2. Earthquake Agency of Dali Bai Prefecture. No. 10 Fuwen Road, Dali 671000, Yunnan, China;
3. Yunnan Earthquake Agency, No. 146 Beichen Road, Panlong District, Kunming 650011, China
At present, short-term earthquake prediction is still one of the most challenging targets worldwide (Eftaxias K. et al., 2003). So, exploring and investigating the earthquake precursors, especially those short-term ones associated with seismic activities, have always been the key problem of short-term earthquake prediction.
Electromagnetic observation, one of the most effective methods of pursuing earthquake precursors, has already attracted more and more public attention because the electromagnetic anomalies before large earthquakes have been investigated and confirmed during the last few decades (e.g., Pulinets S. A. et al., 2004; Molchanov O. A. et al., 2008; Hayakawa M. et al., 2015a, 2015b). Electromagnetic emissions in wide frequency range from ULF (ultra low frequency) to HF (high frequency) band prior to or during seismic activities have been reported by numerous researchers globally. Unusual ULF magnetic signals were observed about two weeks before the Loma Prieta MS7.1 earthquake, on October 17, 1989 (Fraser-Smith A. C. et al., 1990; Bernardi A. et al., 1991). Anomalous electromagnetic emissions were also observed prior to the great crustal MS6.9 earthquake at Spitak, Armenia, on December 7, 1988 (Molchanov O. A. et al., 1992; Kopytenko Y. A. et al., 1993), approximately 30 days before the August 8, 1993 MS8.0 Guam earthquake (Hayakawa M. et al., 1996; Kawate R. et al., 1998) and the great MS8.2 Biak earthquake in Indonesia on February 17, 1996 (Hayakawa M. et al., 2000). Possible seismic related ULF anomalies occurred about two weeks before the L'Aquila MS6.3 earthquake with the distance up to 630km (Prattes G. et al., 2011). Based on the ULF/ELF (extreme low frequency) observations in Nakatsugawa in Japan, Ohta K. et al., (2001) observed abnormal enhancement in ULF/ELF noise intensity one day before and after the famous MS7.6 Chi-Chi earthquake in Taiwan, China on September 21, 1999, and their goniometric directions suggested that those noise propagated from the direction of the earthquake epicenter. Li M. et al., (2013a) reported that obvious ULF electromagnetic abnormity had been observed for several months prior to the large Wenchuan MS8.0 earthquake on May 12, 2008 at Gaobeidian station, 1, 400km away from the Wenchuan epicenter. As the development of ionosphere-Earth integration observation, electromagnetic emissions associated with earthquakes have also been recorded by satellite-borne receivers (e. g., Ouzounov D. et al., 2011; Heki K., 2011; He Y. et al., 2011; Akhoondzadeh M., 2012; Li M. et al., 2012, 2013b; Li M. et al., 2018).
Corresponding to ground-based and satellite-borne techniques for monitoring the precursory electromagnetic emissions associated with seismic activities, the subsurface measurements of electric and magnetic field emissions with frequencies between ULF and VLF (very low frequency) have also attracted greater attention recently. The method avoids the part effect of disturbances outside the Earth's surface. VLF electric field perturbations were observed by borehole antennas instead of terrestrial antennas, 16 days prior to the Chamoli MS6.6 earthquake on March 29, 1999 in India (Singh R. P. et al., 2001).VLF pulse-like electromagnetic signals were also recorded by subsurface antennas before the MS5.8 east Yamanashi Prefecture earthquake (Fujinawa Y. et al., 1997).1 EXPERIMENTAL SYSTEM
The Dali TOA borehole electromagnetic observation locates in the northwest of Dali city and western Erhai Lake. The location is the intersection of NW (north-west) trending Erhai deep fault and NE (north-east) trending wave sub-fault and the center of Dali graben basin. Furthermore, the station is surrounded by farmland and there are no residences within 3km in the north and south directions, while some residences live approximately 300m and 1000m away from the center in the east and west direction, respectively. In measurement area, there are no industry, electrical sparks and ULF radio signal interferes within 3km. However, a pumping station in south-east direction can affect the observation to some extent when it is used seasonally and its influence will be demonstrated in the following part of this paper.
