IC/2001/72 United Nations Educational Scientific and Cultural

IC/2001/72
United Nations Educational Scientific and Cultural Organization
and
International Atomic Energy Agency
THE ABDUS SALAM INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS
SOURCE SCALING FOR THE INTERMEDIATE-DEPTH
VRANCEA EARTHQUAKES
A. Gusev1
Institute of Volcanic Geology and Geochemistry,
9 Piip Blvd, 683006 Petro-Kamchatskii, Russian Federation,
M. Radulian2, M. Rizescu2
National Institute for Earth Physics, P.O. Box MG-2, 76900 Bucharest, Romania
and
G. F. Panza3
University of Trieste, Department of Earth Sciences, Trieste, Italy
and
The Abdus Salam International Centre for Theoretical Physics Trieste, Italy.
MIRAMARE - TRIESTE
July 2001
1
gusev@emsd.iks.ru
mircea@infp.ro; Mihaela.Rizescu@infp.ro
3
panza@dst.univ.trieste.it
Abstract
Earthquake source scaling properties are studied for the intermediate-depth seismicity
nest in Vrancea region, Romania, known as a source of many destructive earthquakes. Some
earlier work indicated high stress drop for Vrancea earthquakes, and our purpose is to check
these findings using wide-band digital records from events of an adequate magnitude range.
We could find body-wave records with acceptable signal/noise ratio for sixteen Vrancea
earthquakes (3.7 < Mw^ 7.4) which occurred since 1976. Recording stations used are local,
regional and teleseismic. To analyze the spectral scaling properties, the common spectral
features of the corner frequency, fc, and the spectral slope, y, have been determined for P- and
S-waves. This was done, both for entire P or S groups and for separate phases like P or pP or
sP, etc. The time-domain durations, T, have been determined from the deconvolved
displacement pulses, and the time-domain fc, equal to T 1 , has been estimated. In all the
variants of the processing - entire group or single pulses, P- or S-waves, spectral domain or
time domain - quite consistent and comparable results have been obtained. The only
systematic difference is between P-wave based and S-wave based estimates, represented by
the usual fcp/fcs ratio of about 1.4. To determine the scaling of fc vs seismic moment, Mo, we
used the Mo values given in the ROMPLUS catalogue, which are consistent with our
estimated spectral levels.
When viewed over the entire magnitude range (M\y = 3.7 -s- 7.4), the fc vs Mw trend
generally agrees with the model of constant stress drop (independent of magnitude).
However, some individual earthquakes of about magnitude 5 show very long durations on
GRF; this may be a peculiar propagation effect for the path to this station. The value of the
constant stress drop, based on the log-average fcs vs Mw trend, is near to 3 MPa. This means
that the scaling of intermediate Vrancea earthquakes over the wide magnitude range is quite
similar to that normally observed for shallow events, with the typical stress drops range of 110 MPa. However, at the upper-magnitude end, this normal trend is violated. The largest
intermediate-depth events (Mw > 6.5) show a clear tendency for higher static stress drops
than the shallow events, and for magnitudes M w > 7, stress drops in excess of 10 MPa can be
expected. Unfortunately, due to the very limited data volume, this conclusion is not
definitive. However, if true, our result means that when one wishes to estimate the strongmotion amplitudes of the largest intermediate Vrancea earthquakes, one cannot merely use
the extrapolation of the strong-motion parameters observed for moderate-magnitude events to
the largest ones on the basis of the standard constant-stress-drop scaling assumption. This
simple approach would lead to significantly underestimated amplitudes.
Introduction
The Vrancea zone, located in Romania, at the sharp bend of the South-Eastern
Carpathians (Figure 1) is a well-defined seismic region in Europe, with unique properties.
The seismicity is concentrated in an extremely narrow, high-velocity, focal volume in the
depth range from 60 to 200 km. A relatively high seismic energy is persistently released (four
shocks with magnitude greater than 7 occurred during the past century) by a seismogenic
process still far from being fully understood. At more shallow levels (0-60 km) the seismicity
here is sporadic and weak (magnitude below 5.5), and seems to be decoupled from the
seismic activity in the subcrustal lithospheric slab. All the major shocks are characterized by
a quite stable reverse faulting focal mechanism with the rupture plane oriented in a NE-SW
direction, parallel to the Carpathians arc, and following the orientation of the epicenters
distribution. The analysis of local short-period data indicates that, for a given seismic
moment, the source area of the intermediate-depth Vrancea events is significantly smaller
than the size usually observed for shallow events, and implicitly the stress drop is higher
(Oncescu, 1989; Radulian and Popa, 1996; Popa and Radulian, 2000). The scaling properties
of the source spectrum are extremely important in seismic hazard studies, because they may
significantly modify the strong motion parameters as compared to the case of "average"
stress drop. For a given magnitude and distance, unusually high stress drop increases
acceleration and velocity amplitudes and decreases the duration of shaking; the effect of low
stress drop is the opposite. As a result, the assumed values of stress drop may have a drastic
effect on the seismic hazard values as computed by the deterministic approach (e.g. Radulian
et al., 2000). Thus, it is of paramount importance to determine reliable source spectral
scaling.
