Experimental studies and finite-element analysis of the seismicity of north china plain

TectonophJsits.

85

Elsevier Scientific

75

(I 982) 75-89 Publishing

Company,

Amsterdam-Printed

in The Netherlands

EXPERIMENTAL STUDIES AND FINITE-ELEMENT SEISMICITY OF NORTH CHINA PLAIN

ANALYSIS

LOO HUANYEN,

and others

SONG

HUIZHEN,

GUO CAIHUA,

LI JIANGUO

OF THE

of Geology, Stcrte Seismologicul Bureuu, Beijing (Chino)

Institute (Received

November

26, 1981)

ABSTRACT

Loo

Huanyen,

Song

finite-element Earthquake The driving

portions,

earthquakes found

factors

Li Jianguo

of North

others.

1982. Experimental

China Plain has been modeled

of the upper of the acting

by the same numerical

deformation

and migration

mantle.

The results obtained

from dehydration

of earthquakes

in the weak regions.

To know more about the source mechanism,

The stresses

and wave travel measurement

along

the fault

perpendicular

plane

devices attached.

having

were detected): nearly

parallel

positions

of the precursors earthquake

crack

starting

failure

behaving

All of these stages, varying

and the foreshocks

prediction

in continental

structure indicate

on the

that weak

cracking.

are of

concentrated

earthquake

compression

results indicate

at the

prediction. and acoustic

that the failure

first at the fault

and later on, the development

from the first one (during a shear

by the

the isostatic

sample, could be divided into 3 stages: (I) creep

(2) a tension

in direction

to the fault plane.

The experimental

a marble

curvilinearly,

(3) as load increases,

direction short-term

no shocks;

to the plane. then extending

at the same end, but different shocks

and

of the concentration

are always

a rock mass was loaded in uniaxial

modes of a fault, not going all the way through

with

and thermal

because

end of the fault and Just above the soft layer. These results are useful in mid-term emission

studies

numerically

consistent

force and the geological

method.

faults and soft layers resulting

in the development

release and plastic

and

China Plain. In: A.L. Hales and Z. Suzuki (Editors),

in the North

to be upwelling

of both the orientation

are analyzed

such as deep-seated

the controlling energy

and

The influences

stress distribution

Caihua.

Tectonoph,vsics. 85: 75-89.

force causing method

Guo

of the seismicity

Prediction.

finite-element viewpoint.

Huizhen,

analysis

the cracking like a main

end,

nearly

of a second crack period,

many

shock. occurs

small in a

in space, may serve to relate the

to the main shock and could thus be used as a guide in plate interiors.

INTRODUCTION

In general, the genetic mechanism of inter-plate earthquakes is a result of underthrusting, collision, or relative motion of the tectonic plates. The intraplate earthquakes, however, are concentrated along the basin or rift borders, far from the plate boundary. They seem to occur in response to a tectonic stress regime in the plate with properties unlike those of plate tectonic theory, unless deformation within 0040-1951/82/CrOOC-oooO/$O2.75

0 1982 Elsevier Scientific

Publishing

Company

76

the plate is assumed. of earthquakes

The most effective ways of studying

in continental

interiors

the genesis and mechanism

should be based upon understanding

of the present tectonic stress field, and the changes understanding bears on a number of scientific questions, forces creating

the stress field and how these relate to the driving

first step is to analyze

qualitatively

the historical

the state

taking place there. This such as the sources of the

evolution

mechanism.

The

of the stress field on the

macroscopic scale, thus gaining insight into its present state. Secondly, it is necessary to establish a deterministic model for the driving force. Thirdly, determining the textures and structures of the continental crust on the basis of geophysical data, especially on the results of seismology, is necessary in order to estimate the stress distribution and, hence, to predict the earthquakes. Finally the experimental study on the failure modes of a rock mass can enable us to develop a physical concept of the earthquake

mechanism

THE EVOLUTION

OF THE STRESS FIELD

Understanding orientation

of the variations

is considered

China-Tarim

for short-term

old block,

predictions.

