TSUNAMI NUMERICAL SIMULATIONAPPLIED TO TSUNAMI EARLY WARNING SYSTEM

ALONG SUMATRA REGION

Wiko SetyonegoroResearch and Development Center BMKG, Jl. Angkasa1 No.2, Kemayoran Jakarta

E-mail: wikosetyonegoro@yahoo.com

ABSTRACT

Western Sumatra region is tectonically active region that often lead to catastrophic earthquakes and tsunamis. When the tsunami disaster will occur and how big run-ups and expansion of the resulting tsunami inundation, could be the arrival time calculation and simulation, it is done by creating a scenarios tsunami modeling before the real tsunami is occured. To generate a tsunami modeling in large quantities will require a variety of mechanisms fault scenarios, subsequently scenarios variations are grouped in segments taken in areas prone to tsunamis, so that the database does not widen. This scenarios is based on the values of fault parameters and modified in such a way as to produce the type of fault models with historical data approach the mechanism of fault conditions in that segment. As another reference to estimate the location of disturbances in these segments, done by observing the deformation history of geological processes and the relative movement of the earth's crust. Then do as well the validity of the calculation results with historical data and the results of a survey on the same segment.

Key words: tsunami, scenarios, modeling, fault

ABSTRAK

Wilayah Sumatera bagian barat merupakan daerah tektonik aktif yang kerapkali menimbulkan bencana gempabumi dan tsunami. Dalam penelitian ini dilakukan perhitungan simulasi run-up, inundasi, waktu, dan waktu kedatangan tsunami, hal itu dilakukan dengan membuat skenario pemodelan tsunami sebelum terjadi tsunami yang sesungguhnya. Skenario tersebut dibuat sebanyak mungkin menjadi sebuah database pemodelan tsunami. Untuk menghasilkan pemodelan tsunami dalam jumlah banyak maka diperlukan berbagai variasi skenario dari mekanisme faultnya, kemudian variasi skenario yang diambil dikelompokkan pada segmen di daerah rawan tsunami, sehingga hasil database tidak melebar. Skenario ini didasarkan pada nilai-nilai parameter fault dan dimodifikasi sedemikian rupa untuk menghasilkan jenis model fault dengan melakukan pendekatan data historis kondisi mekanisme fault pada segmen tersebut. Sebagai acuan lain untuk memperkirakan lokasi gangguan pada segmen tersebut, dilakukan dengan mengamati proses sejarah deformasi geologis dan pergerakan relatif dari kerak buminya. Kemudian dilakukan juga validitas hasil perhitungan dengan data historis dan hasil survei pada segmen yang sama.

Kata kunci: tsunami, skenario, pemodelan, fault

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TSUNAMI NUMERICAL SIMULATION APPLIED TO TSUNAMI......................................................................Wiko Setyonegoro

Naskah masuk : 18 Januari 2011Nasakah diterima : 2 Mei 2011

I. INTRODUCTION 1.1. Active Tectonics

Indonesia is the region of confluence of three major plates such as subduction process in the Western part of Sumatra, the Indian plate moving under the crust subducted to Sumatra with velocity rate 50 mm/ year.

Figure 1. Active tectonic in Indonesia, ploting using

GMT, shown earthquake distribution with

depth parameters around Sumatran trench.

Active tectonics in the Sumatran region has often caused by the earthquake, shown in Figure 1, plots by using GMT (General Mapping Tools), which shows the distribution of areas with shallow earthquakes and has a high intensity. Several events can generate earthquake-tsunami in the western part of Sumatra, and the result will be very destructive. In this case the focus mechanism of data analysis according to the USGS said that Sumatra has a kind of reverse fault side, indicates that subduction activity is not the only cause of earthquakes, but a several "lock" positions on Sumatra, which caused movement of double-couple, which will lead to a series of displacement for the next earthquake. Earthquakes are often associated with serious issues about the potential of tsunami. This research is expected to further useful for determining the location of mitigation in the development and an early warning system efforts in Indonesia. The earthquake that occurred in western part of Sumatra has the potential to cause a tsunami, with the coordinates of the earthquake source in the ocean.

