Earthquake and tsunami scenarios in the South

China Sea Vu Thanh Ca

Center for Marine and Ocean-Atmosphere Interaction Research

Vietnam  Institute of Meteorology, Hydrology and Environment

5/62 Nguyen Chi Thanh, Hanoi, VietnamEmail: vuca@vkttv.edu.vn

Tsunami risks at Vietnam coast There is no tsunami observation system in Vietnam, then no

recorded data of tsunami exist. Historic records: two tsunami events in 1877 and 1882 at Binh

Thuan coast (South Central Vietnam), small earthquake caused small tsunami. No life loss or property damage reported

The tsunami events are mainly based on the memory of coastal residents. Almost all cases of tsunami events are confused with storm surge or wind waves.

Two important events: tsunami in Dien Chau (Nghe An Province, at the end of nineteenth century), caused flood with the depth of about 1.5m (in some report, the flood depth is 5m) and penetrated about 1km inland, and tsunami in Nha Trang in 1923 due to Hon Tro volcanic eruption (the recorded material is missing)

Tsunami risk at Vietnam coast

Primary research results of Paleotsunami (Cao Đình Triều, Ngô Thị Lư et al) show that in the past, there was huge tsunami attacking Vietnam coast;

The time tsunami attacking Vietnam coast is 380 years, 610 years and 960 years ago. Thus the tsunami return period is about 310 years???;

The maximum tsunami runup height could be 18m (????);

Various places with tsunami attacking records

Tra Co

Dien Chau

Hue

Nha Trang

Phan Rang

Vung Tau

Tsunami attacking spots

However, the collected data were not reliable enough, and detailed studies are needed.

The study on the tsunami risk in Vietnam is mainly based on the earthquake data in the South China Sea, especially that from the Manila Trench.

Tsunami source near Vietnamese coast

A major fault stretches from Red River to the South of Vietnam.

In the offshore South Central Vietnam, the earthquake has the maximum magnitude of 6.5 with the faulting mechanism of the strike slip type.

In the Tonkin Gulf and offshore North Central Vietnam, earthquake has the maximum magnitude of 7.5 with the faulting mechanism of the strike slip type.

Volcanic activities are mainly offshore of South Central Vietnam coast.

Possible land slide exists for all Vietnam coastal shelf

Earthquake in the center of South China Sea: magintude of less than 6.5, strike slip faulting mechanism

Risk of tsunami due to land slide and volcanic activities: can be significant for Vietnam, but not yet evaluated

No.

Magnitude

Moment Area

Length Width

Displacement length

MwMo

(Nm)

A (km2)

L (km)

W (km) u (m)

16.5

6.3x10^18 224 28 8

0.94

27

3.5x10^19 708 50 14

1.67

No.

Magnitude

Strike Dip

Rake depth

Mw (®é)

(®é

)(®é

)h

(km)

1 6.5 180 90 -45 6

2 7 180 90 -45 10

Tsunamigenic earthquake scenarios offshore South Central Vietnam

N.

Magnitude Moment Area Length Width

Displacement length

Mw Mo (Nm)A

(km2) L (km) W (km) u (m)

1 6.5 6.3x10^18 224 28 8 0.94

2 7 3.5x10^19 708 50 14 1.67

3 7.5 2x10^20 2239 89 25 2.97

No.

Magnitude Strike Dip Rake Depth

Mw (®é) (®é) (®é) h (km)

1 6.5 57 90 -45 6

2 7 57 78 -45 10

3 7.5 57 78 -45 17

Tsunamigenic earthquake scenarios south of Hainan Island

Manila Trench and Northern Luzon Ridge (Bautista et al, 2001)

H×nh 25.

No.