The main goal of this paper is to analyze the scaling properties of the Vrancea
intermediate-depth earthquakes and to check if the anomalous scaling suggested by the local
short-period data is confirmed by the broadband data recorded at local and global scale. To
this aim, we analyze the set of ninety-three broadband records for sixteen moderate and large
Vrancea intermediate-depth earthquakes.
Observational data
We consider the set of broadband waveforms recorded at teleseismic, regional and
local distances, for sixteen intermediate-depth Vrancea earthquakes which occurred between
1976 and 2000 (3.7 < M w < 7.4) (Table 1).
The use of the data from the station GRF (Grafenberg, Germany), affiliated to the
German Regional Seismic Network, with the longest recording history and located at a
relatively small epicentral distance (about 11°), has been pivotal for the success of this study.
Also highly valuable were the data from the GEOFON station MLR (Muntele Rosu,
Romania), operated jointly by the GeoForschungsZentrum Potsdam and the National Institute
for Earth Physics, Bucharest, located practically in the epicentral zone of Vrancea
earthquakes. Additional data are from representative regional and teleseismic broadband
stations with a reasonable azimuthal coverage for Vrancea earthquakes. We selected stations
situated inside continents to minimize storm/surf microseismic noise. The following
broadband stations (situated at epicentral distances as given in parenthesis) have been used as
well: TRI (9°), AQU (10°), OBN (11°), ARU(22°), BGCA (41°), HIA (59°), CCM (80°),
ANMO (89°). The set of waveforms has been retrieved from the World Wide Web pages of
the IRIS Data Management Center in Seattle, USA and GEOFON Data Center in Potsdam,
Germany.
Estimation of source parameters
For the events listed in Table 1 all usable P- and S-wave records have been processed
in order to estimate the source parameters, both in frequency and time domain. A
computation algorithm, programmed using the MATLAB software package of Math Works,
has been set up to determine interactively the source parameters.
The observed velocity data, corrected for instrument response, are integrated and
differentiated to get the displacement and acceleration time series. The waveforms are plotted
together with the theoretical phases and arrival times, based on the travel times calculated for
the IASP91 Earth model (Buland and Chapman, 1983).
The records of body waves are corrected for attenuation and geometrical spreading.
Attenuation affects both the shape and the spectral properties of the P- and S-wave pulses,
-nft*
therefore to correct for attenuation we consider the attenuation operator €
, where/is the
frequency and t* is the travel time, divided by the quality factor. For the P-wave we assume
that t* varies in the range 0.6 -1.0 s for teleseismic distances (A>20°), and that it is equal to
0.3 s and 0.05 s for regional (A around 10°) and local distances (A around 0.5°), respectively.
For the S-wave the assumed t* value varies from 2.4 s to 4.0 s for teleseismic distances, and
it is equal to 0.6 s and 1.0 s for regional and local distances, respectively. The geometrical
spreading coefficients, as given by relation (Aki and Richards, 1980):
cos iv cos L sin A
A
5
P
(1)
dp
(where x and ^ are the position vectors of the station and focus, respectively, v(£) is the
seismic wave velocity at the focus, ix and i^ are the incidence angle and take-off angle,
respectively, A is the epicentral distance and p is the ray parameter) have been calculated in
an isotropic spherical earth model.
The common "ocfY" spectral model (Brune, 1970, Hanks and Wyss, 1972), that yields
the low-frequency spectral level, the corner frequency fc and the high-frequency spectral
decay y have been used to characterize the spectral shape.
For each phase, the signal and noise time windows are interactively selected, then a
5% cosine tapering is applied and the displacement spectrum is computed using a FFT
algorithm. The signal is noise-corrected by subtracting the estimated noise contribution from
the squared spectrum. The spectrum of the signal is inverse-filtered with the attenuation
operator, corrected for the geometrical spreading factor and scaled to the seismic moment
units. The zero-frequency level, the corner frequency and the slope of the high-frequency
decay are interactively determined after visual inspection of the corrected spectrum. The first
spectral parameter is an estimate of the seismic moment Mo and provides the moment
magnitude by the formula (Kanamori, 1977):
Mw=|aogM0[N.m]-9.1)
(2)
Only the interactive mode allowed us to process records with low signal-to-noise
ratio, with 0.2-0.3 Hz microseismic spectral peak just in the middle of the analyzed frequency
band. A few examples of the analyzed waveforms with high and low signal-to-noise ratios
are given in Figure 2. Signal and noise spectra are also plotted.
