chichi earthquake, taiwan, 21 september 1999. propagation @3 km/sec. slip velocity less
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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
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121°E10 km
1
2
3 km
121°30"E
50 km
Chelungpu Fault
1999 hypocenter
Propagation@3 km/sec.Slip velocity less
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Building Damage
• wrong design
• wrong construction
• wrong location
• engineer
• contractor
• geologist
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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!
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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
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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
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Lateral spreadingLiquefaction in Wufeng
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Liquefaction-induced sand-venting through lateral spreading fissures
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Ground fissuringMSK VIII-IX
Lateral spreading andFoundation settlement
Channel fracturing& collapse
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QuickTime™ and aCinepak decompressor
are needed to see this picture.
Accelerations near the surface rupture
2.5 km/sec
Bruneripple
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Toe collapse and hanging wall accelerations
How fast did the fault slip near the surface?How did toe of thrust deform?
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Footwall
Hanging Wall
Collapsed toe of thrust
Masonry posts still standing=Intensity VII (<0.4 g)
rolled but not thrown
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Minor to moderatedamage (MSKIntensity VIII)
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Masonry wall collapse Masonry pillar“shaken not stirred!”
Toe
footwall
MSK Intensity VIII
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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
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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
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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
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throw
pre-seismic
co-seismic
post-seismic
toe
heave
dip simple shear
collapseaxis
hangingwall rotation
collapseaxis
scarp
collapse ofhanging wall
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inverted, unbroken tile
preseismic
co-seismic
post-seismic
30 cm tile liftedand rotated 180°
Pagoda
un-reinforcedconcretestairway
tileslargelyintact
contact
hillside steepenedflexedand shortened
A B
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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
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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!
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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)
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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!