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.
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421
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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
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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
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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
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,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
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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-
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Ge ol ogi cal form atio n s -- From youngest to o/dest
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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.
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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).
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,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
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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
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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