1.2. Propagate TsunamiA tsunami is a series of water waves (called a

tsunami wave train) that is caused by the displacement of a large volume of a body of water,

such as an ocean. The original Japanese term literally translates as "harbor wave". Tsunamis are a frequent occurrence in Japan, approximately 195 events have been recorded. Due to the immense volumes of water and energy involved, tsunamis can devastate coastal regions. Casualties can be high because the waves move faster than humans

1)can run.

1)Figure 2. Propagate of Tsunami.

Earthquakes, volcanic eruptions, and other underwater explosions (detonations of nuclear devices at sea), landslides and other mass movements, bolide impacts, and other disturbances above or below water all have the potential to generate a tsunami. The propagation of tsunami after disturbances will be faster wave propagating speed in a deep sea, slower wave propagating speed in a shallower sea and decrease in wave

1)length amplification of wave height.(Figure 2) The Greek historian Thucydides was the first

to relate tsunami to submarine earthquakes, but understanding of tsunami's nature remained slim until the 20th century and is the subject of ongoing research. Many early geological, geographical, and oceanographic texts refer to tsunamis as "seismic sea waves". Several meteorological conditions, such as deep depressions that cause tropical cyclones, can generate a storm surge, called a meteotsunami, which can raise tides several metres above normal levels. The displacement comes from low atmospheric pressure within the centre of the depression. As these storm surges reach shore, they may resemble (though are not) tsunamis, inundating vast areas of land. Such a storm surge

2)inundated Burma (Myanmar) in May 2008.

1.3. Tsunami CausesA tsunami can be generated when

convergent or destructive plate boundaries abruptly move and vertically displace fault the overlying water. It is very unlikely that they can form at

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1. HISTORICAL DATA ABOUT FOCAL MECHANISM OF WPHASE MOMENT TENSOR SOLUTION

2. CALCULATION OF CORRECTION OF STRIKE DIRECTION

3. REFERENCE VALIDATION OF GPS AND SEISMOLOGY MEASUREMENTS. (CRUSTAL DEFORMATION)

FAULT PARAMETER DETERMINATION(VALUE & DIRECTION)

TSUNAMI MODELING

TSUNAMI EARLY WARNING SYSTEM

divergent (constructive) or conservative plate boundaries. This is because constructive or conservative boundaries do not generally disturb the vertical displacement of the water column. Fault area related earthquakes generate the

3)majority of tsunami.(Figure 3).

Figure 3. Sumatran fault in Aceh, with 1200 - 1600 km

length and 10 - 15 km depth in western part of

Sumatra, plot using GMT.

Most tsunamis are caused by submarine earthquakes which dislocate the oceanic crust, pushing water upwards. Tsunami can also be generated by erupting submarine volcanos ejecting magma into the ocean. A gas bubble erupting in a deep part of the ocean can also trigger a tsunami. Tsunamis have a small amplitude (wave height) offshore, and a very long wavelength (often hundreds of kilometers long), which is why they generally pass unnoticed at sea, forming only a slight swell usually about 300 millimetres (12 in) above the normal sea surface. They grow in height when they reach shallower water, in a wave shoaling process described below. A tsunami can occur in any tidal state and even at low tide can still

3)inundate coastal areas.

II. PURPOSE1. Determine the correct fault parameter to make

tsunami numerical modeling, through calculation, historical data of earthquake and references validation from research, survey and m o n i t o r i n g ( G P S a n d s e i s m o l o g y measurements)

2. Applying scenarios of tsunami numerical modeling to TEWS (Tsunami Early Warning System), intended to disaster mitigation and prevention for community.

III. OUTLINE1. Observation of the crustal movement,

displacement, deformation, and fault parameters along Sumatra trench.

2. Tsunami modeling for tsunami early warning system as a step tsunami warning against. Modeling tsunami simulation as a supporter of DSS (Decision Support System) which has been done in BMKG 2010.

IV. METHODE AND PROCESSINGTo create the model, necessary information

of focal mechanism (W-phase moment tensor solution), so that the direction and value of fault parameters can be modeled to determine the run-up and innundation in the area around the shoreline, (Figure 4).

4.1. Using Historical DataDetermine fault parameters using historical

data on this study with use data will be used to determine the fault parameters in tsunami scenario is data fochal mechanism of w-phase moment tensor solution. Further data will be used as estimates for determining further more scenarios in the research area, (Figure 4).

