Anatomy of an earthquake series

Date:
August 11, 2021
Source:
GFZ GeoForschungsZentrum Potsdam, Helmholtz Centre
Summary:
A new study focuses on the 2013 seismic sequence at the Castor
platform of a former oil field, about 20 km offshore the coast of
Valencia, Spain.

During the initial phase of the development of a gas storage
facility in the former oil field, thousands of earthquakes with
magnitudes below 4.1 took place after the injection of gas into
the depleted layers of the reservoir.



FULL STORY ==========================================================================
An international team led by scientists at GFZ Helmholtz Centre Potsdam,
in collaboration with colleagues by Spanish, Italian and US institutions,
is publishing a new scientific work on induced seismicity in Europe in
the Journal Nature Communications.


==========================================================================
The study focuses on the 2013 seismic sequence at the Castor platform
of a former oil field, about 20 km offshore the coast of Valencia,
Spain. During the initial phase of the development of a gas storage
facility in the former oil field, thousands of earthquakes with magnitudes below 4.1 took place after the injection of gas into the depleted layers
of the reservoir. While similar gas storage operations worldwide are
typically not stimulating substantial seismicity, the Castor sequence
remains to date the most significant case of seismicity related to this
type of industrial operations in Europe.

The new study employs a combination of advanced seismological techniques applied to an enhanced waveform dataset to better understand the
seismogenic process and the geometry of activated fault, which remained
to date debated.

The new analysis identifies about 3,500 earthquakes, which took place
at shallow depth between September and early October in the vicinity
of the Castor injection platform. The study reveals for the first time
three phases of the crisis. The first phase, accompanying gas injection
from early to mid- September, was characterized by weak seismicity, progressively growing in magnitude. The injection stop marks the
beginning of a second phase, which will last until end of September,
where seismicity slowly migrated towards SW, driven by pore-pressure
diffusion. The third phase, lasting until early October, saw a fast,
backward migration, with the occurrence of all largest earthquakes as
the failure of loaded asperities. Seismicity mostly affected a secondary
fault, located close below the reservoir, and dipping opposite from the reservoir bounding fault.

The study demonstrates that a detailed view of the dynamics of seismic sequences can be resolved even in the lack of a dense local monitoring
network, offering a benchmark for similar future studies elsewhere.

The insights are important also in the light that the Castor project has
been abandoned after the occurrence of the earthquakes and the question
of predictability of the risks and responsibility for such types of
events are under public debate.

========================================================================== Story Source: Materials provided by GFZ_GeoForschungsZentrum_Potsdam,_Helmholtz_Centre. Note: Content may
be edited for style and length.


========================================================================== Journal Reference:
1. Simone Cesca, Daniel Stich, Francesco Grigoli, Alessandro Vuan,
Jose'
A'ngel Lo'pez-Comino, Peter Niemz, Estefani'a Blanch, Torsten
Dahm, William L. Ellsworth. Seismicity at the Castor gas reservoir
driven by pore pressure diffusion and asperities loading. Nature
Communications, 2021; 12 (1) DOI: 10.1038/s41467-021-24949-1 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210811113142.htm

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