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SEISMIC WAVE ATTENUATION IN THE VRANCEA REGION.
PART II. CRUSTAL Q-MODELS
L. ARDELEANU
National Institute for Earth Physics, P.O. Box MG-2, RO-077125 Bucharest – Magurele, Romania,
E-mail: [email protected]
Received May 24, 2017
Abstract. We investigate the shear wave attenuation in the crust beneath the area
from the bending of the Eastern Carpathians by using a procedure based on high-
frequency waveform modelling. We determine 1-D local models of the quality factor
of the medium Q, adequate for simulating the ground motion (waveforms with
maximum frequency 5 Hz) generated by seismic sources of Vrancea region at
locations within the extra-Carpathian zone. The results of the study reveal distinct
attenuative properties among the structural units lying in the study region;
considerably higher attenuation is evidenced beneath the Focşani Basin and the
Vrancea region orogen.
Key words: Seismic wave attenuation, quality factor of the medium, Q-models,
Vrancea region, waveform modelling.
1. INTRODUCTION
The procedure proposed by [1] to estimate 1-D models of the quality factor
of the medium Q in the Vrancea region and adjacent zone is applied to investigate
the crustal attenuation in the extra-Carpathian area.
The algorithm is a non-linear inversion in spectral domain: the amplitude
spectra of high-frequency local records of low-to-moderate magnitude earthquakes
are compared with the synthetic signal spectra, generated for point sources with the
same location and mechanism as the recorded events; the theoretical seismograms
are computed by multimodal summation in layered inelastic media [2, 3].
The non-linear inversion provides a set of models that satisfies the
acceptability criterion – the r.m.s. value of the logarithmic residuals between
theoretical and observed spectra smaller than an a priori defined threshold. The
drawback is that the choice of the threshold is somewhat subjective; it was
determined ad hoc on the basis of synthetic experiments simulating anticipated /
expected errors of the available hypocenter locations and earthquake mechanisms.
Romanian Journal of Physics 63, 807 (2018)
Article no. 807 L. Ardeleanu 2
Specifically, the adopted threshold allows the recovery of the real Q-
structure for simulated depth mislocations of 5 km, or significant deviations of the
focal mechanisms. Since the hypocenter mislocation, and consequently the
incorrect fault plane solution are both effects of using of an inexact velocity model,
the uncertainty of the velocity structure – the main source of discrepancy between
computed and observed waveforms – is implicitly taken into account in the choice
of the threshold level.
To define the representative attenuation structure from the set of acceptable
solutions, the ‘Median Model’ is chosen (following [4]); the half-width of the
corridor of acceptable Q-values provides, at each depth, the model uncertainty.
2. DATA AND VELOCITY MODELS
The waveform data used in the present study are short period velocity
records of low-magnitude crustal earthquakes of Vrancea region, collected by
seismic stations situated at epicentral distances less than 200 km, and equipped
with vertical S13 seismometers (1s free period and damping of 0.7).
The location of the earthquakes (Table 1) and the focal mechanisms are
taken from [5].
Table 1
Earthquake locations
Event
no. Date Origin time
Latitude
[oN]
Longitude
[oE]
Depth
[km]
Local
magnitude
1 30 June 1982 21 : 38 : 47 45.42 26.43 13 3.5
2 21 February 1983 18 : 21 : 05 45.33 26.97 16 3.6
3 21 February 1983 18 : 33: 37 45. 37 27.02 17 3.0
The 1-D models of the elastic parameters of the medium – density and
seismic wave velocities – are compiled by integrating and harmonizing published
results, after a thorough selection and evaluation.
The models consist of several layers for the sedimentary cover and two
layers for the crystalline crust (the upper and the lower crust).
To model the sedimentary cover we used the series of local structures
constructed by [6] for the seismic station sites. For the crystalline crust we
compiled average velocity models along the seismic source – recording station
paths, using the information available from recent literature [7–13].
3 Seismic wave attenuation in the Vrancea region. Part II. Crustal Q-models Article no. 807
The density models are based on the work of [6].
We note that the selection of the data for this first application of the
proposed procedure to investigate the crustal attenuation in the area of interest took
into account the availability of detailed velocity models for the structure beneath
the seismic station sites, as well as the availability of fault plane solutions of small
local earthquakes.
3. 1-D Q-MODEL ESTIMATION
In our study Q ≡ QS – the shear wave attenuation factor, and the relation
SP QQ 2.2 [2] between the quality factors of P- and S-waves is considered in the
generation of the synthetics.