OF NORTH

of both the ancient

the key to understanding formed

CHINA

PLAIN

regional

the current

stress field and its

stress field. The North

1700 m.y. ago, was the initial

form of the China

plate. As pointed out by Zhang Quinwen (1979) and Xu Jiawei (1979), the northeastern part of China was compressed and uplifted by the action of the South Pacific plate in the Middle-Late Mesozoic to form a deep fault system in the NE direction. Since the Cenozoic, the compressive belts in both regions were reactivated by a tensional continental spreading towards the Pacific ocean and deep-seated material was injected into the large pre-existing deep fault system. As a result, the crust thinned and subsided into a series of basins, such as the Song-Jiao and the North China Plains. However, due to the rapid spreading of the Japanese Sea (Uyeda, 1971) these basins had to a certain extent sustained a compressive action, decreasing the amplitude of subsidence and hence reducing the sedimentation. This evidence as well as the examples of Cenozoic fault movement indicate that the direction of principal stress changed

THE DRIVING

from left lateral in the Mesozoic

FORCES

OF THE CONTINENTAL

to a right lateral

TECTONIC

MOVEMENT

slip since Cenozoic.

AND ITS ORIGINS

We emphasize the driving forces resulting from the opposition and interaction of gravity and interior heat energy of the earth, from the point of view of energy transformation and mass transport. Our quantitative analysis starts from the causal relationship of isostasy to the formation of basins, and of their borders to the earthquake location, and from the similarity of the continental marginal basins to the inland

ones. Then using ground

deformation

data of tectonic

elements

in North

China

together

driving

with the finite-element

force acting on the boundaries

method,

the direction

of a finite regional

and magnitude

of the

stress field are determined.

The effect of gravity and interior heat energy on plate tectonics Most of the supporters provides

the driving

difference

of plate tectonics

in space causing

primary

factor.

tectonic

movement,

theory insist that the mantle

force and the changes in geothermal

But Jacoby

movement

of material.

(1978) suggested

may be attributed

mass from below, lateral gravitational

convection

produce

a gravity

In this view, the heat energy is a

that global

to gravitational

differ from the above and may be expressed density

condition tectonics,

or the regional

effect. Our ideas (Loo, 1979)

as follows:

the buoyant

rising of low

sliding and sinking of the re-condensed

deep-seated material can be considered, spatially and temporally, as a unified process of two opposites-the heat energy to break the isostatic equilibrium and the gravity to maintain it. From this viewpoint we discuss the source of the forces that lead to basin formation in more detail below. The relationship In China, and uprising

between basin distribution

and gravity anomalies

all basin areas are generally characterized by high gravity, thin crust of the upper mantle (Loo, 1979) especially the eastern basins which

have NE orientations

and appear

to be an undeveloped

rift nearly

continental margin. In the North China Plain, all large earthquakes mostly along the border of the plain (Fig. 1) formed during continental

parallel

to the

have occurred thinning into

basins or separating into arcs in the Late Mesozoic or the early Cenozoic. In general, engineers ask what the response is at some point to the applied

force

in a specified medium, but the seismologist must ask what driving force and what material specifications will produce an observed result at points on the surface of the earth,

i.e., will account

gravity

for the observations

and magnetotelluric

of ground

deformation,

seismic waves,

sounding.

Our numerical model (Fig. 2) is a block, 180 X 40 km, arbitrarily isolated from the crust continuum of the North China Plain, with the boundary conditions at both the fictitious

ends and bottom

being unknown.

Applying

horizontal

compressive

force to

the lateral ends alone would cause ground surface uplifting which conflicts with the geological phenomenon of continuous depression in the plain area. However, if the unknown driving forces at these fictitious boundaries be idealized with a known rigidity of the external material adjacent to the chosen model multiplied by an unknown displacement at the same boundary, together with the measured annual changes in ground deformation, we are able to solve the equilibrium equation uniquely and successively by initial strain procedure, arriving at the time-dependent stress state and the variation of driving force. The results of numerical modelling (Fig. 2) indicate that driving forces which cause ground subsidence are mainly due to the upwelling of upper mantle material from below. This localized thermal convec-

tertiary

depression

Fig. 1. Positive gravity ing North

China

anomaly

zone and hypocenter

distribution

of strong earthquakes

in and surround-

Bay area (Loo, 1978).

