Seismicity from the deep magma system

doi:10.1126/science.abc2452

GSTDTAP > 气候变化

DOI	10.1126/science.abc2452
	Seismicity from the deep magma system
	Robin S. Matoza
	2020-05-15
发表期刊	Science
出版年	2020
英文摘要	A systematic scan of seismic waveform archives on the Island of Hawai'i has revealed subtle but persistent near-periodic pulses originating within the deep magma plumbing system of Mauna Kea, a dormant volcano that last erupted ∼4500 years ago. On page 775 of this issue, Wech et al. ([ 1 ][1]) report the detection of over a million of the deep (22 to 25 km below sea level) long-period seismic events, which have been occurring continuously and repetitively, often with precise regularity (every ∼7 to 12 min), for at least 18 years. This discovery offers new views into the origin of this mysterious type of deep volcanic seismicity. Seismic data form the backbone of most volcano monitoring networks and play a critical role in understanding how volcanoes work. Volcanic seismicity includes volcano-tectonic (VT) earthquakes (ordinary brittle-failure earthquakes driven by magmatic stresses) and long-period [(LP), 0.5 to 5 Hz] seismicity (volcanic seismicity that is thought to actively involve a fluid in the source mechanism) ([ 2 ][2]). LP seismicity includes individual transient LP events and sustained volcanic tremor signals. LP seismicity at shallow depth (<3 km) in a volcanic edifice is commonly explained by the excitation and resonance of fluid-filled cracks associated with magmatic-hydrothermal interactions or magmatic degassing and is a characteristic signature of unrest and eruption ([ 3 ][3]). Precise regularity in sustained sequences of shallow LP seismicity has been documented at numerous volcanoes worldwide ([ 2 ][2]). In the roots of volcanic systems below this shallow activity, seismicity extends down to mantle depths (to ∼60 km), but linking seismicity to magma pathways is not straightforward. Deep long-period (DLP) seismicity has been observed at depths of 10 to 60 km beneath volcanoes in a range of tectonic settings and has generally been attributed to magma transport in the mid-to-lower crust and uppermost mantle ([ 2 ][2]). The idea that DLP seismicity may represent cooling of magma stalled near the Moho (crust-mantle boundary) has also been proposed ([ 4 ][4]). However, compared to shallow LP seismicity, DLP seismicity is relatively understudied and poorly quantified, largely owing to difficulties in detecting these weak signals in the background noise ([ 5 ][5]). Typically, much of the proposed deep magma transport system appears aseismic, inferred as relatively open flow channels in which quasi-steady magma flow does not generate seismicity ([ 6 ][6]). For example, beneath the active volcanoes on the Island of Hawai'i (an oceanic hotspot), deep (>10 km) and intermediate depth (5 to 15 km) LP seismicity, including deep harmonic tremor, has been recorded for decades at Kīlauea ([ 6 ][6], [ 7 ][7]), and occasional DLP swarms (sequences of repetitive events closely clustered in time and space) have occurred beneath Mauna Loa ([ 8 ][8]). Precise relocation of this repetitive DLP seismicity collapses it to markedly consistent and small source volumes along the presumed magma ascent paths ([ 7 ][7], [ 8 ][8]). This repeated seismic illumination of only a tiny portion of the inferred magma transport system over decades of eruptive changes indicates a source process controlled by stable geologic or magma pathway structure, such as a geometrical conduit discontinuity or a particularly strong barrier to magma flow ([ 6 ][6], [ 7 ][7], [ 8 ][8]). A more complex picture has recently emerged at Mammoth Mountain, California, with swarms of DLPs clustering in the middle and top of a relatively aseismic zone between two arms of migrating brittle-failure earthquakes ([ 9 ][9]). DLP seismicity is challenging to detect with standard seismic network processing and typical noise conditions and is likely underreported, but DLPs have been identified in multiple tectonic settings located at mid-to-lower crustal depths, with most appearing to represent a relatively stable background process ([ 5 ][5], [ 10 ][10], [ 11 ][11]). Between 1989 and 2002, 162 DLPs were detected beneath 11 volcanic centers in the Aleutian arc, occurring both in isolation and as event sequences lasting from 1 