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Source parameters of microearthquakes at Phlegraean Fields (Southern Italy) volcanic area
Physics of the Earth and Planetary Interiors, 47 (1987) 25—42 Elsevier Science Publishers B.V., Amsterdam — Printed in The Netherlands
25
Source parameters of microearthquakes at Phlegraean Fields (Southern Italy) volcanic area Edoardo Del Pezzo, Giuseppe De Natale, Marcello Martini and Aldo Zollo Osservatorio Vesuviano-80056 Ercolano (NA) (Italy) (Received January 2, 1986; revision accepted July 2, 1986)
Del Pezzo, E., Dc Natale, G., Martini, M. and Zollo, A., 1987. Source parameters of microearthquakes at Phlegraean Fields (Southern Italy) volcanic area. Phys. Earth Planet. Inter., 47: 25—42. The seismic activity that occurred at the Pblegraean Fields (Southern Italy) volcanic area during a pronounced episode of ground uplift has been analysed. One hundred and eighty-one three component seismograms from a digital network operating in the period January—May 1984 were processed to obtain seismic moments, source radii and stress drops for 32 microearthquakes (0.7
1. Introduction The ‘Phlegraean Fields’ is an active volcanic area 10 km from the city of Naples, Southern Italy. This region is located within an ancient caldera formed after an ignimbritic eruption about 35000 years ago. Many eruptions have occurred since the formation of the caldera; during the last one (A.D. 1538) a small volcano, Mt. Nuovo, was formed inside the caldera (Fig. 1). Two marked ground uplift episodes took place in the area in 1970 (Corrado et al., 1976) and in 1982, with a cumulative uplift of more than 2 m at the maximum point of ground deformation. The last one began in the second half of 1982 and produced a maximum uplift of 1.7 m (measured at the town of Pozzuoli) in 2 years. An increase of seismic activity followed the ground deformation, reaching a very high daily frequency (Fig. 2) with peaks in October 1983 and April 1984. During the period January 1983 to 0031-9201/87/$0350
© 1987 Elsevier Science Publishers B.V.
September 1984 more than 10000 events, with local magnitude ranging between 0.1 and 4.2, were recorded. A local seismic monitoring network consisting of 22 vertical component stations by Osservatorio Vesuviano and AGIP (Italian Petrol Agency) operates in the area. On 1 January 1984, 12 three-component digital stations, which were owned by the University of Wisconsin (Madison), were installed in the area, in the framework of a scientific cooperation between Osservatorio Vesuviano and the University of Wisconsin. The main purposes of the experiment were to improve earthquake locations and to determine properties of the source and medium from spectral and time domain analyses of seismic signals. A suite of 181 three-component seismograms from 32 selected events were used in this study to estimate seismic source parameters (seismic moments, source radii and static stress drops). This type of analysis has been extensively applied to
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Fig. 1. Volcanological map of Phlegracan &Ads @fter di Vito et al. (1985)).
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27 DAILY 300.0
EARTI-(QUAKE FREQUENCY
,,,,,,,,,,,,,,,,,
~
497, 200.0. U I
May,1983
May,1984 DAYS
Fig. 2. Time history of earthquake activity during the period May 1983—May 1984 given in number of events per day. Each division on the time axis represents 10 days.
many volcanic and tectonic regions (Fletcher, 1980; Frankel, 1981; Archuleta et a!., 1982; Fletcher et al., 1984). A central concern of this study is a microearthquake data set that cover~an unusual local magnitude range (0.7—3.2), which is just below the range covered by researchers cited above. Moreover, estimates from both P-radial and SH motions recorded by many azimuthally well-distributed seismic stations improved the quality of source parameter measurements, The use of an inverse method to determine spectral parameters (low frequency spectral level, corner frequency and high frequency spectral fall off) permitted us to obtain objective estimates and confidence intervals for these source parameters. It is then possible to weight the measurements from a single spectrum by their standard deviations, to reduce bias due to scattered data and to give more reliable error estimates on source parameters.
2. Volcanological setting of the area The history of the Phlegraean Fields volcanism has been characterized by the occurrence of many eruptive episodes, most of them associated with monogenetic pyroclastic vents. The main event that generated the caldera 35 000 years ago, according to Rosi et al. (1983), was an eruption of a huge alkali trachytic ignimbrite followed by a caldera collapse. Stratigraphic sections from a
detailed field survey, geophysical investigations and geothermal deep drilling have reconstructed the structural evolution of the area related to the volcanic activity (La Torre and Nannini, 1980; Rosi et al., 1983). Figure 1, by di Vito et al. (1985), shows the geological map of Phlegraean Fields as deduced from field survey data. Geothermal well data collected inside and on the rim of the caldera reveals the presence of tuffites with trachytic lava intercalations at deeper layers (2000 m) due to volcanic activity (mainly submarine) after the caldera collapse until about 10000 years ago. Shallower layers, which are mainly composed of yellow tuffs and tephra, were probably generated during the recent subaerial activity. In the last 10 000 years all volcanic activity has been confined to the depressed part of the caldera. This was mostly of explosive type (Pliian, Subplinian and Strombolian) and subaerial. Magma— water interactions at depth played an important role in controlling explosive eruptive mechanisms and the differentiation of eruptive products as can be inferred from observed traces of phreato-magmatic eruptions (Rosi et al., 1983). Most of the recent eruptions are related to the post-caldera subaerial activity which started about 4500 years ago. This recent phase was preceded by a local tectonic event, the uplift of the northern sector of the Gulf of Pozzuoli (Fig. 1), which is represented by the ‘Starza marine terrace’ which dates back to about 5000 years ago and is probably due to magma injection at shallow depth
28
(Rosi et al., 1983). All the successive eruptions, up to the last Mt. Nuovo, occurred near and around this local geomorphological discontinuity,
gain ranging information, time code and Omega radio navigation time signal were recorded on 4 tracks audio—grade tape recorders. Six hours of recording time was available for each tape, i.e. a maximum of 360 1-mn-events or 720 30-s-events. A 512 sample (5.12 s) pre-event memory allows the recovery of what triggered the system and pre-event background noise. Amplitude response curve is almost flat to velocity between 1 and 25 Hz. For more details see Powell (1983).
3. Instrwnentation The University of Wisconsin digital recording seismographs are wide dynamic range (106 db) instruments. Earthquake signals are detected by an automatic triggering system and recorded in digital form on 1/4 in. field tapes. Triaxial sensors, which were comprised of HS1O1 type seismometers with a natural frequency of 1 Hz, were used. Geophone signals were preamplified, anti-alias filtered by a 4 poles Butterworth low-pass filter with 24 Hz corner frequency. Each signal was digitized at 100 sample s’ by a wide-range analog-to-digital 12 bit converter provided with a gain ranging amplifier. Sampled data,
4. Seismicity and earthquake locations Figure 3 shows a map of 450 selected epicentres recorded in the period August 1983 to May 1984 by the operating networks displayed in the same figure. Also shown are the locations of the seismic networks installed by Osservatorio Vesuviano and AGIP, consisting of vertical component geo-
A A A
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Fig. 3. Map of 450 selected epicentres of events of Phlegraean Fields; solid triangles are University of Wisconsin digital stations while the open ones indicate the Osservatorio Vesuviano-AGIP survey network. Locations of digital stations are given in Table II.
29
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20.0 68
Fig. 4. Three-dimensional view (a) and North—South section (b) of the events shown in Fig. 3.
30
phones whose signals are telemetered and recorded in analog form on magnetic tapes. The readings from the survey network were performed by high speed playbacks on paper, whereas the digital seismograms where read by a semi-automatic procedure of time picking and accumulation on files of the P and S first arrivals, using graphic displays by an HP-bOO computer. Figure 4a, b shows a 3-D view and a N—S section of the same selected events. These events have been located using first P arrival times from the survey network records and P and S first arrival times recorded by the digital stations during the operating period, The estimated average reading error is about 0.05 s on P wave arrival times from the survey network, 0.02 s for P wave and 0.05 s for S wave readings from the digital stations. The computer program LQUAKE (Crosson, 1981) was used to locate the events. High quality locations were defined according to the following criteria: (1) minimum number of readings 12; (2) maximum RMS residual 0.15 s; (3) maximum azimuthal gap between the recording stations 1500; and
(4) maximum hypocentral error 0.5 km. A homogeneous half space with V~, 3 km ~ and V~ 1.7 km s~ was used as the velocity model according to preliminary results obtained by Scarcella (1984), who applied Crosson’s (1976) inverse technique to find the best layered velocity model at Phlegraean Fields. These results show the lack of evidence of marked discontinuities in the first few kilometres of the crust beneath the seismic network. Direct measurements made in holes drilled at the borders of the Phagraean Fields zone by AGIP agree with low values of P wave velocity ranging between 1.7 and 3.5 km s1 down to about 3.0—3.5 km. The selected sample of seismicity shown in Figs. 3 and 4 is representative of the whole sequence. The earthquakes are concentrated in a small volume (about 4 x 4 x 4 km3) that approximately corresponding to the area of maximum uplift. No spatial variation with time of the hypocentres is evident (del Pezzo et al., 1984). Moreover, the earthquakes (except those located in the Gulf of Pozzuoli) appear clustered along the main tectonic structure in this area: the ‘Starza marine terrace’ (Fig. 1). Figure 5 shows some focal mechanisms inferred from P-wave polarities. A =
=
=
=
=
=
/
_
(
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\~ 2
0
Fig. 5. Wulff projection lower hemisphere focal mechanisms of a set of events (2. (survey network) (after Gaudiosi and lannaccone, 1985).