To construct the input Q-models we use the layer configuration of the
velocity and density structures. The quality factor is allowed to vary in the range 0
to 1500 units; we adopted a step of 25 units for Q-values in the range 0 to 100, a
step of 50 units for Q in the range 100 to 400, a step of 100 for the range 400 to
600, a step of 200 between 600 and 1200, and a step of 300 for Q greater than
1200.
The response of the input structures to point sources having the same
location and mechanism as the recorded events are computed by modal summation
for the maximum frequency of 10 Hz.
Both records and synthetics are then filtered by low-pass with cut-off
frequency at 5Hz, and the synthetics are convolved with the instrument response.
To determine the best-fitting Q-models, the amplitude spectra of observed
and theoretical waveforms – calculated for time windows centered on the most
energetic part of the signals – are smoothed by averaging the amplitudes, for a
bandwidth of 1 Hz. The data misfit is evaluated only over the frequency range
where the signal-to-noise spectral amplitude ratio is greater than 2, in order to
avoid the effect of data noise.
The region from the bending of the Eastern Carpathians has a complex
tectonic structure. Consequently, for particular source-to-station paths the
mismodelling of the real medium by simplified 1-D structures may be severe, and
the synthetic waveforms constructed with average 1-D models may be far from the
true ones, in the considered frequency range.
If, for a specified path, the best-fitting Q-model leads to a r.m.s. value of the
residuals larger than the acceptance threshold, we assume that the overall error
Article no. 807 L. Ardeleanu 4
– mainly result of the poorly modelled velocity structure – is large enough to bias
the solution of the inversion significantly, and discard the result.
In Figs. 1–5 we illustrate the successful application of the inversion
procedure, for different source-station paths of the study region. Figure 6
summarizes the results, showing the attenuation models together with the velocity
structures used in their estimation; the Q-model determined in the first part of the
study [1] is also displayed (path #1).
The earthquakes that provided the observed data are located at depths of
13–17 km, hence the estimates of the attenuation factor are reliable for the
crystalline upper crust and the layers above it [1]. For this depth range
(sedimentary cover and upper crust) Fig. 7 gives another representation of the
results of Fig. 6 – the Q-factor versus the seismic wave velocity.
4. DISCUSSION
Generally, the seismic wave attenuation provides a direct measure of the
Earth’s inelastic properties.
Lateral variations in attenuation, as well as a qualitative relationship between
tectonic province and magnitude of attenuation have been pointed out by a number
of studies. Specifically, the stable continental zones were found to be considerably
less attenuating than the tectonically active regions (e.g. [14–19]).
Our area of interest comprises several juxtaposed tectonic units: the
Carpathian Orogen, the sedimentary Focşani Basin (component of the Carpathian
Foredeep), the stable Moesian and Scythian Platforms.
The ray paths displayed in Fig. 6 span all these crustal provinces: paths #1
and #3 – the Focşani Basin and Scythian Platform; path #2 – the Carpathian
Orogen, Focşani Basin and Scythian Platform; path #4 – the Focşani Basin and
Moesian Platform; path #5 – the Focşani Basin and Carpathian Orogen; path #6 –
the Carpathian Foredeep (Focşani Basin).
The large-scale velocity structure in the region is heterogeneous, and this
heterogeneity is reflected by the 1-D velocity models of Fig. 6.
The seismic attenuation and seismic velocity are in general controlled by the
same factors, such as temperature, composition, water content, partial melt
(e.g.[20–29]). However, the sensitivity of attenuation to these factors is different
from that of velocity (e.g.[20–25]), consequently the elastic and inelastic Earth
models are not identical (e.g. [30, 31]).
5 Seismic wave attenuation in the Vrancea region. Part II. Crustal Q-models Article no. 807
Fig. 1 – (a) Location of epicenter (star) and seismic station (triangle), and earthquake focal
mechanism (event #1 of Table 1). (b) The 1-D velocity and density models. (c) The acceptable
Q-structures; thick grey line – best-fitting model, thick black line – last accepted model. (d)
Normalised amplitude spectra (smoothed by averaging the spectral amplitudes over a bandwidth of
1 Hz) calculated for the observed data and for the theoretical signals generated with the best-fitting
and the last accepted Q-model, respectively (up), and with all acceptable Q-models (down); the
average spectral amplitudes are plotted against the highest frequencies of the 1 Hz smoothing
intervals, the vertical bars indicate the frequency range considered in data misfit evaluation. (e)
Selection of the frequency range for data misfit evaluation: analyzed time windows for signal and
noise (up), and the corresponding amplitude spectra (down).
