The Fenwei rift and its recent periodic activity

Tectonophysics, Elsevier

133 (1987) 257-275

Science Publishers

257

B.V.. Amsterdam

- Printed

in The Netherlands

The Fenwei rift and its recent periodic activity WANG JING-MING Xi’an Geological Institute, (Received

December

Xi’an (People’s Republic of China) 20,1985;

accepted

July 23,1986)

Abstract Wang J.M., 1987. The Fenwei rift and its recent periodic Internal

Processes

and Continental

The Fenwei rift on the southern 600 km in-length bounded

and 30-90

on both

characterized en-echelon typically

by

northern

controlling

southern

rift system of China is marked

structure.

on the neotectonic

mountain

five intervening

downthrows

graben

and Tan Tjong Rie (Editors),

up to 10 km and filled with about

on both sides by abrupt

faults having

an asymmetric

influences

and

In: C. Froidevaux

ranges.

swells

topographic

ranging

by a crescent-shaped

The geometry

extending

kinematics

of the rift and its recent

periodic

is

in a dextral

the underlying

and evolution

deposits,

of the rift valley

east-northeastward

slopes reflecting

valley

7000 m of Cenozoic

from 800 m to 10 km and dipping

The geometry,

movement

Deep

133: 257-275.

sides by majestic

depressions

and bounded

the valley forming

sector of the Jin-Shaan

and

a system of growth

activity.

Tectonophysics,

km in width depressed

six branch

pattern

Rifting.

faults. These are

toward

the centre of

of these faults have had

activity

as the present

overall

form of the rift valley. Estimates

of the amount

calculations response

of extension

across

made on the fault separation to tensile

estimated

stresses,

acting

in a direction

to be 9065 m, since the beginning

Recent

is 4.5 mm/yr

during

various

drainage

and in modem

times is 6 mm/yr. Details

activity

seismic events, ground

fissuring,

occurred

period.

in the historic

of the earthquakes

movement,

consisting

The 22 events of ground-fissuring

recorded

ground

fissures resulted

periodic

and coincident

such as the appearance

is approximately

of streams

during

in the historic

annals

are recognized

of tectonic

and disappearance

activity

of periodic

of lakes, the shifting

of streams

activity.

changes

from available

and lasting

are also concentrated

are the occurrences

on the

such as

which have

historic

records

to begin with a

approximately

200 yrs,

which only minor seismic events occur.

of active seismicity,

from the same cause, both being indications

the rift

phenomena

the last 4000 yrs. Each cycle appears

periods

across

movement

neotectonic

rift-related

or lakes and climatic

of about 600 yrs during

in the

7170 m and the offset rate in modem

in terms of the various

cycles of seismicity

with seven corresponding

with periods

displacement

effects of strike-slip

of a series of violent shocks chiefly of M6-8

period of quiescence

which coincide

the rate of extension

of left-lateral

from the observed

shifting

in the rift region

and then passes on to a prolonged intense activity

The amount

whereas

that the Fenwei rift has been a place of intense

Seven highly periodic

that have occurred

period of intense seismicity

mm/yr.

in the Eocene

The total amount

from is

of the rift were investigated epeirogenic

were obtained

in

of the rift formation

horizons.

periods

from 25 ONW on the west to 70 o NW on the northeast

was also calculated

suggest

geological

of extension

offset since the mid-Pleistocene

These estimates

of more recent

recent

stratigraphic

varying

times it is 8-24

stages of its development

system. The left-lateral

the rift for various

of corresponding

in several distinctive suggesting

tectonic

of the natural and changes

activity

periods

of

that seismic events and of the rift. Moreover,

phenomena

related

of river water

to rifting

from clear to

turbid.

Qinling Mountains in central China. It is the largest and deepest of a series of en-echelon rifts within the Jin-Shaan rift system. Topographically, it is expressed by a crescent-shaped fault valley curving towards the south (Fig. l), known as

Introduction The Fenwei rift is situated on the southern sector of the Jin-Shaan rift system along the front ranges of the Shanxi uplift region and the eastern 0040-1951/87/$03.50

Q 1987 Elsevier

Science Publishers

B.V.

Fig. 1. Schematic

diagram

depression,

.1- Lingbao

8 - Mount

Zongtiao

showing depression.

uplift,

I3 = lower Palaeaoic;

the regional

9 = Emei

14 = upper

geology

4 = Yuncheng platform.

Palaeozoic:

and geomorphology

depression, 10 = Mount

I5 = Mesozoic.

.( = Houma Taer-Mount If, = Triassic.

of the Fennel depression. Jinyan

rift:

I = Xi’an depresslon.

(I = Linden deprrsk~n. uplift.

I - = Quatcrnarv.

7 = Mount

,’ = Ciushi Li uplift.

I I 7 Archaeozoic. I_? = Proterozlc. IX r intruive body.

Fenwei valley, which extends from Baoji, Shaanxi province on the west to Huoxian county, Shanxi

region are both slightly tilted away from the rift valley. The Weihe and Fenhe rivers run from the

province on the east for 600 km in length and 90 km in width, covering an area of more than 40,000 km2.

southwestern and northeastern ends respectively along the axis of the rift towards the centre of the

feature of the Fenwei

from the banks

towards

the side of the plain occur successively the alluvial plain, progressively rising stream terraces, loess

Geologirrrlaadtopocppphicfeatures The most striking

valley (Fig. 4). Outward

rift is a

large-scale lowland valley bounded on both sides by precipitous cliffs and bluffs and high, rugged undulating and sinuous mountain ranges (Wang, 1983). The mean elevation of the Qinling Mountains and Mount Huo on the southeastern border is 2000 m, whereas the Beishan mountainous region on the northwestern border and the Luliang Mountains on the east range from 900 to 1300 m above sea level. In contrast, the central low, flat Fenwei plain is at a mean elevation of 400 m above sea level, being a depression filled with considerable thick loose (Cenozoic deposits (Fig. 2a). The Qinling Mountains and the Beishan uplift

mesas and alluvial terraces; reaching the sinuous ranges of the Qinling and the Beishan mountains on either side of the valley. The transverse is a dustpanshaped

profile of the Fenwei rift valley form tilted towards the south

(Wang, 1984), the southern side being steeper than the northern side. The filling materials consist of river or lake deposits of Eocene to Holocene age, which are as thick as 7000 m near Huxian county. Another characteristic feature of the Fenwei rift is that it consists of six ENE-trending branch depressions with several lineaments forming an undulating pattern (Fig. 1). These six depressions are, from north to south, the Linfen, the Houma, the Yuncheng, the Linbao, the Gushi and the

259

-N Qinlano

mountain Li

m (Legend

is

aLatitudinal

Fig.

