Earthquake Disasters of 13 January and 13 February ...

6 downloads 0 Views 13MB Size Report
Feb 13, 2001 - bounded by the El Desague River to the northwest and by. San Vicente Volcano ..... Calle La Reforma #169, Colonia San Benito, San Salvador.
Earthquake Disasters of 13 January and 13 February 2001, El Salvador Walter Salazar Tokyo Institute of Technology and Universidad Centroamericana Jos6 Sime6n Cafias, El Salvador

Kazuoh Seo Tokyo Institute of Technology

INTRODUCTION El Salvador is a small country in Central America (Figure 1). It has a savanna climate, with daytime temperatures consistently between 25~176 The rainy season extends from May to October, while November to April is generally very dry. Rainfall is about 1,800 mm per year. These weather conditions are favorable for producing sugar and coffee. The population is about 6.0 million with a high concentration in the capital city, San Salvador, where 1.8 million persons are living now. The earthquakes of 13 January and 13 February 2001 demonstrated the high vulnerability in E1 Salvador and strongly affected people from all walks of life. A good set of strong-motion recordings and macroseismic data has been collected and analyzed. In this report we summarize the most important features from these earthquakes.

SEISMICITY AND TECTONICENVIRONMENT On 13 January 2001 an earthquake with a magnitude of 7.8 (M s) and depth of 60 km occurred offshore the El Salvador Pacific coast. This event was associated with the subduction process of the Cocos Plate submerging beneath the continental Caribbean Plate (Figure 2), reaching depths of 300 km below El Salvador (Burbach et al., 1984). This earthquake was the largest in the region since 1899 (see Table 1 and Figure 3). The event ruptured an intraplate normal fault in the Cocos Plate (USGS, 2001). According to the Japanese Society of Civil Engineers (JSCE, 2001) the rupture for 13 January event propagated from east to west, with a fault plane dipping to the northeast (azimuth 306 ~ dip 48 ~ slip-107~ On 13 February another shock occurred with a magnitude of 6.5 (M s) and depth of 7 km, located to the east of Ilopango Lake (Figure 3). According to the USGS (2001) it ruptured on a right-lateral strike-slip fault. Teleseismic analysis revealed a large asperity at the western end of the fault plane of this earthquake (JSCE, 2001), which was characterized by bilateral rupture propagation. The orientation of the fault according to the focal mechanism (azimuth 94 ~ dip 79 ~ slip -179 ~ coincides with that drawn on the geological map prepared by

a German mission (Weber et al., 1978), which shows the fault bounded by the El Desague River to the northwest and by San Vicente Volcano to the southeast. The aftershock distribution also confirms the direction of rupture of the 13 February event (Figure 4). The 13 February event is associated with the uppercrustal seismicity concentrated in the upper 25 km of the continental crust along the active Quaternary volcanoes. In the region they are called "volcanic chain earthquakes" or upper-crustal events because their geographical locations coincide with the belt of volcanoes, but they do not necessarily occur in conjunction with volcanic eruptions. A burst of seismic activity occurred in the volcanic chain region before and after the 13 January subduction event, especially around the epicentral area of the 13 February earthquake (Figure 4). It is interesting to note that after all large subduction earthquakes in the 20th and 21st centuries, destructive volcanic chain events have always occurred within less than four years (Table 1). The possible interaction between them has been analyzed recently by Benito et al. (2002). While subduction zone earthquakes can reach a maximum magnitude of about 8.0, in the volcanic chain the magnitudes are moderate, reaching a maximum value of about 6.7 through historic times. Despite the moderate magnitude of volcanic chain events they have caused a lot of damage, because they have very shallow depths and coincide with the main population centers in E1 Salvador.

DAMAGEDISTRIBUTION According to the Ministry of Economy, Emergency National Committee (COEN, 2001), and Ministry of Foreign Affairs in El Salvador, the 2001 earthquakes together caused an economic loss of US $1.66 billion. About 150,000 houses collapsed and 1,259 casualties were reported. About 1.6 million people were left homeless, which represents 25% of the total population of El Salvador (see Table 2). In many cases, people's houses are the bases for small enterprises that provide family income. It is estimated that the earthquakes affected about 41,400 such businesses.

