Analogy in precursors of dynamic events at different scales

Tectonophysics,

211

152 (1988) 211-282

Elsevier Science Publishers

B.V., Amsterdam

- Printed

in The Netherlands

Analogy in precursors of dynamic events at different scales A.D. ZAVYALOV and G.A. SOBOLEV Institute of Physics of the Earth, U.S.S.R. (Received

Academy

November

of Sciences, B. Gruzinskaya 11,1986;

accepted

IO, Moscow, 123810 (U.S.S.R.)

May 1,198l)

Abstract Zavyalov,

A.D. and Sobolev,

(Editor),

G.A., 1988. Analogy

Seismic Source Physics

This paper

presents

pulses of acoustic field experiments

a study

during

the preparation

of dynamic

Research.

of certain

of seismogenic

and development parameter

of the parameters

under

events at different Tectonophysics,

physical

ruptures,

of dynamic

and seismic emission

the object being loaded. The results obtained prediction

of dynamic

mine shocks and large earthquakes

of elastic pulses of acoustic

is shown that the behaviour

behaviour

the density

It was found that, in all cases, the fault density the number

Prediction

of the time-space

and seismic emission,

before failure in rock specimens,

in precursors

and Earthquake

parameters,

namely,

and the b-value

events at different

as a rule increases,

depend

basis for the further

changes.

on the linear dimensions development

of It of

of studies on the

events.

(3) Failure is a stochastic initiation and development

The Soviet model of avalanche-unstable-rupture formation assumes that the preparation of failure in an inhomogeneous medium repeats itself

volume. The effects of these causes determine larity

qualitatively

failure

scales (Myachkin

et al.,

1975). The following physical factors are considered to form the basis of the failure process at all scales and the associated release of elastic energy: (1) Failure in solids, including rocks, is not an instantaneous act but a thermoactivation process which depends on time and temperature. (2) The development of rock failure in time depends on its inherent heterogeneity and block structure (Sadovsky, 1979; Sadovsky et al., 1982), which have similarities over a wide range of scales. Therefore, failure develops rather unevenly, both in time and in the volume of the object under loading (Kuksenko et al., 1978). 0040-1951/88/$03.50

stage

event the variance

and the b-value undergoes

Introduction

at different

of

mine and

scales. The preparation

varies from 3 to 8; before a dynamic

the physical

the number

in laboratory,

was studied.

study does not qualitatively

strengthen

scales. In: 0. Kulhanek

152: 211-282.

0 1988 Elsevier Science Publishers

B.V.

at different

scales

size and the kinetics

tion and development.

discrete process of the of failures in a given

in the

the simi-

distribution

of failure

This similarity

of

accumulain turn re-

sults in a spatio-temporal similarity in precursor occurrence before a failure and in the release of the elastic energy itself (acoustic emission, mine shocks, and earthquakes). The identification of the precursors of dynamic events would allow us to apply the results of model experiments in failure preparation from one scale to another (e.g. from a laboratory scale to a mining or seismic scale). The fact that some types of failure precursors are scale-independent has been demonstrated theoretically by Petrov (1979) and Chelidze (1984). The present paper analyses the results of studies in the time-space behaviour of certain physical

278

parameters, namely, the number of pulses of acoustic and seismic emission, the density of seismogenic ruptures and the b-value in laboratory, mine and field experiments during the preparation and development of dynamic events of different scales. Some features of failure precursors in rock specimens in the laboratory, and for mine shocks and large earthquakes were studied. SENSOR NUMBER

The number emission

of pulses

of acoustic

and seismic

cl

b

t

Fig. 1. a. Schematic layout of the experiment on a concrete specimen with artificial stress concentrators. b. The curves of the variance of the number of acoustic pulses at various levels of applied load. o =15 MPa is the critical load; 1, 2, 3 and 4 are the numbers of acoustic emission sensors.

