Investigation of Earthquake Precursors Using Ambient Noise

Pranjal, . (2019) Investigation of Earthquake Precursors Using Ambient Noise. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Seismic velocity changes computed by applying the techniques in seismic inter- ferometry, in the Himalayan region in Uttarakhand reveal that considerably large earthquakes can cause a decline in seismic velocity and this may begin a few days before the occurrence of the mainshock itself. Tracking seismic velocities in real time can provide us with valuable information to forecast potentially destructive earthquakes. In seismology, determining the Green's function is one of the most pivotal steps in studying wave propagation. Traditionally this was achieved only by direct measurements, requiring active seismic sources and the occurrence of consider- ably large earthquakes. However, we cannot wait for large earthquakes to occur and do the damage. Moreover, reliance on the occurrence of earthquakes is also problematic because they are infrequent and inhomogeneously distributed. However, in recent times, a number of theoretical studies have proven that the cross-correlation of diffuse wavefields, including, ambient noise, can provide us the Green's function between stations. In our work, we use the known properties of ambient seismic noise, recorded over a period of nine months, to show that changes in seismic velocities can act as a good precursor to large earthquakes. Decreases in seismic velocity a few days prior to the main event suggest that co-seismic damage begins to occur even before the mainshock. This could be a result of foreshocks that take place before the main event. However, the possible reasons for this decline in velocity need to be further studied before arriving at a concrete conclusion. The potential for us to record the seismic velocity changes in this fashion could enhance our ability to forecast earthquakes and help save a number of lives.

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