Of an earthquake event, the essential data is the focal mechanism of fault, such as : length of fault, width, slip, depth, dip, strike accuracy parameters which will determine the maximum run-up that will occur along the Coastline.

This relationship is very closely related to the relationship equation between the Magnitude, and wide area fault. Reference parameters data for fault parameters by focal mechanism of w-phase

4)moment tensor solution from USGS.

Figure 4. Methodology on tsunami hazard potential

4.2. Using CalculationsDetermine fault parameters using

calculation process, there are two assumptions that are used to input parameters fault, according to some cases, as follows:

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TSUNAMI NUMERICAL SIMULATION APPLIED TO TSUNAMI......................................................................Wiko Setyonegoro

1. In assumption fault length is not devided.2. In assumption fault length devided become

subfault.Data which use in tsunami numerical

5)simulation by Etopo2 from bathimetry data).

4.2.1. Length of Fault Not DevidedAssumption on this case, an earthquake fault

is extremely long, then the assumption is a long straight faults by eliminating the momentum correction. Here there is no difference angle correction for strike direction, or in general, there is no correction factor for the actual contours of faults which not exactly straight.

4.2.2 Length of Fault Devided

6)Equation 1. Moment Magnitude

Figure 5. Survey Sites, find an evidentiary maximum

run-up in the western part of Sumatra, through

survey result above is defining the best of fault

parameters as shown in Figure 6 and Figure 7)7.

In case, assumption of fault is rectangle and curvaceous (not exactly straight), The real form of the fault its must be as Figure 7 a, than divide the fault length become a several subfault. Based on Kanamori's equations above, which reduce the wide area (A), should increase Slip. By following fault wide area vs magnitude of Wells and Copper Smith equation (table 2), in this case the fault with a length of 1200 km is divided into several segments, such as red in Figure 7 a and Figure 7 b, without removing the influence of other segments effects, then set value of the slip parameters as accumulation amount of the other slip (table 1). For example, we divide the length of fault into 4 subfaults and do multiply the slip to be 4 times, in case do not want to eliminate the effects of the incident energy of the actual fault deformation

7)which reach shoreline in research areas.(Figure 5) In this case we only running one of the

subfaults (Figure 7 b). Fault with a length of 1200 km, not be run directly in a single running program (Figure 6), but separated into several segments following the plate boundary on Aceh trench, and than running of subfault one by one. Because this is related to the direction of the each segments of

7)faults.Scenarios that will be made in one-time

running of subfault, will be equal to one-time running fault as a whole. So, subfault will represent the entire fault.(Figure 7 b)

Figure 6. Fault with the 1200 km long, straight shape,

before reduce correction of force momentum

factor.

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Figure 7 a). Fault with the 1200 km long, straight shape,

after reduce momentum force correction

factor by devide fault length become a few 8)fault (subfault).

Figure 7 b). Divided fault into 4 subfault, and than

running which using one subfault, to get the

correction of momentum force from strike

directions, the illustration is divide skets A

to B, which running subfault number 3 in 8)skets B.

According Newton's third law, than running for subfault number three (Figure 7 b) become:

F (subfault3) = F1 cos q + F2 cos + F3 + F4

9)Equation 2. Resultan force of slip (disl).

Which F is dislocations of slip forces and q is 9)momentum angle.

Through the Mamoru Nakamura's software, 10)we try to make tsunami numerical simulations.

The essencial parameter for determine the

q

maximum run up of tsunami is ; slip, large area, and direction of the fault which the most of research have a standard value for slip and large area, which almost similar each other. Therefore here we find a

10)several references by table 1.

Table 1. References slip and length of fault

By following Wells and Copper Smith equation which explained the relation between large area of fault and magnitude. So its mean, assumption for for 9.2 SR area of fault is rectangular, than A = 83571.16 km2, input parameters to the Mamoru Nakamura's software for A will be in W x L :

A = W x L 83571.16 = W x 1200 km W = 69.64 km

Which A given by table 1 and L given by table 2 . Than we divide W becomes Aw1 and Aw2, also divide L becomes Al1 and Al2 following Figure 8.

Table 2. Table from Wells and Copper Smith equation,

recommendation wide area of fault.

