Main shocks of natural earthquakes are known to be accompanied by preshocks which evolve following the modified Ohmori’s law in average over many samples. Individual preshock activity, however, is far less systematic for predictive purposes. On the other hand, the microcracks in laboratory rock experiments are always preceded to final rupture. And, previous investigations of field acoustic emissions showed that the activity increases prominently before and after the main shock. But there is no detection of any phenomena to identify the nucleation stage. Here we show that a special underground electric field measurement could detect microcracks. Pulse-like variations were classified into three groups (A, B, C) by frequency. The B-type is suggested to define the nucleation period: activity increases sharply following the modified Omori’s law before the main shock and there is no activity afterward. The B-type is subgrouped into three types possibly corresponding to crack-rupture modes. The variations are supposed to be induced by crack occurrence through electrokinetic effects in the elastic-porous medium. The detection distance is suggested to be several orders larger than that of the acoustic emission due to the effective smallness of dissipation rate, and the waveform can be used to infer the rupture mode.
The identification of anomalous phenomena in each stage of seismic cycle is a prerequisite for understanding physical processes to be applied to earthquake forecasting (e.g., Mogi [
Among all, the preshock has been considered to be the most direct and plausible phenomenon for identifying the earthquake nucleation stage. Previous investigations of preshock activity of natural earthquakes or microcracks activity in laboratory experiments (e.g., [
As for natural earthquakes, however, the evidence is generally limited to average images. There is large extent of variability in particular cases of earthquake with the result of no plausible clue to be applied for prediction of individual earthquake (e.g., [
On the other hand, the rock fracture experiment can always observe the characteristic rapid increase of acoustic emission activity in the nucleation period [
The acoustic emission technique has been widely used in the geotechnical engineering field to examine geological materials since the late 1930s (e.g., Drnevich and Gray [
Electromagnetic approaches have been providing many interesting clues to shed light on preparatory process of earthquakes (e.g., Park et al. [
In the present paper we present an electromagnetic field observation of microcracks by using high quality instruments providing new clues to detect particular features to identify the nucleation stage.
We used electric sensors to observe microcracks instead of ordinary acoustic emission transducer. The sensor in the field is a special antenna made of a vertical casing pipe that is installed into a borehole of 200 to 1,800 m deep. The boreholes are not for electric field measurement but for intrinsic purposes as measurements of seismic motion, ground water level, and chemical contents; ground deformation or pumping of natural gas or waters for drinking; and hot spring belonging to several organizations and private persons (Fujinawa et al. [
Schematic diagram of multiple component electromagnetic field observations [
The electrical circuit is composed of the preamplifier, filtering, and data logging. Additionally a simple counter surge circuit is inserted between sensors and preamplifiers to withstand electric surge induced by lighting discharge. The recording frequencies are DC band (0–0.7 Hz), ULF (0.001–0.7 Hz), and ELF/VLF (1 kHz–9 kHz). The data of VLF band is logged by an event trigger system focused to locate the VLF pulse-like signal for early stage of investigation. Later envelope of electric strength is recorded to monitor the activity of the field. Lower frequency component is recorded continuously with the sampling interval of 2 seconds. The present observation used continuous recording.