~
ML ~ 4.) recorded at a minimum of 15 stations
31
minimum of 15 stations were used for each mechanism (Gaudiosi and lannaccone, 1985). All the focal mechanisms of events aligned on the Starza marine terrace are normal faults dipping 30 o60 0 with minor strike-slip components, while the two events in the Gulf of Pozzuoli show compressive mechanisms. We believe that the seismicity is mainly connected with the activation of pre-existing structures, as a response to magmatic activity,
Solving iteratively the system of eq. 2 by a linear inversion method up to some convergence criterion is satisfied, we obtain the best estimates of the model parameters. We have used, for each inversion step, the S.V.D. algorithm (Lawson and Hanson, 1974) with truncation of small singular values and Levenberg—Marquardt damping (Marquardt, 1963), due to the non-linearity of the problem. At each step we compute the correction
5. Source parameter analysis
~ ~y.= W6y =
(3)
&YW
5.1. Spectral model and the Covariance and Resolution matrices In agreement with the most used theoretical models of seismic sources (Brune, 1970; Madariaga, 1976; Boatwright, 1980), the far-field displacement spectrum V( f) of an earthquake can be described by Log V( f)
=
Log ~ — O.43~JT/Q —0.5 Log(1
+
(f/f~)2~~)
(‘-)
(Boatwright, 1978), where: ~ is the low frequency spectral level; f~is the corner frequency; y determines the high frequency decay of the spectrum; Q is the average quality factor along the ray path from the event to the recording station; and T is the travel time. We can infer these parameters by an iterative non-linear inversion of far-field displacement spectral data. 5.2. Method of inversion Expanding (1) in Taylor series around some *
*
t*
trial estimate x” (~‘ i~ Q*), we can write, for the first order corrections to the starting model
c
2J,TT =
~
(4) R VkVI~T In (3) and (4) k is the rank of the WA matrix, and Sk, Vk, Uk, represent, respectively, the matrices of =
non zero eigenvalues and the corresponding eigenvectors of (WA)T(WA) and (WA)(WA)T (Lawson and Hanson, 1974; Aki and Richards, 1980). The errors on the computed parameters are obtained from the diagonal elements of C matrix. We have tested the developed algorithm with synthetic and real data. It turns out, as it can be seen from the form of eq. 1, that there is a strong correlation between Q and y, so it is impossible to determine both simultaneously. However, by fixing Q, the iterative inversion was very stable, and convergence to a stable minimum was always reached, dependent of the initial model; moreover, the final resolution was always maximum (R I). The quality factor was =
(2)
estimated for Phlegraean Fields area using coda wave techniques (del Pezzo et al., 1985). A Q varying from about 120 at 1 Hz to about 300 at 16 Hz was found. This value of attenuation refers to a volume sampled by coda waves with a linear
where: W is a weight matrix depending on the covariance matrix of the data (C~ WTW); A is the matrix of the partial derivatives of V(f) with respect to the model parameters (dimension: number of spectral points times 4); ~3x is the first order correction to the starting model, 6y is the vector of the residuals (observed V(f) minus the computed ones on the basis of the starting model).
dimension of about 20 km. An S wave composition for coda waves was assumed. A check in this sense was performed in the same paper using an independent and direct method to find S wave Q by the same data set used for coda analysis. The method is based on the hypothesis of a flat S wave spectrum below the corner frequency. Variations in slope were
=
WA 3x
=
,
,
W ~y =
32
treated as due to attenuation. An average S wave Q for the area of Phlegraean Fields resulted of 110 ±50, in good agreement with the low frequency coda Q. In the present study, we have fixed Q at various values in the range of those found by the previous analyses. For each spectrum the value of Q that best fit the data was chosen. However, the ~ and f0 parameters were barely influenced by large variations of Q, mainly due to the short travel times, so ~ and f~are practically unaffected by the assumed Q. By fixing Q, this method proved to be stable, efficient, and useful for routine analysis. The objectivity of the method, and the possibility to estimate statistical errors on the computed param-
eters, make it better than the traditional method of visual determination of spectral parameters. 5.3. Data analysis Thirty-two earthquakes (Table I), recorded in the period January—February 1984 by at least 4 digital station (181 three component records and 181 noise samples from the vertical component) have been analysed. The locations of the events, ranging in local magnitude between 0.7 and 3.2 (O.V. report, 1984), are shown in Fig. 6. They have been selected on the basis of the number and quality of the records plus a good azimuthal distribution of the recording stations.
TABLE I Time (Yr, mo, day, h, mm)
Latitude (degrees,
84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84
4049.92 4049.78 4049.42 4049.89 40 50.26 4049.79 4049.17 4049.85 4049.98 4049.72 4049.48 4049.70 4049.59 4049.91 4049.77 4049.31 4049.42 4049.57 4049.32 4049.30 4049.55 4049.79 4049.85 4049.86 4049.53 4050.51 40 49.67 4049.39 4049.63 4048.75 40 50.51 40 50.36
01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 02 02 02 02 02 02 02 02 02
15 16 16 17 17 18 19 21 22 23 24 24 26 27 27 28 29 30 30 30 30 31 31 10 10 11 15 16 17 18 19 20
03 31 19 00 23 51 02 58 04 48 02 06 04 53 13 39 13 45 05 43 04 24 22 50 20 21 23 43 23 53 21 06 04 22 00 22 00 40 00 44 06 01 00 36 02 20 05 48 1116 03 59 01 59 22 05 20 41 04 02 00 39 01 11
mm)
Longitude (degrees, mm)
Depth (km)
ERR.H (km)
ERR.Z (km)
14 8.46 14 8.52 14 7.59 14 8.53 14 8.04 14 8.65 14 8.86 148.25 14 8.84 14 8.77 14 8.14 14 7.79 14 8.25 14 8.68 14 8.57 14 8.76 147.85 14 8.05 14 7.96 14 7.97 14 8.52 14 8.08 14 8.24 14 8.22 148.25 148.19 146.59 147.54 14 7.54 14 9.40 14 8.16 14 8.39
0.7 2.8 2.1 1.5 3.3 2.6 2.4 1.2 2.0 2.6 0.9 2.3 1.9 1.2 2.4 2.5 2.2 1.7 2.3 2.1 0.7 2.6 2.3 2.5 2.4 1.8 2.9 2.3 2.5 2.3 2.3 2.9
0.2 0.2 0.2 0.2 0.4 0.3 0.3 0.9 0.3 0.3 0.4 0.2 0.6 0.5 0.3 0.8 0.2 0.4 0.2 0.2 0.3 0.4 0.7 0.2 0.2 0.2 0.2 0.1 0.2 0.3 0.2 0.3
0.1 0.1 0.4 0.2 0.5 0.1 0.1 1.8 0.4 0.1 0.2 0.2 0.4 0.8 0.2 0.4 0.1 0.5 0.1 0.1 0.2 0.2 0.4 0.1 0.1 0.2 0.1 0.1 0.1 0.1 0.1 0.1
33 I
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Fig. 6. Epicentre locations (a) and West—East section (b) of the 32 events selected for the source parameter analysis (numbers refer to the order in Table I).
34 0.7648
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(Hz)
Fig. 7. An example of radial and SB seismograms (a) together with their relative displacement spectra (b). Arrows indicate the portion of signals selected for spectral analysis. The continuous line on spectra represents the theoretical model computed by the inversion procedure.
35 1984
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Fig. 7 (continued).
The automatic procedure of data processing that we have developed consisted of the following steps: (1) rotation of the three ground components (V, N—S, E—W) to obtain P-radial and SH motions. P-radial and SH components were used to maximize the amplitude of the direct P wave, and the S component less contaminated by P motions. The azimuth for rotation is computed from the epicentre location; the incidence angle is cornputed by vectorial composition of first P pulse amplitudes by an average of 10 points around the maximum absolute value. 1.28 s of P phase have been selected on the radial seismogram, 2.56 s of S phase on the SH, and some seconds of noise on radial records: (2) multiplication of the P-radial, SH and noise signals with a 10% cosine taper window, removal of linear trends, and determination of amplitude spectra by FFT. The obtained spectra were smoothed by a moving averaging window 5 points long, to improve the resolution on the single estimates; (3) deconvolution of the instrument amplitude response curve in frequency domain and integration to obtain displacement spectra; and
(4) determination of standard deviations on the single spectral estimates from the noise spectrum, following a procedure suggested by Boatwright (1978). Figures 7a, b and 8 show examples of P-radial and SH seismograms and displacement spectra. The best fitting theoretical model computed by the inversion procedure is superimposed on each spectrum (continuous line). Seismic moments have been computed by the low frequency spectral levels ~ using the formula M
=
4irpv~,5R~0~’~ FR~
3 v~ 1.73 v with p 2.2 g cm 5 3 km 51; R hypocentral distance; and F free surface operator 2. Average P and SH radiation patterns KR~,) 0.4, (R~) 0.25 have been used, in agreement with Boore and Boatwright (1984), assuming normal fault mechanisms dipping 300_600, as we observed in the area (Fig. 5). Logarithmic weighted averages over all the records (Archuleta et al., 1982) have been computed to obtain mean low frequency spectral levels nor=
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Fig. 8 (continued).
malized to a distance of 3 km, and seismic moments (see Table III). Source radii have been computed using corner frequencies ~ by weighted logarithmic averages, based on the formulae of both Brune (1970) and Madariaga (1976) models (see Table III) Madariaga model r 0.32v5/f~” 0.21v5/f Brune model r 0.37v5/f =
=
=
Only SH corner frequencies have been used to estimate source radii with the Brune model. Madariaga model was used because it takes into
TABLE II Station Latitude
Longitude
Altitude
Name W02 W03 W04 W05 W06 W07 W10 W14 W15 W20 W21 W11
(degree, mm) 1408.40 1409.17 14 05.03 1411.58 1403.63 1405.74 1405.37 14 09.90 1409.84 1405.89 1407.67 14 07.25
(m) 100 35 70 457 45 170 50 40 150 306 175 82
(degree, mm) 40 49.52 40 50.77 40 50.11 4051.42 4048.59 40 51.58 4046.69 40 47.78 4049.51 40 51.14 40 50.47 40 49.26
38 I~
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Fig. 9. Seismic moment as a function of P-radial (a) and SB (b) corner frequencies. Dashed line is extrapolated from large earthquakes in Southern California (Hanks et al., 1975).
