The Northwest Trending North Boquerón Bay-Punta Montalva Fault Zone (2013)

The Northwest Trending North Boquerón Bay-Punta Montalva Fault Zone; A Through Going Active Fault System in Southwestern Puerto Rico

by Coral Marie Roig-Silva, Eugenio Asencio, and James Joyce ABSTRACT The North Boquerón Bay–Punta Montalva fault zone has been mapped crossing the Lajas Valley in southwest Puerto Rico. Identification of the fault was based upon detailed analysis of geophysical data, satellite images, and field mapping. The fault zone consists of a series of Cretaceous bedrock faults that reactivated and deformed Miocene limestone and Quaternary alluvial fan sediments. The fault zone is seismically active (local magnitude greater than 5.0) with numerous locally felt earthquakes. Focal mechanism solutions suggest strain partitioning with predominantly east–west left-lateral displacements with small normal faults striking mostly toward the northeast. Northeast-trending fractures and normal faults can be found in intermittent streams that cut through the Quaternary alluvial fan deposits along the southern margin of the Lajas Valley, an east– west-trending 30-km-long fault-controlled depression. Areas of preferred erosion within the alluvial fan trend toward the westnorthwest parallel to the onland projection of the North Boquerón Bay fault. The North Boquerón Bay fault aligns with the Punta Montalva fault southeast of the Lajas Valley. Both faults show strong southward tilting of Miocene strata. On the western end, the Northern Boquerón Bay fault is covered with flat-lying Holocene sediments, whereas at the southern end the Punta Montalva fault shows left-lateral displacement of stream drainage on the order of a few hundred meters.

INTRODUCTION Puerto Rico is located in the northeastern boundary of the Caribbean Plate (Fig. 1; French and Schenk , 1997). Along with the 538

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Virgin Islands, it forms part of the Puerto Rico–Virgin Island microplate. The Puerto Rico–Virgin Island microplate is bounded by the Anegada Passage, the Mona Canyon; the 19° N fault zone and the Muertos Trough (Fig. 1; French and Schenk , 1997).

The area is seismically active and has a historic record of earthquakes and tsunamis dating back to 1492 with the arrival of Europeans to the New World (Mann, 2005). Puerto Rico, with over 500 years of recorded history, has experienced damaging earthquakes every century (Clinton et al., 2006). Most of the seismic activity occurs as shallow earthquakes along the Puerto Rico trench and deep subduction earthquakes below the Puerto Rico–Virgin Island platform. Southwestern Puerto Rico shows the greatest concentration of earthquakes inland (Clinton et al., 2006). An earthquake in 1670 is suggested to have occurred in the Lajas Valley area, with a magnitude (M) of 7.5 (Tuttle et al., 2005). However, liquefaction features in northwestern Puerto Rico suggest that the Añasco Valley would be a more appropriate location for an M 6.5 event (Tuttle et al., 2005). A relatively large (M 4.6), strongly felt (VI intensity), shallow (< 6 km) earthquake occurred in the western portion of the valley on 30 May 1987 (Puerto Rico Seismic Network; Dr. James Joyce, oral communication, 2011).

The occurrence of this and other inland earthquakes instigated subsequent investigations aimed at defining the active faults responsible for earthquake activity in the valley and assessing the seismic hazard they present (Geomatrix Consultants, unpublished manuscript, 1988). Despite numerous improvements of the Puerto Rico Seismic Network capabilities and numerous geological studies, only limited success has been achieved defining and recognizing active inland faults related to seismicity.

