A low magnitude seismic sequence near Isernia (Molise, Central Italy) in January 1986

PAGEOPH, Vol. 134, No. 2 (1990) 0033-4553/90/020243-1851.50 + 0.20/0 �9 1990 Birkh/iuser Verlag, Basel

A L o w M a g n i t u d e Seismic Sequence N e a r Isern ia

(Mol ise , Cen t r a l I ta ly) in J a n u a r y 1986

GIULIANA ALESSIO, l CATALDO GODANO, 1 a n d ANTONELLA GORINI 1

Abstract--Following the increase in seismic activity which occurred near Isernia (Molise, Central Italy) in January 1986, a digital seismic network of four stations with three-component, short-period seismometers, was installed in the area by the Osservatorio Vesuviano. The temporary network had an average station spacing of about 8 - I0 km and, in combination with permanent local seismic stations, allowed the accurate determination of earthquake locations during an operating period of about one month. Among the 1517 detected earthquakes, 170 events were selected with standard errors on epicentre and depth not greater than respectively 0.5 and 1.5 km. The most frequent focal depths ranged between 4 and 8 km, while the epicentres distribution covered a small area NE of Isernia of about l0 km 2. A main rupture zone could not be clearly identified from the spatial distribution of the earthquakes, suggesting a rupture mechanism involving heterogeneous materials. The activity was characterized by low energy levels, the largest earthquake, on January 18, 1986, having M D = 4.0. The time sequence of events and pattern of seismic energy release revealed a strong temporal clustering of events, similar to the behaviour commonly associated with seismic swarms.

Key words: Digital seismic network, Central Italy, seismic sequence, swarm, focal mechanism.

Introduction

High-energy earthquakes frequently occur in the central and southern sectors of the Italian Apennine chain. Recent examples include the following earthquakes: September 1979, in Val Nerina, Umbria (M s = 5.8 (NEIS--National Earthquake Information Service)); November 1980, in Irpinia, Campania-Lucania (Ms = 6.9 (NEIS)); April 1984, in Umbria, Perugia--Gubbio (Ms = 5.3 (NEIS)); and May 1984, in the area of Val Comino--Val di Sangro, Abruzzo (M~ = 5.8 (NEIS)).

A new increase in seismic activity began January 15, 1986 near the town of Isernia which, in the Molise region of central Italy, lies within the active seismoge- netic areas of Umbria, Abruzzo (Central Apennines) and Irpinia (Southern Apenni- nes). In addition, Isernia is only 23 km from Alfedena (Abruzzo) where the seismic sequence of May 1984 occurred. The earthquakes affected a small area where moderate seismicity has often occurred in historical times (Table 1), sometimes in

10sservatorio Vesuviano, via Manzoni 249, 80123 Napoli, Italy.

244 G. Alessio et al. PAGEOPH,

Table 1

List o f historical earthquakes in the area between: 41~176 ' latitude north, 14~ ' - 14025 ' longitude east. From P.F.G. (1985). Catalogo dei terremoti italiani dall'anno 1000 al 1980

Year Mo Da Lat. Lon. Int. Mk Epicentral zone

1807 01 28 41 30 14 15 VII 4.6 GALLO 1825 10 27 41 35 14 l0 VI 4.1 MONTERODUNI 1882 06 06 41 30 14 11 VII-VIII 4.8 CAPRIATI 1882 10 21 41 30 14 23 V 3.5 ROCCAMANDOLFI 1898 11 12 41 35 14 13 IV-V 3.3 S. AGAPITO 1900 05 24 41 36 14 20 V 3.5 FROSOLONE 1901 02 24 41 38 14 20 V 3.5 FROSOLONE 1905 07 22 41 36 14 06 V 3.5 COLLI VOLTURNO 1905 08 25 41 40 14 11 IV-V 3.3 FORLI' SANNIO 1914 11 09 41 36 14 12 VI 4.1 COLLI VOLTURNO 1914 12 17 41 35 14 13 VI 4.1 S. AGAPITO 1914 12 19 41 35 14 15 VII 4.6 S. AGAPITO 1917 02 26 41 36 14 06 IV-V 3.3 COLLI VOLTURNO 1919 12 12 41 36 14 12 V 3.5 COLLI VOLTURNO 1920 04 05 41 36 14 12 V 3.5 COLLI VOLTURNO 1925 09 24 41 40 14 09 VII-VIII 4.8 FORLI' SANNIO 1925 09 25 41 40 14 09 IV-V 3.3 FORLI' SANNIO 1925 09 26 41 40 14 09 IV-V 3.3 FORLI' SANNIO 1925 10 06 41 40 14 09 IV-V 3.3 FORLI' SANNIO 1932 12 03 41 35 14 13 V 3.5 S. AGAPITO 1932 12 04 41 35 14 13 V 3.5 S. AGAPITO 1937 12 17 41 35 14 13 V 3.5 S. AGAPITO 1939 05 25 41 35 14 13 IV-V 3.3 S. AGAPITO 1956 11 23 41 35 14 24 IV-V 3.3 MACCHIAGODENA 1956 11 25 41 35 14 24 VI 4.1 MACCHIAGODENA 1956 11 29 41 34 14 24 V 3.5 MACCHIAGODENA 1956 12 02 41 34 14 24 IV-V 3.3 MACCHIAGODENA 1961 01 29 41 36 14 06 V 3.5 COLLI VOLTURNO 1973 10 30 41 40 14 10 VI 4.1 FORLI' SANNIO