Fig. 10. Seismic moments vs. Madariaga’s source radii (a) and Brune’s (b) source radii. Bars indicate standard deviation errors (see the text).
account, properly, propagation and stopping of the rupture process for circular faults. Furthermore, it makes it possible to include in a natural way P-radial spectral estimates (Madanaga, 1976). Brune model, however, was also used to compare our results with previous works, which generally adopted this model. Stress drops have been computed by the aver-
age moments and source radii using the formula (Keilis-Borok, 1959) =
0.44KM )/(r)
3
All the obtained parameters together with estimated errors are shown in Table III. In Fig. 9a, b the plots of M0 vs. corner frequency P-radial and SH are shown. Figure lOa,
39
b shows the obtained scaling laws for the earthquakes of the Phlegraean Fields. Stress drops display a roughly constant pattern, with values around 20 bar (Madariaga model), and 4 bar (Brune model). We do not observe a decrease of stress drops at low seismic moments, that is present in many other areas. Small source radii (few tens of metres) are observed.
6. Discussion The inverse method used to estimate spectral parameters proved itself to be stable and efficient when Q is fixed at a known value. This technique gives estimates of errors on the spectral parameters; generally error estimates are not available when using the standard method of fitting ‘by eye’. Computed standard deviations from the inversion procedure have been used to obtain weighted log-averaged source parameters; the observed standard errors are reasonably small (Table III). This indicates that the method of weighting the single spectral estimates reduces the bias due to more scattered data, Furthermore the use of P-radial and SH displacement spectra revealed a fairly good coherence between the different source parameter estimates. Measurements of high frequency spectral decay have also been obtained by the inversion method: the observed values of y are rather high (around 2.5 on average) both for P-radial and SH spectra. In principle, however, the trade-off between Q and y at high frequencies can bias the measure of spectral fall-off. Measurements of Q~at Phlegraean Fields from coda waves range between 120 at 1.7 Hz to 300 at 16 Hz (del Pezzo et al., 1985): so we do not expect the spectra to be strongly influenced by attenuation at the observed distances (3—12 km). If the observed high frequency fall-offs are related to source properties, this data indicate earthquake ruptures that stopped themselves by a gradual deceleration, according to several kinematic models (Dahien, 1974; Boatwright, 1980). Such a behaviour can be qualitatively interpreted in terms of a ‘seismic gap’ mode of arrest (Hussemi et al., 1975) for fresh ruptures that nucleate in regions of high stress and stop gradually when
encountering lower stress zones. This type of model could be justified in this volcanic area by the presence of a heterogeneous stress distribution in the seismogenic volume. The investigated range of seismic moments is just below that found in most source parameter studies of microearthquakes (Archuleta et al., 1982; Fletcher et al., 1984). The constant pattern of stress drops is different from many other microearthquake sequences that show a tendency for corner frequencies of very small earthquakes (M <2—3; M0 < 1019_1020 dyne-cm) to become constant (Rautian and Khalturin, 1978; Chouet et al., 1978; Archuleta et al., 1982). Recently Brune et al. (1985) have analysed data from the ANZA digital array covering a wide range of seismic moments (1017_1021 dyne-cm). Source parameters from the ANZA array also show an increase of Brune and RMS stress drop with moment. Deviations from constant stress drop are interpreted in terms of source (Aki, 1985; McGarr, 1986) or local attenuation effects (Hanks, 1982). Recently Aki (1985) proposed the self-similarity to be valid within individual ranges of seismic moments. Our data could support this hypothesis, because we observe a nearly constant stress drop within our range of seismic moments, but with considerably smaller values (—~4 bar) with respect to those observed for larger events in other seismic Sequences (— 100 bar) (Aki, 1985).
7. Conclusions The Phlegraean Fields earthquake sequence accompanying the uplift episode 1982—1984 has been analysed. Seismic activity appears to be connected with the main tectonic structures in the area. In particular the earthquakes are roughly aligned along the ‘Starza marine terrace’ around which most of recent volcanic eruptions occurred. The focal mechanisms are mainly normal faults, dipping 30—60 showing a general tensional stress pattern. The spectral analysis of 32 microearth. quakes recorded by a digital three-component network (543 records) has been performed to obtain seismic moments source radii and stress drop estimates. An inverse method to fit displacement O,
40 TABLE III N
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
Time
01150331 01161900 01162351 01170258 01170448 01180206 01190453 01211339 01221345 01230543 01240424 01242250 01262021 01272343 01272353 01282126 01290422 01300022 01300040 01300044 01300601 01310036 01310220 02100548 02101116 02110400 02150159 02162205 02172041 02180402 02190039 02200111
ML
1.1 1.6 1.0 1.6 1.3 1.3 1.9 —
1.5 3.4 1.6 0.7 2.6 1.5 1.7 1.9 1.6 1.5 1.6 1.8 2.3 2.1 1.7 1.9 1.6 0.9 1.9 1.9 1.9 1.9 1.8 1.7
Depth
(~2 0Rad) (cm s)
SD
(~2~ Sh~ (cm s)
SD
~Mo) (dyn cm)
SD
(km) 0.7 2.8 2.1 1.5 3.3 2.6 2.4 1.2 2.0 2.6 0.9 2.3 1.9 1.2 2.4 2.5 2.2 1.7 2.3 2.1 0.7 2.6 2.3 2.5 2.4 1.8 2.9 2.3 2.5 2.3 2.3 2.9
0.29E—5 0.71E—5 0.23E—5 0.32E—5 0.24E—5 0.33E—5 0.26E—4 0.28E—5 0.67E—5 0.43E—3 0.61E—5 0.73E—5 0.1OE—3 0.61E—5 0.73E—5 0.98E—5 0.42E—5 0.13E—5 0.40E—5 0.60E—5 0.43E—4 0.64E—4 0.46E—5 0.23E—4 0.82E—5 0.94E—6 0.21E—4 0.15E—4 0.20E—4 0.12E—4 0.42E—5 0.43E—5
0.23E—5 0.46E—5 0.53E—6 0.13E—5 0.11E—5 0.13E—5 0.23E—4 0.20E—5 0.44E—5 0.20E—3 0.22E—5 0.24E—5 0.48E—4 0.43E—5 0.53E—5 0.11E—4 0.34E—5 0.43E—6 0.22E—5 0.20E—5 0.34E—4 0.23E—4 0.25E—5 0.13E—4 0.65E—5 0.17E—6 0.15E—4 0.14E—4 0.33E—5 0.72E—5 0.23E—5 0.37E—5
0.89E—5 0.38E—4 0.72E—5 0.19E—4 0.23E—4 0.93E—5 0.91E—4 0.93E—5 0.35E—4 0.15E—2 0.26E—4 0.31E—4 0.96E—3 0.28E—4 0.30E—4 0.43E—4 0.22E—4 0.80E—5 0.19E—4 0.33E—4 0.17E—3 0.29E—3 0.24E—4 0.15E—3 0.44E—4 0.93E—5 0.89E—4 0.69E—4 0.14E—3 0.11E—3 0.40E—4 0.29E—4
0.59E—5 0.39E—4 0.24E—5 0.16E—4 0.93E—5 0.83E—5 0.87E—4 1.OE—4 0.30E—4 0.38E—4 0.21E—4 0.24E—4 0.63E—3 0.24E—4 0.29E—4 0.45E—4 0.14E—4 0.54E—5 0.91E—5 0.21E—4 0.12E—3 0.12E—3 0.21E—4 0.92E—4 0.38E—4 0.15E—5 O.91E—4 0.63E—4 0.45E—4 0.53E—4 0.30E—4 0.23E—4
0.83E+18 0.25E+19 0.66E+18 0.11E+19 0.12E+19 0.91E+18 0.79E+19 0.82E+18 0.24E+19 0.13E+21 0.19E+19 0.23E+19 0.45E+20 0.21E+19 0.23E+19 0.33E+19 0.16E+19 0.47E+18 0.14E+19 0.21E+19 0.14E+20 0.21E+20 0.16E+19 0.91E+19 0.30E+19 0.50E+18 0.68E+19 0.52E+19 0.74E+19 0.61E+19 0.19E+19 0.18E+19
0.10+17 0.71+18 0.14+17 0.35+18 0.62+18 0.27+17 0.61+18 0.40+17 0.68+18 0.19+19 0.33+18 0.37+18 0.23+20 0.44+18 0.36+18 0.64+18 0.43+18 0.15+18 0.34+18 0.60+18 0.16+19 0.35+19 0.44+18 0.30+19 0.89+18 0.30+18 0.12+19 0.12+19 0.30+19 0.33+19 0.11+19 0.75+18
9.2 9.5 13.1 11.9 4.3 14.9 10.0 12.9 11.5 3.4 13.8 14.4 5.3 11.9 10.5 11.4 14.9 16.8 14.9 12.1 7.7 6.0 13.9 7.0 14.2 22.0 13.7 13.1 10.1 9.3 17.3 12.5
NS * is the number of stations. The number of records (radial and SH) used to estimate the source parameters is indicated in the brackets.
spectra for obtaining spectral parameters was developed. The method proved to be stable and efficient when the quality factor Q was fixed. One of the advantages of using this procedure is the possibility to obtain error estimates (from the covariance matrix) on the computed spectral parameters. The main result is the lack of a clear moment dependence of stress-drop, within the investigated range of seismic moment (1017_1020 dyne-cm). As a consequence small source radii are observed.
Acknowledgement The authors would like to thank R. Scarpa for many helpful discussions and comments.
References .
.
