Urban Accelerograph Network in the City of Patras, Greece (UPAN) - Network Characteristics and Analyses of H/V Spectral Values

Transcription

1 6 th International Conference on Earthquake Geotechnical Engineering - November Christchurch, New Zealand Urban Accelerograph Network in the City of Patras, Greece (UPAN) - Network Characteristics and Analyses of H/V Spectral Values A. B. Batilas, P.C. Pelekis, G. A. Athanasopoulos ABSTRACT The University of Patras (Urban) Accelerograph Network (UPAN) comprises 8 surface accelerograph stations and a downhole array to a depth of ~7m. The operation of UPAN since 6, allowed the development of a large data base of earthquake recordings at varying local soil conditions of the urban area. The recordings are analyzed by computing acceleration response spectra and H/V spectral ratios. The results of analyses are utilized for studying the effectiveness of using response, instead of Fourier spectra, in the spectral ratio computations. Measured average acceleration response spectra for each soil class are compared to the design spectra of Eurocode 8 for Type- and Type- earthquakes. The results of analyses indicate a gradual increase of site predominant period and of amplification of motion with decreasing distance from the coastal zone of the city. Introduction The operation of dense accelerograph networks in seismically active urban areas allows the collection of data on seismic site response that are valuable for both: (a) improving our understanding of local site effects and (b) for performing seismic microzonation of the studies areas (Berrill et al., ). This paper aims at providing information regarding the installation and operation of the University of Patras Accelerograph Network (UPAN). Data are given regarding the location of accelerograph stations, the type of instruments, the statistics of the available database as well as results of analyses pertaining to the ratio of horizontal to vertical spectral values (H/V method). The results are used for identification of site response characteristics at different areas in the city of Patras. University of Patras Accelerograph Network (UPAN) The Patras accelerograph network of the Geotechnical Engineering Laboratory of the University of Patras UPAN (University of Patras Accelerograph Network) is a dense urban accelerograph network in the city of Patras, Greece (population 6.). UPAN is comprised of 8 free-field digital accelerographs (Etna 8bit, Kinemetrics) and a downhole array (VA-) (Figure ). The operation of the network started in 6 with the installation of QDR units at sites UP-7 ( Trion Symaxon square) and UP-8 (St. Andreas hospital). In 9 the network was expanded with the installation of six more accelerographs (Etna 8bit, Kinemetrics) at UP- (St. Konstantinos church), UP- (Unisol), UP- (Dasilio), UP- (Politeia), UP- (St. Aimilianos) and Civil Engineer M.Sc., Ph.D., Dept. of Civil Engineering, University of Patras, Greece, abatilas@upatras.gr Ph.D., Assoc. Professor of Civil & Structural Engineering, ASPETE, N. Heraklion, Greece, takis_pelekis@yahoo.gr Ph.D., Professor of Civil Engineering, University of Patras, Greece, gaa@upatras.gr

2 UP-6 (Perivola). In the QDR units of UP-7 and UP-8 stations were replaced by Etna 8bit, Kinemetrics units and a common timing system (GPS). Finally, in a downhole array (VA- ) was installed, with surface and underground accelerometers at depths of -m, -m and - 7.m. The accelerograph stations have been installed at sites with varying soil conditions in order to record the corresponding differentiation of ground motion response. The selection of accelerograph stations location was facilitated by the fact that Patras is a coastal city with varying ground conditions with increasing distance from the sea (moving from coastal zone to the foot hills of Panachaikon mountain). V s -depth profiles have been measured in all UPAN stations using surface wave and cross-hole measurements (Vlachakis et al., ). In addition, for some sites of UPAN stations, soil stratigraphy and soil properties are known via exploratory boreholes. In selecting the sites of accelerograph stations, a great effort was put to achieve freefield conditions (Figure ), thus avoiding or minimizing the soil-structure interaction. The orientation of all sensors corresponds to the North: Y and East: X directions. Geology, Tectonics and Geotechnical Conditions The city of Patras is founded on Quaternary deposits, Plio-Pleistocene sediments and recent alluvium (in the southern part of the city). The Plio-Pleistocene sediments are comprised of ) a lower fine-grained material (dark grey silty marls and sandy silts) that extends to a depth greater than m from the surface and ) an upper coarse-grained horizon, consisting of brownishyellow clayey marls with silts, sands, gravels, pebbles and conglomerates, with an average thickness of m (Koukis et al., ) (Figure a). The bedrock of the area is formed by the Olonos-Pindos geotectonic zone, which consists mainly of flysch, radiolarites and thinly-bedded limestones. The depth to the bedrock under the city is not known; based on exploratory boreholes, however it exceeds m from the surface (Koukis et al., ). The seismicity of the region is associated with a number of fault zones that surround the city, as shown in Figure b. The major earthquake sources (faults) are located in the Corinth graben (to the East), the Patras graben (to the West), the Rion graben (to the NE), which connects the above two grabens, the Pyrgos and Abelonas grabens (to the South), the Achaia-Ilia fault (to the SW), the Cephalonia transform fault (to the West) and the Trihonis graben (to the North). Some recent events that shook the urban area and originated from the above earthquake sources include the Aigion earthquake 99 (Athanasopoulos et al., 998), the Patras earthquake 99 (Plicka et al., 998), the Achaia-Ilia earthquake 8 (Margaris et al., ) and the Cephalonia earthquakes ( /). Exploratory boreholes and laboratory testing of soil samples as well as in situ measurements of shear wave velocity were performed at the locations of UPAN accelerograph stations in the framework of the present study. The results, of these measurements combined with similar published data for the Patras area (Koukis et al., ), allowed the identification of surficial soil stratigraphy of the urban area, and provided V s -depth profiles and dynamic properties of soil

