Volcanic earthquakes of Kamchatka: classification, nature of source and spatio-temporal distribution

Tectonophysics,

255

180 (1990) 255-271

Elsevier Science Publishers B.V., Amsterdam

Volcanic earthquakes of Kamchatka: classification, nature of source and spatio-temporal distribution V.I. Gorelchik,

V.M. Zobin and P.I. Tokarev

Institute of Volcanology, Petropavlovsk-Kamchatsky, (Received March 28.1989;

683006 (U.S.S.R.)

revised version accepted November 30,1989)

ABSTRACT Gorelchik, V.I., Zobin, V.M. and Tokarev, P.I., 1990. Volcanic earthquakes of Kamchatka: classification, nature of source and spatiotemporal distribution. Tectonophysics, 180: 255-271. Volcanic earthquakes on Kamchatka can be divided into two large groups: earthquakes with depths of O-40 km generated by stresses which arise during magma migration in the Earth’s crust under volcanos (the first group), and the earthquakes directly connected with the eruptions (volcanic tremor, explosive earthquakes, etc.-the second group). This paper presents a review of some energetic, spectral and spatio-temporal characteristics of the Kamchatkan volcanic earthquakes of the first group and their relationship with volcanic phenomena. Seismicity related to volcanic activity has the following specific features: a local and predominantly swarm-like pattern of earthquake origination; iteration of earthquake swarms in the same seismically active zones; many shallow and relatively small events; a small magnitude limit (up to 5.5-6); the existence of longer-period variations of volcanic earthquake foci as compared to the tectonic one; and a comparatively high value of the slope of the earthquake recurrence plot. At the same time, similarity in behaviour of some parameters of the seismic regime during the preparation and development of eruptions and prior to large earthquakes, as well as the destruction of samples, are noted.

Introduction

hundred years have passed since the first seismic observations were carried out in 1888 in the sum-

Volcanic activity is accompanied by various geological, geophysical and geochemical phenom-

mit zone at Bandai volcano. At present, seismological monitoring is carried out throughout the

ena. Seismic events associated spatially and genetically with volcanic activity and with generation

world, from the northernmost

and ascent of magma are called volcanic earthquakes. They are distinguished from tectonic earthquakes, which are related to movements of blocks in the crust and upper mantle. Since volcanos are always located in tectonically active zones where seismicity is high, the problem arises of discriminating between volcanic and tectonic earthquakes. The problem is solved rather easily when earthquakes are related to volcanic eruptions and their foci are located within the Earth’s crust. However, volcanic earthquakes, related to genesis of magma chambers and movements of magma in the upper mantle, have remained undiscriminated from tectonic ones until recently. One 0040-1951/90/$03.50

0 1990 - Elsevier Science Publishers B.V.

Mayen to the southernmost, Antarctica. Systematic

volcanos

in Jan

Erebus volcano

in

studies on volcanic earthquakes

in

the Soviet Union started in 1946 in the region of the Northern volcanic group, at the Kamchatkan volcanological station. At present two radio-telemetered seismic networks and a network of 11 regional seismic stations run seismic monitoring at most active volcanos on the Kamchatka peninsula. The seismic equipment of the regional and telemetric stations consists of three-component shortperiod seismometers (T, = 1.2 s). Valuable data were obtained as a result of seismological monitoring during the 1955-1956 Bezymianny, 1964 Sheveluch (Tokarev, 1966,198l)

256

V.I. GORELCHIK

SHO

1 BTOUP,

depth>

5 km

14.Vll.i973

Z

II

group,

volc.snlc

tr

KII

II

group,

explosion

earthquake

8.V111.1965;

- --* 3s

E@W

P f9h43m34,2s /

b

30s

J II

C

III

ET AL.

VOLCANIC

EARTHQUAKES

2.57

OF KAMCHATKA

and 1974-1987 Klyuchevskoy eruptions (Gorelchik, 1985; Gorelchik et al., 1988), as well as during the large Tolbachik fissure eruption in 1975-1976 when volcanic earthquake source mechanisms were studied (Zobin, 1979; Fedotov et al., 1984). This paper presents a review of energetic, spectral and spatio-temporal characteristics of volcanic earthquakes from the Kamchatkan volcanos, and discusses their mechanism and relationship with volcanic phenomena.

magma outflow to the surface and explosions to the crater. The foci of these events are located mainly at a distance of not more than 0.5-l km from the active crater of the volcano. Volcanic earthquakes of the second group (explosion earthquakes, volcanic tremor, etc.; see Fig. 1) are used to study the dynamics and mechanism of eruptions. In this study we intend to discuss mainly the first group of volcanic earthquakes.

