{array}{l} \frac{1}{3}(3 - 5{\rm{si}}{{\rm{n}}^2}\varphi){\rm{sin}}{\delta _{\rm{m}}}(3 - 5{\rm{si}}{{\rm{n}}^2}{\delta _{\rm{m}}}) + \\ \frac{1}{2}{\rm{cos}}\varphi (1 - 5{\rm{si}}{{\rm{n}}^2}\varphi){\rm{cos}}{\delta _{\rm{m}}}(1 - 5{\rm{si}}{{\rm{n}}^2}{\delta _{\rm{m}}}){\rm{cos}}{H_{\rm{m}}} + 5{\rm{sin}}\varphi {\rm{co}}{{\rm{s}}^2}\varphi {\rm{cos}}2{H_{\rm{m}}} \end{array} \right\} $ (3)

Similarly, for the sun, we take n=2 and get;

$ {W_{{\rm{S}}2}}\left(P \right) = \frac{3}{4}k\frac{{{M_{\rm{s}}}}}{{{r_{\rm{s}}}}}{\left({\frac{r}{{{r_{\rm{s}}}}}} \right)^2}\left\{ \begin{array}{l} (1 - 3{\rm{si}}{{\rm{n}}^2}\varphi)(\frac{1}{3} - {\rm{si}}{{\rm{n}}^2}{\delta _{\rm{S}}}) + \\ {\rm{sin}}2\varphi {\rm{sin}}2{\delta _{\rm{S}}}{\rm{cos}}{H_{\rm{S}}} + {\rm{co}}{{\rm{s}}^2}\varphi {\rm{co}}{{\rm{s}}^2}{\delta _{\rm{S}}}{\rm{cos}}2{H_{\rm{S}}} \end{array} \right\} $ (4)

And then for the Earth as a whole, we get;

$ {W_{{\rm{whole}}}}\left(P \right) = {W_{{\rm{m}}2}}\left(P \right) + {W_{{\rm{m}}3}}\left(P \right) + {W_{{\rm{S}}2}}\left(P \right) $ (5)

Where, δS and δm denote solar and lunar declination, φ the latitude of the epicenter.

2 OUTGOING LONG-WAVE RADIATION (OLR) DATA PROCESSING

Outgoing Long-wave Radiation, OLR for short, is electromagnetic energy density emitted from the a Earth and its atmosphere out to space in the form of thermal radiation, also known as thermal radiation flux density, in W/m2. By scanning and measuring the Earth and its atmosphere at infrared bands of 10.5-12.5μm with a radiation measuring instrument loaded on the NOAA polar satellite, outgoing long-wave radiation is obtained.

OLR data adopted in this article is taken from global information data broadcast on the US site http://www.emc.ncep.noaa.gov. Because OLR is generated based on infrared band remote sensing, with a waveband approximate to long-wave atmospheric window and weak atmospheric attenuation, and its waveband is close to ground surface long-wave radiation and is sensitive to temperature changes on the sea surface and near-ground surface, it is suitable for monitoring some geological disaster signs related to causes of thermogenic phenomenon.

In order to observe imminent long-wave radiation information field variation characteristics in the Ludian MS6.5 earthquake in 2014, night-time data covering the China land area (20°-45°N, 73°-155°E) from July 31 to August 8, 2014 is selected, forming an OLR information distribution field on the basis of 1°× 1° daily mean grid data. In order to extract daily variation characteristics of the OLR distribution field at the epicenter of Ludian earthquake and its surrounding area before the MS6.5 earthquake, we calculate the imminent OLR value at each grid point (inter-diurnal scale), and get the distribution of value of featured information at each grid point in the radiation enhanced area. The computational formula is as follows:

$ \begin{array}{*{20}{l}} {\Delta {S_i}\left({x, y} \right) = {S_i}\left({x, y} \right) - {S_{{\rm{background}}}}\left({x, y} \right)\;\;\;\;\;(i = {\rm{date}})} \end{array} $ (6)

Where, ΔSi(x, y) denotes numerical increment of OLR at each grid unit location, Si(x, y) OLR at each grid unit location and Sbackground(x, y) represents the OLR value in the fixed background. Tidal force value at the highest point before the earthquake is adopted as the time background in this article (the date July 30 is adopted for this earthquake), and x denotes latitude, y longitude, i grid unit location.

