Geologic and Tectonic Setting
Global Geologic Setting and Plate Tectonics
A seismic hazard assessment should include an evaluation of the global geologic setting. Understanding the behavior and properties of the Earth's surface and underlying layers sets the stage for interpreting the local geologic setting.
Global tectonics is the theory that the earth’s relatively low density crust (lithosphere) is composed of seven or eight major and several smaller rigid fragments. These tectonic plates, both continental and oceanic forms of the crust, move about the earth’s face, driven by convective activity within the higher density and relatively plastic mantle (asthenosphere).
Plates interact at convergent (collisional), divergent (spreading), and transform (lateral) boundaries. Mountains rise and the sea floor is subducted in deep trenches along collisional boundaries, continents and oceans rift and open along spreading center boundaries, while continental and oceanic crust moves laterally along transform boundaries. Earthquakes and volcanic activity occur in association with these plate margins.
Research into regional tectonic conditions should be conducted at the beginning of an SHA to establish the regional seismic framework. It is important to obtain the most recent publications, as the science is dynamic and new information becomes available frequently. Information regarding the global geologic and plate tectonic setting can be obtained from governmental geological agencies for countries of the region. The U.S. Geological Survey (USGS) is a good starting point for any location.
Local Geologic Setting
An understanding of the rock units, stratigraphy, and geologic structure of a region, consistent with the global tectonic setting, needs to be developed to characterize the nature of the existing faults and potential seismic sources of a region. An experienced geologist should assess and summarize the known geologic and faulting conditions found in an available geologic literature, maps, and imagery. When geologic documentation is sparse or requires clarification, detailed surface and aerial reconnaissance of the region should be conducted.
Local Tectonic Setting
Clearly identifying and determining the activity level of faults within and adjacent to a project region is critical for development of the input parameters required to perform an SHA. An experienced seismologist should assess and summarize available literature to identify known seismic conditions in a region. The record of historical earthquakes should be evaluated to develop and verify recurrence parameters for known faults.
Paleoseismicity evaluations are used to supplement seismic monitoring in calculating the seismic hazard of a location. Paleoseismologic reports should be evaluated, or new investigations conducted, to refine the local seismic setting. Paleoseismologic investigations assess the sediments and rocks of an area for indications of ancient and recent earthquakes. Such explorations are usually restricted to geologic regimes that have undergone continuous sedimentation over the last few thousand years, such as swamps, lakes, river beds and shorelines.
Paleoseismic exploration often uses trenching to determine the activity of faults in specific locales. Trenching helps develop parameters needed in modeling faults. In a typical example, a trench is dug across the trace of a suspected active fault in an area of active sedimentation. An experienced geologist can potentially identify evidence of the recency of fault displacement in the walls of the trench. The relative age for the most recent displacement of a fault may be deduced through careful evaluation of cross-cutting fracture and sedimentation patterns. Faults may also be age-dated in absolute terms if dateable carbon or human artifacts are present.
Velocity Structure
The velocity structure is a generalized model of the earth’s crust using layers having different assumed seismic velocities. It is important in a SHA to have a well-defined velocity structure. Several parameters (VS30, Z1.0, Z2.5) are used in most attenuation relationships. The most important parameter is VS30, which is defined as the shear wave velocity in the top 30 meters. This can be estimated based on surface geology or geotechnical data, or directly measured in the field. The preferred approach for detailed design would be directly measured in the field, with more preliminary analyses using estimates from the literature.
Examples
California Institute of Technology - Tectonics Observatory at Caltech
International Association of Seismology and Physics of the Earth's Interior (IASPEI)
Citations:
Revision ID: 5752
Revision Date: 12/14/2022