Volcanic earthquake swarms General characteristics of volcanic earthquakes

A great variety of seismic phenomena which accompany volcanic activity have led to the derivation of a number of classifications of volcanic earthquakes in terms of focal depth, seismogram features, the relationship with different stages of volcanic activity, and various other features (Davison, 1924; Minakami, 1960; Tokarev, 1966,1981). None of these classifications is universal for all volcanos and types of eruptions. Nevertheless, at a first approximation, volcanic earthquakes on Kamchatka can be divided into two large groups. The first group includes earthquakes generated by stresses which arise during magma migration in the Earth’s crust. The source nature of these earthquakes is close to that of tectonic earthquakes. Their foci lie at depths of O-40 km. The earthquakes belonging to this group are used to study stresses and strains in the Earth’s crust in volcanic regions, to outline magma chambers and conduits, to trace the routes of magma movements and to predict the time, place and energy of forth~~g eruptions. Seismograms typical of these earthquakes are presented in Fig. 1. The second group includes volcanic earthquakes directly connected with the eruption itself,

Studies of volcanic earthquakes allow us to point out the following typical features: (a) they tend to occur as swarms; (b) volcanic earthquake foci are located close to the magma chamber and to the conduit of the volcano, which leads to notable limitations in the maximum energy of volcanic earthquakes (maximum magnitude of up to 5.5-6); (c) the seismic process develops at high velocity of; (d) there is repeated faulting at the same fissures and fractures in which the magma rises to the surface. This leads to emergence of a number of earthquakes of the same type, with identical forms of recording, i.e. earthquake families (Tokarev, 1966; Okada, 1983). One can distinguish at least four principal types of earthquake swarms according to their character of occurrence through time, their spatial position and their relations~p with volcanic activity in Kamchatka (Tokarev, 1984). Type I earthquake swarms precede central eruptions at andesitic volcanos. They consist mostly of shallow volcanic earthquakes, the epicentres of which lie at a distance of 3-5 km from the crater and the foci of which are located O-5 km above sea level within the volcanic edifice. Depending on the deep structure of the feeding

Fig. 1. Exampies of volcanic earthquake recording. A. Group I volcanic earthquake with a focal depth of more than 5 km (Tyatya volcano, Kunashir Island, seismic station SHO, epicentral distance 75 km). B. Group I volcanic earthquake with a focal depth of less than S km, and group II volcanic earthquake (volcanic tremor) for Klyuchevskoy volcano, Kamchatka (seismic station KLY, epicentral distance 27 km). C. Group II explosive earthquake (Karymsky volcano, Kamchatka, distance 3 km).

seismic station KII, epicentral

V.I. GORELCHIK

b

a

1 w”

v. Sheveluch

3 rl

51

1

/

:- beginning . of eruption

2 3 4 5 6 7 6 9 0

November,1964

October

v.Klyuchevskoy Bylinkina

Lateral

crater

beginning of eruption

0. 6 I IS . I6 ’ $7 ’ November,1951

r,,,,,,l,,,~,),,,,,,,r , 20 21 ?Z 23 24t=a262726 293031.1

ET AL.

Fig. 2. Examplesof the developmentof volcanic earthquake swarms (types I and II) which preceded volcanic eruptions. a. Type I swarm of volcanic earthquakes preceding the central eruption of Sheveluch (Kamchatka) in 1964. b. Type II volcanic earthquake swarm preceding the flank eruption at Klyuchevskoy (Kamchatka) in 1951: I = the number of earthquakes per day; 2 = maximum energy class.

zones at andesitic volcanos, earthquakes having focal depths of lo-15 km are feasible. The maximum energy class of the swarm events can be Klim = 12.0 f 0.5 *. The swarm duration can be from 7-10 days to 3-5 weeks. The earthquakes of the swarm are related to incessant pressure increase in the upper part of the conduit prior to eruption. During these swarms the total energy, EN, frequency, N, and maximum energy class of earthquakes K,, increase continuously and evenly, reaching their maximum values at the moment of eruption. Type (lateral)

II earthquake eruptions at

swarms basaltic

precede flank and andesitic

* K = log E (Joules) is the energy class of the earthquakes, determined

from short-period

S waves;

K = 4.6+1X4,

where M is the surface wave magnitude (Fedotov, 1972).

volcanos. Very shallow earthquakes predominate in these swarms. The epicentres of the earthquakes lie 3-10 km from the place of formation of a new crater, and the foci are located at a depth of not more than lo-20 km (generally at a depth of O-5 km). Earthquake swarms are caused by magma intrusion and formation of fractures at depth, in which magma rises to the surface. Prior to flank eruptions of central stratovolcanos, the swarms can last for l-10 days, K,, reaching 11.0 f 0.5. In these swarms EN, N and K,, at first sharply increase, and on reaching their maxima, start to decrease gradually;

and a few hours before

flank breakthrough earthquakes with cease. Prior to the eruption of highly liquid magma in rift zones close to shield- and type volcanos, the swarms can last for hours or tens of minutes, and Klim can be erably lower than the value noted above.

the

K z 6.0 basaltic fissureseveral consid-

VOLCANIC

EARTHQUAKES

259

OF KAMCHATKA

02.VII.1975 04h34m

The course of development of seismic activity from swarms of types I and II which precede volcanic eruptions is shown in Fig. 2. The character of the seismicity is rather stable, which enables one to make use of volcanic earthquake swarms to forecast volcanic

eruptions (Tokarev,

1966, 1981, 1985). Type III swarms accompany the Volcanic-type eruptions. They include explosion earthquakes, the foci of which lie at depths of not more than 500 m from the bottom

of the active crater. The epi-

centres are located inside the crater of the volcano. The earthquake

frequency,

N, energy,

EN, and

K max vary, depending upon the intensity of erup-

tion and power of separate volcanic explosions. Type IV swarms occur during and at the end of eruptions, and also after their termination. They can

include

volcanic

earthquakes

with

Fig. 3. Diagram

of the focal mechanism

earthquake (New Tolbachik its correlation

volcanoes,

of a large volcanic

Kamchatka,

1975) and

with the strike of the chain of eruption

cones.