to 30 min ([ 10 ][10]). Between 1980 and 2009, more than 60 DLPs were identified beneath six Cascades volcanic centers, none directly related to volcanic activity ([ 11 ][11]). DLPs occur beneath each of the major volcanic centers in Northern California ([ 5 ][5]). Given the low signal-to-noise ratios of most DLP observations, source mechanism studies have rarely been attempted. Analyses of DLPs at Iwate, Japan, produced variable mechanisms, suggesting a complex magma system at the source region ([ 12 ][12]). ![Figure][13] Origin of deep long-period seismicity Inactive volcanoes can exhibit deep long-period (DLP) seismicity. This activity may arise from pooled, cooling magma at the base of Earth's crust. Magmatic gases exsolve from this pool as the magma crystallizes. Protraction of this “second boiling” is linked to DLP activity. GRAPHIC: V. ALTOUNIAN/ SCIENCE The 1991 eruption of Pinatubo, Philippines, provided new observations connecting DLPs with deep magma transport in a subduction setting and identified DLPs as potentially important eruption precursors ([ 13 ][14]). Prior to the 15 June 1991 eruption, about 400 DLPs were observed between late May and early June 1991, along with >25 hours of low-amplitude DLP tremor in 1- to 10-hour-long episodes ([ 13 ][14]). The DLPs were located at 28- to 40-km depth at the base of the crust below Pinatubo and were temporally correlated with surficial changes and shallow seismicity. The DLP seismicity preceded, by about 1 to 4 hours, shallow (<3 km depth) LP events, tremor, and steam emissions. DLP seismicity increased a few days before the extrusion of an andesitic (volcanic rock type of intermediate silica content, commonly associated with subduction zones) dome containing inclusions of freshly quenched olivine basalt and was accordingly interpreted as resulting from deep basaltic fluid injections into the base of the magma chamber ([ 13 ][14]). A similar increase in mid-to-lower crustal DLP seismicity occurred about 10 months before the 1999 eruption of Shishaldin, Alaska; conversely, 1992 eruptions of Mount Spurr, Alaska, initiated DLP seismicity ([ 10 ][10]). Waveform template matching greatly enhances the detection of DLP seismicity, allowing a more complete investigation of these spatiotemporal relations ([ 14 ][15], [ 15 ][16]). Results from applying this method to 2011–2012 activity at the Klyuchevskoy volcanic group in Kamchatka, Russia, supported the notion that DLP seismicity may (at least in some cases) represent an early eruption precursor and identified systematic connections between deep and shallow LP seismicity and eruptions at multiple volcanoes ([ 14 ][15]). A recent longer study of 17 years of seismicity at Hakone, Japan, reveals similar patterns, with DLPs repeatedly preceding inflation of the volcanic edifice and shallow VT seismicity, and in one case, a phreatic eruption ([ 15 ][16]). The detection of over a million DLPs by Wech et al. is an impressive demonstration of this technique, but as the authors point out, the occurrence of such a prolific number of these events beneath dormant Mauna Kea is surprising. The resulting hypothesis that some (or perhaps millions of ) DLPs are related to crystallization-induced degassing (“second boiling”) of stagnant cooling magma (see the figure) should be tested at other volcanoes in different tectonic settings worldwide. 1. [↵][17]1. A. G. Wech, 2. W. A. Thelen, 3. A. M. Thomas , Science 368, 775 (2020). [OpenUrl][18][Abstract/FREE Full Text][19] 2. [↵][20]1. B. A. Chouet, 2. R. S. Matoza , J. Volcanol. Geotherm. Res. 252, 108 (2013). [OpenUrl][21][CrossRef][22] 3. [↵][23]1. B. A. Chouet , Nature 380, 309 (1996). [OpenUrl][24][CrossRef][25][GeoRef][26][Web of Science][27] 4. [↵][28]1. N. Aso, 2. V. Tsai , J. Geophys. Res. 119, 8442 (2014). [OpenUrl][29] 5. [↵][30]1. A. M. Pitt, 2. D. P. Hill, 3. S. W. Walter, 4. M. J. S. 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领域	气候变化 ; 资源环境
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文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/267707
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Robin S. Matoza. Seismicity from the deep magma system[J]. Science,2020.
APA	Robin S. Matoza.(2020).Seismicity from the deep magma system.Science.
MLA	Robin S. Matoza."Seismicity from the deep magma system".Science (2020).