Ak, K., 1986. Physical Theory of Earthquakes. Proceedings of Strasbourg, 1986 summer school on Seismic Hazard in Mediterranean Regions. May 1985 Columbia University, New York, in press.
41
SD (Hz)
(FcSH)
3.0 2.2 6.6 3.5 1.4 4.4 1.5 2.6 4.1 0.2 2.1 1.8 1.4 3.9 3.2 3.9 6.3 3.8 4.5 2.5 1.8 2.3 8.3 2.2 2.8 4.1 3.9 4.3 3.8 4.3 4.3 2.8
11.1 11.3 13.5 8.8 7.4 15.2 7.7 13.7 8.3 4.2 11.4 12.6 4.6 12.3 8.5 10.0 14.9 17.8 14.2 13.8 9.2 5.5 12.4 6.4 12.0 13.3 11.5 9.8 6.8 8.0 12.4 11.6
KR-MAD)
SD (Hz)
(m)
4.3 3.5 3.2 3.7 3.0 5.1 2.9 1.6 1.3 1.0 3.2 3.4 0.8 3.5 3.4 2.7 2.5 2.0 5.0 4.1 2.6 1.7 5.1 2.6 2.4 2.3 3.8 3.0 2.5 5.3 4.6 3.3
45.7 45.4 31.1 44.3 82.7 30.2 52.8 31.7 45.1 145.4 37.0 35.1 88.5 36.6 48.0 41.3 27.5 24.0 31.4 36.5 55.4 76.1 33.3 68.6 34.6 26.3 36.1 39.6 54.3 53.3 30.9 38.1
SD
SD
K~OM) (bar)
SD
K~B) (bar)
SD
NS(*)
13.6 13.2 6.6 2.6 39.6 6.7 3.7 7.3 2.0 34.0 4.0 4.6 11.9 8.2 5.0 5.9 5.2 5.4 5.8 9.9 16.6 12.6 5.0 11.1 4.4 1.0 4.3 2.7 8.1 6.9 1.3 6.6
58.8 57.7 48.1 73.9 157.0 42.8 84.2 47.6 78.1 286.0 57.1 51.7 164.7 53.0 76.0 64.9 43.7 36.5 45.9 47.1 105.3 142.6 52.5 130.7 54.3 48.8 56.6 66.3 95.6 81.4 52.5 55.9
22.9 18.2 11.4 30.9 85.0 14.4 31.4 5.6 12.7 80.8 16.1 13.8 32.4 15.3 30.1 17.2 7.3 4.1 16.2 13.9 38.2 32.6 21.5 29.5 11.1 8.5 18.8 20.5 34.7 54.2 19.4 16.0
3.8 11.6 9.6 5.7 0.9 14.4 23.6 11.2 11.5 18.3 16.6 23.1 28.5 18.5 9.2 20.7 33.4 15.2 20.3 18.6 34.9 20.7 19.0 12.3 32.1 12.1 62.8 36.7 20.2 17.6 28.3 14.6
3.4 10.7 6.1 2.0 1.4 9.5 5.3 7.7 3.6 12.9 6.1 9.9 18.7 13.0 3.2 9.2 20.9 11.4 12.2 16.0 31.6 10.8 10.0 7.2 15.4 7.4 25.5 11.2 8.3 11.8 16.4 9.7
1.8 5.7 2.6 1.2 0.1 5.1 5.8 3.3 2.2 2.4 4.5 7.3 4.4 6.2 2.3 5.3 8.4 4.2 6.3 8.8 5.2 3.2 4.8 1.8 8.2 1.9 16.4 7.8 3.7 4.9 5.7 4.5
2.1 5.6 1.8 1.5 0.2 5.1 6.5 1.2 1.2 2.1 3.9 5.9 3.4 5.5 2.7 4.3 4.8 2.0 6.9 8.2 5.7 2.2 6.1 1.3 5.6 1.5 16.6 7.5 4.3 10.2 7.2 4.3
4(8] 5[9] 3(6] 7(13] 6(9] 6(11] 7[13] 3(6] 5(10] 2[4] 6(10] 4[8] 5(9] 7(13] 6(12] 7(11] 6(101 6(9] 7(13] 8115] 6112] 5(9] 6(11] 6(11] 5[6] 4(7] 6(11] 5(10] 4(7] 6(10] 6(11] 7(11]
Aki, K. and Richards, P.G., 1980. Quantitative Seismology: Theory and Methods. Freeman and Co., San Francisco, CA. Archuleta, R.J., Cranswick, E., Mueller, C. and Spudich, P., 1982. Source parameters of the 1980 Mammoth Lakes, California, earthquake sequence. J. Geophys. Res., 87: 4595—4607. Boatwright, J., 1978. Detailed spectral analysis of two small New York State earthquakes. Bull. Seismol. Soc. Am., 68: 1117—1131. Boatwright, J., 1980. A spectral theory for circular seismic sources: simple estimates of source dimension, dynamic stress drop and radiated energy. Bull. Seismol. Soc. Am., 71: 69—94.
Boore, D. and Boatwright, J., 1984. Average body-wave radiation coefficient. Bull. Seismol. Soc. Am., 74: 1615—1621. Brune, J.W., 1970. Tectonic stress and the spectra of seismic shear waves from earthquakes. J. Geophys. Res., 75: 4997—5009. Brune, J.N., Fletcher, J., Vernon, F., Haar, L., Hanks, T. and Berger, J., 1985. Low stress-drop earthquakes in the light of new data from the ANZA, California telemetered digital array. Maurice Ewing Series, Vol. 6, Am. Geophys. Union, Geophysics Monograph 37, Washington, DC, pp. 237—245. Chouet, B., Aki, K. and Tsujura, M., 1978. Regional variation of the scaling law of earthquake source spectra. Bull. Seismol. Soc. Am., 68: 49—80.
42 Corrado, G., Guerra, I., Lo Bascio, A., Luongo, G. and Rampoldi, R., 1976. Inflation and microearthquake activity at Phlegraean Fields, Italy. Bull. Volcanol., 40—43: 1—20. Crosson, R.S., 1976. Crustal structure modeling of earthquake data. Part 1. Simultaneous least squares estimation of hypocenter and velocity parameters. J. Geophys. Res., 81: 3036—3046. Crosson, R.S., 1981. LQUAKE: a computer program for hypocenter locations. Tech. Rep. Univ. of Washington, Seattle. Dahlen, F.A., 1974. On the ratio of P-wave to S-wave corner frequencies for shallow earthquake sources. Bull. Seismol. Soc. Am., 64: 1159—1180. Del Pezzo, E., De Natale, G. and Zollo, A., 1984. Space-time distribution of small earthquakes at Phlegraean Fields. Bull. Volcanol., 47: 201—207. Del Pezzo, E., De Natale, G., Scarcella, G. and Zollo, A., 1985.
Q5
of three-component seismograms of volcanic microearthquakes at Campi Flegrei volcanic area—Southern Italy. Pageoph, 123: 683—696. Di Vito, M., Lirer, L., Mastrolorenzo, G. and Scandone, R., 1985. Volcanological map of Campi Flegrei. Edited by: Dipartimento di Geofisica and Vulcanolgia. Universitá di Napoli. Fletcher, J.B., 1980. Spectra from high dynamic range digital recordings of Oroville, California, aftershocks and their source parameters. Bull. Seismol. Soc. Am., 70: 735—755. Fletcher, J.B., Boatwright, J., Haar, L., Hanks, T. and McGarr, A., 1984. Source parameters for aftershocks of the Oroville, California, sequence. Bull. Seismol. Soc. Am., 74: 1101—1123. Frankel, A., 1981. Source parameters and scaling relationships of small earthquakes in the northeastern Caribbean. Bull. Seismol. Soc. Am., 71: 1173—1190. Gaudiosi, G. and lannaccone, G., 1985. Preliminary study of stress pattern at Phlegraean Fields as inferred from focal mechanisms. Bull. Volcanol., 47: 226—231. Hanks, T.C., 1982. Fmax. Bull. Seismol. Soc. Am., 72: 1867—1897.
Hanks, T.C., Hileman, T. and Thatcher, W., 1975. Seismic moments of the larger earthquakes of the Southern California region. Bull. Geol. Soc. Am., 86: 1131—1139. Husseini, MI., Jovanovich, B.B., Randall, M.J. and Freund, J.B., 1975. The fracture energy of earthquakes. Geophys. J., 43: 367—385. Keilis-Borok, V.1., 1959. On estimation of the displacement in an earthquake source dimensions. Ann. Geofis., 12: 205—214. La Torre, P. and Nannini, R., 1980. Geothermal well location in Southern Italy: The contribution of geophysical methods. Boll. Geof. Teor. AppI., 22: 201—209. Lawson, CL. and Hanson, R.J., 1974. Solving Least Squares Problems. Prentice Hall, Englewood Cliff, NJ. Madariaga, R., 1976. Dynamics of an expanding circular fault. Bull. Seismol. Soc. Am., 66: 639—666. Marquardt, D.W., 1963. An algorithm for least squares estimation of non linear parameters. J. Soc. md. Appl. Math., 11: 431—441. McGarr, A., 1986. Some observations indicating complications in the nature of earthquake scaling. Maurice Ewing Series, Vol. 6, Am. Geophys. Union, Geophysics Monograph 37, Washington, DC, pp. 217—226. Osservatorio Vesuviano Report, 1984. Powell, L.A., 1983. Engineering description of the U.W. portable digital seismograph. Proceedings of the Workshop on portable digital seismograph development, IASPEI Meeting, Los Altos, CA. Rautian, T.G. and Khaltunn, V.1., 1978. The use of coda for determination of the earthquake source spectra. Bull. Seismol. Soc. Am., 68: 49—80. Rosi, M., Shrana, A. and Principe, C., 1983. The Phlegraean Fields: structural evolution, volcanic history and eruptive mechanisms. J. Volcanol. Geotherm. Res., 17: 237—288. Scarcella, G., 1984. Metodi di inversione di dati sismici: applicazioni nd Basso Tirreno e nei Campi Flegrei. B.Sc. Physics Thesis, University of Naples.
