New type of earthquake discovered

Date:
December 6, 2021
Source:
Ruhr-University Bochum
Summary:
A research team has documented a new type of earthquake in an
injection environment in British Columbia, Canada. The seismic
events are slower than conventional earthquakes. Their existence
supports a scientific theory that until now had not been
sufficiently substantiated by measurements.



FULL STORY ==========================================================================
A Canadian-German research team have documented a new type of earthquake
in an injection environment in British Columbia, Canada. Unlike
conventional earthquakes of the same magnitude, they are slower and
last longer. The events are a new type of induced earthquake that have
been triggered by hydraulic fracturing, a method used in western Canada
for oil and gas extraction. With a network of eight seismic stations surrounding an injection well at distances of a few kilometres,
researchers from the Geological Survey of Canada, Ruhr- Universita"t
Bochum, and McGill University recorded seismic data of approximately
350 earthquakes. Around ten percent of the located earthquakes turned
out to exhibit unique features suggesting that they rupture more slowly, similar to what has previously been observed mainly in volcanic areas.


==========================================================================
The group headed by Hongyu Yu - first at RUB, later at the Canadian
Geological Survey of Canada - and RUB Professor Rebecca Harrington
describes the results in the journal Nature Communications, published
online on 25 November 2021.

Several theories on the origins of earthquakes To date, researchers
have explained the occurrence of earthquakes in the hydraulic-fracturing process with two processes. The first says that the fluid pumped into the
rock generates a pressure increase substantial enough to generate a new
network of fractures in the subsurface rocks near the well. As a result,
the pressure increase can be large enough to unclamp existing faults
and trigger an earthquake. According to the second process, the fluid
pressure increase from injection in the subsurface also exerts elastic
stress changes on the surrounding rocks that can be transmitted over
longer distances. If the stress changes occur in rocks where faults
exist, it can also lead to changes that cause the fault to slip and
cause an earthquake.

Recently, numerical models and lab analyses have predicted a process on
faults near injection wells that has been observed elsewhere on tectonic faults. The process, termed aseismic slip, starts out as slow slip that
does not release any seismic energy. The slow slip can also cause a
stress change on nearby faults that causes them slip rapidly and lead to
an earthquake. The lack of seismic energy from aseismic slip and the size
of the faults involved make it difficult to observe in nature. Researchers
have therefore not yet been able to document aseismic slip broadly with
any association to induced earthquakes. The work of the current study,
provides indirect evidence of aseismic loading, and a transition from
aseismic to seismic slip.

Modifying the hydraulic fracturing process The German-Canadian research
team interpret the recently discovered slow earthquakes as an intermediate
form of conventional earthquake and aseismic slip - and thus as indirect evidence that aseismic slip can also occur in the vicinity of wells. The researchers therefore dubbed the events hybrid-frequency waveform
earthquakes (EHW).

"If we understand at which point the subsurface reacts to the hydraulic- fracturing process with movements that don't result in an earthquake and, consequently, cause no damage to the surface, ideally we could use that information to adjust the injection procedure accordingly," as Rebecca Harrington, Head of the Hydrogeomechanics Group at RUB, describes one implication of the study.

Not all quakes spread at the same rate "We'd assumed that induced
earthquakes behave like most other earthquakes and have roughly the same rupture speed of two to three kilometres per second," explains Rebecca Harrington. But that doesn't always seem to be the case. While the shaking
from a conventional earthquake of magnitude 1.5 in the researchers'
data set had died down after about seven seconds, an EHW earthquake of
the same magnitude continued to shake for more than ten seconds.

========================================================================== Story Source: Materials provided by Ruhr-University_Bochum. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Hongyu Yu, Rebecca M. Harrington, Honn Kao, Yajing Liu, Bei
Wang. Fluid-
injection-induced earthquakes characterized by hybrid-frequency
waveforms manifest the transition from aseismic to
seismic slip. Nature Communications, 2021; 12 (1) DOI:
10.1038/s41467-021-26961-x ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/12/211206113108.htm

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