One of the nodal planes and a chain of the newly formed cones have the same NE strike: P = compression;

T = extension.

focal

depths of O-20 km, their epicentres being at a distance of lo-20 km or more from the active crater of the volcano. Such swarms reflect the gradual release of stress which ha built up during magma migration and during the eruption (roof subsidence above the peripheral magma chamber, collapse of crater walls, magma withdrawal in the fissure, etc.). EN, N and K,,, which are at a maximum at the beginning of the swarm, gradually decrease with time. The swarms last from a few hours to several months.

two possible fault surfaces solution coincides with or the eruptive fissure which eruptions, and on which

formed (Zobin, 1972, 1979) (see Fig. 3). These data allowed us to identify the sources of relatively large volcanic earthquakes with ruptures in the Earth’s crust, which are generated during the preparation for flank eruptions, and through which the magma rises to the surface. It is suggested that the characteristics

Source nature of volcanic earthquakes The source mechanism of large volcanic earthquakes (M 2 3) is similar to that of tectonic earthquakes of the same intensity, i.e. both tectonic and volcanic earthquakes occur as a result of shear movements of rocks in the Earth’s interior (e.g., Zobin, 1971, 1972; Harada, 1981; Weaver, 1981). The quadrantal distribution of the first P-wave

of the focal mechanism is close to the strike of originates during flank new lateral cones are

quakes,

and

the

of the sources of large earthspace-time

development

of

volcanic earthquake swarms, are dependent on the specific character of formation of these eruptive fissures. Estimations

of

source

parameters

for

large

volcanic earthquakes (seismic moment, source dimensions and stress drop), made on the basis of the spectral characteristics of the body and surface waves, have indicated that the foci of volcanic earthquakes are characterized by more intense long-period radiation, compared to tectonic earth-

motions corresponding to sources of the double couple type has been observed during earthquakes at Sheveluch, New Tolbachik (Kamchatka), Miyake-Sima, Usu (Japan), Mt. St. Helens (U.S.A.), etc. (Fig. 3).

quakes of the same magnitude. This is reflected in the somewhat higher seismic moments of volcanic earthquakes (Zobin, 1979). In terms of the foci of

A study of the focal mechanism of volcanic earthquakes preceding flank eruptions (Klyuchevskoy, New Tolbachik, Tyatya, Miyake-Sima, etc.) has shown that the direction of one of the

volcanic earthquakes, which can be represented by faulting within the eruptive fissure, this means that foci of volcanic earthquakes are formed as a result of the faulting of the “smoother” sides of

V.I. GORELCHIK

260

ET AL.

TORI-SHIMA,

LUYAKE-S~A,~~~~

lo"--

TYATYA,1973 0

I

: 4 P-WAVE SPECTRA DATA

mn

I'

AIO’

10' LENGTH,km

Fig. 4. Correlation of the rupture length of large volcanic earthquakes and the seismic moments of these earthquakes. The length of rupture in earthquake foci is compared to that of eruptive fissures at the surface. In all the cases the length of the eruptive fissure at the surface does not exceed the length of the rupture in the focus of the main volcanic earthquake related to this eruption. The dependence of M,, vs. L for tectonic earthquakes (Ishida, 1974) is given for comparison.

the fissure compared to tectonic earthquake sources. Source dimensions of large volcanic

A comparison of the spectral characteristics

of

during the pre-

waves radiated by earthquakes during central and flank eruptions has shown that in the first case

paration for flank eruptions have been estimated on the basis of P-wave spectra, and then com-

(i.e. in the course of central eruptions) earthquake foci are characterized by a greater amount of

pared with the length of eruptive fissures egressing in the course of these eruptions. It was found that in all the c;ts,s considered (Miyake-Sima, 1962; Beerenberg, 1970; Alaid, 1972; Tyatya, 1973; New Tolbachik, 1975) the length of the eruptive fissure at the surface did not exceed the source dimensions of the main earthquake in the swarm (Fig. 4). The length of rupture in this earthquake source certainly seems to limit the fissure dimension.

long-period radiation and by the relative predominance of a longitudinal wave intensity (Zobin, 1988). The latter effect is illustrated in Fig. 5, which shows plots of log Es - log Ep vs. log Es for volcanic earthquakes related to the Alaid (Kurile Islands) 1972 flank and 1981 central eruptions and to the 1964 Sheveluch (Kamchatka) central eruption. The higher intensity of P-wave radiation (log E,) is noted here for earthquakes

earthquakes

that are generated

VOLCANIC

EARTHQUAKEs

261

OF KAMCHATKA

to the specific geological-structural and the deep structure 3

.

l

El X

X 0

8.5

8.0

Fig.