2

Internal

margin

Huo,

Loess tableland, fault,

as

Fig

o--the

12-Kouzhen-Guanshan Z7-Shichuan

Huoxiat-Lingbao

River fault, fault.

rift basin.

northern IO-fault

22-

the

u. Cross

I---fault

boundary

rift

b. Plane. margin

fault of the rift, 7--Fenhe margin

I3-Hejin-Quwo

IR-Luo

River

Baishui-Dizhai

Ar + r-Archaeozoic

section

on the southern

on the southern fault,

fault,

mountain

3)

River fault,

fault,

Norh

river

Major fault of

3-Weihe

9-Tie1uzhi

2I-~Yaoxian-Mazhao 25-

same

Wei

of the Fenwei

fault,

of the Emei platform,

the Mount River

the

laultlp,].

structure

2-Sanqiao-Weinan

mountain

fault,

and

granite,

14-Shillin

23-Yongfeng-Huaxian

R--fault

Li, II-fault fault,

19---Hejin-Yuncheng Q-Quatemay,

southern

River fault,

of the Mount

fault,

fault,

I -the

boundary

of the Emei platform,

fault

5--fault

of the rift.

on the northern

on the western

margin

on

on the front edge of Mengyuan

ZS-Qishar-Mazhao fault, fault,

P, -Palaeozoic,

20.--Shitou 24-

Yellow

E-Eogene.

fault,

I6-Jing

River

fault,

River

fault,

N,--Miocene,

N2 - Pliocene.

Xi’an respectively, which occur in an en-echelon pattern and often contain a lake marsh or lowland in the centre. As shown in Table 1, five swells alternate with the six depressions in the rift valley. These are represented by, from east to west, Linshi, Mount Taer, the Emei platform, the Zhongtiao Mountains and Mount Li respectively. Like the Baikal rift system at the same longitude, the six en-echelon branch depressions and the five intervening swells of the rift are related to strike-slip movements, the sense of displacement being left lateral. The five swells are formed chiefly by the ancient Archaean and Proterozoic gneisses and Mesozoic granitic rocks, as are the Qinling and Huoshan mountains on te southern border (Figs. 1

and 2). Geophysical survey data such as magnetic anomalies (Sun, 1983) demonstrate that the southeastern part of the rift valley is separated from the northwestern part by the Weihe River and that the Fenhe River fault is underlain by the same rocks, whereas the basement rocks of the northwestern part of the valley appear to be the same as the Cambrian-Ordovician limestones of the Beishan and Luliang mountains on the northern border of the rift valley, as indicated by seismic wave velocities of 5-6 km/set determined from refraction travel-time curves. The topography of the Fenwei valley is the expression of its block-faulted structure. Numerous faults underlie the floor of the valley, which are almost all of the normal type, having high dips

slcpe fault

Soutbem

9

fault

Baoji-the slope

Y icheng

Wenxi

Huoxian

Hejir-Quwo

Kouzhen-Guangshan

Lingbao

Jingyang-East

Xi’ar-Yangguo

Muxiar-Luonan

Houma-Huoxian

Qiangyang-Huoxian

Fufeng-

Guanshan-

mountains-Houma

of zhongtkio

northern

30

110

70

230

60

60

620 200

390

200

520

b P-C- --Pre-Ccttozoic,

E-Eogene,

N-Neogene,

Q-Quatemary.

from Wang (1985): 7 from Lu (1984); 8 from Yang (1983):

Sitihtt fault

13

fatth

Kouzhen-Guangshan

Hejin-Quwo

’ l-6.

Li

fault

Loess mesa

froftt fault

W&tan

fat& of Mount

12

11

10

Tieltui fault

8

boundary

Fenhe River fault

border

Northern

of the Em& platform

Northern

7

9-12,

sktpe fault

of the Etnei platform

Southern

6

5

4

Weihe River fault

Weinan 100

fault

Sanqiao-

Sanqiao-Weinart

420

fault

(km)

Length

rift

Baoji-Sanmenxia

border

faults of the Fenwei

Southern

a

Number

faults and the east-west Location

of the master

Features

Name

1

TABLE

very high

?

60

68

60

60-80

7500 3

1000

1500

‘?

1999 900-300

60-80 angles

700

1000

2400

?

YX15

PC h

Maximum

53-80

70

60-70 ---13 from Yang (1979). S

N

S

N

s

N

N. E or W

S or N

S

60-80

60-RO

N

N or S

60-80

N

(“)

Dip angle

Dip dir.

Attitude

850

800

‘)

1000

1500

7

?

700 ‘)

IOOt-800

1800

‘)

Eh

250

‘, 70

800

1000

‘1

?

? 7

300

800

1000

9

Nh

throw for a given horizon

2000 ‘, ‘,

550

?

98

‘,

100 1

200

7

558

5x0

446

‘,

21x

72X

420

364

x74

291

‘7

600

3575

E

Amount

9

Qh

(m)

91

3.1

250

446

390

?

55

11’ ‘,

21s

‘7

‘,

?

4.:

73

?Ih

.,

‘,

‘,

‘,

(m)

-. __ ..-..

Q

140 ‘>

291

364

‘,

N

of extension

261

TABLE

2

Comparison the Fenwei Name of depression

of subsidence

in the various

branch

depressions

of

rift (from Deng, 1973) Quatemary

Cenozoic

amount

rate of

amount

rate of

(m)

subsidence

(m)

subsidence

(mm&)

(mm/yr)

Xi’all

940

0.47

8300

0.17

Gushi

1295

0.65

7500

0.15

> 0.15

5000

0.10 0.12

Lingbao

>300

Yungcheng

741

0.37

6000

Houma

376

0.19

1800

0.04

Linfen

465

0.23

1570

0.03

of 53-85”. In particular, the floor of the valley is intersected by seven approximately parallel enechelon curved master faults (Table l), giving rise to a series of fault blocks. The downfaulted blocks form graben or fault troughs reflected by the topographic depressions, whereas the upfaulted blocks form horsts expressed by the swells or high ridges such as Mount Li, the Zhongtiao Mountains, Mount Gushan, Mount Jiwang and Mount Taer. In addition, there are other steeply-dipping normal faults, which developed in response to movements on the underlying faults combined with the downward dip-slip displacement on the master faults from the east-west and WNW- and NNE-trending faults formed prior to the rift formation. These are the associated faults of the rift, which result in many minor horsts contributing to an even more intricate rift structure. Among these faults are the six east-west faults (Table 2), which occur in parallel lines, having downthrows ranging from 100-1500 m. Development

of the Fenwei rift

During the Yanshan orogeny in the late Jurassic, arching and subsequent extensional thinning of the crust due to the upwelling of upper-mantle materials occurred in the rift region (Chen, 1981); this produced the original Fenwei arch and the associated fracture zones. The extensional fracturing was most intense along the crest of the arch,

giving rise to the fracture zones 1, 4, 5, and 6 and the Fenhe fault which extended over a large area and dipped toward each other. As the arching continued, the Fenwei uplift was progressively growing and faulting became increasingly active. By the Eocene the original upwarp in the F’reCambrian basement had become a pronounced broad arch and some of the earlier fracture zones such as 1 and the western segment of 3 and 10 and 11 along the crest of the arch had grown into normal faults with considerable downthrows on either side of a central downthrown block, thus having laid the foundations of the structure of the Fenwei rift (Fig. 3, E). Associated with the movement on the master faults was the renewed extensional faulting on the older faults due to the stretching of the crust. The hanging fault blocks dropped down so that these became a series of high-angle normal faults thus contributing to a more complex structure of the rift. Because of the east-west faulting, the sector of the rift from

J

Nl

I

N2

Oinlino

North

mountain

mountain

km

Fig. 3. Schematic

cross-section

ment of the Wei River graben: zones

of the rift

J-Jurassic,

is (the

Nt -Miocene,

of the generation I-6

same

Numbering as in Fig.