Benefitted from a cooperation with Japan, TOA electromagnetic measurement was put into service in November 1992. The depth of the observation well is 800.5m and casing pipes with different diameters reach to 800m underground. The observation system includes a sensor, a channel filter, a channel attenuator and a recorder. The sensor which consists of an 800m perpendicular borehole antenna in the casing pipes and a copper circle with 15m radius was buried 1m sub-surface (Fig. 1(a)). The system includes three observation channels: two ULF bands CH1 (0.01-0.10Hz), CH2 (0.1-1.0Hz), and one VLF band CH3 (1-9kHz). It measures potential difference between the borehole antenna and the copper circle. The measured potential difference is outputted by a drum recorder. The recording pen for each channel can record continuous analog signals as the drum is rotating automatically with a speed of 60cm/hr (see Fig. 1(b)). In general, the baselines of the recordings are stable and only three paralleled smooth lines without perturbation signals are left on the recording paper around the drum. In addition, the amplitudes of background electromagnetic noises of the observation are usually less than 0.05mV. Signals with the magnitudes more than 0.1mV (two times of the background value), and a certain last time are usually considered as anomalous information. Main signals related to seismic activities generally last more than four hours and their magnitudes are more than 0.3mV, which is about seven times of the background ones. The point is that there are different magnification times for three channels CH1, CH2 and CH3 when a signal is outputted by the recorder. For example, 1mV, 10mV and 50mV respectively for CH1 (0.01-0.10Hz), CH2 (0.1-1.0Hz) and CH3 (1-9kHz) if the amplitude of a signal is full of all lattices of the recording paper. In addition, positions of three recording pens can be adjusted manually to avoid the signals out of the recording paper and sometimes, there is a gap of the initial recording time between CH1 and CH2 to avoid a conflict of two recording pens when signal amplitude is large (also see Fig. 1(b)).
Li Wuxian et al. (2003) made an investigation on this TOA electromagnetic observation. 29 MS>5.0 earthquakes in total had been recorded in the ensuing decade after the installment came into service and obvious electromagnetic signals appeared prior to each event. Here are three of these 29 events acts as instances. Firstly, pulse-like signals appeared synchronously in CH1 and CH2 on November 25, 1992 and the main wave information with a 1.0mV magnitude was recorded at the same time in the two channels on November 28, 1992. Alarm signals happened 4 hours before the Yongsheng MS5.4 earthquake on December 18, 1992, 87km away from the Dali TOA station. Secondly, obvious wave-like emissions with irregular amplitudes up to ~1.0mV was recorded only in CH3 instead of CH1 and CH2 on January 9, 18 days prior to the Pu'er MS6.3 earthquake, which occurred on January 27, 1993 and the distance from the Dali station is 303km. Thirdly, the initial weak signals and main continuous impulses with the amplitude up to 3.5mV occurred respectively on August 24 and September 18, 1995, 61 days and 36 days prior to the Wuding MS6.5 earthquake on October 24, 1995, which has an epicentral distance of 201km. These three events are labeled by open circles in Fig. 4. In summary, their results show:
-Borehole TOA electromagnetic system can effectively record MS>5.0 earthquakes within 200km, MS>6.0 events within 300km, and MS>7.0 events within 500km.
- About 70% abnormal fluctuation processes of all events comply with the law of initial anomaly-enhancement anomaly-attenuation information (weak signals)-alarm signals-earthquake occurrence.
- In general, the duration of abnormity increases with the earthquake magnitude (from several days to several months).
- More signals appear in ULF band (CH1 and CH2) while less in VLF band (CH3).
- Coseismic electromagnetic signals are effectively recorded in a range from several seconds to dozens of seconds after the original occurrence time of strong earthquakes, depending on seismic epicentral distances.2 TYPICAL DISTURBANCES AND STATISTICAL RESULTS OF TOA RESPONSES TO STRONG EARTHQUAKES
The system can still not avoid all effects outside and it is affected mainly by thunder, AC/DC (AC, alternating current; DC, direct current) conversion, radio communication and automobile engine.
Fig. 2 shows part of typical disturbances recorded by TOA electromagnetic observational Dali station. In Dali city, dry season is usually from December to April. So disturbances caused by irrigation are seasonal and show a distinguished change process (Fig. 2(a)). Lighting disturbance displays discretely and discontinuously and is deviated from background signals (Fig. 2(b)). AC/DC conversion can lead to a pulse-like impulse in Fig. 2(c). Fig. 2(d) shows a type of civil disturbances and signals are equidistant and constant amplitudes. Although part of interference factors can be distinguished, there is still some uncertainty information like Fig. 2(d) during observations.