420 SeismologicalResearchLetters Volume74, Number4 July/August2003

TABLE 1 Seismological Parameters for Destructive Earthquakes in El Salvador since 1899 Date

Time (UTC)

Lat. (N)

Long. (W)

Depth (kin)

Ms

Source

1

25/03/1899

17:~

13.65~

88.80~

10

5.3

VC

2

19/07/1912

13.87~

89.57~

10

5.9

VC

3

07/09/1915

01:20

13.90~

89.60~

60

7.7

S

4

08/06/1917

00:51

13.82~

89.31 o

10

6.7

VC

5

08/06/1917

02:54

13.77~

89.50~

10

5.4

VC

6

28/04/1919

06:45

13.69~

89.19~

10

5.9

VC

7

21/05/1932

10:10

12.80~

88.00~

150

7.1

S

8

20/12/1936

02:43

13.72~

88.93~

10

6.1

VC

9

25/12/1937

23:50

13.93~

89.78~

10

5.9

VC

0

06/05/1951

23:03

13.52~

88.40~

10

5.9

VC

1

06/05/1951

23:08

13.52~

88.40~

10

6.0

VC

2

07/05/1951

20:22

13.48~

88.45~

10

5.5

VC

3

03/05/1965

10:01

13.70~

89.17~

15

6.3

VC

4

19/06/1982

06:21

13.30~

89.40~

80

7.3

S

5

10/10/1986

17:49

13.67~

89.18~

10

5.4

VC

6

13/01/2001

17:33

13.05o

88.66~

60

7.8

S

7

13/02/2001

14:22

13.62~

88.85~

7

6.5

VC

VC: volcanic chain; S: subduction (after Salazar et al., 1997). The parameters for the 13 January and 13 February 2001 events are taken from USGS (2001) and Centro de Investigaciones Geot~cnicas of El Salvador (CIG), respectively.

Mexico ~

i"

+

Caribbean Sea

16.0" ยง

14.0~ +

12.0o+

I

PacificOcean 91.0"

89.0~

~

f

0

~~--,~La.Paz ~ /.]~n ~Sl,nMiguel~Laun6s~, ~

25 km ~

~

87.0~

,& Figure 1. Geographic location of El Salvador within the Central America isthmus and its political division (departments). IL: Ilopango Lake; CL: Coatepeque Lake; LR: Lempa River.

Seismological Research Letters July/August2003

Volume 74, Number4

421

Volcanic chain Distance (km)

Trench 100 I

200,~

I

300

_e

I

I

~.o .,,. o o ~ " ~ . o ~. . . . " ~ . " ." . " " " c,-anooean D ~ o o 9 oo o o ~) o :9; 9 9. ~ ,_~--__~o,...., o oo o 9"o ~ :,.,,, ; ~ " t ' o ~ oo o 0.s 0""

I

-

o

99

oer~,~ ,,~

1OO

.

o

9

.

.

_ o-y Uo IL 9 o" \

o

oo

\o

9

o

o

r

o

9

o~

v

~o 9

OOo~

c'~ 200

oO

o\~

0:\Oo

121 o

300

.

\

9

E

.

9

o

"~Ja n. 13th M=7.8

\~

"~ Feb. 13th M=6.5 ,A, Figure 2. Cross-section illustrating the hypocenters between 1899-2001 in El Salvador for subduction and upper-crustal events (updated from the earthquake catalog of Salazar etaL,1997). The solid line indicates the upper part of the Cocos Plate (after Burbach etal., 1984).

A Figure 3. Epicenters of destructive earthquakes since 1899. IL: Ilopango Lake; CL: Coatepeque Lake; LR Lempa River. Seismological origin parameters are listed in Table 1. 422

Seismological Research Letters

Volume74, Number4

July/August2003

9

~176

~ o

o

9

oO~

o

~

0

.

.

.

o

~ I

'~;'.

~

o

~~

"% ~0

"

0.0

,

~

o

~

~

0~

~..:~.

,o o

0,,

, ,

~

.

~ ~

0

. .

.:,

"

o

. :..-.~...... ,.

-F

~

~

...

~ 0

~

~

o0

.~

9

, 9

,

-.

~176

0 ~

0:,0.~r ,,.~ ~ ~ ' ~

o::". ~ :~o~,;:,,: ,oo

~

!~176

9. o,

9

~ .

.~

~176

'

.

~ o

~

0:

io~ .

9

~ o

o .

~

~176 :

~ ..-.~ ~ :'.~ ~ ~'0,~f,

.

9

o

~176

~

~ ~

0

~

~,

.

-t-

.,0~

~~176 0"?

o .

~.:..:

" " " o:~176 ..

,

~176

~176176176176176176176

"o~~

o

~

0,+. .... 0 ~

~

,,,_~,,,.,~.,~

" ~ "": ~ ~~ -~-,1 7 6~ 1 7-6 "

.

~

~176o0:~ . 0 -

0~ ~1760:~

,'~,,_~,,~176176176 " ~ 1 7 6 > "' ~ ' ~ r ~ ~ ~ 1 7 6 ~ , o , ~ 1 7 6 1 7 6 1 7 6

. . . .

.

" . ~ 0~

9 9

", 0,o. ~ 0 ~ , ~ " o~ , ' ~ ; ~0 0",~

~176176 ~

~

o

___~'.~-',.,. ~ 1 7 6 1 7 6 1 7 6" ,~

~

~

~

~o

. o,~

~

~~

+

t-~:

"- ~o...=..~,..