25
Source parameters of microearthquakes at Phlegraean Fields (Southern Italy) volcanic area Edoardo Del Pezzo, Giuseppe De Natale, Marcello Martini and Aldo Zollo Osservatorio Vesuviano-80056 Ercolano (NA) (Italy) (Received January 2, 1986; revision accepted July 2, 1986)
Del Pezzo, E., Dc Natale, G., Martini, M. and Zollo, A., 1987. Source parameters of microearthquakes at Phlegraean Fields (Southern Italy) volcanic area. Phys. Earth Planet. Inter., 47: 25—42. The seismic activity that occurred at the Pblegraean Fields (Southern Italy) volcanic area during a pronounced episode of ground uplift has been analysed. One hundred and eighty-one three component seismograms from a digital network operating in the period January—May 1984 were processed to obtain seismic moments, source radii and stress drops for 32 microearthquakes (0.7
1. Introduction The ‘Phlegraean Fields’ is an active volcanic area 10 km from the city of Naples, Southern Italy. This region is located within an ancient caldera formed after an ignimbritic eruption about 35000 years ago. Many eruptions have occurred since the formation of the caldera; during the last one (A.D. 1538) a small volcano, Mt. Nuovo, was formed inside the caldera (Fig. 1). Two marked ground uplift episodes took place in the area in 1970 (Corrado et al., 1976) and in 1982, with a cumulative uplift of more than 2 m at the maximum point of ground deformation. The last one began in the second half of 1982 and produced a maximum uplift of 1.7 m (measured at the town of Pozzuoli) in 2 years. An increase of seismic activity followed the ground deformation, reaching a very high daily frequency (Fig. 2) with peaks in October 1983 and April 1984. During the period January 1983 to 0031-9201/87/$0350
© 1987 Elsevier Science Publishers B.V.
September 1984 more than 10000 events, with local magnitude ranging between 0.1 and 4.2, were recorded. A local seismic monitoring network consisting of 22 vertical component stations by Osservatorio Vesuviano and AGIP (Italian Petrol Agency) operates in the area. On 1 January 1984, 12 three-component digital stations, which were owned by the University of Wisconsin (Madison), were installed in the area, in the framework of a scientific cooperation between Osservatorio Vesuviano and the University of Wisconsin. The main purposes of the experiment were to improve earthquake locations and to determine properties of the source and medium from spectral and time domain analyses of seismic signals. A suite of 181 three-component seismograms from 32 selected events were used in this study to estimate seismic source parameters (seismic moments, source radii and static stress drops). This type of analysis has been extensively applied to
Vokanologicsl
3
2
El
ml
::::::: . .. .
,j::::::::l (P”mlces
Base surge type (Pm,. ces. ashes and lithlc fragments. wlfh lexfural faces 01 Sand WP“as. Massive and Planar). The NYT depos,ts exh,b,,s sed,mentary S,r”Cf”res ,ransl,lonal between a surge deposit and a How depost
deposits
Wl,h Interbedded ash.la”els and ““welded and sonlellrne welded scome).
PyrOClaStIC fall depos115
Domes and lava flows
Fader
Fig. 1. Volcanological map of Phlegracan &Ads @fter di Vito et al. (1985)).
:
27 DAILY 300.0
EARTI-(QUAKE FREQUENCY
,,,,,,,,,,,,,,,,,
~
497, 200.0. U I
May,1983
May,1984 DAYS
Fig. 2. Time history of earthquake activity during the period May 1983—May 1984 given in number of events per day. Each division on the time axis represents 10 days.
many volcanic and tectonic regions (Fletcher, 1980; Frankel, 1981; Archuleta et a!., 1982; Fletcher et al., 1984). A central concern of this study is a microearthquake data set that cover~an unusual local magnitude range (0.7—3.2), which is just below the range covered by researchers cited above. Moreover, estimates from both P-radial and SH motions recorded by many azimuthally well-distributed seismic stations improved the quality of source parameter measurements, The use of an inverse method to determine spectral parameters (low frequency spectral level, corner frequency and high frequency spectral fall off) permitted us to obtain objective estimates and confidence intervals for these source parameters. It is then possible to weight the measurements from a single spectrum by their standard deviations, to reduce bias due to scattered data and to give more reliable error estimates on source parameters.
2. Volcanological setting of the area The history of the Phlegraean Fields volcanism has been characterized by the occurrence of many eruptive episodes, most of them associated with monogenetic pyroclastic vents. The main event that generated the caldera 35 000 years ago, according to Rosi et al. (1983), was an eruption of a huge alkali trachytic ignimbrite followed by a caldera collapse. Stratigraphic sections from a
detailed field survey, geophysical investigations and geothermal deep drilling have reconstructed the structural evolution of the area related to the volcanic activity (La Torre and Nannini, 1980; Rosi et al., 1983). Figure 1, by di Vito et al. (1985), shows the geological map of Phlegraean Fields as deduced from field survey data. Geothermal well data collected inside and on the rim of the caldera reveals the presence of tuffites with trachytic lava intercalations at deeper layers (2000 m) due to volcanic activity (mainly submarine) after the caldera collapse until about 10000 years ago. Shallower layers, which are mainly composed of yellow tuffs and tephra, were probably generated during the recent subaerial activity. In the last 10 000 years all volcanic activity has been confined to the depressed part of the caldera. This was mostly of explosive type (Pliian, Subplinian and Strombolian) and subaerial. Magma— water interactions at depth played an important role in controlling explosive eruptive mechanisms and the differentiation of eruptive products as can be inferred from observed traces of phreato-magmatic eruptions (Rosi et al., 1983). Most of the recent eruptions are related to the post-caldera subaerial activity which started about 4500 years ago. This recent phase was preceded by a local tectonic event, the uplift of the northern sector of the Gulf of Pozzuoli (Fig. 1), which is represented by the ‘Starza marine terrace’ which dates back to about 5000 years ago and is probably due to magma injection at shallow depth
28
(Rosi et al., 1983). All the successive eruptions, up to the last Mt. Nuovo, occurred near and around this local geomorphological discontinuity,
gain ranging information, time code and Omega radio navigation time signal were recorded on 4 tracks audio—grade tape recorders. Six hours of recording time was available for each tape, i.e. a maximum of 360 1-mn-events or 720 30-s-events. A 512 sample (5.12 s) pre-event memory allows the recovery of what triggered the system and pre-event background noise. Amplitude response curve is almost flat to velocity between 1 and 25 Hz. For more details see Powell (1983).
3. Instrwnentation The University of Wisconsin digital recording seismographs are wide dynamic range (106 db) instruments. Earthquake signals are detected by an automatic triggering system and recorded in digital form on 1/4 in. field tapes. Triaxial sensors, which were comprised of HS1O1 type seismometers with a natural frequency of 1 Hz, were used. Geophone signals were preamplified, anti-alias filtered by a 4 poles Butterworth low-pass filter with 24 Hz corner frequency. Each signal was digitized at 100 sample s’ by a wide-range analog-to-digital 12 bit converter provided with a gain ranging amplifier. Sampled data,
4. Seismicity and earthquake locations Figure 3 shows a map of 450 selected epicentres recorded in the period August 1983 to May 1984 by the operating networks displayed in the same figure. Also shown are the locations of the seismic networks installed by Osservatorio Vesuviano and AGIP, consisting of vertical component geo-
A A A
A
A
A
A
A
~ A A A
~
~
°
A
O~
A ~
~:~ -.
A A
-
Fig. 3. Map of 450 selected epicentres of events of Phlegraean Fields; solid triangles are University of Wisconsin digital stations while the open ones indicate the Osservatorio Vesuviano-AGIP survey network. Locations of digital stations are given in Table II.
29
A
I
Z~ I
I
N
___
‘‘.t•~.•+’.~”~
~
*
/
÷
I
/
I
~
~
I
~.
..
.~y
~. •+
~
.
~
..~
‘
•+++.f*.
/
(a)
+
M•~
÷
.+~t1.t÷
1•
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S (b)
Moo
-
2_ 4
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6
0
-
~
0
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-
-
0
0566.: LAT AO—S0.000 LOSS. 64— 8.000 AZIMUTA— 0. DUO LENSTH. 50.0 KM DKPIM— 60.0 KM SC.6L5 FACT. 0.0 WODTM
20.0 68
Fig. 4. Three-dimensional view (a) and North—South section (b) of the events shown in Fig. 3.
30
phones whose signals are telemetered and recorded in analog form on magnetic tapes. The readings from the survey network were performed by high speed playbacks on paper, whereas the digital seismograms where read by a semi-automatic procedure of time picking and accumulation on files of the P and S first arrivals, using graphic displays by an HP-bOO computer. Figure 4a, b shows a 3-D view and a N—S section of the same selected events. These events have been located using first P arrival times from the survey network records and P and S first arrival times recorded by the digital stations during the operating period, The estimated average reading error is about 0.05 s on P wave arrival times from the survey network, 0.02 s for P wave and 0.05 s for S wave readings from the digital stations. The computer program LQUAKE (Crosson, 1981) was used to locate the events. High quality locations were defined according to the following criteria: (1) minimum number of readings 12; (2) maximum RMS residual 0.15 s; (3) maximum azimuthal gap between the recording stations 1500; and
(4) maximum hypocentral error 0.5 km. A homogeneous half space with V~, 3 km ~ and V~ 1.7 km s~ was used as the velocity model according to preliminary results obtained by Scarcella (1984), who applied Crosson’s (1976) inverse technique to find the best layered velocity model at Phlegraean Fields. These results show the lack of evidence of marked discontinuities in the first few kilometres of the crust beneath the seismic network. Direct measurements made in holes drilled at the borders of the Phagraean Fields zone by AGIP agree with low values of P wave velocity ranging between 1.7 and 3.5 km s1 down to about 3.0—3.5 km. The selected sample of seismicity shown in Figs. 3 and 4 is representative of the whole sequence. The earthquakes are concentrated in a small volume (about 4 x 4 x 4 km3) that approximately corresponding to the area of maximum uplift. No spatial variation with time of the hypocentres is evident (del Pezzo et al., 1984). Moreover, the earthquakes (except those located in the Gulf of Pozzuoli) appear clustered along the main tectonic structure in this area: the ‘Starza marine terrace’ (Fig. 1). Figure 5 shows some focal mechanisms inferred from P-wave polarities. A =
=
=
=
=
=
/
_
(
S
C
\~ 2
0
Fig. 5. Wulff projection lower hemisphere focal mechanisms of a set of events (2. (survey network) (after Gaudiosi and lannaccone, 1985).
