Post image for A procedure to estimate the seismic hazard in an urban area: an application to Acireale (Eastern Sicily)

The new paper:

A procedure to estimate the seismic hazard in an urban area:
an application to Acireale (Eastern Sicily)

has been published by Springer-Verlag in Environmental Earth Science. (2011) 64:1777–1786.
The online version of the article is visible here and you can download the PDF from here.

Abstract:

A “standard procedure” to characterize the seismic hazard of a given area was proposed. It is based on a multidisciplinary approach implying: (1) the knowledge of the seismic history of the area; (2) detailed geological surveys; (3)seismic noise measurements; (4) simulations of earthquake scenarios. The downtown of Acireale, a typical baroque town located on Eastern Sicily, was chosen as the “test area”. A catalog of the local seismogenic faults (able to generate earthquakes in historical times) has been compiled, as well as a seismic catalog for the effects of both local and regional earthquakes. The analysis of both catalogs allowed us to make the following conclusions: (1) the most important seismogenic faults affecting the Acireale municipality do not affect the downtown, while the related local earthquakes attenuate their energy (and intensity) in short (few km) distances; (2) the highest seismic intensity (degree X) experienced in Acireale downtown was caused by the 1693 regional earthquake; (3) over the last 140 years, the downtown has experienced the highest intensity value of VII only once, while six times the intensity was VI. On the whole, this implies a moderate seismic hazard. The estimation of the seismic hazard has been also approached by the experimental method of recording seismic noise. Measurements have been performed at seven different sites, where drills gave detailed information on the shallow subsurface geology to obtain HV (horizontal/vertical) spectral ratios. On the whole, the highest site amplification factor was moderate (about 7). A further investigation based on synthetic seismograms (and spectra) produced by simulating two given earthquake scenarios was also performed. The two scenarios are, respectively, representative of the largest expected earthquake in the area (the 1693 shock) and of a moderate (magnitude ca. 5.5) local earthquake (as the 1818 one). Moderate to strong locally expected accelerations were evidenced.

Authors:

Sebastiano Imposa, Francesco Barone, Domenico Bella, Massimo Cristaldi and Stefano Gresta.

Catania, July 14th 2011

We recently update Shakyground website providing a new user interface a a better usability. For any comment or inquire on the website please use our contact form.

Catania, July 10 2009

Version 2 of ShakyGround has been released today. The whole website has been updated. New version of shakyground provides:

  • Better user interface
  • Interoperability with Google Earth for defining Earthquake locations and Source location
  • Interoperability with Microsoft Visual Earth for checking earthquake position within the program
  • New “Project” hierarchy to group together different models
  • Faster speed and code rewritten with .NET framework support
  • New DEMO version does not need a user registration to run. Simply download the product from this page.
  • Use ystem coordinate in WGS84

Users of version one may request an update to version 2. Please send your request using the contact form.

A demo version is available for download. Please register to this website to download the program.

absorption: energy loss of [seismic] waves. These losses are supposed to be caused by energy conversion to heat (inelastic absorption) or scattering, i. e., reflection and refraction of the waves during their propagation.

attenuation: amplitude decrease of [seismic] waves during their propagation. The attenuation of seismic waves is caused by absorption and geometrical spreading due to the decay of the elastic potential of an expanding wave field.

corner frequency: Determined by the intersection of the trends of spectra in the low frequency range and the high frequency range. The corner frequency of seismic spectra are closely related to the size of the seismic source. Common models suppose that the corner frequency is inversely proportional to its extension.

epicenter: Vertical projection of the hypocenter to the earth’s surface. Its coordinates are commonly given in degrees latitude / longitude or in locally defined coordinate systems.

focal depth: Depth of the focus measured with respect to the earth’s surface.

focus: The point of the seismic source where rupture and seismic wave radiation starts. Since the localization of seismic sources is performed reading the onset times, the source coordinates reported in the literature commonly correspond to the coordinates of the focus.

geotechnical parameters: Parameters describing the parameters of rock which are of technical importance. In the context of SHAKYGROUND these parameters are understood as the seismic velocity (or the elastic moduli),the density, the thickness and the inelastic absorption characteristics (Q) of the layers making up the propagation medium.