sequences o f about one month . Moreover , recent studies o f the July 26, 1805

ear thquake have assigned a m a x i m u m macroseismic intensity (XI M.C.S.) to the

village o f Frosolone , near Isernia, and a value o f VI I I M.C.S. to the area studied

in the present paper (PORFIDO e t al . , 1988).

Geological ly, the region is character ized by ou tc ropping Mesozoic l imestones o f

the Laz io -Abruzzo p la t form series, which have been tectonically thrust over the

Molise Basin Flysch (F igure 1) (CARTA DELLE LITOFACIES DEE LAZIO-AaRUZZO

ED AREE LIMITROFE, C N R , 1988). The p la t fo rm deposits and the t ransi t ion to the

pelagic series appear, to be split into highly fractured rigid blocks. The seismic area

is located within such units, where there is evidence for many small faults with no

preferred orientat ion. However , the Matese mounta ins , just south o f Isernia, show

an apenninic ( N W - S E ) orientat ion.

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The aim of this study is to describe the microearthquake activity in the Isernia area during the January 1986 activity, focussing attention on the space-time pattern of the sequence.

Data Acquisition

On 17th January 1986, the Osservatorio Vesuviano (O.V.) established in the Isernia area a mobile seismic network of four digital stations (PCM 5800 Lennartz system) and a Geotech RV 320 Portacorder. These stations, equipped with 3- component, short-period seismometers (Mark L4C-3D), operated for about one month in trigger and gain-ranging modes with a sample frequency of 125 Hz. The coordinates of the digital stations are reported in Table 2, together with those of the local permanent stations of the O.V. regional seismic network and the local permanent stations of the national seismic network operated by the Istituto Nazionale di Geofisica (I.N.G.). The station's location is shown in Figure 1.

The geometry of the mobile network was optimized on the basis of preliminary macroseismic data, preliminary epicentral locations, and the number of stations available. The station spacing was adjusted in order to detect source depths of at least 4-5 km (LEE and STEWART, 1981).

The digital signals recorded by the stations were successively decoded using the 58000 SAS Lennartz system, in order to determine the arrival times of P and S phases. Figure 2 shows an example of a seismic event recorded by station PSH, with the P and S phases as they were identified. Altogether, 1,517 seismic events were detected by the network, of which 170 were well located.

Table 2

List o f seismic stations

Station Latitude N Longitude E Altitude rn

RCS - - Roccasicura 41 ~ '.81 14 ~ 13'.92 757 RNR--Rionero 41 ~ 14 ~ 8'.44 1049 PSL--Peseolanciano 41~ 14~ 804 PSH--Pesche 41~ 14~ 729 BGL--Bagnoli del Trigno 41~ 14~ 659 CLL--Coll i a Volturno 41~ 14 ~ 6'.06 399 SGG--S. gregorio Matese 41~ 14~ 879 MSC--Massico 41~ 13~ 108 DUI- -Duron ia 41~ 14~ 918 SD1--S. Donato V. Comino 41~ 13~ 728

Vol. 134, 1990 Low Magnitude Seismic Sequence Near lsernia 247

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Figure 2 A seismic event recorded by the station PSH. Z = vertical component, N = N - S horizontal component,

E = E - W horizontal component.


