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| Titel |
Local magnitude scale in Slovenia |
| VerfasserIn |
J. Bajc, Ž. Zaplotnik, M. Živčić, M. Carman |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250064461
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| Zusammenfassung |
| The Seismology and Geology Office of the Environmental Agency of the Republic of
Slovenia is recording and processing data about the earthquakes occurring in Slovenia and the
surrounding areas. For the last 30 years the magnitudes MLV of the earthquakes recorded by
the Slovenian seismic stations were computed using the maximum ground velocity on the
vertical component with the attenuation determined to fit the magnitudes recorded by
Wood-Anderson seismograph in Trieste. In the last 15 years the digital seismic network of
the Republic of Slovenia has grown from 6 to 28 permanent seismic stations and
recorded large set of earthquakes, but attenuation relation for the local magnitude
has not been re-evaluated yet. We use a large data set of earthquakes occurring in
the area between 44.6°Â and 47.0°Â N and 12.0°Â and 17.0°Â E in the period from
January 1997 to December 2010 to calibrate the local magnitude equations. An
iterative least-square method is used to determine distance attenuation coefficients and
station corrections for the Wood-Anderson magnitude (MWA). The magnitude
equation is obtained from the automatically determined amplitudes from simulated
Wood-Anderson seismograms of more than 1800 earthquakes. The magnitude span of the
data used is between M = 0.6 and M = 5.2, with all but few tens of events between
M = 1 and M = 3 and the hypocentral distance span from 20Â km to 280Â km,
with less than 0.4Â % of the distances larger than 210Â km. The relation obtained
is
( )
j r[km]–
M WA = log (A [mm ])+ 1.38log 100 km + 3.0+ Cj ,
where A is the maximum amplitude on the horizontal component of the synthetic
Wood-Anderson seismogram, r is the hypocentral distance and index j refers to the station,
so that Cj is the station correction. Station corrections Cj obtained for 30 stations are in the
range from -0.26 to 0.30.
Currently, the MLV of an event is determined as the mean of the MLV magnitudes at
individual stations, using a single magnitude equation with no station correction terms. The
same set of recordings is used to determine the coefficients in the relation for MLV . For the
calibration of the MLV magnitude equation the data of more than 3100 earthquakes is
used.
Attenuation in the calibrated MLV equation is somewhat higher than the one used
presently by the Seismology and Geology Office. However, the MLV values obtained using
the present single equation and the new equation with station corrections do not differ
significantly. The main gain of the introduction of the station corrections is the reduction of
the MLV uncertainty by about 35Â %.
By comparing MLV and MWA as obtained in the first part of the study, we show that the
linear function MWA = 1.06MLV - 0.075 describes well the relation between the two
magnitudes. However, it has limited use, since the magnitude span of the data used is mostly
between M = 1 and M = 3, with just a few tens of events with M > 3. Nevertheless, the
difference between the two local magnitudes does not exceed 0.3 for most of the events with
both magnitudes determined. |
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