This paper presents evidence for a recently characterized, through-going left-lateral fault system that cuts across the entire May/June 2013

doi: 10.1785/0220120115

▴ Figure 1. Geographic and tectonic setting of Puerto Rico. Global positioning system studies show that Puerto Rico moves mostly along with the Caribbean plate with most movement toward the trench (black arrow; not to scale), but with minor differential movement between Puerto Rico and the Caribbean, and major differences in movement between northwest Puerto Rico and northeast Dominican Republic (located west of Puerto Rico, not shown in figure; Jansma et al., 2000; Jansma and Mattioli, 2005). See full text for description on the features defining the Puerto Rico–Virgin Island microplate. southwestern corner of Puerto Rico. The herein named North Boquerón Bay–Punta Montalva fault zone extends from the west coast north of Boquerón Bay, southeast to south of Guánica Bay (Fig. 2). Left-lateral displacement of stream valleys along the southeast end of the fault zone indicates a minimum of 200 m of displacement along this portion of the fault (Fig. 3).

Neotectonics of Southwestern Puerto Rico Southwestern Puerto Rico is characterized by basin and range topography similar to that imaged offshore in the Mona Passage south of Mona Canyon (Joyce et al., 1987). Offshore seismic-reflection studies indicate extensional tectonics and the formation of east–west-trending half grabens (Dolan et al., 1998; Chaytor and ten Brink, 2010). The Lajas Valley is the most extensive and southernmost of the basin and range

features. The valley is also the site of the greatest concentration of shallow inland earthquakes.

The Lajas Valley is an east–west-trending, 30-km-long, 1.5- to 9.0-km-wide linear depression, characterized by closed drainage depressions of the former Laguna Cartagena, Laguna de Guánica, and Ciénaga El Anegado water bodies (Prentice and Mann, 2005). The valley is characterized by abrupt, steeply sloping hills along the south with alluvial fans; the hills along the north are more gently inclined into the valley. The origin of the Lajas Valley is related to block faulting (Mattson, 1960; Veve and Taggart, 1996).

Seismic-reflection profiles along the south side of the Lajas Valley made by Meltzer and Almy (2000) suggest that the valley is bounded by a transtensional strike-slip fault to the south.

Although the fault was identified at depth in seismic profiles offshore of Puerto Rico, Meltzer and Almy (2000) were unable

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▴ Figure 2. Geographical features and location of previous studies. Image shows shaded relief of southwestern Puerto Rico: thick black lines, location of mapped faults; thin black line, approximate location of the South Lajas Valley fault; and black star, location of trench study by Prentice and Mann (2005); also shown, the location of undergraduate research study areas of students from Department of Geology, University of Puerto Rico. See text for full description. to find evidence of the continuation of the fault onshore or that the fault cuts surface sediments (Grindlay et al., 2005).

Surface rupture within the last 5000 years was interpreted

by Prentice and Mann (2005) on a segment of a (previously undocumented) fault scarp near the southern edge of the Lajas Valley (Fig. 2; Prentice and Mann, 2005). Trenching studies revealed two zones of faulting approximately 1 m apart that disrupt Quaternary alluvial fan deposits, radiocarbon dated at ∼5000 years B.P. (Prentice and Mann, 2005). Two colluvial wedges were identified; radiocarbon dating of organic material suggests the occurrence of at least two earthquakes during the past 7000 years, however, no minimum age was established (Prentice and Mann, 2005). Relations indicate normal faulting, valleyside down, with a possible component of strike-slip displacement.

The South Lajas Valley fault is included in the most recent seismic-hazard map of the U.S. Geological Survey as a potential seismogenic source (Mueller et al., 2003). The fault is consid540

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ered to extend from the southwestern edge of the Lajas Valley trending in a generally east–west direction throughout the valley (∼50 km; Fig. 2; Mueller et al., 2003; LaForge and McCann, 2005; Prentice and Mann, 2005). However, evidence of east–west-trending normal faulting indicative of north– south extension has not been supported by composite focal mechanism computed by Huerfano et al. (2005). Active seismicity suggests northeast–southwest compression, northwest– southeast extension, and west-northwest–east-southeast-trending left-lateral faulting.