Article no. 807 L. Ardeleanu 6
Fig. 2 – (a) Location of epicenter (star) and seismic station (triangle), and earthquake focal
mechanism (event #1 of Table 1). (b) The 1-D velocity and density models. (c) The acceptable
Q-structures. (d) Selection of time window for spectrum calculation: observed data (up) and
theoretical seismograms computed using the best-fitting and the last accepted Q-model, respectively;
synthetics are calculated for a scalar moment M0 = 4·1012 Nm. (e) Normalised amplitude spectra
(smoothed over a bandwidth of 1 Hz) calculated for observed data and theoretical signals generated
with the best-fitting and the last accepted Q-model, respectively (up), and with all acceptable
Q-models (down). (f) Selection of the frequency range for data misfit evaluation: analyzed time
windows for signal and noise (up), and the corresponding amplitude spectra (down).
7 Seismic wave attenuation in the Vrancea region. Part II. Crustal Q-models Article no. 807
Fig. 3 – (a) Location of epicenter (star) and seismic station (triangle), and earthquake focal
mechanism (event #3 of Table 1). (b) The 1-D velocity and density models. (c) The acceptable
Q-structures. (d) Selection of time window for spectrum calculation: observed data (up) and
theoretical seismograms computed using the best-fitting and the last accepted Q-model, respectively;
synthetics are calculated for a scalar moment M0 = 2·1012 Nm. (e) Normalised amplitude spectra
(smoothed over a bandwidth of 1 Hz) calculated for observed data and theoretical signals generated
with the best-fitting and the last accepted Q-model, respectively (up), and with all acceptable
Q-models (down). (f) Selection of the frequency range for data misfit evaluation: analyzed time
windows for signal and noise (up), and the corresponding amplitude spectra (down).
Article no. 807 L. Ardeleanu 8
Fig. 4 – For captions see Fig. 3.
9 Seismic wave attenuation in the Vrancea region. Part II. Crustal Q-models Article no. 807
Fig. 5 – For captions see Fig. 3.
Article no. 807 L. Ardeleanu 10
Fig. 6 – (a) Location of investigated ray paths. (b) – (g) Left – the velocity models;
right – the attenuation models: thin lines – the acceptable models; thick lines – the representative
Q-structures; grey area – the estimated uncertainty.
Figure 7 points out noticeable differences between attenuation variations and
velocity variations along the analyzed source-station paths.
The most attenuating ray paths – #5 and #6 – lie predominantly within a
single crustal province – the Focşani Basin. The less attenuating rays travel across
11 Seismic wave attenuation in the Vrancea region. Part II. Crustal Q-models Article no. 807
several provinces, passing through stable platforms: Moesia – path #4, or Scythia –
paths #1, #2 and #3.
Fig. 7 – Shear wave attenuation versus velocity, for different paths of the study region.
The vertical bars show the uncertainty of the estimated Q-values.
Lateral variations of the crustal attenuation in the region from the bending of
the Eastern Carpathians were previously pointed out by [32], who noticed higher
attenuation in the foredeep area (Focşani Basin) and Vrancea orogen,
comparatively to the adjacent Scythian and eastern Moesian Platforms.
Our analysis evidences that the attenuation structure in the shallow crust
differs considerably among the tectonic units.
The results indicate high shear wave attenuation beneath the southwestern
portion of the sedimentary Focşani Basin, as well as beneath the nappes of the
Vrancea region orogen, with Q-values of 100–150 in the crystalline upper crust,
and less than 100 in the shallower layers.
In contrast, the rays crossing portions of stable platforms display optimal
1-D attenuation models with Q-values of 600 to 800 in the crystalline upper crust,
and between 100 and 400 in the layers above it.
Article no. 807 L. Ardeleanu 12
5. CONCLUSIONS
We have used a procedure based on high-frequency waveform modelling to
investigate the shear wave attenuation in the crust beneath the Vrancea region and
adjacent extra-Carpathian zone.
The estimated 1-D (depth dependent) optimal attenuation structures are
adequate for the computation of synthetic seismograms (maximum frequency 5 Hz)
simulating the ground motion generated by seismic sources of Vrancea region, at
locations within the extra-Carpathian area.
Our results evidence that the structural units lying in the study region are
characterized by distinct attenuative properties.
Acknowledgments. The theoretical seismograms were calculated using the computational
packages developed at the Department of Earth Sciences of the University of Trieste, Italy.
The research was supported by the Ministry of National Education, Contract PN09 30, Project
PN09-01 03, and by the Ministry of Research and Innovation, Contract 21N, Project PN 16 35 01 02.
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