Romanian Reports in Physics, Vol. 57, No. 1, P. 141–150, 2005

EARTH PHYSICS

ISOLINE MAPS OF GROUND MOTION ACCELERATIONCAUSED BY THE VRANCEA (ROMANIA) EARTHQUAKE

OF MAY 30, 1990 (MGR = 6.7). COMPARISON WITHTHE MACROSEISMIC INTENSITY MAP

B. D. ENESCU1,2, D. ENESCU1

1 National Institute for Earth Physics, P.O. Box MG-2,Bucharest-Mãgurele, 077125, Romania

2 Disaster Prevention Research Institute, Kyoto University, Gokanosho,Uji, Kyoto 611-0011, Japan

(Received September 28, 2004)

Abstract. A study contributing to the groundwork for tracing the seismic hazard maps of theRomanian territory and neighboring areas is presented. Maximum ground movement accelerationsrecorded at 41 seismic stations have made it possible to work out an isoacceleration map. Estimatesbased on a synthetic isoacceleration calculating method have made it possible to obtain a fairlycomplete isoline system. A number of conclusions are drawn of high significance and use for thefinal tracing of seismic hazard maps.

Key words: ground movement acceleration; macroseismic intensity; directivity function.

1. INTRODUCTION

The paper is part of a series aimed at resolving the problem of seismic hazardin Romania on a novel basis and as near to completeness as possible. Since theseries principally deals with major Vrancea earthquakes, it will also makesubstantial contributions to a better knowledge of seismic hazard in someneighboring regions.

As a first task we will try to resolve the case of the earthquake of May 30,1990 (Gutenberg-Richter magnitude MGR = 6.7), because, as will be proved below,its characteristics differed from those of most moderate and major Vranceaearthquakes. The geographic coordinates of the epicenter in this case wereϕ = 45.8°N and λ = 26.9°E, while hypocenter depth was h = 90 km. The tremorhaving a moment magnitude MW = 6.9, occurred at 10:40:06 GMT. Its peakmacroseismic intensity was I0 = 8.

142 B. D. Enescu, D. Enescu 2

2. MACROSEISMIC DATA AND FAULT-PLANE SOLUTION

A macroseismic map (see Fig. 1, after Radu and Utale, 1990; 1991)1 wastraced out based on replies to a nationwide survey concerning earthquake effects onbuildings, ground and living creatures.

Macroseismic intensity degrees (Fig. 1) were mostly estimated by taking intoaccount replies to survey questionnaires, rather than direct expert observations onthe scene, which were only available in very few cases. Macroseismic intensitieswere estimated on the MSK-64 scale, which virtually coincides with the Mercalliscale. Since the estimates are based on the observations and opinions of hundredsof subjects in a great number of places, their objectivity is evidently scant.

The subjectivity of employees who interpreted the data was another majorsource of errors. A very harmful tendency was noticed, among other shortcomings,of lumping all of the subcrustal Vrancea earthquakes into one category in terms ofisoseismal line direction (i.e. the NE-SW direction), which leads to neglecting thereal directivity of the seismic wave propagation. Such drawbacks were found in thecase of the 1990 earthquake which is investigated in this paper. As a matter of fact,the problem of macroseismic data on Vrancea earthquakes being highly subjectivewas carefully investigated in a previous paper (Enescu [1]).

To obtain the fault-plane solution, Radu and Oncescu (1990)2 used the mostwidespread, reliable method available, namely the one based on the polarity of thefirst arrivals in longitudinal waves.

One of these solutions is shown in Fig. 2a where observation data from awide range of epicentral distances were used. By contrast, short-and medium-epicentral distance observations prevailed over the long-distance ones in obtainingthe fault-plane solution in Fig. 2b.

Account taken of previous researches (Enescu [2]; Müller et al., [3];Constantinescu and Enescu [4; 5]; Enescu and Zugravescu [6] aimed at riddingfault-plane solutions of ambiguities, we find that the fault (rupture) plane in Fig. 2ais the NE-SW directed nodal plane, slanting to the NW. By contrast, in Fig. 2b, it isthe NWN-SES nodal plane with a WSW slant.

3. GROUND MOTION ACCELERATIONS.ISOACCELERATION MAPS

The earthquake was recorded at 36 accelerographic stations is Romania(Table 1) belonging to the Institute for Construction Research at Bucharest and the

1 Radu, C., Utale, A. (1990; 1991), Report CFP/IFA no. 30.86.3, 1990 and Report CFP/IFAno. 30.91.3, 1991. Bucharest-Mãgurele, Romania.

2 Radu, C., Oncescu, M. C. (1990), Report CFP/IFA no. 30.86.3, Bucharest-Mãgurele, Romania.

144 B. D. Enescu, D. Enescu 4

Fig. 2 – Fault-plane solution for the investigated earthquake in two variants,(a) and (b) (after Radu and Oncescu 1990).