N2 -Pliocene,

and developof major fault

2). T-Triassic,

Q -Quatemary.

Xt’an to Sanmenxia in the early Palaeogene tended roughly in an east west direction. pre-Eocene

fracturing

appears

exThe

to have affected

much larger area than the form of the Fenwei at this time. During along

the

movement

faults

the mid-Miocene intensified

propagating

northward

to faults 4 and 5, resulting Y uncheng

Qishan-Mazhao boundary volcanic the

then

with deposition. synsedimentary

and

At the beginning

continuously

giving

birth

the

westward of the

WN W-trending

became

pronounced along the

of the above

towards

trough,

fault troughs

was the

prominent abruptly from the bordering plain. These constitute today the high-standing isolated hills in the rift valley such as Mount Li (Fig. 2a). Mount

northern

and southern

and Mount Taer. which old. Towards the end of downward displacement 6 so that they became the

borders

Since the faults faults,

are a system

of

it is clear that the Fenwet

the youngest

which

is still growing

ments

have never ceased.

structure

and in which

in the crust fault

movr-

of the present

Manifestations

01 Recent tectonic activil)

the

and to-

elevation of the upthrown blocks master faults, giving rise to several some of which even rose uplifts,

Gufeng, Mount Jiwang are now about 5-8 m.y. the Pliocene. large-scale occurred on faults 1 and

rift represents

and continuously

by

wards the west, affecting the present western sector of the rift at Baoji. Associated with the successive formation

1, the major rift faults must

the western

of the Pliocene

progressed

to the Linfen

As shown in Table

have formed contemporaneously

1981).

of the rift (Fig. 3. N,), accompanied activity.

rifting

north,

fault

the

rift

tectonic activity

subsidence

in the generation

while

trough,

(Chen.

a

Recent

rift

valley respectively (Fig. 3. Nz). At the same time, the separate Weihe. Yuncheng, Linbao. Houma and Linfen fault troughs were also integrated into a whole under the unified control of the structure of the rift. During the Quaternary, tectonic activity remained active in the rift but had inherited characteristics. The downthrowing of the faults, together with the subsidence in the depressions and the rising of the swells was still in progress

Fault scarps and triangular traced

for hundreds

facets which can be

of kilometres

are found

on

both valley walls. The fault planes commonly occur in the form of slickensided surfaces which clearly indicate sinistral horizontal movement and downward vertical movement in the rift. Other features

such as hanging

stream

valleys,

hanging

gravel layers at the mouth of streams and vaileyin-valley forms are also commonly found. Most streams

flowing

across

the southern

border

fault

northward into the valley and exhibit sinistral twisting (Fig. 4). Excavations in Weinan and Huaxian on the Weihe River fault have revealed a sandfilled fissure zone produced during the 1556 Huaxian earthquake, which extends for about a hundred kilometres (Hu. 1984). The other associated faults have also had effects on the drainage pattern. The east-west Kouzhen-Guanshan fault is marked by a scar-p between 8-150 m in height in the mid-upper Pleistocene loess and exhibits a dip-slip displacement of 3 m and a sinistral strike-slip component of about 4 m in the recent loess at the Old

and has never ceased since (Fig. 3, Q).

Tributary, Shichuan

Jinyang, the Yeyu. Zhuoyu. and Wenquan streams crossed

Qinyu, by this

In summary, the master faults of the rift appear to have formed successively from south to north. Faults 1 and 2 came into existence in the Eocene, whereas 4 formed in the Miocene and 5 in the Miocene to the early Pliocene. Fault 6 originated chiefly in the late Pliocene-early Pleistocene but grew later into a major normal fault. The average rate of extension across the rift in the period of the Eocene to Present is 0.6 mm/yr at Weinan. while at Yuncheng it is 1.6 mm/yr.

fault have had their downstream portions to the south of the fault congruently shifted to the east, forming an S-shaped meandering pattern (Wang, 1983) (Fig. 4). The Tieluzi fault has also resulted in concordant left-lateral shifting along more than ten streams running across it. In addition, patterns of the master streams in the rift valley such as the Jinhe, the Bahe, the Shichuan and the Luohe indicate control of the WNW-trending faults over their directions, whereas the NNE-trending faults

263

Fig. 4. Left-lateral

horizontal

3-Jing

River,

IO-Yellow

River;

II -Fen

River,

I7-Fenyu

River,

18-Shitou

boundary 14-Shilin

River,

displacement

River,

I-Yeyu

fault of the rift,

River,

of the Fenwei S-Zhuyu

River,

River, 12-Shushui

9-Tieluzhi

River.

Numbers

fault,

rift shown

River,

Z3-Xianyu

in circles:

IO-fault

from the twisted

6-Qingyu-River, I-The

on the souther

drainage

7-Shichuan River,

14-Zhuyu

southern margin

system:

River,

boundary

of the Mount

I-Wei

8-Wenchuian

River,

15-Xiaofuyu

fault

of the rift,

River,

2-Qishui

River, River. 2-the

Li, 12-Kouzhen-Guanshan

9-Luo lo-Ba Northern fault,

fault.

have resulted

in various

the Shichuan, being diverted

the Luohe and the Yellow river into the NNE-trending channels.

A number

of ground

segments

fissures

of the Weihe,

have occurred

in

historical times at various localities along the master faults and the associated faults in the rift, which show the same sense of displacement as on the faults (Wang, 1985b). Geodetic measurements (Hu, 1984) suggest that the bordering mountainous regions on either side of the valley have been rising on a large scale, the change in elevation being marked over extensive areas and characterized by widely spaced contours, whereas the valley has been increasingly subsiding, the ground distortion being characterized by great gradients along the margins of the valley and closed and closely-spaced contours in the interior of the valley. Data from repeated levelling across fault zones in Linfen and Xiaxian suggest that the valley is still stretching, exhibiting pronounced extensional effects (Hu, 1984).