However, Li Wuxian et al. (2003) concluded that earthquake precursory signals are of significant difference from disturbances caused by most of interference factors. First of all, abnormal information lasts for a relative long time and has a developing process. Second, a precursor signal usually has a larger amplitude as well as frequency than that of a disturbance. Furthermore, compared with interference information with a simplex wave form, the wave forms of precursory variations are more complex. They are discrete or continuous impulses sometimes, or irregular square waves added together with instantaneous impulses sometimes. Last but not least, anomalous signals do not appear in a fixed time but randomly and irregularly.
A statistical work has been conducted during the last eight years from 2011 to 2018 according to MS>5.0 earthquakes within 200km, MS>6.0 within 300km, and MS>7.0 events within 500km utilized by Li Wuxian et al. (2003).The results are shown in Table 1. Abnormities appearing within one month are considered as a group-anomaly and Na is used to display the number of anomalous groups. On the contrary, earthquakes taking place within three months after the end of the anomaly are effective ones and their number is shown as Ne. R is the accumulative rate of Ne divided by Na for the time period of statistics. So it is not the same for different time. T1 is the time lag between the start of an anomaly and occurrance time of an event. From Table 1, we can see that for the statistical period during 2011-2018, the corresponding rate R is 15/23=65.2%, that is, 65.2% earthquakes happened during the first three months when TOA abnormity took place.
Obvious electrical changes synchronously appeared at 4:00 am. on May 30, 2018 at Dali TOA station.These changes were observed mainly in CH1 (0.01-0.10Hz) and CH2 (0.1-1.0Hz) observation channels, with magnitudes of ACH1≤0.26mV and ACH2≤0.6mV respectively. Fig. 3(a) shows part of the copy of real-time recording on May 30. Fig. 3(a) clearly shows that electromagnetic oscillations at this time are wave-like variations in CH1 and pulse-like signals in CH2. Small amplitude signals appeared intensively only in CH2 on June 4 (Fig. 3(b)).
From then on, amplitudes of electrical signals became larger and larger in CH1 and CH2. Pulse-like emissions were full of lattices of recording paper for CH2 during June 24-25 and slopped over the recording paper during June 28-29, with the magnitudes being greater than or equal to 10mV (Fig. 3(c)). At the same time, obvious wave-like signals also appeared in CH1 with a maximum magnitude of ~0.6mV (Fig. 3(c)). Fig. 3(d) shows part of copy of real-time observation on July 4 and its display is basically like what Fig. 3(c) described above. This condition remained until the end of July. During this time, anomalous variations appeared more frequently group by group and fluctuations of strengthened information have much longer time duration and larger magnitudes than before on the other hand.
From the end of July, abnormal information tended to attenuate gradually and there were no obvious abnormities appearing at this station in all three channels before the mid-August. During this period, two MS5.0 earthquakes occurred in Tonghai on August 13 and 14, 2018, 303km away from Dali TOA electromagnetic observing station, with the same epicenters located at 24.2°N, 102.7°E and the same focal depths of 6km (Fig. 4). Fig. 3(e) and Fig. 3(f) show part copies of the real-time recordings on August 13 and 14 during the period of two earthquakes events. From Fig. 3(e) and Fig. 3(f), no obvious coseismic response appeared when the two earthquakes occurred, which is not similar to previous events recorded according to Li Wuxian et al. (2003). On the other hand, the TOA system could effectively reflect earthquakes with magnitude ~5.0 within 200km (Li Wuxian et al., 2003). The epicentral distance this time is~303km and it is possible that these two Tonghai MS5.0 earthquakes were lost.
