A reappraisal of seismicity recorded during the 1996 Gj�lp eruption in Iceland using modern seismological methods;A reappraisal of seismicity recorded during the 1996 Gj�lp eruption in Iceland using modern seismological methods

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Title:	A reappraisal of seismicity recorded during the 1996 Gj�lp eruption in Iceland using modern seismological methods;A reappraisal of seismicity recorded during the 1996 Gj�lp eruption in Iceland using modern seismological methods
Authors:	游佳美;Utami, Ika Wahyu
Contributors:	地球科學學系
Keywords:	Gjálp 噴發;冰島;Gjálp eruption;Iceland
Date:	2018-07-18
Issue Date:	2018-08-31 13:10:14 (UTC+8)
Publisher:	國立中央大學
Abstract:	1996年9月30日，在冰島的兩個火山中心 Grimsvötn和 Bárðarbunga之間的7公里長的裂縫發生冰川底火山噴發 (Gjálp噴發)。在噴發的一天半前，巴達本加火山發生了強烈的地震（Mw 5.6）。爆發持續了13天，隨之而來的是大量而密集的小地震（ML <4）和火山震顫。為了詳細研究這些事件，我們使用現代地震學方法分析了由臨時地震網絡HOTSPOT記錄的火山地震信號。為了獲得高度準確的位置，我們採用具有伴隨站校正項的可靠的一維速度模型。然後通過使用新推導的模型計算非線性概率位置來定位所有這些事件。我們將這301個事件的波形相關聯，根據地震目錄和他們記錄的地震圖的相似性來定義關聯事件。接著，我們計算了事件對之間的相對行程時間差，並使用雙差分定位法來解決這些事件之間的震源分離。我們以這種方法，將其中 192個事件重新定位並提高定位精準度，而緊密的火山群集也顯示我們將相對水平位置的不確定性控制於1公里內，垂直定位則小於 2公里。在這項研究中，我們還計算了一個光譜參數，即所有地震信號的質心頻率。此外，我們也計算了主要事件和接下來十個事件的矩張量以了解它們的震源過程。震源機制解顯示這些地震特性為非雙力偶源，而且等向分量不明顯。沿Bárðarbunga火山口邊緣發生的事件為逆斷層，而位於裂縫中的事件則以平移斷層為主。從地震活動模式來看，震央從 Bárðarbunga火山口向噴發地點轉移，可看出有一個橫向遷移穿過 Bárðarbunga火山下方的淺岩漿室延伸的堤壩。質心頻率表明沿Bárðarbunga環斷層和裂縫沿線的大多數事件，其起因有極高的可能是和岩石碎裂有關，而非流體。因此，我們將此結果視為本研究中地震矩張量等向分量不明顯的解釋。;In September 30, 1996, a subglacial eruption, the Gjálp eruption, occurred along a 7-km-long fissure between two volcanic centres, Grimsvötn and Bárðarbunga in Iceland. A strong earthquake in Bárðarbunga (Mw 5.6) preceded the eruption, one and a half days before. The eruption continued for 13 days and it was followed by an intense earthquake swarm with a large number of small earthquakes (ML < 4) and volcanic tremor. To study these events in detail, we analyzed the volcanoseismic signals that were recorded by the temporary seismic network, HOTSPOT, using modern seismological methods. In order to obtain highly accurate locations, a reliable minimum 1D velocity model with accompanying station correction terms was computed. All these events were then located by calculating non-linear, probabilistic locations using the newly derived model. We correlated the waveforms of these 301 events to define linked events based on the catalogue and the similarity of their recorded seismograms. Subsequently, we calculated the relative travel time difference between event pairs and solved the hypocentral separation between these events using the double-difference method. With this approach, 192 events were successfully relocated and we were able to improve the location accuracy by showing a tight clustering and producing relative horizontal location uncertainties of less than 1 km and less than 2 km for the vertical location uncertainties. In this study, we also calculated a spectral parameter, known as the centroid frequency for all earthquake signals. Furthermore, the moment tensor for the main event and 10 events that followed were calculated in order to understand their source processes. All of events we analyzed in this study were best characterized with non-double-couple solutions and insignificant isotropic components. The events along the Bárðarbunga caldera rim show thrust mechanisms while the events located in the fissure are dominated by strike-slip mechanisms. From the seismicity patterns, the shift of epicentres from the Bárðarbunga caldera towards the eruption site was an indication that there was a lateral migration through a dyke extending from the shallow magma chamber underneath the Bárðarbunga volcano. Centroid frequencies indicate that most events along the Bárðarbunga ring fault and along the fissure contained high-frequencies implying rupture of rock, rather than fluid-related source processes. This result thus we consider as an explanation for the insignificant isotropic component of the moment tensors derived in this study.
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