Shake maps, more properly described as "shaking intensity maps," are a tool used by seismologists and civil engineers to summarize the degree of shaking experienced by a geographic region during an earthquake. Shake maps are based on either direct or indirect measurements; direct measurements are generally made by a seismometer or similar accelerometer over time and in the three physical dimensions, while indirect measurements are normally provided by humans reporting objective and subjective experiences. Shake maps commonly use a color gradient to indicate the varying severity of shaking across a region.
Perhaps the best-known examples of shake maps are those produced by the United States Geologic Survey (USGS), which generates both direct measurement shake maps with data from many professional seismic networks and also indirect measurement shake maps based on human reporting. The automatically generated direct measurement shake maps are from the USGS ShakeMap project; the Did You Feel it (DYFI)? human reporting system is an example of community-based science or citizen science. Here are some examples of shake maps produced for some well-known earthquakes:
Both direct and indirect measurement-based shake maps have limitations alongside their strengths. Direct measurement maps are limited by the relatively few number of professional seismic stations (which can cost US$100,000 to install) located even in highly seismically active regions like Japan and California, but they are produced very quickly—usually within minutes. When only a few data points are present, interpolation is used to estimate the shaking around and between them. DYFI-type maps are often based on large numbers of human reports, but the data is often delayed due to broken internet links, and the data is inferential: the numeric values assigned to each data point are derived from answers to questions like "Did anything fall off of a shelf?" rather than a direct measurement by an objective sensor.
The Community Seismic Network project is intended to demonstrate the value of a middle ground between these two important endpoints. For about the same cost as just one single professional quality station, a thousand citizen-science operated direct-measurement stations of moderate quality can be deployed. Our initial target area of the greater Pasadena region currently has four professional seismometer stations which are not well-distributed for generating shake maps of Pasadena. CSN will add one thousand community-based sensors with automatic reporting of shaking data to a remote computing service, and produce shake maps within minutes of the onset of an event. CSN will keep producing updated maps through the lifetime of the event and beyond, until it receives no additional data.
The diagram shows an example random distribution of one thousand stations across greater Pasadena. Even if the reference stations were at each corner of the diagram, there would be no detailed knowledge of the actual shaking occurring in the rectangle itself: existing shake map tools would apply a summary of the limited knowledge that exists today about the subsurface geology of Pasadena, and make crude estimates of the behavior between stations.
In contrast, with hundreds of times the number of accelerometer stations deployed across Pasadena, the average distance between stations drops from about 10 miles to a quarter mile. This is a crucial change in density, as significant (unexpected) variations in a 10 mile distance have been seen in recent California earthquakes.