Displacements

across the Fenwei rift

Amount of extension Individual separations across each fault as measured in a vertical cross-section drawn at right angles to the axis of the rift have been added up. The results (Table 1) show that extension across the master faults near Xi’an has amounted to 6432 m since the Eocene, involving 1032 m in the Neogene and 401 m in the Quaternary deposits. The total amount of extension across the rift, however, is largely due to the contributions of the several associated fault sets. The amount of extension across the WNW-trending Jinhe fault is 98 m and across the NNE-trending Baishui-Dizhai fault it is 371 m. Thus the total amount of extension across all the master and associated faults of the rift since the Palaeogene as measured in a crosssection through Xi’an city amounts to 9065 m, involving an amount of 2510 m since the Neogene and 833 m since the Quaternary.

The same method sure

the

total

southwestern

portion

tion at Yunchen Huoxian,

has also been used to mea-

amount

of extension at Qishan,

across

the middle

and the northeastern

the por-

portion

at

which were 2240 m, 8090 m and 1773 m

respectively.

Guanshan

fault

placement

being in the range of 0.3--b mm/y.

the northern Huoxian

The various

sinistral

displacement

due to local stresses

of the underlying

strike-slip

transcurrent

The pronounced

in the Fenwei

components

and the effects

faults

horizontal

rift are amply

has become a beheaded movements

system. Qinling

the west since the middle Pleistocene (Hou, 1985) (Fig. 4), whereas the shifting of the Fengyu stream in Xi’an is 2 km. Since the late Pleistocene the shiftings

of the Yuanmen

the Zhuyu

tributary

and the Xianyu

in

stream

in Huaxian county have amounted to 0.5,0.25 and 0.6 km respectively. Since the Recent, all the old and new ravines and streams across the segment of the southern border fault between Xianyu and Xiaofuyu river (Fig. 4) have been constantly shifting. By comparing the observed shiftings of the indivudual streams we can obtain a mean left-lateral shifting rate of 17 mm/yr for this section of the fault. Across the southern slope fault of the Emei platform the shifting is 3 km. Left-lateral

displacements

on

the

associated

faults are also reflected by the displaced drainage system. On the Tieluzi fault the left-lateral displacement amounts to 3200 m (Yang, 1983), as is clearly shown by the offset of more than ten streams across this fault (Fig. 4). The total displacement consists of 2600 m which occurred from the Tertiary to the late Pleistocene plus an additional 600 m which has taken place since the late Pleistocene; the rate of displacement as calculated from the latter is 3 mm/yr. In addition, the Yeyu and other streams have been displaced by 2-3 km since the middle Pleistocene across the Kouzhen-

amounts

of sinistral

horizontal

which has occurred

Pleistocene

The

since the middle

is 5.5 km across the western portion

the

Most of the Mountains

bemg 600 m

displacement

Xi’an-Huayin

northward into the valley bend abruptly to the west across the southern border fault. The Shitou stream in Meixian county has shifted 5.5 km to

county,

various

movements by the

stream ax

on the Shilin

across the rift can be calculated.

rift

In

of the rift, the Fenhe River tn

displacement

in the pre-

demonstrated

displacement of the drainage streams flowing from the

of

of JIS-

1979) (Fig. 4).

Thus

fault systems in the rift also exhibit

considerable

Meixian

county

1983). the ratss

fault that crosses it, the displacement

Amount of left-lateral displucement

westward

portion

a result of the left-lateral (Yang,

Cenozoic.

(Wang,

at

northeastern

Baoji,

7.17

portion portion

km and

across 0.6

at Huoxian.

the

km

middle

across

Since

of the

the late

Pleistocene the displacement is 0.7 km near Baoji while in Huaxian it is 1.3 km. The rate of leftlateral mm/yr

displacement Xi’an has been since the middle Pleistocene,

about 7.2 which is

nearly equal to the average rate of 6 mm/yr determined from the modem ground fissures Xi’an.

as in

Amount of vertical displacement The Fenwei rift has been constantly subsiding while the bordering mountains on either side have been continuously rising. The branch depression of Xi’an has subsided 8300 m since the Tertiary and 940 m since the Quatemary, the rate of subsidence being 0.17 and 0.14 mm/yr respectively. Towards the east of the depressions the subsidence becomes less marked (Table 2). In the Linfen depression to the northeast the subsidence has aggregated 1570 m from the Tertiary and 465 m from the Quatemary, 0.03 and 0.23 mm/yr

the rate of subsidence respectively.

The rate of subsidence in historical

being

at some places in the rift

time can be determined

from availa-

ble archaeological data. The remains of the ancient Banpo tribal village in the Xi’an depression which date back to 6000 yrs were buried at depths of l-2 m, indicating a deposit rate of 0.25 mm/yr at this locality, whereas in Lintong, the museum of the First Emperor of the Qing dynasty with its world famous terracotta warriors and horses (2195 yrs old) was exhumed from a depth of 6 m, giving a deposit rate of 2.7 mm/yr. The two figures average 1.5 mm/yr, which is about three times as great as the subsidence rate of 0.47 mm/yr for the

265

whole period buried

of the Quaternary.

Again,

from the

depth (2.7 m) of the Great Wall of the Wei

dynasty,

excavated

in a marginal

part of the Gushi

mm/yr,

the average

progressively

rate

higher

1983). Between

of displacement

from south

tian and the Huayin depression rates of 1, 1.5 and 2.5 mm/yr

for the whole Quatemary

Linfen

Linfen

depression

the rate of deposition

past 1650 yrs as determined (1.6

m) of the

mm/yr,

ancient

Linfen period.

the ratio of the deposit

town

depth

wall

is 1

period

to the subsidence for one locality that of

for the

It can be seen that

rate for a certain

period with

for the

from the buried

about 4.4 times as high as the rate of 0.23

mm for the Quatemary

effects

In the

another

rate for the Quaternary is roughly locality

vertical

historic

in agreement

in the

movement

on

rift.

The

the

old

depression

were subsiding at respectively. The

is one of the greatest

the Shanxi uplifted

1953-1981,

of subsidence

for the years 1964-1976

1.5-2.5

subsided mm/yr

more

the amount

mm/yr. rapidly,

In recent at a rate

86 mm

and the rate being 20-30 years

it has

of about

3.3

the rift are characterized

by

(Hu, 1984).

The faults differential tion

hollows in

region, which subsided

in the period mm and

being (Wang,

1954 and 1978 the Xi’an, the Lan-

depression, the rate of deposit is estimated at 1.7 mm/yr, which is 2.3 times the rate of 0.65 mm/yr at this locality.

to north

for

Zhongtiao

wnnm

vertical Mount

movements.

