Near-Surface Shear Wave Velocity Versus 

Depth Profiles, Vs

30, and NEHRP Classifications 

For 27 Sites in Puerto Rico 

By Jack K. Odum, Robert A. Williams, William J. Stephenson, David M. Worley, 

Christa von Hillebrandt-Andrade , Eugenio Asencio, Harold Irizarry and Antonio 

Cameron 

Open-File Report 2007–1174 

U.S. Department of the Interior 

U.S. Geological Survey 
U.S. Department of the Interior 

DIRK KEMPTHORNE, Secretary 

U.S. Geological Survey 

Mark D. Myers, Director 

U.S. Geological Survey, Reston, Virginia 2007 

For product and ordering information: 

World Wide Web: http://www.usgs.gov/pubprod 

Telephone: 1-888-ASK-USGS 

For more information on the USGS—the Federal source for science about the Earth, its 

natural and living resources, natural hazards, and the environment: 

World Wide Web: http://www.usgs.gov 

Telephone: 1-888-ASK-USGS 

Any use of trade, product, or firm names is for descriptive purposes only and does not 

imply endorsement by the U.S. Government. 

This report has not been reviewed for stratigraphic nomenclature. 

Although this report is in the public domain, permission must be secured from the 

individual copyright owners to reproduce any copyrighted material contained within this 

report. 

ii 
Contents 


Abstract.......................................................................................................................1 


Introduction.................................................................................................................1 


Generalized Tectonic and Geologic Setting ...............................................................2 


Seismic Data Acquisition and Processing...................................................................3 


Seismic Data Interpretation ........................................................................................5 


Discussion of Site Velocities ......................................................................................5 


(1) Escambron, Coastal San Juan, PR Area ...............................................................9 


(2) Carolina Beach, Coastal San Juan, PR Area ......................................................10 


(3) Barbosa Park, Coastal San Juan, PR Area ..........................................................11 


(4) Carolina-UPR Track, San Juan, PR Area............................................................12 


(5) Encantada, Foothills San Juan, PR Area ............................................................13 


(6) San Juan University Track, San Juan PR Area ..................................................14 


(7) Estadio P. Cepeda, San Juan, PR Area ...............................................................15 


(8) Centro de Bellas Artes, San Juan, PR Area.........................................................16 


(9) Barcelonta, PR.....................................................................................................17 


(10) Arecibo Airport, PR ..........................................................................................18 


(11) Arecibo College Track ......................................................................................19 


(12) Montana ............................................................................................................20 


(13) Rincon ...............................................................................................................21 


(14) Mayagüez-El Seco Ball Field............................................................................22 


(15) Mayagüez-UPR Track.......................................................................................23 


(16) Mayagüez-Candelaria........................................................................................24 


(17) Lajas, UPR Agriculture Station.........................................................................25 


(18) Inter-American Univ., San German, PR ...........................................................26 


(19) University of Catolica, Ponce............................................................................27 


(20) Cerrillos Dam, Central Mountains ....................................................................28 


(21) Orocovis, Central Mountains ...........................................................................29 


(22) Caguas Soccer Field .........................................................................................30 


(23) Caguas, Notre Dame ........................................................................................31 


(24) Cayey Observatory............................................................................................32 


(25) Guayama............................................................................................................33 


(26) Humacao CUH Track........................................................................................34 


(27) Fajardo Airport..................................................................................................35 


Summary of Site NEHRP Classifications..........................................................................36 


Acknowledgments..............................................................................................................39


References Cited ................................................................................................................39 


Figures 


1. Cartographically accurate high-resolution side-looking airborne radar mosaic 


image of Puerto Rico ......................................................................................4 


2. Generalized geology of Puerto Rico.....................................................................4 


3. Sites arranged by NEHRP site classification code........ .....................................38 


4. Influence of calculated Vs30 values....... ............................................................39 


Tables 


1. Seismic-refraction-reflection data recording parameters...............................5 


2. Site Categories in NEHRP provisions (BSSC, 1997)................................. 6 


iii 
3. Site locations of investigations, surficial geology, VS 30 velocity.................... 7 


iv

Near-Surface Shear Wave Velocity Versus 

Depth Profiles, VS30, and NEHRP Classifications 

for 27 Sites in Puerto Rico 

by Jack K. Odum1, Robert A. Williams1, William J. Stephenson1, David M. Worley1, 

Christa von Hillebrandt-Andrade 2, Eugenio Asencio3, Harold Irizarry2 and Antonio 

Cameron2 

Abstract 

In 2004 and 2005 the Puerto Rico Seismic Network (PRSN), Puerto Rico Strong 

Motion Program (PRSMP) and the Geology Department at the University of Puerto RicoMayagüez (UPRM) collaborated with the U.S. Geological Survey to study near-surface 

shear-wave (Vs) and compressional-wave (Vp) velocities in and around major urban 

areas of Puerto Rico. Using noninvasive seismic refraction-reflection profiling 

techniques, we acquired velocities at 27 locations. Surveyed sites were predominantly 

selected on the premise that they were generally representative of near-surface materials 

associated with the primary geologic units located within the urbanized areas of Puerto 

Rico. Geologic units surveyed included Cretaceous intrusive and volcaniclastic bedrock, 

Tertiary sedimentary and volcanic units, and Quaternary unconsolidated eolian, fluvial, 

beach, and lagoon deposits. From the data we developed Vs and Vp depth versus 

velocity columns, calculated average Vs to 30-m depth (VS30), and derived NEHRP 

(National Earthquake Hazards Reduction Program) site classifications for all sites except 

one where results did not reach 30-m depth. The distribution of estimated NEHRP 

classes is as follows: three class “E” (VS30 below180 m/s), nine class ”D” (VS30 between 

180 and 360 m/s), ten class “C” (VS30 between 360 and 760 m/s), and four class “B” 

(VS30 greater than 760 m/s). Results are being used to calibrate site response at 

seismograph stations and in the development of regional and local shakemap models for 

Puerto Rico. 

Introduction 

The Caribbean region has a long prehistoric and historic record of high magnitude 

earthquakes that are responsible for large numbers of deaths and property destruction. 

Currently, over 50 million people live within the North American-Caribbean plate 

boundary zone, and projections are for the population to double by 2050 (Mann, 2005). 

With a population of approximately 4 million people occupying an island that measures 

1

U.S. Geological Survey, Geologic Hazards Team, Golden, CO 

2

Puerto Rico Seismic Network, University of Puerto Rico-Mayagüez, PR 

3

Department of Geology, University of Puerto Rico-Mayagüez, PR 
170 by 60 km, Puerto Rico has a population density (433 persons/km2

) comparable to 

Japan and Taiwan (Mann, 2005). As a result of the island’s rugged interior terrain, the 

largest urban centers have developed primarily on the relatively flat, low-lying coastal 

plains, alluvial plains, and terraces along major rivers (fig.1); all these areas tend to be 

vulnerable to intense earthquake ground shaking, liquefaction, and landslides. As is true 

for most developing countries, these most densely populated areas are the location for the 

rapid construction of everything from well engineered high-rise buildings to large 

numbers of one- and two-story, poorly built concrete block houses. 

Numerous studies over the last few decades have clearly established that Vs in the 

upper 30 to 60 m can greatly influence the amplification and duration of earthquake 

ground motions observed at the surface (for example, Borcherdt and Gibbs, 1976; Joyner 

and others, 1981; Seed and others, 1988). The determination of near-surface seismic 

velocities is also motivated by their use in the code provisions that place a special 

significance on shallow Vs (BSSC, 1997). Vs30 data are also a key element in the 

development of shake maps for Puerto Rico. Future earthquakes are unpreventable, so it 

is of utmost importance to gather and disseminate the scientific data that will help 

mitigate at the local level, to the extent that is economically feasible, the effects of these 

future events. 

Generalized Tectonic and Geologic Setting 

The geologic and tectonic history of the rocks that make up the island of Puerto 

Rico spans at least 150 million years. Puerto Rico is part of a shallow subareal platform 

of the easternmost bank of islands forming the Caribbean Greater Antilles island-arc. 

This island group lies along the northeastern edge of the Puerto Rico-Virgin Islands 

microplate, which is located within the active plate boundary created by the convergence, 

and principally left-lateral translation, of the North American and Caribbean plates 

(Mann and others, 2002; McCann, 1985). In the Puerto Rico region, it is estimated that 

the north edge of the Caribbean plate is moving eastward in a strike-slip fashion at a rate 

of 2 cm/yr along the Puerto Rico trench boundary (Dillon and others, 1999) as the eastern 

edge of the Caribbean plate overrides the North American plate. Based upon the regional 

tectonic framework, Puerto Rico is subject to a high degree of seismicity and has a 

history of large earthquakes. Examples of historical events include a magnitude 7.5 

(1946) centered to the northwest, and magnitude 8.1 and 6.9 (1946 and 1953) events 

centered north of Hispaniola (Dillon and others, 1999). Other large historical events are 

the 1787 (M ~8.1) event, which is believed to have an epicenter in the Puerto Rico 

Trench area, and the 1767 (M ~7.5) event, with an epicenter near the Anegada trough 

(Dillon and others, 1999). The USGS National Hazard Map estimates that the probability 

of damaging ground motion for Mayagüez in western Puerto Rico is equivalent to that of 

Seattle, Washington; other Puerto Rico cities also have substantial risk (Dillon and 

others, 1999; Frankel and others, 1996). 

A generalized geologic map showing the distribution of principle lithologic groups 

and the location of major faults is presented in figure 2. The island’s rugged central core, 

including portions of the northeast and southeast margins (approximately 44 percent 

(3,700 km2) of the surface area), primarily is composed of Late Jurassic to Paleocene and 

Eocene age sedimentary rocks (volcaniclastic sandstone, siltstone, ash-flow tuff, breccia 

and conglomerate) and Late Cretaceous and Eocene intrusive plutonic rocks 

(granodiorite, quartz monzonite, and syenite (Bawiec, 2001)). The developing urban 

2

areas of Caguas and Cayey occupy intermountain alluvial valleys that overlie volcanic 

and volcaniclastic rocks. 

