U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the

PUERTO RICO ELECTRIC POWER AUTHORITY

Open-File Report 2007-1259

Stratigraphy Stratigraphy, Structure, and Geologic and

Coastal Hazards in the Peñuelas to Salinas 

Area, Southern Puerto Rico: A Compendium 

Coastal Hazards in the Peñuelas to Salinas 

of Published Literature

, Structure, and Geologic and 

Area, Southern Puerto Rico: A Compendium 

of Published Literature
Stratigraphy, Structure, and Geologic 

and Coastal Hazards in the Peñuelas 

to Salinas Area, Southern Puerto Rico: 

A Compendium of Published Literature

By Jesús Rodríguez-Martínez

Prepared in cooperation with the Puerto Rico Electric Power Authority

Open-File Report 2007–1259

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

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Suggested citation:

Rodríguez-Martínez, Jesús, 2007, Stratigraphy, structure, and geologic and coastal hazards in the Peñuelas to 

Salinas area, southern Puerto Rico: A compendium of published literature: U.S. Geological Survey Open-File 

Report 2007-1259, 27 p.
iii

Contents

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

Introduction.......................................................................................................................................

2

Purpose and Scope ...................................................................................................................

2

Geomorphology of the Study Area ...........................................................................................

2

Geology.............................................................................................................................................

3

Stratigraphy..............................................................................................................................

3

Early Tertiary Age and Older Rocks..................................................................................

3

Middle Tertiary Age Rocks ...............................................................................................

4

Juana Díaz Formation...............................................................................................

4

Unnamed Pelagic Rocks or Deposits........................................................................

5

Ponce Limestone ......................................................................................................

5

Quaternary Age Fan-Delta and Alluvial Deposits............................................................

5

Structure ...................................................................................................................................

6

Great Southern Puerto Rico Fault Zone............................................................................

7

Late Quaternary Faulting ..................................................................................................

7

Geologic Risks...................................................................................................................................

7

Spatial and Temporal Distribution of Seismic Activity.............................................................

8

Potential for Liquefaction, Landslides and Tsunamis...............................................................

9

Summary.........................................................................................................................................

11

References Cited.............................................................................................................................

12

Glossary...........................................................................................................................................14
iv

Figures

1. Map showing location of the Peñuelas to Salinas study area .......................................15

2. Map showing surficial geology of the Peñuelas to Salinas study area .........................16

3. Recent stratigraphic nomenclatures used in the study area ..........................................17

4. Geologic section across southern Puerto Rico fan-delta plain.......................................18

5–15. Maps showing

 5. Major facies of the Ponce Limestone and equivalent beds.....................................19

 6. Thickness of the fan-delta and alluvial deposits......................................................20

 7. Percentage of sand and gravel in the fan-delta plain deposits...............................21

 8. Location and orientation of the Great Southern Puerto Rico Fault zone 

 in the study area ....................................................................................................22

 9. Major tectonic faults and historic earthquakes in Puerto Rico and its 

 vicinity....................................................................................................................23

 10. Geographical distribution of the magnitude of completeness values 

 in Puerto Rico and its vicinity as determined from the Puerto Rico 

 Seismic Network Catalog for the interval January 1986-March 2006 .................23

 11. Seismicity density in Puerto Rico and its vicinity.....................................................23

12. Inland extent of tsunami-induced flooding in the Punta Verraco and 

 Punta Cuchara quadrangles...................................................................................24

 13. Inland extent of tsunami-induced flooding in the Playa de Ponce 

 quadrangle .............................................................................................................25

 14. Inland extent of tsunami-induced flooding in the Santa Isabel quadrangle............26

 15. Inland extent of tsunami-induced flooding in the Salinas quadrangle ....................27

Tables

1. Estimated peak ground acceleration (PGA) in the study area from selected 

 sources. ..........................................................................................................................9

2. Maximum inland extent of tsunami-induced coastal flooding within the 

 study area.......................................................................................................................1
Conversion Factors, Datum, and Acronyms

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

feet per mile (ft/mi) 0.1905 meter per kilometer (m/km)

feet per squared second (ft/s2

) 0.3048 meter per squared second (m/s2)

Area

square foot (ft2

) 929.0 square centimeter (cm2)

square foot (ft2

) 0.09290 square meter (m2)

square mile (mi2

) 2.590 square kilometer (km2)

Datum:

Horizontal Datum - Puerto Rico Datum, 1940 Adjustment

Sea level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929 (NGVD of 

1929) -- a geodetic datum derived from general adjustments of the first-order level nets of both the 

United States and Canada, formerly called “Sea Level Datum of 1929.” 

Acronyms used in this report:

GNPRFZ Great Northern Puerto Rico Fault Zone

GSPRFZ Great Southern Puerto Rico Fault Zone 

PGA peak ground acceleration

PRDNER Puerto Rico Department of Natural and Environmental Resources

PREPA Puerto Rico Electric Power Authority

PREQB Puerto Rico Environmental Quality Board

PRSN Puerto Rico Seismic Network

USGS U.S. Geological Survey

UPRM University of Puerto Rico at Mayagüez

Translations

Spanish English

Lago Lake

Río River
Abstract

The Puerto Rico Electrical Power Authority has 

proposed construction of a pipeline to convey natural 

gas from the municipio of Peñuelas to the Aguirre 

thermoelectric power plant in the municipio of Salinas 

in southern Puerto Rico. To ensure that the geologic 

conditions along the possible routes do not represent a 

threat to the physical integrity of the natural gas pipeline, 

and thus comply with State and Federal regulations, 

the Puerto Rico Electrical Power Authority requested 

the U.S. Geological Survey to provide a synthesis of 

published literature of the geology of the coastal plain in 

the Peñuelas to Salinas area.

The study area is located in part of the Southern 

Coastal Plain of Puerto Rico. In the area that extends 

from the municipio of Peñuelas eastward to the Laguna 

de las Salinas at Ponce, a distance of about 5 miles, the 

study area is underlain by middle Tertiary carbonate 

units. Eastward from the Laguna de las Salinas to the 

pipeline terminus at the Aguirre power plant in Salinas, 

a distance of about 30 miles, the terrain is underlain by 

fan-delta deposits of Quaternary age. The carbonate 

units and the fan-delta deposits are underlain by early 

Tertiary and older-age volcaniclastics with subordinate 

sedimentary rocks and lavas. The Great Southern Puerto 

Rico Fault Zone is the principal geologic structural 

feature in southern Puerto Rico. At present, the Great 

Southern Puerto Rico Fault Zone is considered largely 

quiescent, although it apparently is associated with 

minor earthquakes. There is no evidence of terrestrial, 

late Quaternary faulting within the Peñuelas to Salinas 

area. Seismic activity in this area mostly originates from 

extension zones of more distal shallow sources such as 

Mona Canyon to the northwest and the Anegada Trough 

northeast of the island of Puerto Rico. The magnitude 

of completeness of earthquakes in the study area ranges 

from 2.0 to 2.5. The seismic density for the southern 

coast including the study area is about 0.128 earthquakes 

per square mile, which is close to the average for 

southwestern Puerto Rico.

The estimated maximum peak ground acceleration 

most likely to occur in the study area, due to shallow 

depth seismicity with 2 percent probability of 

exceedance in 50 years, is 9 feet per second squared, as 

obtained by modeling results. The estimated peak ground 

acceleration with 2 percent probability of exceedance 

in 50 years, due to deep seismicity is 7 feet per second 

squared. In Ponce, the probability of exceedance per year 

is higher than 0.1 for the peak ground acceleration values 

less than 1 that result from shallow depth seismicity 

sources such as the Mona Passage extension zone.

The potential for liquefaction due to seismic activity 

may exist in areas near the coastline that have loosely 

to poorly consolidated sedimentary deposits and a 

water table close to or at the land surface. Slope failure 

susceptibility within the study area, due to rainfall and 

seismic activity, may be limited to the area that extends 

westward from Laguna de las Salinas to Peñuelas. In 

this area, foothills with slopes exceeding 10 degrees 

are close to the coastline and are underlain by clayey 

limestone and marls. In the remaining part of the study 

area, eastward from Laguna de las Salinas to Salinas, the 

land is either nearly flat or has a slope of less than 10 

degrees; consequently, the susceptibility to landsliding 

(slope failure) caused by seismic activity and rainfall is 

considered to be minimal or nonexistent.

Based on modeling results from a previous study, 

the estimated maximum inland extent of tsunamiinduced flooding is 2,600 feet in the Laguna de las 

Salinas and Boca Chica, located in Ponce and Juana 

Díaz, respectively. Flooding about 3,000 and 2,800 feet 

from the coastline are estimated for areas near Punta 

Cabullón and Jobos areas, respectively. According to the 

modeling results, the estimated maximum runup of the 

tsunami-induced flooding ranges from 9 to 14 feet for 

the Boca Chica and Punta Cabullón areas, respectively.