Recognition of the North Boquerón Bay–Punta Montalva Fault Zone The Lajas Valley has been the prime focus of several undergraduate research projects conducted by students of the Department of Geology of the University of Puerto Rico, Mayagüez Campus (Fig. 2). De Choudens (1999) interpreted May/June 2013

▴ Figure 3. Geomorphology of the Punta Montalva area. The Punta Montalva fault was identified on the basis of steeply inclined Miocene Ponce Limestone in the Punta Montalva area (a and b) and traced to the Ensenada Las Pardas (b) and the strong lineament (Addarich-Martinez, 2009). Observation of aerial photographs (GS-LR-10 141, 20 January 1951 shown) reveals the presence of left-laterally displaced stream and valley of about 200 m (c and d); also revealed are intermittent streams that align to the lineament. Dashed red, Punta Montalva lineament; dark blue, drainage systems. aerial photos and collected structural data of crosscutting features on alluvial sediments and bedrock along two alluvial streams north of Sierra Bermeja (Fig. 2). Based on kinematic analyses, De Choudens (1999) suggested northeast shortening.

Roig-Silva (2003) analyzed aerial photographs and conducted geological field mapping along the Punta Montalva area in the southeast end of the Lajas Valley (Fig. 2). Roig-Silva (2003) identified steeply inclined limestone strata striking N45°W. In addition, Ocasio (2004) re-analyzed and re-interpreted total intensity magnetic and seismic-reflection data collected by Western Geophysical Company of America and Fugro, Inc. (1974) along the western shore of the Lajas Valley.

Ocasio (2004) interpreted one of the total magnetic intensity

anomalies as the boundary of a previously unmapped west– northwest-trending offshore fault, and named it the North Boquerón Bay fault. The North Boquerón Bay fault is a deep-seated structure within the crust. It also has been suggested that the North Boquerón Bay fault extends inland towards the Lajas Valley (Ocasio, 2004).

Based on the Ocasio (2004) findings, additional investigations were conducted along the western end of the Lajas Valley, near Boquerón (Fig. 2). Multichannel Analysis of Surface Waves (Roig-Silva, 2004; Martinez, 2005, 2006) and highresolution seismic-reflection data (Rivera-Ríos, 2006) were collected in an attempt to delineate the inland extension of the North Boquerón Bay fault (Roig-Silva and Asencio, 2007).

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Their findings suggest that the North Boquerón Bay fault coincides with the boundary of the Boquerón Mangrove Forest; based on field observations and aerial photograph analyses, Kaye (1957) interpreted the boundary as a normal fault.

Geologic mapping conducted at the southeastern end of the Lajas Valley for the preliminary geological map of the Guánica Quadrangle (Addarich-Martinez, 2009) revealed evidence of a fault. Steeply inclined Miocene Ponce Limestone

within the Punta Montalva area was traced to the eastern end of the peninsula at Ensenada Las Pardas (Fig. 3). Here the Miocene Ponce Limestone has a strike of N24°W, and a dip of 59° SE, compared with a strike of N50°E, and shallow dip of 07° SE elsewhere in the area. The inclined strata and the identified lineament (trending N80°W) led Addarich-Martinez (2009) to suggest a fault. Additional observations and the identification of a displaced stream suggest the Punta Montalva fault

Table 1 Details of Images Analyzed Image Type High Resolution Orthophoto Quarter Quadrangles

Details Bands: 3 Linear Unit: 1 m Cell Size: 1,1 More Information: Center for Transportation Research and Education http://www.ctre.iastate.edu/research/ bts_wb/cd‑rom/spatial/doq.htm

USGS-Seamless Data Warehouse and Viewer http://seamless.usgs.gov/about_orthoimagery.php http://seamless.usgs.gov/website/seamless/viewer.htm

Synthetic Aperture Radar-RADARSAT-1

LANDSAT 7 ETM+

Scene ID RSI Order Number Geographical Area Scene Start Time Scene End Time Orbit Orbit Data Type Application Lut Beam Mode Product Type Format Number of Image lines Number of Image pixels Pixel Sampling Scene Centre Corner Coordinated