The relatively flat-lying northern coastal plain, which extends from the 

northwestern coastline nearly to the eastern coast, is visible on the SLAR (side-looking 

airborne radar) mosaic image (fig. 1). A similar, but smaller, coastal plain lies along 

most of the southwest half of the island. These flat regions consist primarily of 

Oligocene and younger sedimentary rocks that unconformably overlie the older volcanic 

and vocaniclasitic rocks (Zapp and others, 1948). Coastal plain rocks (approximately 17 

percent (1,500 km2) of the land surface) are predominantly composed of massive to 

interbedded sandstone, calcareous clastic rocks, and limestone (Bawiec, 2001). The 

relatively flat coastal plains are locally modified by karst solution structure and drainage 

incision as a result of relatively recent island uplift and sea level lowering. 

Quaternary deposits cover approximately 24 percent (2,100 km2) of the exposed 

land surface and consist of beach, swamp, alluvial fan, plain and terrace, and weakly 

consolidated to laterized eolian blanket sands at higher elevation. The largest urban 

developments in Puerto Rico (San Juan and Arecibo on the northern coastal plain, 

Mayagüez on the west coast, and Ponce and Guayama on the south coast) are situated 

predominantly in areas of extensive Quaternary deposits that overlie Tertiary clastic and 

carbonate rocks. A detailed description of the geologic history and lithologic assemblage 

of each terrain block of Puerto Rico can be found in Bawiec (2001). 

Seismic Data Acquisition and Processing 

Shear-wave data were recorded using a linear array of 60 4.5-Hz horizontalcomponent geophones spaced 1.5 m apart. Geophones are single component and oriented 

perpendicular to the profile direction. The shear-wave seismic source consisted of a 

wooden timber with steel caps, oriented at right angles to the direction of the profile 

(parallel to the geophone orientation) placed on pavement or soil beneath the wheels of a 

vehicle. Reversed-polarity seismic energy was produced by striking opposite ends of the 

timber with a 4-kg sledgehammer. A set of reversed seismic S-wave profiles at least 87 

m in length was collected at each site. The shear-wave profile lengths typically result in 

a maximum survey depth range of about 30 to 50 m. Where space permitted, a full 87-moffset shot-point record was obtained at one and or both ends of the profile. All data were 

used in the construction of the depth versus velocity profiles. In cases where reversed 

profiles (looking from opposite ends of the profile) are different, two depth versus 

velocity profiles are plotted and labeled. Differences in interpreted depth and velocity in 

these situations are generally the result of dipping strata. Similar studies to characterize 

near-surface materials using surface seismic methods have been conducted by Williams 

and others (1998, 1999, and 2003) and Odum and others (2003). 

Compressional-wave data were recorded using an in-line spread of 60 8-Hz, verticalcomponent geophones at 1.5-m spacing. Energy for P-wave investigations was generated 

3

Figure 1. Cartographically accurate high-resolution side-looking airborne radar (SLAR) 

mosaic image of Puerto Rico. Visible on the image is the extent of the rugged interior 

terrain that dominates much of the island and the relatively flatter coastal plain platform 

that extends along most of the northern and south-central portions of the island. Most 

major urban centers are located along the island margins. Names and numbers refer to 

city locations and ID site numbers on table 3. Base map modified from Bawiec (2001). 

Figure 2. Generalized geology of Puerto Rico from Renken and others (2002). 

4

by vertically striking a steel plate with a 4-kg sledgehammer. Recording parameters for 

both S- and P-wave surveys are listed in table 1. 

Table 1. Seismic-refraction-reflection data recording parameters. 

Recording system Geometrics Strata Visor 24-bit seismograph (60 channels) 

Sampling interval 0.001 seconds 

Record length 1 second 

Recording format SEG-2 

Geophones 60 4.5-Hz horizontal or 8-Hz vertical 

Geophone array Linear with single phones at 1.5-m intervals 

Source 4.0-kg sledgehammer on wood timber (S-wave) or steel 

plate (P-wave) 

Source array geometry Linear, 87-m array lengths 

Seismic Data Interpretation 

We interpreted refraction data from both the S-and P-wave surveys using the slopeintercept method described by Mooney (1984). Data interpretation generally produced a 

profile column consisting of 2- to 5- distinct velocity layers for each site. In cases where 

no additional layers were detected below about 20 m by refraction methods, the 

maximum imaging depth was approximated by assuming that a higher velocity layer 

would have been detected on the next geophone beyond the end of the profile (Mooney, 

1984). Velocity versus depth profiles and interpreted geology structure columns are 

presented for each site in the discussion section of this paper. Using the results from the 

velocity versus depth profiles we calculated the average Vs to a depth of 30 m (VS30) at 

each site. According to NEHRP guidelines, VS30 is determined by 

n

∑ d i (1) i =1

VS30= n d ∑ V

i

i =1 s i

where di

 is the thickness of the ith layer between 0 and 30 m and Vsi is the velocity of the 

ith layer. The NEHRP building code assigns one of six soil-profile types to a site, from 

hard rock (type A) to soft soils (types E or F), based on the VS30 (table 2). These soil 

profile categories, which are determined for each site in this study, are also part of the 

International Building Code adopted in 2001 (IBC, 2002). 

Discussion of Site Velocities 

In this discussion section, sites are grouped by geographical areas as indicated on 

figure 1. Beginning with the San Juan area on the northeast coastal platform, sites will be 

discussed in a counterclockwise fashion around the perimeter of the island ending with 

the sites on the east coast. 

5

TABLE 2. Site categories in NEHRP provisions (BSSC, 1997). 

Soil profile 

type 

Rock/soil description Average S-wave velocity 

(m/s) top 30 m 

A 

B 

C 

D 

E 

F 

Hard rock 

Rock 

Very dense soil/soft rock 

Stiff soil 

Soft soil 

Special soils requiring sitespecific evaluation 

> 1,500 

760 – 1,500 

360 – 760 

180 – 360 

< 180 

For each site we present a single-page discussion, which contains a depth versus Vs 

and Vp velocity plot, sometimes a photograph showing the survey lay out, location and 

NEHRP classification code. Because Vs layer velocities and thicknesses are important to 

the engineering community and for seismological modeling of site response, a geologic 

interpretation column correlated to Vs layer plot is also presented. The discussion section 

of each page primarily will focus on the lithologic units correlations with the Vs depth 

versus velocity profiles. Table 3 summarizes the velocity information obtained from the 

velocity versus depth profiles and provides additional information on site location, 

surficial geologic map-unit symbols, major subsurface geologic units thought to be 

surveyed, highest velocities recorded in the survey regardless of depth, calculated VS30 

and NEHRP site classification codes (see table 2 for explanation). Geologic names and 

symbols shown on the interpreted geologic columns are taken from geologic quadrangle 

maps and may differ from what is shown in the generalized geologic map (fig. 2). There 

is no correlation between colors shown in columns and the generalized map units in 

figure 2. 

Some of the sites studied in urban areas are located within the infield areas of track 

stadiums or in other urban settings where the original ground surface has been modified 

by the addition of artificial fill (af) and/or one or more layers of engineered compacted 

soil and aggregate. These near-surface layers are seen in the raw seismic data as lowvelocity direct arrivals and refracted phases that are the first arrivals near the seismic 

source position. Shown in many of the velocity versus depth columns are one or two thin 

layers 1 to 3 m thick with velocities faster than the underlying undisturbed geologic 

materials. For the higher-velocity surface layers, we roughly estimate the thicknesses 

because only one or two data points are available to define them, and the thicknesses of 

these layers are not well constrained when a higher velocity layer does not immediately 

underlie the artificial layer. The total thickness of these surface layers is probably less 

than what is calculated. In this study all calculated velocity layers are used in the 

calculation of VS30 and NEHRP site classification. 

6

Table 3. Site locations of investigations, surficial geology, VS 30 velocity, NEHRP site 

classification Code, and geologic unit information for seismic refraction-reflection 

surveys in Puerto Rico. 

Site name 

VS30 

(m/s) 

NEHRP 

soil 

type 

Highest 

recorded 

Vs (m/s) 

Highest 

recorded 

Vp (m/s) 

Imaged(?) 

geologic 

units 

Description Site location 

(1) Escambron, 

San Juan, coastal See 

text * ? 

1,025 @ 

5 m 

2,085 @ 

9 m 

Qe 

Qs 

Tay? 

Eolianite-calcareous reef 

lagoon bay mud? 

Aymamon Limestone? 

N 18o 27’ 59” 

W 66o

 05’ 22” 

(2) Carolina 

Beach, San Juan 

290 D 290 @ 

1 m 

1,645 @ 

2 m 

Qb 

Tay 

Beach deposits (sand) 

Aymamon Limestone 

N 18o 26’ 53” 

W 65o

 59’ 56” 

(3) Barbosa Qb/Qss Beach sand (thin) N 18o 27’ 05” 

Ocean Park, San 285 D 545 @ 2,030 @ QTt Older alluvial deposits W 66o

 02’ 57” 

Juan, coastal 38 m (?) 7 m Tay Aymamon Limestone 

(4) Carolina, 1740 @ 3,324 @ Qal/QTt Alluvium, older alluv. N 18o 23’ 14” 

UPR track, 515 C 12 m 12 m Kf Frailes Fm. Upper Cret. W 65o

 57’ 23” 

San Juan area 

(5) Encantada, 

San Juan area 1,410 B 2212 @ 10 m 

4,214 @ 

9 m 

Qal 

Tg 

Alluvium, older alluv. 

Guaracanal (Andesite) 

N 18o 21’ 22” 

W 65o

 59’ 37” 

(6) San Juan Univ. 440 @ 1,870 @ Qt Alluvium-terrace N 18o 24’ 32” 

track, San Juan 415 C 1.5 m 12 m Tay Aymamon Limestone W 66o

 02’ 44” 

Ta Aguada Limestone 

(7) Estadio P. 535 @ 1,821 @ Af-Qa Artificial fill alluvium N 18o 26’ 15” 

Cepeda, San Juan 173 E ~52 m 5 m Qs/Qa Swamp-older alluvium W 66o

 07’ 31” 

area Tay Aymamon Limestone 

(8) Centro de 450 @ 1,865 @ Qal/Qt Alluvium-terraces N 18o 26’ 15” 

Bellas Artes, 305 D 15 m 10 m Tay Aymamon Limestone W 66o

 10’ 38” 

San Juan area 

(9) Barceloneta 170 E 460 @ 

22 m 

1,790 @ 

9 m 

Qal 

Tay 

Alluvium 

Aymamon Limestone 

N 18o 27’ 30” 

W 66o

 32’ 23” 

(10) Arecibo *630 @ 23 1700 @ Qal/Ql Lagoonal deposits N 18o 26’ 55” 

airport 433 C m 3.0 m QTb Blanket deposits W 66o

 40’ 28” 

Tay Aymamon Limestone 

(11) Arecibo, 

college track 378 C 570 @ 33 m 

3,324 @ 

10.0 m 

QTb 

Tcu 

Blanket deposit 

Camuy Fm. 

