Pageoph, Vol. 113 (1975) Birkh/iuser Verlag, Basel

Application of Electric Method to the Tentative Short-Term Forecast of Kamchatka Earthquakes

By G. A. SOBOLEV 1)

Summary - Tentative forecasts of strong earthquakes on Kamchatka have been carried out con- tinuously since 1972 with scientific purpose. Three methods are being used: electric, ratio Vp/V, and seismostatistical. The main scientific result of the two-year study is the statement that it is possible to predict earthquakes of magnitude t>5.5 some days in advance; eight earthquakes had been predicted from a total number of twelve.

In the course of investigations for earthquake prediction on Kamchatka, a number

of short-term precursors were found that appeared 5-20 days before the earthquakes (SoBoLEV et al. , 1972; SOBOLEV, 1973 ; FEDOTOV et al. , 1972). These precursors became the basis of the program of tentative short-term forecasts, created in December 1971

by S. A. Fedotov and the author, of Kamchatka earthquakes of magnitude >>.5.5.

Three principal methods are being used: electric, Fp/Fs ratio, and seismostatistical. Tentative forecasts have been carried out continuously since 1 January 1972.

Recordings of the electrotelluric field in search of precursors of strong earthquakes

began in 1966 along the eastern coast of Kamchatka. The process of preparation of a large earthquake manifests itself by cracking, accompanied by a change in deformation

velocities and stress in certain parts of the seismoactive area. It would be natural to

expect that, in this case, in the preparation zone, changes would appear in the electric state of the ground. Observations on the mechanics of failure in heterogeneous medium

allow us to suggest that, during the process of preparation of the earthquake, precursors of different duration can appear. The summarized data of precursors (MIACHKIN and ZUBKOV, 1973) shows the presence of a wide range of precursor times, f rom several minutes to several years, for different geophysical fields. We are inclined to believe that

there can exist an almost uninterrupted frequency spectrum of precursors that reflects the fracturing of different-size blocks of rock. In the present work, we report anomalous

changes in electric state of the order of several days. The absence of precursors of shorter duration can be accounted for because the frequency of observations is one hour. Long-term precursors are not seen because the period of research is not sufficient to

remove the effects of seasonal variations and other long-period noise.

Observations of electric precursors are conducted by a network of stations spaced at distances of 100-200 km. As transducers, lead electrodes and electrodes with special

1) Institute of Earth Physics, Moscow, USSR.

230 G.A. Sobolev (Pageoph,

low self-polarization film fixed at a depth of 2 m are used. At all the stations along the

eastern coast of Kamchatka, potential differences are recorded on main and doubling

lines oriented in north-south and east-west directions. The distance between the elec-

trodes is 200 m. At some stations additional lines of different length are employed

with which it is possible to exclude the influence of contact phenomena on the electrodes

mv

so-L / ',

5 0 4 "-.v'-- - k / ~ ~ N'S2 (2) 40 I" - -

' / u--%j v ,~ . , ,~ . \ '~- ~ ' ^ E-W, (3) 15_~ v "

o l -v--

-'~ k 0 pv, . , .~A, . .~ , ,N~l~ ~.i ~ H (6)

4 : ~ Vmox (8)

20"1 ___ / t ~ tmax (9) OO ~ _ _ J ~ _ ~ tmln (10)

/ - 2 0 "

SO: 3 __._-- t,.~ (11)

mm

,11 o . , I I,i (x2) t MAY JUNE JULY Figure 1

Example of electric state of ground change before earthquake with magnitude 6, at station 'Cape Schipunsky'. 1-4 are daily mean values of the difference of voltage between electrodes along main and, doubling lines; 5-7 are for daily mean values of geomagnetic field; 8 is for wind velocity; 9-10 are for soil temperature at the surface; i1 is for rock temperature at a depth of 1.6 m; 12 is for precipitation

and the disturbances due to induced currents. This is achieved by means of cross-

correlation of the data of many measuring lines of several stations and comparison with

geomagnetic variations. Such analysis allow us to distinguish unusual changes of the electrotelluric field, presumably connected with earthquakes.