"~ ~

e0.

o.

":."f-':'

9

~o . . . .

. . . . . .

~

'

~ ,~~ 00~ , . . ~ ~

o

~ .~.~.~176

". ~176176176176176 "

_~'~

o ~

o

~176

~ ~ ~ .. . o "

: :

Oo

~176 ~

"~176 90 .

;~

"0~ . . . . ,o

~ ~ ~ 1 7 6 1 7 69

~

"~ ~

o

.

0

~ ~

9

o~O

o"

o

Fromojan.~176 1o999 t~Jan.~176

~176 ~

+ ~ '~ :i~~ ~:~/~~176176 a9 ~ ~ o~176 :o~." ~176 ~1761761769

90 ~

o

o

1

~176

~ ~ + ~176 o

o

0 o

+

o

,o

~176 ~

o

.~.'o

o

o

. o

o

From Jan. 13th to Feb. 12th 2001 ~

o

+

+

89 ~

90 ~

~

~,

~176

88 o

o

Oo

.t

'1

-.

+

o o

o

Oo~ o

o

o

9

..

.

~

o ~o~

.. o

o %

o

~176176~oo~

9

.~

o o

o o

oo

0"

0

o.

o ~,

o

o

o

From Feb. l~

o

to Dec. 3,1st 2001 OOo

o

o

o

~176

o

o

90 ~

89 ~176

88~

A Figure 4. Epicentral maps (magnitude above 1.0) for different periods of observation. Solid circles depth _ 25 km open circles: depth > 25 km. Based on the database compiled by the Ministry of Environment and Natural Resources of El Salvador. Stars represent the epicenters of the mainshocks of 13 January and 13 February. VC: volcanic chain. Seismological Research Letters July/August2003

Volume74, Number4

423

TABLE2 General Damage Statistics 13 January 2001 Department

Casualties

Collapsed houses

13 February 2001 Homeless

La Libertad 147,708 15,723 685 La Paz 44 17,996 232,135 Sonsonate 101,487 48 10,501 SantaAna 47 112,561 4,823 Cuscatl~n 20 4,282 38,119 Usulutdn 27 29,293 356,391 San Salvador 24 10,372 107,083 San Miguel 2,902 76,665 19 San Vicente 29 5,218 103,086 La Uni6n 1 268 13,094 Ahuachap~n 71,086 0 6,553 2,997 Caba~as 0 309 Moraz~in 498 0 5 Chalatenango 1,250 16 0 Totals 944 108,261 1,364,160 After COEN(2001). See Figure 1 for the political divisions of El Salvador(departments).

Adabe and baharequehouses are the most typical constructions in H Salvador. They are made mainly of mud-clay. Bahareque construction includes bamboo or wood reinforcement in the walls. Such buildings were severely damaged over a wide region. Mixed construction methods using dry clay bricks with small concrete flames and masonry structures of concrete blocks are also very popular. Damage to these types of buildings was not as severe as to adobeand baharequestructures. The damage ratio (collapsed/total houses) is illustrated in Figure 5 for each municipality. The statistics for damaged houses were provided by DIGESTYC (2001). The total inventory of existing houses was taken from the dwelling statistics of 1992 from the same institution. These data were collected after the earthquake of 13 February, and it is difficult to differentiate the damage associated with each event. The nighttime satellite image analysis just after the 13 January event revealed that La Libertad, La Paz, Sonsontate, Usulut~in, and San Salvador suffered severe damage from this event (JSCE, 2001). Centro de Investigaciones Geot&nicas (CIG) of El Salvador provided us the original information they used to make the preliminary isoseismal maps that were published on the Internet for the two events (http://www.snet.g0b.sv/; see Figure 5). Telephone calls were made in the first three days after the mainshocks to local governments, churches, hospitals, local offices of national police, health centers, etc. to assess the preliminary damage distribution throughout the country. We found that moderate damage occurred during the 13 January event, even in the epicentral area of the 13 February