3 UP- Accelerograph Station Lat. ( ) Lοng. ( ) UP UP UP UP UP UP UP UP VA Figure. Satellite image (Google Earth ) of the city of Patras with the locations of accelerographic stations (UPAN) and a photographic view of UP- station Figure. a) Engineering geological map of the city of Patras b) Simplified tectonic map of north Peloponnesus (modified from Koukis et al., ) formations (Vlachakis et al., ). Based on the measured V s -depth profiles, values of V s were estimated for each accelerograph station as shown in Table. It may be seen that the soil conditions (according to EC-8) at stations UP-, UP-, UP- and UP-6 are classified as Class B (EC-8), whereas stations UP-, UP-, UP-7, UP-8 and VA-, as Class C. It should be noted, however, that the soil materials underneath stations UP- and UP-7, with values of V s approximately equal to m/sec, could be classified as a transition from Class C to Class D.

4 Statistics of Earthquake Recordings The operation of UPAN (since 6) helped to develop a large database of earthquake recordings that can be used for studying the effects of local soil conditions, as well as source and path effects, on seismic ground response. The results of such studies can be utilized for improving our understanding of the broad field of site effects and also for a seismic microzonation of the urban area of Patras. Table. V s values and soil classification for UPAN stations. UP- UP- UP- UP- UP- UP-6 UP-7 UP-8 VA- V s (m/s) Soil Class (EC-8) B C B C( D) B B C( D) C C Soil Class (NEHRP) C D C DE C C DE D D At present (Jan, ), the UPAN database comprises recordings of more than seismic events, with epicentral distances.7km to 96km, local magnitudes from. to 6.8, focal depths from km to km and azimuths (with regard to the center of city) from to 6. The graphs of Figure show the distribution of the number of available earthquake recording with regard to magnitude, hypocentral distance, focal depth, azimuth, incident angle and peak ground acceleration, PGA. Further correlations between magnitude, PGA, hypocentral distance and focal depth of recorded earthquakes are shown in the graphs of Figure. According to the above graphs, the majority of recorded earthquakes, have magnitudes from to (weak earthquakes), hypocentral distances less than km, focal depths between and km, azimuths between and, incident angles from and 6, and PGA values less than cm/s (weak motions). Figure. Number of available earthquake recordings as a function of local size (M L ), hypocentral distance (R hyp ), focal depth (H), PGA, azimuth and incident angle (a)

