下地幔可以发生地震吗?
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摘要
大部分的地震发生在比较浅的位置,但是部分地震的深度可以达到600 km以上.目前已有的破裂机制预测地震只存在于上地幔中,但是一些地震的初步定位深度却可以超过670 km(1998年2月9号Okhotsk海地震,地震目录中被定为678 km).研究表明大多数浅震是由于断层失稳,沿着已有的断层面突然滑动引发的,而深震的发震机制目前尚无定论,尤其是下地幔中是否发生地震对于研究深震机制可以提供重要的约束.因为地球中的横向不均匀性,地震的绝对深度有较大的误差,所以我们根据观测地震图中明显的三重值特征,对上述那次深地震进行波形模拟,把相对深度和绝对深度结合起来,最终确定地震实际发生在670公里间断面以上,因此我们认为目前目录中那些最深的地震仍然发生在上地幔中.
Most earthquakes occur in the upper crust(<25 km) while some can take place deeper than 600 km.Previous studies hypothesize that earthquakes only occur in the upper mantle as expected from all available rupture mechanisms(stick-slip for shallow earthquakes,and a few other mechanisms for deep earthquakes).However,some catalogues report earthquakes as deep as 678 km(February 9,1998,Sea of Okhotsk),thus raising hopes for earthquakes to occur in the lower mantle.Due to heterogeneity in the upper mantle,absolute focal depth is not well resolved.We modeled the triplicated P waveform to determine the focal depth relative to the 670 km discontinuity and found that the earthquake actually occurred above the discontinuity.Thus even the deepest earthquakes in current catalogs still occur in the upper mantle.
Most earthquakes occur in the upper crust(<25 km) while some can take place deeper than 600 km.Previous studies hypothesize that earthquakes only occur in the upper mantle as expected from all available rupture mechanisms(stick-slip for shallow earthquakes,and a few other mechanisms for deep earthquakes).However,some catalogues report earthquakes as deep as 678 km(February 9,1998,Sea of Okhotsk),thus raising hopes for earthquakes to occur in the lower mantle.Due to heterogeneity in the upper mantle,absolute focal depth is not well resolved.We modeled the triplicated P waveform to determine the focal depth relative to the 670 km discontinuity and found that the earthquake actually occurred above the discontinuity.Thus even the deepest earthquakes in current catalogs still occur in the upper mantle.
引文
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[11]Shearer P M,Masters T G.Global mapping oftopography on the 660 km discontinuity[J].Nature,1992,355:791-796.
[12]Shearer P M.Global mapping of upper mantlereflectors from long-period SS precursors[J].GeophysJ Int,1993,115:878-904.
[13]Megan P F,Shearer P M.Global mapping oftopography on transition zone velocity discontinuitiesby stacking SS precursors[J].J Geophys Res,1998,103:2 673-2 692.
[2]Meade C,Jeanloz R.Deep-focus earthquakes andrecycling of water into the Earth s mantle[J].Science,1991,252:68-72.
[3]Silver P G,Beck S L,et al.Rupture characteristics ofthe deep bolivian earthquake of 9 June 1994 and themechanism of deep-focus earthquakes[J].Science,1995,268(5207):69-73.
[4]Green H W I,Houston H.The mechanics of deepearthquakes[J].Ann Rev Earth Planet Sci,1995,24:169-213.
[5]Kirby S H,Stein S,Okal E A,Rubie D C.Metastablemantle phase transformations and deep earthquake insubducting oceanic lithosphere[J].Rev Geophys,1996,34:261-306.
[6]Griggs D T,Baker D W.The origin of deep focusearthquakes[M]//Mark H,Fernback S.Properties ofMatter under Unusual Conditions.New York:Wiley,1969:23-42.
[7]Ogawa M.Shear instability in a visco-elastic materialas the cause of deep focus earthquakes[J].J GeophysRes,1987,92:13 801-13 810.
[8]Karato S I.Phase transformations and rheologicalproperties of mantle minerals[M]//Crossleym D J.Earth s Deep Interior.Amsterdam:Gordon andBreach,1997:223-272.
[9]Karato S,Riedel M R,Yuen D A.Rheologicalstructure and deformation of subducted slabs in themantle transition zone:Implications for mantlecirculation and deep earthquakes[J].Phys Earth PlanetInter,2001,127:83-108.
[10]Bina C R.A note on latent heat release fromdisequilibrium phase transformations and deepseismogenesis[J].Earth Planets Space,1998,50:1 029-1 034.
[11]Shearer P M,Masters T G.Global mapping oftopography on the 660 km discontinuity[J].Nature,1992,355:791-796.
[12]Shearer P M.Global mapping of upper mantlereflectors from long-period SS precursors[J].GeophysJ Int,1993,115:878-904.
[13]Megan P F,Shearer P M.Global mapping oftopography on transition zone velocity discontinuitiesby stacking SS precursors[J].J Geophys Res,1998,103:2 673-2 692.
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