N 18o 26’ 55” 

W 66o

 40’ 28” 

(12) Montana 985 B 1220 @ 

8 m 

1,700 @ 

2.5 m 

Qc/QTb 

Tay/Ta 

Colluvium-Blanket dep. 

Aymamon/Aguado Ls. 

N 18o 21’ 03” 

W 66o

 06’ 13” 

(13) Rincon 1,045 B 2385 @ 

16 m 

3,690 @ 

14 m 

af 

Tv 

Artificial fill 

Tertiary volcanic rock 

N 18o 21’ 02” 

W 67o

 16’ 08” 

(14) Mayagüez, 

El Seco ball field 212 D 445 @ 38 m 

1,833 @ 

8 m 

Qal 

Ky 

Alluvium 

Yauco Fm-saprolite 

N 18o 12’ 47” 

W 67o

 09’ 34” 

(15) Mayagüez, 2400 @ 4,400 @ Ky Yauco Fm-saprolite N 18o 21’ 30” 

UPR track 200 D 18 m 11.5 m W 67o

 16’ 06” 

7

Site name 

VS30 

(m/s) 

NEHRP 

soil 

type 

Highest 

recorded 

Vs (m/s) 

Highest 

recorded 

Vp (m/s) 

Imaged(?) 

geologic 

units 

Description Site location 

(16) Mayagüez, 

Candelaria 

200 D 355 @ 

18 m 

1,980 @ 

3.5 m 

Qal 

Ky ?/ Kmr ? 

Alluvium 

Yauco Fm-saprolite 

N 18o 11’ 42” 

W 67o

 04’ 22” 

(17) Lajas, Agri­ 1675 @ 2,735 @ Qaf Alluvial fan N 18o 02’ 03” 

culture Station 435 C 31 m 15 m Kl?/ Kc? Volcanics weathered- W 67o

 9’ 02” 

Cotui Limestone 

(18) San German, 

Inter. Am. Univ. 650 C 1,320 @  6 m 

3050 @ 

6 m 

Kjs Serpentinite N 18o 04’ 58” 

W 67o

 02’ 56” 

(19) Ponce, 505 @ 1,580 @ Qaf Alluvial fan deposit N 17o 59’ 58” 

Univ. Catolica 163 E 26 m 3.5m Tp Ponce Limestone- W 66o

 37’ 10” 

track Tje calcareous sandstone 

(20) Cerrillos 

Dam, mountains 

925 B 1,545 @ 13 

m 

3,640 @ 

15.0 m 

Tm Monderrate Fm N 18o 04’ 33” 

W 66o

 34’ 46” 

(21) Orocovis, 

central mountains 225 D 1,470 @ 34 m 

2500 @ 

10 m 

Ql 

bedrock 

Landslide deposits 

Cret. volcanic rock 

N 18o 10’ 31” 

W 66o

 25’ 59” 

(22) Caguas, 1910 @ 1,820 @ Qt Older alluvial deposits N 18o 15’ 16” 

soccer field 285 D 42 m 6 m Kn Los Negros Fm.- W 66o

 2’ 28” 

 volcanic tuff 

(23) Caguas, Notre 

Dame 395 C 660 @ 22 m 

2,125 @ 

4 m 

Qa? Qt 

Kgc 

Older alluvial deposits 

Granodiorite 

N 18o 13’ 57” 

W 66o

 1’ 38” 

(24) Cayey, 

Observatory 

285 D 800 @ 

100 m? 

1,645 @ 

7 m 

Qal 

Ka 

Alluvium-colluvium 

Volcanic flows-breccia 

N 18o 6’ 42” 

W 66o

 8’ 56” 

(25) Guayama 460 C 1675 @ 

25 m 

3,685 @ 16 

m 

Qaf 

Ka 

Alluvial fan 

Volcanic flows-breccia 

N 17o 58’ 39” 

W 66o

 8’ 56” 

(26) Humacao, 775 @ 2,825 @ Qa/Qaf Alluvium-fanglomerate N 18o 08’ 45” 

CUH track 330 C 25 m 22 m Klg Granodiorite W 65o

 50’ 06” 

(27) Fajardo, 

Airport ball field 425 C 920 @ 45 m 

2,985 @ 

37 m 

Qal 

Kftu 

Alluvium 

Fajardo Fm. 

N 18o 18’ 54” 

W 65o

 39’ 49” 

See figure 1 for site approximate locations. Surficial map units and geologic unit 

abbreviations are taken from 1:24,000 quadrangle maps and might not correlate with the 

generalized geologic units on figure 2. Q – Quaternary; T– Tertiary; K – Cretaceous; J – 

Jurassic. (*) See text discussion for this site. 

8

The location of this site is within 30 m of the ocean front in an area that has been 

artificially filled for construction (fig. 1). Acquisition conditions were poor due to a high 

level of cultural noise and limited working area, which prevented off-end shots needed 

for deeper imaging. All plotted layer depths are poorly constrained and therefore 

represent only rough estimates of the true layer thickness. 

The depth versus velocity curves show two distinct velocity layers in the upper 10 

m. Difference in “S-wave E” and “S-wave W” (indicating data records recorded looking 

east and west along the geophone array) profiles is attributed to dipping strata. Based on 

a first-break phase termination of the P- and S-wave first arrivals, we suspect that there is 

a low-velocity zone underlying the high-velocity layer. Our data suggest that the 

thickness of the high-velocity (Vs=1,025 m/s) layer is at least 2 meters, but, 

unfortunately, say little about the low-velocity layer. We interpret the first layer (Vs=675 

m/s) to be artificial fill consisting of large bedrock pieces and/or concrete rubble. The 

second layer, Vs=1,025 m/s, we interpret to be “reef rock” (eolianite). Eolianite, which 

is Pleistocene to Holocene, consists of well-cemented calcareous fine, to coarse-shell 

fragments and sand. This reef rock is exposed just offshore as is shown by the dark band 

of rock in the foreground of the photo above and is mapped throughout the area with a 

maximum thickness of about 30 m (Pease and Monroe, 1977). The depositional 

processes forming this unit would cause it to stratigraphically transgress and regress with 

sea level fluctuations, thus it is plausible that the imaged unit overlies older lagoonal 

deposits. There is a possible P-wave reflection (from the base of the low-velocity zone?) 

that indicates that the base of the low-velocity zone could be at about 50- to 55-m depth. 

At site 7 (Estadio P. Cepeda, see fig. 1), located on the south west side of the Bahia de 

San Juan, Cretaceous bedrock, believed to be Aymamon Limestone (Tay), was 

interpreted at 52-m depth (Odum and others, in press). Because the depth of imaging is 

less than 30 m, neither a VS30 or NEHRP classification is determined for this site. 

9

The Carolina Beach site lies directly on beach sand (Qb) approximately 0.3 km 

north of the eastern portion of San Juan International Airport and 7 km east of downtown 

San Juan (visible in background center of photo above). Surficial geologic units at this 

site are Holocene to Pleistocene beach and abandoned beach-ridge deposits, which 

consist of fine-to-medium-grain quartz sand and shell fragments generally less than 10 m 

thick (Monroe, 1977). The modern units locally overlie older Pleistocene to Miocene 

(QTt) units consisting of beach, high terrace, and alluvial fan deposits, and these 

unconsolidated units unconformably overlie Miocene Aymamon and Aguada 

Limestones. 

Although two Vs layers were identified (uppermost unit is 1 m thick and not 

shown), we believe that only one geologic unit is present. We interpret this unit 

(Qb/QTt) to be composed of modern and older beach deposits possibly interfingered with 

alluvial fan material at depth. Due to the coarse and unconsolidated nature of this 

deposit, shear-wave energy from the timber placed directly on the beach sand attenuates 

quickly. Although a strong reflection boundary was not observed in the data, the actual 

thickness of the Qb/QTt unit is probably less than the 30 m indicated on the figure above. 

Geologic mapping indicates that this material has a wide distribution in the San Juan 

coastal area (Pease and Monroe, 1977). The VS30 for this site is 285 m/s, which is 

NEHRP soil type “D” (stiff soil). 

10

The Barbosa Park site is located in a similar geologic setting as the Carolina 

Beach site (site 2) with the primary difference being that Barbosa Park is approximately 

100 m inland of the currently active beach and lies in an area of urban development (fig. 

1). 

The Vs depth versus velocity profile for this site shows two layers; the uppermost 

layer with a Vs=280 m/s and the lower layer with a Vs=545 m/s. Based on similarity in 

the geologic setting and layer velocities (Vs=280 m/s at this site and Vs=290 m/s at the 

Carolina Beach site), we interpret the upper layer to be Qb/QTt. Based on geologic 

position and similarity of velocity, we interpret the deeper layer, Vs=545 m/s, to be 

Aymamon Limestone (Tay). The thickness of the Qb/QTt layer is poorly constrained and 

is probably less than the calculated depth of about 38.0 m. The VS30 for this site is 285 

m/s, which corresponds to a NEHRP type classification of “D” (stiff soil). 

11

The track and field complex located within the city of Carolina is located 

approximately four km inland from the coast and at an elevation of approximately 10 m 

(figs. 1 and 2). Near-surface geology is mapped as Pleistocene. Lithologic units are 

believed to be older terrace and alluvial-fan materials (QTt) with a thin veneer of 

Holocene alluvium (Monroe, 1977). These units unconformably overlie the Upper 

Cretaceous Frailes Formation, which is composed of calcareous mudstone, medium- to 

thick-bedded volcanic sandstone, and volcanic breccia (Monroe, 1977). 

Two primary geologic units beneath a thin surficial layer are identified from the 

seismic velocity structure at this location. The unit from 2- to 12-m depth (Vs=280 m/s) 

is interpreted to be the geologic unit QTt; however, the interpreted 10-m thickness of this 

unit possibly includes a weathered zone of the underlying Upper Cretaceous Frailes 

Formation. The Vs and Vp of this unit is similar to that of other interpreted QTt units. 

