60 investigation of earthquake

2012 Abstract Based on the analysis of tectonic feature and geodynamic characteristics of regional faults systems in the southeast Asia, 9 source zones capable of generating tsu-namis af

O R I G I N A L P A P E R

Investigation of earthquake tsunami sources, capable

of affecting Vietnamese coast

Phuong Hong Nguyen•Que Cong Bui•Xuyen Dinh Nguyen

Received: 26 January 2011 / Accepted: 28 May 2012 / Published online: 23 June 2012

 Springer Science+Business Media B.V 2012

Abstract Based on the analysis of tectonic feature and geodynamic characteristics of regional faults systems in the southeast Asia, 9 source zones capable of generating tsu-namis affecting Vietnamese coast were delineated in the South China Sea and adjacent sea areas Statistical methods were applied to estimate the seismic hazard parameters for each source zone, which can be used for the detail tsunami hazard assessment in the future Maximum earthquake magnitude is predicted for the Manila Trench (8.3–8.7), the Sulu Sea (8.0–8.4), and the Selebes Sea source zones (8.1–8.5) Among the source zones, the Manila Trench, west of the Philippines is considered as a most potential tsunami source, affecting the Vietnamese coast The estimated Mmaxvalues were used to develop simple scenarios (with a point source assumption) to calculate the tsunami travel time from each source zone to the Vietnamese coast The results show that for the Manila Trench source zone, tsunami can hit the Vietnamese coast in 2 h at the earliest

Keywords Tsunami source zones South China Sea  Tsunami hazards 

Maximum earthquake magnitude

1 Introduction

The occurence of the Indian Ocean Tsunami on December 26, 2004 has marked a new turning point in tsunami science Many methods have been applied by scientists to assess tsunami hazards for different sea areas in the World The tsunami hazard assessment studies are implemented using two different approaches: deterministic and probabilistic Deterministic tsunami hazard studies involve hydrodynamic modeling of tsunami propa-gation, runup and inundation from a particular source and simulation of known destructive tsunami scenarios (Arcas and Titov 2006) Recent methodology of probabilistic assess-ment, developed by Thio et al (2006), allows us to display results in terms of tsunami

P Hong Nguyen ( &)  Q Cong Bui  X Dinh Nguyen

Earthquake Information and Tsunami Warning Center, Institute of Geophysics, Vietnam Academy

of Science and Technology, 18 Hoang Quoc Viet Street, Cau Giay District, Hanoi, Vietnam

e-mail: phuong.dongdat@gmail.com

DOI 10.1007/s11069-012-0240-3

hazard curves that plot tsunami annual rate of occurrence versus exceedance wave height and tsunami hazard maps that show the peak tsunami wave height that is exceeded in different periods of time along a study coast The method of probabilistic tsunami hazard analysis (PTHA) has been applied to many coastal areas of the countries bordering by the Pacific and Indian Oceans (Tadashi Anaka et al.2007; Burbridge et al.2008; Thio et al

2007

Up to now, most tsunami hazard assessment studies have been focused on the active tectonic sources at regional scale without paying attention to the tsunami sources of medium or local size When assessing tsunami hazard for the southeast Asia, Thio et al (2007) have based only on the mega subduction sources in the sea areas near Japan, the Philippines, south of Indonesia, and Malaysia, but bypassed the sources of medium and small size faults zones in the East Vietnam Sea Likewise, Burbridge et al (2008) paid attention only to the biggest source in terms of the Sumatra–Andaman mega thrust and subduction zone when assessing tsunami hazard for the west coast of Australia As the need for tsunami hazard analysis within each country is arising and becoming more and more urgent, it is necessary to carry out detail studies on the medium and small size active tectonic faults zones in the marginal sea areas, which can be the sources of the local tsunamis that sometime causing damages and losses

This study aims at investigating the tsunami sources, capable of affecting the coast of Vietnam On the basis of the analysis of tectonic feature and geodynamic characteristics of regional faults systems in the southeast Asia, the tsunami source zones were delineated in the East Vietnam Sea and adjacent sea areas Statistical methods were applied to estimate the seismic hazard parameters for each source zone, which can be used for detailed tsunami hazard assessment in the future And in the end, average tsunami travel time was calculated from the source zones to the Vietnamese coast, providing some preliminary vision for tsunami warning in Vietnam in the future