To make one-tsunami scenario, the focal mechanism analysis is required, with the precision of analysis and comparison. In addition to bathymetry data, Slip (disl) which most affects

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TSUNAMI NUMERICAL SIMULATION APPLIED TO TSUNAMI......................................................................Wiko Setyonegoro

large and proportional to propagate maximum run-up of tsunami. There are several reasons to define the slip on Mamoru Nakamura's software. Through trial and error to determine moment tensor solution of earthquakes :

?Choose the best fault parameters to make tsunami scenarios, to fitting with the observation data of Aceh tsunami on December 2004. Based on Kanamori equation.?Run-up tsunami is strong affected by the large of

slip and direction of fault. Based on Wells and Copper Smith equation.

Figure 8. Setting parameters in Mamoru Nakamura's 10)Software.

Actual fault length is around 1200 - 1600 km and slip is around to 15 - 80 m (Equation 1 and

Equation 2), with a few assumptions to eliminate the effect of the moment force from strike direction. Based on :1. Comparison of fault parameter model with data

moment tensor USGS (dip, slip, strike, depth, and magnitude). which means determining fault scenarios based on historical data, so that what has been prepared and is determined not slip too far. This is a validation of model predictions and observations in the past. USGS moment tensor data used as a benchmark to determine the source of earthquakes in the future. Without historical data, the approximate determination of earthquake fault location will be very far off the

11)mark. 2. Calculation of resultant force. After determining

the location of reference for the location of the fault, here is the scenario calculations for the election run up the most valid, namely the calculation of distance and direction of the fault mechanism until the run up through the adjustments that follow the forms of plate boundary fault by GPS measurements references

12)and its strength in generating a tsunami wave. 3. Validations to survey results in Aceh by

following a few research papers. Then, obtained the output of processing results which are described in the next explanation.

4.2.3 Scenarios Simulation result of Run-Up Maximum on Aceh Tsunami 2004

Table 3. Scenarios with each fault parameters to find the correct parameters by ; validation, data and survey, to determine

the correct parameters in Aceh tsunami 2004.

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Model 1 in table 3 (represent by red color in Figure 9, is the smallest of the fault scenario according to table 1 (About Reference for Slip and length of Fault), the result is the run-up max tsunami in Lhoknga not in accordance with the validation survey result. The area fault following table 2.

Model 2 in table 3 (represent by the yellow color in Figure 9, the scenario with the strike of fault position not straight face to Lhoknga (other parameter values the same as Model 1), the result is the run-up max tsunami in Lhoknga validation not in accordance with the validation survey result. The area fault following table 2.

Model 3 in table 3 (represent by the green color in Figure 9), decreasing wide area of faults without increasing the slip, the result showing run-

up maximum tsunami in Lhoknga is not in accordance with the validation survey result. The area fault following table 2.

Model 4, Model 5, and Model 6 in Table 3 (Model 5 represent by orange color, Model 4 and Model 6 represent by pink color), an adjustment value of the slip which following Hanks and Kanamori's equations; Mo = m A S (Equations 1). In case devided fault area (Figure 7 b) and to get the correction of the momentum-force from strike directions. Then obtained a model that can estimate the distribution pattern of maximum run-up from survey result in the respective areas of research. For example, take maximum run-ups in Lhoknga (table 3), validation results ranging from 31-33 m run-ups in Lhoknga.

Figure 9. Scenarios modeling of Aceh earthquakes, December 2004, represent of table 3,plot by GMT.

4.2.4 Scenarios of Each Model

13),14)Figure 10. Validations of maximum tsunami run-up with variations of slip and length.

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TSUNAMI NUMERICAL SIMULATION APPLIED TO TSUNAMI......................................................................Wiko Setyonegoro

4.2.5. Showing Tsunami Simulation Scenarios on Aceh Tsunami 2004

Figure 11. Plot by GMT, using Mamoru Nakamura's Software. Output of scenarios tsunami modeling, for 300 km length,

slip (disl) = 60, in case fault divided, resultant moment force based on Wells and Copper Smith (table 2),

Kanamori's, Mo = m A S (Equation 1) and Newton's third law (Equation 2).

