The Geological Record of the Earhquake cycle in the lower crust

First of all, this is one of the seminar where I can really understand what the speakers actually wanted to deliver to the audience and it is without a doubt has succeeded in keeping me and others as well to focus on his topic.

The seminar is first started by giving the introduction to the audience so that the audience who might have and might have none prior knowledge in the field can understand.

Main Points and Results

The results in experiment trying to correlate the strength of the lithosphere and the stress that act upon it, show that basically the lithosphere is divided into 3 main mechanical zone.

1.       Shallow part of the crust where deformation took place in brittle style and the strength is dependent on frictional resistance

2.       Bottom part of the crust where lithosphere has plastic style deformation and the strength is determined by the plasticity of the mineral in the area

3.       in between the 2 layers where brittle-ductile transition took place showing major rheological discontinuity

Besides the layer based strength, the lithosphere is also affected by the presence of fluid in which the lithosphere is weaker when it is wet and stronger when it is dry.

Water, affecting or weakening the lower crust by three main mechanism

1.       Hydrolytic weakening as water facilitate the movement of dislocation

2.       Nucleation of new weak phases such as mica during metamorphic or hydration reaction

3.       Facilitate grain boundary diffusion and sliding

Earlier hypotheses that suggest the nucleation of earthquakes in the upper mantle is proved to be not true, it has been widely accepted that the nucleation of seismogenic record occurred in the lower crust. It is from seismological evidence of earthquake record with magnitude higher than 5Mw in Africa, India, China and Mongolia, peak at 15 – 40 km depth suggesting lower crust. The common characteristic of the four locations is that they are all have underwent dehydration as a result from partial melting and subsequence of melt removal with geological  record of tectonic Pseudotachylite.

Emplacement of Pseudotachylite enhanced the fluid blocking direction in metamorphism with consequence of partial transformation of Pseudotachylite to Eclogite. The Pseudotachylite provides necessary pathway for liquid. Shear zone and metamorphic reaction is consistently in line with large volume of Pseudotachylite. However, even with this observation, the deformation mechanism is still a mystery to be explored.

Investigation in other regions such as in Northern Norway shows that there is overprint pattern of mylonitic foliation of previous Pseudotachylite by newly formed pristine Pseudotachylite. This shows cyclical and repeating pattern, recording history between brittle and viscous deformation.

All in all, he arrived to the conclusion that the main deformation mechanisms are diffusion creep and viscous grain boundary sliding in mylonitized pseudotachylite. This basically highlighting the roles of lower crustal earthquake in providing path for fluid infiltration hence weakening the dry granulites. 

Next post: Interactions between earthquakes and a ‘critically stressed’ volcano: the 2018 eruption of Sierra Negra, Galapagos Islands

Date: 01/11/2018
Venue: LT 201
Speaker: Prof. Luca Menegon, University of Plymouth