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› Mae 27
› PASSIVE SEISMIC – EXPLORATION FOR THE EVALUATION OF THE
LOCAL SEISMIC RESPONSE – MICRO EARTHQUAKES
This technique is used to obtain information
concerning possible dynamic from the convolution of the frequen-
the study of the spectrums obtained
amplification effects of seismic waves cy of the signal recorded in the domain
for the three components of the
in “ emersion ”. It is based on the recording
of the background noise in ground motion and the application
the time domain and following elaboration
of the signal’s frequencies in (H/V), allows to define and measure
of techniques on spectrum analysis
the domain. It is carried out by placing
a tridimensional geophone on the and the seismic frequency of the
possible local seismic amplifications
ground with low frequency response site. The measurements of the microearthquake
can also be taken in linear
and by recording the seismic noise in
different temporal windows. Later on, “arrays” for the localization of faults.
› NAKAMURA METHOD
A significant part of the damages observed
in destructive earthquakes all
over the world is associated with the
amplification of seismic waves due to
the effects of the local site. The analysis
of the site response is therefore essential
in the evaluation of the seismic
risk in areas subject to earthquakes.
In order to evaluate the effects of the
local site, a series of surveys must be
carried out. Among the empiric methods,
the method of spectrum analyses
H/V on environmental vibrations is
one of the most common. The method,
also called “Nakamura” technique
(Nakamura, 1989), was introduced by
Nogoshi and Igarashi (1971) based on
the initial studies of Kanai and Tanaka
(1961). Since then, many researchers
worldwide performed a large number
of applications.
An important requirement to carry out
the H/ V method consists in a fairly good
knowledge of seismology combined
with basic information on local geological
conditions supported by geo-physical
and geo-technical data. The method
is generally applied in micro-zoning
studies and in the analysis of the local
response of specific sites.
› SEISMIC MONITORING
Seismic monitoring is carried out in areas
subject to risks related to a seismogenic
activity, by acquiring the
seisms with time and recording the
seismograms. Seismic stations are
used able to record in threshold or
continuous type, and low frequency
geophones or seismic accelerometers.
The recording in the long period,
of the earthquakes relative to a
site or fairly large area allows to configure
the seismic scenario of an area
and evaluate the risk and vulnerability
conditions. If the monitoring activity
is supported by specific knowledge
of geological and geo-technical
type, we are talking about seismic Micro-zoning.