ACCELEROGRAPH RECORDINGS OF THE M USA EARTHQUAKE 16 SEPTEMBER, 1972

Size: px

Start display at page:

Download "ACCELEROGRAPH RECORDINGS OF THE M USA EARTHQUAKE 16 SEPTEMBER, 1972"

Cynthia Hudson
5 years ago
Views:

1 115 ACCELEROGRAPH RECORDINGS OF THE M USA EARTHQUAKE 16 SEPTEMBER, 1972 B.Gauir SUMMARY On 16 September, 1972 at UT an earthquake f magnitude ML 5.0 ccurred in sutheast Papua within abut 20 km f a prpsed dam site n the Musa River. The earthquake triggered tw accelergraphs, Musa A and Musa B, ne at the crest and ne at the base f the dam site. The peak grund acceleratins recrded by the accelergraphs were 1.85 m/s^ and 0.39 m/s^ respectively; the peak grund velcities were 87 mm/s and 2.0 mm/s; and the crrespnding mean perids f the grund mtin were abut 0.23 and 0.04 s. The difference in the nature f the accelergrams is attributed t the difference in the gelgical and tpgraphical settings f the accelergraph sites. The upper site has an elevatin f 406 m and cnsists f abut 60 m f weathered sediments verlying fresh ultramafics. The lwer site has an elevatin f 112 m and cnsists f an utcrp f the ultramafic cmplex. 1. INTRODUCTION In June, 1972 the Bureau f Mineral Resurces, Gelgy and Gephysics (BMP), installed 2 accelergraphs (hereafter called "Musa A" and "Musa B"), 600 m apart, ne n weathered sediments n the crest f ne f the abutments and ne n hard ultramafic rck at the base, f a prpsed dam. site n the Musa River in sutheast Papua (Fig. 1). At abut UT n 16 September, 1972 an earthquake f magnitude ML 5.0 ccurred within 20 km f the dam site and triggered the accelergraphs. Despite the clseness f the accelergraphs, the accelergrams differ in bth acceleratin amplitude and perid (Figs. 2 and 3). The triggerings were significant insfar as this was the first time in Papua New Guinea that tw strng-mtin instruments recrded simultaneusly n nearby hard and sft rck sites. 2. GROUND MOTION AT THE ACCELEROGRAPH SITES Table 1 lists the maximum grund acceleratin and velcities in the vertical, nrth-suth, and east-west directins; and the perids f grund mtin at the times f maximum acceleratin. Fig. 2 shws the three cmpnents f bth accelergrams and Fig. 3 is a digitized representatin f bth grund acceleratin and velcity at the Musa A site. 2 Acceleratins greater than 0.3 m/s ( g where g is the acceleratin wing t gravity) were recrded fr abut 11 secnds by the Musa A accelergraph. Acceleratins greater than abut 0.15 m/s^ were recrded fr a duratin f * Department f Minerals and Energy, Prt Mresby Gephysics Observatry, Papua New Guinea. abut 5.5 secnds at the Musa B site. 3. ACCELEROGRAPH SITES Figure 4 shws bth accelergraph-site prfiles dwn t bedrck and includes seismic shear wave velcity, density and thickness f each layer. This data was acquired frm seismic wrk (Bishp et al, in prep.) and drilling (Macias, 1971) in the vicinity f the accelergraph sites. The bedrck which utcrps at the Musa B site is an ultramafic cmplex (seismic shear wave velcity near surface f abut 2500 m/s and density 2.8 x 10 3 kg/m 3 and is als fund 60 m beneath a deeply weathered sedimentary pile at the Musa A site. The 2.5 sec S-P time n bth accelergrams suggests that the earthquake hypcentre was abut 19 km frm the dam site which is much greater than the distance separating the tw accelergraphs (0.6km). Therefre the prpagatin path t the tp f the ultramafics at bth sites culd be cnsidered t be identical and cnsequently the differences between recrded grund mtin at the tw sites is attributed t near surface features f the sites alne. 4. THEORETICAL MODEL TO ACCOUNT FOR ACCELERATION AMPLIFICATION IN LAYERED SYSTEMS Madera (1970) utlined a theretical methd t predict the fundamental perid f the seismic waves and the amplificatin f seismic acceleratin in sil prfiles abve bedrck. He btained gd agreement between cmputed and measured values. The principle is that resnance f seismic waves is set up in the layered system abve bedrck, which amplifies the incming signal. Because it is a resnance effect, the dminating perid recrded n the accelergram shuld be related t the fundamental perid f the layered mdel. On applicatin f Madera's (1970) methd t the prfile with parameters shwn in Fig. BULLETIN OF THE NEW ZEALAND NATIONAL SOCIETY FOR EARTHQUAKE ENGINEERING, VOL.9 N0.2. JUNE 1976