Casualties 0 58 0 0 165 0 4 0 87 0 0 0 1 0 315

Collapsed houses 0 20,399 0 0 15,467 20 0 19 8,425 0 0 420 0 0 44,750

Homeless 0 94,496 0 0 109,836 0 1,370 230 66,443 0 0 2,638 0 0 275,013

shock. In many cases the second event caused the total collapse of houses previously damaged in the municipalities of San Vicente, Verapaz, Guadalupe, TepetMn (Department of San Vicente), Cojutepeque (Cuscatkin), and Jerusaldn (La Paz). Heavy damage was observed to the west and the east of the epicenter for the 13 February event. The damage distribution can be compared with the seismic zonation, which is included in the seismic code for E1 Salvador. Zone 1 is considered to have higher seismic hazard than Zone 2 (Figure 5). The zonation appears reasonable with the damage induced by the earthquakes of 2001. We visited some of the damaged areas from 28 January to 2 February to investigate the pattern of damage. Here we summarize a few of our observations. Comasagua in La Libertad is a town on a hilltop near Santa Tecla. Houses in the town were heavily damaged (see Figure 6) or totally collapsed. Moreover, the landslides along access roads in the mountain area left the town isolated. Usually the town is only a few minutes away from Santa Tecla by car, but it took more than two hours to get there after the earthquake. Some of the people remained in the town preparing temporary houses, and others evacuated to a refuge at Las Delicias stadium, which the army prepared in Santa Tecla. We visited Santiago de Marfa in Usulut~in (Figure 7) and it was clear that in this location, bahareque structures performed much better than adobe structures. A strong-motion record was obtained with a PGA of 0.8 g on the horizontal

424 SeismologicalResearchLetters Volume74, Number4 July/August2003

,& Figure 5. Damage ratio (collapse/total houses) caused by both events (13 January and 13 February)in each municipality. Based on the database of DIGESTYC (2001). The epicenter solutions and isoseismal maps of VII MM after CIG (2001). Projections of rupture faults in the surface after JSCE (2001). Zone 1 is considered to have a higher seismic hazard than Zone 2 in El Salvador's seismic code. IL: Ilopango Lake.

,& Figure 6. Damaged house in Comasagua (La Libertad) with reinforced concrete frames and reinforced brick walls. Seismological Research Letters July/August2003

Volume74, Number4

425

,A, Figure 7. A typical baharequestructure in Santiago de Maria (Usulut~.n). Baharequestructures performed much better than adobestructures during the strong shaking. and 0.43 g on the vertical component. This town is also located on the top of a hill. We also observed landslides throughout wide areas of the country. The landslide distribution is superposed with a simplified geological map (Weber et al., 1978) for the country (Figure 8). We note the landslides occurred in the youngest geological formations and that they occurred inside Zone 1. The youngest pyroclastic flows belonging to the San Salvador formation were deposited in successive eruptions from Ilopango Lake. This volcanic ash is called Tierra Blanca (white soil) and has been dated to about A.D. 260. These soils are especially sensitive to erosion by surface water (SchmidtThomd, 1975). Our investigations confirm that substantial erosion is taking place continuously in the country (Figures 9 and 12). The worst destruction was found in Santa Tecla, La Libertad, where one landslide occurred in a residential area on 13 January and killed about 500 people (Figure 10). The landslide, which took place just after the earthquake, was about 100 m in height, 100 m or more in width, and 250-300 m in length. Presumably the landslide began at the top of the hill and people did not have enough time to escape. There were many landslides of similar scale in this region, but only one among them caused heavy damage. Only 500 m away from

426

Seismological Research Letters Volume74, Number4

the landslide a strong-motion record was obtained with a PGA of 0.5 g on the horizontal and 0.25 g on the vertical component. Another large landslide was found along the Pan-American Highway in northern San Vicente (Figure 11); it shut down the most important traffic route in the country for about eleven months. Rapid development around San Salvador proceeded without consideration for such natural disasters. OPAMSS (Oficina de Planificaci6n del Area Metropolitana de San Salvador) has been concerned about the rapid development (Figure 12) and has been trying to incorporate seismic microzonation in the city. Heavy rains have also produced big disasters in the past. For example, in September 1982 four days of intensive rain triggered a mudflow from the slopes of San Salvador Volcano, killing around 500 people in a residential area. Many people live in ramshackle houses in ravines where landslides occur commonly during both earthquakes and heavy rains. We visited some reinforced concrete buildings in downtown San Salvador. The quality of these constructions appeared good, but they revealed slight damage on outer nonstructural walls and some joints. According to statistics presented by Flores (1996), 77% of the buildings that were severely damaged by the destructive volcanic chain earth-

July/August2003

Ge ol ogi cal form atio n s -- From youngest to o/dest

,~,~:,:--,-:~'~Sedimentary '":~'~ deposits San Salvador-it includes Tierm Blencd ! I Cuscatlan ii Balsamo i:i:i:i: Morazan-Ch alaten ango and Metapan

Las Colinas Landslide

(Figure 10)i: ,.. ... ..

14.0 ~ + ~

0

25 km

Pan-American

.. ..~..... ..,...

Highway Landslide

(Figure 11) 13.0 ~ +

-I-

+

90.0 ~

89.0 ~

J

88.0 ~

iliiii!84184 ....

Erosion in Sant~ Cruz Analquito l,Figure 9) ......

....ili ~ii i........ i l; ........~ii i i ~ ,.................... ~,~

~

i~ii ..... ,~,iiii,:~iiii!~ii~;i,ji~i,,i~:,iii!: ~ '":. ....

