Electromagnetic anomaly before earthquakes measured by electromagnetic experiments

395 Earthq Sci (2009)22: 395−402 Doi: 10.1007/s11589-009-0395-5 Electromagnetic anomaly before earthquakes measured by electromagnetic experiments∗ Guoze Zhao Ji Tang Yan Zhan Lifeng Wang Jijun Wang Qibin Xiao and Xiaobin Chen Institute of Geology, China Earthquake Administration, Beijing 100029, China Abstract Three experiments are carried out for earthquake monitoring using electromagnetic (EM) methods in recent years. Some earthquakes occurred in chance of the measurement time period for each experiment and the anomalies were recorded before the shocks. The observation at a site 20 km away from the epicenter of Zhangbei MS6.2 earthquake in 1998 shows that the apparent resistivity decreases in the strike direction before and/or during the earthquake and the resistivity increases in the decline direction. This anomalous variation in apparent resistivity accounts for about 20%. The apparent resistivities at a site in the epicentral area decrease in the strike and decline directions before and/or during the earthquake and increase after shocks. The experiments using artificial electromagnetic signals with super low frequency carried out in 1999 show that the resolution and stability of electric and magnetic spectra are improved. The spectra of electric and magnetic fields and apparent resistivity at the Baodi station began to anomalously change two days before the Qian’an earthquake with 120 km distant to the station. The anomalous variation of electric and magnetic spectra is about twice as great as normal variation and the apparent resistivity changes by about 20%. The measurements in active seismic area of Yunnan province in the year 2005 indicate that the electric and magnetic spectra anomalously change by one order before the Taoyuan earthquake about 100 km away from the observatories. But the measurements at the sites in Beijing area 2 000 km away from the epicenter do not show any anomaly. Key words: electromagnetic anomaly; apparent resistivity; artificial SLF method; EM experiment; magnetotellurics CLC number: P318.6 Document code: A 1 Introduction Since the 1970s the anomalous change of resistivity in the crust prior to earthquakes has been studied and the valuable data have been obtained (Reddy et al, 1976; MT Group of Institute of Geology, State Seismological Bureau, 1981; Tang et al, 1998; Zhao et al, 2003). Although it is debatable whether or not the earthquake can be predicted, in other words, it can be announced for time, position and amplitude of an earthquake before the earthquake (Tselentis and Melis, 1996), the increasing data of earthquake monitoring have shown that the electromagnetic anomalies indeed exist before disastrous earthquakes and the events can be predicted if the relationship between these electromagnetic effects and the physical process in the crust can be understood (Johnston and Uyeda, 1999). ∗ Received 1 April 2009; accepted in revised form 27 May 2009; published 10 August 2009. Corresponding author. e-mail: Bleier and Freund (2005) reported that accurate earthquake warnings are, at last, within reach. Instead of coming from the mechanical phenomena that have been the focus of decades of the study, they will come from electromagnetic phenomena. The positive relation amounts to about 80% between earthquakes and electromagnetic (EM) anomalies detected by using satellite observation (Parrot, personal communication, 2006). Several satellites have been launched in recent years aimed at test of EM monitoring on the earthquake (Parrot, 2002; Zhao et al, 2007a, b). Since the Xingtai earthquake occurred in 1966 electromagnetic methods have been applied to earthquake monitoring in China. However most of data come from observations using traditional electric and magnetic methods (Ma et al, 1982; Zhao et al, 2007b). This paper will present the observed data using the electromagnetic (EM) methods similar to magnetotellurics (MT) but different from traditional electric and magnetic methods. In the EM methods the data of electric and magnetic components are recorded at each site simultaneously. The 396 depth in the crust detected by observed EM data is related to the signal frequency and is larger than those by traditional electric methods. More accurate geoelectric structure beneath observation sites can also be obtained (MT Group of Institute of Geology, State Seismological Bureau, 1981). Prior to early 1990s the magnetotelluric method was used to monitor the Tangshan earthquake (MS7.8, 1976) and earthquakes occurred in central Qilianshan in northwestern China. The anomalous variations in resistivity before and after earthquakes were recorded (MT Group of Institute of Geology, State Seismological Bureau, 1981; Zhang et al, 1987, 1994; Wang et al, 1993). During the last decade either equipments or data processing of the EM method are improved so that the data quality is enhanced and more confident information is achieved (Tang et al, 1998; Zhao et al, 2003, 2005; Qian et al, 2009). A few experiments on monitoring crustal resistivities and electromagnetic anomalies will be presented in this paper by natural and artificial electromagnetic methods. The data from a newer observation in Yunnan seismic area will be analyzed along with the previous observed data (Tang et al, 1998; Zhao et al, 2003). The data of electrical field, magnetic field and/or resistivity presented in the paper cover the whole measurement time periods without artificial selection. Fortunately, the earthquakes occurred in chance of all these measurements and anomalous variations of resistivity and electromagnetic fields were recorded although it is the only experiment carried out by our group with purpose to test the possibility of EM methods for earthquake monitoring. It is expected that it will richen practical examples for earthquake monitoring by using electromagnetic method, which is helpful to the studies on earthquake prediction using these experiments. 2 Resistivity variations before and after the Zhangbei earthquake Zhangbei MS6.2 earthquake occurred in the conjunction area of Zhangjiakou-Penglai seismic belt with Shanxi seismic belt in North China on January 10, 1998. The epicenter is located at Meimaocun village (MMC) in Zhangbei county. The focus was at about 12.8 km depth (Yang and Chen, 2004). Magnetotelluric (MT) measurements were carried out on 1 June 1994 at the site 511 in Daijiafangzi of Wanquan county, about 20 km away from the epicenter of the Zhangbei earthquake. MT observations were car- Earthq Sci (2009)22: 395−402 ried out repeatedly at the site 511 from January 13 to 14 and from 20 to 21, 1998 after the earthquake. The observations were also carried out at site Meimaocun village (MMC) in the epicentral area. MT measurements were performed neither for longer time period nor for more sites due to the difficult conditions, e.g., very cold climate (−30 °C). The equipment MMS-03E from Metronix was used in the MT measurement. The electrodes were (...truncated)