5 Magnitude, M L 7 6 UPAN's sensitivity (α) Hypocentral distance, R hyp (km) Hypocentral distance, R hyp (km) (b) 6 8 Focal depth, H (km) 7 6 PGA (cm/s ) Magnitude, M L (c). Hypocentral distance, R hyp (km) (d) 6 8 Focal depth, H (km) Figure. Graph of (a) magnitude-distance distribution, (b) source distance-focal depth distribution, (c) PGA-source distance distribution, and (d) magnitude- focal depth distribution Preliminary Analysis of Earthquake Recordings When analyzing recorded earthquake accelerograms the most frequently used tool is the acceleration response spectrum (usually normalized with respect to PGA value). The diagrams of Figure show the average (N varying from 88 to ) normalized response spectra for the accelerograph stations UP- to UP-8 computed for recorded earthquakes of Type-, which according to EC-8, are events with M.. The diagrams also depict shade areas corresponding to the region from median to +s.d. It may be seen that in all cases a predominant period of shaking can be identified, varying between.sec and.8sec and increasing with decreasing V s values. Each diagram also includes the design spectrum of Eurocode (EC-8) corresponding to soil class determined on the basis of V s values given in Table. It is observed that in all cases the code overestimates the normalized median acceleration response spectra, although agrees well with the +s.d. curve. Similar diagrams for Type- earthquakes (M>.) are shown in Figure 6. It has to be noted, however, that the normalized response spectra of Figure 6 have been computed, based on a very small number of recorded earthquakes ( to ), resulting in an increased uncertainty, compared to Type- earthquakes. Nevertheless, it is interesting to note that in this case the predominant periods of shaking are significantly increased (.sec to.8sec), especially for the softer sites (V s =m/sec to m/sec) and that the EC-8 design spectra underestimate the measured normalized median spectral values. It is generally accepted that a response spectrum in addition to local soil conditions at the recording station also reflects the effects of source mechanism and of travel path of seismic waves (Borzgonia and Bertero, ). In the case of UPAN the very small distance between the recording stations, justifies the assumption that source and path effects are similar for all accelerograph stations. Thus, any differentiation in the shape and amplitude of response spectra at different stations (i.e. different locations of the city) would be expected to result from different ground conditions.

6 UP- (N=) S=. V s =8m/s UP- (N=7) S=. V s =6m/s UP- (N=6) S=. V s =m/s S a /PGA..... UP- (N=) S=. V s =m/s..... UP- (N=) S=. V s =7m/s..... UP-6 (N=88) S=. V s =6m/s..... UP-7 (N=7) S=. V s =m/s..... UP-8 (N=) S=. V s =m/s..... Median (geometric mean) EC-8 (Type - M<. ) + s.d Period, sec Figure. Comparison of average recorded acceleration response spectra (% damped) with EC-8 Type (M.) elastic response spectra UP- (N=) S=. V s =8m/s UP- (N=) S=. V s =6m/s UP- (N=) S=. V s =m/s S a /PGA..... UP- (N=) S=. V s =m/s..... UP- (N=) S=. V s =7m/s..... UP-6 (N=) S=. V s =6m/s..... UP-7 (N=) S=. V s =m/s..... UP-8 (N=) S=. V s =m/s..... Median (geometric mean) EC-8 (Type - M>. ) + s.d Period, sec Figure 6. Comparison of average recorded acceleration response spectra (% damped) with EC-8 Type (M>.) elastic response spectra

7 In order to remove, however, the source/path effects, it is possible to estimate the horizontal to vertical spectral ratios (HVSR) at each station (Nakamura 989, Lermo and Chavez-Garcia, 99; Theodoulidis and Bard, 99; Mucciarelli et al., ). In the present study the spectral ratio for each earthquake was estimated using % damped response spectra instead of Fourier spectra (Yamazaki and Ansary, 997; Zhao et al., 6). Both types of average spectral ratios (i.e. response HVRSR spectra and Fourier HVSR spectra) were estimated for all recorded earthquakes and are shown in the diagrams of Figure 7. It may easily be seen that the agreement between the two types of spectral ratios (HVSR vs. HVRSR) is very good. Furthermore, it is observed that the shape of H/V ratio curves varies between different stations, in terms of both, predominant period and amplitude. In addition at sites of UP- and UP-, the H/V spectral ratio takes low values (~) and shows a flat response, with no identifiable predominant periods. It should also be mentioned that H/V spectral ratios show an increase of predominant periods and amplification as the distance from the coastal zone is decreased. The correlation between response spectra, H/V spectral ratios and local soil stratigraphy at the accelerograph stations is the subject of on-going investigations. UP- (N=6).sec.sec UP- (N=7) UP- (N=6) HVSR UP- (N=).7-.sec.-.98sec UP- (N=7).-.6sec.-.sec UP-6 (N=8) HVRSR... UP-7 (N=7).7-.sec... UP-8 (N=86).sec Figure 7. Spectral ratio HVSR compared to response spectral ratio HVRSR at the stations of UPAN Conclusions The operation of the urban accelerograph network in the city of Patras, Greece (UPAN) started in 6 and has allowed the development of a large database of earthquake recordings, that are analyzed in terms of average response spectra and H/V spectral ratios at each station. The analyses indicate a differentiation of surface response as a function of distance from the shoreline