Jinan

-_---

----

r Driving

Fig. 2. Relationship

between

the 1966 Xintai earthquake.

driving

force, ground

surface

displacement

force

and stress state of focus zone of

tion would

seem to be induced

More detailed paper. Although driving

discussion

about

the foregoing

by the deep subduction the finite-element

statement

forces, the problem

We will now discuss the lithological how these correlate THE

INFLUENCE

TRAPLATE

CRUST

with the strain

OF THE

has explained

of response

of the medium

character

of the cold Pacific

analysis

will appear

the origin

and

mode

to them remains

and structure

Plate.

in a separate of the

unsolved.

of the earth’s crust and

and stress state.

LITHOLOGICAL

CHARACTER

AND

STRUCTURE

OF THE

IN-

ON THE STRESS FIELD

The stress distributions are strongly influenced by the pre-existing tectonic framework, and both deformation and failure of the geological medium depend on the coupling between the acting forces and structures. Although our understanding of the tectonic processes at continental plate boundaries has increased tremendously in the last 10 years, little is known about the present tectonic activity within the plate. The continental crust of China is composed of numerous small blocks, which are in motion relative to each other. They are accompanied by other tectonic deformations of the faulting systems. Therefore, the rigid plate postulate can not be justified. Moreover, seismic wave monitoring and magnetotelluric sounding provide evidence of a low velocity layer correlating well with a low resistivity layer at about of recent huge 15-20 km depth in the North China Plain. Most hypocenters earthquakes are located just above these layers and are closely related to the upper mantle upwelling. The mechanism of these earthquakes and their effects on the stress state still need further Experimental

study.

studies on the formation

mechanism

of the low velocity-low

resistivity

layer Condie (1976) pointed butions in the continental

out that existing models for electrical conductivity districrust are nonunique and complex, and that two possible

explanations of the origin of this layer are generally considered, namely, dehydration and partial melting. However, experimental data are not consistent with partial melting in the crust at less than 20 km depth. We have made experimental determinations of the electrical conductivities of hornblendite, granite and basalt cores, 2 mm thick and 6 mm in diameter,

in a solid medium

press at temperature

and

pressure conditions equivalent to depths of 15-20 km. We also crushed the granite and the hornblendite and recompacted the samples in parallel tests. Among the three uncrushed rocks (Figs. 3, 4) only the slope of the temperature curve of the hornblendite specimen changed due to dehydration (Fig. 5). It’s electrical resistivity at 650°C is about 31.0 Q-m, which is still higher than the value measured in the field of a few Q-m. The runs of heating-cooling-reheating for the crushed granite were

Pressure

\

7.1 kb

32

24 Temp~~ature,(1/T)104,

Fig. 3. Electric

conductivity

of hornblendite.

Fig. 4. Electric

conductivity

"K-'

of granite

more sensitive to the cooling cycle. The electrical resistivity at the same temperature would drop to a value of 15.8 Q-m or less (Fig. 6). According to our optical analysis (Fig. 7) the microstructure of the specimens changed during heating and cooling runs. Thus their physical properties are temperature-dependent at high pressure (see also Wong, 1979). In others words, they are structure state-dependent due to thermal cracking of the grains. Kato (1978) measured the compressive velocity, VP, for serpentine

at a pressure

dehydration with increase

during

of 5 kbar and a temperature

heating

of temperature.

of 600°C. He reported

would cause VP to increase At 600°C

suddenly

the wave velocity

that the

and decrease

dropped

again

to a minimum

and kept nearly constant during cooling (Fig. 8). Kern (1978) also reported the decrease of VP for granite at temperatures above 600°C accompanied by a sharp and change in the slope of VP - T curve. VP is a sensitive indicator of microcracking of very small changes in water content (Johnson, 1978). It is clear from the measurements of electrical conductivity and wave velocity for different rock samples and from consideration of the orientation of the intergranular thermal stresses that, during heating cycles, dehydration would induce grain diffusion and aggregate loosening as a result of thermal cracking. Thus the elastic moduli and fracture strength would decrease, causing low resistivity and low velocity. We conclude that the crack-weakening

caused by the thermal

activated

energy is the origin of this kind

a .0

c I-

1

4000

C

Wave

number

(cm-1

1

Fig. 5. Identification of homblendite under high pressure and high temperature; (a) before test. (b) after test, (c) infrared spectra analysis.

of layer sliding

in the continental and diffusion

phenomenon.

to the geothermally layers.

along

Therefore,

crust.

