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| Titel |
Macroseismic investigation of the 2008-2010 low magnitude seismic swarm in the Brabant Massif, Belgium. The link between macroseismic intensity and geomorphology |
| VerfasserIn |
Koen Van Noten, Thomas Lecocq, Bart Vleminckx, Thierry Camelbeeck |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250076207
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| Zusammenfassung |
| Between July 2008 and January 2010 a seismic swarm took place in a region 20Â km south
of Brussels, Belgium. The sequence started on the 12th of July 2008 with a ML
= 2.2 event and was followed the day after by the largest event in the sequence
(ML = 3.2). Thanks to a locally installed temporary seismic monitoring system
more than 300 low magnitude events, with events as low as ML = -0.7, have been
detected. Results of the relocations of the different hypocenters and analysis of
the focal mechanisms show that the majority of these earthquakes took place at
several km’s depth (3 to 6 km) along a (possibly blind) 1.5 km long NW-SE fault
(zone) situated in the Cambrian basement rocks of the Brabant Massif. Remarkably,
60 events (0.6 -¤Â ML -¤Â 3.2) were felt, or heard only sometimes, by the local
population. This was detected by the “Did you feel it?” macroseismic inquiries
on the ROB seismology website (www.seismology.be). For each event a classical
macroseismic intensity map has been constructed based on the average macroseismic
intensity of each community. Within a single community, however, the reported
macroseismic intensities locally often vary ranging between non-damaging intensities of
I and IV (on the EMS-98 scale). Using the average macroseismic intensity of a
community therefore often oversimplificates the local intensity, especially in hilly areas in
which local site effects could have influenced the impact of the earthquakes at the
surface.
In this presentation we investigate if the perception of the people of how they experienced
the small events (sound, vibrations) was influenced by local geomorphological site effects.
First, based on available borehole and outcrop data a sediment thickness map of the
Cenozoic and Quaternary cover above the basement rocks of the Brabant Massif is
constructed in a 200Â km2 area around the different epicenters. Second, several electrical
resistivity tomography (ERT) profiles are conducted in order to locally improve the
sediment thickness map and to determine if any fault trace can be localized. Finally, the
relationship between the variation in macroseismic intensity and the local sediment
thickness above the Cambrian basement is investigated in function of the distance to the
hypocenter.
This methodology allows deducing if macroseismic intensities are higher on the eroded
hill slopes and in the incised valleys, at which the sediment and alluvial cover is the thinnest
(30 m), and if thus macroseismic intensity can be related
to geomorphology. |
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