Magnitude Moment Area Length Width

Displacement Length

Mw Mo (Nm) A (km2) L (km) W (km) u (m)

16.5

6.30957E+18

240 17 14 0.88

27.0

3.54813E+19

741 35 21 1.60

37.5

1.99526E+20

2291 73 31 2.90

48.0

1.12202E+21

7079 151 47 5.28

58.5

6.30957E+21

21878 313 70 9.61

69.0

3.54813E+22

67608 646 105 17.49

Tsunamigenic Earthquake Scenarios: Manila Trench

No.Magnitude Strike Dip Rake Depth

Mw (®é) (®é) (®é) h (km)

1 6.5 177 15 90 4

2 7.0 177 15 90 5

3 7.5 177 15 90 8

4 8.0 177 15 90 12

5 8.5 177 15 90 18

6 9.0 177 15 90 27

Ryukyu Trench

No. Magnitude Moment Area Length Width Displacement

Length

Mw Mo (Nm) A (km2) L (km) W (km) u (m)

1 7 3.5x10^19 708 50 14 1.67

2 7.5 2x10^20 2239 89 25 2.97

3 8 1.1x10^21 7079 158 45 5.28

4 8.5 6.3x10^21 22387 282 79 9.39

5 9 3.5x10^22 70794 501 141 16.71

Tsunamigenic Earthqake scenarios: Ryukyu Trench

No.Magnitude Strike Dip Rake Depth

Mw (®é) (®é) (®é) h (km)

1 7 87 15 90 3

2 7.5 87 15 90 5

3 8 87 15 90 8

4 8.5 87 15 90 14

5 9 87 15 90 24

Tsunami earthquake scenarios for Vietnam coast

The tsunami generation model: the Okada’s double couple source model (1985, 1997).

Numerical model for the tsunami propagation in the South China Sea

Based on MOST( Tito & Gonzalez, 1997)

0

cos

cos

1

vdud

Rt

fvd

R

gh

R

gu

R

vu

R

u

t

u

coscoscos

fud

R

gh

R

gv

R

vv

R

u

t

v

cos

Numerical modeling

Discretization schme: Finite volume methods with geological coordinate;

The computed results by the model are used as the boundary conditions for tsunami inundation computation on Vietnam coast.

Scenario Checking results

• Computations carried out to check the risks of tsunami due to different earthquake scenarios;

• Only earthquake with magnitude greater than 7.5 south of Hainan Island generate significant tsunami at Vietnam coast;

• The earthquakes offshore South Central Vietnam do not generate tsunami

Scenario checking results

• Earthquake with magnitude greater than 8 can generate significant tsunami at Vietnamese coast.

Determined 12 tsunamigenic earthquake scenarios with corresponding parameters

Results of tsunami computation at the Vietnam coast

Fig 1 Tsunami height according to scenario 1 (earthquake M=8 at Manila Trench)

• Fig 2. Tsunami propagation time from the source (hours)

Results of tsunami computation at the Vietnam coast

Fig 3 Tsunami height according to scenario 3 (earthquake M=8.5 at Manila Trench)

Fig 4 Tsunami height according to scenario 5(earthquake M=9 at Manila Trench)

Results of tsunami computation at the Vietnam coast

Fig 4 Tsunami height according to scenario 10 (earthquake M=9.0 at Ryukyu Trench)

Fig 4 Tsunami height according to scenario 11 (earthquake M=7.5 south of Hainan)

Remarks The risk of tsunami at Vietnam coast is small; Results of computation show that the most

dangerous earthquakes are that with magnitude of greater than 8 in the Manila Trench;

The possible strongest earthquake in the Manila Trench have the magnitude of 8.5. Thus, tsunami risk at Vietnam coast exists;

An earthquake with the magnitude of 9 in Ryukyu Trench and earthquake with magnitude of 7.5 at south Hainan island may cause significant tsunami at Vietnam coast;

A numerical Model for tsunami runup based on spacial averaging operator

Wetting/Drying grid mesh technique is often used when simulating tsunami intrusion inland

It is very difficult to simulate the tsunami intrusion inland with small grid mesh due to excessive computational time and memory;

Use of large grid mesh can create significant error if it is assumed that the grid mesh at the tsunami front is fully wet

fThe spatial averaged value of a physical quantity is defined as

wSw

fdsS

f1

The relationship between ensemble average and spatial average is obtained by modifying the formula of Crapsite et al (1986) and Hiraoka (1990) and is expressed as