2 Main seismotectonic characteristics of the southeast Asia

The southeast Asia, in particular the South China Sea has diverse tectonic structures and complicate geodynamic development history On the one hand, this is a transition zone between the Eurasian and the Australian plates; on the other hand, it plays a role of a boundary separating the Pacific Ocean from Indian Ocean The lithosphere plates in the region move relative to each other, with the tendency to converge at the East Vietnam Sea: the Pacific plate moved westward, the Indian–Australian plate moved north–northwest-ward In the mean time, the Indian plate moved rapidly northward and collided with the Asia plate

The collision between plates have resulted in this region some main active structures as follows:

• A series of mega subduction zones: the Sunda Trench stretching over 8,000 km from the northwestern boundary of southeast Asia to the eastern part of Timor island (A, Fig.2); the Philippines Trench stretching over 3,000 km along the east coast of the Philippines archipelago (F, Fig.2) Some smaller subduction zones can be listed as the Manila Trench subduction zone (1,150 km, M, Fig.2), the subduction zone in the eastern boundary of Sulu Sea (over 650 km long, SN, Fig.2)

• A series of marginal seas, formed due to the back-arc spreading: the Celebes Sea (formed 54–42 Ma b.p.), the East Vietnam Sea (32–15.5 Ma b.p.), the Sulu Sea

(19–17 Ma b.p.), the Molucca sea (40–35 Ma b.p.), the Banda Sea (20–15 Ma b.p.), and the Makassar Sea (23–17 Ma b.p.), (Fig.1)

• Major strike-slip faults such as the Sagaing sub-longitudinal fault which is a dextral fault in the present period with average slip rate of 20 mm/year (1, Fig.2); the Red river northwest–southeast trending fault which is also a dextral fault in the present period with relatively low slip rate (5, Fig.2); the Sumatra northwest–southeast trending fault which has is a dextral fault with slip rate of from 15 mm/year to the southeast to 25 mm/year to North–northwest (2, Fig.2); the Philippine sub-longitu-dinal fault (on Philippines islands) which is a sinistral fault with slip rate in the middle section of 35 mm/year (20, Fig.2) Average slip rate of sub-plates inside the region relatively each to other is about 10 mm/year

• Paired occurrence of subduction and strike-slip fault zones: the pair of mega subduction zones and strike-slip fault with the same name of Sumatra; the pair of subduction and strike-slip fault zones with the same name of Philippine, the pair of Timor subduction zone (the Eastern section of the Sumatra subduction zone) and Sorong strike-slip fault Figure2shows principal tectonic structures in the southeast Asia As can be seen from this map, the Vietnamese coast has a specific situation as if it is ‘‘blocked’’ inside the East Vietnam Sea The ‘‘East Vietnam Sea’’ term hereafter indicates the sea area surrounded by Chinese continent from the north, by dense island systems of Thailand and Malaysia from the southwest, of Indonesia and Malaysia from the south, and by the Philippines archi-pelago from the east (see Fig.1) The Vietnamese coast will most probably be affected, and therefore, most considerably, by tsunamis generated from the sources inside the East

Fig 1 Seismotectonic map of the study area with the East Vietnam Sea and its adjoining seas labeled

Vietnam Sea It can hardly be affected by destructive tsunamis, originated in the central Pacific Ocean, from the Sea of Japan and East China Sea in northeast side and even from the mega subduction zones as the Sundaland and the Philippines, too In fact, the 2004 Sumatra tsunami caused no harm to the Vietnamese coast as it was blocked by the dense island arc of Indonesia and Malaysia

Up to now, the only tsunami data available in the Bac Bo Gulf, East Vietnam Sea is reported by Integrated Tsunami Database for the World Ocean (ITDB2005) This event is shown in Fig.3, recorded in January 5, 1992 and is described that it was caused by ‘‘a series of small earthquakes with the maximum being estimated at 3.7 and with a focal depth of 8–12 km occurred off the southwest coast of Hainan Island, China The tide rose abnormally beginning at 14:30 and continued until 17:00 Beijing time It was recorded by four gages around the island including at Beibu Bay with the maximum at Yulin Station near Sanya Port with a period of about 20 min and a maximum amplitude of 80 cm Fishing boats in various harbors were damaged by ramming, and many had broken anchor chains’’ (Lander et al.2003and Lin et al.1993) However, the fact that a 3.7 magnitude

Fig 2 Tectonic sketch-map of the southeast Asia (adapted from Tija 2008) 1 Terrain boundaries:

A Sunda Trench; B Banda Trench; C Jaya thrust; D Papua Trench; E Yap Trench; F Philippine Trench;

