Lower crustal earthquakes near the Ethiopian rift induced by magmatic processes

Lower crustal earthquakes at plate boundaries and intraplate settings occur at depth where deformation is normally expected to occur in a ductile manner. Here we use the available earthquake catalogs and compute theoretical predictions for a range of conditions for the occurrence of lower crustal earthquakes beneath the Main Ethiopian Rift (MER) and adjacent north-western (NW) plateau. Yield strength envelops are constructed using information on geothermal gradient, strain rate, and composition constrained by geophysical observations. Our models suggest that away from the MER beneath the NW plateau the depth distribution of earthquakes in the lower crust is best explained by strong mafic lower crustal rheology and hydrostatic fluid pore pressure conditions. In the same region the effective elastic thickness is similar to seismogenic thickness showing that the lower crust has long-term strength and hence can physically support brittle deformation. On the contrary, in the central MER the seismogenic thickness is much larger than the effective elastic layer thickness implying that the lower crust has no long-term strength. Here our models show that both hydrostatic and near-lithostatic fluid pore pressures fail to explain the observed seismicity and instead a combination of near-lithostatic pore fluid pressure and transient high strain rate due to the movement of fluids provide a plausible mechanism for the occurrence of seismicity in the lower crust. Our interpretations are supported by occurrence of swarms of deep earthquakes beneath the MER, as opposed to more continuous background deep seismicity away from the rift. Using time-depth progression of earthquakes, we estimate permeability values of 5.9 × 10−15 m2 and 1.8 × 10−14 m2 at lower crustal depth. The range of permeability implies that seismicity can be induced by pore-pressure diffusion, likely from fluids sourced from the mantle that reactivate preexisting faults in the lower crust. Our thermo-rheological models explain the first order differences in lower crustal earthquakes both directly beneath and outboard of the rift valley.

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Lower-crustal earthquakes in southern Tibet are linked to eclogitization of dry metastable granulite

Nature Communications ◽

10.1038/s41467-018-05964-1 ◽

2018 ◽

Vol 9 (1) ◽

Cited By ~ 9

Author(s):

Feng Shi ◽

Yanbin Wang ◽

Tony Yu ◽

Lupei Zhu ◽

Junfeng Zhang ◽

...

Keyword(s):

Southern Tibet ◽

Crustal Earthquakes ◽

Lower Crustal

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Deep lower crustal earthquakes in central India: inferences from analysis of regional broadband data of the 1997 May 21, Jabalpur earthquake

Geophysical Journal International ◽

10.1046/j.0956-540x.2001.01584.x ◽

2002 ◽

Vol 148 (1) ◽

pp. 132-138 ◽

Cited By ~ 34

Author(s):

N. P. Rao ◽

T. Tsukuda ◽

M. Kosuga ◽

S. C. Bhatia ◽

G. Suresh

Keyword(s):

Central India ◽

Crustal Earthquakes ◽

Lower Crustal

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Lower crustal earthquake facilitated by overpressurized frictional melts

10.5194/egusphere-egu2020-5204 ◽

2020 ◽

Author(s):

Xin Zhong ◽

Arianne Petley-Ragan ◽

Sarah Incel ◽

Marcin Dabrowski ◽

Niels Andersen ◽

...

Keyword(s):

Wall Rock ◽

Rapid Quenching ◽

Elastic Model ◽

Shear Rigidity ◽

Geological Time ◽

Differential Stress ◽

Crustal Earthquakes ◽

Fluid Pressurization ◽

Frictional Melting ◽

Lower Crustal

Earthquakes are among the most catastrophic geological events that last only several to tens of seconds. During earthquakes, many processes may occur including rupturing, frictional sliding, pore fluid pressurization and occasionally frictional melting. However, little direct records of these fast processes remain preserved through geological time. During rapid shearing, frictional melt may form that lubricates the rocks and facilitates further sliding. The frictional melt layer may quench quickly within seconds to minutes depending on its thickness. After quenching, the product pseudotachylyte preserves valuable information about the conditions when the frictional melt was generated. Here, we study pseudotachylyte from Holsn&#248;y Island in the Bergen Arcs of Western Norway, an exhumed portion of the lower continental crust. The investigated pseudotachylyte vein is ca. 1-2 cm thick and free of injection veins along the 2 m visible length of the fault. The pseudotachylyte matrix is made up of fine-grained omphacite (Jd38), sodic plagioclase (Ab83) and kyanite with minor rutile and sulphides. Many dendritic garnets are found within the pseudotachylyte showing gradual grain size reduction towards the wall rock. This suggests that the garnets crystallized during rapid quenching. The stability of epidote, kyanite and quartz in the wall rock plagioclase, and omphacite and albitic plagioclase together with quartz in the pseudotachylyte matrix constrains the ambient P ca. 1.5-1.7 GPa and T ca. 650-750&#176;C. Using Raman elastic barometry, the constrained pressure condition from quartz inclusions in the dendritic garnets in the pseudotachylyte is > 2 GPa. Based on an elastic model (Eshelby&#8217;s solution), it is not possible to maintain 0.5 GPa overpressure within a thin melt layer by thermal pressurization or melting expansion. A potential explanation is that GPa level differential stress was present in the wall rocks and the melt pressure approached the normal stress when shear rigidity vanished during frictional melting. Our study illustrates how overpressure can be created within frictional melt veins under conditions of high differential stress, and offers a mechanism that facilitates co-seismic weakening during lower crustal earthquakes. &#160;

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Dynamic earthquake rupture in the lower crust

Science Advances ◽

10.1126/sciadv.aaw0913 ◽

2019 ◽

Vol 5 (7) ◽

pp. eaaw0913 ◽

Cited By ~ 14

Author(s):

Arianne Petley-Ragan ◽

Yehuda Ben-Zion ◽

Håkon Austrheim ◽

Benoit Ildefonse ◽

François Renard ◽

...

Keyword(s):

Lower Crust ◽

Failure Process ◽

Wall Rock ◽

Ductile Deformation ◽

Earthquake Rupture ◽

Seismic Slip ◽

Crustal Earthquakes ◽

Sequence Of Events ◽

Deformation Processes ◽

Lower Crustal

Earthquakes in the continental crust commonly occur in the upper 15 to 20 km. Recent studies demonstrate that earthquakes also occur in the lower crust of collision zones and play a key role in metamorphic processes that modify its physical properties. However, details of the failure process and sequence of events that lead to seismic slip in the lower crust remain uncertain. Here, we present observations of a fault zone from the Bergen Arcs, western Norway, which constrain the deformation processes of lower crustal earthquakes. We show that seismic slip and associated melting are preceded by fracturing, asymmetric fragmentation, and comminution of the wall rock caused by a dynamically propagating rupture. The succession of deformation processes reported here emphasize brittle failure mechanisms in a portion of the crust that until recently was assumed to be characterized by ductile deformation.

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