Tabel 4. Output of tsunami numerical modeling in Aceh tsunami, December 2004. List of analysis in each research areas,

scenario for 300 km length, slip (disl) = 60 m, in case fault divided, resultant moment force based on Wells and

Copper Smith (table 2), Kanamori's, Mo = m A S (Equation 1) and Newton's third law (Equation 2).

13),14)

13),14)

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Figure 12. Maximum tsunami height vs ocean bottom in Aceh region, output of Mamoru Nakamura's software, showing

how tsunami generate by ocean bottom. Scenario for 300 km length, slip (disl) = 60 m, in case fault divided,

resultant moment force based on Wells and Copper Smith (Table 2), Kanamori's, Mo = ? A S (Equation 1) and 13),14)Newton's third law (Equation 2).

4.2.6. Validation

Figure 13. Validation maximum run-up between model vs survey references in each area. Residue obtained from the

measurement is 6.95 m. Residue is too large for a considerable distance. The pattern of the curve shows the

suitability of the maximum run-up at each point. To predict ccurately, it needed more fault scenarios parameters 13),14)obtained from the reference GPS measurements, focal mechanisms and moment tensor data.

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TSUNAMI NUMERICAL SIMULATION APPLIED TO TSUNAMI......................................................................Wiko Setyonegoro

V. TSUNAMI HAZARD POTENTIALIndonesian government, with the help of

donor countries, has developed a Tsunami Early Warning System for Indonesia (Indonesian Tsunami Early Warning System - InaTEWS). The system is centered on Meteorology, Climatology, and Geophysical Agency (BMKG) in Jakarta. This system allows BMKG sending a tsunami warning in the event of an earthquake that caused the tsunami potential. Current system is being perfected. In the future, this system will be issued a warning level 3, in accordance with the results of

15)calculations DSS (Decision Support System).Development of tsunami early warning

system was involved many parties, both central government agencies, local governments, international agencies, non-governmental agencies. Coordinator of the Indonesian side is the Ministry of Research and Technology (RISTEK). Meanwhile, the designated agency responsible for issuing Info Earthquake and Tsunami Warning is BMKG. This system is designed to be issued a tsunami warning in time of at least 5 minutes after

15)the earthquake occurred.Early Warning System has 4 components :

1. Knowledge of Hazard and Risk,2. Forecasting, Warning, and Reaction, 3. Observation (Monitoring of the earthquake and

sea level),4. Integration and Dissemination of Information,

15)Preparedness, (Figure 14).

Tsunami early warning system is a series of complex work systems involving many parties, on the regional, national and international tributaries

14)of the society. In the event of an earthquake, the events

were recorded by means of seismograph (earthquake recorder). Earthquake Information (strength, location and time events) are sent via satellite to BMKG Jakarta. BMKG will release the next Quake Info delivered through technical equipment simultaneously. Earthquake data included in the DSS to consider whether the earthquake has the potential to cause a tsunami. The calculation is based on millions of modeling scenarios that have been made beforehand. Then, may issue BMKG tsunami warning info. This earthquake data will also be integrated with data from devices other early warning systems (GPS, Buoy, Obu, and Tide Gauge) to confirm whether a tsunami had actually been formed. This information is also transmitted by BMKG. BMKG deliver tsunami warning information through an intermediary institutions, which include (Local Government and the Media). This intermediary institutions that transmit warning information to the public. BMKG also convey information via SMS alerts to mobile phone users who are registered in the database BMKG. Earthquake Info ways of delivery for now is via SMS, facsimile, telephone, email, RANET (Internet Radio), FM RDS (Radio RDS have facilities / Radio Data

15)System) and through the website of BMKG.

Figure 14. Chart of monitoring procedures of tsunami early warning system in Indonesia. 15)

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5.1 ProceduresSystem Seiscom-P BMKG releases tsunami

potency earthquake parameter. Observation censor buoys, tide gauge & GPS gives verification report waving tsunami. Simulation of tsunami is implemented automatically by DSS based on earthquake parameter input Seiscom-P. DSS gives image of tsunami analysis consisting of ; location of tsunami impact coast, arrival time, wave height, level commemoration of tsunami & alternative of decision. Information of commemoration of tsunami is sent through communications 5 in 1 system in BMKG to evacuate community (Figure

15)14).