Data Analysis

Location of Events and Source Mechanism

Most of the seismic events had low magnitudes, with only a few reaching values comparable with the maximum magnitude of 4.0. As a result, many events were recorded by only two or three stations. Of these, station PSH was the nearest to the active zone (on average < 3 km from the epicentral area (Figure 1)) and recorded most of the events without saturation of the signals, as allowed by the digital instrumentation used.

The P time picking accuracy is typically less than 0.05 sec for the digital stations and 0.1 sec for the permanent analog stations. The events with at least six arrival times (P and S) detected at four or more stations were selected for location analysis; the arrival times were weighted inversely to the uncertainties in the onset of the P phase.

Since no detailed information is available for this specific area, a half-space with Vp = 5.0 km/sec has been assumed for the velocity model. This value is reasonable because (a) after a series of tests using different velocities on a sample of events, it yielded the smallest residual times in locating earthquakes, and (b) it is in good agreement with ScArtAscIA'S (1982) results which show a velocity of 6 km/sec at about 8 km depth along the profile Pescara-Benevento.

A Vp/vs ratio was evaluated from Wadati diagrams of single earthquakes in this sequence. P and S arrival times of 72 events were used to obtain a mean value of vp/vs = 1.75, which is comparable, even if slightly smaller than values obtained for earthquake sequences in different seismotectonic domains of the Apennine region (DESCHAMPS et al., 1984; HAESSLER et al., 1988).

Hypocentral determinations were made using the C.H.E.A.P. (Calcul Hypocen- tral avec l~tablissement Automatique de Probabilitr) computer program of TARAN- TOLA and VALETTE (1982). This program maximizes the probability density function a(x, y, z) of the hypocentre, considering the covariance matrix of data errors and the errors introduced by imprecision of the velocity model. Such imprecision can be accounted for using the covariance matrix Cr of the theoretical error on the velocity model as follows

(C T ) / j = 17 2 exp( - 1/2((r i -- rj ) 2/D 2))) (1)

where ri - rj is the relative distance between stations i and j and D is the correlation distance of theoretical errors. Usually D is assumed to be equal to the average wavelength of the signal. Both the covariance matrix of experimental errors Ct and the covariance matrix Cp of the error on the a priori hypocentral coordinates must be added to C~, where Cp is defined as

(a~ 0 0

~

Vol. 134, 1990 Low Magnitude Seismic Sequence Near Isernia 249

The probability density function of the hypocentre is thus

a(x, y, z) = k exp( - 1/2((t0 - ti(x, y, z))r(Ct + CT) -t(t0 -- ti(x, y, Z))

+ ( p - p o ) C ; l ( p _ p0))) (2)

where p and Po are the positions of, respectively, the hypocentre and the a priori hypocentre, to is the observed arrival time from which the weighted mean of the observed times is subtracted, and t i is the travel time from which the weighted mean of the travel times is subtracted.

To verify that the solution obtained was the absolute maximum, the probability density function a(x, y, z) around the hypocentre over a fixed grid was calculated with the C.H.E.A.P. program. An example of the results is given in Figure 3 for the epicentre and depth of the main event; it is evident that no secondary maximum of a(x, y, z) is present.

Using 20 observations (12 P-time readings and 8 S-time readings), the hypocentral parameters of the main shock, located with the C.H.E.A.P. program, were:

Date To Lat. N Long. E Depth (km) Md

01/18/1986 09:32 45.00 41~ ' 14~ ' 7.93 4.0

Figure 4 shows the epicentral distribution of events whose hypocentres have been located with errors ~<0.5 and ~< 1.5 km for, respectively, horizontal position and depth. Most of the epicentres are concentrated in an area of about 10 km 2 to the NE of Isernia. They are arranged in clusters NW of station PSH, without any preferential alignments of events (Figure 4). The events also show a clustering with depth. Thus, although the maximum depth of the seismic sources was 12 km, projection of all the hypocentres onto, for example, NW-SE and NE-SW oriented cross-sections, reveals a concentration of hypocentres at depths between about 4 and 8 km (Figure 5). As a result of these clustering patterns, no space trends can be inferred from either epicentral or hypocentral distributions. In addition, no temporal variation of the average focal depth was recognised during the sequence.

