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| Titel |
Focal mechanism and depth of the 1956 Amorgos twin earthquakes from waveform matching of analogue seismograms |
| VerfasserIn |
A. Brüstle, W. Friederich, T. Meier, C. Gross |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1869-9510
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| Digitales Dokument |
URL |
| Erschienen |
In: Solid Earth ; 5, no. 2 ; Nr. 5, no. 2 (2014-10-31), S.1027-1044 |
| Datensatznummer |
250115343
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| Publikation (Nr.) |
 copernicus.org/se-5-1027-2014.pdf |
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| Zusammenfassung |
Historic analogue seismograms of the large 1956 Amorgos twin earthquakes
which occurred in the volcanic arc of the Hellenic subduction zone (HSZ) were
collected, digitized and reanalyzed to obtain refined estimates of their
depth and focal mechanism. In total, 80 records of the events from 29
European stations were collected and, if possible, digitized. In addition,
bulletins were searched for instrument parameters required to
calculate transfer functions for instrument correction. A grid search based
on matching the digitized historic waveforms to complete synthetic
seismograms was then carried out to infer optimal estimates for depth and
focal mechanism. Owing to incomplete or unreliable information on instrument
parameters and frequently occurring technical problems during recording, such
as writing needles jumping off mechanical recording systems, much less
seismograms than collected proved suitable for waveform matching.
For the first earthquake, only seven seismograms from three different stations at Stuttgart (STU), Göttingen (GTT)
and Copenhagen (COP) could be used. Nevertheless, the waveform matching grid search yields two stable misfit
minima for source depths of 25 and 50 km. Compatible fault plane solutions
are either of normal faulting or thrusting type. A separate analysis of 42
impulsive first-motion polarities taken from the International Seismological
Summary (ISS bulletin) excludes the thrusting mechanism and clearly favors a
normal faulting solution with at least one of the potential fault planes
striking in SW–NE direction. This finding is consistent with the local
structure and microseismic activity of the Santorini–Amorgos graben. Since
crustal thickness in the Amorgos area is generally less than 30 km, a source
depth of 25 km appears to be more realistic.
The second earthquake exhibits a conspicuously high ratio of body wave to surface wave amplitudes suggesting an intermediate-depth event located in the
Hellenic Wadati–Benioff zone. This hypothesis is supported by a focal
mechanism analysis based on first-motion polarities, which indicates a
mechanism very different from that of the first event. A waveform matching
grid search done to support the intermediate-depth hypothesis proved not to
be fruitful because the body wave phases are overlain by strong surface wave
coda of the first event inhibiting a waveform match. However, body to surface wave amplitude ratios of a modern intermediate-depth event with an epicenter close to the island of Milos observed at stations of the German
Regional Seismic Network (GRSN) exhibit a pattern similar to the one observed
for the second event with high values in a frequency band between 0.05 Hz
and 0.3 Hz. In contrast, a shallow event with an epicenter in western Crete and
nearly identical source mechanism and magnitude, shows very low ratios of
body and surface wave amplitude up to 0.17 Hz and higher ratios
only beyond that frequency. Based on this comparison with a modern event, we
estimate the source depth of the second event to be greater than 100 km. The
proximity in time and space of the two events suggests a triggering of the
second, potentially deep event by the shallow first one. |
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