Prior observation has confirmed that the system has high robustness against both meteorological and urban noise enabling us to get large signals related to the crustal activities (Fujinawa et al. [
The observation network has been built and operated since 1989, and the sites situate at several kinds of geological conditions nearly uniformly in central Japan (Figure
Locations of the electromagnetic field observation sites (⚫) in central Japan utilizing borehole antenna 150–1200 m long as the principal sensor. Those sites have been constructed during 1989~1996. Every site measures the vertical component of the electric field. Additionally mutually perpendicular short dipoles of 20~60 m long were used for measurement of horizontal electric component at five sites (Hasaki, Chikura, Koufu, Nagaoka and Sagara) by 2002. And the induction type magnetometers were used at two sites (Nagaoka, and Sagara) by 2002 [
The ULF band characteristic waveform was first identified by the temporally rapid recording at the time of volcanic eruption activities in 1990 at Izu-Oshima (Figure
(a) Records of electric field anomalies on 11 October, 1990 related to the small volcanic activities in the Izu-Oshima Island by the borehole antenna [
The present observation was conducted using detectors having larger dynamic range. The experiment was originally to investigating the coseismic signals with propagating velocity much larger than the seismic wave (Fujinawa et al. [
Geographical relationship between the observation site Hasaki (◆) and the rupture zone 500 km × 200 km (dashed line). Calculated displacement distribution (↗) of the Tohoku Earthquake (★, magnitude 9.0). Also shown are the two largest preshocks, which occurred two days before (⚫, M7.3) and one day before (⚫, M6.8) the earthquake (revised from Suzuki et al. [
To construct diurnal monitoring of DC and AC components we averaged in the interval of 1 sec and/or resampled by 150 Hz from the original data sampled 4.5 kHz (by 11th March or 18 kHz after 24th March). Figure
Comparison of two kinds of monitoring data. (a) Raw data on 25th March 2011 sampled by 18 kHz and averaged for 1 second. Small drift of about 0.003 mV is due to the limit of circuit performance. (b) The same raw data through resampling by 150 Hz and averaged for 1 second.
Simple average (1 sec)
150 Hz sampling and average (1 sec)
Figure
Monitoring records of normal state and active state. (a) DC channel record on March 31, 2011 corresponding to the normal state without any short period fluctuation of large strength except for a diurnal variation due to the earth and ocean tide. There were no conspicuous environmental noises to make it difficult to detect anomalous signals. (b) DC channel record on March 25 with pulse-like fluctuations around 1:00 and 23:00. The signals are very similar to those found in the analog record at several cases of crustal activities [
Whole pulse-like variations with strength greater than 2 mV about 40 times the background noise level of 0.025 mV were picked up from the raw data. There were a total of 225 events. The successive large pulses corresponding to nearby lightning have not been detected. Most of the events are isolated in the sampled interval of 100 ms except cases superimposed small events as described in the discussion section. The largest event is of the coseismic variation corresponding to the main shock reported previously [
Waveform of the pulse-like particular signals. Every event has large strength compared with noise level with the result of none data filtering. (a) DC band (type A). Type A has a duration of several tens of seconds to several tens of minutes and a height of about 2-3 mV. The form was first identified by the temporally rapid recording at the time of volcanic eruption activities in 1990 at Izu-Oshima [
DC: A-type
AC: B-1 type
AC: B-2 type
AC: B-3 type
AC: C-type
The type of an event (Figure
Types B and C events were firstly identified in this observation of increased dynamic range in our method. They have a much smaller duration time from 10 ms to several tens of ms for type B and 0.3–0.5 ms for type C compared with that of A-type of several minutes.