Stratigraphy, Structure, and Geologic and Coastal Hazards 

in the Peñuelas to Salinas Area, Southern Puerto Rico: 

A Compendium of Published Literature

By Jesús Rodríguez-Martínez
Introduction

The Puerto Rico Electric Power Authority (PREPA), 

as part of its efforts to diversify the fossil fuel sources to 

its thermoelectric power generation plants, has proposed 

the construction of a pipeline to convey natural gas from 

storage facilities in the municipio of Peñuelas to the 

Aguirre thermoelectric power plant in the municipio of 

Salinas in southern Puerto Rico (fig. 1 at end of report). 

The proposed paths for the route that eventually will be 

selected by the PREPA for the natural gas pipeline all 

lie along the fan-delta complex of the Southern Coastal 

Plain of Puerto Rico. The pipeline will cross the coastal 

plain along municipios from Peñuelas to Salinas, with 

the exception of portions between the municipios of 

Peñuelas and Ponce where the coastal plain is narrow 

or absent and the foothills extend to the coastline. To 

ensure that geologic conditions along the possible 

route(s) do not represent a threat to the physical integrity 

of the natural gas pipeline, and thus comply with State 

and Federal regulations, the PREPA requested the U.S. 

Geological Survey (USGS) to conduct a study of the 

geology along the proposed route(s) of the natural gas 

pipeline. This study was conducted to: (1) describe the 

surface and subsurface distribution of the main rock 

units existing along the path of the proposed pipeline, 

(2) determine the magnitude and distribution of 

present and historical seismic activity in the study area,

(3) assess the potential for land liquefaction and 

landslides, and (4) assess the potential for the occurrence

of tsunamis along the coastal portion of the Peñuelas to 

Salinas area. This study will help the PREPA assess 

the probable geologic threats to the proposed natural 

gas pipeline along the Southern Coastal Plain of Puerto 

Rico. This study will also help the USGS improve the 

delineation of the hydrogeologic framework of the 

Southern Coastal Aquifer System of Puerto Rico.

Purpose and Scope

The purpose of this report is to document the 

geology in the Peñuelas to Salinas area using available 

published scientific literature. The specific objectives 

of the report are to: (1) describe and present the surface 

and subsurface distribution of the main rock types in 

the Peñuelas to Salinas area along the coastal plain, and 

(2) describe the potential for seismic and other geologic 

risks in the study area, including land liquefaction, 

landslides, and tsunami-induced coastal flooding.

The objectives of the report are achieved primarily 

by integrating the results of previous geologic studies 

conducted within the study area along the Southern 

Coastal Plain of Puerto Rico. These studies, conducted 

by the USGS and other Federal agencies, as well as 

State and private entities, include but are not limited to, 

geologic maps, hydrogeologic assessments, geophysical 

surveys, and localized geologic assessments prepared 

as part of environmental impact statements. The 

published literature used in this report was prepared 

primarily by the USGS, the University of Puerto Rico 

at Mayagüez (UPRM), the Puerto Rico Department 

of Natural and Environmental Resources (PRDNER), 

and the Puerto Rico Environmental Quality Board 

(PREQB). Unpublished USGS data were also used, 

and include lithologic logs from water well drilling and 

other stratigraphic data from deep exploratory test holes 

drilled within the study area.

Geomorphology of the Study Area

The study area occupies the southernmost portion 

of the Southern Coastal Plain of Puerto Rico and 

includes the coastal portions of all municipios between 

the municipios of Peñuelas and Salinas (fig. 1). In the 

westernmost part of the study area, between Peñuelas 

and Laguna de Salinas, the Southern Coastal Plain is 

about 5 miles long and is either very narrow or absent 

and coastal plain foothills of Tertiary age carbonates 

are incised by alluvium-filled stream valleys along the 

Río Tallaboa and Río Matilde (Monroe, 1980). From 

Ponce to Salinas, a distance of about 30 miles, the 

study area lies within the fan-delta1 complex of the 

Southern Coastal Plain (Renken and others, 2002). The 

topographic slope in the fan-delta complex is lowest 

near the coastline, ranging from 5 to 20 feet per mile 

(ft/mi), and steepest near the apex of each fan where it 

ranges from 13 to 26 ft/mi (Renken and others, 2002). 

A narrow coastal zone separates the Southern Coastal 

Plain from the Caribbean Sea, and consists mostly of 

mangrove swamps, salt flats, and minor wetlands and 

hypersaline lagoons. Bedrock hills (locally known as 

cerros), the result of normal faulting in the basement 

rocks (described later), protrude above the Southern 

Coastal Plain in the municipios of Santa Isabel and 

Salinas (Renken and others, 2002).

Caliche is a distinctive feature in soils of the study 

area, as well as the rest of the Southern Coastal Plain, 

particularly in areas underlain by carbonate rocks such as 

those in the highlands of Peñuelas and Ponce (Monroe, 

1980; Renken and others, 2002). Unique to semiarid to 

arid climates such as that of the study area, Caliche is a 

residue created by the evaporation of water saturated in 

calcium bicarbonate. Caliche forms a surficial crust with 

a thickness that ranges from a few inches to several feet 

and acts as a barrier that, together with the arid climate, 

has substantially limited the development of karst 

topography along the Southern Coastal Plain of Puerto 

Rico (Giusti, 1978).

1

 Terms in bold format are defined in the glossary.

 Stratigraphy, Structure, and Geologic and Coastal Hazards in the Peñuelas to Salinas Area: A Compendium of Published Literature
Trunk streams that traverse the fan-delta 

plain exhibit a straight to slightly sinuous channel 

morphology. Two such examples are the Río Nigua 

at Salinas, which is relatively straight and extends 

southwest from the apex of the Salinas fan apex, and 

the Río Coamo channel that drains the Coamo fan delta 

within the municipio of Santa Isabel.

Geology

The rocks underlying the wedge of middle Tertiary 

and Quaternary age rocks in the study area are early 

Tertiary and older in age (Monroe, 1980; Renken and 

others, 2002). These rocks generally are not exposed 

along the South Coastal Plain in the Peñuelas to Salinas 

area, except where exposed locally by normal faulting 

(Renken and others, 2002; Berryhill, 1960). The rocks 

are predominantly volcaniclastic, complemented by 

lesser amounts of sedimentary rocks and lavas, and are 

considered equivalent in age to rocks in the Cordillera 

Central north of the Southern Coastal Plain (Renken 

and others, 2002). The stratigraphic and structural 

relation of the early Tertiary age and older rocks in the 

Peñuelas to Salinas area and the rest of the Southern 

Coastal Plain, have been determined or inferred from 

outcrop exposures in foothills north of the study area, 

the Cordillera Central, and from analysis of geophysical 

data collected for coastal assessments of oil and seismichazard potential.

In contrast to the geology of early Tertiary age and 

older rocks, which generally are poorly understood, 

the geology of the middle Tertiary and Quaternary 

formations in the Peñuelas to Salinas area is well studied 

(Monroe, 1980). This discrepancy exists because the 

main aquifers in the Southern Coastal Plain of Puerto 

Rico are composed of middle Tertiary and Quaternaryage rocks, and thus all hydrogeologic studies of the 

Southern Coastal Plain have focused on these rocks. 

Carbonate and clastic strata of the Juana Díaz Formation 

and Ponce Limestone of middle Tertiary age underlie the 

western part of the study area from Peñuelas to Ponce 

(fig. 2 at end of report; Monroe, 1980). The eastern part 

of the study area, which extends from Ponce to Salinas, 

is underlain by fan-delta deposits consisting of gravel, 

sand, silt, and clay, predominantly of Quaternary age 

(Renken and others, 2002).

The Great Southern Puerto Rico Fault Zone 

(GSPRFZ) is the principal structural feature along the 

southern coast of Puerto Rico, and it contains a wide 

variety of fault structures (Renken and others, 2002; 

Krushensky and Monroe, 1978). The fault zone is 

formed by parallel to subparallel sinistral faults oriented 

northwest-southeast that extend diagonally across the 

south-central part of the island, and beneath the fan-delta 

plain and the Isla Caja de Muertos shelf offshore from 

southern Puerto Rico (Renken and others, 2002). The 

sedimentation pattern in the Southern Coastal Plain has 

largely been determined by the GSPRFZ. The structural 

dip of the middle Tertiary age rocks is generally 

southward, except where locally reversed northward 

by normal faulting that probably is associated with the 

GSPRFZ (Monroe, 1980).

Stratigraphy

The recent (post 1969) stratigraphic nomenclatures 

used by previous studies to represent the stratigraphy of 

the south coast of Puerto Rico are presented in figure 3 

at end of report. The nomenclature used in this report is 

adopted from Renken and others (2002), because unlike 

previous nomenclatures, it incorporates and properly 

defines the Quaternary-age deposits along the Southern 

Coastal Plain of Puerto Rico.