We interpret the higher velocity (Vs=1,740 m/s) unit to be the Frailes Formation (Kf). 

The VS30 for this site is 515 m/s, which is NEHRP type “C”. 

12

This site is located approximately 10 km inland from the ocean front, at an 

elevation of approximately 30 m, and lies within the southern foothills of the San Juan 

area. Portions of the athletic field where the survey was conducted have been excavated 

to flatten the area, and weathered volcanic bedrock is exposed in the surrounding hills 

(see photo above). To calculate the VS30 for this site, we used an average of the shearwave data from the two profiles to generate the depth versus velocity profile (blue line). 

These profiles should be considered to be an average for this site because, as was true 

with the P-wave profiles, the east and west profiles look different. 

Depth versus velocity curves and geologic interpretation of Vs layers are shown 

above. Geologic mapping indicates that the hills are composed of Tertiary volcanic and 

volcaniclastic rock units (Pease, 1968). The layer from 0 to 4 m, Vs=540 m/s, is 

interpreted to be soil, slope colluvium (Qc), and probably includes some weathered 

bedrock material. The layers from 4 to 11 m, 1,320 m/s, and below 11 m, 2,212 m/s, are 

interpreted to be units in the Guaracanal Andesite with the upper layer representing 

weathered material and/or an interbedded unit (example given, breccia and tuff). The 

VS30 for this site is 1,410 m/s, the highest value measured in this study, and corresponds 

with NEHRP class “B” (rock). 

13

This site is located at an elevation of 25 m and approximately 5.5 km from the 

coast line (figs. 1 and 2). The geologic setting is similar to the Carolina-UPR track site in 

that it is several kilometers inland from the ocean front and is in the vicinity of hills 

composed of Tertiary and older bedrock. Geologic mapping indicates the surficial unit at 

this site is QTt (Monroe, 1977). 

Beneath a thin layer of artificial fill (af) and soil, only one velocity layer was 

identified from the seismic refraction-reflection data. Although the surficial map unit is 

QTt, the calculated unit thickness at this site is uncertain and may be less than 30 m as 

plotted. Approximately 200 m to the north of the site is an outcrop of Aymamon 

Limestone (Tay). The Vs of 440 m/s is higher than what has been determined for the QTt 

unit at other sites and lower than the typical values of 525-630 m/s that have been 

calculated for the Aymamon Limestone (Odum and others, in press). We speculate that 

the material imaged at this site may consist of unconsolidated QTt material and that the 

QTt may be partially cemented by calcareous enriched ground water from the underlying 

partially weathered Aymamon Limestone (Tay). Information from geotechnical borings 

are needed to better constrain the thickness of the QTt at this site. The VS30 for this site 

is 415 m/s, which makes it a NEHRP class “C” site (very dense soil-soft rock). 

14

The Estadio Pedro Cepeda stadium is built on an artificial fill surface, which 

overlies bay mud and swamp deposits associated with Bahía de San Juan (Kaye, 1959). 

This site has a complex stratigraphic sequence as is indicated by the presence of five 

interpreted velocity layers that include a low velocity unit bounded on either side by 

higher velocity layers. We interpret the layers from 0- to 7-m (Vs= 405 m/s) and 7- to 9- 

m (Vs=525 m/s) depth to be engineered artificial layers associated with construction site 

stabilization over the bay and swamp deposits. From approximately 9- to 26-m depth (Vs 

= ~120 m/s) there is a low-velocity layer which we interpret to be bay mud, swamp, and 

peat (Qs). At this location a thickness of 17 m or more is very reasonable for this unit, as 

Kaye (1959) reported that a logged water well at the U.S. Naval Reservation (3.5 km 

away on the northeast side of Bahía de San Juan) penetrated 25 m of bay mud and swamp 

material (Qs). The Vs determined for this Qs unit is similar to those determined for 

similar lithologic deposits at other locations (in example 55-100 m/s San Francisco bay 

mud (Fumal, 1978)). The layer from ~26- to ~52-m depth (Vs=~350 m/s) is interpreted 

to be older alluvium and alluvial fan and terrace deposits (QTt), which are expected to be 

found in this stratigraphic position. The deepest layer, below ~52 m (Vs= ~535 m/s), is 

interpreted to be the Aymamon Limestone (Tay). The VS30 for this site is 173 m/s, the 

lowest value measured in this study, and corresponds with a NEHRP category “E” (soft 

soil). 

15

This profile was acquired on the flood plain of the Río de Bayamon, at an 

elevation of 3 m, and approximately 4 km west of site 7 (Estadio P. Cepeda) (fig. 1). 

This location, approximately 2 km from the coast, has a stratigraphy of bedrock units of 

middle Tertiary marine sedimentary rocks, predominantly limestone with interbedded 

sandstone, which have been incised by river drainage and modified by karst processes 

(fig. 2). Overlying this eroded surface are river alluvium, swamp, and lagoonal deposits 

of Pleistocene and Holocene age (Briggs, 1965, 1966; Briggs and Akers, 1965; Zapp and 

others, 1948). 

Vs and Vp depth versus velocity curves along with a lithologic interpretation of 

the Vs profile are presented above. The velocity unit (Vs=225 m/s) from 0 to 14.5 m is 

interpreted to be predominantly recent river alluvium (Qa) with possibly some sections 

that include older alluvial and terrace units, as well as, buried swamp and organic 

material. We interpret the velocity layer below 14.5-m depth (Vs=450 m/s) to be 

weathered Aymamon Limestone (Tay), which is exposed in river bluffs and isolated hills 

near the survey site. The Aymamon Limestone is very fine- to fine-grained pure 

limestone that is commonly thick bedded, chalky, and locally coarsely fragmental 

(Briggs, 1968). A velocity of 440 m/s was interpreted for weathered Aymamon 

Limestone in the San Juan area and 450 m/s at Barceloneta (site 9). It is possible that the 

Aguada Limestone (Ta), which underlies the Aymamon, is partially sampled at this site. 

The VS30 for this site is 350 m/s, which corresponds to a NEHRP class “D” site. 

16

This profile was acquired on the flood plain of the Río Grande de Manati, at an 

elevation of 5 m, near the town of Barceloneta (fig. 1). This location is 3 km from the 

coast and 22 km west of San Juan. Bedrock units in this area are middle Tertiary marine 

sedimentary rocks, predominantly limestone with interbedded sandstone, which have 

been incised by river drainage and modified by karst processes (fig. 2). Overlying this 

eroded surface are upland plateau eolian Blanket deposits, river alluvium, swamp, and 

lagoonal deposits of Pleistocene and Holocene age (Briggs, 1965; Briggs and Akers, 

1965; Zapp and others, 1948). 

The velocity unit (Vs=120 m/s) from 0- to 4.5-m is interpreted to be recent river 

alluvium (Qal), and it is possible that this section includes buried swamp and organic 

material. The unit from 4.5- to 23-m depth, Vs=145 m/s, is interpreted to be older 

alluvial (Qal) deposits. We interpret the velocity layer below 23-m depth, Vs=450 m/s, 

to be weathered Aymamon Limestone (Tay), which is exposed in river bluffs less than 

half a kilometer from the survey site. A Vs of 450 m/s was interpreted for weathered 

Aymamon Limestone at site 8 in the San Juan area. The VS30 for this site is 170 m/s, 

which makes it a NEHRP class “E” site. 

17

This site is located approximately 3 km inland, is partially on the flood plain of 

the Río Grande Arecibo, and occupies a similar geographical position as the Barceloneta 

site (site 9). The shear-wave velocity versus depth curve shows two shallow units 

(Vs=135 and 260 m/s), which are interpreted to be artificial fill, colluvium, and alluvium. 

The unit from 2.5- to 23-m depth, Vs=455 m/s, is interpreted to be Blanket deposits 

(Qtb). Blanket deposits, Miocene-to-recent in age, are stranded sands that have been 

laterized concurrently with solution of the underlying limestone (Briggs, 1966). Blanket 

deposits, friable to well cemented in exposure and as much as 30 m thick, are found in 

discontinuous patches overlying the Aymamon Limestone (Tay) and the Aguada 

Limestone (Ta) in this area. Although it is possible that some of the material in the lower 

part of this velocity layer might be weathered Aymamon Limestone, we believe that a 

difference of 180 m/s indicates two different lithologies. Therefore, we interpret the 

velocity layer below 23-m depth, Vs=630 m/s, to be the Aymamon Limestone (Tay). A 

poorly constrained reflection observed in the seismic data suggests that there may be a 

velocity layer boundary at approximately 66-m depth. The Vs for this unit is 1,730 m/s. 

It is speculative but this reflection may be from the top of the Aguada Limestone, which 

underlies the Aymamon Limestone. The VS30 for this site is 433 m/s, which corresponds 

to NEHRP type classification “C” (very dense soil-soft rock). 

18

The Arecibo College track site is located approximately 9 km west of the airport 

site (site 10) and sits on a bluff 17 m above the river flood plain. The primary surficial 

unit at this site is Quaternary to Tertiary Blanket deposits (QTb) (Briggs, 1966). The 

depth versus velocity interpretation shows that beneath 1 m of artificial fill (af) there are 

four Vs layers. The shallowest of these layers from 1.0- to 3.5-m depth, Vs=300 m/s is, 

probably Holocene unconsolidated soil composed of colluvium, alluvium, possibly wind 

blown dune material, and probably includes some of the underlying QTb unit. Layers 

from about 3.5 to 16.0 m and 16.0 to 33.0 m (Vs=375 and 425 m/s, respectively) are 

interpreted to be Blanket deposit sands (QTb). Blanket deposits, Miocene to recent in 

age, are stranded sands that have been laterized concurrently with solution of the 

underlying limestone (Briggs, 1966). The deepest velocity unit imaged (below 33.0 m), 

Vs=570 m/s, is interpreted to be Cumay Limestone (Tca), which has been mapped in 

outcrop near the site. However, it is possible that erosion removed the Cumay Limestone 

and the unit imaged is the Aymamon Limestone (Tay). The layer 4 velocity at this site 

(Vs=570 m/s) is similar to that determined for the interpreted Aymamon Limestone at 

Arecibo Airport (Vs=630 m/s). The VS30 for this site is 376 m/s, which is at the lowvelocity end of NEHRP soil type C (very dense soil-soft rock). 