Green’s function: Function describing the transmission of the seismic wavefield from the source to the receiver. It corresponds to the transfer function / impulse response of the propagation medium.

hypocenter: The location of the focus. Its coordinates are the epicentral coordinates plus the focal depth.

intensities: See macroseismic intensities.

macroseismic intensities: Measure of earthquake effects felt at the earth’s surface. The macroseismic intensities are an ordinal measure described in the macroseismic scales. Note that the energy released by an earthquake cannot be directly determined from this measure.

macroseismic scales: Description of the effects which are assigned to the values of the macroseismic intensities. The most common scales (MCS = Mercalli-Cancani-Sieberg, MKS = Medvedev-Karnik-Sponheuer) consist of 12 degrees.

magnitudes: Metric measure representing the energy released during an earthquake. It is determined by reading the maximum amplitudes on seismograms and applying appropriate corrections for the attenuation of seismic waves on their way from the seismic source to the receiver. The most common magnitudes are the surface wave magnitude MS(determined from surface wave amplitudes with a period of 20 s) and the local magnitude Ml or MWA (obtained from a record with a short period seismometer such as the WOOD ANDERSON seismometer).

Q: Material characteristics of absorption. It is defined as the energy decay of seismic waves per wavelength. In the context of Shakyground Q is assumed to be related to the energy losses due to inelastic absorption and constant over a wide frequency range.

radiation pattern: azimuth dependence of seismic wave amplitudes. In a strict sense the radiation pattern is defined for seismic waves with a period tending to infinity using a point source model. The radiation pattern for P- and S-waves shows 4 symmetric lobes separated from each other by 2 perpendicular nodal planes, where the amplitudes vanish. In practice the distribution of seismic wave amplitudes with respect to azimuth is complicated by the fact that the source has a finite extension, by effects of rupture propagation (Doppler effect) and wave propagation.

response spectrum: A set of values consisting of the maximum responses of single mass oscillators with varying natural frequencies and a given damping. The single mass oscillators are supposed to have one degree of freedom of movement. The response spectra of most seismic regulation codes are based on a damping coefficient of 5 % of critical, other common values are 1 %, 2% and 10 %, respectively.

seismic moment: The integral of all dislocations taking place across the source plane, weighted by the local shearing moduli. It is one of the basic parameters for the source model used in Wimsimul.

seismic source: The origin of seismic waves. In Shakyground the seismic source is supposed to be an earthquake, with a sudden release of elastic energy due to rupture of rock.

seismic spectrum: The spectral representation of the seismic signal.

shear fracture: Fracture characterized by two blocks sliding with respect to each other. Shear fractures typically make angles with the principle direction of stress of about 30 to 45 degrees, depending on the coefficient of friction. In earthquake theory the seismic source is supposed to represent a shear fracture. As a consequence of this fact the most dominating radiation of earthquake sources are shear waves.

source plane, area: The area which ruptures during an earthquake.

source radius: In the widely used circular source model the half diameter of the source area. It is estimated from the corner frequency of the seismic spectrum.

stress drop: The stress released during an earthquake. The stress drop expresses the relation between the ruptured source area and the amount of dislocation taking place on this area. Whereas it is difficult to determine the amount of stresses released on the single points of the seismic source, the global stress drop can be derived from the seismic moment and the source radius.

During run time the numerical kernel of SHAKYGROUND performs a number of checks in order to ensure a proper processing of the model parameters. As quite normal in numerical processing, the formulae are valid only within certain ranges and may become unstable it these limits are violated. In most cases the limits are wide enough to cover the user’s demands, nevertheless it may happen that users try to overstretch the underlying concept, or, for unfortunate combination of model parameters the numerical instability is encountered in an unforeseen manner. In order to limit problems due to numerical instabilities which may even cause program failure SHAKYGROUND tries to fix the possible reasons adjusting the critical parameters in a suitable way. SHAKYGROUND reports the critical situations encountered during run time writing an integer number, the so-called “Message Code”to the file “simul.log”. The single events are coded as follows:

1 = correction of layer thickness (happens if thickness of layer stack is less than focal depth)

2 = danger of numerical overflow in absorption terms

4 = overcritical reflection encountered

8 = time window probably too short

16 = upper frequency limit of response spectrum calculation adjusted.