Type B pulses are subdivided into three types depending on their waveform (Figures
Similar electric variation without stepped decays had been previously reported in association with acoustic emission in rock deformation experiments [
The simple relaxation type electric variation was also deduced in the analytical estimation by Fenoglio et al. [
Type B-2 (Figure
Type B-3 (Figure
Type C (Figure
In the crack theory [
Figure
The Tohoku Earthquake, magnitude 9.0, occurred on March 11, 2011. There are several time periods (grey horizontal line) without data due to logger problems before the main shock and regional electric power shutoff. (a) Time evolution of number of pulse-like signal per hour of type A (green). The unit is shown on the left hand side. Almost all type A variations occurred after the main shock. (b) Time evolution of type B (red) including B-1, B-2, and B-3. The type B variation is suggested to be particularly useful for imminent prediction. The bold continuous curve attached by
From these results we can understand the dominance of the positive polarity in diurnal monitoring record on March 9 (Figure
As for the B-type variations, there are only several events per day on March 3 and 4. The number of events started to increase on the 7th, had a prominent peak on the 9th, a pronounced lull on the 10th, and recovered considerably on the morning of 11th until the earthquake occurred. The lull does not mean there are no activity of microcracks. The smaller the threshold is, the more the number of events are. The feature is in agreement with the evolution of space-time distribution of microcracks of rock fracture experiment [
The activity of the type B phenomena evolved similarly to those of the acoustic emissions just before the rupture in the rock experiment [
Fitting the modified Ohmori law
The three types, B-1, B-2, and B-3, occurred in nearly steady percent, 33, 46, and 21% for the whole period. However the compound type (B-3) decreased its role in the more active period (from 9 to 11, March) with the result of increase of activity of tensile type (B-1) and shear type (B-2)
Evolution of the cumulative numbers
Occurrence of higher frequency events of C type and low frequency A type after the main shock can be interpreted according to the after effect stage of the seismic cycle. It is supposed that there is a strong adjustment of stress and strain in the rupture area inducing normal closure and shear slips (e.g., Yoshioka and Scholz [
The main shock occurred on March 11 some 50 km south of the foreshock at almost the same region as predicted and was made public several years before. There occurred two foreshocks of major class, one of magnitude 7.3 (March 9) and other of 6.8 (March 10). The microcracks might be imagined to be associated with the foreshocks that occurred two days before the Tohoku earthquake when the activity of microcracks was largest. There were no coseismic electric variations corresponding to these foreshocks themselves as detected previously in 2002 [