3 TIDAL FORCE NICHE AND ANOMALOUS THERMAL CHANGES 3.1 Seismogenic Structure

According to the measured results from the China Earthquake Networks Center (http://www.ceic.ac.cn/), at 04:30 p.m., August 3, 2014, the MS6.5 earthquake occurred in Ludian County in Zhaotong City, Yunnan Province. The earthquake focus (initial rupture point) is located at (103.3°E, 27.1°N), at a depth of about 12km. The seismogenic fault, the NW-striking Baogunao-Xiaohe fault, is a NW-striking secondary strike-slip fault of the NE-striking Zhaotong-Ludian fault system. The focal mechanism shows that the Ludian earthquake is a left-lateral strike-slip type (Xu Xiwei et al., 2014). The distribution of the epicenter location and active faults in the quake zone is shown in Fig. 1.

Fig. 1 Epicenter location of the Ludian earthquake and distribution of active faults in the earthquake area
3.2 Tidal Force Changes

Using formula (5), we calculate changes of celestial tidal force with time in the Ludian area before and after the earthquake, and draw a diagram, as shown in Fig. 2. It can be seen from Fig. 2 that changes of celestial tidal force experienced four periodic changes; peak-valley-peak, which are marked A, B, C and D. The phase position of the tidal force niche during the earthquake has a certain relationship with the earthquake type. Earthquakes of normal fault type mostly occur around the peak value of the tidal force niche, and earthquakes of strike-slip type mostly take place in the transition stage (Ma Weimin et al., 2011). For the Ludian MS6.5 earthquake, the tidal force value stayed in the transition stage, which is in accordance with the understanding that the seismogenic structure is strike-slip fault type.

Fig. 2 Temporal variation curve of tidal force in the Ludian earthquake
3.3 Temporal and Spatial Variation Characteristics of OLR before the Earthquake

Although the occurrence of an earthquake is associated with celestial tidal force, it is still difficult to determine whether the tidal force is one of the causes of the earthquake. The reason is that we cannot determine the imminent tectonic stress state and its correlation with changes of tidal force, and it is believed that only when seismogenic tectonic stress achieves the imminent stage, tidal force can possibly trigger earthquakes (Chen Ronghua et al., 2004). Therefore, determining whether the in-situ stress has reached a critical state will be a key problem.

Tidal force niche cycles are shown in Fig. 2. On the basis of Formula (6), using OLR data on July 30 as background value, OLR data thereafter day by day is subtracted from the background value to obtain the OLR changes. Thus, the continuous daily variation distribution of the OLR radiation field on a national scale is obtained, as shown in Fig. 3.

Fig. 3 Spatial-temporal evolution of temperature anomaly increase of the MS6.5 earthquake in Ludian, China

In the Chinese mainland, obvious long-wave radiation enhancement only appears in the Ludian area. It can be seen from Fig. 3 that since July 31, the central enhancement area for OLR daily average incremental field extends from the northeast end of the quake zone to SW, along the NW-striking secondary strike-slip fault of NE-striking Zhaotong-Ludian fault system, approaching the maximum value the day before the earthquake on August 2. The Ludian MS6.5 earthquake took place at 04:00 p.m., August 3, thereafter radiation enhanced area went back rapidly, and the central OLR daily average enhancement area disappeared after August 6, returning to normal. The distribution of anomalous OLR enhancement area in the Ludian MS6.5 earthquake conforms with that of active faults, basically spreading along the Zhaotong-Ludian fault. The earthquake took place at the top of the extending and shifting direction of anomalous OLR enhancement, which may indicate the migration direction of the local stress field in tectonic activities.