~
ML ~ 4.) recorded at a minimum of 15 stations
31
minimum of 15 stations were used for each mechanism (Gaudiosi and lannaccone, 1985). All the focal mechanisms of events aligned on the Starza marine terrace are normal faults dipping 30 o60 0 with minor strike-slip components, while the two events in the Gulf of Pozzuoli show compressive mechanisms. We believe that the seismicity is mainly connected with the activation of pre-existing structures, as a response to magmatic activity,
Solving iteratively the system of eq. 2 by a linear inversion method up to some convergence criterion is satisfied, we obtain the best estimates of the model parameters. We have used, for each inversion step, the S.V.D. algorithm (Lawson and Hanson, 1974) with truncation of small singular values and Levenberg—Marquardt damping (Marquardt, 1963), due to the non-linearity of the problem. At each step we compute the correction
5. Source parameter analysis
~ ~y.= W6y =
(3)
&YW
5.1. Spectral model and the Covariance and Resolution matrices In agreement with the most used theoretical models of seismic sources (Brune, 1970; Madariaga, 1976; Boatwright, 1980), the far-field displacement spectrum V( f) of an earthquake can be described by Log V( f)
=
Log ~ — O.43~JT/Q —0.5 Log(1
+
(f/f~)2~~)
(‘-)
(Boatwright, 1978), where: ~ is the low frequency spectral level; f~is the corner frequency; y determines the high frequency decay of the spectrum; Q is the average quality factor along the ray path from the event to the recording station; and T is the travel time. We can infer these parameters by an iterative non-linear inversion of far-field displacement spectral data. 5.2. Method of inversion Expanding (1) in Taylor series around some *
*
t*
trial estimate x” (~‘ i~ Q*), we can write, for the first order corrections to the starting model
c
2J,TT =
~
(4) R VkVI~T In (3) and (4) k is the rank of the WA matrix, and Sk, Vk, Uk, represent, respectively, the matrices of =
non zero eigenvalues and the corresponding eigenvectors of (WA)T(WA) and (WA)(WA)T (Lawson and Hanson, 1974; Aki and Richards, 1980). The errors on the computed parameters are obtained from the diagonal elements of C matrix. We have tested the developed algorithm with synthetic and real data. It turns out, as it can be seen from the form of eq. 1, that there is a strong correlation between Q and y, so it is impossible to determine both simultaneously. However, by fixing Q, the iterative inversion was very stable, and convergence to a stable minimum was always reached, dependent of the initial model; moreover, the final resolution was always maximum (R I). The quality factor was =
(2)
estimated for Phlegraean Fields area using coda wave techniques (del Pezzo et al., 1985). A Q varying from about 120 at 1 Hz to about 300 at 16 Hz was found. This value of attenuation refers to a volume sampled by coda waves with a linear
where: W is a weight matrix depending on the covariance matrix of the data (C~ WTW); A is the matrix of the partial derivatives of V(f) with respect to the model parameters (dimension: number of spectral points times 4); ~3x is the first order correction to the starting model, 6y is the vector of the residuals (observed V(f) minus the computed ones on the basis of the starting model).
dimension of about 20 km. An S wave composition for coda waves was assumed. A check in this sense was performed in the same paper using an independent and direct method to find S wave Q by the same data set used for coda analysis. The method is based on the hypothesis of a flat S wave spectrum below the corner frequency. Variations in slope were
=
WA 3x
=
,
,
W ~y =
32
treated as due to attenuation. An average S wave Q for the area of Phlegraean Fields resulted of 110 ±50, in good agreement with the low frequency coda Q. In the present study, we have fixed Q at various values in the range of those found by the previous analyses. For each spectrum the value of Q that best fit the data was chosen. However, the ~ and f0 parameters were barely influenced by large variations of Q, mainly due to the short travel times, so ~ and f~are practically unaffected by the assumed Q. By fixing Q, this method proved to be stable, efficient, and useful for routine analysis. The objectivity of the method, and the possibility to estimate statistical errors on the computed param-
eters, make it better than the traditional method of visual determination of spectral parameters. 5.3. Data analysis Thirty-two earthquakes (Table I), recorded in the period January—February 1984 by at least 4 digital station (181 three component records and 181 noise samples from the vertical component) have been analysed. The locations of the events, ranging in local magnitude between 0.7 and 3.2 (O.V. report, 1984), are shown in Fig. 6. They have been selected on the basis of the number and quality of the records plus a good azimuthal distribution of the recording stations.
TABLE I Time (Yr, mo, day, h, mm)
Latitude (degrees,
84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84 84
4049.92 4049.78 4049.42 4049.89 40 50.26 4049.79 4049.17 4049.85 4049.98 4049.72 4049.48 4049.70 4049.59 4049.91 4049.77 4049.31 4049.42 4049.57 4049.32 4049.30 4049.55 4049.79 4049.85 4049.86 4049.53 4050.51 40 49.67 4049.39 4049.63 4048.75 40 50.51 40 50.36
01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 02 02 02 02 02 02 02 02 02
15 16 16 17 17 18 19 21 22 23 24 24 26 27 27 28 29 30 30 30 30 31 31 10 10 11 15 16 17 18 19 20
03 31 19 00 23 51 02 58 04 48 02 06 04 53 13 39 13 45 05 43 04 24 22 50 20 21 23 43 23 53 21 06 04 22 00 22 00 40 00 44 06 01 00 36 02 20 05 48 1116 03 59 01 59 22 05 20 41 04 02 00 39 01 11
mm)
Longitude (degrees, mm)
Depth (km)
ERR.H (km)
ERR.Z (km)
14 8.46 14 8.52 14 7.59 14 8.53 14 8.04 14 8.65 14 8.86 148.25 14 8.84 14 8.77 14 8.14 14 7.79 14 8.25 14 8.68 14 8.57 14 8.76 147.85 14 8.05 14 7.96 14 7.97 14 8.52 14 8.08 14 8.24 14 8.22 148.25 148.19 146.59 147.54 14 7.54 14 9.40 14 8.16 14 8.39
0.7 2.8 2.1 1.5 3.3 2.6 2.4 1.2 2.0 2.6 0.9 2.3 1.9 1.2 2.4 2.5 2.2 1.7 2.3 2.1 0.7 2.6 2.3 2.5 2.4 1.8 2.9 2.3 2.5 2.3 2.3 2.9
0.2 0.2 0.2 0.2 0.4 0.3 0.3 0.9 0.3 0.3 0.4 0.2 0.6 0.5 0.3 0.8 0.2 0.4 0.2 0.2 0.3 0.4 0.7 0.2 0.2 0.2 0.2 0.1 0.2 0.3 0.2 0.3
0.1 0.1 0.4 0.2 0.5 0.1 0.1 1.8 0.4 0.1 0.2 0.2 0.4 0.8 0.2 0.4 0.1 0.5 0.1 0.1 0.2 0.2 0.4 0.1 0.1 0.2 0.1 0.1 0.1 0.1 0.1 0.1
33 I
0.4
Iso
141
5
(a) I. wos
A ~
w20
A
kp3
~
A’
W2’
1~’~~~’ A(c~~G!
W04
0
-
‘IL-) 1~
0
;~-1CJ1)~
I
tL~w10
NEST—EAST 0.0
0415
1.4100
5
SECTION
PHLEGRAEAN
FIELDS ______________
__________________________________________________________________
II
(b) ~J~21 —1.0
[951
91° [94
rita -2.0..
~jaa
[9 29 —3.0
~
[!129 [1113
ms9l2° ~ E]~jta
[I)~
m23
[9
[1122
~
06
912
- C!]
[932
[95
—.40 B.E
I
7.5
I
8.5
9,5
Fig. 6. Epicentre locations (a) and West—East section (b) of the 32 events selected for the source parameter analysis (numbers refer to the order in Table I).
34 0.7648
~
_____________________________________________________ 0.00
3.00
6.00
9.00
12.00
15.00
9.00
12.00
15.00
SH Component
0.6185
-
K
0.00
~
3.00 1984.01.30
19e4.o1.30
—~. 0
,,.
6.00
Radial component
06: 01 sta W20
08:01
Sta W20
SH
r-r—r—rrr-
(b) —4.0 0
as
UI *
E
a 3 ,M
6 4 01 0 J
-
—7.0
—8.0 -0.3
0.0
0.3
0.8
0.9
Log (Frequency
1.2
1.5
1.8
(Hz)
Fig. 7. An example of radial and SB seismograms (a) together with their relative displacement spectra (b). Arrows indicate the portion of signals selected for spectral analysis. The continuous line on spectra represents the theoretical model computed by the inversion procedure.
35 1984
01
30
06: 01
Sta 1420
~adia1
—3.0 (b)
~3
-4.0.
as
01
* E
a as
0
-
-5.0.
a E 4 DI
—6.0.