P0434576 05-2150-001 Puerto Rico Dec 08 1999 10:22:28.404 Dec 08 1999 10:22:45.415 21364 Descending Definitive Mixed SAR Standard 3 Path Image (SGF) RADARSAT CEOS 9088 8893 12.500 m 17°450 N 067°070 W 18°210 35:63″ N 18°090 40:45″ N 67°320 04:60″ W 66°300 15:36″ W 17°210 16:20″ N 17°090 17:87″ N 67°440 40:59″ W 66°430 13:16″ W 2000-11-13 17:3459800 −67:0151410 18:2826923 18:0329814 −67:6868608 − 65:9750845 16:6532845 16:4071267 −68:0480129 − 66:3496801 14.250 57.000 28.500

ACQUISITION_DATE SCENE_CENTER_LAT SCENE_CENTER_LON Corner Coordinates

GRID_CELL_SIZE_PAN GRID_CELL_SIZE_THM GRID_CELL_SIZE_REF

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has a strike-slip component (Addarich-Martinez, 2009; RoigSilva et al., 2009).

Investigations along the Punta Montalva fault conducted

by Rivera-Santiago (2009) identified a series of geomorphological features showing left-lateral displacement. Such features include a left-lateral-displaced stream (by ∼200 m), intermittent streams that align parallel to the lineament of the fault, and a displaced stream valley exhibiting left-lateral displacement (also ∼200 m; Fig. 3).

CHARACTERIZATION OF THE FAULT ZONE AND FIELD WORK Geomorphology Analysis of high-resolution digital orthophoto quarter quadrangles (DOQQs), synthetic aperture radar (SAR), RADARSAT-1, and Landsat 7 Enhanced Thematic Mapper Plus (Landsat 7

ETM+; Table 1) were used in the generation of lineament maps of southwestern Puerto Rico (Roig-Silva, 2010). An average orientation of N84°E was obtained from the analysis of the DOQQs; however, the mode was N20°E. Integrating analysis of the SAR image gave an average lineament orientation of N86°E and mode of N18°W, whereas incorporation of the Landsat image gave a N68°E orientation with N33°E as the mode. Although the lineaments are lacking field validation, a density map (Bruning et al., 2011) was generated (Fig. 4).

The data suggest north to northeast structural grain for the Lajas Valley.

Southeastern Lajas Valley–Punta Montalva At the southeast end of the Lajas Valley, the Punta Montalva fault is geomorphologically characterized by a strongly defined topographic lineament. Several streams in the area exhibit structural control. One of these streams shows left-lateral

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Figure 4. Lineament density map of southwestern Puerto Rico. Lineaments identified in high-resolution ortho-images, synthetic aperture radar RADARSAT-1 and Landsat 7 ETM+ were combined in a density map for southwestern Puerto Rico. Locations of mapped faults, field sites visited, and proposed faults (South Lajas Valley fault, Punta Montalva fault, and North Boquerón Bay fault) in the study area are shown. The rose diagram shows the preferential northeasterly orientation of lineaments.

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deflection of about 200 m whereas other intermittent streams align with the lineament of the proposed Punta Montalva fault.

Deflection can be observed in 1950s aerial photographs and USGS topographic maps (Fig. 3). A shutter ridge across the mouth of two streams has trapped alluvial sediment in a fan behind the ridge along the fault. The valley shows ∼200 m leftlateral displacement as well.

Central Lajas Valley-Lajas Site Northwest of Punta Montalva and into the Lajas Valley, a projection of the Punta Montalva fault intersects a small bedrock hill of the highly deformed Cretaceous Parguera Limestone Formation. The outcrop (trending north–south) located on the west wall of the hill is cut by large west-northwest-trending vertical faults (Fig. 5). Stratigraphic relations indicate leftlateral displacement (Fig. 5a,b). Other structures at the site are consistent with northeast–southwest shortening.