The spectral shape has been estimated on all usable components and separately for P,
pP, sP, S and sS phases, whenever they could be unequivocally identified. Our tests show that
the spectra obtained when considering isolated single wave pulses or wave groups
(containing P, pP and sP, for example) are consistent with each other, both as corner
frequency and spectral slope. The values obtained from the different components of motion
also show a good consistency. Therefore, the seismic moment, corner frequency and spectral
slope for P- and S-waves have been estimated as averages of all the reliable values
determined from the different phases and components of motion (Table 2).
The correction for attenuation is performed, in the frequency domain, on the
displacement spectrum. To calculate the corresponding phase correction, that follows from
the requirement of causality, a program module from Claerbout (1976) was used. The pulse
duration is interactively estimated in the time domain, where the pulse is low-pass filtered,
with a cutoff frequency of 2-4 Hz (typical for the teleseismic case), using a 4 th order
Butterworth filter, to remove the high-frequency noise produced by the normalization with
the attenuation operator. For the time-domain analysis, only isolated P- and S-phases have
been considered, and the corresponding results are given in Table 2.
Only a few estimates have been published for the corner frequencies and source
durations of Vrancea intermediate depth earthquakes. From the broadband recordings of the
1977 event, Fuchs et al. (1979) give, for S-waves, fc = 0.07 Hz, in agreement with our results.
The source durations for the strong events of 1977, 1986 and 1990 (first and second shocks)
determined from teleseismic waveforms inversion (see Tavera (1991) for references) are also
compatible with our determinations. From the strong motion data of the 1986 event Oncescu
(1989) gives, for S-waves,/ c = 0.23 Hz. This value, relatively high if compared with ours, can
be explained by the fact that only the asperity, and not the whole source area, is seen by the
accelerogram record. The difference is natural because analogue strong motion data cannot
provide a good coverage of the low-frequency part of the spectrum. The computation of the
source parameters for small to moderate Vrancea events, using local data (Oncescu, 1986;
Rizescu, 1999; Radulian and Popa, 1996), gives values of/c similar to the ones we obtain for
the MLR station.
The comparison of the Mo and M w values that we have determined, from the zerofrequency spectral levels, with the corresponding values given by the Romanian earthquake
catalogue (Figure 3) shows, in general, an acceptable agreement, with no systematic bias. We
prefer to use moment magnitudes (Mw) from the regional catalogue, since we consider these
estimates, based on all available data for each event, more reliable than our spectral
estimates, based, in many cases, on the data of a single station.
Scaling relationships and their discussion
We now try to establish the relationship fc vs M w that defines the source scaling. The
observed Mw -fc relation is represented in Figure 4 (for P-waves) and Figure 5 (for S-waves).
Linear trends that represent the characteristic constant-stress-drop average behavior,
following Brune (1970), are also plotted. For S-waves:
log/ cS = -0.5M w +C s
(3)
where Cs=2.25 or 2.6 for 1 or 10 MPa stress drop, respectively. The relationship of this kind
was first established by Thatcher and Hanks (1973) as the average empirical trend for
moderate-to-large shallow California earthquakes. Since that paper, a lot of work has been
done following Brune's spectral approach, and for practically all studied sets of moderate or
moderate-to-large shallow earthquakes, (3) holds at least approximately. However, the values
of the average stress drop vary significantly, covering the range 0.5 - 10 MPa. A certain part
of this scatter probably reflects some differences in processing and analysis procedures;
another contribution is the natural dispersion between various data sets. To derive the
analogue of (3) for P-waves, we assume the typical empirical value of the corner frequency
ratio:
f(P/fcs=l-4
(4)
and obtain:
(5)
where Cp=2.4 or 2.75 for 1 or 10 MPA stress drop, respectively. The coefficient 0.5 in
relations (3) and (5) expresses the standard, constant stress drop source scaling.
Assuming this kind of scaling, we approximate our data points with a linear relation
with the fixed slope value equal to -0.5, and with the intercept value obtained from the bestfit (dashed lines in Figures 4 and 5). The corresponding values of the stress drops for P- and
S-waves are 3.1 MPa and 3.2 MPa, respectively. These values of stress drop are within the
interval usually observed for shallow earthquakes when Brune's approach is used. They are
also close to the characteristic values for the inter-plate earthquakes after Kanamori and
Anderson (1975). Though this result is evidently a good first approximation, some deviations
are seen at the upper-magnitude end, and will be discussed later. Another interesting feature
is seen in the magnitude range from 5 to 5.5, where the dispersion of our estimates seems to
be unusually high.
