Engineering Seismology

Empirical and analytical methods are used to assess site specific earthquake hazards.
Seismological surveys are used to determine dynamic parameters of existing buildings and structures in order to assess design procedures and performance of structures.


Determining Site Amplification Effects:

The Geophysical Institute of Israel offers services for measuring and estimating site amplification for strategic sites. Local geological site conditions play an important role in the determination of the site-specific seismic hazard. The phenomenon of seismic wave amplification in sedimentary deposits and topographical conditions is a major factor affecting overall damage and loss during strong earthquakes. GII's Seismologists are engaged in various projects for estimating site amplifications at various locations both in Israel and overseas.


1. Earthquake zonation map for the cities of Israel :


Based on analysis of some 4,500 microtremor measurements at various locations within 15 cities of Israel, GII seismologists have prepared microzonation maps in terms of amplification and fundamental frequency across the area investigated.





2. Earthquake Site Response study for proposed bridges in Israel:

During the years, almost every significant bridge planed by Netivei Israel (Ma'az) were surveyed evaluating the Site Specific Response (to earthquakes) i.e. using horizontal-to-vertical spectral ratio from ambient noise measurement to approximate fundamental resonance frequency of the subsurface and its amplification as well as evaluation of the geotechnical characteristics (shear-wave velocity and thickness of sediments) for one-dimensional analysis of site effects by detailed comparison of the analytical site response functions and experimental spectral ratios (stochastic optimization algorithm).


An example : bridges along rout no. 71

A N-S cross-section in the area of Yechezkel Junction (Rout No. 71) based on geological, geophysical borehole information and the results of ambient noise measurements.

 

A site response function (dotted line) evaluated by analytical calculation of the sub surface model versus Spectral Ratio (full line) as resulted from ambient noise measurements at the Yechezkel Junction Site (right – south end; left – north end)



3. Topographic site effects:

Predominant frequency and amplification factors were determined for the high plateau on several mountains in Israel. The investigated sites exhibit amplification which can be attributed to topography. In places the site exhibits a relatively strong amplification effect (up to 4) in the frequency range of about 2.5 to 3.5 Hz. The data collected at different sites indicate that topography can have a significant effect on seismic signals. These observations are important in terms of the seismic safety of facilities built on topographical prominences.

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4. Empirical estimates of model parameters of full scale structures:

The ability to evaluate the vulnerability of existing structures during an earthquake, especially in regions where strong earthquakes are very rare, depends on the model of the structure according to its dynamic characteristics.
Relatively simple analysis of microseisms (ambient noise), explosions and earthquakes recorded by seismometers placed on the structures, facilitates determination of the response frequencies of the structure, the mode shapes and the damping ratios under linear conditions (weak motions).
We have obtained high resolution results in determining natural frequencies and have estimated the mode shapes and damping ratios of the first modes of an hotel at a Dead Sea resort, prefabricated residential buildings and various school buildings in Eilat, Israel.