Sierra Bermeja To the west of Laguna Cartagena, two exposures within a stream channel cutting into alluvial fan deposits show evidence of Quaternary displacement (Fig. 6; De Choudens, 1999; Roig-Silva, 2010). Along the downstream exposure, northeasttrending normal faults displace alluvial sediment (Fig. 6).

Along the 10-m-long outcrop a fault displacement shows an accumulative normal faulting displacement of 33 cm. The faults trend north-northeast with vertical to almost vertical dips. Structural data show northeast–southwest shortening (Fig. 6).

Ground penetrating radar (GPR) data collected next to the outcrop show evidence for the faults to extend at least 1 m below the stream bottom (Fig. 6). However, the faults do not seem to disrupt surface sediments.

The second outcrop along this alluvial fan channel shows erosional features that trend northwest–southeast; these features are parallel to the North Boquerón Bay fault and the Punta Montalva fault projections. Although further investigation is

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Figure 5. Central Lajas Valley site. Following the trend of the Punta Montalva fault towards the northwest from Punta Montalva area (a and b) reveals the presence of deformed Parguera Limestone. The outcrop shows a series of fault features with two major faults showing reverse faulting (c), and reverse left-lateral displacement (d). Dot, movement toward the reader; X, movement away from the reader.

Locations of the faults along the outcrop are shown in (e). Geological mapping by Volckmann (1984) shows the location of a fault bend next to the outcrop (b); black, faults. Kinematic analyses based on the structural features found suggest northwest to southeast extension (f).

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▴ Figure 6. Sierra Bermeja: Alluvial fan stream channel. Image showing features found along an alluvial stream channel in Sierra Bermeja: (a) location of the field site; (b) topographic map of the area; (c) high-resolution DOQQ of the stream with location of the outcrop and ground penetrating radar (GPR) line; (d) image of the outcrop (red arrows indicating relative displacement); (e) stereonet of faults and deformation features showing north-northeast to south-southwest direction of shortening (blue arrows); and (f) GPR profile collected along stream bottom.

needed, the erosional gully coincides with a 70-m apparent leftlateral displacement of the stream observed in aerial photos and the topographic map (Fig. 7). Structural data collected are interpreted to represent north to northeast shortening.

GPR data collected in the vicinity of a road west of the alluvial channel suggest a series of faults along the line (RoigSilva, 2010). A northwest–southeast trending erosional feature along the road is spatially correlated with one of the faults identified in the GPR line (Fig. 7). No other correlation has been established.

Northwest Lajas Valley–Boquerón Continuing the trend of the Punta Montalva fault towards the northwest and away from Sierra Bermeja, there is the Bo-

querón area. Geophysical data collected in the Boquerón Public Beach (Roig-Silva, 2004; Martinez, 2005; Rivera-Ríos, 2006) were interpreted to mark the inland continuation of the North Boquerón Bay fault (proposed by Ocasio, 2004).

The abrupt boundary of the Boquerón Mangrove Forest and the alluvial deposits of the Lajas Valley are interpreted as the surface expression of the North Boquerón Bay fault.

GPR data collected at the Boquerón area show faulted reflectors that are spatially correlated with a lateral decrease in shear-wave velocities recorded by Roig-Silva (2004) and Martinez (2005) using Multichannel Analysis of Surface Waves.

The faulted reflectors also are correlated with pinch-out sediments imaged by Rivera-Ríos (2006) using high-resolution seismic reflection. These reflectors are interpreted to represent the

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▴ Figure 7. Ground penetrating radar along the Reparto Saman Road. Reparto Saman Road runs parallel to the alluvial stream channel: (a) location with respect to the Lajas Valley; (b) pink, length of the road; blue, a small section of the line shows faulted reflectors); (c) dashed red, faulted reflectors shown by GPR data correlate with an erosional gully trending northwest.

North Boquerón Bay fault (Fig. 8; Roig-Silva and Asencio, 2007; Roig-Silva, 2010).

tially vertical planes with dip-slip motion or sub-horizontal nodal planes.

