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| Titel |
A seismological study of shallow weak earthquakes in the urban area of Hamburg city, Germany, and its possible relation to salt dissolution |
| VerfasserIn |
Torsten Dahm, Sebastian Heimann, Wilhelm Bialowons |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250034746
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| Zusammenfassung |
| In the night from 8/9 April 2009, shortly after midnight on Maundy Thursday before Easter,
several people in Gross-Flottbek, Hamburg, felt unusual strong ground shocks so that some of
them left their houses in fear of earthquake shaking. Police and Fire Brigade received phone
calls of worried residents, and few days later Internet pages were published where people
reported their observations. On 21 April 2009 at about 8 p.m. local time a second
micro-earthquake was felt. Damage to buildings or infrastructure did not occur to our
knowledge. The Institute of Geophysics, University of Hamburg, installed from 22 April to
17 May 2009 three temporal seismic stations in the epicentral area. Seismological data from
two close-by stations at the Deutsches Elektron-Synchrotron (DESY) in about 1 km and the
Geophysical Institute in about 7 km distance were collected and integrated to the temporal
network.
The events occurred above the roof of the shallow Othmarschen Langenfelde salt diapir
(OLD), in an area known for active sinkhole formation and previous historic ground
shaking events. The analysis of the seismological data recovers that three shallow
micro-earthquakes occurred from 8 to 21 April at a depth of about 100m, the largest
one with a moment magnitude of about MW 0.6. Depth location of such shallow
events is difficult with standard methods, and is here constrained by waveform
modeling of surface waves. Earthquakes occurring in soft sediments within the
uppermost 100 m are a rare phenomena and cannot be explained by standard models.
Rupture process in soft sediments differ from those on faults in more competent rock.
We discuss the rupture and source mechanism of the earthquakes in the context of
previous historic shocks and existing sinkhole and deformation data. Although
the event was so weak, the rupture duration was unusual long and possibly 0.3 s.
Three possible models for the generation of repeated micro-earthquakes in Gross
Flottbek are developed and discussed, implying quit different hazards for subsidence,
ground motion and sinkhole formation. Our favored model postulates that roof
failure occurs in an existing soil cavity beneath the epicenter at a depth of about 100
m. Other models bearing a smaller geo-hazard cannot be disproved with the data
available, but future geophysical experiments may be planned to resolve this question. |
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