,,,,-::~

:;iiiii............... ii!~ ~!i..li.i.i.i{.i{.i.::::i..i.::~, ..~,,~,:::::::t iii:~':i

....

.^. .^ :-:^.^

..............

.....

:i~:~:,,,!iiiiiiiiii, .....,iiiiiili:~:~ ~

:,iii,li'~ "~iii~ii~ili,i}iliiiii~i!i,,~ ~,:....::

San Vir ......

:::::::::::::: i ::i

...............

:,,~;i~i~;,/.....

,~,:,

o'~:'~"~i ~'-~'-'~ i;iiil

~:~ ....:i!,~!i,i,:,:!:~,ji~::i~i~i,~........ ii~i~i,~i;: ~ i;~i~i....i~,i:'i :;~:,:,i~i~,:~ i i~'~': ii~ii~!i...... iiii~

............. ....

,

~ii~.......................... ~,i~i'i~........... i . . _

:~:~,:.

N

i

.

i i

:

':~ii~:' :i ~: ii !i

%iiii!iiiiiiiiiiiiiiiiiiiiiii!iiiiiiiiiiiiiiiiiiiili!:!i!iiiii. ~iiiii!iiiii!i:i!!i!iiiii!~,:

~!~i~i~i~i~i~i~i~i~i~;~i~i~i~i~=,!~=:!='~~`!=~=' ~:~``~:~~' ~' :~:~ i i i!i i i i !i i i ~'

: .......!ii~i i!"

~ii~il~i~i~ii,i~i~,i~:~:i:i~i!i!i!i~i:~

ii ,& Figure 8. Landslide distribution (solid circles) and main geological formations in El Salvador. Top: 13 January earthquake. Bottom: 13 February earthquake (square in the top figure). Database compiled by Ministry of Environmentand Natural Resources of El Salvador. IL: Ilopango Lake.

Seismological Research Letters July/August2003

Volume 74, Number4

427

A Figure 9. Large-scale erosion in the volcanic ash deposits around Santa Cruz Analquito. The photo was taken from west to east. Erosion can be seen in Figure 12 for a wide region. See location in Figure 8.

quake of 10 October 1986 in the downtown area have not received any repairs yet, and some of them are still in use. We confirmed this situation in our survey. A building damage evaluation was made by the Ministry of Public Works (MOP) and the National Association of Engineers and Architects of El Salvador (ASIA) in the metropolitan region of San Salvador. They classified the damage using four flag categories: green (no damage), yellow (moderate damage), orange (severe damage), and red (to be demolished or already collapsed). Table 3 lists the buildings classified as red. In the boundaries of the Lempa River (see location in Figure 3), liquefaction was observed over a wide area. A very old rail bridge suffered heavy damage due to settlement in the foundations (Prof. Jon Cortina UCA, personal communication, 2001).

428

Seismological Research Letters Volume 74, Number4

,A Figure 10. Landslide in the Residencial Las Colinas in Santa Tecla. Picture was taken two weeks after the earthquake of 13 January, when the rescue operations and the demolition had already concluded. See location in Figure 8.

STRONG MOTION Strong-motion data were provided by Universidad Centroamericana UCA (Bommer et al., 1997; UCA, 2001), Geotdrmica Salvadorefia GESAL, Centro de Investigaciones Geotdcnicas CIG, USGS (2001), and Instituto Nicaraguense de Estudios Territoriales INETER (Figure 13). A total of 36 records for 13 January and 21 records for the 13 February event was analyzed. The analog records were corrected with a baseline correction and band-pass filtered between 0.20-25 Hz. The records from digital instruments were corrected with a band-pass filter of 0.05-50 Hz. In Table 4 and Table 5 the peak values for acceleration and velocity are shown for the three components of motion. In Figures 14 and 15 the time history acceleration for the north-south component is presented. The stations are arranged according to the age of the geological formations underlying the stations (youngest to oldest) and distance from the surface rupture projection.

July/August2003

A Figure 11. Large-scale landslide affecting an important section of the Pan-American Highway in northern San Vicente at Curva La Leona. The landslide shut down traffic for about eleven months. See location in Figure 8.

A Figure 12. Rapid urban development has taken place on the top of the hills to the east of Ilopango Lake. Seismological Research Letters July/August2003

Volume 74, Number4

429

TABLE 3 Buildings Classified with Red Flag (to be demolished or already collapsed) in the Metropolitan Region of San Salvador due to the 13 January 2001 Earthquake Building Name