First-Motion Focal Mechanisms The Puerto Rico Seismic Network has documented at least 70 felt-earthquake events within the southwestern Puerto Rico area from 1986–2008. Of these, 11 events were located within our study area (Table 2).

First-motion focal mechanism solutions suggest strain partitioning along the North Boquerón Bay–Punta Montalva fault zone. The focal mechanisms in the southeast Lajas Valley suggest a component of compression and a component of leftlateral strike slip. Conversely, the northwest focal mechanism solutions suggest great variability, for which several show essen-

DISCUSSION

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The Punta Montalva fault, located in the southeastern end of the Lajas Valley, is defined and identified using remote sensing, geophysical data, geological field mapping, and seismic activity, whereas the North Boquerón Bay fault, located in the northwestern end of the Lajas Valley, is identified on the basis of shallow subsurface geophysics, remote sensing, and seismic activity. Local seismic events and geomorphological features can be associated with the Punta Montalva fault and the North Boquerón Bay fault. First-motion focal mechanism solutions of May/June 2013

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Figure 8. Ground penetrating radar: Lines collected in the Boquerón area. (a) location of the Boquerón area, (b) location of the GPR lines collected; (c, d, and e) GPR lines show the location of the North Boquerón Bay fault (dashed red and labeled NBBF in the profiles; arrows indicate relative sense of motion). The lines are located close to the site of previous studies by the Department of Geology, University of Puerto Rico, Mayagüez (see Fig. 3).

local events suggest strain partitioning with a component of compression and another of left-lateral strike slip along the Punta Montalva fault.

The geomorphology of the Lajas Valley suggests a northeasterly preferential orientation. A post-Miocene tectonic change from north–south extension to northwest–southeast extension has been documented in various fault studies in southern and northwestern Puerto Rico (Larue and Ryan, 1991; Larue et al., 1991; Hippolyte et al., 2005; Mann et al., 2005). East–west-trending basin and range topography may be remnant and inherited from Miocene–Pliocene tectonic setting of Puerto Rico. Preferential Holocene activation of older fault systems has been suggested by Mann et al. (2005).

Projecting both the North Boquerón Bay fault and Punta Montalva fault inland (towards the central area of the valley), a

series of geomorphological and geological features are present that suggest both faults are part of a bigger fault zone cutting through the Lajas Valley. Field observations suggest the North Boquerón Bay–Punta Montalva fault utilizes pre-existing westnorthwest–east-southeast paleo tectonic bedrock structures to cut across the regional structural grain and deformed Miocene limestone, as well as displaced Quaternary stream valleys and alluvial fan deposits. The fault exhibits left-lateral strike-slip motion.

Continuing the Punta Montalva fault trend from southeast Lajas Valley towards the northwest brings it into alignment with a bedrock fault. The fault is postulated to curve northward into small hills that lie between the Parguera synclinal hills and the eastern end of Sierra Bermeja (Fig. 5; Volckmann, 1984). The exposure in the hill along the fault bend is deformed Parguera

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Table 2 Selected Felt Earthquakes in Southwestern Puerto Rico for Which First-Motion Focal Mechanisms were Computed Solutions are Shown in Figure 9 Event Number 07 26 30 37 39 42 56 49 52 53 60 70

Date (yyyy/mm/dd) 1991/12/26 1999/11/13 2000/08/19 2003/03/22 2003/03/22 2003/03/23 2007/03/15 2004/11/04 2006/03/08 2006/04/18 2007/05/01 2008/03/28

Hh:mm:ss.ss 07:29:17.30 22:27:46.02 08:15:51.06 19:44:49.42 20:20:53.69 23:16:33.11 03:21:59.65 02:03:22.59 17:06:06.30 13:31:21.33 03:27:02.72 21:15:05.14

Limestone. Structural data collected are consistent with northeast–southwest shortening in a left-lateral restraining bend.