National Institute for Earth Physics at Bucharest-Mãgurele (Danci 1997)3.Accelerographs used are of SMA-1 type with film recording for along threecomponents (two horizontal ones and a vertical one). We also used data recorded atfive Bulgarian accelerographic stations (Table 1). We have denoted by amax themaximum value of ground motion acceleration along the strongest component,while amax. res. is the maximum resultant acceleration recorded at the same station(Table 1). Small differences were found (Table 1) between amax and amax.res..

Table 1

No. Seismic station Station code amax [cm/s2] amax.res [cm/s2]

0 1 2 3 4

1 Adjud ADJ 87 93

2 Arges ARR 18 25

3 Bacãu BAC 133 149

4 Baia BAA 89 95

5 Bârlad BIR 148 157

6 Bolintin V. BLV 209 220

7 Braºov BRS 48 51

8 Brãila BRL 81 86

9 Brãneºti BRN 139 146

(continues)

3 Danci, G., (1997), Ph. D. Thesis, Library of the Bucharest University.

5 The Vrancea (Romania) earthquake of May 30, 1990 145

Table 1 (continued)

0 1 2 3 4

10 Bucharest BUC 137 146

11 Buzãu BUZ 192 204

12 Carcaliu CFR 164 173

13 Cãlãraºi CLS 112 119

14 Câmpina CMN 236 250

15 Câmpulung M. CMP 47 50

16 Cernavodã CVD 107 124

17 Constanþa CTS 48 51

18 Craiova CRV 38 40

19 Feteºti FTS 100 107

20 Focºani FOC 109 116

21 Galaþi GLT 98 104

22 Giurgiu GRG 114 121

23 Iaºi IAS 106 111

24 Muntele Roºu MLR 66 70

25 Oneºti ONS 232 247

26 Periº PRS 224 240

27 Piteºti PTS 46 49

28 Ploieºti PLS 85 91

29 Râmnicu Sãrat RMS 164 175

30 Slobozia SLB 133 142

31 Surduc SDR 97 100

32 Tulcea TLC 94 100

33 Turnu Mãgurele TRM 112 119

34 Vaslui VSL 124 132

35 Vãleni de Munte VLM 154 164

36 Vrâncioaia VRI 157 158

Bulgarian seismic station

0 1 2 3 4

37 Kavarna KAV 36 46

38 Provadia PRV 48 49

39 Ruse RUS 112 115

40 Shabla SHB 33 33

41 Varna VRN 35 35

By representing Table 1 accelerations on the macroseismic map, manydisagreements were found between the two categories of data (seismic intensities

146 B. D. Enescu, D. Enescu 6

and accelerations), which are put down to the scant objectivity and reliability of themacroseismic data (Fig. 1).

A first attempt at drawing isolines of the ground motion acceleration wasmade by Mandrescu [7], who used amax values and made the comparison with thefault-plane solution in Fig. 2a. The resulting isolines were very approximately traced.

In Fig. 3, we have represented the amax accelerations. Their distribution onthe map (Fig. 3) appears to confirm the fault-plane solution in Fig. 2b. This comesas no surprise, given that accelerographic stations are within short and very shortdistances from the hypocenter.

Besides, the solution in Fig. 2b is largely based on observation data fromvery short-, short-and medium-epicentral distances, rather than long distanceswhere errors in determining the polarity of the first arrivals in the P wave are muchmore likely to occur. As a matter of fact, for all their incompleteness and inaccuracyeven Mandrescu’s isolines [7] tend to agree with the solution in Fig. 2b.

In an attempt to improve the reliability of the isolines and make sure they areas close to complete as possible, we used a method developed and tested by Enescuand Smalbergher [8] to estimate the directivity functions of the seismic source. Toapply this method, we used the focal mechanism solution shown in Fig. 2b, and theequations imposed by this method were calibrated using values of the maximumrecorded acceleration. Values leading to anomalies hard to account for wereeliminated in this stage.

By taking into account the distribution of the maximum acceleration valuesshown in Fig. 3 and our estimates obtained by the above method, we could tracethe isoaccelerations of 35, 50, 70, 100, 125, 150, and 170 cm/s2 (see Fig. 3).

If we examine the data in Fig. 3, we find the insoline routes to be in goodagreement with the maximum acceleration values. Of a total 41 values ofmaximum acceleration only as few as nine are more or less divergent from theisolines we traced. Five of them (namely the ones recorded in Adjud, Brãila,Feteºti, and Galaþi) are lower than the values shown by the nearest isolines. Suchcases do not really hamper the study of seismic hazards, as isoline-provided valuesbeing larger they ultimately ensure a higher safety of the buildings. On the onehand, one must not carry it too far in admitting higher values ungroundedscientifically just for the sake of overensuring constructions, since this wouldneedlessly push up costs. In fact, this is what seismic hazard studies are for:ensuring building safety at reasonable costs. On the other hand, it is possible that atsome points where values lower than normal were once recorded, they may be“normal” or even higher than normal at a future earthquake. Therefore, as will beshown in further papers, such situations should be also taken into account to makesure that seismic hazard studies achieve their goal.