Address

Colegio InmaculadaConcepci6n Archivo del Viceministerio de Vivienda y Desarrollo Urbano

la. Calle Ponientey 2a. Ave. Nte, SantaTecla Km. 51/2, Carreteraa SantaTecla, Complejo la Lechuza,en instalaciones DUA, frente al Estado Mayor Direcci6n de Urbanismo y Arquitectura (DUA), Oficinas Administrativas, Km. 5 1/2, CarreteraSantaTecla, Complejo La Lechuza,frente ai Estado Departamentode Servicios Generales Mayor Condominio REGIS,Edificio A, B, C y D Barrio Candelaria,San Salvador Hospital Rosales(Tapial) 25 Av. Norte, entre la. Calle Ponientey Ave. Roosevelt Hospital San Rafael (construcci6n antigua) Nueva San Salvador, La Libertad Colegio Fdtima 4a. Calle Orientey 7a. Avenida Sur, NuevaSan Salvador, La Libertad Muro de Parque Residencial Moriah Parque Residencial Moriah, San Salvador Boulevard Tutunichapa,Edificio A2 Centro Judicial Isidro Men6ndez Calle Marconi y Avenida Los Diplom~ticos, Barrio San Jacinto, San Escuela de Educaci6n Especial (Tapial) Salvador Colegio Bautista (Tapial) Avenida Cuba y Avenidael Cocal, Barrio San Jacinto, San Salvador Colegio Internacional (Tapial) Calle La Reforma#169, Colonia San Benito, San Salvador Zona FrancaSan Bartolo Leader Garmentsde El Salvador (Pared Norte 2o. nivel) 12 Avenida Norte Sur No. 2-12, NuevaSan Salvador Kiosko de Pinturas Protecto Colegio Sagrado Coraz6n (Cuerpo 1) 13 Calle Poniente,Calle El Mirador, contiguo a Hotel El Salvador,Colonia Escal6n, San Salvador Colegio San Francisco (Cuerpo 1) Calle Circunvalaci6n, final PasajeNo.2, Colonia Lomas de San Francisco, San Salvador Escuela ParroquiaSantiagoAculhuaca (Centro de Oraci6n Calle Morazdn No. 26, Ciudad Delgado 25 Avenida Norte Peluqueria Universitaria Colegio Santa In6s (Capilla, internado de novicias y bodega) 3a. Ave Sur y 4a. Calle Poniente, SantaTecla Iglesia Santa Catarinade Apopa Iglesia Santa Catarina Complejo Parroquial Santiago Texacuangos(Escuelae Iglesia) Plaza Central SantiagoTexacuangosfrente a la Alcaldfa 23 Av. Sur y Caile Arce COLATINO(Bodega) Av. Manuel Gallardo 1-3 SantaTecla. NuevaSan Salvador Local PNC Hotel Cuscatldn(Pared en Sector de Gradas) Calle Concepci6n#675. San Salvador 75 Av. Norte y Calle Colonia San Miguel. San Ram6n, Mejicanos Condominio Montecristo Sobre Boulevard del Ejercito Unidad Medica de Ilopango (Pared de casetasubestaci6n el6ctrica) Edificio B (M6dulo 4 y 5) Centro Urbano5 de Noviembre.20 Av. Nortey Calle 5 de Noviembre,San Salvador Centro Educativo Denver(Area Primaria y Parvularia) Lomas de San Francisco. Calle a Huizucar#85 San Salvador Carretera Panamericana,Ilopango Cdrcel de Mujeres Instituto Polit6cnico Nazareth Alameda Roosevelt#2818. Media cuadra al Ponientede la Universidad Francisco Gavidia, San Salvador Unidad de Salud Habitat- Confie CarreteraTroncal del Norte, Km. 6.5 frente a Urbanizaci6nBrisas del Norte. Ciudad Delgado Residencial Alpes Suizos (Casa Comunal, talud pasaje Hannover) Final Calle Real. Colonia Quezaltepeque.NuevaSan Salvador. Casa Comunal, casa45 poligono A-8. Source of information: MOP-ASIA-FESIARA(2001)

430 SeismologicalResearchLetters Volume74, Number4 July/August2003

Guatema /

~

Honduras

f'~f

14.

UAR

ST/- A. 'w ~ -

- AQC

/)

CA ~ P / ' N ' ~ S T R B E R ~ I ~

A MG ~ ~

l

~ Pacific 12.0 ~ +

Nicaragua

ABOAN

LEAN~ ~

JUIN

Ocean

+

+ 89.0 ~

"~

DCAN _

+

~

GPJ

AVS (VF)t ~\/ ~ TroncaldelNorte//

9 DB

87.0 ~

N . k.,

13"44' +

Boulevarddel

ffl

"1 l-

OB

Epicenterof October10th1986earthquake

UC

0 13039 ' + 89"16'

1 +

2 km +

+

+

+ " ~

+ 89o09 '

,& Fioure 13. Locationof the strong-motionstations (blacktriangles)that recordedthe 13 Januaryand 13 Februaryearthquakes.Top: CentralAmerica;bottom: San SalvadorCity; lines indicatethe main streetsafter 0PAMSS(2002). Opentriangles representthe stationsthat recordedthe 10 October1986 earthquake in San Salvador.SitesVS, RS, and SS composea vertical array of accelerographs(i.e.,RF indicatesan accelerographin the bottom of the borehole).

