Automatic Felt Maps: How to Rapidly Know When and Where an Earthquake has been Felt?

We propose a new tool which automatically maps within 20-25 minutes of the event occurrence, the geographical areas in which the earthquake has been felt. Brutal increases of the number of instant visitors connecting on the EMSC web site are automatically detected. Such a synchronized action involving many individuals can only be explained if these individuals share a common reason to visit the EMSC web site at the very same time. We assume that they have just experienced the same earthquake. After filtering out the IP addresses known to be from seismological institutes, a geographical location is assigned to each IP address comprised in this burst. The felt maps are then obtained by plotting the observed cluster. A statistical approach is being implemented to assess the significance of change in the visitors' origin in comparison to the average audience in the region. This methodology has been tested since end-2004 on about 20 earthquakes. The felt maps have been distributed to the seismologists of the country(ies) affected by the given event who validated the approach. Obviously, it can only work when a significant number of individuals with Internet access have actually felt the event. In practice, this service works mainly for European earthquakes and the proximity of an urbanised region is the key parameter even when compared to the magnitude. A felt map was produced following a ML3.3 event as it occurred close to Zagreb (Croatia). Despite these limitations, the felt maps is the quickest way offered to seismologists to automatically and easily collect information about the effects of an earthquake. It can work independently of any seismological data and theoretically (we hopefully had not such a case yet) it should be able to map areas where individuals are unable to access the Internet following the earthquake shaking.