21 research outputs found
Seismic scattering and absorption parameters in the W-Bohemia/Vogtland region from elastic and acoustic radiative transfer theory
In this study, frequency-dependent seismic scattering and intrinsic attenuation parameters for the crustal structure beneath the W-Bohemia/Vogtland swarm earthquake region close to the border of Czech Republic and Germany are estimated. Synthetic seismogram envelopes are modelled using elastic and acoustic radiative transfer theory. Scattering and absorption parameters are determined by fitting these synthetic envelopes to observed seismogram envelopes from 14 shallow local events from the October 2008 W-Bohemia/Vogtland earthquake swarm. The two different simulation approaches yield similar results for the estimated crustal parameters and show a comparable frequency dependence of both transport mean free path and intrinsic absorption path length. Both methods suggest that intrinsic attenuation is dominant over scattering attenuation in the W-Bohemia/Vogtland region for the investigated epicentral distance range and frequency bands from 3 to 24 Hz. Elastic simulations of seismogram envelopes suggest that forward scattering is required to explain the data, however, the degree of forward scattering is not resolvable. Errors in the parameter estimation are smaller in the elastic case compared to results from the acoustic simulations. The frequency decay of the transport mean free path suggests a random medium described by a nearly exponential autocorrelation function. The fluctuation strength and correlation length of the random medium cannot be estimated independently, but only a combination of the parameters related to the transport mean free path of the medium can be computed. Furthermore, our elastic simulations show, that using our numerical method, it is not possible to resolve the value of the mean free path of the random medium
Single-station cross-correlation analysis of ambient seismic noise: application to stations in the surroundings of the 2008 Iwate-Miyagi Nairiku earthquake
We introduce the single-station cross-correlation (SC) technique of processing ambient seismic noise and compare its results with the established cross-correlation (CC) and autocorrelation (AC) techniques. While CC is the correlation of the signals of two seismic stations with each other and AC is the correlation of a signal with itself, SC is the correlation of two different components of a single three-component seismic sensor. The comparison of the three different correlation techniques shows that CCs give the best results at frequencies below 0.5âHz and that SCs give the best results at higher frequencies. In all three processing techniques, ambient seismic noise is correlated in order to reconstruct the Green's function describing the wave propagation between the first and the second sensor. By relating the coda parts of the daily Green's functions with the long-term reference Green's functions, shear wave velocity changes are determined. Here, we apply this technique to the data of 20 seismic stations in the surroundings of the fault zone of the Iwate-Miyagi Nairiku earthquake (MW=6.9), which occurred on 2008 June 13, UTC (2008 June 14, Japan Standard Time) in the northern part of the Japanese island Honshu. The data range from 2008 January to 2011 June and therefore include the Tohoku earthquake (MW=9.0), which occurred on 2011 March 11, off the coast of northern Honshu. The data are analysed in five different frequency ranges between 0.125 and 4.0âHz. The data show coseismic velocity changes for both earthquakes followed by a post-seismic velocity recovery. In general, the coseismic velocity changes increase with frequency. For the Iwate-Miyagi Nairiku earthquake, the strongest velocity changes occur close to the fault zone. Quickly recovering coseismic velocity changes can be separated from changes not recovering during the study period. For the Tohoku earthquake, the complete area is affected by coseismic velocity changes. A modelling of the depth of the coseismic velocity changes indicates that the Iwate-Miyagi Nairiku earthquake can be explained either by large shallow velocity changes or by small, but deep changes. For one station, the observations can only be explained by assuming deeper changes. For the Tohoku earthquake, the modelling shows that different parts of the study area are affected in different ways, some showing shallow changes, others deeper changes. Furthermore, seasonal velocity variations occur, which are compatible for the different stations above 0.5âHz, with velocity maxima in autum
Fast and robust earthquake source spectra and moment magnitudes from envelope inversion
With the present study we introduce a fast and robust method to calculate the
source displacement spectra of small earthquakes on a local to regional scale.
The work is based on the publicly available Qopen method of full envelope
inversion which is further tuned for the given purpose. Important source
parameters -- seismic moment, moment magnitude, corner frequency and
high-frequency fall-off -- are determined from the source spectra by fitting a
simple earthquake source model. The method is demonstrated by means of a data
set comprising the 2018 West Bohemia earthquake swarm. We report moment
magnitudes, corner frequencies, and centroid moment tensors inverted from short
period body waves with the Grond package for all earthquakes with a local
magnitude larger than 1.8. Moment magnitudes calculated by envelope inversion
show a very good agreement to moment magnitudes resulting from the
probabilisitc moment tensor inversion. Furthermore, source displacement spectra
from envelope inversion show a good agreement with spectra obtained by multiple
taper analysis of the direct onsets of body waves, but are not affected by the
large scatter of the second. The seismic moments obtained with the envelope
inversion scale with corner frequencies according to . Earthquakes of the present data set result in a smaller
stress drop for smaller magnitudes. Self-similarity of earthquake rupture is
not observed. Additionally, we report frequency-dependent site amplification at
the used stations.Comment: Version after peer-revie
Coseismic drop of seismic velocity caused by the 2023 TurkeyâSyria earthquakes
The Mw 7.8 earthquake in Turkey on 6 February 2023 was extraordinary for various reasons. It originated in depth of only 10 km, ruptured along a fault plane around 300 km long and the surface was covered by an extensive network of high-quality seismic instruments. The strong motions resulted in a vast number of tragic casualties and huge material losses in Turkey and Syria. However, abundant and proximate seismic observations of this event and numerous aftershocks give an opportunity to deepen the understanding of earthquake processes. In this study, we carried out an assessment of coseismic changes of seismic velocity using Passive Image Interferometry. We used data from one strong-motion and twenty-four broadband sensors. We observed coseismic drops of seismic velocity, which reached up to -1.79 per cent at a location directly at the ruptured East Anatolian Fault Zone. Along the Mw 7.8 earthquake fault, we observe frequency dependence of the velocity changes. At frequencies above 0.5 Hz, the velocity drops seem to be higher at locations close to the ruptured faults than in the more distant areas
Comparison of multiple lapse time window analysis and qopen to determine intrinsic and scattering attenuation
This study compares the results of Multiple Lapse Time Windows Analysis (MLTWA) and full envelope inversion (Qopen) to determine intrinsic and scattering attenuation of the crust using the region around the central part of the LeipzigâRegensburg fault zone in Germany as an example. We use 18 of the regionâs strongest earthquakes from 2008 to 2019 with a magnitude between 1.4 and 3.0 in the frequency band range between 3 and 34âHz. The determined attenuation values of both methods are similar within their error bars. The inverse quality factors of the shear wave are relatively low compared to other regions, with values of 3.2 Ă 10â4 to 8.7 Ă 10â4 for Qâ1i and 1.4 Ă 10â4 to 2.8 Ă 10â4 for Qâ1scâ , respectively. As a by-product of Qopen, we also obtain the energy site amplification of the stations used in the inversion as well as source displacement spectra and moment magnitudes of the inverted earthquakes. Several combinations of inversion parameters were tested for MLTWA, with Qâ1i and Qâ1sc providing the lowest trade-off. Likewise, we investigated the influence of window length on the results of Qopen. We found a dependency of the results on the length, if the windows are shorter than 30âs. For longer time windows, the dependence disappears, and the result becomes independent of window length