Horizontal Peak Ground Acceleration Attenuation Relationship. Way and Argumentation of its choice.

Transcription

1 Horizontal Peak Ground Acceleration Attenuation Relationship Way and Argumentation of its choice. by Joseph Leonov The attenuation relationship is an essential and as it has been recognized, crucial piece of information for the performance of a probabilistic seismic hazard analysis. The development of an attenuation low is based exclusively on the recorded seismic events. The seismic activity along the Dead Sea Fault system is moderate. Consequently, despite of the relatively dense array of strong motion instruments operating in the region, there are no sufficient acceleration data to develop a regional attenuation function and in the process of seismic hazard assessments for the East Mediterranean Region (EMR), we have to use empirical equations that were developed elsewhere. One of the most used attenuation function, for rock soil conditions was that of Joyner and Boore (1981,1982) fig. 2. The arguments for such a choice was a similarity found in the estimated Q value for the California region and the EMR territory. The necessity to reconsider the existing attenuation low and the consequent seismic hazard re-analysis stemmed from the 22-th November 1995 Gulf of Aqaba earthquake of magnitude 7.1 that occurred on the Aragonese fault, 70 km south of the towns of Eilat and Aqaba. This event, at present, is the strongest one ever recorded in our region. Surprisingly, it triggered strong motion accelerometers installed at distance more than 400 km. Away. Ten stations (tabl. 2) of the Israeli strong motion network and few others in Jordan and in Saudi Arabia, registered this event. From the Seismology and the related areas point of view, this event brought twofold benefit: - - enhancement of the events data-bank;

2 - - enlightening the inaptitude of the JB_82 attenuation relation with respect to the new supplied data. The discrepancy between the predicted and observed values is especially well pronounced for the intermediate and long distances (fig 3). The inconsistency, at first, has been attributed to the local site effect (site amplification factor) presented inherently into onsurface, free-field recorded observations. For this reason, it was imperative to make clear what is the part of the local site effect in any of the available, meaningful records. Assuming none of the sites, with registered strong motion records, behaved nonlinearly, the effect of the local site conditions is eliminated knowing the transfer function of the particular site. Thus, on the base of the Convolution theorem, deconvolving the available record derives the input counterpart on the elastic half-space of the on-surface recorded event. As a result, a necessary task in the process of site effect reduction is to find the transfer function of every instrumented site where strong motion data have been recorded. The transfer function of each such place has been revealed empirically by the use of Nakamura s and HVSR techniques, utilizing free-field measurements performed in stations close vicinity, within period of about a month. Detailed description of how the empirical site transfer (response) function and its numerical counterpart have been determined is clarified in Zaslavsky et al (2002). An example of the result of site effect exclusion is shown in fig. 4 for the strong motion station located in Eilat. For most of the places, the received soil/site amplification factor is in the vicinity of 1.5. Apparently, the exclusion of the local geological conditions influence from the on-surface record does not yield perceptible changes in the PGA values. In such a way, the unsuitability of the JB_82 attenuation low, for the study area could be clearly deduced (fig.5). Unambiguously, the revealed discrepancy between the acquired data and the selected attenuation relation provokes a reassessment of the seismic hazard for the EMR with respect to the new acquired data. Focusing solely on an attenuation relationship, the arising dilemma was either to develop new one or to adopt an existent. An attempt to develop specific one has been made in 1994/95 by Y. Gitterman et al. The information gap into intermediate distances data has been filled with results from differentiated velocity transducers records (fig. 6). A close look on