The rate of eleva-

Li is 3 mm/yr,

Mountains

have

risen

whereas

the

37 mm in the

peneplane indicate that the Qinling Mountains on the southern border of the rift have risen 2500 m

past 17 yrs (Hu, 1984)

since

coincides with the rate of displacement on the eastern segment of the Weihe River fault along the northern slope of the Zhongtiao Mountains. Again

Mount mm/yr

the Palaeogene, Taibai, whereas

the rate of movement

the highest the amount

for

peak, being 0.062 of elevation for the

elevation

being

the corresponding

2.2 mm/yr

which

rate of

approximately

period of the Neogene to the Present ranges from 1400-1900 m, the corresponding elevation rate being 0.7-l mm/yr. To the north, the Beishan Mountains on the northern border have risen 1200 m from the Neogene to Present, the elevation rate being 0.6 mm/yr. As for the branch uplifts within the rift valley, the fault block of Mount Li has risen 900 m during the Quatemary, the rate of

the Mount Taer-Mount Jiuyun uplift rose 30 mm in the years 195551971, the rate of elevation being 1.9 mm/yr, while the average rate of rise of the Linshi uplift to the north for the past 17 yrs is 1 mm/yr.

movement being 0.9 mm/yr, whereas for the Linshi uplift in the northern part of the valley the rate is 0.17 mm/yr.

Eversince the early Eocene the Fenwei rift has been subjected to tensile stresses, which have been particularly pronounced since the Quaternary. The author has studied extensively thousands of tectonic joints and 2275 corresponding loess gullies in the loess-covered parts of the rift valley.

Pattern 1983)

of modern

crustal displacement

(Wang

The modem diastrophic movement is marked by inherited characteristics, the direction of movement being closely related to the morphology of the youngest structure in the crust. The subsidence in the grabens is far greater than that of the shoulders and the inner swells of the rift and the greatest depressions occur along the master faults. The pattern of the modem crustal displacement thus coincides with the distribution of the uplifts and depressions in the rift. Geodetic measurements obtained before the year 1972 suggest that the rift valley is descending relative to the bordering mountains at a rate of 3

Fractures and tensile stresses in the rift

Two conjugate joint sets are recognized (Wang, 1985a), based on these studies, which exhibit regular synchronous deviations in directions along the axis of the rift (Table 3). While in general the strikes of the two joint sets deviate anticlockwise from east to west, they nevertheless turn clockwise from Xi’an to Qishan, being in accordance with the change in the trend of the axis of the rift along this segment. The orientation of the tensile stress axis, as determined according to the conjugate joint sets (Fig. 5), is approximately perpendicular to the axis of the rift and also deviates as the trend of the rift changes, being consistent with tensile

26h

‘I ABLE 3 Regular

deviations

of the strike of loess tectonic Joints and the tcnsilc principal

Locality

N-S ShearJoints strike

stress T-axis different

localities

E-W shear Joints sense of

strike

Direction scnsc of

displacement

displacement

Feng-Xiang

N5”E

N7S” W

Qishan

N20”E

N65” W

Qianxian

N15”W

Xi’an

N15” W

right-lateral

of the 7‘.ax15 N3S”W bi22” w

N85”E

left-lateral

N52” W

NXO”E

left-lateral

N7.5 I’W

Jingyang tight-lateral

Kouzhen

N5”W

N80”E

left-lateral

N52”W

Laogou

N5”W

N85”E

left-lateral

NSO” w

N1O”E

N9O”W

Kangqiao

N4O”W

Heyang Xishangzhang

N5”E

right-lateral

NX5”W

Hanzhuang

N20”E

righl-lateral

N7O”W

N2S”W

N55” W

NIS” W

Wanrong

N2S”E

stress orientations of N40” W, N45OW and N19“W for Heyang (Chen, 1981) the Northern and the Qinling mountains @hang et al., 1983) respectively, as derived from the pattern and preferred orientations of the streams of the Weihe drainage system.

Fig. 5. a. Distribution of earthquakes

and ground

Seismic stress field (mapped

to the date of Tables 4 and 5).

according

fissure

left-lateral

N4O”W

According to the stress interpretations of the Xi’an ground fissures of the Tang dynasty (before 800 A.D.) (Wang, 1985b). the Weinan fissures of 1556 Huaxian earthquake and the recent fissures in Xi’an the tensile stress orientation for these

in the Fenwei

River (Wang.

1983; Seismo-Geological

Brigade,

1979). h.

TABLE

4

Focal mechanism

solutions

for some earthquakes

Time of

Epicentre

earthquake

in the Fenwei

rift a

Focal-

Magnitude

AxisP

depth

(MS)

strike

(km) Jan. 13,1965

Yuangu

Dec. 18,1967

Puxian

AxisT dip

AxisN

strike

strike

dip

angle

dip

angle

angle

10

5.5

78

28

321

6

59

57

Shanxi

30

5.4

40

21

308

7

201

68

Shanxi

Sep. 30,197l

Hejing Shanxi

30

3.3

246

46

147

8

50

43

Oct. 12, 1972

Longxian

20

2.2

267

17

155

50

9

34

Dec. 2,1973

Heyang

20

2.4

288

62

162

17

85

21

May 31,1974

Longxian

32

3.7

94

52

218

25

321

30

Dec. 25,1975

North

29

2.3

220

66

323

6

56

23

Feb. 3,1976

Chunhua

Shaanxi

3.1

262

36

172

0

81

53

Jun. 24,198O

Longxian

Shaanxi

23

4.3

301

15

155

83

33

16

a From

Shaanxi Shaanxi Shaanxi

of Weinan

Hou (1985).

places

is N72O W, NM0 W and

tively. In addition, tained

focal

for earthquakes

Periodicity of modem diastrophic activity

N55” W respec-

mechanism

solutions

that originated

From a careful review of the characteristics of the Fenwei rift and the accompanying phenom-

ob-

in the rift in

recent years (Table 4) combined with comprehensive fault plane solutions for small-magnitude

ena, such epeirogenic

shocks (Table

and climatic changes, which occurred during the last 4000 yrs of recorded history, it can be seen that the modem activity of the Fenwei rift involved prolonged stages of quiescence characterized by slow and gradual movements, as well

5) show that the tensile stress orien-

tations range from N62”W to N18”E. These values are consistent with all the results as determined from loess tectonic joints, the drainage system, ground fissures as well as geodetic measurements. This suggests that the tensile perpendicular to the longitudinal axis of have been the dynamic forces responsible development of the Fenwei rift and tectonic

TABLE

activity

as seismic movement,

events, ground fissuring, shifting of streams or lakes

as short stages of pronounced and abrupt activity. The two periods alternated with each other, thus making up several complete cycles.

stresses the rift for the modem

(Fig. 5b).

5

Comprehensive

fault plane solutions

for small magnitude

earthquakes

recorded

in the middle

and western

portions

of the Fenwei rift

since 1971 Area or location seismograph

of

AxisT

AxisP

Time of records

observatory

strike

dip angle

strike

AxisN dip angle

strike

Linfen Shanxi

1971-1972

a

230

38

328

10

71

Hancheng

1971-1975

b

241

58

359

18

98

1972-1974

b

288

47

22

4

’

Longxian

Shaanxi Shaanxi

Yaoxian

Shaanxi

1975-1976

Zhouzhi

Shaanxi

June 30,1983

’

of Jingyang

Apr. 27-Nov. 1983 ’

30,

North

a Jinyan

(pers. commun.,

1976); b Ding Wenyu

dip angle 50 25

116.5

42.5

270

4

2

33

176

58

49

48

165

21

267

36

81

31

321

40

196

35

(pers. commun.,

1985); ’ Jiang Jialan

(pers. commun.