G Maluku East trough; H Maluku West trough; J Gorontalo Trench; K Cotabaco Trench; SN Sulu-Negros Trench; M Manila Trench; N East Luzon trough; R Ryukyu Trench; S-NW Sabah trough; 2 Regional Faults: 1 Sagang; 2 Sumatra; 2a-Bangkalis; 3 Three Pagodas; 4 Mae Ping; 5 Red river; 6 109 meridian; 7 Ranong; 8 Khlong Marui; 9 Peusangan; 10 Mentawai; 11 Axial Malay; 12 Sakala; 13 Adang/Paternoster; 14 Palu; 15 Matano; 16 Tarera-Aiduna; 17 Ransiki; 18 Sorong-Irian; 19 Gorontalo; 20 Philippine; 21 Ulugan;

22 Balabac; 23 West Baram line; 24 West Balingian line; 25 Sorol; 26-North of the East Vietnam Sea 3 Exotic terraines/Microcontinents: AS Andaman spreading centre; Pa Paracel; Ls Luconia Shoals; Ba Bacan; Si Silembu; Na Natal; MB Macclesfield Bank; ES East Sabah; Sr Seram; Nt Timor North; SK Sikuleh; Cal Calamian block; DG Dangerous grounds; Su Sula spur; TB Tukangbesi; Sb Sumba

earthquakes can generate a tsunami causes doubts from many people and makes this event seem to be unsubstantiable

In this paper, only tsunami sources capable of causing damage to the Vietnamese coast are considered In the East Vietnam Sea, the most potential tsunami sources are considered

to be: (1) the Manila subduction zone; (2) the West of the East Vietnam Sea faults zone; (3) the North of the East Vietnam Sea faults zone and (4) the Northwest Borneo-Palawan fault zones (Fig.4) Detail of these sources is discussed below

2.1 The Manila subduction zone

The zone consists of several trenches stretching along the west coast of the Philippines from latitude 20N to latitude 12N (marked as M in Fig.2) The Manila Trench forms the convergent margin between the Philippines sea plate and the Sunda plate from central Luzon to Taiwan Within this zone, many strong earthquakes have occurred, some of which has magnitude of 8.2 According to Bautista et al (2006), during period from 1589

to 2005, at least six tsunamigenic earthquakes have occurred in this zone, causing con-siderable losses of lives and properties These earthquakes are listed below:

• West Luzon offshore earthquake in 1677 (Ms 7.3, generating a tsunami with about 1 m run-up)

• Pasig River (Manila) earthquake in 1828 (Ms 6.6, generating a tsunami with about 1 m run-up)

Fig 3 Historical Tsunamis in the southeast Asian Region 1600’s—2006 The only historical tsunami recorded in the Bac Bo Gulf, East Vietnam Sea is caused by a series of 3.7 magnitude earthquakes and caused no casualties (ITDB 2005 )

• Agno earthquake in 1872 (Ms 6.8, generating a tsunami with about 1 m run-up)

• Agno earthquake in 1924 (Ms 7.0, generating a tsunami with about 1 m run-up)

• San Esteban earthquake in 1934 (Ms 7.6, generating a tsunami with about 1 m run-up)

• Iba—Palauig earthquake in 1999 (Ms 6.8, generating a tsunami with about 1.5 m run-up)

The run-up measurements given above are observed data taken from a number of existing references to historical tsunamis generated in the areas surrounding the Philippine Islands, including local chronicles, scientific reports, and partly from mareograph data Among the references, the most valuable are the bulletins of a seismological station, established as a part of the Manila Central Observatory in 1865 and operated continuously until World War II It is also known that during the interval from about 1900 until the start

of World War II, the seismological station was directed by two scientists: first by Miguel Saderra Maso and then by the W C Repetti, both of whom were familiar with tsunamis and their relationship to earthquakes, and who were alert for their occurrence (Wiegel and

Fig 4 A sketch map of tsunami source zones in the South China Sea, capable of affecting the Vietnamese coast (see Sect 3 for explanation) The numbers indicate the following sources zones: 1a Riukiu—Taiwan; 1b West Taiwan; 2a North Manila Trench; 2b Central Manila Trench; 2c South Manila Trench; 3 The Sulu Sea; 4 The Selebes Sea; 5 The South Banda Sea; 6a The North Banda Sea 1; 6b The North Banda Sea 2; 7 North of the East Vietnam Sea; 8 Northwest Borneo-Palawan; 9 West of the East Vietnam Sea

ASCE1980) Despite of the fact that some historical events listed above, particularly those occurred in the nineteenth century are considered ‘‘probable’’ or ‘‘doubtful’’ in the chronicle catalog of local tsunami of the Philippines (Nakamura 1978), this evidence proves the existence of the tsunamis generated in the Manila subduction zone

