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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.


Related in: MedlinePlus

(a) Map of Iceland showing the extent of neovolcanic zones as gray areas and main fracture zones as thick black lines. White areas indicate permanent glaciers. HOTSPOT stations are shown as colored circles with the number of each station enclosed in the circle. Green circles represent stations included in this study, purple circles represent stations with significant data gaps and red circles are all the remaining HOTSPOT stations. The red square shows the location of the study area. (b) Map of the NW part of the Vatnajökull glacier. Solid lines represent the outlines of calderas, dashed lines represent the outlines of central volcanoes. The red square shows the location of the Gjálp fissure. The red star signifies the epicenter of the 29 September (Mw~5.6) earthquake that occurred at 10:48:17 (GMT). The black dots are the epicenters of the earthquakes that followed the mainshock 20 minutes later. The seismic activity migrated closer to the Gjálp area about 3 hours later and these events are shown as blue dots. All locations and origin times are taken from ref. 12. The maps were created using GMT version 4.5.11 (http://gmt.soest.hawaii.edu/) and were combined together using Adobe illustrator CS6 (http://www.adobe.com/products/illustrator.html).
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f1: (a) Map of Iceland showing the extent of neovolcanic zones as gray areas and main fracture zones as thick black lines. White areas indicate permanent glaciers. HOTSPOT stations are shown as colored circles with the number of each station enclosed in the circle. Green circles represent stations included in this study, purple circles represent stations with significant data gaps and red circles are all the remaining HOTSPOT stations. The red square shows the location of the study area. (b) Map of the NW part of the Vatnajökull glacier. Solid lines represent the outlines of calderas, dashed lines represent the outlines of central volcanoes. The red square shows the location of the Gjálp fissure. The red star signifies the epicenter of the 29 September (Mw~5.6) earthquake that occurred at 10:48:17 (GMT). The black dots are the epicenters of the earthquakes that followed the mainshock 20 minutes later. The seismic activity migrated closer to the Gjálp area about 3 hours later and these events are shown as blue dots. All locations and origin times are taken from ref. 12. The maps were created using GMT version 4.5.11 (http://gmt.soest.hawaii.edu/) and were combined together using Adobe illustrator CS6 (http://www.adobe.com/products/illustrator.html).

Mentions: The Vatnajökull glacier is an area far from the inhabited southern coast of Iceland which is exposed to bad weather conditions throughout the year, making its seismological monitoring problematic (Fig. 1a). Underneath the glacier lie several active volcanoes that have erupted repeatedly in historical times. Eruptions from these volcanoes often produce large quantities of glacial melt water which drain in the southern coast of Iceland causing catastrophic floods known as “jökullhlaups”678. In 1996 a series of spectacular seismic and volcanic phenomena took place in the NW part of Vatnajökull affecting two of its most active volcanoes, namely Bárdarbunga and Grimsvötn. These phenomena started on 29 September with a shallow (~3.5 km) Mw~5.6 earthquake located near the rim of the Bárdarbunga caldera and whose moment tensor exhibited thrust faulting with a large non-double-couple component91011. Several smaller earthquakes followed and their epicenters delineated the western rim of the Bárdarbunga caldera suggesting the failure of a ring fault1213 (Fig. 1b). The next two days high-amplitude volcanic tremor appeared in the seismic records and earthquake activity migrated to the SE between the Bárdarbunga and Grimsvötn volcanoes, marking the onset of the subglacial eruption. The eruption broke through the ice cap on 2 October creating a 7 km long fissure which was named “Gjálp”14. Seismic activity continued following a declining trend until the end of October, while in early November the water that melted during the eruption drained underneath the glacier in the southern coast of Iceland.


Reduction of randomness in seismic noise as a short-term precursor to a volcanic eruption
(a) Map of Iceland showing the extent of neovolcanic zones as gray areas and main fracture zones as thick black lines. White areas indicate permanent glaciers. HOTSPOT stations are shown as colored circles with the number of each station enclosed in the circle. Green circles represent stations included in this study, purple circles represent stations with significant data gaps and red circles are all the remaining HOTSPOT stations. The red square shows the location of the study area. (b) Map of the NW part of the Vatnajökull glacier. Solid lines represent the outlines of calderas, dashed lines represent the outlines of central volcanoes. The red square shows the location of the Gjálp fissure. The red star signifies the epicenter of the 29 September (Mw~5.6) earthquake that occurred at 10:48:17 (GMT). The black dots are the epicenters of the earthquakes that followed the mainshock 20 minutes later. The seismic activity migrated closer to the Gjálp area about 3 hours later and these events are shown as blue dots. All locations and origin times are taken from ref. 12. The maps were created using GMT version 4.5.11 (http://gmt.soest.hawaii.edu/) and were combined together using Adobe illustrator CS6 (http://www.adobe.com/products/illustrator.html).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f1: (a) Map of Iceland showing the extent of neovolcanic zones as gray areas and main fracture zones as thick black lines. White areas indicate permanent glaciers. HOTSPOT stations are shown as colored circles with the number of each station enclosed in the circle. Green circles represent stations included in this study, purple circles represent stations with significant data gaps and red circles are all the remaining HOTSPOT stations. The red square shows the location of the study area. (b) Map of the NW part of the Vatnajökull glacier. Solid lines represent the outlines of calderas, dashed lines represent the outlines of central volcanoes. The red square shows the location of the Gjálp fissure. The red star signifies the epicenter of the 29 September (Mw~5.6) earthquake that occurred at 10:48:17 (GMT). The black dots are the epicenters of the earthquakes that followed the mainshock 20 minutes later. The seismic activity migrated closer to the Gjálp area about 3 hours later and these events are shown as blue dots. All locations and origin times are taken from ref. 12. The maps were created using GMT version 4.5.11 (http://gmt.soest.hawaii.edu/) and were combined together using Adobe illustrator CS6 (http://www.adobe.com/products/illustrator.html).
Mentions: The Vatnajökull glacier is an area far from the inhabited southern coast of Iceland which is exposed to bad weather conditions throughout the year, making its seismological monitoring problematic (Fig. 1a). Underneath the glacier lie several active volcanoes that have erupted repeatedly in historical times. Eruptions from these volcanoes often produce large quantities of glacial melt water which drain in the southern coast of Iceland causing catastrophic floods known as “jökullhlaups”678. In 1996 a series of spectacular seismic and volcanic phenomena took place in the NW part of Vatnajökull affecting two of its most active volcanoes, namely Bárdarbunga and Grimsvötn. These phenomena started on 29 September with a shallow (~3.5 km) Mw~5.6 earthquake located near the rim of the Bárdarbunga caldera and whose moment tensor exhibited thrust faulting with a large non-double-couple component91011. Several smaller earthquakes followed and their epicenters delineated the western rim of the Bárdarbunga caldera suggesting the failure of a ring fault1213 (Fig. 1b). The next two days high-amplitude volcanic tremor appeared in the seismic records and earthquake activity migrated to the SE between the Bárdarbunga and Grimsvötn volcanoes, marking the onset of the subglacial eruption. The eruption broke through the ice cap on 2 October creating a 7 km long fissure which was named “Gjálp”14. Seismic activity continued following a declining trend until the end of October, while in early November the water that melted during the eruption drained underneath the glacier in the southern coast of Iceland.

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.


Related in: MedlinePlus