Within minutes after the 1992 Landers earthquake, seismic activity increased at widely scattered locations across the western United States at distances outside the aftershock zone (i.e., greater than 1–2 fault lengths) up to and beyond 1000 km from the Landers epicenter (Hill et al. 1993, Anderson et al. 1994). This remote triggering raised the question of whether this was an unusual event or just the first major earthquake to occur in a region so densely covered with seismic stations that ample remotely triggered small events could be detected. This motivated the search for earthquake catalogues yielding evidence of remote triggering associated with historic seismicity.
What triggers aftershocks?
Dynamic stress changes trigger aftershocks that rupture during the passage of the seismic waves, but the vast majority aftershocks occur during the days, weeks and months after the mainshock. While dynamic stress changes cannot directly trigger aftershocks that rupture long after the passage of the seismic waves emitted by the mainshock, they may initiate secondary changes to failure thresholds that lead to delayed triggering.
Aftershock zone is much larger than previously thought
For millennia we have known that the region around a significant earthquake will suffer aftershocks. In just the past decade or two, we have discovered that the aftershock zone is much larger. In fact, it extends across the entire surface of the Earth.
Earthquakes are triggered by other remote seismic events as a reaction to long-traveling seismic waves that temporarily stress the crust. Dr. Tom Parsons and his team at the United States Geological Survey (USGS) investigate a global array of sample regions with quality earthquake recording networks, and identify remotely triggered earthquakes on all continents and in every tectonic setting.
Studying earthquake rate changes at the global level is challenging because so much coincidental spontaneous activity happens, thus we require observation of highly significant temporal and spatial anomalies before concluding that a remote triggering episode has occurred.
We see an array of responses that can involve immediate and widespread seismicity outbreaks: delayed and localized earthquake clusters, to no response at all.
About 50% of local earthquake catalogs studied by Parsons and his team showed possible (localized, delayed) remote triggering, and around 20% showed probable (instantaneous, broadly distributed) remote triggering. In any single study region, at most only about 2-3% of the 260 global mainshocks of magnitude greater or equal to 7.0 caused strongly significant local earthquake rate increases.
However, the combined cases of probable remote triggering imply a 56% probability gain during the 24 hours after seismic waves from remote earthquakes arrive, and the largest event in that group reaches magnitude 6.7.
One curious feature is that delays of hours or days after seismic waves pass-through are found that are difficult to reconcile with the transient stresses imparted by seismic waves. We note that these delays are proportional to magnitude, and that nucleation times are best fit to a fluid diffusion process if the governing earthquake rupture process involves unlocking a magnitude-dependent critical nucleation zone.
The physics behind dynamically triggered quakes
It is well established that distant earthquakes can strongly affect the pressure and distribution of crustal pore fluids. The Earth’s crust contains hydraulically isolated, pressurized compartments in which fluids are contained within low permeability walls. We know that strong shaking induced by seismic waves from large earthquakes can change the permeability of rocks. Thus the boundary of a pressurized compartment may see its permeability rise.
Previously confined, over-pressurized pore-fluids may then diffuse away, infiltrate faults, decrease their strength, and induce earthquakes. Magnitude-dependent delays and critical nucleation zone conclusions can also be applied to human-induced earthquakes.
Evidence suggests future dynamically-triggered quakes may be predictable
Dynamic triggering of earthquakes by seismic waves is a robustly observed phenomenon with well-documented examples from over 30 major earthquakes.
Professor Emily Brodsky and her team from the University of California, Santa Cruz, use a variety of tools from geoscience and non-geoscience fields, such as statistics, to show that predictions of future seismicity rates based on seismic wave amplitudes are theoretically possible and may provide similar results to purely stochastic prediction schemes.
Although the triggered earthquakes are predominantly small, large earthquakes are occasionally possible. Since seismic waves can travel efficiently through the Earth, dynamic triggering can connect distant faults resulting in coupled disasters around the globe.
The take away for reinsurers
If this new evidence on how earthquakes interact over long distances means earthquakes can be linked many thousands of kilometres apart, it may present a challenge to the reinsurance assumption that globally distributed earthquakes are independent.
WRN Seismic Seminar in London February 23rd
The upcoming Willis Research Network Seminar on the 23rd of February promises to be an interesting event challenging our view of risk about earthquake potential.
The seminar will focus on the latest topics currently driving the scientific community, working closely with governments and private institutions to deepen our understanding of seismic risk, and help increase preparedness and resilience.
Dr. Tom Parsons, from USGS, Menlo Park, CA and Prof Emily Brodsky, from University of California, Santa Cruz, continue to pioneer research in the field of dynamic stress triggering, and will present the state of art and latest developments, as well as clear influences of global interactions on earthquake occurrence rates in specific regions.
Tom Parsons joined the USGS in 1992. He is a senior research geophysicist who develops earthquake and tsunami forecast methods and assessments at global scale and for specific regions. He has been especially interested in incorporating space geodesy, advanced statistical methods in paleoseismology, and stress transfer into earthquake rupture forecasts. He studies interactions between earthquakes at a variety of scales, with particular emphasis on observing and modeling global trends. He also runs numerical experiments on earthquake fault mechanics and crustal deformation using finite element techniques to learn more about earthquake initiation and deformation.
Emily Brodsky is a professor of Seismology at the UC Santa Cruz. Her studies of earthquake triggering, hydrogeology, fault zone structure and friction require using tools from a variety of geoscience disciplines including seismology, hydrogeology, structural geology, and rock mechanics. Her group also utilizes tools from non-geoscience fields such as experimental rheology, statistics, and materials science. The exploration of the mechanics underlying earthquakes has led into studies of other geophysical processes that involve some combination of friction, failure and fluid flow.