X

active crater,

X

velocities

the

centre,

earthquake foci migrate along fractures in the direc-

tion of magma movement. These phenomena have

X

been observed at Kilauea, Hawaii (Decker, 1978),

9.0

9.5

I

I

I

10.0

intensity

11.0 11.0'09Ls (I)

10.5

of P- and S-wave

radiation

from the central crater, and

during the 1972 Alaid flank eruption (Kuril Islands). relative intensities

were estimated

at seismic stations

comparative

around

which enables one to forecast the place of erupthe system of near-surface

1

5. Comparative

recordings

are localized

or a new future eruptive

tion. Occasionally,

;:

X

during the 1981 Alaid eruption, S-wave

I

l

x I

I

cases the earthquakes

/

/

X

few tens or hundreds of km2 in area. In major

/

. /’ /x

/:

1-

feeding

The dimensions of epicentral areas amount to a

/

2-

of the magma

zones. .

.

environment

relations

situated near the volcano.

of P- and S-wave

maximum

were used. I, 2 = Data on volcanic

the 1972 (I ) flank eruption

Sheveluch central eruption (Kamchatka, the volcanic

The

ground

earthquakes

and 1981 (2) eruption

Alaid central crater; 3 = data on volcanic Plots characterizing

P- and

from short-period

of

from the

earthquakes

of the

1964) (Fedotov,

1972).

earthquakes

of two eruption

types are clearly distinguished.

related to central eruptions. Earthquakes related to central eruptions of all the volcanos in the world (Zobin, 1988) are also noted for higher seismic moments, M,,, (long-period estimate of foci intensity), MS values being equal. The difference in the level of seismic wave radiation during earthquakes related to flank and central crater eruptions may be interpreted as an effect of different nature of the fault rupture formation. During eruptions from the central crater, “revival” of the existed eruptive fissure apparently occurs, and during flank eruptions the emplacement of a new fissure occurs. In this case, rupturing along the already existing fissure will occur more “smoothly”. As a result, more intense long-period radiation will be generated. Spatial distribution of volcanic earthquakes The spatial distribution of earthquake foci in the region of a volcano or a volcanic group is due

and at Krafla,

Iceland,

where magma can flow

horizontally from the caldera for a distance of 65 km (Einarsson and Branddotter, 1980). In such cases seismic regions have a form that is elongated in one direction, and the new eruptive centres arise in the head part of the seismic activity front. The seismicity of groups of active and extinct volcanos which form so-called long-living volcanic centres, having a common system of feeding magma chambers, is made up of the sum of the seismically

active zones which exist within

the

centre and can cover a territory of a few thousand km’. We take as an example the seismicity features in the region of the Klyuchevskoy volcanic group, or the Klyuchevskoy long-living volcanic centre. The Klyuchevskoy volcanic centre includes about 20 large volcanos of Quaternary age. It is characterized by having the highest level of volcanic activity from recent times up to present day. As can be seen from the map of epicenters (Fig. 6a, b) the principal crustal seismicity of the Klyuchevskoy volcanic group in concentrated in a few seismic zones, identified by high concentrations of epicentres (Gorelchik,

1985). The position

of the seismic zones is controlled by the distribution of volcanos,

which are primarily

active-

Klyuchevskoy (Central zone around the summit crater), Bezymianny (Bezymianny zone) and Plosky Tolbachik (Tolbachik zone)-and by the structural features in the region, and fractures with a NE and NW strike (zones located close to seismic stations PDK, APH and KOZ; see Fig. 6a, b). The distribution of earthquake foci by depth is different under volcanos having different feeding

V.I. GORELCHIK

a

4 K=logEJ

‘\, .\...G_’ ..

Fig. 6a. For legend see p. 264.

H,km

ET AL

VOLCANIC

EARTHQUAKES

OF KAMCHATKA

263

b

.

/-

/’ .

Fig. 6b. For legend see p. 264.

0

SVL

”

.

V.I. GORELCHIK

264

zone

structures

and,

lava compositions. correlation

correspondingly,

according

different

to deep seismic

sounding

data, at a

depth of between 10 and 20 km (Balesta,

The character of the temporal

Just as at Klyuchevskoy,

of seismicity and volcanic activity for

ity

various structural layers is also different.

around

ET AL.