1983).

26X

Periodic seismic activity in the Fenwer riji Seismicity

the past

been subjected

Earthquake

example,

which caused

lives and is known

in general

During

Huaxian

4000 yrs the rift region

to a total of 49 earthquakes

has

of M 5

destructive history,

of 1556 (Wang,

occurred

the loss of 830,000 human

worldwide

earthquakes

as one of the most

so far recorded

at the very bend

River fault where the fault trace turns

6) (Geophysical

ward from its general

Sciences,

Chinese

Academy

of

1976), five are of M 7 or larger and one

eral, seismicity

east-west

was more

in human

of the Weihe

or larger, of which 15 are of M 6 or larger (Table Institute,

lY80). for

northeast-

direction.

intense

In gen-

in the depres-

is of M 8.0. At least 80% of the shocks of M 7 or

sions than in the uplifts and still more pronounced

larger,

and frequent

or 60% of the shocks

occurred fault

of M 6 or larger,

on the Weihe River and the Fenhe

near

River

the axis of the rift (Fig. 5b), whereas

western larger

near

depressions

faults

of the rift.

The

M 8.0

of the eastern

and

than in those of the middle

por-

tion of the rift. In fact, all the shocks of M 7 or

only 27% of the M 6 or larger shocks originated the border

in the depressions

portions occurred

in the Weihe

in the southwestern

River

and

Linfen

and northeastern

TABLE 6 Cycles of earthquake activity in the Fenwei rift Date of earthquake

Epicentre

Magnitude (M)

Interval of time (in yrs)

Oct. 23. 1815 Sep. 28.1704

Sanmenxia Longxian

6.8 6.0

May 18,169s

Linfen

7.4

Jun. 30.1642

The North of PingJu

6.0

May 15.1568

the Norheast of Xi’an

6.8

Jan. 23. 1556

Huaxian

8.0

Jan. 19.1501

Chaoyi

T

291 t

219

Lintong Aug. 10.1487 ------------------------------------~----Huoxian Sep. 17,1303

(837)

Cycle of seismicity number

stage

7

Period of quiescence

6

Period of active seismicity

l

7.0 6.3 7.8

Aug. 25. 1291

Linfen

6.5

Jan. 24.1209

Fushan

6.5

Feb. 4.867

Xiangfen

5.5

May 27. 793

Weinan

6.0

Dec. 13,600

Xi’an

5.5

Spring, 8 A.D.

Xi’an

5.5

----_-----_----_-________________ I 620

Penod of quiescence

t 267

5 (859) *

Nov. 11.7 B.C.

North of Changan

June, 131 B.C.

Xi’an

* 5.0

Qishan

+ 7.0

780 B.C. 1177 B.C.

Qishan

f 5.0

QiShl

k5.0

1767 B.C.

The boundary of Henan

6.0

2300 B.C.

and Shaanxi province

6.0

* Numbers indicate the length of each cycle of seismicity.

Period of quiescence

592 Period of active seismicity

t 239

6.0

1189 B.C.

Period of active seismicity

4 (888) *

Period of quiescence

649 Period of active seismicity

t &3txl

3 Period of quiescence

V (800)

(800) ’ The second cycle of seismicity ?

The first cycle of seismicity

269

sectors of the rift respectively, whereas seismic activity was much less marked and frequent in the Linbao, Yuncheng and Houma depressions in the middle sector. Slight tremors are frequent in the rift. Since 1957 the Weihe River depression has received a total of 21X shocks smaller than M 5, which mostly occurred in swarms. Round about the time of the Tangshan E~thqu~e of 1976. Chuuhua, Yaoxian and Xinping counties in Shaanxi province each experienced an earthquake swarm, which consisted of 10, 5 and 24 minor shocks respectively, the largest magnitude being smallei than M 3. In addition, Zhouzhi county was affected by a series of tremors of about M 1 in 1982, while in Jinyang two other earthquake swarms were recorded in 1983, consisting of 44 and 22 shocks respectively. The foci of the above five swarms are roughly located on the same line, being perpendicular to the axis of the rift. In 1965, the Houma depression was affected by the M 5.5 and M 5.1 Yuanqu earthquakes that originated outside the rift in the southern and northern border areas respectively, for which the suggested T o~entation is 3210, whereas the T o~~ntation for the &f 5.5 and M 5 Puxian earthquakes is 322”. Again, in 1975 two earthquake swarms occurred in Yicheng and Jiangxian within the rift, the diurnal frequency being 5 and 6 and the largest magnitude being iw 3.2 and 3.1 respectively. The line joining the above four locations and the T orientations suggested for them are also both perpendicular to the axis of the rift (Table 4), suggesting that tensile stresses act at deep levels in the rifti_Focal mechanism studies (Gao, 1979; Xu et al., 1983) also prove that most of the earthquakes in the Fenwei rift originated by mechanisms of normal faulting in response to a horizontal tension, the tensile stress axis being approximately horizontal and at right angles to the rift axis. Most of the earthquakes in the rift originated at depths between 10 and 30 km, while the foci of the deepest and the shallowest shocks are 48 and 5 km below the surface respectively. Thus they are all of the shallow-focus type. Periodic pattern of seismic activity

As shown in Table 6, the earthquakes of M 5 or

larger that have occurred in the Fenwei rift during historical times fall into seven cycles of seismicity, which are markedly cyclic and repeated at relatively short time intervals of between 790 and 860 yrs (Wang, 198513). Historical records for the first three cycles are scarce or lost as they were so long ago, yet the time intervals between them can still be discerned. As for the latter four cycles, there is abundant historical evidence, which shows that stages of intense seismicity take about 140-270 yrs while the intervening periods of quiescence are much longer, lasting about 590-650 yrs. It can also be noted that the stages of quiescence of cycles 3,6 and 7 each involve a short climax stage of moderate seismicity, which lasts 12, 95 and 47 yrs respectively, although the actual duration for the former might be longer than 12 yrs due to lack or loss of historical records of that period. For the latter two climax stages the largest representative seismic events are M 7.8 and M 6.8 respectively. Since these climax periods of seismicity correspond to the seismic events that occurred along the NNE-WNW-trending faults in the eastern part of North China, they must have been due to intense activity on these structures extending into the rift and intersecting the rift faults. The interference of the rift structures appears to have been responsible for their scattered distribution in time and the great variation in earthquake magnitude. Such climax stages have not yet been found in the other four cycles of seismicity. In brief, the Fenwei rift in itself is characterized by cyclic seismic activity, not withstanding some random climax periods of seismicity in response to tectonic activity outside the rift.