Seismological ResearchLetters July/August2003 Volume74, Number4 431

TABLE4 Peak Ground Acceleration and Velocity for the 13 January Earthquake Distance (km)

N-S

Vertical

E-W

Station Owner

Epi

Hyp

Rup

Acc (cm/s2)

Vel (cm/s)

Acc (cm/s2)

Vel (cm/s)

Acc (cm/s2)

Vel (cm/s)

ULLB

UCA

74

81

40

1138.2

55.3

621.5

16.6

565.4

36.0

QC

CIG

90

96

73

142.6

23.8

115.8

6.5

181.5

12.8

A

CM

CIG

165

168

131

12.8

1.8

--

13.6

1.9

A

OB

CIG

89

94

55

407.3

35.0

288.4

9.6

380.8

25.0

S

HSRF

UCA

90

96

56

489.3

57.1

242.0

18.1

477.3

33.8

S

CSBR

UCA

89

95

56

154.5

25.2

163.3

14.7

195.1

31.4

S

RS

CIG

90

96

56

313.8

25.6

323.2

12.8

309.0

22.2

S

RF

CIG

90

96

56

215.9

16.4

194.7

11.0

213.0

15.6

S

ST

CIG

92

97

57

616.1

48.3

467.5

19.8

705.4

42.3

S

ESJO

UCA

92

97

58

295.6

25.4

149.7

12.5

273.0

17.2

S

SS

CIG

92

98

59

273.9

17.9

175.8

10.4

256.1

20.3

S

DB

CIG

93

98

60

226.5

25.0

152.0

8.4

235.7

19.8

S

VS

CIG

95

100

61

272.9

38.7

215.3

10.5

294.0

19.0

S

HSGT*

UCA

83

89

61

~

~

~

SM

CIG

83

89

67

798.04

26.9

421.7

16.0

717.0

Geology A

S 40.9

S

PGB

GESAL

83

89

67

450.6

22.3

234.8

16.1

363.5

24.3

S

UARM

UCA

109

113

74

593.3

49.7

219.3

19.8

446.2

53.2

S

MG

CIG

105

109

90

118.7

10.9

89.4

4.8

124.7

11.4

S

SA

CIG

135

139

102

141.4

18.3

53.0

4.8

83.3

9.8

S

AH

CIG

147

151

114

192.9

20.6

114.9

8.1

144.3

13.6

S

UPAN

UCA

81

87

47

175.2

9.5

86.7

8.4

153.2

9.2

C

USPN

UCA

76

83

49

567.8

37.4

432.3

18.7

485.6

27.0

C

UTON

UCA

97

102

64

231.1

23.3

202.3

9.9

268.3

23.5

C

CU

CIG

133

137

120

81.4

10.6

60.7

4.0

84.8

8.9

C

HSTR

UCA

68

75

44

257.3

12.3

251.1

11.6

302.4

22.5

B

CA

CIG

118

122

87

119.3

17.8

47.3

4.1

101.0

19.6

B

SE

CIG

112

117

90

86.5

7.9

54.7

4.6

63.6

8.9

B

CHAN

INETER

200

203

195

89.4

6.4

41.0

2.2

67.8

4.8

LEAN

INETER

232

234

227

39.0

2.4

25.2

1.4

36.7

2.6

ESAN

INETER

283

285

273

13.8

2.4

9.1

1.0

10.4

2.3

RAAN

INETER

300

302

295

56.0

3.8

21.4

1.5

44.7

3.9

DCAN

INETER

306

308

303

44.8

3.1

27.4

1.8

43.7

3.3

MGA

INETER

307

309

304

33.1

2.6

13.8

1.1

42.3

2.7

JIAN

INETER

322

324

314

6.3

0.8

3.9

0.5

5.3

0.9

GRAN

INETER

342

344

339

8.7

1.8

5.6

0.9

9.0

1.3

BOAN

INETER

362

363

356

3.4

0.6

1.7

0.5

3.4

0.5

JUIN

INETER

400

401

397

2.6

0.6

1.7

0.5

3.0

0.6

--

--

The stations are arrangedaccording to age of the geological formations underlyingthem (youngestto oldest) and distancefrom the rupture projection (Rup) of the large asperity shown in Figure 13, JSCE (2001). A: sedimentarydeposits; S: San Salvador; C: Cuscatl~n; B: B~lsamoformation. *Only the first 15 seconds of the digital record are available due to malfunction of the instrument. 432 SeismologicalResearch Letters Volume74, Number4 July/August2003

TABLE 5 Peak Ground Acceleration and Velocity for the 13 February Earthquake Vertical

N-S

Distance (km)

Acc (cm/s2)

Vel (cm/s)