Instead, weak electromagnetic emissions appeared in two ULF channels again about 10 days after the two events and the information which was collected on August 26 and 27, 2018 strengthened gradually. Fig. 3(g) displays a copy of part of the record collected on August 27, 2018. From Fig. 3(g), CH1 and CH2 recorded electrical emissions with amplitudes of ACH1≤0.2mV and ACH2≤0.3mV respectively. Then the emissions decayed to be background quickly. During all these periods, CH3 (1-9kHz) recorded no obvious information. On September 8, 2018, a notable earthquake with MS=5.9 occurred in Mojiang, Yunnan, China, with an epicenter located at (101.5°E, 23.3°N), 300km away from Dali station (Fig. 4), and its focal depth is 11km. Fig. 3(h) is a copy of part of the record on the same day as the Mojiang MS5.9 earthquake occurred. It displays the most important point that obvious coseismic variations have been clearly recorded in CH1 and CH2. This is the key characteristic among most strong earthquakes recorded before (Li Wuxian et al., 2003).4 DISCCUSION AND CONCLUSIONS
Throughout the total fluctuations of the electrical emissions at Dali TOA station at this time, initial anomalous signals appeared at the end of May, 2018 in CH1 and CH2, with amplitudes of ACH1≤0.26mV and ACH2≤0.6mV. During the last 10-day of June to the middle July, a great number of main abnormities occurred in CH1 and CH2, with amplitudes of ACH1 up to ~0.6mV and ACH2 ~10mV respectively. Then, the signals tend to decay but emissions appeared again at the end of August with maximum amplitudes of ACH1 ~0.2mV and ACH2 ~0.3mV. The Mojiang MS5.9 earthquake occurred ten days after these alarm signals, on September 8, 2018. It is clearly that this variation process complies with the law of initial anomaly-enhanced anomaly-attenuating information (weak signals)-alarm signals-earthquake occurrence, which was confirmed by Li Wuxian et al. (2003).
Dali electromagnetic observation station includes three recording channels covering ULF (0.01-1.00Hz) band and VLF (1-9kHz) band. During the entire fluctuation process of electrical emissions, electrical information was mainly collected from ULF band of CH1 and CH2 instead of VLF band of CH3 channel, which recorded no obvious electromagnetic variations during about three months from June to August, 2018 at Dali TOA station.
It has been proposed that an earthquake is a dynamic evolution process and electromagnetic anormaly emissions are accompanying at every stages of its preparation, development and occurrence. In order to investigate the production mechanism of these electromagnetic signals associated with seismic activities, numerous rock-pressure simulation experiments indoor and explosive experiments in the field have been implemented worldwide. Laboratory experiments by Qian Shuqing et al.(1996, 2003) and Hao Jinqi et al. (2003) indicate that electromagnetic signals are always recorded when a rock sample is subjected to a dynamic stress. There is a close connection between produced signal and the formation of micro-cracks in a rock. Frequency band range of these electromagnetic emissions is so wide that covers ULF, VLF, HF and even VHF. Furthermore, the shape of the ULF electric and magnetic anomaly varied obviously in early-, mid- and late-term of the experiments. The work of Freund (Freund F. et al., 1982, 2007; Freund F., 2002, 2009, 2010; Scoville J. et al., 2015) has shed new light on the production of current and electromagnetic signals in stressed rocks although Dahlgren R.P. et al., (2014) proposed that there is much difference between produced current and electromagnetic signals when dry and fluid-saturated gabbro samples are stressed. However, it is impossible to confirm the right state of rocks with a depth of 10km or even several hundred kilometers.
Up to now, no clear explanation has been given although several physical mechanisms have been proposed to interpret the generation of EM emissions and electrical currents observed either during seismic activities or in laboratory experiments. These include the electrokinetic and magnetohydrodynamic, piezomagnetism, stress-induced variations in crustal conductivity, microfracturing etc. (Draganov A. B. et al., 1991; Park S. K., 1996; Fenoglio M. A. et al., 1995; Egbert G. D., 2002; Simpson J. J. et al., 2005). Whatever the physical mechanism of electromagnetic generation is, a commonsense obtained from experimental rock-pressure and seismic electromagnetic records is that these probable electromagnetic emissions are characterized by a wide frequency range from DC-ULF, VLF and LF to VHF. However, since the record of obvious ULF (f=0.01-10.00Hz) magnetic signals before the Loma Prieta MS7.1 earthquake, on October 17, 1989 (Fraser-Smith A. C. et al., 1990; Bernardi A. et al., 1991), more attentions have been paid to the ULF band. The ULF band is of particular interest because only EM signals in the ULF range and at lower frequencies can be easily recorded at the Earth's surface without significant attenuation compared with "high" frequency emissions that might be emitted at epicenter depths at more than 10km, even several hundred kilometers. This is a probable reason why VLF band at Dali TOA recorded no electrical anomaly before the Mojiang MS5.9 earthquake.