As noted earlier, unconsolidated to lithified, 

clastic, and carbonate deposits of middle Tertiary 

and Quaternary age are better understood than other 

stratigraphic units in the study area because they 

have been studied more than older units within the 

Southern Coastal Plain of Puerto Rico. In ascending 

order (oldest to youngest), the middle Tertiary and 

Quaternary age stratigraphic units are the Juana Díaz 

Formation (Oligocene age), unnamed pelagic deposits 

(early Miocene), Ponce Limestone (middle Miocene to 

Pliocene (?) age), and unnamed units of gravel, sand, 

silt, and clay of Pleistocene and Holocene age (figs. 2 

and 3). The middle Tertiary and Quaternary age rocks in 

the study area are underlain by undifferentiated igneous 

rocks (volcaniclastics, lavas, and intrusives) and minor 

sedimentary units of early Tertiary age and older. These 

older rocks are exposed or present at shallow depths 

locally within the study area where exposed by normal 

faulting, as noted earlier. The contact between the two 

rock sequences is highly unconformable, the result of 

substantial erosion prior to sea transgression and the 

onset of carbonate deposition.

Early Tertiary Age and Older Rocks

The volcaniclastics (sandstone, siltstone, breccia, 

and conglomerate) are by far the largest group among 

the early Tertiary age and older rocks (fig. 2), as 

indicated by examination of blocks upthrown by normal 

faulting and exposures of time-equivalent units in the 

foothills of the Cordillera Central to the north (Renken 

and others, 2002). The volcaniclastics are complemented 

by secondary amounts of igneous intrusives and lavas. 

Limestone and other non-volcaniclastic sedimentary 

rocks are minor constituents within this age-group of 

rocks.

Geology 
The lithologic character and stratigraphic relations 

of the early Tertiary age and older rocks are not well 

understood in the Peñuelas to Salinas area. These rocks 

are hundreds to more than 1,000 feet (ft) beneath the 

land surface and are either exposed or present at shallow 

depths where upthrown by normal faulting, such as at 

Cerro Aguirre in the municipio of Salinas (Berryhill, 

1960) (fig. 2). However, examination of cores obtained 

from exploratory borings as part of the site assessment 

for the proposed nuclear plant in Aguirre, Salinas, and 

examination of the upthrown blocks exposed by normal 

faulting indicate that the texture and mineralogical 

composition of the early Tertiary age and older rocks 

within the study area are similar to those of their time 

equivalents in the Cordillera Central to the north (Puerto 

Rico Water Resources Authority, 1972).

The early Tertiary age and older rocks generally 

become shallower east of Juana Díaz as the middle 

Tertiary basin gradually pinches out (fig. 4 at end of 

report). In the Salinas area, these rocks are overlain 

directly by Quaternary age deposits. This eastward 

shallowing trend is interrupted locally by normal 

faulting, either by downward or upward movement, as 

evidenced in the USGS test hole SC-2 in Salinas (fig. 4; 

for lithologic log of this test hole see plate 3 in Renken 

and others, 2002). These rocks are present about 850 ft 

below land surface in oil test well CPR-3, drilled near 

the coast in Ponce (fig. 4; detailed stratigraphy in U.S. 

Geological Survey, unpub. data). In agricultural wells 

east of the study area, these rocks are present mostly at 

depths of less than 100 ft below land surface (Renken 

and others, 2002; U.S. Geological Survey, unpub. data).

Middle Tertiary Age Rocks

In ascending order (oldest to youngest), the 

middle Tertiary units in the study area are the Juana 

Díaz Formation of Oligocene age, an unnamed pelagic 

unit of early Miocene age, and the Ponce Limestone 

of middle Miocene to presumably Pliocene age (figs. 

2 and 3). Stratigraphic boundaries between these 

two geologic units have been revised several times 

as a result of macrofaunal and microfaunal studies 

(mainly of foraminiferal assemblages and their age 

ranges), recognition of mappable lithofacies, and the 

identification of unconformities.

Juana Díaz Formation

Within the study area, the Juana Díaz Formation 

is continuously exposed along the coastal parts of the 

municipios of Peñuelas and Ponce, except where locally 

covered by alluvium in the valleys of the Río Tallaboa, 

Río Pastillo, and Río Cañas (fig. 2). East of Ponce, the 

exposures of the Juana Díaz Formation within the study 

area are limited to outliers, such as those at Cerro de los 

Muertos in Santa Isabel and Cerro Aguirre in Salinas 

(Berryhill, 1960).

The lower or basal part of the Juana Díaz Formation 

consists of clastic mudstone or equivalent facies 

composed of conglomerates that include boulders, 

cobbles, sand, and mudstone of possible alluvial fan 

or fan-delta origin. The coarser clastic facies (cobbles 

and boulders) are commonly expressed as cut and 

fill structures that resulted from high energy events 

(Renken and others, 2002). Deposition of these facies 

was probably in response to uplift and increased erosion 

of the underlying early Tertiary age and older rocks 

during a low sea-level stand (Renken and others, 2002; 

Monroe, 1980). Some of the lithofacies of this basal unit 

apparently were deposited in a shallow sea environment, 

probably a shelf setting, as indicated by the marine 

fossils present in the rock. As observed at outcrops, an 

angular unconformity separates the basal clastic facies 

of the Juana Díaz Formation from the older underlying 

rocks. The basal clastic facies are well exposed north of 

the study area, along Highway 52 in the municipio of 

Juana Díaz. The basal deposits, however, are not exposed 

within the study area and are overlain by younger units 

(Monroe, 1980).

In the western part of the study area that includes 

the municipios of Peñuelas and Ponce, the basal part 

of the Juana Díaz Formation grades upward into a 

limestone sequence deposited in a shelf-type marine 

environment, as indicated by the presence of reef, fore 

reef, and back-reef fossils (Frost and others, 1983). 

Eastward from Ponce, the upward transition is from 

basal terrigenous to carbonate slope deposits indicative 

of a deeper marine environment than that of rocks in the 

Peñuelas to Ponce area. Generally, the limestone of the 

Juana Díaz Formation is highly fossiliferous, containing 

a diverse faunal assemblage of coral, coralline algae, 

large benthic foraminifers, and bivalves deposited during 

alternating periods of reef growth and destruction (Frost 

and others, 1983).

Limited lithologic data and structural dislocation 

in the form of tilting and complex fault relations make 

the estimation of thickness in the Juana Díaz Formation 

difficult. The Juana Díaz Formation ranges from 656 to 

more than 2,296 ft thick in the Peñuelas to Santa Isabel 

area (U.S. Geological Survey, unpub. data; Todd and 

Low, 1976; Monroe, 1980). The Juana Díaz Formation 

may be thickest in the Río Tallaboa, Peñuelas to Playa 

de Ponce area (fig. 2), with a thickness ranging from 

1,300 to about 2,200 ft (Monroe, 1980; Renken and 

others, 2002). At well CPR-3, an oil exploratory well 

drilled near the coast in the Punta Cabullón area (fig. 

4; U.S. Geological Survey, unpub. data), the top of the 

Juana Díaz Formation was estimated to be about 1,600 

ft below land surface, with an estimated thickness of 

1,200 ft (Renken and others, 2002). The Juana Díaz 

Formation thins eastward to less than 660 ft in the 

 Stratigraphy, Structure, and Geologic and Coastal Hazards in the Peñuelas to Salinas Area: A Compendium of Published Literature
municipio of Salinas where it lies buried, except at 

Cerro Aguirre where it crops out (Berryhill, 1960). At 

outcrops, the thickness of the clastic beds of the basal 

part of this formation may range from 330 to about 1,400 

ft, whereas the thickness of the limestone facies may 

range from about 500 to more than 1,950 ft (Renken and 

others, 2002).

Unnamed Pelagic Rocks or Deposits

An unnamed carbonate-rock sequence of early 

Miocene age unconformably overlies the Juana Díaz 

Formation and crops out locally in the north-central 

part of the southern middle Tertiary belt. This rock 

sequence includes the Angola Limestone, composed 

of island-slope chalk beds described by Seiglie and 

Bermúdez (1969), the pelagic Río Tallaboa marlstone 

described by Seiglie and Moussa (1974), and an upper 

“new formation” described and defined by Frost and 

others (1983). The chalk and marlstone beds crop out in 

a few localities within the Peñuelas to Ponce area, but 

these units or their time-equivalent strata presumably are 

buried beneath the rest of the study area (Moussa and 

Seiglie, 1970). Moussa and Seiglie (1970) believe an 

unconformity separates the Angola Limestone and Río 

Tallaboa units and their time equivalents, whereas Frost 

and others (1983) have suggested continuous deposition 

during the early Miocene. Renken and others (2002) 

referred to this unnamed carbonate sequence as the 

unnamed pelagic deposits.

Ponce Limestone

The Ponce Limestone of Miocene and Pliocene 

age unconformably overlies the unnamed carbonate 

unit described above, and where this unit is missing, 

the Ponce Limestone unconformably overlies the Juana 

Díaz Formation (Monroe, 1980). The Ponce Limestone 

consists mostly of yellowish-orange, soft to moderately 

hard fossiliferous limestone, and within the study area 

it crops out almost continuously from Peñuelas to Río 

Pastillo in the municipio of Ponce (Monroe, 1980). 