2.2 The West of East Vietnam Sea faults zone

This fault system, known under many different names, is often referred to in Vietnamese literature as ‘‘the 109 meridian deep-seated fault system’’ (numbered 6 in Fig.2) Orig-inated from the south Hainan island area, the major fault line goes down south, passing about 550 km along the Central Vietnam’s coast through the Tuy Hoa shear zone The less active part of the fault is extended down south in sub-longitudinal direction with a length

up to 700 km

The system consists of at least 3 main fault lines, which have a submeridianal direction and sink step by step into the sea These faults slope westwards, down under the Kontum massif, with an angle of 30–40 and depth of over 100 km Besides the major faults, there are many smaller ones, breaking through the Earth crust and appearing on the surface in the form of normal faulting, while in the lower part, they have a reverse form of faulting

This is a deep-seated fault zone, playing the role of boundary between the Indo-sinian geoblock and the East Vietnam Sea oceanic crust Geological evidence shows that its main activities have terminated in early Miocene, and during the present time, the fault becomes less active The Vung Tau earthquakes of 2005 might related with this fault zone

2.3 The North of East Vietnam Sea faults zone

This is a passive continental margin zone of Atlantic type, with a series of normal faults forming grabens and depressions extending in NE–SW or ENE–WSW direction The faults have lengths ranging from some hundreds to a thousand km and can generate medium earthquakes and tsunamis The most active faults are observed in the margin of the East Vietnam Sea oceanic crust

2.4 The Northwest Borneo—Palawan faults zone

This is a thrust fault zone, located in the Northwest of Sabah trough (S in Fig.2) The left part of the fault is considered to be active, while the segment along Palawan is considered

to be inactive Medium earthquakes have been observed along this fault zones, with maximum magnitude of 6.0

2.5 Other potential tsunami sources

Besides the mentioned tectonic elements within the East Vietnam Sea, some smaller subduction zones in the Sulu and Banda Seas can be considered as the tsunami sources, dangerous to the Vietnamese coast Among these, the Sulu-Negros Trench (SN in Fig.2) is

a short convergence zone along the western margin of the central Philippines, and the Cotabato Trench (K in Fig.2) is another short trench system, located along the south-western coast of Mindanao (Thio et al.2007)

3 Tsunami source zones, capable of affecting the Vietnamese coast

The tsunami source zones are defined on the seismotectonic basis In this paper, a tsunami source zone is defined along seismically active faults by summing all the possible rupture zones caused by maximum earthquakes, which might occur within a given zone In other words, this is the projection of tectonic fault plans counting from the deepest active layer to the sea’s surface However, while delineating a seismic source zone boundary, this prin-ciple is rather flexible and sometimes extended, depending on certain observed earthquake epicenter distributions, a set of faults or related volcanic arcs, particularly in cases of scattered earthquake data The acceptable boundary for a seismic source zone has to maintain all seismotectonic characteristics of the zone as a whole, namely the azimuthal location, direction of main geological structures, and cluster of earthquake epicenters The following tsunami source zones have been delineated in the East Vietnam Sea and adjacent sea areas:

1 The Taiwan Sea source zone;

2 The Manila Trench source zone;

3 The Sulu Sea source zone;

4 The Selebes Sea source zone;

5 The North Banda Sea source zone;

6 The South Banda Sea source zone;

7 The North of East Vietnam Sea source zone;

8 The Northwest Borneo-Palawan source zone;

9 The West of the East Vietnam Sea source zone;

The source zones were divided into sub-source zones and coded by numbers as shown

in Fig.4and Table1

Table 1 Earthquake subcatalogs

of the tsunami source zones in the

South China Sea (after

fore-shocks and afterfore-shocks removal)

Tsunami source zone Observation

period

Number of earthquakes

Observed

M max

1a Riukiu—Taiwan 1965–2008 89 7.2

2a North Manila Trench 1958–2006 36 8.2 2b Central Manila Trench 1872–2008 193 8.0 2c South Manila Trench 1974–1993 16 6.2

4 The Selebes Sea 1964–2007 139 8.0

5 The South Banda Sea 1998–2006 29 6.3 6a The North Banda Sea 1 1608–2008 156 7.6 6b The North Banda Sea 2 1966–2007 61 6.5

