chichi earthquake, taiwan, 21 september 1999. propagation @3 km/sec. slip velocity less

Taichung

Wufeng

Takeng

Fengyuan ShihkangTungshih

SunMoon L.

ChiChi

Nantou

Tsaotun

LukuChusan

MingchienShuili

Taiping

epicenter

20 km

Taichung

Wufeng

Takeng

Fengyuan ShihkangTungshih

SunMoon L.

ChiChi

Nantou

Tsaotun

LukuChusan

MingchienShuili

Taiping

epicenter

20 km

Taichung

Wufeng

Takeng

Fengyuan ShihkangTungshih

SunMoon L.

ChiChi

Nantou

Tsaotun

LukuChusan

MingchienShuili

Taiping

epicenter

20 km

ChiChi Earthquake, Taiwan, 21 September 1999

121°E10 km

1

2

3 km

121°30"E

50 km

Chelungpu Fault

1999 hypocenter

Propagation@3 km/sec.Slip velocity less

Building Damage

• wrong design

• wrong construction

• wrong location

• engineer

• contractor

• geologist

Wrong Engineering• Ground floor too weak

• Building not symmetrical

• Too close to next building

• Foundations not deep enough

• Accelerations too large (inappropriate seismology and geology input)

Conclusion: Taiwanese Engineering is Exceptionally good. 95% success. Most buildings held!

Construction problems in concrete structures

• Weak cement (should be strong)

• Brittle steel (should be ductile)

• Insufficient stirrups in columns

• Wrong positioning of steel

• Weak wall/column bonding

Geology & seismology

• Where are the faults?

• How often do they move?

• Liquefaction?

• Basin Resonance?

• Accelerations near fault? (Brune ripple)

• Focussing effects? (Heaton pulse)

Important effects in the 1999 Chichi Earthquake

Lateral spreadingLiquefaction in Wufeng

Liquefaction-induced sand-venting through lateral spreading fissures

Ground fissuringMSK VIII-IX

Lateral spreading andFoundation settlement

Channel fracturing& collapse

QuickTime™ and aCinepak decompressor

are needed to see this picture.

Accelerations near the surface rupture

2.5 km/sec

Bruneripple

Toe collapse and hanging wall accelerations

How fast did the fault slip near the surface?How did toe of thrust deform?

Footwall

Hanging Wall

Collapsed toe of thrust

Masonry posts still standing=Intensity VII (<0.4 g)

rolled but not thrown

Minor to moderatedamage (MSKIntensity VIII)

Masonry wall collapse Masonry pillar“shaken not stirred!”

Toe

footwall

MSK Intensity VIII

curboffset5.6 m

C.enlarged view ofChungKuan lane

fault strikeN15°E

N60°E

curb

curbfootwall

hangingwallup 3.3 m

Takeng River

B≈4 m

C=3.3 m

D =3.5 m

A≈4 m

Chelungpu F.

footwall down hanging wall up

Hm

Hn

Hw

1 km

X9X7

X28

T30

T7

T25

T92

T120

T50

T50XXX120 t8

t7

t13

t5

V30

V20

T370

T40B

Wufeng

X130

V30

pre-seismic

post-seismic

pre-seismic

post-seismic

C

DA

X11

Chelungpu F.

R10°

R15°

R20°

R15°T45 t15 t13

X17T10

V5

t20

T60

T30T20

t7

t10

t7

X133figure 6

xx

Conclusions on thrust emplacement

1. Slip was relatively slow (velocities 1-2 m/sec)2. Accelerations were less than 0.5 g on the toe of the thrust

Brune ripple apparently not applicable to thrust faults in western Taiwan

throw

pre-seismic

co-seismic

post-seismic

toe

heave

dip simple shear

collapseaxis

hangingwall rotation

collapseaxis

scarp

collapse ofhanging wall

inverted, unbroken tile

preseismic

co-seismic

post-seismic

30 cm tile liftedand rotated 180°

Pagoda

un-reinforcedconcretestairway

tileslargelyintact

contact

hillside steepenedflexedand shortened

A B

2.5 m localizedthrusting

1.5 m distributedshortening & rotation

thrust-parallel tension gashes

prograde rotationof hillside treesand stairways

local southwarddirected thrustingover unconfinedsouthern slope

20°

≈0.3 msurficialshortening

80 m

150 m

10°

Pagoda 1

Shrine

Figure 8

50-m-WIDE ZONE OF ROTATION & SHORTENING

Conclusions

• Structures must not be built on faults• Width of zone damage zone 50 m on

hanging wall near bedrock• Width of severe damage 1 km in sediments

Observation:Many schools and bridges built near or across faults!

recommendations

• Need better acceleration data from fault zones (make fault zones parks)

• Instrument and map thrust faults to north

• Need to ensure that contractors understand the need for tied stirrups, and correct density. in concrete columns.

• Need to avoid regions of known liquefaction (make fissured zones into parks)

High tech. reccommendation

• Inexpensive accelerometers could be included in every new building in Taiwan as a “Black Box” recorder like those used in aircraft.

• Cost could be less than $100 given Taiwanese ingenuity and experience!

• This would make contractors realize that they will have to answer for bad construction practices!