Since the single events can occur simultaneously the final message code reported by SHAKYGROUND is given by the sum of the codes of the single events. For example a message code of 17, which occurs quite often, corresponds to a “correction of layer thickness” (1) plus the adjustment of the upper frequency limit of the response spectrum calculation (16).

You have now completed the editing of your input sheets. Click now the “Space Shuttle” icon on the tool bar. SHAKYGROUND asks for your patience and after some time of processing displays the standard output screen (see Fig. 14). The graphics show three response spectra, i. e., the average obtained over all simulations, the spectrum corresponding to the average plus one standard deviation (often referred to as “84%-spectra”) and the peak-hold spectra, which represent the highest values encountered during the whole number of simulations. Note that due to the stochastic source model there always be a statistical fluctuation of the response spectra even if the model parameters were not subjected to a random fluctuation.

Together with this graphics a table is presented reporting the most important signal parameters as maximum and RMS-values of ground acceleration, velocity and displacement. The strong motion duration is defined in terms of a HUSID plot, and corresponds to the time window where

of the total signal power, and a(t) is the acceleration time series (see Fig. 15).
Finally two magnitude values are given. MS is the surface wave magnitude deduced from the seismic moment after Geller (1976). MWA is the WOOD-ANDERSON magnitude and corresponds roughly to Ml as obtained on common short period recordings. MWA is estimated on the base of the synthetic traces, MS on the other hand on the base of an input parameter, i. e., the seismic moment. Similar to the response spectra the average of these signal parameters is given together with the values of average +one standard deviation and the peak hold values. For the reason explained earlier there will be always a statistical fluctuation of these values (besides MS) even if no random fluctuation was applied to the input parameters of the model.
On the right hand side of your screen you note two buttons labeled “Save Results” and “View Results”. Clicking “Save Results” the contents of the screen are saved to the model data base, for examples “Simul.mdb”. You can also choose to “Save [the]Results in ASCII”. SHAKYGROUND writes the spectra and the contents of the table with the signal parameters to a textfile in your current working directory. This which can be edited with any editor and easily imported to other programs like EXCEL for further processing. Choosing this option SHAKYGROUND asks you to specify the filename; if it alraedy exists you may either overwrite it or append your actually produced results to former ones.
Selecting “View Results” you have the possibility to scroll through the actually created time series and response spectra. Your screen should look like Fig. 16. Note the list-box with the arrow on its right. Use the arrow to get a list of available traces. SHAKYGROUND shows the numbers and you may click one of them in order visualize the corresponding traces.

Fig. 14. SHAKYGROUND’s standard output screen.

Then select the type of traces you wish to see clicking the flip-flop boxes. You may also select the length of the time window you want to see. SHAKYGROUND suggests a default value which should permit you to see all significant parts of the signal. You may choose to see the a longer or shorter window by specifying a corresponding number of points in the box labeled with “Time window”. You can choose the part of the visualized response spectra in a similar way specifying the number of points in the box labeled “RS Window”.

Now click the field with the traces or the response spectrum shown on your screen. The graphic routines used in SHAKYGROUND permit to select the field and to adjust its size. You also may click the strings like “Acceleration (m/s^2)”, “Displacement (m)” and so on and move them around on the field. If you desire to export some graphics to an other application, (e. g., MS Word where you are processing a document) select the area of interest, then press the “Print” key on your keyboard. Doing so you copy the graph to the clipboard of your computer. Now move to your application and point the mouse to the place where you wish to paste the graph.

Fig. 15. Visualizing Synthetic traces (acceleration, speed, displacement)

If there are no created traces or response spectra SHAKYGROUND prompts you a message. Return to the previous screen clicking the little X in the upper right corner of the screen (attention: use the X on the same row as the Strings “Input” and “Output”, otherwise you terminate the SHAKYGROUND session).