1981).

the surface seismic activ-

Bezymianny

usually

precedes

and

zones), seismic activity is observed

accompanies the eruptions. In other seismic zones of the Klyuchevskoy

within a depth interval of O-30 km. In the upper crustal layers, at depths shallower than 5 km, the

or more from the craters of active volcanos, surface

greater number of earthquakes occur mainly dur-

earthquakes

In zones of recent basaltic volcanism (Central and Tolbachik

ing premonition

(short-term

centre, which are located at distances of 20-30 are scarce,

depths of lo-20

precursors) and dur-

and events

without any visible relationship

seismicity

but during some large eruptions

low. In

with focal

km prevail. These generally occur

ing eruptions. During all other periods the level of at these depths is extremely

km

with eruptions, an increase

in

mid- and lower-crustal layers, the seismic activity

seismic activity can be observed within the whole

shows a correlation

not only with the eruptions

volcanic centre. For instance, during the prepara-

but also with the volcanic process as a whole, including the processes in the conduit and magma chamber or in the system of chambers (Ivanov et

sure eruption (LTFE) in June-September 1975, the area affected by seismic faults originally con-

al., 1981; Fedotov et al., 1984; Gorelchik,

tion and development of the large Tolbachik

fis-

stituted 400 km2. During and after this eruption the seismicity increased over a territory of 5000

1985;

Gorelchik et al., 1988). Earthquakes having a focal depth of more than 5 km can occur at any state of

km2 (contour

R in Fig. 6a, b) (Fedotov

et al.,

1984). Widening of the area of seismicity occurred twice, in 1975-1976 and again in 1976-1978

the volcano, but the most of them are generally recorded in the periods between eruptions, preceding an increase in summit crater or flank eruptions and lasting from a few months to a year (mid-term

(Fedotov et al., 1984). The velocity of seismic activity propagation away from the eruption re-

precursors).

gion is estimated as 1.95 + 0.35 km/month. It is likely that not only magma chambers beneath the

In the zone of andesitic volcanism (e.g., at Bezymianny, during the last 15 years), seismic activity related to growth and destruction of the extrusive dome was relatively low (K,, = 8). It

Northern and Southern Breakthroughs of the new Tolbachik volcanos and Plosky Tolbachik, but also the overall system of chambers feeding the Klyuchevskoy volcanic group, which is assumed to

was concentrated at a very shallow depth, commonly above sea level (see Fig. 6b). Earthquakes with focal depths of more than 5 km under the

be located in the lower-crustal layers or in the crust-mantle transition layer, were involved in the

volcano rarely occur. In this case, the probable

process of the Tolbachik

lower limit of the focal depth in the crust appears to be limited by the roof of a large (10 x 16 km)

this eruption, an area of about 36 km2 was covered with eruption products. This eruption is thought

magma

to have caused deformations

chamber

Fig. 6 (pp. 262-263). the Klyuchevskoy Kharchinsky,

which

been

Maps of earthquake volcanic

group

ZR = Zarechny,

Nikolka);

4 = active

Kizimen);

5 = calderas;

eruption.

Tectonic

preceded

the formation

27 to July 5.1975;

has

volcanos

epicentres

in 1971-1985.

PL = Bbzhny craters;

8 = established;

II = from August

2 to August

which a system of intermediate

Dahry

stations: Ploskye,

magma

I = permanent; KM = Kamen,

2 = temporary; 3 = extinct volcanos ( HR = ZM = Ziminy, 0. TL = Ostry Tolbachik, NK =

KLY = Klyuchevskoy,

7 = Northern

of cones of the Northern

and splitting of the

with H d 30 km, K > 7.5 (144 > 2) (a) and K < 7.5 (M < 2) (b) for the region of Seismic

and

(SV = Sheveluch,

6 = volcanic

disturbances:

established,

eruption. As a result of

(N)

9 = presumed; and Southern 27,1975; chambers

and

BZ = Bezymianny, P. TL = Plosky Tolbachik, Southern (S) Breakthroughs of the large Tolbachik

10 = outlines Breakthroughs

of the epicentral

regions

of the large Tolbachik

III = from September

1 to 17,1975);

feeding the Klyuchevskoy

volcanic

of earthquake

fissure eruption

II = the outline group is proposed

swarms

KZ = fissure which

(I = from June

of the region under to exist.

VOLCANIC

EARTHQUAKES

roof above the intermediate the propagation region

265

OF KAMCHATKA

chambers

of seismic activity

of the Klyuchevskoy

and its future

which led to

over the whole

volcanic

group.

relationships

between

and

volcanic

of the Northern

accompanied

by an intense

tremor

seismic phenomena

earthquakes. amplitude

struction

ods of repose, eruption

volcano preparation

which correspond

tics of seismic

activity.

during

the preparation

LTFE

in Kamchatka

by peri-

is characterized and

development

to definite Variations and

amplitude.

started,

simultaneously

ferences

of the

at the beginning

the outpouring

with sharp

of lava in the area of the Southern lasted

and

was

moderate volcanic tremors and plete absence of earthquakes.

in

of lava activity,

amplitude

An incessant

shown in Fig. 7. Several days before the formation

prediction

the recon-

(from 0.2 to 6 pm at a distance

for 479 days,

and

volcanic

by an increase

of the large cones of the Northern and Southern Breakthroughs, earthquake swarms occurred at their places of emergence. Their development in space and time provided a good basis for the successful

in

with the explosive

crater).

was

tremor

of the eruption.

activity

When

was recorded

from the active

et al., 1984) are

regime

was accompanied

tremor a tremor

in seismicity

development

(Fedotov

of

characteris-

Breakthrough volcanic

(see Fig. 7) reflected

of explosive

of the eruption

explosive-effu-

Variations

of the dynamic

The upsurge Each specific

The violent

sive activity explosive

Temporal

course.