Because

the dehydration

grain

boundaries,

it should

the low velocity-low

tectonic

activity

resistivity

and the seismicities

is an initiating

not be regarded layers

in basins

factor

in

as an isolated are ascribed

are in turn affected

by these

x2

-3. i E ”

Pressure

7.8 kb

Cooling

b b$

-5.

x .r .> z -0’ .-E

% 0 rc 3’ 9 ~ \

” ‘5 -7

\

:

\

. Reheating

iLl

-9 8

16 Temperoture,(llT)104,

Fig. 6. Electric

conductivity

Fig. 7. Identification

24

32 “K-l

of crushed

of granite

granite.

under high ten iperature

and high pressure;

(a) before

test, (b) after test.

6.0.

Dehydration

4.0 OA Fig. 8. VP measurements

of serpentine

(After Kato,

1978).

The influence of soft inclusions in the earth’s crust on the stress distribution The inclusions

mentioned

here refer to faults

and low velocity-low

resistivity

layers. The two-dimensional finite-element analysis of the geological profile of Fig. 2 indicates anomalous horizontal stress in the relatively strong medium just overlying the low-velocity

layer.

If a dipping

fault

ends

nearby

but

does

not

cross

0.7

b‘ \ t? 0

0.2

- -.-

O.Ul/

0 Imax/

Fig. 9. Effect of intermediate

E element

principal

modulus

stress 0, on the maximum

shear stress 7max at fault ends.

the

84

underlying

soft layer, the stress would be highly concentrated

may have been the cause of the 1966 Xingtai

at the fault tips. This

earthquake.

A typical

three dimen-

sional finite element analysis gives a similar result to that of the two dimensional analysis (Fig. 9). Obviously the soft inclusions are easily deformed, inelastically or plastically, to distribute and transfer the stress from the weak portions towards the adjacent strong region, thereby triggering an earthquake. On the other hand, stresses could not be transmitted very far because of the consumption and absorption of energy by the soft portions. Thus mechanical properties must be controlling factors in earthquake migration. In addition, the relationship between the fault orientation and intermediate principal stress, a,, is analyzed. As the maximum principal stress u, is constant, whether uz is parallel to the fault or not, the shear stress at both the lateral and lower end of a fault always shows a tendency

to increase

with uz. But when a3 is parallel

the fault, the shear stress at the lateral end is decreased lower end. This can be explained

by the influence

somewhat

to

and is less at the

of the underlying

soft layer.

These numerical results indicate that the orientation of driving forces and the geometry of geological structures are the key points in the quantification of the regional tectonic prediction. EXPERIMENTAL ROCK

stress

STUDIES

state

and,

therefore,

ON THE PROCESSES

are useful

in mid-term

OF DEFORMATION

AND

earthquake

FAILURE

OF A

MASS

The rupture strength pressure (depth), loading

of a rock mass is controlled by temperature, confining rate and interstitial water pressure. In the last 10 years, the

effect of water in the rock mass has attracted suggested

that hydraulic

controlling

earthquakes,

fracturing

studies

scientific

attention,

are not only useful

but also in determining

and it has been for predicting

and

the stress state at several kilometers

depth. The work in this field, however, is still in the embryonic stage. The relation between the geometry of the rock structure, the confining pressure and time will be considered in detail below. Structural types and rupture mechanism Fracture mechanics receive a wider and wider application in studies of the mechanics of rock mass. However, applied stresses in the laboratory are usually compressive rather than tensile as in the geological environment. The fracture response of rock faults, filled with materials and loaded with compressive stress, differs considerably from tensile cracking. In applying fracture mechanics to rock mechanics, these differences must be taken into account. Bernaix (1974) has given a detailed analysis of the effect of the size, numbers and geometry of the empty cracks on the characteristics of the deformation and rupture

x5

of rocks. result

If small

in ruptures

in size, numerous of ductile

and with a scattered

shear character

and release energy

shocks. When the cracks are large, nearly parallel echelon”