3 the comparison between the curves this relationship produces and the data at our disposal, after the Gulf of Aqaba EQ, shows its indecency. Naturally, the Gulf of Aqaba and the subsequent several, digitally recorded earthquakes (tabl 1) enhanced the strong motion database but unfortunately not as much as necessary, in particular after ignoring some of the records owing to the poor quality. Hence, as a whole, the data continue to be scarce, with gaps for particular magnitude and distance values. (fig.7) The naturally imposing conclusion is that a data of such a quantity could not provide adequate and consistent base for a reasonable relationship derivation. Accordingly, we were forced to make use of an already derived attenuation relationship. More recently derived relationships have been sought (tabl. 3). State-of-the-art developments are based on more quantitative, qualitative and reliable information as well as encompass more factors having influence on the phenomenon of the ground motion attenuation. Most of the selected expressions have been created within the last decade (fig. 8). All they, in principal, account for the effect of such factors like magnitude, distance, more current of them consider also fault mechanism nature and local site geological conditions. With regards to the long distant PGA values and the correspondent magnitudes, four representatives from the selected collection appear to be more appropriate than the rest (fig 9). In our opinion the most fitting one is that of Boore - Joyner Fumal (BJF). This attenuation law is a notable exception in the way it deals with the local site conditions. Most of the nowadays relationships approach the soil classification in rather simplifying manner e.g., rock/shallow soils, deep stiff soils and soft soils. At variance with them, the BJF relationship incorporates site factor that is essentially a physics characteristic of the local geology, representing it in terms of the average shear wave velocity, measured over the upper 30 m (100 ft). This as well as the equation term referring to the fault mechanism type is built-in features making BJF relationship attractively versatile for adoption (fig 10), (fig 11), These figures also depict that the data values (w.r.t magnitude and distance) are found beneath the graphs of the relevant relationship, as the overestimation is not prominent - the most of the data fall within plus/minus one standard deviation and the prediction seems favorably close.

4 However, some restrictions on this relationship application have to be considered that are underlined by the authors and are attributed to the nature of the data used for its derivation,. The recommendations are to be used with magnitudes above 5.5 and the distances not greater than 100 km. Despite the limitations on the distances, as far as the local data is concerned, the equation is surprisingly suitable also for the long distances. Concisely, the BJF attenuation low of 94 has been chosen and accordingly implemented into the Probabilistic Earthquake Hazard Analysis procedure aiming at renovation of the present Hazard Map. REFERENCES Ambraseys, N. N., Simpson K.A., Bommer, J. J. Prediction of horizontal response spectra in Europe, Earthquake Engineering and Structural Dynamics, 25, 1996, pp Boore D. M., Joyner W. B., Fumal T. E. Equations for estimating horizontal response spectra and peak acceleration from western North American earthquakes: A summary of recent work, Seismological Research Letter No 1, vol. 68, p Campbell K. W. Strong motion Attenuation Relations: A ten-year Perspective, Earthquake Spectra, No.4, vol. 1, p Campbell K. W. Empirical near-source attenuation relationship for horizontal and vertical components of peak ground acceleration, peak ground velocity, and pseudo-absolute acceleration response spectra, Seismological Research Letter No 1, vol. 68, p Fukushima Y., Tanaka T. Acceleration of strong earthquakes ground motion in Japan, BSSA No. 4, vol. 80, p Fukushima Y., Tanaka T. Site-dependent attenuation relations of seismic motion parameters at depth using borehole data, BSSA No. 6, vol. 85, p

5 Gitterman Y., Zaslavsky Y., Shapira A. Analysis of strong records in Israel, Proceedings of XVII-th regional European seminar on earthquake engineering Haifa, Israel 5-10 Sep., Joyner W. B., Boore D. M. Peak horizontal acceleration and velocity from strong motion records including records from the 1979 Imperial Valley, California, earthquake, BSSA No 6. vol. 71, p Munson C. G., Thurber C. Analysis of the attenuation of strong ground motion on the island of Hawaii, BSSA No. 4, vol. 87, p Sadigh K., Chang C.-Y., Egan J. A., Makdisi F., Youngs R. R. Attenuation relationship for shallow crustal earthquakes based on California strong motion data, Seismological Research Letter No 1, vol. 68, p Sharma M. L. Attenuation relationship for estimation of peak ground horizontal acceleration using data from strong motion arrays in India, BSSA No. 4, vol. 88, p Toro G. R., Abrahamson N. A., Schneider J. F. Model of strong ground motions from earthquakes in Central and Eastern North America: Best estimates and uncertainties, Seismological Research Letter No 1, vol. 68, p. 41

6 Fig 2.

7 Table 2: Station PGA Year code Magnitude (cm/sec^2) BNR KFR MIZ HAT IZR (1) DA DA (2) DA DA TUG BET MIF NET ALM JER 8.33 ALM ALN 8.82 ASQ EIL (1) GOS HAC HAD KGM 37.1 MIZ SVT (2) EIL KGM (3) EIL LAV RDG TMR 4.78 GOS (1) KIT ZEF (2) GOS (3) KIT RAM

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12 Table 1: No. date M m Seismic Moment Z/D Latitude Longitude (local) (body) components Region Arad Galilee Arad Arad Samaria Samaria Moav Arad Gulf of Eilat Gulf of Eilat Gulf of Eilat East Mediter Beirut Golan Golan Parran

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17 Hazard Map: