Mw 5.5 WEST OF GIBRALTAR on December 17th 2009 at 01:37 UTC

An earthquake of magnitude Mw 5.5 occurred on 17/12/2009 west of Gibraltar Strait, off Cape Saint Vincent at 01:37 (UTC and Portuguese time). More information on the main shock is available on the EMSC web site and from the Instituto de Meteorologia, Seismologia Lisbon, Portugal. The event occurred off the coast of South-Western Portugal.

The epicenter lies 256 km South of Lisbon, 183 km South-West of Faro. This event is the largest event in the region since the Mw 6.1 of 12/02/2007. This earthquake took place 80 Km North East of the 12/02/2007 main shock. It occurred in the region where the the 1755 Lisbon Earthquake presumably occurred.

This earthquake has been widely felt in Portugal and in Morocco, as our macroseismic map and feltmaps show (Figure 3). It has been felt in Spain as far as Madrid (almost 800km away) in upper floors.

Answer our questionnaire

If you have experienced this earthquake, you are invited to answer the online questionnaire.

Your testimony will be useful to update our Macroseismic map (on the right) which maps the effects of the earthquake.

View the results of the questionnaires

Send us pictures or videos

If you were in the area and have taken photos or videos of earthquake effects (on buildings, ground, landscape), please send us your files. You can send them by email, MMS or upload them directly from your computer. Just click on the link below :
Send us pictures or videos of the main shock effects


EMSC Web site Traffic variation

Over the past 5 years, the EMSC has developed mechanisms to use information captured from the EMSC web traffic to rapidly detect felt earthquakes and to map the extents of their effects. When an earthquake is felt, people turn to the internet to find out more. This in turn produces a surge in web traffic which is automatically detected (Figure 1.) Using Digital Element’s IP localization technology, we are able to identify the location from which the traffic originates. From this information, we automatically build “feltmaps” (Figure 2) – maps ofwhere the earthquake has been felt. Significant absence of traffic from areas can indicate severe damage in those regions. Using these methods, we can detect the occurrence of an event in populated areas typically within 2-3 minutes, generally before we are able to publish the first automatic earthquake location using seismological data. These mechanisms respond to community input in the form of increased web traffic to rapidly detect events that are significantto the population and to map the extents of the effects.

Figure 1: Traffic on EMSC web site before and after the earthquake occurrence.
Figure 2: Animation of the feltmaps in the first 20 minutes after the earthquake occurrence. Red dots represent the places where a significant increase of traffic has been observed during the time window, which is due to people who just felt the earthquake and goes on the internet to find more information.

Figure 3: Comparison between Macroseismic maps (left) and Feltmap 5 minutes after earthquake occurrence (right). The area where the earthquake was felt compares well with the one derived from variations of web traffic.
Figure 4: Zoom on surge of web traffic after the earthquake occurrence. The number of unique IP's on EMSC web site goes up by a factor of 10 within 5 minutes after the earthquake occurrence



Epicenter location and regional seismicity

Figure 5: Epicenter Location
Figure 6: Moment Tensors Solutions
Figure 7: Past Regional Seismicity as from the ISC catalogue (1964-1998) and EMSC catalogue (1998-today)



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Aftershocks distribution from the main shock until 21/12/2009 08:00 UTC.