7 North of the East Vietnam Sea

8 Northwest Borneo-Palawan

9 West of the East Vietnam Sea

4 Statistical estimation of seismic hazard parameters

for the tsunami source zones

4.1 Earthquake data analysis

In this paper, the Gumbel’s Extreme Values method and the Maximum Likelihood method were used for estimating the seismic hazard parameters Detail description of the esti-mation methods can be found in Phuong (1991,1997,2001); Kijko (1984); Kijko et al (1987) The following parameters were estimated for each tsunami source zone:

• Expected maximum earthquake magnitude Mmax;

• Constants a, b in the Gutenberg–Richter magnitude-frequency relation and their deductive values k,b;

• Mean return period T(M) of the strong earthquakes with magnitude M

A catalog of 6,267 earthquakes, which consists of both historical and instrumental data

up to 2007 was compiled for the study area (Fig.3) Earthquake data were grouped for each source zone and the subcatalogs were treated for completeness and homogeneity Table1 lists the subcatalogs of all source zones after removal of foreshocks and after-shocks As the subcatalogs were obtained from the observational data, they are different for different zones, and in some cases, it was difficult to apply statistical methods because of lack or incompleteness of data For this reason, both mentioned methods were applied in order to obtain the best choice of the sought parameters

4.2 Estimation of seismic hazard parameters by the Extreme Values method

The extreme value theory was first applied to estimate the seismic hazard parameters for earthquake source zones in Vietnam in 1991 (Phuong 1991) The theory is formulated under the following assumptions (Gumbel1958):

1) The prevailing condition must be valid in the future;

2) The observed largest values are independent of each other

Let X be a random variable with a probability function of F(x):

FðxÞ ¼ PfX  xg The probability that x will be the largest among n independent samples from the same distribution F(x) will be:

G Xð Þ ¼ P X1  x; X2  x; ; Xn  xf g ¼ Fnð Þ;x which is the exact distribution function of the largest value When the random character of the earthquake occurrence is taken into account, it is possible to consider the largest annual earthquake magnitudes in a given time period as a random series with distribution G(x)

In most cases, the initial distribution function F(x) is not known It is then necessary to deal with the asymptotic forms of distributions introduced by Gumbel, who considered three asymptotic distributions of extreme values

The first asymptotic distribution of the largest values is of the form:

where b1and u are the distribution parameters to be determined and b1[ 0 If we sub-stitute lna1= b1u and take twice the logarithm of both sides of (IV.1), we obtain:

ln½lnG1 xð Þ ¼ lna1 lnb1x ð2Þ The second asymptotic distribution of the largest values is of the form:

G2ðxÞ ¼ exp  un e

x e

where e is the lower limit of largest values, b2 is the shape parameter, and un is the characteristic largest value

The third asymptotic distribution of the largest values is of the form:

G3ðxÞ ¼ exp  x x

x u

where x is the upper limit of the largest value x, and b3 and u are the distribution parameters to be determined

In the first asymthotic distribution, the variate is unlimited in both directions In the second asymthotic distribution, a lower limit for the variable X exists As for assessment of seismicity of a region, only upper threshold of earthquake magnitude is interested, the second asymthotic distribution is therefore ruled out in our case In addition, as there exists

an upper limit in the third asymthotic distribution of the largest values, for the occurrence

of maximum earthquake magnitudes, the third asymthotic distribution naturally has a better physical meaning for a probabilistic model than either the first or the second distribution (Yegulalp and Kuo1974)

If we substitute a3¼ x  uð Þb3 and take twice the logarithm of both sides of (4), we have:

The estimation of the parameters ai, bi, i = 1,3 is obtained by least square method using (2) and (5) The subcatalogs listed in Table1were used for least squares fit procedure In this study, only the third asymptotic distribution was applied to parameter estimation, and the obtained x values correspond to the sought maximum earthquake magnitudes of the seismic source zones

4.3 Estimation of seismic hazard parameters by the maximum likelihood method 4.3.1 Extreme magnitude distribution applied to the macroseismic part of the catalog The available earthquake catalogs usually contain two types of information: macroseismic observation of major seismic events that occurred over a period of a few 100 years and complete instrumental data for relatively short periods of time Let us assume the Poisson occurence of earthquakes with the activity rate k and the doubly truncated Gutenberg– Richter distribution F(x) of earthquake magnitude x The doubly truncated exponential distribution can be represented by:

F xð Þ ¼ P X  xð Þ ¼A1 AðxÞ

A1 A2

where A1¼ exp bMð minÞ; A2¼ exp bMð maxÞ; Ax¼ exp bxð Þ; Mmax is the maximum regional magnitude value, Mminis the threshold magnitude and b is a parameter The above assumption implies that earthquakes of magnitudes greater than x can be represented by a