The time domain estimates behave in a quite similar way. In fact the (1/T) vs Mo
relationships, plotted in Figures 6 (P-wave) and 7 (S-wave), are very close to the fc vs M w
relationships of Figures 4 and 5. This consistency is also visible when looking at the
correlation between fc and the source duration (Figures 8 and 9). The/ C and the inverse of the
source duration closely follow the line with the slope 1, only slightly shifted towards smaller
1/T values. On the average, fcP/(l/TP)=1.0 and f cS /(l/T s )=l.l.
Thus, the scaling revealed in the first approximation for Vrancea intermediate-depth
earthquakes follows the general trend of shallow earthquakes. The ratio of the corner
frequency values, derived from P- and S-waves is close, on the average, to the value of 1.4,
quite typical for shallow earthquakes. The relationship between the inverse pulse durations
for P and S-waves is similar. The dispersion of the values obtained with the analysis in the
frequency domain and in the time domain, is comparable both for P- and S-waves (Figures 4
and 6, 5 and 7, respectively). The good match between the results of frequency-domain and
time-domain analyses adds an additional weight to our general conclusions.
When viewed over the entire earthquake size range (Mw = 3.7 -s- 7.4), the observed fc
vs Mw trend generally agrees with the model of constant stress drop (independent of
magnitude). However, at the larger-magnitude end, this model becomes invalid. The largest
events (Mw > 6.5) show a clear tendency for stress drops that are larger than those for
moderate-magnitude ones (or, for typical shallow events). From the visual inspection of.
Figures 4 and 5, one can expect, for Mw > 7, stress drop values as high as 10-20 MPa. This
possibility is very important for prediction of future strong motion, because at a given Mw,
increased stress drop produces larger peak velocity and acceleration values and response
spectra, and smaller durations.
The problem of the existence of a simple and theoretically plausible scaling law is a
general one, and has very important consequences on seismic hazard estimation issues.
Theoretical source models usually assume constant stress drop and constant rupture velocity,
resulting in a simple fc vs Mw scaling, like the one described by equations (3) and (5).
However, natural earthquakes need not follow these or other similar assumptions. In the case
of Mexican earthquakes (Singh et al 1989), it was found that when strong motion amplitudes
are extrapolated from moderate to large earthquakes, according to the constant stress drop
model, the resulting estimates are in excess with respect to the values actually observed. The
situation in the case of the Romanian earthquakes seems to be the opposite: the extrapolation
from moderate earthquakes would rather underestimate the strong-motion amplitudes of the
largest events. In such a situation, using empirical and not "theoretically-based" scaling may
lead to more reliable results. One such case is the study of Moldoveanu and Panza (1999)
who, using a kind of empirical source scaling modelled successfully the main characteristics
of the accelerogram recorded in Bucharest for the strong 1990 earthquake (Mw=6.9).
In some cases, the S-waveform at GRF station is unipolar, whereas the P-waveform of
the same event is of comparable duration but bipolar (approximately doubling the value of/ c ),
as seen in the example in the center of Figure 2. Quite hypothetically, this fact may be
explained by a difference between the propagation mode of P- and S-waves, around 11°
along this particular path, with a single branch for S-wave and multiple branches for P-wave.
The distributions of the estimated P-wave and S-wave spectral slopes as functions of
magnitude are plotted in Figures 10 and 11, respectively. The /values for S-wave are in the
range 1.8 - 2.0, while the / values for P-wave are generally larger, with a tendency to increase
with increasing earthquake size. This tendency, not clearly defined, is however hardly an
artifact due to the approximations in our procedure, and deserves some discussion.
Our /estimates are in fact somewhat questionable because (1) true t values for the
actual paths are unknown and (2) we assume that t is frequency dependent and this model is
too simple. The first factor can hardly cause the dependence of / on magnitude, while the
second factor may provide an explanation. Indeed, our analysis suggests that the true
teleseismic t may significantly decrease between 1 and 6 Hz, reaching values as low as 0.1 0.2 s at 5 - 6 Hz. In such a case, our correction for attenuation is poor and our /estimates are
based on somewhat biased, too gradually sloping spectra. The magnitude of this bias depends
on the actual frequency band in which the data are analyzed that, in turn, depends on the
signal-to-noise ratio. At the largest magnitudes, the utilizable frequency band is the widest,
and the mentioned bias is the largest as well, thus leading to an underestimate of /. Therefore,
when we are ignoring the frequency dependence of t* we must introduce an artificial
decrease, with increasing magnitude, of the /estimates. The actual tendency is the opposite
one. This may indicate that the increase, with increasing magnitude, of the value of /for the
P-wave spectra is a genuine phenomenon. The lack of a similar tendency in S-wave is not too
relevant because the S-wave spectra of the larger events are teleseismic and are thus of
relatively lower reliability.