In recent years ground fissures have been discovered at Cuifeng in Zhouzhi, Lantian and Zhuyu in Huayin along the southern border fault; at Gaoling, Weinan, Xiezhou (Wang, 1985b), Dongguo, Xiaxian and Jiangxian (Fig. 5a) along the Weihe River fault; at Dali, Yongji, Dongrhang and Taocun in Yuncheng along the fault on the southern slope of the Emei platform at Xuedian in Wanrong and Xinjiang the fault on the northern slope of the Fmei platform, at Hancheng and

Hejin along the northern border fault, at Ruicheng, Pinglu and Yunqu along the fault on the northern slope of the Zhongtiao Mountains, at Xi’an and Beiyao in Weinan along the east-west fault on the southern slope of Mount Li. at Longquan in Jinyang on the east-west Kouzhen-Guanshan fault, and at Chang’an and Lintong along the NNE-trending B~shui-Dial fault. Most ground fissures coincide with the underlying pre-Cenozic faults in orientation and direction and sense of displacement, suggesting that they have developed in response to modern movements on these pre-existing structures (Wang, 1985b). The Taocun and Yuncheng ground fissure, for instance is in line with the fault on the southern slope of the Emei platform. This fissure, with a general strike of N55”E. has its southern wall lowered with reference to the northern side and has a length of 10 km and width ranging from 1 to 15 cm. It first appeared in 1975 at the Panpo village in the late Pleistocene loess deposits and reappeared in 1976 and 1980. Another example is the Liw~g-Caicun fissure zone in Wanrong county extending from Liwang to Caicun for a distance of 20 km with some interruptions, one of which is the Xuedian fissure 1.5 km long and 0.4-2 m wide which appeared after a storm in 1983. It appears to be the reappearance of the ground fissure of 1959. Still another is the Chang’an ground fissure which has typically a strike of N30-60°E and a length of 40 km. Especially noteworthy are the seven Xi’an ground fissures following the trend of the east-west fault on the southern slope of Mount Li which have a total length of 26.3 km, the longest individual fissure being as long as 7 km, and a strike N80”E with dip angles lower than 70-80” in a southerly direction. They occur as evenly spaced parallel zones and are characterized by feather joints of various orders and conformable left lateral displacements combined with relative downward movements on the southern side. The individual fissures often extend in a zig-zag pattern. Since 1959 these fissures have reappeared many times as in 1964, X976 and 1984. For these past 26 yrs, the total amount of downthrow on the southern side is 38 cm, giving an average rate of 15 mm/yr, while the amounts of left-lateral displacement and extension

across the fissures add up to 11.5 cm and 45.5 cm, the co~~pond~g average rates being 4.5 mm/yr and 18 mm/yr, respectively. Their characteristics, amounts and rates of displacement and distribution patterns all suggest coincidence with the regional east-west faults (Table 3). The features of the above-mentions ground fissures demonstrate that the present day activity of the Fenwei rift is guided chiefly by the pre-existing shearextension faults (Gao, 1979: Zhang et al., 1981). Fissuring has caused great destruction in Xi’an and proved to be a veritable geologica hazard. The loss of money solely from the southern suburb fissure through its damage to building amounts to 720 thousand yuan/yr. Many ground fissures have occurred in Xi’an during the historical period. As recorded in the annals of history, in 1486 in Xi’an “Countless dwelling houses collapsed and sank into the deep chasms appeared in the earth”. In 788 A,D. in the Tang dynasty, “the Temple of Hanyuan with its stairs and balustrades and more than 30 rooms collapsed of itself with no cause at all”. A fissure of the Tang dynasty has now been found near the ruins of this temple (Fig. 6; it is buried at a depth of 1.9 m and has a strike of N63”E and a width of 0.1-13 cm, the southern side being relatively depressed. It is filled with dark brown subclay. The whole fissure zone consists of 3-6 minor fissures having a total width of I2 m and is 220 m from a present-day ground fissure. In Xianyang, which is situated along the very Weihe River fault, we discovered, another ground fissure at a depth of 2 m in the displaced ruins of the Imperial Palace of the Qin dynasty, which apeared in the Han dynasty of at least 1800 yrs ago. It has a strike of N85” E and a width of 7-35 cm. Still another example is the WeinanHuayin ground fissure, produced by the Huaxian

Fig. 6. Sketch of Tang dynasty’s ground fissure in test pits near domestic building of a factory in the northern suburb of Xi’an.

271

earthquakes in length

of 1556 (Wang, and

extends

form with a general 5-20

1980), which is 70 km

in en-echelon

strike N80”E.

or zig-zag

The fissure

is

cm in width and filled with white fine sand,

the northern

side being

depressed

0.1-5

m (Fig.

ments

over the last 4000 yrs are listed in Table 7.

Their

general

in the following (1) Zoning They

occur

Yuncheng

7). The ground

fissures recorded

in historical

docu-

characteristics

rift valley. fissuring

statements: and regional

over

extensive

and Houma

in the Fenwei

found

the same

affecting

at more a zone more

Such fissures correspond

occurrence ground

relatively

that

simultaneously

localities,

these 26 historic in several

depressions

to the underlying

(2) Periodic

in space.

of the Weihe,

occurred

than 100 km in length.

distribution areas

It is sometimes event

than ten different orientation

may be summarized

fault system. in time.

fissures

short

in

time

The

total

of

are concentrated intervals

repre-

senting periods of marked fissuring activity, which each involve at least two to five events of fissuring and last 84-135

yrs, whereas

the intervening

inter-

vals, marked by little fissuring activity, are periods of quiescence lasting 578-607 yrs. These two kinds of periods make up a complete cycle of fissuring activity, the whole duration being 750-800 yrs. (3) Coincidence with seismic events. A comparison of Tables 6 and 7 clearly demonstrates that the active periods of fissuring are accompanied by frequent seismic events corresponding to seismically active periods. Both occur contemporaneously and last for approximately the same length of time. The quiescent periods of fissuring also coincide with the corresponding periods of seismicity. This suggests that they resulted from the same cause, both related to fault movements in the rift. While

the earthquakes

rapid slip on the faults, the fissures response to slow fault creep.

resulted

from

developed

in

(4) Some quiescent periods of fissuring activity may involve relatively short intervals of time

Fig. 7. The water fault

excavated

Ruiquan

Middle

(The fault nearly and below

school,

is tenso-shear

vertical, cutting

Han-Qin

and sand eruption in the drainage Weinan

one, striking

the

cultural

layers

The upper

ground

surface.

along the seismic

in front

county,

with 0.7 m downward

dynasty.) modem

hollow

ditch

N60”E,

Tang-Song

NW, wall, and

of the cavities is 1.68 m

The seismically

tilled fault scarp is 3.3 m in height.

province.

dipping

fall of the upper

of Ming,

surface

of the gate of

Shaanxi

covered

sand-

marked by relatively intense activity, as for instance represented by the modem fissures and the fissures of the late Han dynasty, which correspond to the active periods of fissuring along the NNEWNW- and E-W-trending faults in the eastern part of North China. As is the case of the seismic activity, so these exceptional periods of fissuring activity must have been due to the intense activity on the above-mentioned structures superimposed on the structures of the Fenwei rift.