Papers by Vinogradov (1964), Nersesov et al. (1976) Mogi (1962) and others that appeared in the 1960’s and 1970’s describe characteristic time distributions of the number of elastic pulses and of the number of background earthquakes in seismogenic regions and draw conclusions on the desirability of monitoring these parameters to help predict large dynamic events. New possibilities for studying the behaviour of acoustic (seismic) pulses during macrorupture preparation (mine shocks, earthquakes) are available, with more sophisticated equipment for the acoustic emission method widely used in laboratory and mine experiments, and with improved recording possibilities for seismographic networks, with equipment ever more sensitive to acoustic and seismic pulses and offering ever greater time resolution.

f 12. MINE

w u IO-

SHOCK

i\

u

i

DEC.lY61 1 JAN. 1962 1 FEB.1962 1 MAR. 1962 1

Fig. 2. Variance of the number of acoustic emission pulses during the preparation period preceding a mine shock in the Anna mine, Czechoslovakia (after Vinogradov, 1964). Level I is the mean variance value for the observation period and level 2 is its root-mean-square deviation. The shaded area indicates the period when the variance exceeds the standard deviation.

8 /

t

f Ex.7,8 I.........(( 14 16

,,,,,,,,,,, 18

20 22 OO 02 27.04.1984

04

06 ')

14

16

18 20

28.04.1V84

22

00

02

04 06 08 lOT, hours 29.04.1984

Fig. 3. Variance of the number of acoustic emission pulses during failure in hard-rock massifs in the mine. The Lower arrows indicate the times of expIosions in the supporting rock massifs, the upper arrows indicate the times of the largest dynamic events. The rest of the legend is the same as for Fig. 2.

219

K=l3.5,13.6,13.

64

Fig. 4. Time variations

66

of the variance

68

of background

70

72

earthquakes

74

76

80

78

in the epicentral

areas

of some Kamchatka

earthquakes.

The

legend is the same as for Fig. 2.

In recent laboratory experiments, using a 200 X 100 x 50 cm concrete specimen filled with granite pebble and in which artificial stress concentrators had been inserted, we found an increase in the variance of the number of time intervals of acoustic activity of various levels when approaching the instant of barrier failure (Fig. 1). The phenomenon was most pronounced around sensors 3 and

noted by many workers. It can be explained by the progressive appearance of local instability in the deformed volume of material as it approaches macrofailure. The b-value The distribution of the number of dynamic events according to their energy (recurrence curve)

4, which monitor the lower part of the barrier approaching failure. A characteristic increase in

is a general

the variance of the number of acoustic pulses was observed by Vinogradov (1964) in 1962 before a

process on various scales, from rock specimens earthquakes. One of the main characteristics

shock in the Anna

this distribution

mine in Czechoslovakia

(Fig.

2). A study

of the failure

of hard rock massifs

in

mines has revealed a similar effect of increased variance of the number of elastic pulses (Fig. 3) before some of the largest dynamic events. A study of seismicity in the preparation areas of several large earthquakes in Kamchatka by Zavyalov et al. (1980) also revealed increased variance in the number of background earthquakes which precede the main shock (Fig. 4). Increased variance of various physical parameters before failure of rock specimens has been

statistical

law which governs

is the parameter

the recurrence curve). Papers by Vinogradov (1977),

Mogi

(1962)

to of

b (the slope of

(1964)

and

the failure

Zhurkov

others

dealing

et al. with

. 4; 161.0

-

1

Fig.

5. The

Vinogradov,

b-value 1964).

i

prior

j

to

4

failure

condltional time

in lime

brick

(after

280

earthquake,

b 1.0 MINE

caused

SHOCK

while

the

by coalescing

of larger

ruptures.

subsequent

ruptures

An anomalous

slope of the recurrence can be regarded approaching DEC.1961 1 JAN.1962 1 FEB.19621 MAR.1962 Fig. 6. The b-value

during

shock

mine,

in the Anna

the preparation

stage

Czechoslovakia

(after

‘.

