Earthq Sci DOI 10.1007/s11589-017-0180-9 RESEARCH PAPER Correlation between abnormal trends in the spontaneous fields of tectonic plates and strong seismicities Da-Cheng Tan . Jian-Cun Xin Received: 18 January 2017 / Accepted: 6 April 2017 Ó The Author(s) 2017. This article is an open access publication Abstract Tectonic activities, electrical structures, and analysis (Geller 1996; Wyss et al. 1997; Uyeda 2000). electromagnetic environments are major factors that affect Complex electromagnetic environments, dynamically rich the stability of spontaneous fields. The method of corre- observation data, developing observation techniques and lating regional synchronization contrasts (CRSC) can theoretical understanding, and basic dysfunctional short- determine the reliability of multi-site data trends or short- impending predictions of catastrophic earthquakes cause impending anomalies. From 2008 to 2013, there were three credibility problems in geoelectric field observations and strong earthquake cluster periods in the North–South short-impending predictions (Geller 1996; Huang 2006). seismic belt that lasted for 8–12 months. By applying the If the confirmation of the presence of background vari- CRSC method to analyze the spontaneous field ESP at 25 ation trends in the geoelectric fields of the site is possible sites of the region in the past 6 years, it was discovered that before the occurrence of catastrophic earthquakes, then the for each strong earthquake cluster period, the ESP strength implementation of corresponding short-impending geo- of credible anomalous trends was present at minimum 30% electric field information analysis will have a more credible of the stations. In the southern section of the Tan-Lu fault basis. Geoelectric field observation data comprise sponta- zone, the ESP at four main geoelectric field stations showed neous fields, telluric fields, and interference information, significant anomalous trends after June 2015, which could which determine the necessity of reliable data analysis. The be associated with the major earthquakes of the East China VAN method (Varotsos and Alexopoulos 1984a, b)isa Sea waters (MS 7.2) in November 2015 and Japan’s short-impending earthquake prediction method based on Kyushu island (MS 7.3) in April 2016. geoelectric field anomalies, which relies on the long and short polar distances in the same location to explore seis- Keywords Spontaneous field Á Anomalous trend Á mic electric signals (SESs) and initiate predictions thereof Reliability Á Strong earthquake Á Correlation (Ma 2008). However, the VAN method is only used for short-impending earthquake prediction with obvious con- troversies (Geller 1996; Huang 2006), which is based on 1 Introduction the criticality model that electric signals are emitted when in the future earthquake focal area the gradually increasing In seismic predictions, geoelectric fields are mainly applied stress reaches a critical value so that the existing electric in disputed and short-impending precursory information dipoles due to defects exhibit a cooperative orientation (Varotsos 2008). In the analysis of strong regional seis- micities, it is a common method for the statistical analysis D.-C. Tan (&) Á J.-C. Xin Lanzhou Institute of Seismology, China Earthquake of seismic catalogs. To improve the reliability of precur- Administration, Lanzhou, 730000, China sory seismic anomaly statistics, Huang introduced a fea- e-mail: [email protected] sible RTL (region-time-length) statistical method (Huang 2006; Huang and Liu 2006). This approach requires a D.-C. Tan Lanzhou National Observatory Geophysics, Lanzhou 730000, relatively stable data background. In a region with intense China 123 Earthq Sci tectonic activities, frequent strong quakes, and poor data Liu 2006). The device comprises 8 pairs of mutually stability, such as the vicinity of the North–South seismic orthogonal long (several kilometers) and short (tens of belt in recent years, prediction using the VAN method or meters) polar distances. Mainland China’s devices are surveying the precursory anomalies using the RTL method basically laid out in double-triangle shapes (also called is very difficult. L-shapes) as shown in Fig. 1c. The ratio of the long and In the North–South seismic zone, the time quasi-syn- short polar distances in the same direction is about 1.5, chronization phenomena occurs in the turning trends of with the long distances mostly being 300–400 m. spontaneous fields at multiple sites near the same fault zone Geoelectric field observation data usually includes (Tan et al. 2014). Similar phenomena also occur near the spontaneous fields, telluric fields, and interference infor- Tan-Lu fault zone. This study introduces the correlating mation. ESP represents spontaneous field, ET represents regional synchronization contrasts (CRSC) method for telluric field, and ER represents the signal interference determining the reliability of spontaneous field anomaly factor. The