Conclusions
The scaling relations for the Vrancea subcrustal earthquakes have been analysed using
both frequency- and time-domain measurements from broadband recordings. The data set is
formed by sixteen events which occurred between 1976 and 2000 (3.7 < Mw^ 7.4), recorded
by ten stations spanning the epicentral distance range from 0.5° to 90°. The two kinds of
measurements (frequency- and time-domain) are well correlated over the whole magnitude
interval considered, and the inverse of the pulse duration is a good estimate of the corner
frequency. The ratio of the corner frequency values, as well as the ratio of the inverse pulse
duration values, deduced from P-and S-waves, are close to the value 1.4, a typical one for
crustal events.
The earthquake size scaling with corner frequency or source duration follows, in the
first approximation, the general trend observed for shallow earthquakes. For the whole
magnitude range (Mw between 3.7 and 7.4) our data distribution is well approximated by a
linear dependence with slope 0.5, typical for crustal earthquakes. The large dispersion in the
5-5.5 magnitude interval is partly explained by the anomalously low fc and 1/T values
obtained with Grafenberg station (GRF). This station is the only one that provides
observations for a broad magnitude range, including the largest magnitudes, so it is crucial
for our study, but unfortunately strong path effects might contaminate its recordings of
Vrancea earthquakes.
For the largest magnitudes range (Mw > 6.5) our study evidences a deviation of the
source scaling towards larger stress drops. This result has significant consequences on
seismic hazard estimation, due to Vrancea intermediate-depth earthquakes. It may mean that
the empirical scaling of strong motion parameters may in certain cases be closer to reality
than the "theoretically-based" ones that use physically sound but in fact too simplistic
assumptions. The applicability of a simple scaling is further questioned by some indication
that the spectral slope increases with increasing magnitude. Unfortunately, these very
important results should be considered as preliminary ones because of the limited data set,
which is available.
Acknowledgements
This research is a contribution to the NATO SfP project 972266 "Impact of Vrancea
Earthquakes on the Security of Bucharest and other Adjacent Urban Areas (Ground Motion
Modelling and Intermediate-Term Prediction)" and to the UNESCO-IUGS-IGCP project 414
"Realistic Modelling of Seismic Input for Megacities and Large Urban Areas". We used the
Tau software to calculate travel times and SPHERAY software to compute geometrical
spreading
coefficients,
available
on
Internet
at
www.orfeus.knmi.nl
and
www.geoscope.ipgp.jussieu.fr, respectively.
References
Aki, K., Richards, P.G., 1980. Quantitative seismology. 2 volumes, W.H. Freeman, San
Francisco, 932p.
Brune, J.N., 1970. Tectonic stress and the spectra of seismic shear waves from earthquakes. J.
Geophys. Res., 75, 4997-5009.
Buland, R., Chapman, C.H., 1983. The Computation of Seismic Travel Times, Bull. Seism.
Soc. Am., v. 73, pp. 1271-1302.
Claerbout, J. F., 1976, Fundamentals of Geophysical Data Processing, McGraw-Hill, N.Y
Fuchs, K., Bonjer, K.-P., Bock, G., Cornea, I., Radu, C , Enescu, D., Jianu, D., Nourescu, A.,
Merkler, G., Moldoveanu, T., Tudorache, G., 1979. The Romanian earthquake of March 4,
1977 II. Aftershocks and migration of seismic activity, Tectonophysics, 53, 225-247.
Hanks, T.C., Wyss, M., 1972. The use of body-wave spectra in the determination of seismicsource parameters. Bull. Seism. Soc. Am., 62, 561-589.
Kanamori, H., 1977. The energy release in great earthquakes. J. Geophys. Res., 82, 29812987.
Kanamori, H., Anderson, D.L., 1975. Theoretical basis of some empirical relations in
seismology, Bull. Seism. Soc. Am., 65, 1073-1095.
Moldoveanu, C.L., Panza, G.F., 1999. Modelling, for microzonation purposes, of the seismic
ground motion in Bucharest due to the Vrancea earthquake of May 30. In: Vrancea
Earthquakes: Tectonics, Hazard and Risk Mitigation, F. Wenzel, D. Lungu (eds) & O. Novak
(co-ed.), 85-98, Kluwer Academic Publishers, Dordrecht, Netherlands.
Oncescu, M.C., Marza, V.I., Rizescu, M., Popa, M., 1999. The Romanian earthquake
catalogue between 984-1997, in: Vrancea Earthquakes: Tectonics, Hazard and Risk
Mitigation, F. Wenzel, D. Lungu (eds) & O. Novak (co-ed.), 43-47, Kluwer Academic
Publishers, Dordrecht, Netherlands.