2 a fundamental perid f abut 0.26 s was cmputed fr the Musa A site. This value cmpares reasnably well with the measured mean Musa A accelergram perid f 0.23 s at the time f peak acceleratin (Table 1). Because the prpagatin paths thrugh the ultramafics are cnsidered t be identical, it is reasnable t assume that the input signal at the tp f the ultramaf ics beneath Musa A is the same as that registered at Musa B. Therefre Madera's (1970) mdel culd be used t estimate the peak acceleratin amplificatin at Musa A directly with respect t Musa B. When this was carried cut an amplificatin rati f 2 was btained. When extreme estimates f layer density and internal damping were substituted, the maximum pssible amplificatin rati attainable was 2.7, which is still significantly less than the measured acceleratin rati f 4.5. Hence Madera T s (1970) mdel des nt fully accunt fr the amplificatin f acceleratin which tk place at the Musa A site. 5. TOPOGRAPHICAL FACTOR Davis & West (1973) have bserved the effects f tpgraphy n grund mtin at statin sites carefully selected s that the nly difference between them was tpgraphy. Their results indicate that the amunt f amplificatin and the perids at which it ccurs vary with the size f the muntain, and are prbably a functin f the relatin between the wavelengths f the incming seismic signal and the dimensins f the muntain. The suggested causative mechanism fr this amplificatin is resnance f the muntain set up by shear waves when the wavelength f these waves are f the same rder as the dimensins f the muntain. Musa A and B sites are lcated respectively near the crest and base f a 400 m high muntain. Assuming a shear wave velcity f 3.5 km/s and using a perid f 0.25 s (crrespnding rughly t the perid at peak acceleratin) a wavelength f abut 900 m is btained, which is abut half the width f the muntain. Using the results btained by Davis & West (1973) fr a muntain f similar size (Mt. Butler, Nevada), it was fund that the average amplificatin rati f grund mtin at the crest relative t the base was abut 2.5. nt yet been analysed but it is anticipated that this amplificatin is again due t the cmbinatin f the weathered sedimentary pile and the tpgraphy at the Ynki site. 7. CONCLUSIONS 1. An earthquake f magnitude ML 5.0 triggered the tw accelergraphs at the Musa River dam site n 16 September, The maximum acceleratin recrded by the Musa A accelergraph at the crest f the dam was 1.85 m/s 2 at a perid f 0.23 s. An acceleratin in excess f 0.3 m/s 2 was sustained at the site fr abut 11 s. The maximum acceleratin recrded by the Musa B accelergraph at the base f the dam site was 0.39 m/s 2 at a perid f 0.04 s. An acceleratin in excess f 0.3 m/s 2 was sustained fr abut 2 secnds. 2. The 2.5 s S-P time interval n the accelergrams indicates that the separatin between the earthquake hypcentre and the dam site was abut 19 km. 3. It is suggested that the 4.5:1 rati f amplitude f grund acceleratin which ccurred at the Musa A site relative t that f the Musa B site can be explained by the cmbined effect f the lithlgy and tpgraphy. 8. REFERENCES Bishp, I. D., Pllck, E. J. and Pettifer, G. R. (in prep.), "Musa River hydrelectric-scheme seismic survey, P.N.G., 1970", Bur. Miner. Resur. Aust. Rec. (unpubl.). Davis, L. L., and West, L. R. (1973), "Observed effects f tpgraphy n grund mtin", Bull. seism. Sc. Amer. 63, Macias, L. F. (1971), "Gelgy f the Musa Grge dam site area. Eastern Papua", Gelgical Survey f Papua New Guinea Nte n Investigatin, Madera, G. A. (1970), "Fundamental perid and amplificatin f peak acceleratin in layered systems", Dept. Civ. Eng., Massachusetts Inst, f Tech. Rpt. N. R69-15, Sils Publ. N Hence the ttal amplificatin rati f the grund mtin at Musa A relative t Musa B when the cmbined effect f lithlgical and tpgraphical factrs are taken int accunt has been theretically estimated at abut 5, which is in reasnable agreement with the measured rati f abut YONKI DAMSITE ACCELEROGRAMS On 20 September, 1974 in the eastern highlands f Papua New Guinea, an amplificatin factr f maximum grund acceleratin amplitude f abut 8 was registered between a weathered sedimentary accelergraph site (Ynki) and a hard rck site (Intake) abut 1 km apart. Preliminary maximum recrded acceleratin and perid at the Ynki site were 2.3m sec 2 and 0.17 sees respectively, and at the Intake site were 0.28m sec" 2 and 0.09 sees respectively. Recrds have

3 117 TAELE 1. SUMMARY OF GROUND MOTION AT MUSA GORGE 16 SEPTEMBER/ 1972 Cmpnent Max. accel. Max. velcity Perid (s) m/s 2 mm/s P wave S wave MUSA A Z NS EW Vectr Resultant Mean 0.23 MUSA B Z NS EW Vectr Resultant Mean 0.04

4 FIGURE 1: LOCALITY OF MUSA RIVER 00

5 t Secnds L 0 cm VERTICAL SCALE (apprx) HORIZONTAL COMPONENTS: 1cm = 1.7ms -2 VERTICAL COMPONENTS: lcm* 1.1ms -a. FIGURE 2 : COPIES OF MUSA RIVER ACCELEROGRAMS

6 ACCELERATION cm/s 2 VELOCITY cm/s ACCELERATION VELOCITY ACCELERATION cm/s 2 VELOCITY cm/s GV C DO c z Q H O z cn DO m«>h *T O m s C m > > m 3D O > x

7 121 FIGURE 4: THE MUSA PROFILES Depth MUSA A MUSA B (m) ~ ~ Cs = 171 m/s Y = 2.2 x 10 3 kg/m 3 Cs = 285 m/s y = 2.5 x 10 3 kg/m 3 in 6 CD lh " ' <D ^ Cs = 456 m/s t * Y = 2.25 x 10 kg/iri 24 Cs = 1311 m/s 61 Y = 2.35 x 10 3 kg/m 3 Cs = 2565 m/s u Cs = 2280 m/s H Y = 2.8 x 10 3 kg/m 3 y = 2.8 x 10 3 kg/m 3 n 4J Ntatin Cs = seismic shear v/ave velcity Y = density Nte (1) Density values are estimates supplied by Macias (pers. cmm.) wh examined the drill cres. (2) The Musa A accelergraph site is 300 m abve the Musa B site.

Differentiation Applications 1: Related Rates

Differentiation Applications 1: Related Rates Differentiatin Applicatins 1: Related Rates 151 Differentiatin Applicatins 1: Related Rates Mdel 1: Sliding Ladder 10 ladder y 10 ladder 10 ladder A 10 ft ladder is leaning against a wall when the bttm