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Title Attenuation of Coda Waves in the Source Area of the 1990 July 16 Luzon Earthquake, Philippines Author(S)

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Title Attenuation of Coda Waves in the Source Area of the 1990 July 16 Luzon Earthquake, Philippines Author(S) View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Kyoto University Research Information Repository Attenuation of Coda Waves in the Source Area of the 1990 Title July 16 Luzon Earthquake, Philippines Author(s) KANAO, Masaki; ITO, Kiyoshi Bulletin of the Disaster Prevention Research Institute (1992), Citation 42(2): 31-51 Issue Date 1992-06 URL http://hdl.handle.net/2433/124987 Right Type Departmental Bulletin Paper Textversion publisher Kyoto University Bull. Disas. Prey. Res. Inst. , Kyoto Univ. , Vol. 42, Part 2, No. 365, June, 1992 31 Attenuation of Coda Waves in the Source Area of the 1990 July 16 Luzon Earthquake, Philippines By Masaki KANAO and Kiyoshi ITO (Manuscript received on December 15, 1991) Abstract Coda Q values are determined by using small aftershocks of the Luzon earthquake of July 16, 1990 in Philippines (M = 7.8) recorded by temporary observations. The observations were conducted from August to December in 1990 as a cooperative project between the Disaster Prevention Research Institute, Kyoto University and the Philippine Institute of Volcanology and Seismology. The temporary stations were set at 21 sites, in an area of more than 150km along the Philippine fault and around the city of Baguio. 16-88 events, having S-P time less than 5.0s were analyzed at each station. The single scattering model of coda generation was applied to the band-pass filtered RMS coda amplitudes in five frequency bands from 2 to 32Hz. The time window was taken for all the events from more than twice the arrival time of S wave to 25s from origin time, which indicates that the scattering area contributing to the coda excitation is within a radius of approximately 45km. By fitting the coda Q to the power law, Q,=Qef, the averaged values of coda Q and frequency exponent n were obtained as Q,,= 68, and n=1.06 around the city of Baguio. On the other hand, Qe= 165, and n=0.84 in the southern part of the fault where the rupture of the main shock initiated. The lower value of coda Q and the higher value of n in the northern part suggest the existence of large heterogeneities, such as small cracks in the crust made by active aftershock activity around the city of Baguio. As for the southern part of the area, fault movement is considered to be smooth enough not to develop more cracks than in the northern area. The frequency dependence of coda source factors was determined to examine the difference in site amplifications according to respective geological aspects at each observation station. The temporal change in coda Q and coda source factors during four months after the occurrence of the main shock were not obvious when considering their errors. I. Introduction Coda parts of S waves are considered to consist of S waves scattered by many heter- ogeneitiesdistributed randomly in the lithosphere7' 8' 25' 26' 33) The decay rate of coda amplitude with time shows the attenuation property of the upper lithosphere. The quality factor determinedfrom the decay of coda amplitude with time is called coda Q (Q). Coda Q values have been determined in various regions in the world' S,6' 9' 10, 17, 20, 23, 27). Regionaldifferences in Q, have also been revealed by the studies of n in the formula of Q= Qofn, where f is frequency. The relationship among tectonic features, seismicity and coda Q has also been pointed out by analyzing coda waves. For example, Jin and Aid 14)searched the regional variations in coda Q for the oceanic lithosphere. They found that younger oceanic lithosphere has a higher frequency exponent n value than older one, which is similar to the continents where the active regions have higher n values. Thus coda Q and n values are 32 M. KANAOand K. ITO effective in determining the attenuation characteristics and their relation to tectonics in studied area. In and around the Philippine Islands, geophysical studies of crustal velocity structure for the Manila Trench and its fore arc basin were carried out by means of multichanned seismic reflection prospecting11,18). Attenuation of earthquake—shaking intensity with epicentral distance in the Philippines was determined from the data of 83 earthquakes 34). The result was quite similar to that in the San Andreas fault region. On the other hand, Acharya 2.'3) suggested that the attenuation of ground motion in the Philippines is less severe than in the San Andreas area. Since then, no detail analyses of seismic waves have been performed to reveal the structures of velocity and attenuation in Luzon Island. In particular, Q values have never been determined in the Philippines by using small local earthquakes. On July 16, 1990, a large earthquake with a magnitude of 7.8 occurred in the central part of Luzon Island. The event was a large shallow intra—plate earthquake common along the