composition of the observed value of geo- trends. Time corresponding phenomena were found electric field can be written as between the trends of dynamic spontaneous field anomalies E ¼ ESP þ ET þ ER: ð1Þ and strong cluster earthquakes in the statistics of the trend variation of 25 stations on the North–South seismic belt When the conditions of the electromagnetic environ- and 70 earthquakes of MS 5.0 and above from 2008 to ments and the observation systems are ideal, the observed 2013. In the Tan-Lu fault zone, correspondence existed value of the i-th minute is set as Ei, and the daily mean between the trend anomalies in 2015 and increased tectonic value of the strength of ESP calculated using the value data activities in the region of Japan. of minutes in one day can be expressed as formula (2). When formula (2) is applied for calculation, the relatively stable main composition of ET, namely the effect of tidal 2 Characteristics of trend variation in the spontaneous geoelectric field, is eliminated (Tan et al. 2012): field strength 1 X1439 E Á E : ð2Þ SP 1440 i Although the basic principle of geoelectric field observa- i¼0 tion devices is originated from the VAN method, their The spontaneous field on day j is set as ESP(j), and the layout can be diverse. In general, the observation devices ESP daily jump can be expressed by formula (3): based on the VAN method comprise 2–3 pairs of elec- DE ¼ E À E : ð3Þ trodes, respectively, in the EW and NS directions. The SPðjÞ SPðjþ1Þ SPðjÞ polar distance ranges from tens to hundreds of meters. In general, when the determination of the impact of Moreover, 2–4 pairs of 1–10 km-long pole distance can be electromagnetic environment and observation devices at laid with appropriate configuration of their electrodes the observation site is difficult, the reliability of results (Varotsos et al. 1991, 1993). In 1990, the Sino-French calculated via formulae (2) and (3) must be confirmed. Electromagnetic Cooperation Project established the When plotting the curves of ESP and DESP, to show their observing devices at the Songshan (SHN) station in long-impending trends, substantial kick data in a short Tianzhu, Gansu, as shown in Fig. 1a (polar distance of duration (less than ten days) are deleted. several kilometers was not laid). Japan Institute of Physical The observation environments and device systems of Chemistry’s electrode layout models installed on the new two stations in Shandan, Gansu, and Haian, Jiangsu, are isle of the Izu islands are as shown in Fig. 1b (Huang and favorable. Within the range of 100 km, no earthquake of Fig. 1 Schematic of various geoelectric field orthogonal observation devices. a Orthogonal devices based on the VAN method, b orthogonal devices of Japan’s new isle, and c China’s orthogonal devices 123 Earthq Sci MS 5.0 or above occurred in the past 5 years. The ESP South seismic belt also appeared in the southern section of patterns at these two sites are stable and their annual the Tan-Lu fault zone in correspondence to the ESP chan- rangeability does not exceed 100 mV/km, with a slightly ges of the stations in Jiashan, Anhui and Lingyang, Shan- larger jump of DESP before and after the summer season, as dong, as shown in Fig. 3c. shown in Fig. 2a, b. The locations of the two stations are shown in Fig. 2c. Figure 2a, b reflects that ESP, and DESP are relatively stable in the local areas where the earthquake 3 The reliability of spontaneous field anomalies activities are weak, thus showing the annual change pattern. 3.1 Reliability analysis method for short-impending Figure 2d indicates the two regions studied here. Fig- and trend anomalies of spontaneous field ure 2e, f is the trend curves of ESP and DESP of the Lugu lake and Luo’ci stations on the southern section of the At present, there are at least two methods for determining North–South seismic belt, respectively. The locations of the reliability of short-impending anomalies of sponta- these two stations are shown in Fig. 2c, wherein the ESP neous fields ESP. The first method is based on the principle and DESP curves show their significant range ability and of the VAN method for calculating the ratio of data vari- violent jumps; however, the drastic changes in 2013–2014 ation of long-range and short-range polar distance at the showed better time corresponding phenomena. In recent same station in the same direction. If the ratio is close to years, strong seismicities violently occurred in the North– one, the variation is considered to be a SES. Considering South seismic zone. The long-term ESP stability of most Changli station in Fig. 3a as an example, after zeroing stations in the vicinity was poor, with large magnitudes of treatment on the short polar distance data, the variation changes and unclear cyclical annual changes, whereas ranges of EEW(L), EEW(S), ENS(L), and ENS(S) during the phenomena like persistent fluctuation, leap, and bound time interval 15:54–16:13 were almost equal to each other.