Outcropping strata of Ponce Limestone north of the 

study area contain abundant marine fossils, such as 

molds of gastropods, pelecypods, coral heads, and large 

foraminifera, all of which are indicative of deposition 

in shallow-water lagoon and back-reef environments. 

Samples collected from the Kewannee oil test wells 

(CPR1, 2, and 3 in fig. 4) indicate that shallow-water 

beds of the Ponce Limestone grade west to east from 

a shallow carbonate facies to terrigenous sand, gravel, 

and mudstone interbedded with some limestone, as 

evidenced in lithologic logs from oil test well Kewannee 

CPR-3. Within the study area, between Playa de Ponce 

and Santa Isabel, the Ponce Limestone is replaced by 

time-equivalent terrigenous clastics buried beneath fandelta deposits of Quaternary age.

As with the Juana Díaz Formation, structural 

complexities and a limited availability of water-well 

lithologic data make it difficult to accurately estimate 

the thickness of the Ponce Limestone. Within the study 

area, the limestone facies and equivalent clastic strata 

appear to be thickest in the Juana Díaz to Santa Isabel 

area (1,000 to more than 1,300 ft) (Monroe, 1980; fig. 

4). The Ponce Limestone in the Río Tallaboa area was 

estimated to be 2,788 ft by Monroe (1980). This value 

is considered excessive by Frost and others (1983) and 

Renken and others (2002), who believe the estimate is 

inconsistent with the regional trend east of Peñuelas. 

Both later studies attribute the initial overestimate 

by Monroe (1980) to complex fault relations and 

vegetative cover, each of which can result in erroneous 

interpretation of geologic relations.

Quaternary Age Fan-Delta and Alluvial Deposits

The middle Tertiary section is overlain by fan-delta 

and alluvial deposits of Quaternary age. In the municipio 

of Salinas, however, the Quaternary age deposits are 

directly underlain by early Tertiary and older-age rocks 

in the vicinity of Aguirre.

The part of the study area between Ponce and 

Salinas lies within the fan-delta plain along the 

southern coast of Puerto Rico. This plain was formed 

by the coalescence of four larger and seven smaller 

fan deltas along an area that extends from Ponce to the 

municipio of Patillas, about 16 mi (miles) east of Salinas 

(Renken and others, 2002). The fan-delta deposits are 

predominantly Quaternary in age and consist of gravelly 

sand (including boulders and cobbles), and silt (Renken 

and others, 2002). Poorly developed soil caliche is 

exposed in isolated areas north of the study area, and its 

presumed existence in the subsurface below land surface 

is consistent with semiarid conditions that existed 

during the Pleistocene (Renken and others, 2002). The 

strata deposited within a fan-delta environment are 

dominated by streamflow, sheetflow, and gravity-driven 

depositional processes. Alluvial deposits are restricted 

to areas near streams, and are dominated by streamflow 

features (Renken and others, 2002).

In the municipio of Peñuelas and the westernmost 

part of Ponce, fan-delta deposits are replaced by timeequivalent alluvial deposits that occupy the narrow 

coastal valleys of Río Tallaboa and Río Matilde. 

These alluvial deposits presumably are underlain 

by the Ponce Limestone or time-equivalent units. 

Eastward from Playa de Ponce to Salinas, the fandelta deposits unconformably overlie unconsolidated 

to poorly consolidated clastic sediments, as well as 

minor limestone and marl deposits of shallow-marine 

conditions that are, at least in their lower part, timeequivalent to the Ponce Limestone (figs. 4 and 5 at end 

Geology 
of report; Renken and others, 2002: Monroe, 1980). The 

clastic-sedimentary units directly beneath the fan-delta 

deposits are Pleistocene or Holocene in age (Renken and 

others, 2002). Eastward from about Salinas, the fan-delta 

deposits are underlain by early Tertiary and older age 

rocks (Renken and others, 2002; fig. 4).

The delta plain occupies the low-lying part of the 

Southern Coastal Plain of Puerto Rico. Most of the fan 

deltas between Ponce and Salinas do not exceed an 

altitude of 160 ft above sea level at their apex, except 

for the Capitanejo fan in Juana Díaz, which is up to 

260 ft above sea level (fig. 1; U.S. Geologic Survey 

topographic quadrangles of Playa de Ponce, Santa 

Isabel, Salinas, and Central Aguirre). The surface of 

the different fans slopes coastward with a gradient that 

ranges from 130 ft/mi within the apex or proximal 

upland channels to 10.5 ft/mi near the coast. From the 

apex to coastline, fan radii average nearly 32,800 ft. East 

of Ponce, all of the fan deltas contain a well-developed, 

semi-radial, subaerial morphology (Renken and others, 

2002). Pre-Miocene strata protrude above the delta plain 

at a few localities, such as Cerro Aguirre in Salinas and 

Cerro de los Muertos in Santa Isabel (fig. 2; Berryhill 

1960; Glover, 1971).

The fan deltas within the study area and along the 

rest of the Southern Coastal Plain are separated from 

the Caribbean Sea by a narrow, marginal marine zone 

of supratidal or salty scrub flats, marsh and mangrove 

swamps, and beach deposits. Fan-delta coastal zones are 

largely characterized by broad sand and gravel beaches 

of terrestrial origin (Renken and others, 2002). The most 

extensive area of mangroves, marshes, and tidal flats 

within the study area is located along the Salinas fandelta margin. Mangrove swamps of substantial aerial 

extent occupy the margins of Laguna de Las Salinas in 

Ponce, particularly along the northern margin close to 

Highway 2. Beaches between El Tuque in Ponce and Río 

Tallaboa in Peñuelas and along the southern edge of the 

Playa de Salinas at Salinas are the only areas within the 

study area that consist partly of skeletal carbonate sand, 

with bioclastics derived from fringing reefs (Frost and 

others, 2003).

Renken and others (2002) used water-well logs 

and other well drilling data to estimate the thickness of 

fan-delta and alluvial deposits and map the configuration 

of the underlying contact (fig. 6 at end of report). 

Fan-delta deposits usually do not exceed 33 to 98 ft in 

thickness within both the proximal part of the fan and 

inland alluvial channels that extend downward from the 

fan apex. The thickness of the fan-delta sequence east 

of Ponce increases from less than 10 ft at its landward 

margin to between 150 and about 330 ft in most coastal 

areas (figs. 4 and 6). The fan-delta sequence and other 

associated Quaternary age deposits may locally thicken 

to about 490 ft where they overlie the structural basin 

in the Playa de Ponce to Bahía de Rincón area in the 

municipios of Santa Isabel and Salinas (figs. 2 and 

4). The actual thickness of the Quaternary deposits 

within this basin, however, is poorly known because of 

limited control and lack of age indicators, such as faunal 

assemblages. The fan-delta deposits thin considerably 

east of Bahía de Jobos and are less than 130 ft thick.

Renken and others (2002) recognized three scales 

of cyclical deposition within the vertical sequences of 

fan-delta deposits from exposed and subsurface sections 

in the Southern Coastal Plain, including the study area. 

The small-scale pattern, from less than 3 to 70 ft thick, 

is characterized by a progressive-upward change in 

grain size within a discrete bed. The intermediate-scale 

pattern, from 50 to 230 ft thick, is characterized by a 

progressive change in bed thickness. The large-scale 

pattern, from 1,310 to 1,640 ft thick, is characterized 

by overall changes in bed thickness that occurs within 

the entire coarsest grained section of gravel, sand, and 

silt. Small-scale cyclicity is generally caused by shortterm depositional events, such as stream and flood flow, 

that result either in a fining-upward sequence caused by 

aggradation or lateral accretion within channels, or in 

a coarsening-upward sequence caused by channel bar 

migration.

A lithofacies map for the fan-delta plain shown as 

plate 1D in Renken and others (2002) shows that sand 

and gravel deposits are concentrated as lobes, each 

separated by interlobe areas of fine-grained detritus. 

The percentage of gravel and sand content generally 

decreases with distance down the fan. As indicated in 

figure 7 at end of report, gravel and sand percentages 

are highest in proximal areas closest to the fan apex and 

along inland alluvial channels where flows are restricted 

and flow velocities are greatest. The gravel and sand 

content exceeds 40 percent within the proximal parts 

of the fan deltas of Salinas, Coamo, and Capitanejo 

in Juana Díaz. Within the proximal part of the Río 

Portugués-Bucaná fan delta the sand and gravel content 

exceeds 60 percent.

Structure

The GSPRFZ is the principal geologic structural 

feature in the southern part of Puerto Rico. The middle 

Tertiary section is inclined toward the south, except 

where faulting was reversed and as a result the rock 

sequence fragmented in blocks. The structural setting 

and associated tectonic movements controlled fan-delta 

and alluvial-fan deposition in the study area during preQuaternary and Quaternary periods. Late Quaternary 

faulting has only been observed in offshore areas south 

of the study area, and has not affected Quaternary 

alluvial deposits along the Southern Coastal Plain.