It has been revealed that the great rupture consist of three subevents: main asperity and subsequent two events [
Anomalous electric signals corresponding to main shock are shown in the lower sheet of Figure
Coseismic electric field variation for the Tohoku earthquake (3.11) at Hasaki (lower sheet) and the strong motion seismograph (UD component) at nearest K-net observation site CHB005 (upper sheet, (1): EW, (2): NS, (3): UD). The vertical line “org”, “P”, and “S” are origin time of 3.11, P wave arrival time, and S wave arrival time at CHB, respectively. There is very faint electric phase corresponding P arrival time and none phase corresponding to S arrival. The first large negative peak coincides with the peak acceleration and the second prominent peak did not correspond to any phase.
The fault displacement shows a simple distribution [
Aftershock occurred in an extensive area in and around the main rupture of 3.11. During 3 weeks after the main shock there occurred as many as 427 aftershocks with a magnitude larger than 5.0 in and around the rupture zone of 3.11. In this period there occurred A- and C-type variations of highest frequency of some 5 kHz (Figure
Electromagnetic emission observation in VLF bands has been widely conducted to investigate the nucleation process. It is generally reported that there are considerable correlation between occurrence time of earthquakes and those of emission: the strength of electromagnetic emission increase before and/or after earthquake (e.g., [
The AE monitoring for rock burst is usually conducted with the spacing of several tens of meter (e.g., Hasegawa et al. [
The detection distance of seismoelectromagnetic signals (SES) by the borehole antenna is suspected to be some three orders much larger than that of the acoustic emission method [
Fujinawa et al. [
The characteristic electric field variation induced by cracks through electrokinetic mechanism can be more systematically discussed on the basis of formulation of Pride [
There are some events in B types which have clear primary phase preceding main phase as illustrated in Figure
Sample waveform of events having the primary phase (“P” in the figure) before the main phase (“S” in the figure). The EM mode is suspected at around the origin time
The S-P time of events A20110308_04h49m03s (Figure
Catalog B-type Pulse 2011-03-03
Number | Second | Hour | Type | Strength (mV) | Duration (ms) | Comment |
---|---|---|---|---|---|---|
3-March | ||||||
1 | 74089.15 | 20.58 | B-2 | −3.5 | 40 | P phase? |
2 | 76994.65 | 21.39 | B-3 | −5.5 | 30 | P phase |
3 | 77752.56 | 21.60 | B-1 | 2 | 25 | |
| ||||||
4-March | ||||||
4 | 63758.70 | 17.71 | B-3 | 11.5 | 30 | |
5 | 64522.10 | 17.92 | B-2 | 5 | 40 | |
6 | 65485.30 | 18.19 | B-2 | −11.5 | 10 | |
7 | 65937.20 | 18.32 | B-2 | 12 | 10 | |
8 | 81148.30 | 22.54 | B-2 | −3.0 | 10 | P phase |
| ||||||
6-March | ||||||
Till 14:00 (no-data) | ||||||
8 | 50571.1 | 14.05 | B-1 | 2 | 30 | |