It can be seen from Fig. 3 that celestial tidal force has an inducing effect on active faults with in-situ stress in the critical state. Anomalous OLR enhancement is significant before the earthquake, and the anomaly area goes through the evolution process of initial OLR rise→ strengthening→ attenuation→ abnormal peaking→ attenuation→ returning to normal, which is basically consistent with earthquake abnormal heating changes and abnormal temperature increase before the earthquake (Ma Weiyu et al., 2012).

In addition, in order to know whether OLR has a similar changing process in a longer period of time during the imminent period of earthquake in the study area, we analyzed the changes of OLR with various cycles before and after the earthquake in the same area, using the above OLR processing methods according to indications of celestial tidal force cycles (Fig. 2), such as the OLR diagram in stages A, B before the earthquake and stage D after the earthquake (Fig. 4). The image shows that no obvious OLR enhancement changes appear in this region, thus it can be seen that the occurrence of OLR anomalies may be associated with the Ludian MS6.5 earthquake.

Fig. 4 Image of the space-time evolution of long wave radiation in A, B stages before the earthquake and the D stage after the earthquake
4 DISCUSSIONS

OLR, manifesting electromagnetic energy density emitted from the Earth and its atmosphere to outer space, is a radiation physical quantity directly reflecting the underlying land surface properties and energy change parameters, and its wavebands are concentrated near the atmospheric window, which receives little interference from the cloud layer, therefore, OLR data is chosen as the research object. OLR image changes for the Ludian MS6.5 earthquake show significant anomalous changes before the earthquake.

The data processing method in this article differs greatly from previous statistical methods. Blackett M. et al. (2011) for the same earthquake, uses the same statistical method to process remote sensing data from the same data source and gets infrared anomalies before the earthquake. However, the results vary largely as a different time scale and background date are chosen. This is because an earthquake is a rare event of small probability, which conflicts with traditional statistical algorithm. Traditional statistical methods have not yet associated earthquakes with the mechanical foundation of infrared remote sensing. According to tide value change cycles, we obtain the OLR enhancement image before the earthquake, which goes through the evolution process of initial OLR rise→strengthening→ abnormal peaking→attenuation after the earthquake→returning to normal, and we get OLR anomalies according to tidal force cycles, which show its uniqueness within the scope of the Chinese mainland. The results are not only consistent with the OLR anomalies before the Lushan MS7.0 earthquake obtained using the same method (Ma Weiyu et al., 2014), but also correspond to the process of radiation changes in the process of rock breaking under the stress loading (Wu Lixin et al., 2004).

The influence of tidal force on earthquakes is mainly reflected under the condition of being in the critical state. By continuous observations of OLR changes during earthquake-free period before and after the earthquake, the results show that at other stages with a similar celestial tidal force niche when an earthquake occurs, such as stages A, B and D, no obvious OLR anomalies are displayed in the corresponding period, so there is no earthquake, indicating that celestial tidal force has obvious inducing effect on active faults in the critical state (that is, OLR increase-reaches abnormal peak-attenuation). However, its inducing effect is only an external factor that causes earthquakes, not a decisive factor. Tectonic activity is the internal factor determining if an earthquake will occur.

How the change of tidal force will modulate and induce the occurrence of an earthquake and affect long-wave radiation anomalies are still unknown, so more earthquake examples should be accumulated for more elaborate analysis. However, the existence of this phenomena and some knowledge achieved from research shows that using satellite remote sensing technology to obtain ground surface radiation information and analyzing and extracting local strong earthquake abnormal signs by the dynamic value of radiation field will help to improve the level of earthquake prediction.

This paper has been published in Chinese in the Journal of Seismological and Geomagnetic Observation and Research, Volume 37, Number 6, 2016.

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