 Stratigraphy, Structure, and Geologic and Coastal Hazards in the Peñuelas to Salinas Area: A Compendium of Published Literature
Great Southern Puerto Rico Fault Zone

The GSPRFZ consists of parallel to subparallel 

sinistral faults that strike northwest and extend 

diagonally across the south-central part of the island 

(Glover, 1971). Geophysical data indicate that the 

GSPRFZ extends beneath the fan-delta plain and the Isla 

Caja de Muertos shelf (Glover, 1971; Garrison, 1969), 

possibly forming the southern wall of the Whiting Basin 

(Western Geophysical Company of America, 1974). At 

least 12 northwest striking faults have been identified 

and mapped as part of the GSPRFZ (fig. 8 at end of 

report). Northwest of Ponce, the width of the fault zone 

between the northernmost and southernmost sinistral 

faults is more than 29,500 ft wide (Glover, 1971). The 

actual width of the GSPRFZ coastward, however, is not 

known because it is concealed by the fan-delta plain. 

The strike of this fault zone varies from N 54o

 W to N 

55o

 W, where it crosses the Cordillera Central mountain 

range to the northwest and west of Ponce (Glover, 1971; 

Krushensky and Monroe, 1975, 1978, 1979), to N 75o

W where it underlies the fan-delta plain (Renken and 

others, 2002).

There is no consensus between previous studies 

concerning the age of the GSPRFZ. Glover (1971) has 

postulated that the first sinistral movement occurred 

during the early Cretaceous about 144 million years 

before present (Ma), with additional movements 

occurring during Maestrichtian (about 70 Ma) and 

Eocene (about 57 Ma). McIntyre (1971, 1975) has 

suggested that initiation of the GSPRFZ (fig. 8) occurred 

after the middle Eocene (about 45 Ma). Krushensky 

and Monroe (1975, 1978) have constrained sinistral 

movement to Eocene to Oligocene (55-35 Ma) with 

horizontal movement continuing along the Lago 

Garzas and Bartolomei faults (fig. 2) as late as the early 

Miocene (about 23 Ma). Sinistral movement along these 

two faults could correspond with the development of 

the large structural and depositional basin that underlies 

the fan-delta deposits between Ponce and Santa Isabel 

(Renken and others, 2002). Other faults of the GSPRFZ 

that strike east-west and have a normal slip component 

displace rocks of the Ponce Limestone and Juana Díaz 

Formation that are exposed west and north of Ponce. 

Renken and others (2002) have suggested that the pattern 

of buried bedrock highs and lows are relict features 

of a braided sinistral fault system. These highs and 

lows supposedly originated as horst and graben fault 

blocks, formed by movement along the GSPRFZ that 

were subjected to subaerial erosion and then buried 

and preserved as paleotopographic ridges and valleys 

beneath the fan-delta plain.

Late Quaternary Faulting

As of March 2005, no evidence of terrestrial, late 

Quaternary (since about 10,000 years ago) faulting has 

been found along the southern coast of Puerto Rico 

(Mann and others, 2005). In an aerial photography study, 

Geomatrix Consultants (1988) detected the presence of 

numerous, predominantly east-west trending lineaments 

on the coalesced alluvial fan complex between Río 

Coamo and Río Jueyes in the municipio of Santa 

Isabel (fig. 1). No evidence of deformation of the late 

Quaternary alluvial fan deposits (fan-delta deposits), 

however, was recognized in the field during the study 

by Geomatrix Consultants (1988). As of March 2007, 

no comprehensive study of late Quaternary faulting has 

been conducted along the southern coast of Puerto Rico 

(E. Asencio, University of Puerto Rico at Mayagüez, 

written commun., 2007).

A geophysical survey conducted during 2000, 

however, revealed evidence of late Quaternary seafloor 

faulting in three offshore areas along the southern coast 

of Puerto Rico (fig. 8; Mann and others, 2005). Late 

Quaternary seafloor faulting is indicated by a recent 

scarp with about 66 ft of relief on the seafloor of the 

offshore Ponce Basin that bounds the ridge where the 

island of Caja de Muertos is located. Late Quaternary 

seafloor faulting also was observed along the offshore 

extensions of the Esmeralda and Río Jueyes faults, 

both of which are strands of the GSPRFZ. This late 

Quaternary seafloor faulting apparently pinches out 

landward in a scissor-like manner and does not displace 

terrestrial fan-delta deposits.

Geologic Risks

The GSPRFZ is considered to be largely quiescent 

and is associated only with small earthquakes (McCann, 

1985). The magnitude of completeness for earthquakes 

in the study area is less than the average for the island, 

although seismic density in the study area is close to 

the highest regional average for the island (Clinton and 

others, 2006). Mueller and others (2003) determined 

that the highest peak ground acceleration (PGA), due 

to shallow seismicity that may occur in the study area, 

with a 2 percent probability of exceedance in 50 years, 

is about 9 feet per squared second (ft/s2

). The highest 

peak ground acceleration, due to deep seismicity that 

may occur in the study area, with a 2 percent probability 

of exceedance in 50 years, is about 7 ft/s2

 (Muller and 

others, 2003). The occurrence of at least one earthquakerelated tsunami has been documented in the study area, 

and affected the municipio of Ponce during the second 

half of the 19th century (Neumann, 1913). Modeling 

results by Mercado and Justiniano (2007) indicate that 

Geologic Risks 
the inland extent of flooding induced by a tsunami 

within the study area can range from about 2,600 to 

3,000 ft. A study of the susceptibility of the study area 

to liquefaction due to seismic activity has not been 

done. However, the potential for liquefaction due to 

seismic activity may exist at localities within the study 

area having loose to moderately indurated sediments 

and a water table close to or at the land surface; these 

localities are generally near the coastline. The potential 

for rainfall-induced landslides within the study area is 

none to minimum, except in the area west of Laguna de 

las Salinas in Ponce (fig. 1), which has a moderate to 

high susceptibility of landsliding due to rainfall (Larsen 

and others, 2004).

Spatial and Temporal Distribution of Seismic 

Activity

The island of Puerto Rico has a long history of 

damaging seismic activity and associated tsunamis. 

The island is bounded on all sides by major tectonic 

faults (fig. 9 at end of report). Historical earthquakes 

with damaging effects on Puerto Rico have occurred 

in 1615, 1670, 1751, 1776, 1787 (an approximate 

magnitude 8.0 event that originated in the Puerto Rico 

Trench), 1867 (an approximate magnitude 7.3 event 

that originated in the Anegada Trough), and 1918 (an 

approximate magnitude 7.3 event that originated in the 

Mona Canyon). [The Richter scale used to measure 

the magnitude of earthquakes was invented in 1934.] 

The earthquakes of 1943 (approximate magnitude 

7.5) and 1946 (approximate magnitude 7.8) also 

caused substantial damage in the Dominican Republic 

(Shepherd and Lynch, 1992). The 1867 and 1918 

earthquakes were followed by tsunamis that affected the 

southern and western coasts, respectively.

Puerto Rico lies on a microplate located between 

the obliquely subducting North American and Caribbean 

plates. The main sources of seismic activity in Puerto 

Rico are at the defined boundaries of the microplate: 

the subduction zones to the north (Puerto Rico Trench) 

and south (Muertos Trough), and zones of extension at 

the Anegada Trough to the east and in Mona Canyon 

to the west (fig. 9). All of these regions have produced 

earthquakes with magnitudes above 7.0 throughout the 

history of the island (Asencio, 1980; Moya and McCann, 

1992).

In addition to these offshore sources, Prentice 

(2000) recently found evidence of two surface-rupturing 

events, each with a component of strike-slip motion, 

that occurred within the last 5,000 years along the South 

Lajas Fault—an inland, normal fault in southwestern 

Puerto Rico (fig. 9). According to La Forge and McCann 

(2005), this 31.3-mile-long fault segment can produce 

magnitude 7.0 seismic events and potentially could 

be the western extension of a fault zone that extends 

eastward toward the municipio of Peñuelas. The Great 

Northern (GNPRFZ) and Southern Puerto Rico fault 

zones, considered active from the early Cretaceous to 

early Tertiary, are now considered largely quiescent and 

apparently are associated with small earthquakes that 

may represent inherent zones of weakness (McCann, 

1985). These two fault zones have an unknown potential 

for large magnitude events; however, no evidence of 

terrestrial Holocene (late Quaternary) rupturing has been 

identified along these fault systems (McCann, 1985).

The average magnitude of completeness for the 

earthquake database of the Puerto Rico Seismic Network 

(PRSN) for the January 1986 to March 2006 interval 

is 3.5 (Clinton and others, 2006). The majority of the 

seismic events in Puerto Rico occur at depths of less 

than 82,000 ft (Clinton and others, 2006). This relatively 

shallow seismicity is concentrated in areas along Mona 

Canyon, the region between the Puerto Rico Trench 

and the 19o

 Fault zone, the Sombrero Fault zone, and 

across the southern part of Puerto Rico (including 

the study area) (Clinton and others, 2006). The deep 

seismicity, greater than 82,000 ft below land surface, 

occurs mainly in the Dominican Republic and in the 

northern part of Puerto Rico (Clinton and others, 2006). 