8 and of the stiffness of ground materials. The measured normalized average response spectra at each accelerograph station were compared to the EC-8 normalized elastic design spectra for the corresponding soil classes. The comparison indicates that EC-8 either overestimates or underestimates the response, depending on the type of earthquake (i.e. for magnitude M. and >., respectively). H/V spectral ratios computed by utilizing response spectra (with no further smoothing) were found to be almost identical to corresponding H/V Fourier spectral ratios. References Athanasopoulos G. A., Pelekis P. C. and E. A. Leonidou (998), Effects of surface topography and soil conditions on the seismic ground response -including liquefaction - in the Egion (Greece) /6/99 earthquake, Proceedings, Eleventh European Conference on Earthquake Engineering. Paris, France, (in CD-ROM), Rotterdam: Balkema. Berrill J., Avery H., Dewe M., Chanerley A., Alexander N., Dyer C., Holden, C., and Fry B., (), The Canterbury Accelerograph Network (CanNet) and some Results from the September, M7. Darfield Earthquake, Proceedings of the Ninth Pacific Conference on Earthquake Engineering Building an Earthquake- Resilient Society, -6 April,, Auckland, New Zealand. Borzgonia Y. and Bertero V., (), Earthquake Engineering: From Engineering Seismology to performancebased engineering, CRC Press LLC, ISBN ),. Eurocode 8: Design of structures for earthquake resistance - Part : General Rules, Seismic Actions and Rules for Buildings (EN 998-), CEN, Brussels,. Koukis G., Sabatakakis N., Tsiambaos G., and Katrivesis N., (), Engineering geological approach to the evaluation of seismic risk in metropolitan regions: case study of Patras, Greece, Bulletin of Engineering Geology and the Environment, 6: 9-. Lermo J. and Chavez-Garcia, F.J., (99), Site effect evaluation using spectral ratios with only one station, Bulletin of Seismological America, 8 (): 7-9. Margaris B., Athanasopoulos G., Mylonakis G., Papaioannou C., Klimis N., Theodoulidis N., Savvaidis A., Efthymiadou V. and Stewart J. (), The 8 June 8 Mw6. Achaia-Ilia, Greece Earthquake: Source Characteristics, Ground Motions, and Ground Failure, Earthquake Spectra, 6 (): 99-. Mucciarelli M., M.R. Gallipoli, (), The HVSR Technique from Microtremor to Strong Motion: Empirical and Statistical considerations, th World Conference on Earthquake Engineering, Vancoyver, B.C., Canada, August -, paper No.. Nakamura Y., (989), A method for dynamic characteristics estimation of subsurface using microtremor on the ground surface, QR of RTR, (). Plicka V, Sokos E, Tselentis A-G, Zahradnik J (998), The Patras earthquake ( July 99): relative roles of source, path and site effects, Journal of Seismology : 7 9. Theodoulidis N.P., and Bard P.-Y., (99), Horizontal to vertical spectral ratio and geological conditions: an analysis of strong motion data from Greece and Taiwan (SMART-), Soil Dynamics and Earthquake Engineering, : Vlachakis V., Pelekis P., Batilas A., and Athanasopoulos G., (), Shear Wave Velocity Measurements in the Area University of Patras Accelerograph Network (UPAN) Using Active & Passive Surface Wave Methods, SGEEP, Austin Texas, -6 March,. Yamazaki F., and Ansary M.A., (997), "Horizontal-to-Vertical Spectrum Ratio of Earthquake Ground Motion for Site Characterization", Earthquake Engineering and Structural Dynamics, 6: Zhao J.X., Irikura K., Zhang J., Fukushima Y., Somerville P.G., Asano A., Ohno Y., Oouchi T., Takahashi T., and Ogawa H., (6), "An Empirical Site-Classification Method for Strong-Motion Stations in Japan Using H/V Response Spectral Ratio, Bulletin of Seismological America, 96 (): 9-9.