Clicking the “Response Spectrum” sheet your screen should display something similar to Fig. 10. You will note three fields, i. e. “Frequencies” “Type of Response Spectrum”, and “Damping”. In the “Frequency” field you indicate the highest and lowest frequency of the response spectrum together with the stepwidth of response spectrum calculation. Note that SHAKYGROUND cuts down the upper frequency limit to a value corresponding to 1/6 of the digitization frequency. If you specify, for example, a digitization frequency of 200 Hz (see the “General” sheet) the uppermost possible value of the response spectrum will be 3313Hz. This limit has been introduced for reasons of numerical stability. You can rise this limit by choosing a higher digitization frequency together with a larger number of points of the synthetic seismogram. For more details about how to choose the digitization frequency and seismogram length see the paragraph concerning the “General” sheet.

The Response Spectrum Sheet.

The choice of the “Type of Response Spectrum” to be calculated is quite simple. Just click the box of your desired response spectrum type. Choose the damping value of the response spectrum using the box named “Damping” at the bottom of the sheet. The damping is understood in fractions of 1 instead of %. For a 5% response spectrum choose a value of 0.05 in the “Damping” box.

The choice of the seismogram length and the digitization frequency requires some care. From the viewpoint of computing time and numerical exactness it is desirable to work with short seismogram length and, on the other hand, with high digitization frequencies. The attempt to match both demands at the same time, however, may cause problems due to an effect known as “wrap around effect” or “alias in the time domain”. The “wrap around effect” resides in the periodicity of the Fast Fourier Transform. It happens if the duration of the signal is longer than the time window given by the seismogram length divided by the digitization frequency. The parts of the signal, which would fall outside the time window, appear at its beginning, interfering with the signals occurring there for true physical reasons.The effect of wrap around may occur essentially for two reasons: First, the duration of the Green’s function is longer than the selected time window. This typically happens if the sources are distant from the receiver, i. e., the traveltime of the direct signal becomes longer than the time window. You can backtrace this effect from the “eventcode” produced by SHAKYGROUNDs numerical kernel. The eventcode is reported in the file “simul.log”. A further reason for wrap around may be the choice of seismic source moment and global stress drop yielding a large source with a low corner frequency and a long source duration. Actually this phenomenon is not reported by SHAKYGROUNDs eventcode, but can be easily identified by comparing the “Strong motion duration” to the seismogram length. If the strong motion duration is close to the seismogram length then you should indeed suspect the presence of “wrap around effect”. You can convince yourself about the validity of your choice writing the single synthetic seismograms to disk and visualizing them one by one.

Select now the “General” sheet with your mouse. Your screen should look like Fig. below. This sheet is organized in 5 logical units, which are the parameters for the synthetic “Seismogram Information”, the “Simulation Parameters”, the “Statistics [of] Source” parameters, “Statistics [of] Layer” parameters, and the “Absorption mode”.

The General Sheet with the parameters controlling the nmerical calculus.