70

72

Fig. 7. The b-value

prior

The density of seismogenic

of a mine

of July 28, 1976.

acoustic

emission

in

T, years

7.8, Tangshan,

China,

marked

period

of reduced

underground and

shafts,

model

and

Based

shock

(Fig.

materials,

6). As a rule,

by a high b-value b, when

on the kinetic and

the

solids

is not

rather

the continuation

ture,

that

isolated

is followed

the

dynamic

approach

becomes

the object

a

by a

occurrence,

concentrated

under

loading from

(Kuksenko

one stage

large-

event

mensions

the

is reached

in

failure

factor

here is the concentration

of December

(3) is the root-mean-square

15, 1971.

crite-

The possibility of using the concentration criterion to predict a macrofailure has been demon-

failure

in

some

rock

materials and K, before dy-

specimens

in

the

I

68

70

72

74

of the b-value (solid line) in a 200 X 200 km zone that contains

respectively.

the

,:'....".:

66

earthquake

source;

in the formation of a higher rank.

namic

M=7.7

(say,

rion K, = L/I,, where 1, is the average rupture length and L, = N* -‘I3 is the mean distance between ruptures.

large

K=13.6

et al., 1978).

occurs when the crucial N* of appropriate di-

weak seismicity

I

on

micro- to macro-ruptures) concentration of ruptures

The critical

to the failure

in the area of the imminent

points

to an other

strated for a number of technical rock specimens. The critical value

Fig. 8. Time variations

but (Zhur-

to

at certain

model of earthquake preparation, we believe that the increase in b is connected to the growth of

64

in

in stages proof minute di-

micro-ruptures)

interaction between them results of a rupture (or several ruptures)

avalanche-unstable-cracking

b 0.6,

of

failure

of a long process

(submicro-,

The transition

occurs. Similar changes in b were also observed in studies of seismicity in various seismically active areas of the world (Figs. 7 and 8) (Zavialov et al., 1980; Zhang and Fu, 1981). process

have led to the conclusion a sudden,

of our knowl-

of the strength

scale (macro-) ruptures. At a given moment, the failure process, so far quasihomogeneous in na-

of rock specimens

the case of a mine

and deepening dependence

solids

mensions

present a regular pattern of behaviour of the b-value during the period preceding failure in a specimen about to macrorupture (Fig. 5) and in period

ruptures

kov, 1968). This process proceeds gressing, gradually from ruptures

0

78

76

to the M=

earthquake

loading

“,

74

scale, of an

Vinogradov,

MB=?.8

1.

in the

macrofailure.

The development

68

change

curve, on whichever

as one of the precursors

edge of the time

I

is

I

1964).

b

decrease

and the formation

Levels

I and

2 are the long-term

error of current

b-values.

(background)

76 the epicentre b-value

78

T, YEARS

of the M = 7.7, Ust-Kamchatsk and

its mean

square

deviation,

281 Log

laboratory experiments was found to be between 3 and 5 (Zhurkov et al., 1977; Sobolev and Rummel, 1982). After having developed a method which permitted the use of the concentration criterion (Sobolev and Zavyalov, 1981, 1984), we found that the epicentral regions of future large earthquakes are characterized by a lower parameter of seismogenic rupture density K,, (Fig. 9), an analogue to the concentration criterion. This method was tested using data from Kamchatka, Caucasus. Central Asia and Southern California.

Li,Cm

cm

Fig. 10. Mean instant

distance

of failure

L, between

in the object

line) and the experimental mers; (2) crystals,

data

polycrystal

(4) rocks; (5) computer

under

cracks

of size I, at the

loading.

Theory

(solid circles)

metals;

simulation;

(straight

for: (I)

(3) composite

poly-

materials;

(6) earthquakes.

The values of K,, before large earthquakes in these regions were found to be in the range of 5 - 12. Figure 10 illustrates the scale invariance of K,; here, all experimental points, from experiments using polymers to actual earthquakes, are well fitted by a straight line. In other words, over the whole range of scales of L, and I;, which reaches 15 orders, the value of the failure criterion K* (the slope of the straight line) remains unchanged (Petrov, 1984).

+ i-

!

Conclusions

The available experimental data indicate clear similarities in the appearance of certain dynamic failure precursors at different scales. This suggests that the development of the failure process is controlled by one physical mechanism, whether in the laboratory or under field conditions and that this mechanism is obviously based on the concept of kinetic strength of solids.

I-

Fig. 9. Map of K,, as of July 1, 1971 based K > 8.5, H d 100 km earthquakes

ing epicentres

value of K,,

standard

error.

Kamchatka,

class

July 1, 1971 to July prior

to large

1, 1977.

earthquakes

and

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