Because of the low signal-to-noise ratio and the relatively low number of stations available, it has not been possible to obtain good fault plane solutions for single events. A composite solution, however, can be computed for events with clear polarities, similar waveforms and approximately the same location. This well-tested procedure (MENDIGUREN, 1980; KAGAN and KNOPOFF, 1985) assumes that such events have a similar focal mechanism and, although it yields results which average the polarization directions, it does provide useful information on the average stress field in the area.

The program Utilized was derived from WICKENS and HODGSON (1967), incorporating modifications made at the Osservatorio Vesuviano Computer Centre and using the same velocity model as for the locations. The program assumes a

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length and width are 30 and 20 kin, respectively.

double couple source and determines the nodal planes and stress axes which best fit

the observed data. The result (Figure 6) shows a predominant strike-slip mechanism with a small

dip component; the strike and the dip of the A and B planes are N 256.6~ = 69.4 ~ and N 360.1~ ~3 = 58.0 ~ respectively.

Vol. 134, 1990 Low Magnitude Seismic Sequence Near hernia

N

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PLANE B

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253

DATE ORIGIN TIME LAT N LONE DEPTH MAG

860118 01:22 49.07 41 36.50 14 14.94 5.15 3.6

860118 08:49 32.99 41 36.13 14 13.92 5.92 3.1

860118 09:32 45.00 41 36.57 14 15.27 7.93 4.0

860118 15:48 23.23 41 32.20 14 12.85 5.40 3.3

860118 21:08 35.92 41 36.85 14 15.05 6.88 2.2

860119 21:56 13.90 41 32.62 14 14.56 6.10 3.6

860120 17:33 46.37 41 38.13 14 15.44 5.97 2.5

860121 08:20 43.12 41 38.72 14 13.39 5.66 2.9

860123 71:41 54.95 41 37.76 14 15.90 5.89 2.2

860124 04:08 24.76 4~ 37.29 14 15.01 9.23 2.8

860124 07:17 58.52 41 37.51 14 14.93 4.16 1.7

860204 22:55 53.27 41 37.88 14 14.89 1.76 3.1

860207 03:16 36.87 41 37.72 14 16.05 5.32 1.6

860208 03:48 10.36 41 37.90 14 13.11 6.16 1.2

860209 18:12 09.57 41 38.39 14 13.02 4.75 0.6

Figure6 Composite fault plane solution obtained from P-wave polarity. Data are plotted on a Wulff stereo- graphic projection, lower hemisphere. The events used in computing the solution are listed in table above

graph. N and 6: strike and dip of plane. + compression, - dilatation.

Time Series and Energy Release

Local magni tudes of the events were calculated from seismogram dura t ions for

the stat ion S. Gregorio Matese (SGG) , a pe rmanen t s tat ion of the O.V. seismic

regional network, using the empirical relationship:

MD = -- 1.85 + 2.70 log(t) + 0.001D (3)

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where t is the signal duration in seconds and D is the epicentral distance in kilometres; their respective coefficients are determined by correlating signal duration with the Wood-Anderson magnitude (I.N.G., Rome) for a set of selected earthquakes.

The seismic energy of the events was obtained from the GUTENBERG-RICHTER (1942) relationship:

log E = 9.9 + 1.9ML -- 0.024M 2. (4)

The largest values of daily energy release occurred during the first four days of the sequence (Figure 7), in good agreement with the pattern of the daily number of events; the corresponding maximum values of these parameters, attained on the same day, are 21 �9 10 ~6 erg and 410 events.

For statistical analysis of the time series associated with this seismic sequence, different probability distributions were examined for describing the normalized

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N. OF EVENTS PER TIME UNIT

Figure 8 Probability distribution of the number of events per unit time for a three-hour sampling interval and a minimum magnitude threshold of M/> 1.5. The simple Poisson distribution (dotted curve) and the

generalized Poisson distribution (full curve) are also shown.


frequency distribution of the number of events per unit time. As shown in Figure 8, the frequency distribution cannot be interpreted as a simple Poisson process; the lambda parameter for this distribution, which gives the mean rate of the occurrence of the events, is 4.3 events/3 hours.

A considerably better agreement with the experimental data is obtained using a generalized Poisson process, that is a model of Poisson-distributed clusters; the parameters k and E indicate, respectively, the mean rate of occurrence of clusters and the cluster size distribution (SHLIEN and TOKSOZ, 1970). The low value of 1.8 obtained for E would be consistent with a swarm type of seismic activity (UTSU, 1970; DE NATALE and ZOLLO, 1986); it would also agree with the fact that some events of comparable magnitude (Mma x = 4.0) were observed during the early days of the sequence.