—7.0 -0.3
0.0
0.3
0.9 Log [Frequency
0.6
1.2
1.5
1.8
(Hz) I
Fig. 7 (continued).
The automatic procedure of data processing that we have developed consisted of the following steps: (1) rotation of the three ground components (V, N—S, E—W) to obtain P-radial and SH motions. P-radial and SH components were used to maximize the amplitude of the direct P wave, and the S component less contaminated by P motions. The azimuth for rotation is computed from the epicentre location; the incidence angle is cornputed by vectorial composition of first P pulse amplitudes by an average of 10 points around the maximum absolute value. 1.28 s of P phase have been selected on the radial seismogram, 2.56 s of S phase on the SH, and some seconds of noise on radial records: (2) multiplication of the P-radial, SH and noise signals with a 10% cosine taper window, removal of linear trends, and determination of amplitude spectra by FFT. The obtained spectra were smoothed by a moving averaging window 5 points long, to improve the resolution on the single estimates; (3) deconvolution of the instrument amplitude response curve in frequency domain and integration to obtain displacement spectra; and
(4) determination of standard deviations on the single spectral estimates from the noise spectrum, following a procedure suggested by Boatwright (1978). Figures 7a, b and 8 show examples of P-radial and SH seismograms and displacement spectra. The best fitting theoretical model computed by the inversion procedure is superimposed on each spectrum (continuous line). Seismic moments have been computed by the low frequency spectral levels ~ using the formula M
=
4irpv~,5R~0~’~ FR~
3 v~ 1.73 v with p 2.2 g cm 5 3 km 51; R hypocentral distance; and F free surface operator 2. Average P and SH radiation patterns KR~,) 0.4, (R~) 0.25 have been used, in agreement with Boore and Boatwright (1984), assuming normal fault mechanisms dipping 300_600, as we observed in the area (Fig. 5). Logarithmic weighted averages over all the records (Archuleta et al., 1982) have been computed to obtain mean low frequency spectral levels nor=
=
=
=
=
=
=
=
36 ~~j4
03 3006:01 I
5~* 005
SM
SIn
lSOa 01 30 00:01
111111)
III
II
WOO
8*03.1
III)
I
10
10
:111111
hull
2
1
1
1111:11
0 SLUM 02 10 05:48 I
00
—3
S~A015
I
0M~ 1
111111
10
~,e0a.nc SM
3004 02 10 00:46
I
I
i~iiii
I
i 50*
015
8.03.1
hIII~ I
111111
-
20
I
~
ISSA 01 2*20:26
51*
~ 202
05
~
II
20
0
hIll
F.~.Qo.fl0o
6
I
11111) ~
hill
I
110,
I 0
1684 01 26 20:21 I
20
10•~ 10
II
I
510 005
I
10
I
I
1111111 F,..q*.no
I
I
v
10550
102
660162
1111111
lull
1111111
111111
111111
1
Fig. 8. Examples of P-radial and SH displacement spectra. Best fit models from the inversion procedure (continuous line) are also shown.
37 —2
15504
0*
20 20:21
10
514 *20
5*
*564
IIllIJ
I
11111
0* 22 20:21
I
504 020
8.05.1
111111
I
I
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10
-
-2
10 10 -5 55‘-5
—7
so~f :
11111
I
o
10 05:45
I
I
I
504
10
.04
1
F.~oqo2fl0y 5*
02
I
I
I
I I I
—o
2
‘0
1
I
I I I
I 5
10
*4
1
111111 2
110
ISo.. 02
-
10
II
a 10
*0 05:42
I
.10.
I
I
I
514
*12
00 I
504
8.02,1
I I I
I
I
I I I
I
.
~10 l~
‘~‘
—b
1
10
-
‘N
10.
00
I
10
I
1
llhl~
I
11111
0
—71).-
2
111:11
F.~oq..oflA 10
0
1
10
I
I
lIll~
______________________________________________________ 0 5
10
F.~o4n60C2
1°~, 1
2
10
Fig. 8 (continued).
malized to a distance of 3 km, and seismic moments (see Table III). Source radii have been computed using corner frequencies ~ by weighted logarithmic averages, based on the formulae of both Brune (1970) and Madariaga (1976) models (see Table III) Madariaga model r 0.32v5/f~” 0.21v5/f Brune model r 0.37v5/f =
=
=
Only SH corner frequencies have been used to estimate source radii with the Brune model. Madariaga model was used because it takes into
TABLE II Station Latitude
Longitude
Altitude
Name W02 W03 W04 W05 W06 W07 W10 W14 W15 W20 W21 W11
(degree, mm) 1408.40 1409.17 14 05.03 1411.58 1403.63 1405.74 1405.37 14 09.90 1409.84 1405.89 1407.67 14 07.25
(m) 100 35 70 457 45 170 50 40 150 306 175 82
(degree, mm) 40 49.52 40 50.77 40 50.11 4051.42 4048.59 40 51.58 4046.69 40 47.78 4049.51 40 51.14 40 50.47 40 49.26
38 I~
1021
-
1020
-
I
I
(III!]
I
I
I
I
1
111111
I I
20
1
920031
1
I
-
10
-
20
-
19
1008
6
0 FM
$11
111(11
1
Hz
1
I
10
20
-
S
I
I
I I
I
I
a
I
2
I
((II /
Ib)
.
Il~~’~
/////
-
55/
/_//~~~
10~°
/ I
0
111111
Soul-se RadIus
I
-
I
I
10
I Ib)
5
-
I
1 Madal-teQa
Ill
II
III!
~
I 10
1
Hz
I
~
1 FM
I
-
jo20
::“::;~s~
0
1009
-
-
0
I
Io(
(41
1:1: 10
I
10
// ~ I
~
/~~/(
I
(III
I
6
I
2 81-use
Sou,~oe 9.101148
1
rn~)
Fig. 9. Seismic moment as a function of P-radial (a) and SB (b) corner frequencies. Dashed line is extrapolated from large earthquakes in Southern California (Hanks et al., 1975).
Fig. 10. Seismic moments vs. Madariaga’s source radii (a) and Brune’s (b) source radii. Bars indicate standard deviation errors (see the text).
account, properly, propagation and stopping of the rupture process for circular faults. Furthermore, it makes it possible to include in a natural way P-radial spectral estimates (Madanaga, 1976). Brune model, however, was also used to compare our results with previous works, which generally adopted this model. Stress drops have been computed by the aver-
age moments and source radii using the formula (Keilis-Borok, 1959) =
0.44KM )/(r)
3
All the obtained parameters together with estimated errors are shown in Table III. In Fig. 9a, b the plots of M0 vs. corner frequency P-radial and SH are shown. Figure lOa,
39
b shows the obtained scaling laws for the earthquakes of the Phlegraean Fields. Stress drops display a roughly constant pattern, with values around 20 bar (Madariaga model), and 4 bar (Brune model). We do not observe a decrease of stress drops at low seismic moments, that is present in many other areas. Small source radii (few tens of metres) are observed.
6. Discussion The inverse method used to estimate spectral parameters proved itself to be stable and efficient when Q is fixed at a known value. This technique gives estimates of errors on the spectral parameters; generally error estimates are not available when using the standard method of fitting ‘by eye’. Computed standard deviations from the inversion procedure have been used to obtain weighted log-averaged source parameters; the observed standard errors are reasonably small (Table III). This indicates that the method of weighting the single spectral estimates reduces the bias due to more scattered data, Furthermore the use of P-radial and SH displacement spectra revealed a fairly good coherence between the different source parameter estimates. Measurements of high frequency spectral decay have also been obtained by the inversion method: the observed values of y are rather high (around 2.5 on average) both for P-radial and SH spectra. In principle, however, the trade-off between Q and y at high frequencies can bias the measure of spectral fall-off. Measurements of Q~at Phlegraean Fields from coda waves range between 120 at 1.7 Hz to 300 at 16 Hz (del Pezzo et al., 1985): so we do not expect the spectra to be strongly influenced by attenuation at the observed distances (3—12 km). If the observed high frequency fall-offs are related to source properties, this data indicate earthquake ruptures that stopped themselves by a gradual deceleration, according to several kinematic models (Dahien, 1974; Boatwright, 1980). Such a behaviour can be qualitatively interpreted in terms of a ‘seismic gap’ mode of arrest (Hussemi et al., 1975) for fresh ruptures that nucleate in regions of high stress and stop gradually when
encountering lower stress zones. This type of model could be justified in this volcanic area by the presence of a heterogeneous stress distribution in the seismogenic volume. The investigated range of seismic moments is just below that found in most source parameter studies of microearthquakes (Archuleta et al., 1982; Fletcher et al., 1984). The constant pattern of stress drops is different from many other microearthquake sequences that show a tendency for corner frequencies of very small earthquakes (M <2—3; M0 < 1019_1020 dyne-cm) to become constant (Rautian and Khalturin, 1978; Chouet et al., 1978; Archuleta et al., 1982). Recently Brune et al. (1985) have analysed data from the ANZA digital array covering a wide range of seismic moments (1017_1021 dyne-cm). Source parameters from the ANZA array also show an increase of Brune and RMS stress drop with moment. Deviations from constant stress drop are interpreted in terms of source (Aki, 1985; McGarr, 1986) or local attenuation effects (Hanks, 1982). Recently Aki (1985) proposed the self-similarity to be valid within individual ranges of seismic moments. Our data could support this hypothesis, because we observe a nearly constant stress drop within our range of seismic moments, but with considerably smaller values (—~4 bar) with respect to those observed for larger events in other seismic Sequences (— 100 bar) (Aki, 1985).
