Abstract
In order to verify the duration magnitude MD we calculated local magnitude ML values of 288 earthquakes oc- curring from October 2002 to April 2003 at Mt. Etna. The analysis was computed at three digital stations of the permanent seismic network of Istituto Nazionale di Geofisica e Vulcanologia of Catania, using the relationship ML = logA+alog∆−b, where A is maximum half-amplitude of the horizontal component of the seismic record- ing measured in mm and the term «+alog∆−b» takes the place of the term «−logA0» of Richter relationship. In particular, a = 0.15 for ∆<200 km, b =0.16 for ∆<200 km. Duration magnitude MD values, moment magnitude MW values and other local magnitude values were compared. Differences between ML and MD were obtained for the strong seismic swarms occurring on October 27, during the onset of 2002-2003 Mt. Etna eruption, charac- terized by a high earthquake rate, with very strong events (seismogram results clipped in amplitude on drum recorder trace) and high level of volcanic tremor, which not permit us to estimate the duration of the earthquakes correctly. ML and MD relationships were related and therefore a new relationship for MD is proposed. Cumula- tive strain release calculated after the eruption using ML values is about 1.75E+06 J 1/2 higher than the one cal- culated using MD values.
Highlights
In order to know the size of an earthquake without considering the produced effects, Richter proposed the definition of magnitude and related it to the maximum amplitude of the ground displacement
The quantity «−logA0» is defined empirically with respect to a reference earthquake, which describes the variation of maximum amplitude (A) of the event related to the epicentre distance (∆)
The DaDisp worksheet used to simulate a seismic signal recorded by standard Wood-Anderson seismometer. a) Velocity seismic signal recorded by geophone; b) Discrete Fourier Transformation (DFT) of velocity seismic signal; c) velocity response curve of geophone; d) displacement response curve of geophone; e) Wood-Anderson response curve; f) velocity spectrum divided by displacement response curve; g) corrected spectrum multiplied by Wood-Anderson response curve; h) Wood-Anderson simulated seismic signal
Summary
In order to know the size of an earthquake without considering the produced effects, Richter proposed the definition of magnitude and related it to the maximum amplitude of the ground displacement. The «local magnitude» ML (Richter, 1935), is defined with the relationship. The quantity «−logA0» is defined empirically with respect to a reference earthquake, which describes the variation of maximum amplitude (A) of the event related to the epicentre distance (∆). To evaluate magnitude in a more practical approach, principally when the recording of strong earthquakes is clipped in amplitude, empiric relationships were developed using the duration of the seismic event by Solov’ev (1965), Tsumara (1967) and many other authors
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