)()(11

xdlnxfSx

fS

Sx

f

l

iwii

With Sw is the wetted surface inside a grid mesh (a cell), ds is a surface element area

0S

SS wWith As the effective surface function

Derivation of main equations of the model

• The main equations of the model are derived by applying the above-mentioned spatial averaging procedures to the shallow water equations and continuity equation

0

ty

q

x

q yx

0

/

/

2

2

xcx

t

xt

yxxx

Qqd

f

y

dqd

y

x

dqd

xxgd

d

qq

yd

q

xt

q

0

/

/

2

2

ycy

t

yt

yxyy

Qqd

f

y

dqd

y

x

dqd

xygd

d

q

yd

qq

xt

q

Spacial averaged governing equations

Continuity equation

0

t

S

y

qf

x

qf yxxy

0

/1

/1

11

2

2

xcx

t

xt

yxxx

Qqd

f

y

dqSd

yS

x

dqSd

xSxgd

d

qSq

ySd

Sq

xSt

q

Momentum equations

i,j i,j+1 i,j-1

i-1,j

i+1,j

I

I I

i+1,j+1

x2

x

y

y1

0

uij

ui+1j vij

vij-1

i-1,j+1 i-1,j-1

i+1,j-1

A B

C D

Onshore

Offshore

x1

x

y

Wetting – Drying cells with wave front

Experiments by Ting and Kirby (1994, 1995, 1996)

Wave generator

0.4m

35

10.38m

Fig. 5 (a) C o mputed and observed p hase-averaged w ater surfacee levat io n at (x-xb)/h b=7.462. Spilling breaker. Fig. 5a

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0 0.2 0.4 0.6 0.8 1

t/T

(z-<

z >)/

h

ObservedComputed

Computed and observed phase-averaged water surface elevation at (x-xb)/hb=7.462. Spilling breaker.

Fig. 5 (a) C o mputed and observed p hase-averaged w ater surfacee levat io n at (x-xb)/h b=7.462. Spilling breaker. Fig. 5a

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0 0.2 0.4 0.6 0.8 1

t/T

(z-<

z >)/

h

ObservedComputed

Computed and observed phase-averaged water surface elevation at (x-xb)/hb=7.462. Spilling breaker.

Fig. 6a

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0 0.2 0.4 0.6 0.8 1t /T

(z-<

z >)/

h

ObservedComputed

Computed and observed phase-averaged water surface elevation at (x-xb)/hb=10.528. Spilling breaker

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0 1 2 3 4 5 6 7 8 9 10111213

Horizontal Distance (m)

Hei

ght

(m)

BedComp. EtaavComp. EtamaxComp. EtaminComp. WavehObs. WavhObs. EtaavObs. EtamaxObs. Etamin

Comparison between observed and computed time averaged wave height, highest, lowest and mean water surface elevation for spilling breaker. Experimental data from Ting and Kirby (1994, 1995).

Fig. 8a

- 0.4

- 0.2

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

t / T

(z-<

z>)

/hm

ObservedComputed

Computed and observed phase-averaged water surface elevation at (x-xb)/hb=6.494. Plunging breaker.

Fig. 8d

- 0.5

0

0.5

1

0 0.2 0.4 0.6 0.8 1

t /T

<u>k

/c3 (X

10-3

)

ObservedComputed

Computed and observed phase-depth averaged relative advective transport rate of TKE in the horizontal direction at (x-xb)/hb=6.494. Plunging breaker.

Experiments by Mase and Kobayashi (1993)

Wave generator

0.47

m 20

1

Wave runup meter

Computed and observed wave runup height. T=2.5s, =0.1.

Fig. 11a

- 0.05

- 0.025

0

0.025

0.05

0 5 10 15 20 25

Time (sec)

Wat

er

Surf

ace E

leva

tion (

m)

ObservedComputed

One dimensional runup

3D wave runup on a conical island (Brigg et al, 1995)

Thank you