9 | 53094.25 | 14.75 | B-1 | 3.8 | 40 | |
10 | 53336.8 | 14.82 | B-1 | 16.0 | 40 | |
11 | 64179.30 | 17.83 | B-1 | 3.3 | 40 | Clear P phase |
12 | 65995.55 | 18.33 | B-2 | 6.5 | 14 | |
13 | 70869.25 | 19.69 | B-2 | 3.5 | 20 | |
14 | 75902.15 | 21.08 | B-2 | 9.5 | 25 | |
15 | 83206.30 | 23.11 | B-3 | −2.2 | 40 | |
16 | 84237.55 | 23.40 | B-2 | 8.3 | 25 | Subevents |
| ||||||
7-March | ||||||
16 | 16950.85 | 4.71 | B-2 | 2 | 15 | |
17 | 20103.65 | 5.58 | B-2 | 3.3 | 12 | |
18 | 64481.80 | 17.91 | B-2 | 2.1 | 13 | |
19 | 68579.95 | 19.05 | B-1 | 2.2 | 37 | |
20 | 69144.05 | 19.21 | B-3 | 4.2 | 40 | |
21 | 69559.45 | 19.32 | B-2 | 2.1 | 9 | |
22 | 79177.40 | 21.99 | B-3 | 7.9 | 45 | |
| ||||||
8-March | ||||||
23 | 17343.05 | 4.82 | B-2 | 8.0 | 35 | P, S phase |
24 | 21232.45 | 5.90 | B-3 | 12.4 | 45 | P, S phase |
25 | 21512.30 | 5.98 | B-1 | 4.1 | 37 | HF. At 1st step |
26 | 26099.85 | 7.25 | B-2 | 2.1 | 4 | |
27 | 26871.50 | 7.46 | B-3 | 5.7 | 37 | |
28 | 31388.05 | 8.72 | B-1 | 7.0 | 45 | Subevent in coda |
29 | 33307.20 | 9.25 | B-1 | 4.1 | 60 | P, S phase? |
30 | 33624.30 | 9.34 | B-1 | 8.5 | 60 | |
31 | 39163.85 | 10.88 | B-1 | 5.1 | 43 | |
32 | 39727.80 | 11.04 | B-1 | 4 | 40 | |
33 | 42297.30 | 11.75 | B-2 | 2 | 7 | |
34 | 44458.00 | 12.35 | B-2 | 2 | 15 | |
35 | 75881.30 | 21.08 | B-2 | 2.2 | 8 | |
36 | 79821.30 | 22.17 | B-3 | 5.8 | 30 | |
37 | 80090.35 | 22.25 | B-3 | −3.8 | 45 | |
38 | 80203.15 | 22.28 | B-2 | 6.2 | 20 | |
39 | 82117.00 | 22.81 | B-3 | 11.5 | 50 | |
40 | 82234.60 | 22.84 | B-1 | 3 | 40 | |
41 | 82617.35 | 22.95 | B-3 | 5.8 | 40 | |
42 | 83904.35 | 23.31 | B-2 | 2.7 | 10 | |
43 | 83904.40 | 23.31 | B-2 | 5 | 10 | |
44 | 85307.20 | 23.7 | B-2 | 4.1 | 10 | |
45 | 85885.95 | 23.86 | B-2 | −11.7 | 25 | |
46 | 86122.75 | 23.92 | B-3 | −2.8 | 30 | P, S phase? |
47 | 86738.30 | 24.09 | B-2 | 2.2 | 10 | |
48 | 86953.40 | 24.15 | B-2 | 4.2 | 15 | |
49 | 87379.20 | 24.27 | B-2 | 2.1 | 10 | |
50 | 87590.00 | 24.33 | B-2 | 6.0 | 10 | |
51 | 89027.35 | 24.73 | B-1 | 5 | 45 | |
| ||||||
9-March | ||||||
52 | 2493.70 | 0.69 | B-1 | 2.5 | 47 | |
53 | 3817.85 | 1.06 | B-2 | 3.7 | 33 | High freq.com at 1st step 1.5 kHz |
54 | 4109.35 | 1.14 | B-3 | 8.3 | 43 | |
55 | 5327.10 | 1.48 | B-3 | 6 | 44 | |
56 | 5374.60 | 1.49 | B-1 | 2.2 | 32 | |
57 | 6221.80 | 1.73 | B-3 | 2.1 | 40 | |
58 | 6272.40 | 1.74 | B-3 | 3.5 | 50 | Complex |
59 | 6454.95 | 1.79 | B-3 | 5.4 | 60 | |
60 | 6684.75 | 1.86 | B-3 | 8.9 | 30 | P, S phase? |
61 | 7091.50 | 1.97 | B-3 | 18.3 | 40 | P, S phase? |
62 | 8501.15 | 2.36 | B-1 | 3 | 30 | |
63 | 11404.10 | 3.17 | B-1 | 3.5 | 35 | P, S phase? |
64 | 12040.80 | 3.34 | B-1 | 3.5 | 40 | P, S phase? |
65 | 12185.25 | 3.38 | B-1 | 8.0 | 20 | |
66 | 12983.65 | 3.61 | B-1 | 7.5 | 50 | P, S phase? |
67 | 14067.05 | 3.91 | B-2 | 6.3 | 30 | P, S phase? |
68 | 14206.80 | 3.95 | B-1 | 5 | 40 | P, S phase? |
69 | 16891.85 | 4.69 | B-3 | 11.3 | 30 | P, S phase? |
70 | 16892.00 | 4.69 | B-2 | 4.1 | 10 | |
71 | 19113.05 | 5.31 | B-2 | 4.5 | 10 | P, S phase? |