Philippine Fault, which, in fact, has caused many large historical earthquakes in Luzon Island. The ground ruptures by the earthquake extended about 150km from Gabaldon (GBD) to Digdig (DIG) as shown in Fig. 1. The seismic strong motions caused extensive damage along the Philippine Fault and around the city of Baguio. In order to study the distribution of after- shocks in detail, the observations of aftershocks were conducted two times as a cooperative project between the Disaster Prevention Research Institute, Kyoto University (DPRI), and the Philippine Institute of Volcanology and Seismology (PHIVOLCS). Temporary observation stations were set along the earthquake fault and in the vicinity of Baguio City. Many aftershocks were recorded and about 200 hypocenters were determined accurately by Ohkura et cd.21). Many records of the aftershocks have sufficient excitation of coda waves to determine coda Q. Seismograms of these temporary observations were used for the determination of coda Q in the crust and upper mantle. The records at 13 observation stations were used for the analyses of coda waves. The regional difference in coda Q values in the aftershock area of the Philippine earthquake is discussed in relation to the aftershock activities. Coda source factors were also determined from the data. Under the assumption that the averaged value of coda source factors of many events at a station indicates the site effect near the station, the frequency dependence of coda source factors for all the observation stations are studied to reveal the relationship between the frequency dependence of coda source factor and the surface geology. Furthermore, site amplicication is discussed on the basis of the coda source factor. Temporal change in Q,, is also discussed. 2. Data and Analyses 2. 1 Data Seismic wave forms obtained by the two observations of the aftershocks of the 1990 Philippine earthquake of July 16 were used for the analyses of coda Q in this paper. The first observation was conducted from Aug. 30 to Sep. 4, 1990. Twelve temporary stations were set along the Philippine Fault and in the vicinity of Baguio City. The Locations of the observation stations are listed in Table 1(a) and shown in Fig. 1 by solid triangles and Attenuation of Coda Waves in the Source Area of the 1990 July 16 Luzon Earthquake, Philippines 33 . , 1 t I 11–N i 17.0 N la- - TAG I I 50kin SAB/BUR _ A I\ ATK/AT2 PUG A-1 BAY A BAG * M0.0 1 CP1AM 16 .0 N —ss''-DIG CAR d A APB1/PB2 11111 i Phillipine 1\47.8 II,,\iiki.. LUP`-) "Dfault • • ...Zi: rur; II TAL Apity f Io,G. FMSGBD !111\11 \ I I l% 120.0 E 121.0 E Fig. 1. Map of Luzon Island showing the locations of temporary stations. Solid triangles show the stations set for the first observation. The open triangles show the stations set for the second observation. The half solid triangles show the stations set for both obvervations. Stations SAB, ATK, and PB1 were moved to more adequate neighboring places BUR, AT2 and PB2, respectively,during the observations. The large star shows the epicenter of the 16 July 1990 Luzon earthquake (USGS), The small star shows the epicenter of the largest aftershock. The thick lines show the faults. half–solid triangles. The seismograms analyzed were recorded with high–gain vertical–component short period (2Hz) velocity–type seismometers at all stations except the station CAR. A horizontal–com- ponent was also set at CAR as the vertical one. Three types of records were used for the analyses. One is digital seismograms recorded in floppy discs at nine stations, another is digital seismograms recorded on audio magnetic tapes at stations GBD and CP1, and the other is FM records at the station CAR. The seismograms at CAR were digitized on a personal computer for the following analyses. Details of the recording systems are described by Ohkura et aL21). The second observation was conducted from Nov. 17 to Dec. 7 by staff members both of S ' \ 34 M. KANAOand K. ITO Table 1. Locations of observation stations ; (a) stations of the first observation, (b) stations of DPRI in the second observation, and (c) stations of PHIVOLCS in the second observation. The abbreviation codes corres- pond to those in Fig. 1. station code lat. (N) long (E) Height (km) (a) Palayan PLY 15.5865 121.1194 0.090 Gabaldon GBD 15.4501 121.3354 0.125 Talavera TAL 15.5999 120.9713 0.045 Fort Magsaysay FMS 15.4363 121.0758 0.100 Pantabangan 1 PB1 15.8143 121.1058 0.150 Pantabangan 2 PB2 15.8110 121.1070 0.220 Lupao LUP 15.8473 120.9499 0.160 Digdig DIG 15.9489 121.0007 0.300 Carranglan CAR 15.9547 121.1055 0.265 San Manuel SAM 16.1327 120.6862 0.100 Camp One CP1 16.2353 120.5265 0.120 Baguio BAG 16.4162 120.6210 1.520 Atok ATK 16.5058 120.6806 1.824 (b) Sablan SAB 16.4965 120.4867 0.460 Burgos BUR 16.5183 120.4630 0.460 Atok ATK 16.5058 120.6806 1.824 Atok2 AT2 16.4983 120.6733 1.750 Pugo PUG 16.3321 120.4765 0.100 Camp One CP1 16.2353 120.5265 0.120 (c) Tagudin TAG 16.9180 120.4590 0.100 Bayombong BAY 16.4830 121.1300 0.320 Baguio BAG 16.4162 120.6210 1.520 DPRI and PHIVOLCS.
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