This seismicity is caused by the oblique subduction 

of the North American plate beneath the Puerto Rico 

microplate (Clinton and others, 2006). The magnitude 

of completeness in the southern part of Puerto Rico, 

including the study area, ranges from 2 to 2.5 for the 

January 1986 to March 2006 period (fig. 10 at end of 

report, Clinton and others, 2006).

The areas of highest seismic density in and near 

Puerto Rico for the January 1986 to March 2006 period 

are: (1) southwestern Puerto Rico between the GSPRFZ 

and the coast, (2) the region between the Puerto Rico 

Trench and the 19° fault zone, and (3) the Sombrero 

seismic zone (Clinton and others, 2006; fig. 11 at end of 

report). As noted earlier, seismic density in the Peñuelas 

to Salinas area averages about 0.128 earthquakes 

per square mile, which is close to the average for the 

southwestern part of Puerto Rico (Clinton and others, 

2006). The seismic density parameter is based on 

detected earthquakes of all magnitudes and depths.

The USGS recently conducted a probabilistic 

seismic hazard assessment for Puerto Rico and the U.S. 

Virgin Islands (Mueller and others, 2003) for use in 

earthquake-damage mitigation, response planning, and 

for the adoption of stricter building codes and structural 

design standards. The methodology used for the study, as 

described by Frankel and others (1996, 2002), is based 

primarily on (1) gridded and smooth historical seismicity 

generalized using exponential magnitude distributions, 

and (2) specific fault sources with published slip-rate 

or recurrence information. These sources include 

the main subduction interface south of the Puerto 

 Stratigraphy, Structure, and Geologic and Coastal Hazards in the Peñuelas to Salinas Area: A Compendium of Published Literature
Rico Trench, the Mona Canyon and Anegada Trough 

extensional zones, the Septentrional Fault (the major 

plate boundary structure in the Dominican Republic), 

and the South Lajas Fault (fig. 9). Where information 

on seismicity or fault components was incomplete, 

the seismic hazard assessment was supplemented with 

sources based on geodetic or other deformation data. 

For this methodology, earthquakes were assumed to 

occur randomly in time so that probabilistic ground 

motions represent time-independent seismic hazards. 

The results of this seismic hazard assessment by Mueller 

and others (2003) were published in a series of maps of 

probabilistic ground motion for three measures: peak 

ground acceleration, 1.0-second spectral acceleration, 

and 0.2-second spectral acceleration. Each measure was 

published for 2 percent and 10 percent probabilities 

of exceedance in 50 years, which correspond to return 

times of 2,500 and 500 years, respectively.

Mueller and others (2003) excluded the Muertos 

Trough and related structures offshore from southern 

Puerto Rico in their assessment because their rates of 

activity are poorly known. In their assessment model, 

LaForge and McCann (2003) considered the activity 

rates of these structures low compared to those of the 

other structures in and near Puerto Rico and the U.S. 

Virgin Islands. They agreed, however, with Mueller and 

others (2003) that the proximity of these structures to 

the southern coast of Puerto Rico makes them important 

subjects for future research.

For simplicity, only the probabilistic PGA estimates 

obtained from the study by Mueller and others (2003) 

are presented and discussed in this report. Probabilistic 

PGA is considered by Mueller and others (2003) to be a 

parameter that, by itself, may indicate the seismic hazard 

potential for a given area. The highest PGA most likely 

to occur in the study area, with 2 percent probability 

of recurrence, is due to shallow depth seismicity (table 

1). In Ponce, the probability of exceedance per year is 

greater than 0.1 only for PGA values below 1 ft/s2

 caused 

by shallow seismic activity (less than 31.3 miles deep) 

from the Mona Passage extension zone. In general, PGA 

values in the Ponce area have a higher probability of 

exceedance per year for seismic activity that originates 

from shallow sources, such as the Mona Passage 

extension zone, than from other sources.

Potential for Liquefaction, Landslides 

and Tsunamis

Seismic activity is itself a driving force for the 

occurrence of other hazards. Along a coastal zone, such 

as the Peñuelas to Salinas area, these earthquake-induced 

hazards can produce liquefaction, landslides (slope 

failure), and tsunamis; slope failure can result from 

liquefaction as well as seismic activity. As of 2006, an 

assessment of the liquefaction potential of the Peñuelas 

to Salinas area has not been conducted. No evidence has 

Table 1. Estimated peak ground acceleration (PGA) in the study area from selected sources.

[ft/s2, feet per second squared]

Estimated PGA in 

the study area

(ft/s2)

Source of seismicity

2 percent probability of exceedance in 50 years

9 Shallow (less than 31.3 miles below ground surface)

7 Deep (greater than 31.3 miles below ground surface)

7 Deep subduction, predominantly from south of the Puerto Rico 

Trench

3 Subduction beneath Hispaniola

5-7 Active extensional zones in the Anegada and Mona Passages

13 Combination of all sources 

10 percent probability of exceedance in 50 years 

5-7 Combination of all sources 

Geologic Risks 
been found that indicates liquefaction has occurred in 

historical times within the study area. It is reasonable to 

assume, however, that seismically-induced liquefaction 

may be a potential hazard at locations within the study 

area that have loose and poorly consolidated sedimentary 

deposits and a shallow water table—such areas are 

generally near the coast

As of 2006, landslides (slope failure) due to seismic 

activity have not been documented within the study area. 

The possibility of landslides in the study area may be 

limited to areas west of Laguna de las Salinas, based on 

the presence of foothills near the coastline in that area. 

It is assumed that chances of landslides due to seismic 

activity east of Laguna de las Salinas are probably 

minimal or none.

A recent landslide assessment of the municipio of 

Ponce indicates that the coastal area between Laguna 

de las Salinas and Peñuelas has a moderately high 

susceptibility to landslides triggered by rainfall (Larsen 

and others, 2004). This area is underlain by the Juana 

Díaz Formation and the Ponce Limestone. Locally, both 

of these geologic units have similar lithologies; each 

consisting predominantly of highly clayey limestone 

that grades to marl. The topographic slope in this 

area exceeds 12 degrees, which was determined to be 

the approximate threshold value for rainfall-induced 

landslides (Larsen and others, 2004). Between Laguna 

de las Salinas and Peñuelas, the coastal plain is either 

narrow or non-existent, except in the lower valleys 

of Río Tallaboa and Río Matilde. The coastal area of 

Ponce east of Laguna de las Salinas is not susceptible 

to landslides caused by rainfall because the area has 

low topographic relief characterized by slopes generally 

less than 10 degrees (Larsen and others, 2004). It is 

reasonable to assume that eastward from Ponce to 

Salinas, the landslide susceptibility along the coastal 

plain due to rainfall is also minimal or nonexistent, 

because slopes in this area are generally less than 10 

degrees.

One of the primary sources of tsunamis is offshore 

seismic activity. For example, the 7+ magnitude 1918 

earthquake, caused by a rupture in the Mona Passage 

extension zone, resulted in a tsunami that caused a 

substantial loss of property and human lives in the 

western part of Puerto Rico, particularly the municipios 

of Mayagüez and Aguadilla (Clinton and others, 2006). 

A tsunami caused by an earthquake on November 18, 

1867, affected the municipio of Ponce (Neumann, 1913).

The approximate maximum inland extent of the 

tsunami flood caused by the largest probable seismic 

event in the Peñuelas to Salinas area was determined 

from a modeling study by Mercado and Justiniano 

(2007) and is presented on figures 12 through 15 at 

end of report. The maximum inland extent of tsunamiinduced coastal flooding within the study area, according 

to Mercado and Justiniano (2007), is shown in table 

2. The expected runup, or maximum vertical height 

onshore above sea level of tsunamis, ranges from 9 ft in 

the Boca Chica area (USGS Santa Isabel quadrangle) to 

14 ft in the Punta Cabullón area (USGS Playa de Ponce 

quadrangle) (Mercado and Justiniano, 2007).

Table 2. Maximum inland extent of tsunami-induced coastal flooding within the study area.

[Data from Mercado and Justiniano modeling results are based on seismic activity produced within 

the immediate vicinity of Puerto Rico and assume that uniformed displacement occurs along the 

entire length of the fault where the seismic activity originates. A worst-case scenario is considered by 

assuming a maximum displacement along the fault.]