72 | 19198.05 | 5.33 | B-1 | 3 | 35 | P, S phase? |
73 | 21171.70 | 5.88 | B-1 | 2 | 30 | |
74 | 21657.55 | 6.02 | B-1 | 3.5 | 40 | |
75 | 22136.65 | 6.15 | B-1 | 4 | 35 | |
76 | 26479.25 | 7.36 | B-1 | 3.5 | 35 | |
77 | 27832.15 | 7.73 | B-1 | 2.2 | 40 | |
78 | 29191.10 | 8.11 | B-1 | 3 | 30 | |
79 | 30824.20 | 8.56 | B-2 | 2.2 | 20 | |
80 | 31511.55 | 8.75 | B-2 | 2.2 | 10 | |
81 | 33641.20 | 9.34 | B-2 | 4.3 | 10 | |
82 | 33649.35 | 9.35 | B-1 | 2 | 35 | |
83 | 35323.25 | 9.81 | B-1 | 4 | 45 | |
84 | 37181.60 | 10.33 | B-2 | −2.1 | 10 | |
85 | 39193.76 | 10.89 | B-2 | 4 | 10 | |
86 | 42674.60 | 13.8 | B-2 | 4 | 40 | |
87 | 54068.55 | 15.02 | B-2 | 2 | 10 | |
88 | 55135.70 | 15.32 | B-1 | 11.3 | 40 | |
89 | 66156.10 | 18.38 | B-2 | −5.1 | 10 | |
90 | 66775.25 | 18.55 | B-2 | 5.1 | 30 | |
91 | 67365.35 | 18.43 | B-1 | 4 | 40 | |
92 | 67574.65 | 18.77 | B-2 | 3 | 27 | Complex |
93 | 67651.80 | 18.79 | B-2 | 2.8 | 10 | |
94 | 67766.85 | 18.82 | B-1 | 3.7 | 33 | |
95 | 67999.75 | 18.89 | B-1 | 3 | 30 | Most complex |
96 | 68063.10 | 18.91 | B-3 | 10.8 | 45 | |
97 | 68211.65 | 18.95 | B-2 | 5 | 15 | Most complex |
98 | 68801.6 | 19.11 | B-3 | 11.9 | 60 | |
99 | 69211.45 | 19.23 | B-1 | 3.5 | 35 | |
100 | 69769.60 | 19.38 | B-3 | −4.0 | 35 | |
101 | 70573.30 | 19.6 | B-1 | 3.1 | 35 | |
102 | 71162.05 | 19.77 | B-2 | 2.9 | 28 | Complex |
103 | 71496.80 | 19.86 | B-1 | 4.1 | 50 | Subevent |
104 | 71694.70 | 19.92 | B-2 | 3.5 | 10 | |
105 | 71939.50 | 19.98 | B-2 | 2.8 | 10 | |
106 | 72573.75 | 20.16 | B-3 | −4.2 | 25 | Complex |
107 | 72754.65 | 20.21 | B-3 | −16.0 | 40 | |
108 | 73382.65 | 20.38 | B-2 | 4 | 10 | |
109 | 73409.05 | 20.39 | B-2 | 2.2 | 8 | |
110 | 73619.15 | 20.45 | B-1 | 3 | 40 | Complex |
111 | 73893.35 | 20.53 | B-2 | 3.9 | 10 | |
112 | 74918.70 | 20.81 | B-3 | 9.2 | 35 | |
113 | 75750.80 | 21.04 | B-1 | −4.0 | 30 | Subevent |
114 | 76353.05 | 21.21 | B-2 | 9.7 | 10 | Subevent |
115 | 76732.30 | 21.31 | B-2 | 3.9 | 10 | Subevent |
116 | 77958.25 | 21.66 | B-2 | 4.3 | 15 | |
117 | 78709.60 | 21.86 | B-3 | 10.5 | 40 | |
118 | 78908.15 | 21.92 | B-2 | −7.6 | 10 | Typical B-2 type |
119 | 79258.25 | 22.02 | B-3 | 2 | 33 | P? subevent? |
120 | 80042.45 | 22.23 | B-2 | 2.1 | 9 | P? subevent? |
121 | 82451.20 | 22.9 | B-2 | 4 | 10 | |
122 | 82632.30 | 22.95 | B-3 | 6.5 | 30 | P? subevent? |
123 | 83138.70 | 23.09 | B-2 | −10.1 | 10 | |
124 | 84541.40 | 23.48 | B-3 | 11.0 | 30 | |
125 | 84795.30 | 23.55 | B-3 | 6.5 | 35 | |
126 | 84841.35 | 23.57 | B-1 | 4 | 30 | Subevent |
127 | 85341.90 | 23.71 | B-1 | 4.4 | 34 | |
128 | 85352.70 | 23.71 | B-2 | 6 | 10 | |
129 | 85374.55 | 23.72 | B-1 | 3 | 45 | |
130 | 87677.00 | 24.35 | B-1 | 5 | 38 | P phase? |
131 | 88837.15 | 24.68 | B-2 | 3.1 | 10 | |
| ||||||
10-March | ||||||
132 | 3800.15 | 1.06 | B-2 | 4.1 | 20 | P phase? |
133 | 4262.10 | 1.18 | B-2 | 2.2 | 22 | P phase? |
134 | 5869.60 | 1.63 | B-2 | 3.7 | 15 | Subevent |
135 | 7020.15 | 1.95 | B-2 | 2.5 | 15 | |
136 | 7870.35 | 2.19 | B-2 | 2 | 10 | |