By contrast, special attention must be paid to anomalies such as the maximumacceleration values recorded at four stations (Bolintinul din Vale, Câmpina, Periº

9 The Vrancea (Romania) earthquake of May 30, 1990 149

and Oneºti), which are much higher than those indicated by the closets values. Wetherefore take into account these four anomalies (Fig. 3).

Using a relation obtained by Enescu [1] for subcrustal Vrancea earthquakes,between amax and the macroseismic intensity, I

log amax = 0.2712 I + 0.1814, for 5 ≤ I ≤ 9 (1)

we turned the isoacceleration map in Fig. 3 into a map of isoseismal lines (Fig. 4).Figs. 3 and 4 indicate the earthquake under investigation is characterized by

two areas of maximum acceleration and intensity southwest of the epicentral area(in Muntenia) and a maximum in the area around Oneºti (in Moldova), whichconfirms a finding by Atanasiu (see Saulea [9]) of there being two maxima of themacroseismic intensity, one on each side of the epicentral zone. These maximashould probably be put down to regional or local amplifications.

4. CONCLUSIONS

The main conclusions arising from our results in this paper are as follows:1. The distribution of the maximum values of ground motion acceleration is

in disagreement with the macroseismic intensity distribution (Fig. 1). This provesmacroseismic intensity estimates are of a highly subjective nature.

2. Data in Fig. 3 reveal a good agreement between the isolines we traced andthe recorded values of maximum acceleration.

3. Data in Figs. 3–4 confirm an earlier conclusion of Enescu and Smalbergher[8] that the general shape of the isoseismal lines and isoaccelerations depends on thefocal mechanism. In the case under investigation, the general outline of the isolinesin Figs. 3–4 agrees with the focal mechanism solution in Fig. 2b where the fault planeis NWN-SES directed and slanting to the WSW. In Figs. 3–4 we notice isolines arenot definitely directed either NW-SE or NE-SW. This is because the focal mechanismsolution (Fig. 2b) shows a nodal plane, which is supposed to be the fault plane,along the NWN-SES direction, and another nodal plane along the NE-SW direction.It is the direction of not just one but both nodal planes that leads to the generalshape of the isolines; this in our case is the direction of isolines in Figs. 3–4.

4. Atanasiu’s conclusion on the existence of maxima (Figs. 3, 4) one on eachside, or just on one side, of the epicentral zone is confirmed. These maxima areprobably originating in some regional or local amplifications.

5. Some maximum acceleration values (specifically, 5, or 12%) are found tobe lower than those indicated by the closest isolines (Fig. 3). These are obviouslynot serious anomalies.

6. Conclusions 4 and 5 above point to the necessity of a seismic microzoningbased on the most effective methods available.

150 B. D. Enescu, D. Enescu 10

7. Differences between the isoseismal line maps in Figs. 1 and 4 prove thatmacroseismic maps based on replies to survey questionnaires are hardly subjective.At the same time, this could to some extent sustain the view that maximumaccelerations do not entirely reflect the effects of an earthquake.

8. The relatively high values of the maximum acceleration recorded inDobruja (at Cernavodã, Carcaliu, Tulcea and Baia), which sparked some panicamong some experts who were familiar with the seismic parameters that had beenused in the design of the Cernavodã nuclear power plant, were largely generated bythe characteristics of the focal mechanism of this earthquake. But the Vranceastrong earthquakes (MGR > 6.7) are characterized by NE-SW directed fault planes.

9. The most significant conclusion of this study is that our methods proved tobe effective, and the results we got will make a substantial contribution toobtaining the final seismic hazard maps for the territory of Romania and even somesurrounding areas.

REFERENCES

1. D. Enescu, St. Cerc. Geof., 35, 15–27 (1997).2. D. Enescu, Com. Acad. R.P.R., 12, 12, 1279–1290 (1962).3. G. Müller, K. P. Bonjer, H. Stöckl, D. Enescu, Journal of Geophysics, 44, 203–218 (1978).4. L. Constantinescu, D. Enescu, Rev. Roum.Géol., Géophys., Géogr., Série Géophysique, 28,

19–32 (1984).5. L. Constantinescu, D. Enescu, The Vrancea earthquakes within their scientific and technological

framework (in Romanian), Ed. Academiei, Bucharest, 230 p (1985).6. D. Enescu, D. Zugravescu, Rev. Roum. Géophysique, 34, 17–34 (1990).7. N. Mandrescu, St. Cerc. Geof., 34 (1996).8. D. Enescu, V. Smalbergher, Rev. Roum. Géol., Géophys., Géogr., Série Géophysique, 24, 2, 235–

254 (1980).9. E. Saulea, St. Cerc. Astr., Seismol., 6, 2, 297–313 (1961).