USGS:Science for a changing world

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

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Earthquake Seismology and Engineering Joint Working Group Report

Earthquake Seismology

Leaders – C. Mueller (USGS-EHZ), C. von Hillebrandt (U. Puerto Rico)


A. Frankel (USGS-EHZ), Chesley Williams (Risk Management Solutions), Gail Atkinson (Carlton U. Ottawa), Dariush Matazedian (Carlton U. Ottawa), C.B. Crouse (Dames and Moore), Alexander Allman, Roland LaForge (U.S. Bureau of Reclamation, Denver), Joe Litehiser (Bechtel, CA), Kenneth Campbell (EQE, Evergreen, CO)


E.V. Leyendecker (USGS-EHZ), J.M. Cruzado (U. Puerto Rico)


Mehmet Celebi (USGS-EHZ)


1. Methodology and Input to New Seismic Hazard Maps

In the time frame of the new seismic hazard maps (less than one year) the earthquake panel concentrated on two issues.

Most of the discussion involved the southwest Puerto Rico seismic zone, which was considered part of the Mona Passage extensional zone in the draft maps. Should the southwest zone be treated separately? If so, what shape? Should UPR network data be incorporated into the hazard estimates? Elsewhere, the hazard mappers have found that regional and global catalogs are sufficient to determine the rates of the hazard-controlling earthquakes in the smoothed-seismicity part of the hazard calculation (that's probably magnitude 5-6 in the southwest zone). University of Puerto Rico network data, however, suggest somewhat different statistics for the southwest zone compared to the whole region: a higher b value, and only one earthquake with magnitude greater than 5 since 1900. Consistent treatment of the network data here might lower the hazard relative to the draft maps.

The other main issue addressed by the panel was catalogs. The draft maps used a PDE-based catalog, assumed complete to magnitude 4.5 after 1963, for the smoothed-seismicity part of the hazard calculation. The hazard mappers doubt that catalog differences will have much effect on the hazard, but two comparisons were discussed. First, the IPGH catalog of Tanner and Shepherd, which uses different magnitude-conversion rules than the draft maps, should be considered. Second, the Engdahl - van der Hilst - Buland relocation catalog lists improved depths for earthquakes near Puerto Rico, but is complete only down to magnitude 5.2. Should it be added to the mix?

The panel also discussed the deformation rate used for the Mona Passage zone, wondering if 5 mm/yr might be better than the10 mm/yr rate that was used in the draft maps. They also reviewed the GPS data, and decided that 1 mm/yr for the Anegada Passage zone looked OK.

2. Develop a 5-Year Plan for Research

Members of the earthquake panel agreed that the highest research priority in a five-year time frame should be given to the creation of new strong ground motion attenuation relations (pga and several spectral periods) for Puerto Rico. Atkinson and Motazedian have begun work on empirical attenuation relations, which will take several years to complete. Concerning the new hazard maps, it was pointed out that, to the extent we are unsure about using a western or eastern North America attenuation relation for Puerto Rico, using just a western rule with its published uncertainty might underestimate the true uncertainty. Perhaps a modified eastern rule (eastern Q, but 50-bar stress drop?) could be constructed in time for inclusion in the new maps. The work of Atkinson and Motazedian might also lead to improved rules for converting traditional instrumental magnitudes to moment magnitudes. The panel recommended that, in support of these efforts, pre-1991 network data should be transcribed from 9-track tape onto more stable archival media (UPR, Carleton U.), and that instruments in the University of Puerto Rico network should be calibrated (UPR, USGS-Golden).

The hazard mappers would prefer to have rate information on specific offshore structures, but expect that GPS will remain the best source of information on deformation rates offshore in a 5-year time frame. They were encouraged by the presentation of the paleoseismology group, who offered the possibility of getting recurrence rates and maximum magnitudes for onshore faults. The panel wondered if there is any way to estimate seismic vs. aseismic deformation. Finally, the panel noted that site effect observations and near-surface geophysical measurements will be needed if local (urban-area) hazard maps including site response are ever anticipated, but didn't consider that a priority.

3. Foster cooperation between the U. S. Geological Survey (Geologic Hazards Group in Golden, CO), Carleton University (Atkinson & Motazedian - attenuation and magnitudes), and the Univ. of Puerto Rico (von Hillebrandt et al - network data).



  • Spectral acceleration values
  • Time histories
  • Directivity effects
  • Duration of strong ground motion


Liquefaction susceptibility maps should be prepared for urban areas. This is an important aspect of geotechnical investigations.


  • Update and improve types of site conditions in Puerto Rico. This is a first step in determining site amplification factors for use in determining site response.

  • Site effect observations and near-surface geophysical measurements will be needed, if local (urban-area) hazard maps, including site response, are ever anticipated.


An expanded strong motion program is needed. This includes both free field instrumentation and building instrumentation. The priority is placed on free field instrumentation. Site locations should be carefully selected and the type of site classified by soil profile and shear wave velocity. Sites should include different types of material in order to evaluate site response.

Weak ground motion instrumentation can be used to obtain site response instrumentation. This type of instrument can be deployed on different types of sites to obtain site amplification in short periods of time. While it can be argued that this type of data might not be suitable for strong ground motion, it can be quite useful.


Fragility curves should be developed for Puerto Rico construction types and details.


Laboratory testing should be done on selected types of mass-produced construction.


Code enforcement is limited due, in part, to a limited number of inspectors. Inspection of projects to ensure that what was designed is actually built is an important component of earthquake design and improvement is encouraged.


Tsunami design should be considered in construction projects. This can be done through different means such as relocation or barriers, such as sea walls or wave energy dissipation devices. Maps of wave height and velocity would also be useful.


  • A campaign to make the public more aware of the consequences of poor earthquake parameters.

  • Map data should be presented in different ways that would make it easier for the public to understand. An example would be a map portraying the probability of exceeding a code level design in the next 50 years.

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