In addition, clear coseismic signals has been recorded when the Mojiang MS5.9 earthquake took place. It is not precedented that coseismic phenomena have been mentioned. Li Wuxian et al. (2003) has shown clear coseismic signals appeared ~23s after the original time of Yongsheng MS5.4 earthquake on December 18, 1992. The epicentral distance is ~87km (in Fig. 4). In light of these, the average speed of coseismic information is of ~3.8km/s, which is in the speed range of seismic waves in lithosphere (~3.2-7.0km/s). Fortunately, Tang Ji et al. (2008) established two observation systems, including one MT (magnetotelluric) instalment produced by Metronix Canada and one ELF measurement produced by Russia in the Wenchuan MS8.0 aftershock area on May 22, 2008, 10 days after the Wenchuan main shock occurred on May 12, 2008. Their results confirmed that coseismic emissions were included in all components of electrical and magnetic fields and they arrived synchronously together with seismic waves. These results were consistent with those of Karakelian D. et al., (2002), who observed that coseismic ULF signals exist on all components of the magnetic and electric fields and begin with the arrival of seismic waves. Mogi T. et al., (2000) found coseismic changes in the electric field and inferred that these changes began at the arrival time of a seismic wave by examining the relationship between variations in the electric field and earthquakes in Sumatra, Indonesia. Similar results has also been attained by Nagao T. et al., (2000), who concluded that electric field changes started at the arrival times of seismic waves, rather than the original times of earthquakes, and claimed that the generation mechanism of changes was related to the electrokinetic effect. On the other hand, Iyemori T. et al., (1996) reported a possible mechanism associated with coseismic geomagnetic variations of an induction effect due to crustal dynamo mechanism. However, Matsushima M. et al., (2002) considered that such an electromagnetic variation should propagate faster than the seismic wave itself. They also proposed seismo-dynamo effect of electromagnetic induction in the electrically conducting Earth caused by ground motion in the Earth's magnetic field (Honkura Y. et al., 2000).
Unfortunately, the point is the coseismic response recorded during the Mojiang MS5.9 earthquake shows undistinguished waves (Fig. 3(h)) because of the recording pen used now is too abraded to leave clear lines on the recording paper. The rotating speed the recording drum is 1cm per minute, which makes signal waves combined and undistinguished. At the same time, the beginning time of the coseismic signals is probably not accurate on account of TOA instrument aging. However, we still consider that this coseismic information arrived together with seismic waves based on the description by Li Wuxian et al. (2003).
In summary, among three observation channels (CH1 0.01-0.10Hz, CH2 0.1-1.0Hz and CH3 1-9kHz) at Dali TOA electromagnetic observation station, two ULF channels recorded obvious electrical abnormities from the end of May to the end of August, 2018. Initial weak signals with magnitudes of ACH1≤0.26mV and ACH2≤0.6mV firstly appeared on May 30, 2018. From then on, abnormal information gradually strengthened either in magnitude or in occurance frequency and it reached its climax with a maximum magnitude of ACH1 ~0.6mV and ACH2 ~10mV from the end of June to the mid July, 2018. The information started to decrease from the end of July and only weak signals occasionally occurred till the end of August, the time that obvious anomalies were recorded again in two ULF channels, with maximum magnitudes of ACH1 ~0.2mV and ACH2 ~0.3mV respectively. 10 days later, the Mojiang MS5.9 earthquake occurred on September 8, 2018, 300km away from Dali TOA station and a coseismic response has also been recorded during this period. Thus, these ULF electromagnetic abnormities can be attributed to this Mojiang event to some extent.
Meanwhile, on the basis of the previous works, the corresponding results show borehole TOA electromagnetic observation in Dali are able to response effectively to some degree to earthquake events within a limited range. The related data have been put into daily use effectively and play more and more important role in regional earthquake prediction. Borehole observation including electromagnetic and resistivity methods has attracted more and more attentions and it will be apromising apporach to search more natural hazard precursors.ACKNOWLEDGMENTS
We are grateful to Prof. Wang Haitao and Prof. Liu Guiping for their help in the preparation of this work.
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