Publication Date:
2017-04-04
Description:
Since the late 1960s - early 1970s, seismologists started studying the elastic properties
of the Earth crust looking for signals from the Earth interior indicating that a large
earthquake is coming. To be useful for prediction a signal needs to: 1) occur before
most large earthquakes and 2) occur only before large earthquakes. Up to now, no
one has ever found such a signal, but since the beginning of the search, seismologists
developed theories that included variations of the elastic property of the Earth crust
prior to the occurrence of a large earthquake. The most popular is the theory of the
dilatancy: when a rock is subject to stress, the rock grains are shifted generating
micro-cracks, thus the rock itself increases its volume. Inside the fractured rock,
fluid saturation and pore pressure play an important role in earthquake nucleation, by
modulating the effective stress. Thus measuring the variations of wave speed and of
anisotropic parameter in time can be highly informative on how the stress leading to a
major fault failure builds up. In 1980s and 1990s such kind of research on earthquake
precursors slowed down and the priority was given to seismic hazard and ground
motions studies, which are very important since these are the basis for the building
codes in many countries. Today we have dense and sophisticated seismic networks to
measure wave-fields characteristics: we archive continuous waveform data recorded
at three components broad-band seismometers, we almost routinely obtain highresolution
earthquake locations. Therefore we are ready to start to systematically look
at seismic-wave propagation properties to possibly reveal short-term variations in
the elastic properties of the Earth crust. One seismological quantity which, since the
beginning, is recognized to be diagnostic of the level of fracturation and/or of the pore
pressure in the rock, hence of its state of stress, is the ratio between the compressional
(P-wave) and the shear (S-wave) seismic velocities: Vp/Vs. Variations of this ratio
have been recently observed and measured during the preparatory phase of a major
earthquake. In active fault areas and volcanoes, tectonic stress variation influences
fracture field orientation and fluid migration processes, whose evolution with time
can be monitored through the measurement of the anisotropic parameters. Through
the study of S waves anisotropy it is therefore potentially possible to measure the
presence, migration and state of the fluid in the rock traveled by seismic waves,
thus providing a valuable route to understand the seismogenic phenomena and their precursors. On the other hand, only in the very recent times with the availability
of the continuous seismic records, many authors have shown how it is possible to
estimate the relative variations in the wave speed through the analysis of the crosscorrelation
of the ambient seismic noise. In this paper we first analyze in detail these
two seismological methods: shear wave splitting and seismic noise cross correlation,
presenting a short historical review, their theoretical bases, the problems, learning,
limitations and perspectives. We, then, compare the main results in terms of temporal
trends of the observables retrieved applying both methods to the Pollino area (southern
Apennines, Italy) case study.
Description:
Published
Description:
257-274
Description:
4T. Fisica dei terremoti e scenari cosismici
Description:
JCR Journal
Description:
restricted
Keywords:
earthquake prediction
;
04. Solid Earth::04.06. Seismology::04.06.01. Earthquake faults: properties and evolution
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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