dif-

of 13 km outpouring

Breakthrough

accompanied the almost

by com-

Differences in eruption mechanisms can affect the temporo-spatial development of seismic activ-

of the onset of the eruption

Ap(T =O.fH.lc)

t

N(n,rZS) 100,

T

SB

SO 60

I

y

K

115

10.5

tYIrki% t

9.5 a3 1.5

I

III

JULY

UGUST

SRPTEMBER

Fig. 7. Temporal variations of the average amplitude (A) of volcanic tremor at seismic station TLB and the daily number (N) of earthquakes with K 2 7.5 (M >, 2) during June 27-September

30, 1975. Arrows indicate the time of formation of the eruptive cones;

SB, NB, MF = breakthroughs of the Southern and Northern boczas (1st cone) and of the main flow (2nd cone).

V.I. GORELCHIK

266

ity in the region of the volcano. c are shown

the locations

of the swarms occurring Southern tember

in December

1976, relative

and the change during Northern located through,

in June-July

1975, and after termination in position

these swarms. around

Sep-

at all stages of swarm

swarm

swarm relative

further

Again,

to another,

in the direction

(see

of the epicentral

zone took place to the southwest

suggestive

of the place,

crater.

The

eruption,

Southern

feeding

Breakthrough occurred in the southwestern margin of the preceding swarm. It is proposed (Fedotov et al., 1984) that the Northern Breakthrough was due

1975

regions

of this

displacements, northeast

one to the

of the proposed

intru-

characteristics

of the

and

in the volcanic time

of the deep

and

region are

mechanism

structure

of

of volcano

zones.

Parameters of the seismic regime

to a rise to the surface of magma from the lower layers of the crust or from the crust-mantle transition layer where the magma chambers are located which feed both Plosky Tolbachik and the cinder cones zone where the LTFE occurred. During Southern Breakthrough basalts were intruded

to

of the Southern

the

Thus, the spatio-temporal

the South-

of seismicity

Tolbachik

related

sion. distribution

Plosky

After

1976, an

obviously

the epicentral

(see Fig. 8b) the displacements

the

swarms.

south

from

of the

of fissures

in December

emerged,

em Breakthrough of

the region growth

(see Fig. 8c) revealed

southwest,

Break-

development

intrusion

Breakthrough.

for the

the cones of the Northern

Fig. 8a). In the swarm which preceded

of the LTFE

earthquake

zones were

preparation

from The

by earthquake

termination magma

centres

of these zones in space

the epicentral

fissures

Breakthrough.

was accompanied

and

of the eruption

to the eruptive

During

Breakthrough

and

through

Northern

zones

prior to the Northern

Breakthroughs

an angle

In Figs. 8a, b and

of the epicentral

ET AL.

Concentration

criterion for seismically

active faults

Wsd

This cumulative parameter to seismology quite recently

the at

1975

has been introduced (Sobolev and Zavia-

1976

ATLB

LTLBA

i'::: 27VI (::I

28.W

~~329-3O.Vl 0

l-5.w

Fig. 8. Position of the epicentral zones of the earthquake swarms during the large Tolbacbik fissure eruption relative to the eruption centres, and their development in space during the swarm: 1 = active volcanos;

2 = extinct volcanos;

3 = collapse of the Plosky

Tolbachik crater; 4 = eruption centres; 5 = zones of development of areal volcanism; 6 = seismic stations.

VOLCANIC

EARTHQUAKES

261

OF KAMCHATKA

lov, 1981), as a modification

of the concentration

where K = log E(J)

is the energy class of earth-

of solid bodies (Zhurkov

quakes, N is the number of earthquakes within a

et al., 1980). Its value can be determined as the

given energy class K + i, and A and Y are con-

ratio of the average inter-fault distance to average

stants. The slope coefficient,

fault length in a certain seismically active volume:

b-value of the frequency-magnitude

criterion of destruction

log N(M)

KS, = N; 1’3/L

y, corresponds

distribution

= a - bM, b = 1.5y, approximately.

In the volcanic

regions

of Kamchatka,

N* = N,/V

long-term values of y for earthquakes

L=$,

are approximately

depths of lo-30 I=1 where K,, is the concentration the number of earthquakes

criterion and N* is occurring

in a unit

volume. Ii is the fault length of a seismic event of and can be evaluated energetic class Ki, (Riznichenko, 1976) using the empirical equation log Ii = 0.244Ki - 2.666. The values of KS, calculated in this manner for the LTFE in Kamchatka during June 27-September 17, 1975 are shown in Fig. 9 (Gorelchik and Zavialov, 1986). It has been shown that, as during tectonic earthquakes, a large amount of destruction of the medium occurring during the formation of magma-supplying channels in the course of the Northern and Southern Breakthroughs of the LTFE,

or during

the

collapse

of

the

Plosky

Tolbachik summit caldera, is preceded and accompanied by an accumulation of fractures in some volume of rocks. This leads to the formation

of

zones of decreased KS, values at destruction sites. Zones of decreased KS, values at volcanos are characterized

by smaller dimensions as compared

to similar zones in tectonically active regions. KS, values calculated for the LTFE region and Plosky Tolbachik crater are within the range of 5-10 (Gorelchik and Zavialov, 1986), i.e. they are close to similar estimate obtained in laboratory for model materials and rocks (Zhurkov et al., 1980), and also similar to values obtained prior to violent earthquakes in Kamchatka (Sobolev and Zavialov, 1981). Variation of the volcanic earthquake frequency-energy relationship The frequency-energy monly written: log N(K)

=A

- yK

relationship

is com-

to a

the

with focal

km are equal to 0.4-0.5,

i.e. they

the same as for tectonic earth-

quakes (Gorelchik

et al., 1981;

Tokarev,

1981).

