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Reduction of randomness in seismic noise as a short-term precursor to a volcanic eruption

View Article: PubMed Central - PubMed

ABSTRACT

Ambient seismic noise is characterized by randomness incurred by the random position and strength of the noise sources as well as the heterogeneous properties of the medium through which it propagates. Here we use ambient noise data recorded prior to the 1996 Gjálp eruption in Iceland in order to show that a reduction of noise randomness can be a clear short-term precursor to volcanic activity. The eruption was preceded on 29 September 1996 by a Mw ~5.6 earthquake that occurred in the caldera rim of the Bárdarbunga volcano. A significant reduction of randomness started occurring 8 days before the earthquake and 10 days before the onset of the eruption. This reduction was observed even at stations more than 100 km away from the eruption site. Randomness increased to its previous levels 160 minutes after the Bárdarbunga earthquake, during which time aftershocks migrated from the Bárdarbunga caldera to a site near the Gjálp eruption fissure. We attribute this precursory reduction of randomness to the lack of higher frequencies (>1 Hz) in the noise wavefield caused by high absorption losses as hot magma ascended in the upper crust.

No MeSH data available.


Diagrams showing the temporal variation of the dominant (red line) and centroid (blue line) frequency of ambient seismic noise for the same period as the PE variations.All other symbols are the same as in Fig. 3. Note that the logarithm base 10 of the frequency is plotted in the y-axis.
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f4: Diagrams showing the temporal variation of the dominant (red line) and centroid (blue line) frequency of ambient seismic noise for the same period as the PE variations.All other symbols are the same as in Fig. 3. Note that the logarithm base 10 of the frequency is plotted in the y-axis.

Mentions: The dominant and centroid frequencies of noise were also calculated for the purpose of understanding its power spectrum characteristics and their variations. In order to do this, we first estimated the power spectrum of the data using the Fast Fourier Transform for the same time windows that PE was calculated. The dominant frequency is defined as the frequency with the highest power in the data, while the centroid frequency represents the weighted mean of all the frequencies in the data (see Methods section). We find that for stations HOT23 and HOT25, the dominant frequency was rather stable around 0.25 Hz, while for station HOT14 the dominant frequency varied mostly between 0.01–0.25 Hz (Fig. 4). For stations HOT23 and HOT25 the centroid frequencies exhibit a large range between 0.2–4.8 Hz and 0.2–4.5 Hz respectively, while station HOT14 showed a centroid frequency range between 0.05–4.5 Hz. For station HOT23, the centroid frequencies dramatically shifted to become almost the same as the dominant frequencies approximately 6.6 days before the Bárdarbunga earthquake. This shift was also observed at stations HOT25 and HOT14, 6.2 and 6.9 days respectively before the earthquake (cf. Fig. 4) correlating well with the observed minimum in PE at the same stations. The dominant and centroid frequencies increased immediately after the Mw = 5.6 mainshock at all three stations and especially at HOT23 that was the closest station to the Gjálp fissure.


Reduction of randomness in seismic noise as a short-term precursor to a volcanic eruption
Diagrams showing the temporal variation of the dominant (red line) and centroid (blue line) frequency of ambient seismic noise for the same period as the PE variations.All other symbols are the same as in Fig. 3. Note that the logarithm base 10 of the frequency is plotted in the y-axis.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5121890&req=5

f4: Diagrams showing the temporal variation of the dominant (red line) and centroid (blue line) frequency of ambient seismic noise for the same period as the PE variations.All other symbols are the same as in Fig. 3. Note that the logarithm base 10 of the frequency is plotted in the y-axis.
Mentions: The dominant and centroid frequencies of noise were also calculated for the purpose of understanding its power spectrum characteristics and their variations. In order to do this, we first estimated the power spectrum of the data using the Fast Fourier Transform for the same time windows that PE was calculated. The dominant frequency is defined as the frequency with the highest power in the data, while the centroid frequency represents the weighted mean of all the frequencies in the data (see Methods section). We find that for stations HOT23 and HOT25, the dominant frequency was rather stable around 0.25 Hz, while for station HOT14 the dominant frequency varied mostly between 0.01–0.25 Hz (Fig. 4). For stations HOT23 and HOT25 the centroid frequencies exhibit a large range between 0.2–4.8 Hz and 0.2–4.5 Hz respectively, while station HOT14 showed a centroid frequency range between 0.05–4.5 Hz. For station HOT23, the centroid frequencies dramatically shifted to become almost the same as the dominant frequencies approximately 6.6 days before the Bárdarbunga earthquake. This shift was also observed at stations HOT25 and HOT14, 6.2 and 6.9 days respectively before the earthquake (cf. Fig. 4) correlating well with the observed minimum in PE at the same stations. The dominant and centroid frequencies increased immediately after the Mw = 5.6 mainshock at all three stations and especially at HOT23 that was the closest station to the Gjálp fissure.

View Article: PubMed Central - PubMed

ABSTRACT

Ambient seismic noise is characterized by randomness incurred by the random position and strength of the noise sources as well as the heterogeneous properties of the medium through which it propagates. Here we use ambient noise data recorded prior to the 1996 Gjálp eruption in Iceland in order to show that a reduction of noise randomness can be a clear short-term precursor to volcanic activity. The eruption was preceded on 29 September 1996 by a Mw ~5.6 earthquake that occurred in the caldera rim of the Bárdarbunga volcano. A significant reduction of randomness started occurring 8 days before the earthquake and 10 days before the onset of the eruption. This reduction was observed even at stations more than 100 km away from the eruption site. Randomness increased to its previous levels 160 minutes after the Bárdarbunga earthquake, during which time aftershocks migrated from the Bárdarbunga caldera to a site near the Gjálp eruption fissure. We attribute this precursory reduction of randomness to the lack of higher frequencies (>1 Hz) in the noise wavefield caused by high absorption losses as hot magma ascended in the upper crust.

No MeSH data available.