19

This site is located approximately 3 km south of the northern coast and at an 

elevation of 140 m (fig. 1). There was a field error while acquiring data at this site that 

involved P-wave geophones being used in part of the S-wave array. As a result, there is 

no reversed S-wave data and the S-wave profile was pieced together from portions of the 

intended S-wave record. Due to this error, the S-wave interpretation must be considered 

as a rough approximation. 

The depth versus velocity profiles for this site show three velocity layers. We 

interpret the layer from 0 to 2.5 m (Vs=370 m/s) to be Qc (colluvium) along with Qtb 

(Blanket deposit). Geologic mapping (Monroe, 1969) identifies QTb material near this 

site and the Vs=370 m/s is similar to that interpreted for some of the Blanket deposits at 

the Arecibo College track (site 11). The layer from 2.5- to 7.5-m depth, Vs=995 m/s, is 

interpreted to be Tay (Aymamon Limestone) based on geologic mapping (Monroe, 

1969). The Aymamon Limestone, described as a massive, thick-bedded fossiliferous 

unit, is generally indurated into finely crystalline dense limestone (Monroe, 1969). The 

Vs for Tay, at this location, is higher than that interpreted at other areas. This may be the 

result of a higher degree of weathering of this unit at other sites and/or it is possible that 

at the other sites an upper formational unit with different physical properties is sampled. 

The velocity layer below 7.5-m depth, Vs=1,220 m/s, may be Tay, or it may be Aguada 

Limestone (Tau), which is described as a hard limestone. A poorly constrained layer, 

Vs=1730 m/s, was interpreted from a deep reflector at the Arecibo Airport (site 10) and 

was interpreted to be Aguada Limestone. VS30 for this site is 985 m/s, making it a 

NEHRP class “B” site. 

20

This site lies in the hills directly above the city of Rincon, which lies along the 

western coast of Puerto Rico (fig. 1). The P-wave data show very different results 

comparing profiles from either end of the survey. This area has been cut and filled to 

form the athletic field, and thus bedrock is nearer the surface on the east end of the profile 

(see photo above). To calculate the VS30 for this site, we used an average of the shearwave data from the two profiles to generate the depth versus velocity profile (blue line). 

These profiles should be considered to be an average for this site because, as was true 

with the P-wave profiles, the east and west profiles look different. 

No detailed geologic map exists for this area. The generalized geologic map (fig. 2) 

and geologic quadrangle mapping to the southeast of the Rincon quadrangle (Curet, 

1986) indicate that the hilly area where this site is located is probably composed of 

Tertiary and Cretaceous volcanic, volcaniclastic, and sedimentary rocks. Bedrock is 

exposed in the excavated cliff face directly behind the site (see photo above). We believe 

that the entire profile samples weathered and more competent bedrock similar in 

composition to those mapped to the southeast. We note that the velocity layer below 16- 

m depth, Vs=2385 m/s, has essentially the same velocity (Vs=2400 m/s) as that 

determined for the Yauco Formation (Ky) at University of Puerto Rico-Mayagüez track 

(site 15). Of additional interest is the large velocity contrast at this site (at approximately 

16-m depth) which is similar to the depth (16.5 m) of the large velocity impedance at site 

15. VS30 for this site is 1,045 m/s, which makes this a NEHRP class “B” site. 

21

The Mayagüez El Seco baseball field site is located on an artificial-fill platform a 

few meters above sea level. The simplified near-surface stratigraphy of this area consists 

of unconsolidated to weakly consolidated alluvium (Qal) overlying Cretaceous 

interbedded volcaniclastic and sedimentary rock (Curet, 1986). These Cretaceous rocks 

have been altered by the tropical to subtropical environment to produce near-surface 

variably-thick sections, depending upon rock type, of highly weathered bedrock 

(saprolite). 

Although six Vs layers were identified at this site we believe that only three 

primary geologic units are represented over the interpreted 40-m depth. We interpret the 

first two layers (0 to 1.5 m, Vs=230 m/s and 1.5 to 3.0 m, Vs=648 m/s) as artificial fill 

(af) layers. The uppermost layer is composed of compacted soil, and the lower unit is 

likely composed of large boulder-sized and smaller rock pieces. Beneath the fill layers is 

a section of unconsolidated alluvial and near-shore marine material (Qal) (3.0 to 8.0 m, 

Vs=150 m/s and 8.0 to 20.0 m, Vs=172 m/s). It is expected that this section is composed 

of typical interbedded fluvial, coastal plain, and near-shore marine interbedded sediments 

that consist of gravel, coarse- to fine-grained sand, silt and clay. It is also possible that 

within the Qal unit are zones of organic swamp and lagoonal sediments that have been 

identified at other nearby locations and have similar low shear-wave velocities. The slight 

velocity increase at 8.0 m may represent an older, more consolidated unit and/or a change 

in lithology character. 

Geologic mapping indicates that the unconsolidated Qal unit overlies bedrock that 

is probably the Yauco Formation (Ky). This Upper Cretaceous unit is composed of 

interbedded, calcareous, volcaniclastic sandstone, siltstone, mudstone, claystone, 

limestone, and subordinate breccia and conglomerate (Curet, 1986). As stated earlier, 

this unit exhibits varying degrees of weathering, resulting in variable thicknesses of 

saprolite. We interpret the lower two layers (20.0- to 36-m depth, Vs=340 m/s and 

greater than 36.0 m, Vs=445 m/s) to be weathered Ky bedrock. The VS30 for this site is 

212 m/s, which is NEHRP class “D” (stiff soil). 

22

This site is located within the main track facility at the University of Puerto RicoMayagüez, see figure 1. The geographic position of this site places it near surface 

exposures of the Yauco Formation (Ky), which outcrop less than 50 m from the profile 

site near the white building in the photograph above. 

The Vs versus depth profile identifies three distinct velocity layers. The layer 

from the surface to 2.5-m depth, Vs=230 m/s, we interpret to consist of modified soil and 

artificial fill (af) along with possibly a thin veneer of unconsolidated material (Qal). The 

velocity from 2.5- to 16.5-m depth, Vs=140 m/s, is interpreted as saprolite derived from 

the weathering of the Yauco Formation (Ky). Located on the UPRM campus, and a few 

hundred meters from our profile, is a 30-m-deep borehole drilled by Jaca and Sierra 

Testing Laboratories (2002) that indicates that beneath one meter of artificial fill is 29 m 

of saprolite. Results of their Standard Penetration Test (SPT) at this drill hole site were 

as follows: average of 9 blows per ft (bpf) from 0- to 5-m depth, 15 bpf from 5- to 12-m 

depth, 53 bpf from 12- to 16-m depth, and >100 bpf below 16 m (Jaca and Sierra Testing 

Laboratories, 2002). The SPT data indicate a distinct physical property change in the 

bedrock at approximately 16.0-m depth, which correlates with the dramatic increase in 

shear-wave velocity (Vs=140 m/s to Vs= 2,400 m/s) that we interpret at approximately 

the same depth. The VS30 velocity for this site is 200 m/s, which is NEHRP class “D” 

(stiff soil). It is likely that a 2.0-Hz site resonance will be generated at this site during an 

earthquake based upon the high impedance boundary at 16.5-m depth. Potential 

earthquake resonance frequency, f, is calculated from the zero-offset reflection traveltime 

f=1/2T, where T is the two-way traveltime of the reflection. 

23

This site is approximately 0.5 km from the Bahía de Mayagüez and just west of 

Highway 2. This traffic noise is responsible for the data degradation that limited the 

imaging depth at this site. Geographic position and geologic mapping indicate that the 

primary surficial units in this area are Qal (interbedded fluvial, coastal plain, and nearshore marine sediments), which consist of coarse- to fine-grained sand and gravel and silt 

and clay. Also present in the area, and immediately east and south of the investigation 

site, are swamp deposits (Qs) and extensive areas of mangroves. 

Two velocity layers are identified in the upper three meters of this site (see 

velocity versus depth graph above). We interpret the velocity from 0 to 2 m, Vs=200 

m/s, to be artificial fill (af). This layer overlies a 1-m- thick layer, Vs=325 m/s, which we 

speculate is another fill placed into what was probably a swamp area. Beneath the fill is a 

15-m-thick, Vs=145 m/s, layer interpreted to be Qal. From 18- to ~30-m depth is a layer 

(Vs=355 m/s), which we interpret to be saprolite (weathered Upper Cretaceous bedrock 

(Ky)). The VS30 velocity for this site is 200 m/s, which is at the low end of a NEHRP 

class “D” (stiff soil). 

The stratigraphic section and seismic velocity columns for the Candelaria and El 

Seco (site 14) sites are very similar. Not only is the upper section of weathered bedrock 

similar (Vs=340 and 355 m/s), but the depth from the surface to the boundary between 

saprolite and less weathered bedrock is essentially the same at 20 and 18 m, respectively. 

24

This survey site lies on the relatively flat surface of Valle de Lajas south of a 

small mountain range composed of Upper Cretaceous volcanic rocks (fig. 1). We 

interpret the shear-wave velocity layer from 0- to 5-m depth, Vs=160 m/s, to be modern 

alluvial (Qal) material. The unit from 5- to 11-m depth, Vs=485 m/s, is interpreted to be 

older Quaternary terrace (Qt) and alluvial fan (Qaf) sequences composed of coarsegrained materials. It may also include some underlying weathered bedrock. Based upon 

geologic mapping, the unit from 1- to 31-m depth may represent Upper Cretaceous 

basaltic volcanic rock (Kl- Lajas Formation) that is partly weathered (Volckmann, 1984). 

The Vs velocity of 768 m/s is similar (800 m/s) to that determined for formation Ka 

(andesite flows) at Cayey Observatory (site 24). Alternatively, the unit may be composed 

of Upper Cretaceous limestone (Kc-Cotui Formation or Ksl-Sabana Grande Formation), 

which are exposed in low hills 1.5 km to the north (Volckmann, 1984). The unit below 

31 m depth, Vs=1675 m/s, is interpreted to be volcanic rock (Kl/Kr). The VS30 for this 

site is 435 m/s making this a NEHRP class “C” site. 

