USGS:Science for a changing world

Colegio de Ingenieros y Agrimensores, San Juan, March 23-24, 1999

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Paleoseismology and Active Faults Working Group Report

Leaders – C. Prentice (USGS-EHZ), J-C. Moya (Consultant)


J. Joyce (U. Puerto Rico), M. Tuttle (U. Maryland and consultant), R. Updike (USGS-EHZ), S. Johnson (USGS-EHZ), T. Pratt (USGS-EHZ), G. Mattioli (U. Puerto Rico), E. Schweig (USGS-EHZ), P. Jansma (U. Puerto Rico).


Puerto Rico lies within the actively deforming plate boundary zone between the Caribbean and North American plates. Current models of seismic hazard include sources offshore to the west (i.e., Mona Passage and Septentrional fault), to the north (Puerto Rico trench), to the southeast (Muertos Trough and Anegada Passage). None includes any potential onshore source, because no onshore Quaternary fault has yet been documented in the published literature. Historic earthquake records, current levels of microseismicity, and recent GPS results all point to significantly greater deformation offshore and limited displacement onshore in Puerto Rico and the Virgin Islands. However, given the plate tectonic setting and available offshore geophysical data, the presence of onshore Quaternary faults seems very likely. Current tectonic models all suggest that onshore deformation ought to be occurring. Furthermore, there is low-magnitude seismicity throughout the island. If onshore faulting is occurring, identifying and characterizing active faults would have a significant impact on seismic hazard maps. The apparent contradiction between paleoseismologic evidence used to infer onshore active faulting and evidence cited above, including GPS geodesy, suggests that additional GPS geodetic work focused on specific potentially seismogenic onshore structures is essential.

The group wishes to stress the following two points:

  1. In Puerto Rico, neotectonic studies are essential to better characterize seismic hazard.
  2. It is necessary to integrate marine geophysical surveys with onshore work, because the land masses are part of much larger, mostly submerged tectonic blocks.


HIGHEST PRIORITY - Paleoliquefaction studies, reconnaissance and detailed site studies


To define paleoearthquake chronology

To possibly provide a test of seismic hazard– maps – levels of ground shaking and recurrence times.

To possibly provide a test of source delineation from the South Puerto Rico Slope Fault Zone (SPRSFZ) and Muertos Trough.

Proposed activities

Field reconnaissance aimed at identifying Quaternary faults – search for paleoseismic sites. Establish existence of potential onshore sources. Develop paleoseismic chronology and slip rates.

Quaternary mapping (marine terraces, fluvial terraces and basins, beach ridges, etc.) Establish framework for understanding neotectonic deformation of the island.

Interpretation of aerial photography – identify possible Quaternary faults.

GPS monitoring of specific onshore faults.


NEXT HIGHEST PRIORITY- Paleotsunami deposits and modeling of tsunami potential.


Help establish tsunami recurrence rates (See also Tsunami Report)

Proposed activities

Nearshore high-resolution imaging. Identify important targets for onshore studies (see Marine Geology Report).

Other necessary activities

Updated digital topography. Existing maps are decades old, modern methods could significantly improve understanding of modern landforms and processes (including tsunami inundation zones). Some efforts in this direction have already begun (see section on State of Knowledge).

Laser Altimetry and IFSAR surveys to obtain high resolution topographic data over potentially tectonically active regions. Some efforts in this direction have already begun (see section on State of Knowledge).

Onshore high-resolution geophysical surveys including ground penetrating radar (GPR) in regions of suspected Quaternary faulting – help target paleoseismic excavations.

Related marine geology and geophysics activities

Studies to determine if SPRSFZ is seismically active.

Improved velocity model for earthquake relocation using the local network and offshore temporary OBS networks.

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