Oncescu, M.C., 1986. Some source and medium properties of the Vrancea seismic region,
Romania. Tectonophysics, 126, 245-258.
Oncescu, M.C., 1989. Investigation of a high stress drop earthquake on August 30, 1986 in
the Vrancea region. Tectonophysics 163, 35-43.
Popa, M., Radulian, M., 2000. Test of the empirical Green's function deconvolution on
Vrancea (Romania) subcrustal earthquakes. Studia Geoph. et Geod. 44, 403-429.
Radulian, M., Popa, M., 1996. Scaling of the source parameters for the Vrancea intermediate
depth earthquakes. Tectonophysics, 261, 67-81.
Radulian, M., Vaccari, F., Mandrescu, N, Panza, G.F., Moldoveanu, C.L., 2000. Seismic
hazard of Romania: deterministic approach. Pure appl. geophys. 157, 221-247.
Rizescu, M., 1999. A completely automated system for seismological data acquisition,
processing and exchange, PhD Thesis, Institute for Atomic Physics, Bucuresti, 219 p.
Singh, S.K., Ordaz, M., Anderson, J.G., Rodriguez, M., Quaas, R., Mena, E., Ottaviani, M. &
Almora, D., 1989. Analysis of near source strong-motion recordings along the Mexican
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Tavera, J., 1991. Etude des mecanismes focaux de gros seismes et seismicite dans la region
de Vrancea, Roumanie, Raport de stage de recherche.
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Table 1. Selected intermediate-depth Vrancea earthquakes. Locations and
magnitudes are taken from the Romanian Earthquake Catalog ROMPLUS (Oncescu et al.,
1999), where Mw values are moment magnitudes from teleseismic observations for Mw > 6.0
and moment magnitudes or duration magnitudes, from short-period recordings, converted to
moment magnitude scale for Mw < 6.0. MW-HARVARD are moment magnitudes as given by the
Harvard CMT catalogue on the web site of the Harvard University.
Date and origin time
1976/10/01
1977/03/04
1979/05/31
1979/09/11
1981/07/18
1985/08/01
1986/08/30
1990/05/30
1990/05/31
1995/09/06
1995/09/19
1997/11/18
1998/03/13
1999/04/04
1999/04/28
2000/04/06
17:50:43.2
19:21:54.1
07:20:06.3
15:36:54.2
00:02:58.6
14:35:04.3
21:28:35.7
10:40:06.4
00:17:47.9
10:58:45.9
19:32:21.4
11:23:16.3
13:14:37.2
01:21:12.6
08:47:56.9
00:10:39.9
Latitude
(deg. N)
45.68
45.77
45.55
45.56
45.69
45.73
45.52
45.83
45.85
45.53
45.63
45.78
45.51
45.70
45.47
45.69
Longitude
(deg. E)
26.49
26.76
26.33
26.30
26.42
26.62
26.49
26.89
26.91
26.39
26.58
26.72
26.33
26.44
26.32
26.68
Depth
(km)
146
94
120
154
166
93
131
91
87
120
143
126
165
146
145
136
Mw
6.0
7.4
5.3
5.3
5.5
5.8
7.1
6.9
6.4
4.1
4.1
4.6
4.7
3.7
5.3
5.0
Mw-Harvard
7.5
5.2
5.1
5.2
7.2
6.9
6.3
5.2
5.4
5.1
Table 2. The source parameters (corner frequency, high frequency spectral slope, pulse duration and moment magnitude) from P- and Swaves recorded at the stations listed in the last column. The standard deviation and the number of observations (in parenthesis) are also given, when
at least two determinations are available.
