Seismic Peak Acceleration Maps

Peak horizontal acceleration at each station is contoured in units of percent-g (where g = acceleration due to the force of gravity = 981 cm/s/s). The peak values of the vertical components are not used in the construction of the maps because they are, on average, lower than the horizontal amplitudes and ground motion prediciton equations used to fill in data gaps between stations are based on peak horizontal amplitudes. The contour interval varies greatly and is based on the maximum recorded value over the network for each event.

Maps - General Information

• The Red star usually located near the center of the map is the epicenter.
  
• Small unfilled circles, shown on some maps, are points where strong motion values were estimated and used to fill gaps in the station distribution.
• The colored triangles indicate reporting stations. In California, for example:
  

  a)   Red triangles are stations from the Caltech/USGS digital telemetered network.

  b) Blue triangles represent California Geological Survey (CGS) and California Strong Motion Instrumentation Program (CSMIP) dial-up stations.

  c)   Yellow triangles represent stations from the ANZA Regional Network.

  Green triangles represent National Strong Motion Project (NSMP) dial-up stations or, on historical maps, non-digital stations from which strong motion records were digitized

  Contour  of Peak Ground Acceleration of Honshu Mega Thrust: Fraction of Contour is: 

  >90 contour
  140.969 38.3694 90
  141.002 38.3768 90
  141.019 38.3637 90
  141.036 38.3429 90
  141.043 38.3303 90
  141.036 38.3105 90
  141.03 38.297 90
  141.002 38.2639 90
  141.002 38.2637 90
  140.969 38.2404 90
  140.946 38.2303 90
  140.936 38.2262 90
  140.927 38.2303 90
  140.902 38.2522 90
  140.895 38.2637 90
  140.902 38.2831 90
  140.908 38.297 90
  140.929 38.3303 90
  140.936 38.3384 90
  140.962 38.3637 90
  140.969 38.3694 90
  >90 contour
  140.636 36.6435 90
  140.655 36.6303 90
  140.669 36.6077 90
  140.673 36.597 90
  140.669 36.5905 90
  140.641 36.5637 90
  140.636 36.5604 90
  140.628 36.5637 90
  140.602 36.5792 90
  140.588 36.597 90
  140.602 36.6205 90
  140.612 36.6303 90
  140.636 36.6435 90
  >120 contour
  141.036 38.7741 120
  141.054 38.7637 120
  141.069 38.7503 120
  141.082 38.7303 120
  141.069 38.7036 120
  141.064 38.697 120
  141.036 38.678 120
  141.004 38.697 120
  141.002 38.6983 120
  140.985 38.7303 120
  141.002 38.7536 120
  141.015 38.7637 120
  141.036 38.7741 120

For moderate to large events, the pattern of peak ground acceleration is typically quite complicated, with extreme variability over distances of a few km. This is attributed to the small scale geological differences near the sites that can significantly change the high-frequency acceleration amplitude and waveform character. Although distance to the causative fault clearly dominates the pattern, there are often exceptions, due to local focussing and amplification. This makes interpolation of ground motions at one site to a nearby neighbor somewhat risky. Peak acceleration pattern usually reflects what is felt from low levels of shaking up to to moderate levels of damage.