25

This site is located in southwest Puerto Rico and lies within a low mountainous 

area between two large alluvial valleys (figs. 1 and 2). The survey was conducted on an 

athletic field constructed on exposed Lower Cretaceous bedrock, serpentinite (Kjs); see 

photo above (Volckmann, 1984). The graph above shows two distinct velocity layers 

that we interpret to be weathered (saprolite) Kjs (Vs=200 m/s, from 0- to 11-m depth) 

and unweathered bedrock (Vs=1,320 m/s below 11-m depth). The VS30 for this site is 

650 m/s, which makes this a NEHRP class “C” site. The NEHRP description of a class 

“C” site defines it as very dense soil-soft rock, which is somewhat misleading in this case 

as geologic maps and the exposure would indicate that this site is located on bedrock. In 

locations where weathering depths are less, this material would be NEHRP class “B”. 

26

Although four Vs layers are determined from the data at this site (figs. 1 and 2), 

we believe that only two primary geologic units are sampled. The uppermost layer (0 to 

1.5 m) probably represents a combination of turf, soil, and artificial fill. The velocity of 

87 m/s is very low for naturally occurring geologic materials. Due to the thinness of the 

unit, it has little effect on the calculated VS30 or NEHRP site-classification value. Two 

layers with similar velocity were determined for the intervals 1.5 to 8.5 m (Vs=140 m/s) 

and 8.5 to 26.0 m (Vs=160 m/s). We believe that both of these units are associated with 

river alluvium and alluvial fan deposits. The change in Vs at 8.5 m probably represents a 

change in overall physical property (grain size) characteristics and/or a transgression into 

an older alluvial fan or Pleistocene terrace(?) unit. Velocities determined at this site are 

consistent with those interpreted for other Qaf and Qal deposits. An alluvial fan unit 

thickness of approximately 25 m is not unreasonable for the depositional environment at 

this site. At the town of Salinas, 35 km to the east, a similar geologic environment is 

present, and geologic contouring of the base of the permeable alluvium beneath the 

alluvial fan varies from 15- to 38-m depth (McClymonds and Ward, 1966). A single unit 

interpretation for this site is supported by the constant VP velocity of 1,580 m/s, which is 

typical of water-saturated, unconsolidated to weakly consolidated coarse-grained 

lithologies that dominate alluvial fan deposits. We believe that the highest velocity unit 

(Vs=505 m/s), below 26.0-m depth, represents the Ponce Limestone (Tp-Miocene). An 

alternative interpretation is that erosion has removed all, or most, of the Ponce Limestone 

unit, and the lower velocity layer is sampling all, or part of, the Juana Diaz Formation 

(Oligocene to Miocene). This unit, described as calcareous sandstone overlain by chalky 

limestone, may be seismically similar to the overlying Ponce Limestone (Krushensky and 

Monroe, 1975). The VS30 for this site is 163 m/s and classifies as a NEHRP class “E” 

(soft soil). 

27

This survey was conducted on an excavated bedrock bench above, and to the 

southwest of, the dam at Lake Cerrillos. The 89-m-long profile passed within 5 m of 

PRSN’s broadband seismic station, CELP. Surface conditions at this site consisted of a 

thin (10 to 15 cm) veneer of soil containing loose cobbles and coarse material. Exposures 

in areas excavated for the dam show dipping bedrock units and fractured rock in the near 

surface (see dipping strata and seismograph station in photo). Although only the 

Monserate Formation (Tm) was imaged at this site, three distinct Vs layers were 

calculated (Krushensky and Monroe, 1975). The shallowest layer from 0 to 4 m, Vs=365 

m/s, is interpreted to be loose surface debris accumulated during dam construction and 

probably includes some highly fractured bedrock. The intermediate velocity layer from 4 

to 13 m, Vs=865 m/s, likely correlates with weathered and fractured rock related to 

construction blasting to form the bench area and to inherent natural fractures. We 

interpret the higher velocity layer from 13 to 50 m, Vs=1,545 m/s, to represent a 

transition to more competent bedrock. The VS30 of this site (925 m/s, NEHRP “B”) 

strongly reflects the influence of the shallowest and intermediate velocity layers that 

predominantly consist of soil, rock rubble, and transitionally fractured and weathered(?) 

bedrock. 

28

Data at this site were acquired on a landslide deposit resting on the flank of a 

bedrock knob in the central mountainous area (figs. 1 and 2). Bedrock in this area 

consists of Upper Cretaceous volcanic sequences. Knobs of volcanic rock are often 

surrounded by extensive landslide deposits and in most areas the near-surface bedrock 

has been weathered. 

The Vs and Vp velocity versus depth curves for this site show primarily two 

distinct velocity units. We interpret the upper two units from the surface to 34-m depth, 

Vs=155 and 270 m/s, to be colluvium (Qc), landslide (Ql) and weathered bedrock 

material. Below 34 m, we interpret the Vs=1,470 m/s unit to be relatively unweathered 

volcanic bedrock of the Malo Breccia (Kma/Kmaf). The Malo Breccia is composed of 

very thick bedded andesite and basaltic pryoclastic and tuff material (Briggs, 1971). The 

VS30 for this site is 225 m/s making it a NEHRP class “D” site (stiff soil). The Vs value 

of 270 m/s may be a good representative estimate for the widespread, thick landslide and 

colluvial sections found throughout the upper mountainous central core area of Puerto 

Rico. 

29

This profile was acquired in an intermountain valley approximately 20 km south 

of the coast (figs. 1 and 2). The area is rapidly developing as a suburb of San Juan. The 

simplified near-surface stratigraphy consists of unconsolidated to weakly consolidated 

alluvium (Qal), older alluvial terrace (Qt), and alluvial fan (Qaf) deposits overlying 

Cretaceous interbedded volcanic and volcaniclastic bedrock. In places these Cretaceous 

rocks have been altered by the tropical to subtropical environment to produce variably 

thick sections, depending upon rock type, of highly weathered bedrock (saprolite) in the 

near surface. 

We interpret the layer from 0- to 7-m depth, within 200 m/s, to be alluvial and 

terrace material (Qt). The layer from 7- to- 42 m depth, Vs=325 m/s, is believed to be 

older terrace and/or alluvial fan and probably weathered bedrock. The Vs of 325 m/s is 

within the range of older terrace deposits (Qt), as well as saprolite soil identified during 

the 2004 surveys in Puerto Rico (Odum and others, in press). We interpret the layer 

below 42-m depth with a Vs=1,910 m/s, to be Upper Cretaceous Los Negros Formation 

(Kn). The Los Negros Formation is composed of thick-bedded to massive basalt tuff, 

which may show some degree of alteration in proximity to the Caguas pluton (Rogers, 

1979). 

VS30 for this site is 285 m/s making it a NEHRP class “D” site. There is a 

possibility of a strong site resonance at about 1.7 Hz based on the reflection time from the 

high velocity contrast boundary between the older terrace deposits and Kn at 42-m depth. 

30

This profile was acquired in an intermountain valley approximately 20 km south 

of the coast (figs. 1 and 2). The site is 5.5 km southeast of the Caguas soccer field area 

(site 22) and is geographically similar to that site. The simplified near-surface 

stratigraphy of this area consists of unconsolidated to weakly consolidated alluvium 

(Qal), older alluvial terrace (Qt), and alluvial fan (Qaf) deposits overlying Late 

Cretaceous intrusive rock (Rogers, 1979). In places, these Cretaceous rocks have been 

altered by the tropical to subtropical environment into variably thick sections, depending 

upon rock type, of highly weathered bedrock (saprolite) in the near surface. 

The Vs and Vp depth versus velocity curves for this site are presented above 

along with a lithologic interpretation of the Vs profile. The shallowest layer with a Vs= 

235 m/s, from 0- to 6- m depth, is interpreted to be modern soil and alluvial material 

(Qal?-Qt). The layer from 6 to 21 m, Vs=395 m/s, is interpreted to be predominantly 

older alluvial terrace and alluvial fan material and may include some weathered bedrock. 

The unit below 21 m, Vs=660 m/s, is interpreted to be weathered Late Cretaceous Caguas 

pluton, which is composed primarily of granodiorite. The velocity for this plutonic rock 

is similar to that determined for the Late Cretaceous San Lorenzo granodiorite (775 m/s) 

profiled near Humacao (Odum and others, in press). The VS30 for this site is 395 m/s 

making it a NEHRP class “C” (very dense soil-soft rock) site. 

31

This profile was acquired on the grounds of the Caguas Observatory, which is the 

site of a USGS geomagnetic instrument and a PRSN seismograph station. The city of 

Cayey is approximately 20 km north of the ocean and is constructed within a mountain 

valley (figs. 1 and 2). The observatory grounds lie on a gently sloping alluvial fan and 

alluvial terrace surface, which is covered by various thicknesses of colluvium. 

Underlying bedrock consists of Cretaceous volcanic and volcaniclastic rocks that show 

various depths of weathering depending on rock type. 

The Vs- and Vp-depth versus velocity curves for this site are presented above 

along with a lithologic interpretation of the Vs profile. The uppermost three velocity 

layers, 180 m/s, 245 m/s and 305 m/s, from 0- to 20-m depth, are interpreted to be 

modern soil and colluvium (Qc), and alluvial fan and terrace (Qaf and Qt) sections. The 

unit below 20- depth is geologically mapped as Formation A, which consists of 

interstratified volcanic breccia, conglomerate, and volcaniclastic sandstone and siltstone 

(Berryhill and Glover, 1960). Depth of weathering of these units is unknown. Poorly 

constrained reflection data hints at a velocity layer, Vs=800 m/s, below 100-m depth. 

The VS30 for this site is 285 m/s making it a NEHRP class “D” (stiff soil) site. 

32

This site lies a few kilometers inland from the southern Puerto Rico coast on the 

gently sloping surface of an alluvial fan (figs. 1 and 2). No geologic map exists for this 

area, but it is believed that the near-surface geology is similar to mapped geology a few 

kilometers to the west and consists primarily of alluvial fan material over Cretaceous 

volcanic sequences. 

