Glossary

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.