For earthquakes taking place in the upper-crustal layers, and within the volcanic from 0.5-0.6

to 1.5-2.5,

edifice,

y varies

due to sharp non-uni-

formities of the physical properties of rocks, and to the variability

of the strain field close to the

volcano. These data correlate well with experimental studies on destruction processes, which have revealed a direct link between y and the degree of non-uniformity of samples (Vinogradov, 1964). On the whole, the seismic regime of active volcanic regions where swarm seismicity predominates, with large numbers of shallow and relatively small events, is characterized by increased average values of the coefficient compared to tectonic regions.

y as

The value of y changes both in space and time, depending not only upon the physical properties of crust but also upon the character of processes occurring in seismic zones beneath volcanos and within the volcanic centre. Acute variations in slope coefficient out the area were observed

y through-

in various

zones which arose during the LTFE.

seismic

The average

value of y for the region of the Klyuchevskoy volcanic group, calculated for earthquakes of K > 7.5, constituted 0.7 for the period of preparation and development of the LTFE (June 27-September 17, 1975) (Gorelchik et al., 1981). In the eruption region at that time there existed zones of low (0.4-0.5) and high (1.2-2.5) values of y (Gorelchik and Zavialov, 1986) (Fig. 10). The low values were noted in the Northern and Southern Breakthrough regions, where the strain increased during formation of new volcanos; while the high values

were

noted

in

the

region

of

Plosky

Tolbachik, where the summit crater collapsed as a result of magma withdrawal and strain drop in the magma chamber beneath the volcano.

V.I. GORELCHIK

268

ET AL.

27.W - 2.Vll(before07h00m)l,

I16

1

r 116

1ZKM

I

156

168

le.0

80

,

Fig. 9. Maps of K,, for the region of the large Tolbachik fissure eruption (June 27-September for five main stages of eruption process:

18, 1975). Maps of K,, were compiled

(a) the stage of gradual development of seismic activity which preceded the largest

earthquakes on July 2, 1975; (b) formation of the 1st cone; (c) formation of the 2nd cone; (d) formation of the 3d cone,the Northern Breakthrough;

(e) formation of the 8th cone, the Southern Breakthrough (Fedotov et al., 1980). K,, calculations were made for unit

volumes of 12

x

to the

x

in Fig.

of KS,: I = 6.1-8.0;

of the 6

2 = 8.1-10.0;

system of co-ordinates;

7= study area; 8= Plosky Tolbachik

Northern Breakthrough;

VIII,

(K=12,

ML,=

Southern Breakthrough);

km2 calculation grid, the axis orientations of

x

3 = 10.1-12.0; volcano;

4 = 12.1-14.0;

5 = 14.0. Other notation:

9= cones of the Tolbachik

6 = Cartesian

fissure eruption (I,

ZZ, ZZZ,

10 = collapse of the Plosky Tolbachik crater; II = epicentres of the largest

5) earthquakes in the region of the large Tolbachik fissure eruption which occurred on July 2, 1975.

In many swarms of volcanic earthquakes such as those that preceded the Sheveluch eruption in 1964 (Tokarev and Gorelchik, 1974), the breakthrough of flank craters named after B.I. F’iip at

Klyuchevskoy in 1966, the Northern and Southern Breakthroughs during the LTFE in 1975 (Gorelchik et al., 1981), the submarine dacitic eruption in the region of the Curacoa Reef in 1973

VOLCANIC

EARTHQUAKES

269

OF KAMCHATKA

1971-1975

a

Y 180

80 156 I

168

D

Y -

12tiM

,

18D

168

15

Fig. 10. Maps of y in the region of the large Tolbachik fissure eruption over the period of June 27-September intervals of O-20 km; (b) for O-5 km. y-values: 6 = 0.91-1.00; 12

x

12

x

7 = 1.01-1.10; 20 km3 (a) and 12

8 = 1.11-1.20; x

12

X

1 = 0.41-0.50;

9 = 1.21-1.30;

2 = 0.51-0.60;

10 = 1.31-1.40.

3 = 0.61-0.70;

18,1975:

4 = 0.71-0.80;

(a) for depth 5 = 0.81-0.90;

y-values were calculated for seismoactive volumes of

5 km3 (b), with the number of earthquakes, N b 20. Other symbols are the same as in Fig. 9.