Larger left-lateral faults trend west-northwest toward the Sierra Bermeja. No evidence of recent fault displacement was observed. Seismic events within the area suggest east-

Lat DD 18.030 17.934 17.917 18.036 18.029 18.005 17.988 18.030 17.968 17.994 18.042 17.918

Long DD −67.310 −66.964 −66.939 −67.190 −67.168 −67.155 −67.059 −67.124 −66.970 −67.072 −67.219 −66.944

Depth (km) 20.0 10.7 3.9 3.9 5.2 0.8 5.1 12.50 7.93 10.75 34.68 7.28

Magnitude 3.3 4.1 3.6 3.4 3.3 3.5 3.55 2.82 3.14 3.23 3.92 3.02

northeast–west-southwest compression, whereas outcrop data shownorth-northeast–south-southwest to northeast–southwest shortening.

At the western end of Sierra Bermeja the fault coincides with bedrock shear zones. Structural data collected suggest

▴ Figure 9. Location of selected felt earthquakes and first-motion focal mechanism solutions. Asterisks, earthquake locations; horizontal location error shown in kilometers; mapped faults shown in black. Earthquakes along the Punta Montalva area suggest north-northwest to south-southeast direction of extension, whereas earthquakes located at the Boquerón area suggest south-southwest to east-northeast direction of extension.

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left-lateral displacement of the bedrock fault and northeast– southwest shortening. A few hundred meters to the west a ∼70-m left-lateral displacement was observed at a local stream.

An erosional feature located at the stream bend trends westnorthwest and parallel to the co-alignment of the Punta Montalva fault and the North Boquerón Bay fault. Exposures downstream show displaced alluvial sediment with normal displacement and oriented to the northeast. GPR data collected show that these faults do not cut surface sediments.

CONCLUSIONS The North Boquerón Bay fault has been mapped based on shallow subsurface geophysical data, and the fault extends towards the northwest, offshore of Boquerón Bay. Studies conducted by Dietrich (1995) identify several faults offshore of Boquerón Bay. Of these mapped faults one correlates with the North Boquerón Bay fault topographic lineament north of Boquerón Bay. Results from side-scan sonar data collected

by Grindlay et al. (2005) in Boquerón Bay documents a carbonate platform inclined 3° to the south along the projection of the North Boquerón Bay fault. Along this side-scan line faulting is observed. Nonetheless the faults do not appear to cut surface sediments (Grindlay et al., 2005).

Based on data collected in this investigation and previously published data, the Punta Montalva fault and the North Boquerón Bay fault are interpreted to form a major through-going left-lateral strike-slip fault zone that cuts across the southwest corner of Puerto Rico. The fact that the fault zone cuts across the geomorphic grain of the area and follows bedrock faults suggests the fault is a young incipient system that has developed by the reactivation of older extinct faults in favorable orientation. Major displacement has occurred on the Punta Montalva fault (∼200 m) relative to apparent displacement measurements in the alluvial fan channel (∼70 m; Roig-Silva et al., 2009; Roig-Silva, 2010).

Seismicity in southwestern Puerto Rico suggests that the fault system may be active. Many recorded earthquakes (including some reported as felt) are located parallel to deformational features that have been related to the Punta Montalva and North Boquerón Bay fault system. Accurate dating of displacement and strain accumulation along the fault is needed to assess the seismic-hazard potential of the fault.

ACKNOWLEDGMENTS Partial funding for this investigation was provided by the Department of Geology of the University of Puerto Rico, Mayagüez Campus; the Society of Exploration Geophysicists for the Project of Special Merits Ground Penetrating Radar Field Experience conducted with the University of South Carolina and the University of Puerto Rico; and the Puerto Rico Seismic Network. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

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Coral Marie Roig-Silva U.S. Geological Survey Eastern Geographic Science Center 12201 Sunrise Valley Drive MS 521 Reston, Virginia 20192 U.S.A. Eugenio Asencio James Joyce Department of Geology, Physics Building Rm F419 University of Puerto Rico-Mayaguez Carr. 108 Km 0.6 Mayaguez, Puerto Rico 00681

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