High Resolution Seismic Imaging Glossary

• Active Method - A disturbance is produced at a point on the surface by a blow with a heavy hammer or shaking by a source truck. The disturbance propagates outward as a series of wave fronts similar to ripples on a lake. The passage of the wave fronts by each point on the surface is marked by the motion of that surface, which can be measured and recorded by arrays of sensitive recording instruments. Using the time interval between impact and wave arrival, you can determine the velocity of the wave. Wave velocities are important in seismic imaging and modeling.
• Passive Method - No disturbance is created with this method. Instead, sensitive recording instruments are set out in arrays to simply "listen". In an urban area, common things like pedestrian traffic, automobile traffic, and airplanes can serve as a source of disturbance waves that propagate beneath the surface. We can measure the time interval of these "microtremor" disturbances between a set of sensors and determine the speed of the waves.
• Seismic Imaging - A seismic source can disturb the ground either in the vertical direction to generate P-waves, or in the horizontal direction to generate S-waves. From the echoes that are recorded as the seismic waves bounce back to the surface from the various geologic structures beneath, we can create a subsurface image and estimate how fast seismic waves travel beneath a site. Both the image and the wave speeds are important to more accurately determine how the soils and rock will behave during an earthquake.

  See also reference:

  • Odum, J.K., Williams, R.A., Stephenson, W.J., and Worley, D.M., 2004, High-resolution imaging within the urban environment: "What's beneath our streets?" U.S. Geological Survey Fact Sheet, FS 2004-3017.
    URL: http://pubs.usgs.gov/fs/2004/3017/fs-2004-3017.html
• Earthquake Hazards - An earthquake hazard is anything associated with an earthquake that may affect the normal activities of people and their community. This includes surface faulting (displacement that reaches the earth's surface during slip along a fault), ground shaking (such as waves that are generated by sudden slip on a fault and travel through the earth and along its surface), landslides (movement of surface material down a slope), liquefaction (water-saturated sediment temporarily loses strength and acts as a fluid), tectonic deformation (rock-deforming processes and resulting structures that occur over large sections of the lithosphere), and tsunamis (sea wave of local or distant origin that results from large-scale seafloor displacements associated with large earthquakes, major submarine slides, or exploding volcanic islands).
• Site Characterization - Using the data we collect from a field project area, we can estimate its dynamic soil properties (for example, S-wave speed and P-wave speed), site response (localized ground shaking), and liquefaction potential. These observations can be incorporated into hazard maps and be used to educate local residents and guide land use decisions.
• Sedimentary Basin - These are large valleys filled with unconsolidated sand, clay and other erosional and depositional deposits whose base is often a hard bedrock surface. Information on the depth and shape of sedimentary basins is essential for predicting how the ground will shake in future earthquakes and why different parts of the basin shake more strongly than others, work that could lead to improved forecasts of future ground shaking during earthquakes.
• Ground Motion Modeling - Ground motion is the movement of the earth's surface from earthquakes. Ground motion is produced by seismic waves that are generated by sudden slip on a fault and travel through the earth and along its surface. Based on this process, we try to model what might happen in the event of an earthquake.
• Hazard Maps - Regions will behave differently during an earthquake because of their differing rock types, tectonic settings, regional fault characteristics, etc. Hazard maps generally use a color scale to highlight an area's level of earthquake hazard. Below is an example of a seismic hazard map for the United States.