Vel (cm/s)

Acc (cm/s2)

Vel (cm/s)

19.4

3.4

16.9

2.0

26.1

3.4

A

89.4

4.6

35.8

3.1

91.8

4.5

A

426.0

14.7

230.9

4.1

232.7

6.2

S

15

105.3

26.0

121.3

6.9

139.1

22.4

S

36

21

100.5

11.3

54.3

4.3

93.1

9.7

S

37

38

22

31.5

4.4

30.7

2.9

69.1

6.0

S

36

37

22

135.5

14.6

59.8

3.1

71.6

6.4

S

CIG

39

40

24

104.5

7.3

72.0

2.9

108.8

10.0

S

ESJO

UCA

40

41

25

121.4

18.6

51.3

2.8

97.7

6.6

S

SS

ClG

42

43

27

64.7

5.3

44.8

2.1

65.7

7.7

S

VS

CIG

41

42

27

74.8

6.8

45.5

3.4

60.5

6.0

S

VF

CIG

41

42

27

40.2

3.7

30.0

2.7

42.1

5.2

S

Epi

Hyp

Rup

CIG

31

32

16

ULLB

UCA

54

55

40

HSGT

UCA

7

10

CSBR

UCA

30

31

DB

CIG

35

BER

GESAL

CI

CIG

OB

Station

Owner

QC

1.3

Acc (cm/s21

E-W

~

Geology

UC*

CIG

43

44

28

40.2

1.9

59.5

6.7

S

RS

CIG

43

44

28

57.2

3.7

33.7

1.7

64.3

5.3

S

RF

CIG

43

44

28

42.8

2.9

25.6

1.6

39.7

4.6

S

HSRF

UCA

47

48

32

46.8

6.8

22.5

2.0

39.8

4.8

S

ST

CIG

50

51

35

37.7

5.8

18.3

1.9

42.7

5.6

S

UARM

UCA

72

72

58

28.4

4.2

25.9

1.4

36.1

2.3

S

USPN**

UCA

9

11

UPAN

UCA

36

37

21

182.4

9.7

43.8

2.1

104.8

UTON

UCA

34

35

21

247.7

25.0

238.0

10.5

HSTR

UCA

12

14

9

401.5

20.1

256.8

9.7

0.2

~

C 4.8

C

339.5

30.3

C

296.9

20.2

B

The stations are arrangedaccording to age of the geological formations underlyingthem (youngestto oldest) and distancefrom the rupture projection in the surface (Rup) shown in Figure 13. A: sedimentarydeposits; S: San Salvador; C: Cuscatl~n; B: Bdlsamoformation. *Only two components of motion are available. **Malfunction. For the 13 January event the highest peak value of 1.16 g was observed at station ULLB, which is located on the coast and was the closest station to the event. It might also reflect the sedimentary deposits at the site. The high-frequency content of the signal suggests that the thickness of the deposits might be very shallow. However, among the other stations the highest peak values were always recorded on the youngest materials. The horizontal response spectra (5% damping) are compared for stations with similar distances from the source. For the 13 January earthquake the record at UARM station contains more long-period energy than records from other stations (Figure 16A). We could not find in our survey any topographical or geological reasons to explain this signal. The town is located on a small hill and the soil conditions did not appear soft. Although stations UPAN and USPN are located in the same geological formation, significant differences in the spectral ordinates are observed (Figure 16B). A similar case is also

observed for the 13 February event for UTON and UPAN stations (Figure 16C). We observe high spectral ordinates (more than 1.5 g) at HSGT station in San Vicente City in the shortperiod range. Severe damage occurred in this area in the 13 February shock (Figure 16D). A detailed study of damage distribution in San Vicente City and its relation with microtremors is presented in the work ofAbeki et al. (2002). It is clearly desirable to have more detailed information about the deep soil structure and also to study the geomorphological conditions at the accelerograph stations. As mentioned earlier, heavy damage was observed at the tops of some hills, including the towns of Comasagua and Santiago de Marfa (Figures 6 and 7). For a better understanding of the characterization of subduction and volcanic chain earthquakes, one should also use the data to separate the effects of the source, path, and site effects (Andrews, 1986; Iwata and Irikura, 1988; Hough, 1997).

Seismological ResearchLetters July/August2003 Volume74, Number4 433

Distance from the source

km 1200 F 0 -1200 L

150 600

[

-15

-6oo L

. . . . . . . .

..........- . . . . . . . .

~

QC

73

CM

131

OB

55

HSRF

56

CSBR

56

RS

56

RF

56

ST

57

ESJO

58

SS

59

]~oo

- ooI e os,ts (A)

San Salvador formation (S)

I#1

E

30O

vU

~

tO r~ ".--

0 -300

VS*

61

PGB

67 ...... -1 600

UARM

74

800

--~ U U

Sedimentary

o -800