137 | 8116.80 | 2.25 | B-2 | 2 | 10 | |
138 | 17156.75 | 4.77 | B-3 | 12.0 | 40 | |
139 | 24115.35 | 6.70 | B-1 | 5 | 50 | |
140 | 24348.45 | 6.76 | B-1 | 5.5 | 37 | |
141 | 75609.20 | 21.00 | B-2 | 10.5 | 35 | P phase? |
142 | 84056.55 | 23.35 | B-2 | 2 | 15 | |
| ||||||
11-March | ||||||
143 | 13276.85 | 3.69 | B-1 | 3.5 | 45 | |
144 | 13400.30 | 3.72 | B-1 | 2 | 35 | |
145 | 15506.5 | 4.31 | B-3 | 4.2 | 45 | P, S phase |
146 | 18325.85 | 5.09 | B-2 | 2.2 | 20 | P, S phase |
147 | 21801.45 | 6.06 | B-2 | 4 | 20 | P, S phase |
148 | 22395.30 | 6.22 | B-2 | 7.0 | 10 | |
149 | 22503.65 | 6.25 | B-2 | 8.0 | 10 | |
150 | 22628.50 | 6.29 | B-2 | 3 | 10 | |
151 | 22891.70 | 6.36 | B-2 | −7.0 | 15 | |
152 | 23432.95 | 6.51 | B-2 | 5.5 | 10 | P-S 40 ms |
153 | 23485.85 | 6.52 | B-2 | −11.0 | 35 | |
154 | 25421.75 | 7.06 | B-2 | 4 | 15 | P, S phase |
155 | 25658.20 | 7.13 | B-1 | 12.5 | 35 | |
156 | 27287.35 | 7.58 | B-1 | 2 | 35 | |
157 | 27319.50 | 7.59 | B-2 | −8.5 | 10 | |
158 | 27385.00 | 7.61 | B-1 | 2 | 35 | P, S phase |
159 | 28639.15 | 7.96 | B-1 | 3.2 | 36 | |
160 | 34295.7 | 9.53 | B-2 | 2.6 | 20 | |
161 | 34564.55 | 9.60 | B-2 | 4.1 | 10 | P, S phase |
162 | 36804.60 | 10.22 | B-2 | 2.5 | 10 | |
163 | 45290.80 | 12.58 | B-1 | 2 | 35 | P, S phase |
Just before the main shock occurrence there appeared some events superposed by numerous small events as illustrated in Figure
A sample waveform of the event of B-2 superimposed with many small events suggesting connection of groups of small scale cracks. This type appeared only on the most active day of microcrack, that is, on 9th March (Figure
There appeared undulation of microcracks activity after the most active period around 9th March. Similar phenomena have been reported in laboratory experiments and field observation (e.g., [
There is a disagreement in the VLF activity between some of previous results and present cases. It is reported that there is no VLF after the main shock in the report of [
Here we present the case that the activity is dominated by the B types in the final stage superimposed numerous complex microcracks, which suggests appearance of different regime of the nucleation process but without drastic phase transition.
We suggest that the microcracks occurred in the southeastern peripheral of the great rupture of length 500 km in relation to the nucleation stage of the main rupture. We can imagine that the accelerated strain accumulation is not restricted to the so-called asperity region at around the epicenter of foreshock or main shock (Figure
We showed that the electromagnetic method can detect microcracks preceding natural earthquakes by a special underground antenna as an alternative to acoustic emission measurement. The nucleation stage of a crustal rupture can be identified by monitoring the electric field variations focusing on particular waveforms in the selected frequency ranges. These pulse-like phenomena are suggested to be plausible precursors for an imminent prediction.
