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Q: Why are there often different magnitudes reported for the same earthquake?
A: When an earthquake occurs, the first information that is processed and relayed is usually based on a small subset of the seismic stations in the network, especially in the case of a larger earthquake. This is done so that some information can be obtained immediately without waiting for all of it to be processed. As a result, the first magnitude reported is usually based on a small number of recordings. As additional data are processed and become available, the magnitude and location are refined and updated. Sometimes the assigned magnitude is "upgraded" or slightly increased, and sometimes it is "downgraded" or slightly decreased.
Sometimes the earthquake magnitude is reported by different networks based on only their recordings. In that case, the different assigned magnitudes are a result of the slight differences in the instruments and their locations with respect to the earthquake.