(Gibowitz et al., 1974) etc., a decrease in the value of y or b was noted prior to the largest earthquake in a swarm. The N(K) distribution for explosive earthquakes has a non-linear form. From observational data at Karymsky (Tokarev, 1981, 1985) it has been established that N(K) dependence on a double logarithmic scale is best approximated by a parabola. A parabolic form for plots of explosive earthquake recurrence, with a peak at definite energy class values, and sharp distinction of these from N(K) distributions for tectonic and volcanic earthquakes, can be explained by the fact that explosive earthquakes are related not to shear movements of rocks in a focus but to explosions of gases in the conduit. A peak in the plot indicates that the optimum conditions for an explo-

sion are created in the vent of the volcano at a certain rate of matter supply (magma or gases) form depth. The feasibility of the explosion decreases with increasing deviation from these conditions. Conclusions Seismicity related to magmatic activity and, correspondingly, to volcanic eruptions or to intrusive activity, has the following specific features: a local and predominantly swarm-like pattern of earthquake origination; reiteration of earthquake swarms in the same seismically active zones; a large number of shallow and relatively small events; and the existence of longer-period radiation of volcanic earthquake foci as compared to

V.I. GORELCHIK

270

earthquakes

tectonic

of the same intensity.

The

features of volcanic earthquake foci are related to the nature of eruptive fissures through which the magma rises to the surface. The distinctive

fea-

tures of the seismic regime of these volcanos are: a small magnitude limit (up to 5.5-6);

localization

of regions of increased seismic activity and rapid waning of activity with distance from the volcano; and a comparatively high value of the slope of the earthquake recurrence plot ( y). in the be-

y for volcanic earthquakes

and the density parameter faults during the preparation

deflation of the Krafla volcano in NE Iceland. J. Geophys. Res., 47: 160-165. Fedotov,

S.A.,

1972.

Kamchatka

Energy

Earthquakes

Classification

of the Kurile-

and the Problem of Magnitudes.

Nauka, Moscow, 116 pp. (in Russian). Fedotov,

S.A.,

Gorelchik,

Seismological Tolbachik

studies

V.I. on

and Stepanov,

the mechanism

fissure eruption, 1975-1976.

V.V., of

1980.

the large

Bull Volcanol., 43:

73-84. Fedotov, S.A., Gorelchik, Chubarova,

V.I., Zobin, V.M., Stepanov, V.V.,

O.S. and Shirokov, V.A., 1984. Seismological

data on mechanism and development of an eruption.

At the same time, the similarity haviour of coefficient

ET AL.

Ksr, for seismogenic and development of

eruptions and prior to large earthquakes, as well as the destruction of samples in the laboratory, and the shear character of faults in the volcanic and tectonic earthquake foci, confirm the proposed similarity of destruction processes at various scales, which are governed by one and the same physical mechanism. This makes studies of the seismic regime of volcanos and the process of formation of eruptive fissures important not only for the prediction of eruptions and for improved understanding of the physics of volcanic process, but it also enables one to make use of volcanos as a kind of “natural laboratory” to observe the processes of destruction and splintering of rocks under natural conditions.

S.A. Fedotov (Editor), The Large Tolbachik tion. Nauka, Moscow, pp. 389-448 Gibowicz,

S.J.,

Latter, J.H.

quake swarm associated

G.K.,

1974. Earth-

with volcanic eruption,

Curacoa 23

443-475.

Gorelchik, V.I., 1985. Seismic activity of Klyuchevskoy preparation

and

development

Breakthrough in March-June

of

the

during

Predskazanny

1983. Volcanol. and Seismol.,

1: 71-87 (in Russian). Gorelchik,

V.I. and Zavialov,

A.D.,

1986. Behavior

of the

density of seismogenic ruptures parameter during preparation and development of the large Tolbachik tion. Volcanol. Seismol., 6: 60-66 Gorelchik,

V.I.,

Seismicity

Zobin,

V.M.

fissure erug

(in Russian).

and

Chubarova,

and dynamic characteristics

O.S.,1981.

of the earthquake

foci in the region of the large Tolbachik

fissure eruption in

1975-1976.

(in Russian).

Gorelchik,

Volcanol. Seismol., 3: 73-98

V.I., Chubarova, O.S. and Garbuzova,

V.T., 1988.

Seismicity in the region of the Northern volcanic group in Kamchatka,

1971-1983.

Volcanol.

Seismol., 1: 90-100

(in

Russian). Harada, T., 1981. Stress field in Usu volcano deduced from focal mechanism

Acknowledgements

(in Russian).

and Sutton,

Reef Area, Northern Tonga, July 1973. Ann. Geofis., (3-4):

In:

Fissure Erup-

solutions.

Bull. Volcanol.

Sot.

Jpn.,

2:

93-10. Ishida, M., 1974. Determinations

The authors would like to express their thanks to Professor S.A. Fedotov for his valuable and helpful comments, and to Dr. E.S. Aster for his review of the paper.

212. Ivanov, B.V., Gorelchik,

V.I., Andreev, V.N., Maximov, A.P.,

Stepanov, V.V. and Chirkov, A.M., 1981. The 1972-1974 eruption

of

volcanol.,

44(l):

Minakami,

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