GEOPHYSICAL EXPLORATION FOR POTENTIAL TRENCH SITES AND CHARACTERIZATION OF NORTH BO QUERON BAY-PUNTA MONTALVA FAULT ZONE: A CAPABLE FAULT SYSTEM IN SOUTHWESTERN PUERTO RICO

摘要

We characterized potential paleoseismic trenching sites of Near-Surface Faults along North Boqueron Bay-Punta Montalva Fault Zone (NBB-PMFZ) in Guanica, P.R., using high resolution ground penetration radar by imaging and mapping the locations of fractures and faults that have displaced near surface sediments. GPR Line-1 can be characterized by a 4-m thick sequence of Quaternary deposits (with velocities of .19 m/ns) that overlays the Ponce Limestone (with velocities of about .16 to .17 m/ns) mapped as as bioclastic grainstone, wackstone and packstone.. The most important structural and stratigraphic features observed in GPR Line-1 suggests the following: (1) near surface Quaternary deposits are folded and fault-tilted within a 35-m wide deformed zone. (2) reflectors within the interpreted Ponce limestone (4-18 m) in depth are generally horizontal to subhorizontal, except at the fault zone. No deformation has been observed on GPR Line-2 and GPR Line-3. We interpreted the deformed near surface Quaternary deposits on Line-1 as a positive strike-slip flower structure. We conclude, that at least on Line-1 the geophysical data provides a reasonable interpretation of the presence of coseismic surface rupture on NBB-PMFZ and might be a potential site for trenching.