The depth versus velocity curve for this site shows four velocity layers. We 

believe, however, that only two geologic units are imaged. We interpret the uppermost 

layers, 0- to 4.5-m (Vs=225 m/s) and 4.5- to 10- m depth (Vs=345 m/s) to be alluvial fan 

material (Qaf). Water wells drilled to the base of the alluvium in the neighboring 

quadrangle show the thickness of fan material ranging from thin (couple of meters) near 

the mountains to greater than 40 m near the coast (Berryhill, 1960). The velocities of 

these upper two units are consistent with those interpreted for other alluvial fan deposits 

on the island. The unit from 10- to 2-m depth, Vs=620 m/s, is interpreted to be Ka 

Formation, which consists of volcanic andesite flow and interstratified volcaniclastic 

material. This is the same volcanic material that is found at the Cayey Observatory (site 

24). The velocity for this layer may include some Qaf material, as well as weathered Ka 

and/or volcaniclastic material. The velocity layer beneath 25-m depth, Vs=1,675 m/s, is 

also interpreted to be more competent Ka material. The VS30 for this site is 460 m/s, 

which makes it a NEHRP class “C” (very dense soil-soft rock) site. 

33

This survey was conducted on the Community University-Humacao track in the 

city of Humacao, which is located approximately 6 km inland from the east-central coast 

(fig. 1). The majority of the urban area, which includes the track complex, is built upon 

alluvial fan (Qaf) material deposited by the Río Humacao as it exits the mountainous area 

a few kilometers to the west. The Qaf unit (Holocene to Pleistocene) consists of

unconsolidated to weakly consolidated, poorly to well sorted, clay- to boulder-sized 

material in fans and stratified valley-fill deposits as much as 25m thick (M’Gonigle, 

1978). The Qaf material unconformably overlies a complex of bedrock units including 

granodiorite of the San Lorenzo Formation (Upper Cretaceous) and lava and 

volcaniclastic rocks of the Pitahaya Formation (Lower Cretaceous). 

Two near-surface Vs layers were identified (0 to 1.5 m, Vs=325 m/s and 1.5 to 

2.5m, Vs= 490 m/s). We interpret these upper two Vs layers to be compacted artificial 

fill (af) and engineered soil associated with the construction of the campus track complex. 

We interpret the unit from 2.5- to 25.0-m depth, Vs=290 m/s, to be alluvial fan (Qaf) 

material. Geologic mapping indicates a typical thickness of 25 m for the Qaf unit in this 

area, which correlates well with the 21-m thickness determined from the seismic 

refraction-reflection data. We interpret the unit below 25.0 m, Vs=775 m/s, to be 

weathered San Lorenzo Formation granodiorite (Klg). Jaca and Sierra Testing 

Laboratoreis (2002) drilled a 30 m deep borehole at a site located on a hill 3 km from our 

profile. They reported that only weathered San Lorenzo granodiorite was encountered in 

the borehole. Their downhole velocity calculations showed a range of Vsfrom 450 to 760 

m/s, similar to our measurements, with a 1,220 m/s measurement at the very bottom of 

the hole. The calculated VS30 for this site is 330 m/s and classifies it as NEHRP “D” 

(stiff soil). 

34
This site lies southwest of the city of Fajardo at an elevation of approximately 20 

m (fig. 1). The ball field, across from the airport, is constructed on an alluvial fan or 

terrace surface of a small tributary to the Río Fajardo, which has a large alluvial plain to 

the southeast of the site. Nearby hills contain outcrops of the upper unit of the Fajardo 

Formation (Kfsu). We believe that this Kfsu unit is the bedrock sampled by the seismic 

data. The upper unit of the Fajardo Formation is composed of thin-bedded, locally 

cherty, tuffaceous siltstone and sandstone that may contain some calcareous layers near 

its top (Briggs and Aguilar-Cortes, 1980). 

Although five shear-wave (Vs) layers were identified, we believe that only three 

primary geologic units are represented over the imaged 50-m depth. The upper two 

layers (0 to 0.5 m, Vs=100 m/s, and 0.5 to 2.0 m, Vs=210 m/s) represent a thin layer of 

turf and modified soil and a thicker layer of unconsolidated colluvium and alluvium, 

respectively. Based on the geomorphic position of this site, the layer from 2.0 to 10.0 m, 

Vs=365 m/s, is interpreted to represent weakly consolidated and possibly weakly 

laterized clay, as well as fine- to coarse-grained colluvium, alluvium and alluvial fan 

material (Qaf-Qal). Because of the higher Vs, we speculate that some of these materials 

may be older, and more consolidated, than the recent Qal material in the Río Fajardo 

flood plain. It is also possible that this layer may contain weathered bedrock. The 

velocity layers from 10-to 44-m depth, Vs=550 m/s, and 44- to 50-m depth, Vs=920 m/s, 

are both believed to represent the upper unit of the Fajardo Formation with the 67 percent 

difference in velocity being either the result of weathering or a change in formation 

lithology. The VS30 for this site is 426 m/s and corresponds to NEHRP soil type “C” 

(very dense soil-soft rock). 

35

Summary of Site NEHRP Classifications 

Site characterization data were acquired at 27 sites located within and around 

urban areas, intermountain valleys, and at two PRSN broadband seismograph stations 

throughout Puerto Rico. Geologic maps indicate that near-surface units range in age 

from Holocene (bay mud, beach, and alluvial deposits) to Cretaceous (volcaniclastic 

sediments and plutonic rock). Overall data quality is considered to be good and velocity 

columns to a depth of at least 30 m were estimated for all but the Escambron site (site 1, 

see text for an explanation). In this study, all interpreted layer thicknesses and velocities, 

to a depth of 30 m, were used in the calculation of VS30 for a specific location. This 

means that calculations include the velocities and calculated thicknesses of the one or two 

near-surface thin layers of artificial fill even though these layer thicknesses are often 

poorly constrained. From the calculated velocity data, VS30 and NEHRP site 

classifications were determined, based on the table 2 categories, and are shown in table 3 

along with the depth and value of the highest Vs and Vp determined for each site. Figure 

3 groups sites by NEHRP classification and shows the following distribution: three class 

“E” (VS30 below180 m/s), ten class ”D” (VS30 between 180 and 360 m/s), nine class “C” 

(VS30 between 360 and 760 m/s), and four class “B” (VS30 greater than 760 m/s). 

At all three NEHRP class “E” sites, data imaged thick sections (15 to 25 m) of 

modern alluvial and/or lagoonal material. Site 7 (Estado P. Cepeda), in the San Juan 

area, is located on a high velocity artificial-fill pad that overlies a thick section (17 m) of 

low velocity bay mud and swamp deposits. All three of these sites have VS30 values that 

are close to the lower class “D” boundary indicating that in these areas the thickness of 

the low velocity unit is the major factor in determining the site NEHRP class. However, 

the thick (9 m) artificial fill layers at site 7 elevates the VS30 value toward the class “D” 

boundary. 

For the 10 class “D” sites, the lowest three VS30 values were found in the 

Mayagüez area where geologically mapped “bedrock” (Cretaceous) has been deeply 

weathered by the tropical environment to produce a saprolite soil. Saprolites are 

transitional by nature and show an increase in both strength and velocity with depth. 

Velocities for a formation are variable with depth depending on site location as seen at 

site 15 (that is, as low as VS =140 m/s in the upper 17 m with a sharp jump to 2,400 m/s 

below 17 m). Such sharp changes in velocities can produce local site resonance in the 

event of an earthquake. If the saprolite zone at these sites were a few meters thicker, then 

the site classification would be “E” rather than “D”. Other class “D” sites have thick 

sections of unconsolidated beach, alluvial deposits, weathered bedrock, or landslide 

material. In the case of site 26, Humacao-CUH track, the VS30 value of 330 m/s is close 

to the lower NEHRP class “C” boundary. It is the thicknesses of the alluvial and alluvial 

fan units at this location that are the primary factors for the class “D” rating, as the 

underlying Cretaceous intrusive granodiorite has a VS of 775 m/s. 

All sites that were interpreted to be NEHRP class “C” typically have a thick 

section of Pleistocene to Tertiary, weakly indurated blanket, alluvial fan and older terrace 

deposits, which in all cases except one, have Vs between 360 and 760 m/s. The one 

exception is site 4 (Carolina track) where the unconsolidated unit is thin, and the VS30 is 

highly influenced by the underlying Tertiary bedrock. Additionally, all class “C” sites 

imaged either a Tertiary formation (limestone and/or clastic sedimentary rock) at depth or 

more competent intrusive or volcaniclastic bedrock beneath an upper weathered zone. 

36

All of the “C” sites had VS30 velocities in the lower to middle range of this category and 

therefore were not close to being considered borderline “B” sites. All four class “B” sites 

imaged Tertiary or older intrusive or volcaniclastic bedrock near the surface. 

In general, while there may be a wide range of individual velocities within a 

column, it is often the thickness of the layer overlying Tertiary or older bedrock that is 

the major factor in determining whether a site is classified as “C” or “D” (for example, 

site 15). At “B” class sites, the classification is clear at the sites sampled; however, the 

weathering depth is variable and needs to be considered when extrapolating this 

classification category to the surrounding areas. 

When analyzing and comparing individual and grouped VS30 values, two factors 

involved in the data calculation need to be remembered: (1) in this study all units at a 

survey site, including artificial fill (af), are used in the calculation of VS30 and the 

subsequent NEHRP class category and (2) near-surface and thin velocity layers, often 

consisting of one or two layers of artificial fill, are sometimes poorly constrained with 

respect to thickness; however, the calculated thickness values are used in calculating 

VS30. Neither factor played a significant role in the calculated site classifications during 

this study. They, however, do have an effect, and under certain circumstances could 

change a site’s NEHRP classification. Figure 4 examines the effect of site specific 

presence and thickness of artificial fill units on VS30 calculations at a class “D” site (El 

Seco, site 14). The first column shows the results as determined in this report. The next 

two hypothetical calculations show the changes in calculated VS30 value by first 

removing the second unit af layer and then by removing both layers. The resulting 

change in the VS30 value is a reduction of approximately 3 percent and 0 percent 

respectively, at this site. In this case there was only a minor shifting of the Vs30 value 

toward the next lower site classification. It is recommended that when VS30 values and/or 

class designations are being projected beyond the limits of where they were obtained that 

factors such as surface layer thickness and velocity be examined for relevancy and 

modified as appropriate. 

37

Figure 3. Sites arranged by NEHRP site classification code in order of increasing VS30. 