form, the angle between

distribution,

the maximum

in small swarms

to one another principal

the cracks

and aligned

of

in “en

stress axis and the crack

plane being rather small, the brittle and tensile cracks develop easily and release energy in the form of moderate and smaller shocks. On the other hand if cracks are “en-echelon” but inclined at larger angles to the maximum principal stress direction and displaced with respect to each other to some extent, shear rupture takes place and energy

is released

in a large earthquake

4-quadrant responsible

distribution of tension and compressive stresses. These phenomena are for the regional anomalous ground deformation and groundwater regime

and can serve as a useful tool for mid-term

with a larger focal region,

prediction

of earthquakes.

and with a

Therefore

they

are worthy of further study. Mogi (1978) also proposed that the creep failure of a filled fault would generate neither a mainshock nor foreshocks; that the more cracks there are present, the more foreshocks will occur; and that the poorer the connection between the cracks is, the larger the magnitude of the earthquakes will be. The ideas and concepts ments stage

and partly

of the research.

cursors,

In order

more systematic

Experimental

stated above are derived

partly

from tests on a few rock blocks/masses, research

to fully

understand

remains

from photoelastic

experi-

and are only the initial

the possible

earthquake

pre-

to be done along this line.

study of the relation between the mainshock

and foreshock

areas

Efforts at predicting earthquakes in our country reveal that the mainshock and precursors (or foreshocks) do not necessarily fall in the same region. An experimental study of the relationship between the precursors and foreshocks and the main fracture would be an important factor in predicting the location of a large earthquake. For this purpose, a 2-mm thick saw-cut, inclined 45” to the axis of a marble specimen

of 5 X 5 X 12.5 cm, was extended

and rock powder

mixture.

gauges,

emission

acoustic

characteristics

The specimen and

velocity

to its center

line and filled with epoxy

was loaded in uniaxial

compression.

measuring

were attached.

equipment

of the failure modes in these experimental

Strain The

studies are as follows (Figs.

10-13): (1) Fault creep without shocks: It is assumed that the strength of the interface between the filled material and the marble has cohesive and frictional components. At low stress level, creep-slippage along the contact face took place with the result that the cohesion was overcome without any changes in acoustic emission and in wave travel time. During this stage, the axial strain and lateral strain, not parallel to the fault plane, were much higher than the other lateral strain, parallel to the fault plane, with the latter even approaching zero. These phenomena may be equivalent to precursors such as ground surface displacements and groundwater level variations. (2) The small shocks due to tension cracks at fault tip: As a result of fault

86

Fig. IO. Development Fig.

of tensile cracks

I 1. Shear failure occurred

nearby

at fault end of marble cracked

uniaxial

compression.

zone.

creeping, a tearing-off crack started to develop perpendicular to the fault plane. The propagation

at the existing of this primary

fault tip nearly crack was stable

and its path was curvilinear. With increase in stress level, a second tension crack developed again at the same tip and extended downward to the edge of the specimen, but without any stress drop before complete rupture. During cracking, the

Fig. 12. Relationship

between

stress and strain.

87

Fig. 13. Relationship

between

stress, strain,

acoustic

emission

and wave travel time.

wave travel time and the acoustic emission increased with time and loading. At a certain stress level, the acoustic emission dropped down to a minimum-equivalent to a silent period prior to a large earthquake. (3) The mainshock critical adjacent breaking

level,

rupture

by overall shearing

shear rupture: took place

As the compressive

at a zone

connected

load reached to the fault

a tip,

to the cracked zone with angles relative to the fault from 30” to 60”. Before through, the acoustic emission and wave travel time increased appreciably.

This is obviously due to the redistribution of both the shear stress from the sliding plane and the tensile stress from the cracked zone to the neighbouring region capable of sustaining an increase of stress. Despite the fact that such an uniaxial experiment in which the confining pressure effect is not considered may not truly reflect the actual situation, it can nevertheless be used as a basis for future studies. Applying the present results to earthquake prediction in the continental interior suggests that large shocks should be preceded first by fault creep and then by a number of small shocks in tensile cracked zones trending

in different

guides in relating main shock.

directions

the locations

from the shear failure plane.