 Location Maximum inland 

extent (feet)

USGS 

quadrangle

Figure in 

this report

Laguna de las Salinas 2,600 Punta Cuchara 12

Boca Chica 2,600 Santa Isabel 14

Juana Díaz 2,600 Santa Isabel 14

Punta Cabullón 3,000 Playa de Ponce 13

Jobos National Estuarine Research 

Reserve near Aguirre 2,800 Salinas 15

10 Stratigraphy, Structure, and Geologic and Coastal Hazards in the Peñuelas to Salinas Area: A Compendium of Published Literature
Summary

The Puerto Rico Electrical Power Authority has 

proposed the construction of a pipeline to convey 

natural gas from storage facilities in the municipio of 

Peñuelas to the Aguirre thermoelectric power plant in 

the municipio of Salinas. In order to ensure that the 

geologic conditions along the probable routes do not 

represent a threat to the physical integrity of the pipeline 

and thus comply with State and Federal regulations, the 

Puerto Rico Electrical Power Authority requested the 

U.S. Geological Survey to evaluate and synthesize the 

geology of the coastal plain in the Peñuelas to Salinas 

area.

The proposed pipeline route traverses the fandelta complex of the Southern Coastal Plain of Puerto 

Rico with the exception of areas between Peñuelas 

and Ponce where the coastal plain is narrow or absent 

and the foothills extend to the coastline. A narrow 

coastal zone that consists of mangrove swamps, salt 

flats, wetlands, and hypersaline lagoons, separates 

the Southern Coastal Plain from the Caribbean Sea. 

The study area is underlain in its western part, from 

Peñuelas to Ponce, by the carbonate and clastic strata 

of the Juana Díaz Formation and Ponce Limestone of 

middle Tertiary age. The eastern part of the study area, 

which extends from Ponce to Salinas, is underlain by 

fan-delta deposits, mostly consisting of gravel, sand, silt, 

and clay, that are mostly Quaternary in age. The rocks 

underlying the middle Tertiary and Quaternary age rocks 

are early Tertiary and older in age. The underlying rocks 

are predominantly volcaniclastic, with lesser amounts 

of sedimentary rocks and lavas. The underlying rocks 

are assumed to be equivalent in age to the rocks in the 

Cordillera Central that lies to the north of the Southern 

Coastal Plain.

The Great Southern Puerto Rico Fault Zone is the 

main geologic structure on the southern coast of the 

island of Puerto Rico. Geophysical data indicate that the 

Great Southern Puerto Rico Fault Zone extends beneath 

the fan-delta plain and the Isla Caja de Muertos shelf. 

This system of faults predominantly displaces early 

Tertiary and older age rocks, but some of its components 

that strike east-west may displace rocks of the Juana 

Díaz Formation and Ponce Limestone exposed west and 

north of Ponce. As of 2006, no evidence of terrestrial, 

late Quaternary faulting in the southern coast has been 

documented in the literature. Geophysical evidence, 

however, indicates the occurrence of late Quaternary 

seafloor faulting at three offshore localities along the 

southern coast. This late Quaternary seafloor faulting 

apparently pinches out landward in a scissor-like manner 

and does not displace terrestrial fan-delta deposits.

Puerto Rico lies on a microplate located between 

the obliquely subducting North American and Caribbean 

plates. The main sources of seismic activity in Puerto 

Rico are at the defined boundaries of the microplate: 

the subduction zones to the north (Puerto Rico Trench) 

and south (Muertos Trough), and zones of extension at 

the Anegada Trough to the east and in Mona Canyon 

to the west. The Great Northern Puerto Rico Fault zone 

and the Great Southern Puerto Rico Fault Zone are now 

considered to be largely quiescent and only seemingly 

associated with small earthquakes. The magnitude of 

completeness for earthquakes in the south coast of 

Puerto Rico that includes the study area ranges from 

2 to 2.5, as determined by the Puerto Rico Seismic 

Network database. The seismic density of the study 

area is about 0.128 earthquakes per square mile, which 

is about the same as that of southwestern Puerto Rico. 

The estimated peak ground acceleration, a measure 

of seismic intensity, with 2 percent probability of 

exceedance in 50 years, most likely to occur along the 

southern coast (including the Peñuelas to Salinas area) 

due to shallow depth seismicity is 9 ft/s2

. The estimated 

peak ground acceleration, with 2 percent probability of 

exceedance in 50 years, most likely to occur along the 

southern coast due to deep seismicity (deeper 31.3 miles) 

is about 7 ft/s2

. In Ponce, the probability of exceedance 

per year is higher than 0.1 only for those peak ground 

acceleration values less than 1 ft/s2

 that result from 

shallow seismicity in Mona Canyon. In general, peak 

ground acceleration values in the municipio of Ponce 

have a higher probability of exceedance per year when 

caused by seismic activity from shallow sources such as 

the Mona Passage extension zone.

An assessment of the liquefaction potential in the 

study area has not been conducted as of 2006. Likewise, 

no evidence of the occurrence of liquefaction in 

historical times has been documented in the study area. 

However, areas near the coastline that are characterized 

by loosely to poorly consolidated sediments and a water 

table close to or at land surface may be susceptible to 

liquefaction caused by seismic activity.

As of 2006, no evidence of slope failure due to 

seismic activity has been documented within the study 

area. The possibility of slope failure due to seismic 

activity in the study area is limited to the area westward 

from Laguna de las Salinas, based on the presence 

foothills close to coastline. Eastward from Laguna de 

las Salinas, the possible routes for the proposed natural 

gas pipeline are either over land that is either nearly flat 

or has a minimal slope, consequently, it is reasonable 

to assume that eastward from Ponce to Salinas, the 

probability of landslides along the coastal plain is 

minimal to nonexistent.

Summary 11
The area extending westward from Laguna de las 

Salinas to Peñuelas is considered to have a moderate to 

high susceptibility to rainfall-induced landslides. This 

area has topographic slopes that exceed 10 degrees 

and is underlain by clayey limestone to marl within 

the foothills. Eastward from Laguna de las Salinas, the 

coastal area is either flat or with a topographic slope 

less than 10 degrees, consequently, the susceptibility 

to rainfall-induced landslides is considered minimal to 

nonexistant.

The occurrence of at least one tsunami caused by 

an earthquake has been documented for the second 

half of the 19th century, and occurred in the municipio 

of Ponce. Modeling results indicate that the maximum 

inland extents of tsunami-induced coastal flooding 

within the study area may be about 2,600 ft at Laguna 

de las Salinas in Ponce and Boca Chica in Juana Díaz, 

3,000 ft in Punta Cabullón in Ponce, and about 2,800 ft 

in the vicinity of the Jobos National Estuarine Research 

Reserve in the Aguirre locality of Salinas. Runup ranges 

from 9 ft in the Boca Chica area to 14 ft in the Punta 

Cabullón area.

References Cited

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144 p.

Berryhill, H.L., Jr., 1960, Geology of the Central Aguirre 

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Bold, W.A. van den, Neogene ostracoda from southern 

Puerto Rico:Caribbean Journal of Science , v. 9, 

no. 3-4, p. 117-133.

Clinton, J.F., Cua, G., Huérfano, V., Von HillebrandAndrade, C.G., and Martínez-Cruzado, J., 2006, 

The current state of seismic monitoring in Puerto 

Rico: Seismological Research Letters, v. 77, no. 5, 

September/October 2006, p. 532-543.

Frankel, A., Mueller, C., Barnhard, T., Perkins, D., 

Leyendecker, E., Dickman, N., Hanson, S., and 

Hopper, M., 1996, National Seismic Hazard Maps: 

Documentation June 1996: U.S. Geological Survey 

Open-File Report 96-532, 70 p.

Frankel, A.D., Petersen, M.D., Mueller, C.S., Haller, 

K.M., Wheeler, R.L., Leyendecker, E.V., Wesson, 

R.L., Harmsen, S.C., Cramer, C.H., Perkins, D.M., and 

Rukstales, K.S., 2002, Documentation for the 2002 

update of the National Seismic Hazard Maps: U.S. 

Geological Survey Open-File Report 02-420, 33 p.

Frost, S.H., Harbour, J.L., Beach, D.K., Realini, 

M.J., and Harris, P.M., 1983, Oligocene reef tract 

development, southwestern Puerto Rico: Sediment IX, 

University of Miami, 144 p.

Garrison, L.E., 1969, Structural geology of the Muertos 

Insular Shelf, Puerto Rico: U.S. Geological Survey 

Open-File Report, unnumbered, 9 p.

Geomatrix Consultants, 1988, Geological-seismological 

evaluation to assess potential earthquake ground 

motions for the Portugués Dam, Puerto Rico: Report 

submitted to the U.S. Army Corps of Engineers, 92 p.

Giusti, E.V., 1978, Hydrogeology of the Karst of Puerto 

Rico: U.S. Geological Survey Professional Paper 

1012, 68 p., 2 pls.

Glover III, Lynn, 1971, Geology of the Coamo 

area, Puerto Rico, and its relation to the volcanic 

arch-trench association: U.S. Geological Survey 

Professional Paper 636, 102 p., 4 pls.

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, 1 sheet.

Krushensky, R.D., and Monroe, W.H., 1978, Geologic 

map of the Peñuelas and Punta Cuchara quadrangles, 

Puerto Rico: U.S. Geological Survey Miscellaneous 

Investigations Series Map I-863, 1 sheet.