38

Figure 4. Influence of calculated Vs30 values at a site where one or more artificial 

surface high velocity layers (in example, artificial fill) exists. First column shows results 

from this report; second column shows results if both artificial layers have Vs of lowest 

fill; third column shows results of natural setting with no artificial fill. 

Acknowledgments 

We wish to thank the students from the University of Puerto Rico-Mayagüez 

geophysics and engineering classes and the staff of PRSN-Mayagüez for their generous 

contributions of time, effort, and enthusiasm during the data acquisition phase of this 

study. Without their efforts in the field, let alone the endurance of long van rides and 

early morning departures on weekends, the collection of data would have been far less 

enjoyable! The USGS would also like to recognize the financial support provided by 

Puerto Rico Seismic Network and Puerto Rico Strong Motion Program, which made this 

two year study possible. This paper was improved by comments from Richard Dart, Beth 

Burton and Eugene Schweig. 

References Cited 

Bawiec, J.W., 2001, Geologic terrains of Puerto Rico, in Bawiec, W., ed., Geology, 

geochemistry, geophysics, mineral occurrences and mineral resource assessment for 

the Commonwealth of Puerto Rico: U.S. Geological Survey Open-File Report 98-38, 

CD. 

39

Berryhill, H., L., Jr. and Glover, L., 1960, Geologic map of the Cayey quadrangle, Puerto 

Rico: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-319, 

scale 1:20,000. 

Berryhill, H., L., Jr., 1960, Geology of the Central Aguirre quadrangle, Puerto Rico: U.S. 

Geological Survey Miscellaneous Geologic Investigations Map I-318, scale 1:20,000. 

Borcherdt, R.D., and Gibbs, J.F., 1976, Effects of local geological conditions in the San 

Francisco Bay region on ground motions and the intensities of the 1906 earthquake: 

Seismological Society of America Bulletin, v. 66, p. 467-500. 

Building Seismic Safety Council (BSSC), 1997, NEHRP, recommended provisions for 

seismic regulations for new buildings, Part 1-Provisions (FEMA Federal emergency 

Management Agency 302), 290 p. 

Briggs, R.P., 1965, Geologic map of the Barceloneta quadrangle, Puerto Rico: U.S. 

Geological Survey Miscellaneous Geologic Investigations Map I-421, scale 1:20,000. 

Briggs, R.P., 1966, The blanket sands of northern Puerto Rico, in Transactions, 3rd 

Caribbean Geologic Conference, Jamaica 1962, p. 60-69. 

Briggs, R.P., 1968, Geologic map of the Arecibo quadrangle, Puerto Rico: U.S. 

Geological Survey Miscellaneous Geologic Investigations Map I-551, scale 1:20,000. 

Briggs, R.P., 1971, Geologic map of the Orocovis quadrangle, Puerto Rico: U.S. 

Geological Survey Miscellaneous Geologic Investigations Map I-615, scale 1:20,000. 

Briggs, R.P., and Akers, J.P., 1965, Hydrogeologic map of Puerto Rico and adjacent 

islands: U.S. Geological Survey Hydrologic Investigations Atlas Map HA-197, scale 

1:240,000. 

Briggs, R.P., and Aguilar-Contes, Eduardo, 1980, Geologic map of the Fajardo and Cayo 

Icacos quadrangle, Puerto Rico: U.S. Geological Survey Miscellaneous Geologic 

Investigations Map I-1153, scale 1:20,000. 

Curet, A.F., 1986, Geologic map of the Mayagüez and Rosario quadrangles, Puerto Rico: 

U.S. Geological Survey Miscellaneous Investigations Series Map I-1657, scale 

1:20,000. 

Dillon, W., ten Brink, U., Frankel, A., Rodriguez, R., and Mueller, C., 1999, Seismic and 

tsunami hazards in the northeast Caribbean addressed at meeting, EOS, Transactions, 

American Geophysical Union, v. 80, no. 26, p. 309-10. 

40

Frankel, A., Mueller, C., Barnhard, T., Perkins, D., Leyendecker, E.V., Dickman, N., 

Hanson, S., and Hopper, M., 1996, National seismic-hazard maps: Documentation 

June 1996, U.S. Geological Survey Open-File Report 96-532, p.110. 

Fumal, T. E., 1978, Correlations between seismic wave velocities and physical properties 

of near-surface geologic materials in the southern San Francisco Bay Region: Santa 

Cruz,University of California, M.S., 113 p. 

IBC, 2002, International Building Code 2003: International Building Council, 672 p. 

Jaca and Sierra Testing Laboratories, 2002, Report on the geophysical exploration 

performed at six seismic station sites for the downhole and direct seismic 

investigation project, Puerto Rico: Report No. 5737, submitted September 10, 2002 to 

Jose A. Martinez Cruzado, University of Puerto Rico, Mayaguez, 20 p. 

Kaye, C. A., 1959, Geology of the San Juan metropolitan area, Puerto Rico: U.S. 

Geological Survey Professional Paper 317-A, 48 p. 

Krushensky, R.D., and Monroe, W.H., 1975, Geologic map of the Ponce quadrangle, 

Puerto Rico: U.S. Geological Survey Miscellaneous Investigations Series Map I-863, 

scale 1:20,000. 

Mann, P., 2005, Introduction, in Mann, P., ed., Active tectonics and seismic hazards of 

Puerto Rico, the Virgin Islands, and offshore areas: Geological Society of America 

Special Paper 385, p. 1-12. 

Mann, P., Calais, E., Ruegg, J.C., DeMets, C., Jansma, P.E., and Mattioli, G.S., 2002, 

Oblique collision in the northeastern Caribbean from GPS measurements and 

geologic observations: Tectonics, v. 21, no. 6, p. 1057. 

McCann, W.R., 1985, Earthquake hazards of Puerto Rico and the Virgin islands: U.S. 

Geological Survey Open-File Report 85-731, p. 53-72. 

McClymonds, N.E., and Ward, P.E., 1966, Hydrologic characteristics of the alluvial fan 

near Salinas, Puerto Rico: U.S. Geologic Survey Professional Paper 550-C, p. 231­

234. 

M'Gonigle, J.W., 1978, Geologic map of the Humacao quadrangle, Puerto Rico: U.S. 

Geological Survey Miscellaneous Geologic Investigations Map I-1070, scale 

1:20,000. 

Mooney, H.M., 1984, Handbook of Geophysical Exploration: Volume 1, Seismic, Bison 

Instruments, Inc. 

41

Monroe, W.H., 1969, Geologic map of the Moca and Isabela quadrangle, Puerto Rico: 

U.S. Geological Survey Miscellaneous Geologic Investigations Map I-565, scale 

1:20,000. 

Monroe, W.H., 1977, Geologic map of the Carolina quadrangle, Puerto Rico: U.S. 

Geological Survey Miscellaneous Geologic Investigations Map I-1054, scale 

1:20,000. 

Odum, J.K., Williams, R.A., Stephenson, W.J., and Worley, 2003, Near-surface S-wave 

and P-wave seismic velocities of primary geological formations on the piedmont and 

Atlantic coastal plain of South Carolina, U.S. Geological Survey Open-File Report 

03-043, 14 p. (http://pubs.usgs.gov/of/-03-043). 

Odum, J.K, Williams, R.A., Stephenson, W.J, von Hillebrandt-Andrade, C., Asencio, E., 

and Cameron, A., in press, Detailed description of near-surface shear wave velocity 

versus depth profiles, VS30, and NEHRP classifications for fourteen sites in Puerto 

Rico: 17th Caribbean Geologic Conference, July 18-22, San Juan, Puerto Rico, Special 

Paper, CD. 

Pease, M.H., Jr., 1968, Geologic map of the Aguas Buenas quadrangle, Puerto Rico: U.S. 

Geological Survey Miscellaneous Geologic Investigations Map I-479, scale 1:20,000. 

Pease, M.H., Jr., and Monroe, W.H., 1977, Geologic map of the San Juan quadrangle, 

Puerto Rico: U.S. Geological Survey Open-File Report 75-252, scale 1:20,000. 

Renken, R.A., Ward, W.C., Gill, I.P., Gomez-Gomez, Fernanado, and RodriguezMartinez, J., 2002, Geology and hydrogeology of the Caribbean Islands aquifer 

system of the Commonwealth of Puerto Rico and the U.S. Virgin Islands: U.S. 

Geological Survey Professional Paper 1419, 140 p. 

Rogers, C.L., 1979, Geologic map of the Caguas quadrangle, Puerto Rico: U.S. 

Geological Survey Miscellaneous Geologic Investigations Map I-1152, scale 

1:20,000. 

Seed, H.B., Romo, M.P., Sun, J.I., Jaime, A., and Lysmer, J., 1988, the Mexico 

earthquake of September 19, 1985--relationships between soil conditions and 

earthquake ground motions: Earthquake Spectra, v.4, p. 687-729. 

Volckmann, R.P., 1984, Geologic map of the San German quadrangle, Puerto Rico: U.S. 

Geological Survey Miscellaneous Geologic Investigations Map I-1558, scale 

1:20,000. 

42

Williams, R.A., Stephenson, W.J., and Odum, J.K., 1998, Surface-seismic imaging for 

NEHRP soil profile classifications and earthquake hazards in urban areas, in 

Dakoulas, P., Yegian, M., and Holz, R. D., (eds), Geotechnical Special Publication 

no. 75: Geotechnical Earthquake Engineering and Soil Dynamics III, v. 1, American 

Society of Civil Engineers, Proceedings of a Specialty Conference, Seattle, WA, p. 

166-177. 

Williams, R.A., Stephenson, W.J., Frankel, A.D., and Odum, J.K., 1999, Surface seismic 

measurements of near-surface P-and S-wave seismic velocities at earthquake 

recording stations, Seattle, Washington: Earthquake Spectra, v. 15, no. 3, p. 565-584. 

Williams, R.A., Stephenson, W.J., Odum, J.K., and Worley, D.M., 2003, Seismic 

velocities from high-resolution surface-seismic imaging at six ANSS sites near 

Memphis, Tennessee: U.S. Geological Survey Open-File Report 2003-218, 11 p. 

Zapp, A.D., Bergquist, H. R., and Thomas, C.R., 1948, Tertiary geology of the coastal 

plains of Puerto Rico: U.S. Geological Survey Oil and Gas Preliminary Map 85, scale 

1:62,500, Summary in Petróleo Interamericano, 7, no. 1, p. 52-54. 

43