7. Conclusions The Phlegraean Fields earthquake sequence accompanying the uplift episode 1982—1984 has been analysed. Seismic activity appears to be connected with the main tectonic structures in the area. In particular the earthquakes are roughly aligned along the ‘Starza marine terrace’ around which most of recent volcanic eruptions occurred. The focal mechanisms are mainly normal faults, dipping 30—60 showing a general tensional stress pattern. The spectral analysis of 32 microearth. quakes recorded by a digital three-component network (543 records) has been performed to obtain seismic moments source radii and stress drop estimates. An inverse method to fit displacement O,
40 TABLE III N
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
Time
01150331 01161900 01162351 01170258 01170448 01180206 01190453 01211339 01221345 01230543 01240424 01242250 01262021 01272343 01272353 01282126 01290422 01300022 01300040 01300044 01300601 01310036 01310220 02100548 02101116 02110400 02150159 02162205 02172041 02180402 02190039 02200111
ML
1.1 1.6 1.0 1.6 1.3 1.3 1.9 —
1.5 3.4 1.6 0.7 2.6 1.5 1.7 1.9 1.6 1.5 1.6 1.8 2.3 2.1 1.7 1.9 1.6 0.9 1.9 1.9 1.9 1.9 1.8 1.7
Depth
(~2 0Rad) (cm s)
SD
(~2~ Sh~ (cm s)
SD
~Mo) (dyn cm)
SD
(km) 0.7 2.8 2.1 1.5 3.3 2.6 2.4 1.2 2.0 2.6 0.9 2.3 1.9 1.2 2.4 2.5 2.2 1.7 2.3 2.1 0.7 2.6 2.3 2.5 2.4 1.8 2.9 2.3 2.5 2.3 2.3 2.9
0.29E—5 0.71E—5 0.23E—5 0.32E—5 0.24E—5 0.33E—5 0.26E—4 0.28E—5 0.67E—5 0.43E—3 0.61E—5 0.73E—5 0.1OE—3 0.61E—5 0.73E—5 0.98E—5 0.42E—5 0.13E—5 0.40E—5 0.60E—5 0.43E—4 0.64E—4 0.46E—5 0.23E—4 0.82E—5 0.94E—6 0.21E—4 0.15E—4 0.20E—4 0.12E—4 0.42E—5 0.43E—5
0.23E—5 0.46E—5 0.53E—6 0.13E—5 0.11E—5 0.13E—5 0.23E—4 0.20E—5 0.44E—5 0.20E—3 0.22E—5 0.24E—5 0.48E—4 0.43E—5 0.53E—5 0.11E—4 0.34E—5 0.43E—6 0.22E—5 0.20E—5 0.34E—4 0.23E—4 0.25E—5 0.13E—4 0.65E—5 0.17E—6 0.15E—4 0.14E—4 0.33E—5 0.72E—5 0.23E—5 0.37E—5
0.89E—5 0.38E—4 0.72E—5 0.19E—4 0.23E—4 0.93E—5 0.91E—4 0.93E—5 0.35E—4 0.15E—2 0.26E—4 0.31E—4 0.96E—3 0.28E—4 0.30E—4 0.43E—4 0.22E—4 0.80E—5 0.19E—4 0.33E—4 0.17E—3 0.29E—3 0.24E—4 0.15E—3 0.44E—4 0.93E—5 0.89E—4 0.69E—4 0.14E—3 0.11E—3 0.40E—4 0.29E—4
0.59E—5 0.39E—4 0.24E—5 0.16E—4 0.93E—5 0.83E—5 0.87E—4 1.OE—4 0.30E—4 0.38E—4 0.21E—4 0.24E—4 0.63E—3 0.24E—4 0.29E—4 0.45E—4 0.14E—4 0.54E—5 0.91E—5 0.21E—4 0.12E—3 0.12E—3 0.21E—4 0.92E—4 0.38E—4 0.15E—5 O.91E—4 0.63E—4 0.45E—4 0.53E—4 0.30E—4 0.23E—4
0.83E+18 0.25E+19 0.66E+18 0.11E+19 0.12E+19 0.91E+18 0.79E+19 0.82E+18 0.24E+19 0.13E+21 0.19E+19 0.23E+19 0.45E+20 0.21E+19 0.23E+19 0.33E+19 0.16E+19 0.47E+18 0.14E+19 0.21E+19 0.14E+20 0.21E+20 0.16E+19 0.91E+19 0.30E+19 0.50E+18 0.68E+19 0.52E+19 0.74E+19 0.61E+19 0.19E+19 0.18E+19
0.10+17 0.71+18 0.14+17 0.35+18 0.62+18 0.27+17 0.61+18 0.40+17 0.68+18 0.19+19 0.33+18 0.37+18 0.23+20 0.44+18 0.36+18 0.64+18 0.43+18 0.15+18 0.34+18 0.60+18 0.16+19 0.35+19 0.44+18 0.30+19 0.89+18 0.30+18 0.12+19 0.12+19 0.30+19 0.33+19 0.11+19 0.75+18
9.2 9.5 13.1 11.9 4.3 14.9 10.0 12.9 11.5 3.4 13.8 14.4 5.3 11.9 10.5 11.4 14.9 16.8 14.9 12.1 7.7 6.0 13.9 7.0 14.2 22.0 13.7 13.1 10.1 9.3 17.3 12.5
NS * is the number of stations. The number of records (radial and SH) used to estimate the source parameters is indicated in the brackets.
spectra for obtaining spectral parameters was developed. The method proved to be stable and efficient when the quality factor Q was fixed. One of the advantages of using this procedure is the possibility to obtain error estimates (from the covariance matrix) on the computed spectral parameters. The main result is the lack of a clear moment dependence of stress-drop, within the investigated range of seismic moment (1017_1020 dyne-cm). As a consequence small source radii are observed.
Acknowledgement The authors would like to thank R. Scarpa for many helpful discussions and comments.
References .
.
Ak, K., 1986. Physical Theory of Earthquakes. Proceedings of Strasbourg, 1986 summer school on Seismic Hazard in Mediterranean Regions. May 1985 Columbia University, New York, in press.
41
SD (Hz)
(FcSH)
3.0 2.2 6.6 3.5 1.4 4.4 1.5 2.6 4.1 0.2 2.1 1.8 1.4 3.9 3.2 3.9 6.3 3.8 4.5 2.5 1.8 2.3 8.3 2.2 2.8 4.1 3.9 4.3 3.8 4.3 4.3 2.8
11.1 11.3 13.5 8.8 7.4 15.2 7.7 13.7 8.3 4.2 11.4 12.6 4.6 12.3 8.5 10.0 14.9 17.8 14.2 13.8 9.2 5.5 12.4 6.4 12.0 13.3 11.5 9.8 6.8 8.0 12.4 11.6
KR-MAD)
SD (Hz)
(m)
4.3 3.5 3.2 3.7 3.0 5.1 2.9 1.6 1.3 1.0 3.2 3.4 0.8 3.5 3.4 2.7 2.5 2.0 5.0 4.1 2.6 1.7 5.1 2.6 2.4 2.3 3.8 3.0 2.5 5.3 4.6 3.3
45.7 45.4 31.1 44.3 82.7 30.2 52.8 31.7 45.1 145.4 37.0 35.1 88.5 36.6 48.0 41.3 27.5 24.0 31.4 36.5 55.4 76.1 33.3 68.6 34.6 26.3 36.1 39.6 54.3 53.3 30.9 38.1
SD
SD
K~OM) (bar)
SD
K~B) (bar)
SD
NS(*)
13.6 13.2 6.6 2.6 39.6 6.7 3.7 7.3 2.0 34.0 4.0 4.6 11.9 8.2 5.0 5.9 5.2 5.4 5.8 9.9 16.6 12.6 5.0 11.1 4.4 1.0 4.3 2.7 8.1 6.9 1.3 6.6
58.8 57.7 48.1 73.9 157.0 42.8 84.2 47.6 78.1 286.0 57.1 51.7 164.7 53.0 76.0 64.9 43.7 36.5 45.9 47.1 105.3 142.6 52.5 130.7 54.3 48.8 56.6 66.3 95.6 81.4 52.5 55.9
22.9 18.2 11.4 30.9 85.0 14.4 31.4 5.6 12.7 80.8 16.1 13.8 32.4 15.3 30.1 17.2 7.3 4.1 16.2 13.9 38.2 32.6 21.5 29.5 11.1 8.5 18.8 20.5 34.7 54.2 19.4 16.0
3.8 11.6 9.6 5.7 0.9 14.4 23.6 11.2 11.5 18.3 16.6 23.1 28.5 18.5 9.2 20.7 33.4 15.2 20.3 18.6 34.9 20.7 19.0 12.3 32.1 12.1 62.8 36.7 20.2 17.6 28.3 14.6
3.4 10.7 6.1 2.0 1.4 9.5 5.3 7.7 3.6 12.9 6.1 9.9 18.7 13.0 3.2 9.2 20.9 11.4 12.2 16.0 31.6 10.8 10.0 7.2 15.4 7.4 25.5 11.2 8.3 11.8 16.4 9.7
1.8 5.7 2.6 1.2 0.1 5.1 5.8 3.3 2.2 2.4 4.5 7.3 4.4 6.2 2.3 5.3 8.4 4.2 6.3 8.8 5.2 3.2 4.8 1.8 8.2 1.9 16.4 7.8 3.7 4.9 5.7 4.5
2.1 5.6 1.8 1.5 0.2 5.1 6.5 1.2 1.2 2.1 3.9 5.9 3.4 5.5 2.7 4.3 4.8 2.0 6.9 8.2 5.7 2.2 6.1 1.3 5.6 1.5 16.6 7.5 4.3 10.2 7.2 4.3
4(8] 5[9] 3(6] 7(13] 6(9] 6(11] 7[13] 3(6] 5(10] 2[4] 6(10] 4[8] 5(9] 7(13] 6(12] 7(11] 6(101 6(9] 7(13] 8115] 6112] 5(9] 6(11] 6(11] 5[6] 4(7] 6(11] 5(10] 4(7] 6(10] 6(11] 7(11]
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