The rapid evolution of the seismic sequence is pointed out by the variation with time of the 'b'-value (GUTENBERG and RICHTER, 1944) computed from the noncumulative magnitude-frequency distribution. The whole sequence is characterized by an average value of b = 0.85 + 0.02. In detail, however, b increased quickly during the first days of the sequence (Figure 9), later tending to a constant value. The physical meaning of the 'b' coefficient is usually related to the stress level applied to the medium; therefore the observed 'b' variation may be related to the stress-drop after the major shocks.

The observed changes in b might also reflect a variation in the fractal dimension of the seismic fracturing process, as the distribution of events, at first confined effectively to a plane, begins to extend into the surrounding crust in several directions (i.e., the distribution of events evolves from a planar (2D) to volumetric (3D) geometry). However, such an interpretation must remain speculative since, because of the very small crustal volume involved in the Isernia sequence, the hypocentral location errors are relatively large for a clear discrimination between planar and volumetric crustal failure.

Discuss ion and Conclusions

The January-February 1986 seismic sequence, which occurred near Isernia (Molise, Central Italy), affected a small area (about 10 km 2) in the Central-Southern Apennines. The earthquakes had low magnitudes (Mma x = 4.0) and their epicentral distribution shows a clustering of events, without any regular geometry. Similar features have been observed for other seismic sequences in the Central Apennines (DESCHAMPS et al., 1983-1984; FERRI and GORINI, 1986) and may be attributed to the effect of complex fracture mechanisms.

Most of the events were concentrated at depths between 4 and 8 km, broadly coinciding with the overthrust contact between limestones of the Lazio-Abruzzo platform series and the underlying Molise Basin Flysch (Figures 1 and 10), which have different mechanical properties (MOSTARDINI and MERLINI, 1986).

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A composi te fault plane solution indicates that the Isernia sequence was related

to a strike-slip failure mechanism with a small dip component . Al though such a

mechanism contrasts with the normal faulting commonly associated with large

ear thquakes in Central Italy, it does not appear to be unusual for small ear thquakes

located along transit ion zones or lateral segments o f the moun ta in chain (GAs-

PARINI et al., 1980, 1985; DESCHAMPS et al., 1984b; HAESSLER et al., 1988;

WESTAWAY et al., 1989). Indeed, for events involving areas as small as the Isernia

sequence, minor structures are likely to strongly influence the rupture mechanism

and provide fault plane solutions unrelated to the regional stress field (JACKSON,

1983).

Acknowledgements

The authors would like to thank Giuseppe De Natale, Giovanni Iannaccone,

Marcello Mart ini and Aldo Zollo who provided some o f the comput ing programs

used in this paper. Thanks are also extended to Chris topher Ki lburn who carefully reviewed the

final version o f the manuscript .

REFERENCES

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CARTA GEOLOGICA d'ITALIA--Scala l:100.000, Foglio 161 (Isernia). DE NATALE, G., and ZOLLO, A. (1986), Structural Analysis and Clustering Features of the Phlegrean

Fields Earthquake Sequence (May 1983--May, 1984), Bull. Seismol. Soc. Am. 76 (3), 801-814. DESCHAMPS, A., GASPARINI, C., IANNACCONE, G., and SCARPA, R. (1983), Nuove ipotesi sulla

geometria della sorgente del terremoto umbro del 19 settembre 1979, Atti del II Convegno, vol. I, Gruppo Nazionale di Geofisica della Terra Solida (C.N.R.) Roma, 12-14 dicembre 1983.

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4 Figure 10

Geological cross-section of the Isernia area section (Section n. 11 of MOSTARDINI and MERLINI, 1986). The limits of the Carta Geologica d'Italia (Foglio 161-Isernia) are reported above the section. See text

for discussion of the correlation between tectonics and focal depths.


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KAGAN, Y. Y., and KNOPOFF, L. (1985), The Two-point Correlation Function of Seismic Moment Tensor, Geophys. J. R. Astr. Soc. 83, 636-657.

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(Received July 11, 1989, revised/accepted June 1, 1990)

A low magnitude seismic sequence near Isernia (Molise, Central Italy) in January 1986

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