These may be used as

of precursor

to the location

and foreshocks

of the

The relation of strain rate to rupture time In the uniaxial compression test we could disclose the characteristics of rock rupture differing in stage and in space, which are important for understanding the earthquake mechanism and which possess a certain value in earthquake prediction. But we can not exactly determine the rupture time yet, due to the lack of knowledge of the relationships between the strain, the stress, the confining pressure, the temperature and the time. In general, the shear strength of rocks is proportional to the confining pressure and strain rate, and a decreasing function of temperature. It is

88 Highconfiningpressure 1

Fig.

14. Schematic

confining

pressure

representation

of the relationship

between

deviator

stress

6, -0s.

strain

rate (.

and temperature.

obvious that development of information such as that shown in Fig. 14 for various types of rocks in the laboratory is necessary in order to make numerical simulations of earthquake prediction. With this background, the physical and mathematical simulation can be mutually verified and together may be useful in forecasting the specific time, place and intensity of a large earthquake. For instance, we made a three dimensional finite element simulation in connection with the uniaxial test described above. Considering only the different rigidity of the material filling the fault and neglecting the mechanical properties of its contact relation with adjacent rocks, the numerical simulation showed the same tension zone location at the fault tip as that in the test, but the time at which the tension stress develops is still dependent

on the level of deviator

model,

adding

rigidity

along

mathematical addition

“joint the

element” fault

plane

models basically

and

there

must

in the

we must modify

tension

zone

until

adjust

the physical

state. To solve creep problems,

of the appropriate

be a method

the numerical

face and must repeatedly

reach an identical

to a clear understanding

relationships,

stress. Therefore,

on the contact

the and in

stress-strain-time-temperature

to represent

the

internal

and

external

boundary conditions and a micromechanistic approach to provide valuable insight into the bounding mechanism related to the shear resistance and creep movements of a rock mass. REFERENCES

Bernaix,

J., 1974. Properties

Condie,

K.C.,

1976. Plate Tectonics

and Crustal

Jacoby,

W.R.,

1978. Role of gravity

in plate tectonics.

Johnson,

of rock and rock masses.

B. et al., 1978. Thermal

19th Rock Mechanics

cracking

Symp., p. 25.

In: Advances

Evolution.

in Rock Mechanics.

Pergamon.

New York, N.Y., 28X pp.

In: Rock Slides and Avalanches.

of rock, subjected

to slow, uniform

temperature

pp. 707-727. changes.

U.S.

89

Kato,

S., 1978. VP measurements

of serpentine

rocks at 5 kb and 600°C.

Lecture

preprints,

Jpn. Seismol.

Sot., No. 2 (in Japanese). Kern,

H., 1978. The effect

velocities

of high temperature

in quartz-bearing

and high confining

and quartz-free

igneous

and

pressure

metamorphic

on compressional

rocks.

wave

Tectonophysics,

44:

185-204. Loo Huanyen

et al., 1978. Numerical

and seismicities Collected viewpoint

1979. Discussion

on driving

of earth’s crust dynamics.

K., 1978. Rock mechanics

Geosciences,

Vol. 8, Iwanami

between

Bay area.

current

In: North

tectonic

China

stress field

Block Tectonics

and earthquakes.

S., 1971. The New Views of the Earth. T.F. and Brace, W.F.,

Tectonophysics,

activation.

and mechanisms

of earthquakes

in China

from

the

1: I - 10 (in Chinese).

In: H. Kanamori

(Editor),

Physics

of Earthquakes.

1979. Thermal

(in Japanese). expansion

of rocks:

Some measurements

at high pressure.

57: 95-I 17.

1979. Strike-slip

Quinwen,

forces

Shoten, Tokyo (in Japanese).

Wong.

Xu Jiawei,

China

Seismol. Geol.,

Uyeda.

Zhang

of the relationship

the North

Works (in Chinese).

Loo Huanyen, Mogi,

simulation

in and surrounding

of Tan Lu fault and its relevance

1979. On the tectonic

(in Chinese,

unpubl.).

’

history

of northeastern

to geology

and mines. (in Chinese,

China geosynclinal

unpubl.).

area from viewpoint

of