_~

ground

dynasty

Tlx

lst~of~~ The8thyearafDabe

l-k 6th year of Qianfu

TbeIs1yurofTi -flte2ndyrudLalg+g The34tbye4uofthereign Jiying The22thye4rofcben&ua

dynasty ground fissures

Taog

TheendoftbehIingDynasty

mag

fissures

_

.__I-

lYSY-1983

March 879 May 835 June 834

Lantian Xika Xrra

--.~--.

The eaatem suburb of Xi’an

July 1486

-____

Colt@ Xiwan& Weinan The North of xi’an W&an, Xi’an

Gaazhk-Jiangj

sxt, xi’oa, Jiaqyi-Ganzhai as&bove

In the suburbs of Xi’an Temple of Hanyuandi-

‘Rx

Place of occurrence

Int&emid4ikofthe 17tilccatwy June 1621 May 1568 January 15%

1915-1921

round 1938

Modem

P-d

Gregorian calender date

Name of ground fissures

--

associated phenomena p

Modem time

fissures asd tbe

Time of occurrence

Historic gmmd

TABLE 7

fissure

~___.

- -~-

--...

Fissuriq

=mkg FiSSUlkg

Land slides and water

_

qection

. ..“. .

FkStKillg RI 6.8 earthquake Tht Huaxian earthquake fissure

FiSIiItg

Fisnuine

FiSStKkg

Ground

^_I..~-

.~

Phenomena

Duration

of

135

-T-------

(VW

cycle

a From Wang (1985).

ground fissures

Pre-Qin dynasty

Early Han dynasty ground fissures

dynasty ground fissures

Late Han

Fissuring

Xi’an Xi’an

131 B.C.-8 A.D. 194 B.C.-188 B.C. 266 B.C.-476 B.C.

The reigns of Emperor Wudi and Chengdi

Emperor Hui

The spring and Autumn period The end of the reign of Emperor Jie of the Xia dynasty yellow Emperor the 16th century B.C. 4ooo B.C.

Fissuring

Xi’an

107-125 May, 105

Emperor An The 1st year of Yuanxing

Houshi, $uoyang Fufeng, Fengxiang

Hedong Jingyang, Yunyang, Hedong

Fissuring Fissuring

Fissuring

Fissuring Fissuring Fissuring Fissuring

168-189 September 158

“-

The reign of Emperor Lindi The 1st year of Yanxi

Xi’all

767-788

M 6 earthquake River overflows and land slides in Ankang 14 shocks

The reign Da&Zhengyuan

Xi’an, Weinan Xi’an

May 793 February 788

The 9th year of Zhenyuan The 4th year of Zhengyuan

I

1 293 A

,t 84

I 578

112

274

Other cyclic phenomenu

associated

with the rifting

suffered both

The cyclic crustal activity been accompanied various

natural

the third period

phenomena of intense marsh

as a result of rapid

Xuanpu

of the Fenwei

by changes

1100 B.C., several valley

in Longxian

rift has

in the occurrence in the valley.

seismicity lakes

beginning

appeared

subsidence.

of

During in

in the

The marsh

(Shi, 1963) at this time had

the Jinhe

rift, as suggested Weihe

quiescent

period

the area

experienced disappeared

lasting was

from

tectonically

770 to 476 B.C., quiet

and

only

slight uplift. Meanwhile, Lake Xuanpu and the Weihe River as the main river

a rapid marsh

by the

at this time Increased and

in addition

the Weihe turbid.

The

the

came

River sixth

into

became quiescent

slight elevation

of the

by the fact that no ponds existed

after the year 960 A.D. and again the

River was

This is shown

and Juangzipo

once more witnessed

in Chang’an

during

pro-

seismically

and

period

Lake Weishan

in Shandong

number

Meipi

Accordingly.

than the present

on to the fourth

Qujiang.

existence.

700 km2, which was even larger

It then passed

in Chang’an

clear

an area of about

when

subsidence.

in size and

ponds

This

vince.

rapid

fact that ponds

uctiuity

turned

in turn

turbid

and

followed

the Jinhe

by an

active

clear. stage

which the rift was once again subsiding rate. Accordingly. Zhaoyuqi

at

the Salt Lake and the

reappeared

between

the

years

1271 and 1644 during Yuan-Ming time and the Lupo Bank north of the Weihe River sank to form

active in its work

a navigable marsh lake, whereas the Luohe stream, originally meandering into the Weihe River, was

of erosion so that loess was vigorously attacked and the river water became turbid (Shi. 1977). On

diverted, following a straight channel directly into the Yellow River. The beginning of the quiet stage

the contrary, the Jinhe River outside the rift region with a west-northwesterly direction was nevertheless clear at this time, indicating a tectonically stable situation in that part of the crust. The active stage in the fourth cycle began about 330 B.C. during the period of the Warring Kingdoms when the crust of the rift region underwent renewed subsidence. Many natural lakes successively came into existence in the vicinity of

of the seventh cycle of seismicity was approximately the end of the Ming dynasty. During this

flowing

over the valley became

period, the crust was again relatively stable so that the Salt Lake migrated to the east and became smaller, whereas lakes Zhaoyuqi and Lupo were completely absent and the Luohe stream repeatedly shifted its channel, now flowing into the Weihe, now into the Yellow River. In conclusion

many

natural

phenomena

in the

Chang’an (the present Xi’an) such as the ponds Biaochi and Gaochi towards the end of the Qin

rift have changed as seismic and fissuring events occurred periodically. Such changes of opposite

dynasty. The Salt Lake in Yuncheng at the end of the Han dynasty and the marsh Zhaoyuqi in the Taiyuan rift north of the Fenwei rift also appeared. The channel of the Weihe River broadened and the water became clear as a result of rapid subsidence, whereas the area outside the rift, drained by the Jinhe River, was then subjected to intense loess erosion because of rapid elevation, so that the river water becames turbid. During the quiet stage in the fifth cycle, the rift region again experienced slight elevation. By the year 250 A.D., in the Shu Han period, the Salt Lake of Yuncheng had migrated southward to Yongji, whereas Lake Zhaoyuqi bacame smaller and the Weihe River once more became turbid and the Jinhe River clear (3101581 A.D.). Then followed the active stage of the fifth cycle when the crust once more

extremes were approximately concurrent with active and quiescent periods of seismicity and fissuring which in turn coincided with the corresponding stages of crustal activity of the rift. Thus the periodicity of the modern activity of the Fenwei rift in suggested. The cycle of rifting appears to have begun with a stage of marked extension when the crust was subjected to rapid stretching and consequent vigourous subsidence due to the direct effect of gravity. As a result, marshes or lakes appeared widened and deepened, the river water of the Weihe became clear and seismicity intensified. Then followed a stage of little extension when the crust underwent slight elevation instead of subsidence. Accordingly, lakes diminished in size, the Weihe River became turbid, and there was little seismic and fissuring activity.

275

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Yi

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