An example of anomalous change of the electric state of ground before an earth- quake is shown in Fig. 1. On 27 May 1972 a deep earthquake occurred with magnitude 6,

Vol. 113, 1975) The Tentative Short-Term Forecast of Kamchatka Earthquakes 231

its source situated under Kamchatka at the epicentral distance of 250 km from the station 'Cape Schipunsky'. Approximately one week before the earthquake, synchron- ous changes of electrotelluric field level, including all components, were observed (graphs 1-4, Fig. 1). Meteorological factors in this region and the geomagnetic field did not change noticeably before the earthquake.

If we assume the values of the electrotelluric field on 20 May to be normal, then, at the moment of the earthquake, the changes of the field on both east-west lines were 10-13 mV and on north-south lines 8-10 inV. Hourly records during that period, given in Fig. 2, show the identity of azimuth change of the field along the lines.

E - W I

N - S z

MAY 1972 Figure 2

Hourly records of mean and doubling lines showing the change of electric state of ground before earthquake with magnitude 6

In the majority of cases, the changes of electrotelluric field before an earthquake do not exceed the background noise. This makes it impossible to distinguish precursors through the recording of one measuring line. As a result, standardization of methods is needed to conduct the joint processing of several synchronous records.

The recognition of anomalous changes of the electrotelluric field was made by computer analysis (Fig. 3). The algorithm employed for this purpose decreases the high-frequency noise and creates a 'zerc' line of seasonal variations for each measuring line. The deflection of real changes of the field from the 'zero' line are then calculated. To increase the signal-noise ratio, the data are averaged on all the measuring lines for each station. To increase stability of this analysis, we used multiple averaging on variable

232 G . A . Sobolev (Pageoph,

time windows. Maximums of the outlet function correspond to an alarm period. Figure 3 shows the unprocessed records of four components for one station situated in the central part of the Kamchatka seismoactive area, and the results of analysis of these

records by means of two different programs. The arrows show the moments of all earth-

quakes with magnitude/>5.5 that occurred closer than 250 km from the station. The numbers near the arrows correspond to the numbers of the earthquakes in Fig. 4. One

can see that most earthquakes were preceded by an anomalous increase of the processed

I0 mv I

E-W 2 0 o -

E-W i 0 o -

N-SI, 2 0 ~

N-S I

1 9 7 2 1 9 7 5 1974

Figure 3 Detection of the Kamchatka earthquakes with magnitude/>5.5 by the computer programs exemplified by the electric station 'Cape Schipunsky'. The numbers near the arrows serve to identify the corres-

ponding earthquakes in Fig. 4

signal. The programs are not good enough to exclude all seasonal variations, as those for example which appeared in May 1972 and 1973. But the main maxima before the earthquakes are not connected with such variations.

The tentative short-term forecast of the time and place of Kamchatka earthquakes was based on data from electrotelluric stations, and changes of the ratio of p- and s- wave velocity calculated for each seismological station according to the method developed by L. B. Slavina.

Individual values of the parameter x = Vp/Vs for each station as a function of time were estimated using the relation x = (Ts - p ) / ( T p - o) + 1, where Ts - p is for the difference in the time of arrival of p- and s-waves to the station; Tp - o is for the travel

Vol. 113, 1975) The Tentative Short-Term Forecast of Kamchatka Earthquakes

�9 / ~ .... ~ . ~ ~ _ - ~

K A M s

AV,

I Qio 08

22 0

SPN

014 Qi5

Qz~

P A C I F I C O C E A N

233

SEA oF O K H O T S K

. .

i I I I I

I /

Figure 4 Map of epicenters of the Kamchatka earthquakes for which the computer forecast was made for the station 'Cape Schipunsky'. 1 is the axis of deep-sea trench; 2 is the seismic station; 3 is the electric station; 4 is the joined seismic and electric station. Magnitude of earthquakes: 5 is approximately 5.5; 6 is approximately 6; 7 is greater than 6; 8 is greater than 7; 9 is the epicenters of earthquakes before which considerable anomalies were observed; 10 those with less-distinct anomalies; 11 those with questionable anomalies; 12 those with no anomalies. The presence of anomalies for the electric and

Vp/V~ method is depicted by symbols, according to the scheme in the right lower corner

time of p-waves from the source to the station. The moment of earthquake occurrence was determined with the help of data from all Kamchatka seismological stations.