The “Seismogram Information” concerns the “Length” of the synthetic seismograms given in points. For internal reasons of this number must be a power of 2. The second item of interest is the “Digitization frequency” given in Hz. If you select for example 200 Hz the spacing of points in the synthetic seismograms will be 0.005 s. With a length of 4096 points the seismogram length expressed in seconds or, in other words, the time window will be 4096/200 = 20.48 s. Limits imposed by SHAKYGROUND are 16384 points for the seismogram length and 1000 Hz for the digitization frequency.
In the field “Simulation parameters” you have two boxes where you select the “Number of Simulations” to perform during a run, and the “Seed Value..” for the “..Random Gen[erator]”. The seed value should be an integer. It initializes the random generator at the beginning of a SHAKYGROUND session. The number of simulations are important for the degree of statistical stability of your simulations. A good compromise between the needs of short computing times and statistical stability of SHAKYGROUNDs output parameters can be 50 simulations. In this case SHAKYGROUND will perform 50 simulations with the model parameters specified by the user,varying them according to the choices explained above, then produce a statistics of a number of output parameters and of the response spectra. For testing purposes you may select a small number of simulations, such as 3 which is the default. A choice of less than 2 simulations is blocked by SHAKYGROUND, since otherwise the standard deviation would not be defined any longer.The next two fields “Statistics Source” and “Statistics Layers” concern the manner of how to perform the random parameter variation. You may choose a “Uniform Distribution” where all values within a given range have the same probability. The uniform distribution has finite limits, i. e., certain values cannot be exceeded. In the gaussian distribution the average values have the highest probability of occrurence, however, in theory there is no upper or lower limit of possible values. For the sake of numerical stability SHAKYGROUND limits in any case the parameter variation with respect to its lower boundary in the sense that a value less than 5% of the average is not permitted.
The last field “Absorption mode” you have the choice between an “acausal” or “causal”, absorption model. The acausal absorption model causes the appearance of a little amount of signal energy before the seismic signals physical arrival time. The reason for this phenomenon resides in the fact that the acausal absorption model is zero-phase, in other words there is no phase shift since the seismic velocities are assumed independent of frequency. In the causal mode a velocity dispersion is assumed according to a model developed by Futterman (1962). Even though appearing more reasonable from a theoretical viewpoint one could argue the causal absorption model objecting that the true phase shifts cannot be described correctly neither by a zero-phase nor by Futterman’s model. After all, as experience has shown the results in most cases are affected only to a minor degree by the choice of the absoprtion mode.

In this sheet (Fig.below) you may access the specification concerning the receiver position both with respect to its coordinates as well as to its position within the layer stack. The coordinates are entered in the frame named “Receiver position”. There are two text boxes where the coordinated expresses in UTMX and UTMY can be edited. As in the “Source” sheet, UTMX and UTMY are metric coordinates as in systems like, e. g, Gauß-Krüger or Gauß-Boaga.  SHAKYGROUND calculates the epicentral distance of the receiver and displays it in the frame “Epicentral distance” in the box labeled with “Calculated (m)”. Since the epicentral distance results automatically from the source and receiver coordinates it cannot be edited. What can be done, however, is to subject the epicentral distance to a random fluctuation in the same way as is done for the source and layer parameters. In the spin-box (box with the double horizontal arrow) the user specifies the bandwidth of the random fluctuation in percent of the value calculated. In other words, if the calculated epicentral distance is 10,000 m and the user specifies a random fluctuation of 15% then the actual epicentral distance used during the simulations will float around the calculated one according to a standard deviation of ± 1500 m. SHAKYGROUND reports the absolute value of the fluctuation in a text-box, where, however, editing is disabled. Note that in the concept of SHAKYGROUND the receiver position is linked with the layer stack which are actually present on the “Strata” sheet. If a geological model is loaded from the catalog, SHAKYGROUND automatically loads also the corresponding receiver position linked to that layers. If you wish to save the receiver coordinates (and its vertical position within the layer stack) you have to do this using the “Strata” sheet (click the “Strata” sheet then “Save in Catalog”). Or you may use the option to save the whole model, i. e., the whole bunch of the 6 sheets, by clicking the floppy-disk icon on the toolbar underneath the title “Models”.

Finally SHAKYGROUND offers on option to consider the wave field in some depth instead of a position directly on the earth’s surface. This option can be useful if the foundations of a building are situated at some depth. The seismic loading at depth is often lower due to destructive interference of upgoing and downgoing waves. In the frame “Receiver position within layer stack” SHAKYGROUND permits to pose the receiver at one of the layer interface of the geological model. SHAKYGROUND displays the model schematically. The first position is the “Surface” which is also the default. Clicking one of the subsequent layer names moves the receiver to the lower boundary of that layer. At a first glance there seems to be a limitation since the receiver position is linked to a layer boundary. This limit, however, can be easily circumvented by the creation of a virtual layer subdividing an existing layer into two layers with identical impedance and Q values, and choosing the thicknesses appropriately. Note again that the receiver position can be saved totgether with the layer parameters using the “Strata” sheet, or by saving the “Mode”, i. e. the whole stack of SHAKYGROUNDs input sheets.

The Receiver Sheet with coordinates, and the vertical position within the layer stack.