5.2 Action Plan in BMKGIn this study, examples of research on

tsunami modeling has been done will be applied to the Figure 14 above. Work plan with the tsunami scenario produces more than a million scenarios, complete with risk and vulnerability modeling. With a full predictive modeling tsunami, the potential hazard of tsunami information intended for the evacuation process will run as fast as possible, (Figure 14).

5.3 How it WorksModels are made in the form of tables of data

with sequently of each parameters. Locations have been grouped according to the fault position which nearest to the shoreline. Model that has been made in the classification sequence: nearest distance to fault, strike direction, extensive fault, and slip (dislocation). Created tools (software) specifically to identify the best simulation of the millions of tsunami scenarios. Procurement of special instruments that will be placed on Local Government and the regional tsunami warning center of BMKG. Expected to accelerate the evacuation decision for the community.

VI. CONCLUSIONSValidation measurements carried out

between the model and the survey would be more accurate for areas which close to the fault position. Instead validation with remote areas of the fault, the value of validation will be slightly off the mark. This is caused by a correction factor of bathymetry data, and stike angle of the fault.1. To calculate the maximum tsunami run-up with a

very long fault, it would be more effective if used

subfault, and running the program be repeated several times following the actual fault length.

2. Error factor which often occurs when creating the scenarios modeling is consisting of ; moment force from angle position of faults (F-strike) (Figure 7 b), the definition of fault length (L), the nearest distance between the fault with shoreline, and correction factor from the bathymetry condition (ocean bottom).

VII. ACKNOWLEDGEMENTSThanks so much to JICA and RSVD -

Nagoya University for organizing this training course on operating management of Earthquake, Tsunami and Volcano eruption observation system. For Prof. Kimata who arranged all training program, and also for all teachers who give knowledge which has supported this paper. Lastly, thanks for all friends and colleagues at JICA and RSVD for all support and cooperation along this training.

VIII. REFERENCES1) Fradin, Judith Bloom, & Dennis Brindell (2008).

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H i s t o r i c T s u n a m i . ( 2 0 0 9 ) . (http://www.answers.com/topic/historic-tsunami#2004:_Indian_Ocean), accessed on 24 September 2009.

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5) NOAA, National geophysical data center , GEODAS Grid Translator. (2009). (http://www.ngdc.noaa.gov/mgg/gdas/gd_designagrid.html), accessed on 18 August 2009).

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Implications of the 26 December 2004 Sumatra-Andaman Earthquake on Tsunami Forecast and Assessment Models for Great Subduction-Zone Earthquakes, Bulletin of the Seismological Society of America, 97(1A), S249-S270.

8) Kenji Hirata. (2006). "Tsunami Source Model from Satellite Altimetry", Program for Deep Sea Research, IFREE, Japan Agency for Marine-Earth Science and Technology, Natsushima, 2-15, Yokosuka 237-0061, Japan.

9) Addison-Wesley. ; Kleppner, Daniel; Robert Kolenkow (1973). An Introduction to Mechanics. McGraw-Hill. pp. 133-134. ISBN 0070350485.

10) Nakamura Mamoru (2006). Source fault model of the 1771 Yaeyama tsunami- Southern Ryukyu island Japan inferred from numerical simulation, Pure Appl. Geophys., 163, 41-54.

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Stevens, R.McCafrrey, C.Subarya. et al. Crustal motion in Indonesia from Global Positioning System measurements, Journal of Geophysical Research,108(B8) , doi:10.1029/2001JB000324, 2003.

13) Jose C. Borrero. (2005). Field Survey northern Sumatra and Banda Aceh, Indonesia and after the Tsunami and Earthquake of 26 December 2004, Department of Civil Engineering, University of Southern California, Los Angeles, CA 90089-2531, USA.

14) Eric L. Geist, Vasily V. Titov, Diego Arcas, Fred F. Pollitz, & Susan L. Bilek. (2007). Implications of the 26 December 2004 Sumatra-Andaman Earthquake on Tsunami Forecast and Assessment Models for Great Subduction-Zone Earthquakes, Bulletin of the Seismological Society of America, 97(1A), S249-S270.

15) Wikipedia, The Free Encyclopedia, "Tsunami Article".(2009).(http://id.wikipedia.org/wiki/Tsunami), accessed 27 October 2009.

Y. Bock, L. Prawirodirdjo, J. F. Genrich, C.W.

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