Event
date & time
1976/10/01
17:50
1977/03/04
19:21
1979/05/31
07:20
1979/09/11
15:36
1981/07/18
00:02
1985/08/01
14:35
1986/08/30
21:28
1990/05/30
10:40
1990/05/31
00:17
1995/09/06
10:58
1995/09/19
19:32
1997/11/18
11:23
1998/03/13
13 :14
1999/04/04
01:21
1999/04/28
08:47
2000/04/06
00:10
fcP
(Hz)
0.35
+0.10 (3)
0.16
+0.06 (3)
0.30
± 0 . 0 1 (2)
0.4
±0.06 (2)
0.2
+0.06 (3)
0.19
+0.11 (30)
0.30
±0.13 (21)
3.66
1.55
1.67
4.03
0.52
±0.27 (8)
0.84
±0.52 (7)
fcs
(Hz)
0.26
±0.06 (3)
0.1
±0.02 (3)
0.26
±0.04 (2)
0.32
+0.03 (2)
0.28
+ 0 . 0 1 (2)
0.33
0.13
+0.02 (3)
0.11
±0.05 (19)
0.14
±0.07 (16)
2.43
±0.04 (2)
2.33
±0.04 (2)
1.36
± 0 . 2 8 (2)
1.18
± 0 . 0 6 (2)
2.9
+0.42 (2)
0.57
± 0 . 5 1 (6)
1.08
+ 0 . 9 2 (4)
YP
Ys
TP
(S)
1.97
±0.1 (3)
2.73
± 0 . 1 5 (3)
2.40
±0.26 (2)
1.87
± 0 . 5 8 (2)
3.25
±0.32 (3)
2.11
±0.45 (30)
2.05
±0.43 (21)
1.98
1.57
2.11
1.89
2.21
+ 0 . 2 6 (8)
2.54
±0.44 (7)
1.42
±0.22 (3)
1.80
± 0 . 1 5 (3)
1.78
± 0 . 1 1 (2)
1.90
±0.08 (2)
1.72
±0.24 (2)
1.84
2.02
±0.10 (3)
1.94
+0.22 (19)
1.98
± 0 . 4 6 (16)
2.13
±0.14 (2)
1.87
+0.06 (2)
1.92
±0.12 (2)
1.93
+ 0 . 1 (2)
1.72
± 0 . 0 8 (2)
2.0
±0.17 (6)
2.16
±0.30 (4)
2.8
± 0 . 8 (2)
10.1
± 1 . 1 (3)
2.6
+0.0 (2)
2.0
+ 0 . 1 (2)
4.4
+0.2 (3)
5.1
MWP
Mws
Recording
Station
4.7
±0.2 (3)
7.0
±0.2 (3)
4.8
GRF
+0.1 (3)
6.9
GRF
+0.1 (3)
5.0
GRF
+0.1 (2)
4.8
GRF
+0.1 (2)
4.9
GRF
±0.1 (2)
5.0
GRF
(s)
12.9
±3.7 (2)
3.5
3.4
± 0 . 8 (2)
3.5
± 2 . 1 (2)
5.9
5.1
±0.0 (2)
4.8
±0.1
9.2
±1.1 (3)
9.2
(2)
7.1
±0.2 (3)
6.7
±0.2 (27)
6.0
+0.2 (16)
±0.2 (13)
5.9
± 0 . 1 (6)
4.6
MLR
3.8
+0.1 (2)
4.0
MLR
+2.6 (18)
4.1
±2.6 (10)
8.7
±1.4 (13)
0.6
±4.5 (13)
0.5
0.3
+ 0 . 1 (2)
0.5
1.0
±0.0 (2)
0.6
4.7
0.7
±0.2 (2)
1.1
4.8
±0.2 (2)
0.3
0.4
2.0
+ 0 . 1 (2)
3.9
±0.7
(5)
1.8
±0.9 (6)
±2.7 (6)
2.9
±1.9 (6)
6.8
±0.1
6.7
5.2
±0.3 (8)
5.1
±0.2
(7)
AQU,
HIA,
AQU,
HIA,
GRF, ARU,
CCM, ANMO
GRF, ARU,
CCM, ANMO
± 0 . 0 (2)
5.0
± 0 . 1 (2)
MLR
5.0
MLR
+0.1
4.0
GRF
(2)
3.7
± 0 . 1 (2)
5.1
+ 0 . 2 (6)
5.1
+0.2 (4)
MLR
MLR, T R I , GRF,
OBN, ARU, BGCA
MLR, T R I , GRF,
ARU
21°
24°
*
27°
**
* -
•
*
*
48°
*
*
30°
'v|
*
• *
+»
48°
•*v
i
* *
"BaiaMare
+ * •
.
t'
* • ** *<>'^>
ttfoa
****V
******* ***:***•
i. •
46°
*****
*•
*
* * •
*
***
\
y in
••••**
,
*
i
*
s
44°
* * *•••••/•KOTtodw
* i ••**
LEGEND
OGEOFON station MLR
•*•
**
•"
.
a
i~
44°
mWmsmmm
*
Q Subcrustal cqs. Mw>4
•
21°
•
MB—1
%***" t»
+ • •
46°
24°
27°
30°
Figure 1. Vrancea seismogenic zone. Intermediate-depth earthquakes with magnitudes
Mw > 4.0 from the up-to-date ROMPLUS catalogue are plotted, together with the location of
the GEOFON station MLR.
Mw - Romplus catalog
Figure 3. Moment magnitudes determined in this study versus moment magnitudes of
ROMPLUS catalogue. The bisecting line is also plotted.
