- The Red star usually located near the center of the map is the epicenter.
- The colored triangles indicate reporting stations:
- Blue triangles represent the OGS stations.
- Red triangles are stations from the INGV digital telemetered network.
- Black triangles represent the ZAMG stations.
- Orange triangles represent the ARSO stations.
- Magenta triangles represent the PAT stations.
- Green triangles represent the SI stations.
- Yellow triangles represent the UNITS stations.
The legend bar at the bottom of the instrumental intensity maps explains the colors .
In the popup window, the earthquake information includes the event date, time, location coordinates in degrees latitude and longitude, and hypocentral depth in kilometers.
The station information includes the station code and name, the agency that manages the station, the station location coordinates in degrees latitude and longitude, and the peak acceleration and velocity values for each component of ground motion (when available). When the peak ground motion maps are made, the value from the peak horizontal component of ground motion is used as the value for the station. This value is highlighted in bold in the station information.
Components from many stations are defined by three letter codes. The last letter indicates the orientation (Z = vertical, N = horizontal north, E = horizontal east). The first two letters indicate the instrument class:
FBA's (force balance accelerometers) are designed to record extremely large ground motions and can accurately record waves from very large earthquakes. However, ground motions from small and moderate earthquakes are often too small to trigger these instruments or rise above instrument noise. On the other hand, Broadband seismic sensors can record extremely small ground motions and accurately record waves from earthquakes that range from very small up to moderately large. A number of stations have both FBA and broadband sensors. For ShakeMap, the network tends to emphasize FBA recordings for large ground motions and broadband recordings for small ground motions.
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.
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.
Peak Velocity Maps
Peak velocity values are contoured for the maximum horizontal velocity (in cm/sec) at each station. As with the acceleration maps, the vertical component amplitudes are disregarded for consistency with the regression relationships used to estimate values in gaps in the station distribution. Typically, for moderate to large events, the pattern of peak ground velocity reflects the pattern of the earthquake faulting geometry, with largest amplitudes in the near-source region, and in the direction of rupture (directivity). Differences between rock and soil sites are apparent, but the overall pattern is normally simpler than the peak acceleration pattern. Severe damage, and damage to flexible structures is best related to ground velocity. For reference, the largest recorded ground velocity from the 1994 Northridge (Magnitude 6.7) earthquake made at the Rinaldi Receiving station, reached 183 cm/sec.
Spectral Response Maps
Following earthquakes larger than magnitude 5.5, spectral response maps are made. Response spectra portray the response of a damped, single-degree-of-freedom oscillator to the recorded ground motions. This data representation is useful for engineers determining how a structure will react to ground motions. The response is calculated for a range of periods. Within that range, the Uniform Building Code (UBC) refers to particular reference periods that help define the shape of the "design spectra" that reflects the building code.
ShakeMap spectral response maps are made for the response at three UBC reference periods: 0.3, 1.0, and 3.0 seconds. For each station, the value used is the peak horizontal value of 5% critically damped pseudo-acceleration.
Rapid Instrumental Intensity Maps
As an effort to simplify and maximize the flow of information to the public, Wald et al. (1999b) have developed a means of generating estimated Modified Mercalli Intensity maps based on instrumental ground motion recordings. These "Instrumental Intensities" are based on a combined regression of peak acceleration and velocity amplitudes vs. observed intensity for eight significant California earthquakes (1971 San Fernando, 1979 Imperial Valley, 1986 North Palm Springs, 1987 Whittier, 1989 Loma Preita, 1991 Sierra Madre, 1992 Landers, and 1994 Northridge).
From the comparison with observed intensity maps Wald et al. (1999b) find that a regression based on peak velocity for intensity > VII and on peak acceleration for intensity < VII is most suitable. This is consistent with the notion that low intensities are determined by felt accounts (sensitive to acceleration). Moderate damage, at intensity VI-VII, typically occurs in rigid structures (masonry walls, chimneys, etc.) which also are sensitive to high-frequency (acceleration) ground motions. As damage levels increase, damage also occurs in flexible structures, for which damage is proportional to the ground velocity, not acceleration. By relating recorded ground motions to Modified Mercalli intensities, we can now estimate shaking intensities within a few minutes of the event based on the recorded peak motions made at seismic stations.
A descriptive table of Modified Mercalli Intensity is available from ABAG (Association of Bay Area Governments). A table of intensity descriptions with the corresponding peak ground acceleration (PGA) and peak ground velocity (PGV) values used in the ShakeMaps is given below.
For additional information see the ShakeMap Manual