Krushensky, R.D., and Monroe, W.H., 1979, Geologic 

map of the Yauco and Punta Verraco quadrangles, 

Puerto Rico: U.S. Geological Survey Miscellaneous 

Investigations Series Map I-1147, 1 sheet.

La Forge, R.C., and McCann, W.R., 2005, A seismic 

source model for Puerto Rico, for use in probabilistic 

ground motion hazard analysis: Geological Society 

of America Special Paper 385: Active Tectonics and 

Seismic Hazards of Puerto Rico, the Virgin Islands, 

and offshore areas: v. 385, No. 0, p. 223-248.

Larsen, M.C., Santiago, M., Jibson, R., and Questell, E., 

2004, Map showing susceptibility to rainfall-triggered 

landslides in the municipality of Ponce, Puerto Rico: 

U.S. Geological Survey Scientific Investigations Map 

I-2818, 1 sheet.

Mann, P., Hippolyte, J.P., Grindlay, N.R., and Abrams 

L.J., 2005, Neotectonics of southern Puerto Rico and 

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Special Paper 385, p. 173-214.

McCann, W.R., 1985, On the earthquake hazards of 

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12  Stratigraphy, Structure, and Geologic and Coastal Hazards in the Peñuelas to Salinas Area: A Compendium of Published Literature
McIntyre, D.H., 1971, Geologic map of the La Plata 

quadrangle, Puerto Rico: U.S. Geological Survey 

Miscellaneous Investigations Series Map I-660, 1 

sheet.

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quadrangle, Puerto Rico: U.S. Geological Survey 

Miscellaneous Investigations Series Map I-918, 1 

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p. 331-340.

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Monroe, W.H., 1980, Geology of the middle Tertiary 

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Fernando, Rodríguez-Martínez, Jesús, and others, 

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Islands Aquifer System of the Commonwealth 

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References Cited 13
Glossary

deep seismicity—is seismicity that originates at a depth 

greater than 31.3 miles. This type of seismicity generally 

originates from activity in the lower zone of the crust 

such as the deeper interaction of the Caribbean and 

North American plates.

fan delta—is an alluvial fan that progrades into a 

standing body of water from an adjacent highland. Fan 

deltas are characterized by their semi-radial or fan-like 

shape, concave-upward radial and convex-upward crosssectional profiles, and the relatively narrow space they 

occupy between a highland mountain front and a nearby 

water body (Wescott and Ethridge, 1980; McPherson and 

Moiola, 1987).

magnitude of completeness—is the lowest magnitude 

at which 100 percent of the seismic events in a spacedtime volume are detected (Clinton and others, 2006). 

Magnitude of completeness can be considered as a 

quality-control parameter that assesses the spatial and 

temporal uncertainties of the earthquakes database. 

These spatial and temporal uncertainties in the database 

arise from such factors as limitations of networks in 

detecting certain signals, use of diverse equipment to 

detect and record signals, and processing of acquired 

signals with a variety of software and assumptions.

peak ground acceleration (PGA)—is the maximum 

acceleration (velocity rate of change with time) 

experienced by a particle in the ground during the course 

of the earthquake motion. It is a measure of earthquake 

ground motion and is expressed in percent of the gravity 

(G) force value of 32.1 feet per second per second.

probability of exceedance (PE)—is the probability 

that a specific ground motion (peak acceleration) is 

exceeded in a determined number of years. A peak 

ground acceleration with a PE of 2 percent in 50 years 

is that acceleration with a return time of approximately 

2,500 years. A peak ground acceleration with a PE of 10 

percent in 50 years is that acceleration with a return time 

of approximately 500 years. All ground motions (peak 

ground accelerations) from all sites and all magnitudes 

corresponding to a particular probability of exceedance 

are contoured and presented in a map.

shallow seismicity—is seismicity that originates at 

depths less than 31.3 miles in the Caribbean area, 

shallow seismicity caused mainly by extensional activity 

in the Mona and Anegada Troughs, and by stress at 

shallow depths that result from the interaction of the 

Caribbean and North American plates (subduction) 

in the vicinity of the Puerto Rico Trench. Shallow 

seismicity is mostly limited to the mid and upper crust 

regions.

spectral acceleration—is the approximated acceleration 

experienced by a building during the course of the 

earthquake motion. Likewise, it is expressed in percent 

of gravity (G). This acceleration is only due to those 

components of the ground motion that have the same or 

close natural periods as those of the building.

14  Stratigraphy, Structure, and Geologic and Coastal Hazards in the Peñuelas to Salinas Area: A Compendium of Published Literature
Figure 1. Location of the Peñuelas to Salinas study area. Insert shows extent of the Southern Coastal Plain of Puerto Rico (adapted from Renken and others, 2002). 

Stratigraphy, Structure, and Geologic and Coastal Hazards in the Peñuelas to Salinas Area: A Compendium of Published Literature Figure 1 15
Figure 2. Surficial geology of the Peñuelas to Salinas study area (modified from Monroe, 1980). 

Stratigraphy, Structure, and Geologic and Coastal Hazards in the Peñuelas to Salinas Area: A Compendium of Published Literature Figure 2  16
Figure 3. Recent stratigraphic nomenclatures used in the study area (modified from Renken and others, 2002).

Stratigraphy, Structure, and Geologic and Coastal Hazards in the Peñuelas to Salinas Area: A Compendium of Published Literature Figure 3  17
Figure 4. Geologic section across southern Puerto Rico fan-delta plain (adapted from Renken and others, 2002). 

Stratigraphy, Structure, and Geologic and Coastal Hazards in the Peñuelas to Salinas Area: A Compendium of Published Literature Figure 4  18
Figure 5. Major facies of the Ponce Limestone and equivalent beds (adapted from Renken and others, 2002). 

Stratigraphy, Structure, and Geologic and Coastal Hazards in the Peñuelas to Salinas Area: A Compendium of Published Literature Figure 5  19
Figure 6. Thickness of the fan-delta and alluvial deposits (adapted from Renken and others, 2002).

Stratigraphy, Structure, and Geologic and Coastal Hazards in the Peñuelas to Salinas Area: A Compendium of Published Literature Figure 6  20
Figure 7. Percentage of sand and gravel in the fan-delta plain deposits (adapted from Renken and others, 2002).

Stratigraphy, Structure, and Geologic and Coastal Hazards in the Peñuelas to Salinas Area: A Compendium of Published Literature Figure 7  21
Figure 8. Location and orientation of the Great Southern Puerto Rico Fault zone in the study area (modified from Renken and others, 2002). 

Stratigraphy, Structure, and Geologic and Coastal Hazards in the Peñuelas to Salinas Area: A Compendium of Published Literature Figure 8  22
Figure 11. Seismicity density in Puerto Rico and its vicinity (adapted from Clinton and others, 2006). 

http://redsismica.uprm.edu/spanish/tsunami/media/PRMonitoring.pdf 

Figure 10. Geographical distribution of the magnitude of completeness values in Puerto Rico and its 

vicinity as determined from the Puerto Rico Seismic Network Catalog for the interval January 1986-

March 2006 (adapted from Clinton and others 2006).

http://redsismica.uprm.edu/spanish/tsunami/media/PRMonitoring.pdf

Figure 9. Major tectonic faults and historic earthquakes in Puerto Rico and its vicinity (adapted from Clinton and others, 2006). 

Stratigraphy, Structure, and Geologic and Coastal Hazards in the Peñuelas to Salinas Area: A Compendium of Published Literature Figures 9, 10, 11  23
Figure 12. Inland extent of tsunami-induced flooding in the Punta Verraco and Punta Cuchara quadrangles (adapted from Mercado and Justiniano, 2007). http://coastalhazards.uprm.edu/index.html

Stratigraphy, Structure, and Geologic and Coastal Hazards in the Peñuelas to Salinas Area: A Compendium of Published Literature Figure 12  24
Figure 13. Inland extent of tsunami-induced flooding in the Playa de Ponce quadrangle (adapted from Mercado and Justiniano, 2007). http://coastalhazards.uprm.edu/index.html

Stratigraphy, Structure, and Geologic and Coastal Hazards in the Peñuelas to Salinas Area: A Compendium of Published Literature Figure 13  25
Figure 14. Inland extent of tsunami-induced flooding in the Santa Isabel quadrangle (adapted from Mercado and Justiniano, 2007). http://coastalhazards.uprm.edu/index.html

Stratigraphy, Structure, and Geologic and Coastal Hazards in the Peñuelas to Salinas Area: A Compendium of Published Literature Figure 14  26
Figure 15. Inland extent of tsunami-induced flooding in the Salinas quadrangle (adapted from Mercado and Justiniano, 2007). http://coastalhazards.uprm.edu/index.html

Stratigraphy, Structure, and Geologic and Coastal Hazards in the Peñuelas to Salinas Area: A Compendium of Published Literature Figure 15  27