The analysis of time dependence of parameter x was made by computer (SoBOLEV and SLAVINA, 1974). The algorithm includes the calculation of differences between the current value of x and the average value A x = x~ - 2 , the introduction of weight coefficients Pt to reduce the effect of accidental errors, and separate summing of positive and negative A x along variable time intervals. At the outlet we have the function

,

i=1 i=i

234 G.A. Sobolev (Pageoph,

The maximum of the outlet function correspond to the alarm period as in the case of electric forecast. In Fig. 4, the map of computer forecasts of Kamchatka earthquakes, according to data from only one station (Cape Shipunsky), is shown. The solid and lined circles denote the earthquakes before which both electrotelluric (ETF) and Vp/V~ outlet functions exceeded the alarm levels. One can see that most earthquakes with precursors occurred within 250 km of the recording stations. Some of the earthquakes in the south, for example number 10, were predicted on the basis of the southern stations records.

It is possible to estimate the success of the electric method for the tentative short- term forecast. 235 separate forecasts were made and recorded in informal proceedings of the Institute from 1 January 1972 to 20 April 1974. Thus the forecasts were made for 3 to 4 days in advance, twice a week. The data of five electrotelluric stations were used.

Table 1

Method N Np L(%) I

ETF 12 8 29 2.30 Vp/V, 14 10 56 1.28 ETF+ Vp/V~ 12 7 22 2.64

The numerical results are summarized in Table 1, where the data from the Vp/Vs

method are also given for comparison. The forecasts were made in Petropavlovsk- Kamchatskij by A. A. Chromov for the ETF method, and by L. B. Slavina for the Vp/Vs method. In addition, these two methods were combined for short-term forecasts. The conditional alarm forecasts were given when alarm periods of both methods coincided. In Table 1 N is the number of earthquakes with magnitude >15.5 which occurred during the period for which forecasts had been made by means of a given method; Np is the number of predicted earthquakes; L is the ratio of the sum of products of alarm time and the square of alarm region to the product of total time of forecast and total square of region; I is the efficiency of the forecast, i.e. the ratio of the density of earthquakes during alarms to the average one. The main result of the two-year study is the statement that it is possible to predict earthquakes with magnitude/>5.5 some days in advance with a probability two times higher than chance guessing. It is very interesting that I tends to increase as a result of a combination of the two methods. This increase is accounted for by reduction of time of false alarms.

From these results it would seem that the attempt to conduct a continuous forecast program on Kamchatka is, at this time, of scientific interest only. The efficiency of forecast is too low for practical purposes.

REFERENCES

FEDOTOV, S. A. et al. (1972), Progress of earthquake predietion in Kamchatka, Tectonophysics 14 (3/4). MIACHKIN, V. J., and ZOBKOV, S. J. (1973), Svodnijgraphikpredvestnikov zemletriasenij, Izvestia AN

SSSR, No. 6.

Vol. 113, 1975) The Tentative Short-Term Forecast of Kamchatka Earthquakes 235

SOBOLEV, G. A. (1973), Perspectivy operativnogo prognoza zemtetriacenij po electrotelluricheskim nabludeniam, Sbornick, predvestniky zemletriacenij, VJNJTJ.

SOBOLEV, G. A., MOROSOV, V. N., A~O MI~UNOV, N. J. (1972), Eleetrotelluricheskoe pole i sdnoe zemletriasenie na Kamchatka, Izvestia AN SSSR, Physica Zemly, No. 2.

SOBOLEV, G. A., and SLAVINA, I_. B. (1974), Bistrie izmenenia electricheskich i seismicheskich svoistv sredy v seismoactivnom raione, DAN SSSR 215, No. 5.

(Received 22nd November 1974)