11
GRF N, 1986/08/30 21:28
-
1E»015 -
Frequency (Hz)
0.1
Frequency (Hz)
Frequency (Hz)
Figure 2. Examples of the considered waveforms and spectra of Vrancea intermediate-depth earthquakes. The signal (P-wave) and noise
spectra are plotted together with the chosen spectral model. The example on the left represents a moderate earthquake (Mw 4.7) recorded at the local
station MLR, the example at the center of the figure represents the strong 1986 event (Mw 7.1) as recorded by the Grafenberg station, at regional
distance. The example on the right represents a moderate earthquake (M w 4.7), which could not be used in this study because of the low signal to
noise ratio, as recorded at the Grafenberg station.
Stations
+
D
GRF
Mw=-2log(fc)+5.13
RMS=0.417
CCM
ARU
•
ANMO
O
HIA
A
AQU
•
MLR
•
OBN
*
TRI
0.01
0.1
1
10
Fc - P waves (Hz)
Figure 4. M\y (from ROMPLUS catalogue) versus P-wave corner frequency for
Vrancea intermediate-depth earthquakes. The dashed line represents the data best fit, and it is
defined by the equation given in the upper right-hand corner. The fit RMS (computed as the
squared root of the residual mean square) is also given. The solid lines represent the
approximate average trends for shallow earthquakes, for 1 and 10 MPA stress drop.
13
Stations
+
GRF
Mw=-2 log(fc)+4.84
ARU
RMS=0.298
•
ANMO
o
A
•
e
*
HIA
AQU
MLR
BGCA
TRI
•»
\
\
3.5
0.1
0.01
1
10
Fc - S waves (Hz)
Figure 5. Mw (from ROMPLUS catalogue) versus S-wave corner frequency for
Vrancea intermediate-depth earthquakes. The dashed line is the data best fit, and it is defined
by the equation given in the upper right-hand corner. The fit RMS (computed as the squared
root of the residual mean square) is also given. The solid lines represent the approximate
average trends for shallow earthquakes, for 1 and 10 MPA stress drop.
14
Stations
+
D
GRF
O
ARU
•
ANMO
o
A
Mw=-2 Iog(1/T)+5.02
RMS=0.394
CCM
HIA
AQU
•
MLR
•
OBN
*
TRI
0.01
Figure 6. Mw (from ROMPLUS catalogue) versus the inverse of the P-wave pulse
duration for Vrancea intermediate-depth earthquakes. The dashed line is the data best fit, and
it is defined by the equation given in the upper right-hand corner. The fit RMS (computed as
the squared root of the residual mean square) is also given. The solid lines represent the
approximate average trends for shallow earthquakes, for 1 and 10 MPA stress drop.
15
Stations
+
GRF
•
ANMO
o
A
HIA
AQU
•
MLR
e
*
TRI
Mw=-2log(1/T)+4.61
ARU
BGCA
RMS=0.428
6.5
•
X
\
\
iX
•
•\
4.5
\
0.01
0.1
1
\
10
1/Tau - S waves (Hz)
Figure 7. Mw (from ROMPLUS catalogue) versus the inverse of the S-wave pulse
duration for Vrancea intermediate-depth earthquakes. The dashed line is the data best fit, and
it is defined by the equation given in the upper right-hand corner. The fit RMS (computed as
the squared root of the residual mean square) is also given. The solid lines represent the
approximate average trends for shallow earthquakes, for 1 and 10 MPA stress drop.
16
log(i/T) = log(ic) - 0.004
RMS=0.126
Log(fc) - P waves
Figure 8. Inverse of the pulse duration versus fc for P-wave of Vrancea intermediatedepth earthquakes. The best fitting line, defined by the equation given in the upper right-hand
corner, is plotted.
Iog(1/T) = log(!c) - 0 . 0 3 6
BHS=0.100
Log(fc) - S waves
Figure 9. Inverse of the pulse duration versus fc for S-wave of Vrancea intermediatedepth earthquakes. The best fitting line, defined by the equation given in the upper right-hand
corner, is plotted.
17
Stations
+
D
++
GRF
CCM
ARU
ANMO
O
HIA
A
AQU
•
MLR
A
OBN
*
TRI
+
•
to
+
r
CD
> 2.5
to
A
±+
5
CL
B
•
CD
it
•
i
* +
O
•
•
Mw
Figure 10. Slope of the high-frequency decay for P-wave spectra of Vrancea
intermediate-depth earthquakes versus Mw (from ROMPLUS catalogue). The stations
providing data are identified by different symbols.
Stations
+
GRF
•
ANMO
A
AQU
•
MLR
BGCA
o
e
ARU
HIA
TRI
o
2.5
*
A
4
+
o -^
•
•
+
•
1
1
1
•
A
4.
+
•
1
Mw
Figure 11. Slope of the high-frequency decay for S-wave spectra of Vrancea
intermediate-depth earthquakes versus Mw (from ROMPLUS catalogue). The stations
providing data are identified by different symbols.
18