Earthquakes are understood to be fracture of crustal rocks or stick slip of neighboring blocks. Preshocks occur generally in the rupture preparatory stage, and the activity has accelerated phase preceding main rupture. But that feature appears when averaged for many earthquakes. Preshock activity of individual earthquake is less regular to be used for confident predictive purpose.
Microcracks are always detected in the nucleation stage of accelerated deformation in almost all laboratory experiments. There are several efforts to investigate microcracks related to natural earthquakes by means of acoustic emission measurements. But they have not succeeded to find particular phenomena to identify the nucleation stage though the increase of acoustic emission intensity has been detected just half a day before and after main shocks.
We have been observing electromagnetic field changes for investigating the field characteristics related to earthquake activity and seismic wave in order for earthquake prediction and for application for early warning by using special underground antenna. The measurement method has been proved to be robust to the natural and man-made noises by observational investigation since 1989. Here we show that the instrument could detect microcracks appearing in the nucleation stage of the Tohoku Earthquake. Whole large pulse-like variations were grouped into three (A, B, and C) by frequency and waveforms and found to characterize different phases of the earthquake occurrence, the preparation, main shock, and after shock processes. Those variations are grouped by time constants: type A with duration of several to several ten minutes just the same as the pulse-like signals detected previously at the time of volcanic eruption and seismic swarms, B of several hundred Hz, and C of several kHz. The A type was identified in the early stage of observation at the time of volcanic eruption in 1992 and has been suggested to be induced by seismic swarms and volcanic eruption. The B type variations are subgrouped into three waveform: type B-1 of similar to the GUV except in the stepped decay in the relaxation phase after sharp rise, type B-2 of wave packet similar to acoustic emission, and type B-3 of superposition of B-1 and B-2. Different types of electric waveforms of the B type are suggested to correspond to crack rupture modes of tensile and shear and to be generated by confined water movement through the electrokinetic effects on the ground of previous investigations on laboratory experiments. The earthquake preparation period is characterized by the appearance of characteristic microcracks: beginning period of the nucleation period is characterized by rare occurrence of A types, nucleation period is by B types and particular evolution of the subgrouped types. The second kind of cracks (B type) is shown to occur only in the nucleation period: activity increases sharply before the earthquake following the modified Ohmori’s law. A and C occur at the stress arrangement period after the main shock. Field monitoring of microcracks activity by EM method is shown to be practically possible to investigate the nucleation stage providing a break-through for the short term prediction method.
We picked up 224 events under the condition of larger event than three times of back ground level. All of the events are grouped in one of three groups without any undefined ones originating from urban and natural noises. It is very unique point of e-field observation by the borehole antenna. Among the three types the B type is the most interesting from the point of view of identification of the nucleation stage of natural earthquake. We present a catalog of whole B-type events consisting of 163 events. Almost all events are compounded by B-1 and B-2. But events containing dominantly the tensile mode are grouped into B-1, and the shear modes are grouped into B-2. The grouping is done not quantitatively but qualitatively. The results of grouping exist in Table column “Number,” sequential number of events of the type B, column “Second,” occurrence time (JST) in second of the event, column “Hour,” occurrence time (JST) in hour, column “Type,” sub-group type of the B event, column “Strength,” peak height of the pulse, in milivolt, column “Duration,” width of pulse, in milisecond, column “Comment,” several features of waveforms: appearance of P phase, association of subevent, complex type, and so forth.
This work was supported by JST Grant AS2121347A to OKI Engineering Co., Ltd. The authors thank T. Matsumoto and Y. Okada (NIED) for support to use the borehole and F. Freund and W. C Lai, K. Hattori, and Q. H. Huang for valuable discussions and comments. Detailed and valuable comments by Editor Professor J. P. Makris and reviewer K. Eftaxias are greatly appreciated. The authors used data of K-NET at CHB005.