During the 75 years before the great 1906 earthquake on the San Andreas fault, the San Francisco Bay area suffered at least 14 shocks of moment magnitude (Mw) equal to or exceeding 6; these occurred on all major faults, and included two events of Mw 6.8. (Stein 1999, Nature).
Following my previous blog on how globally distributed earthquakes need not be independent (dynamic stress triggering), there is also growing evidence that large earthquakes can inhibit or promote failure on nearby faults for years to centuries (static stress triggering), and that static (or permanent) stress transfer plays a central role in this interaction.
- Stress increases (‘trigger zones’) associated with earthquake ruptures promote aftershocks and subsequent mainshocks
- Stress decreases (‘stress shadows’) inhibit aftershocks and subsequent mainshocks.
The location of these zones depends on the rupture geometry; the duration of these zones depends on the fault stressing rate or background seismicity rate.
Aftershocks (earthquakes that happen after a mainshock) can sometimes be larger or more damaging than their mainshock (e.g, Kumamoto and Christchurch). Static stress triggering acts over short distances (generally less than <200 km) and time periods from minutes to decades. By translating this stress change into probability change, seismic hazard appears to be strongly influenced by earthquake interaction, according to Ross Stein, CEO and cofounder, Temblor, Inc., Adjunct Professor of Geophysics, Stanford University.
Coulomb stress transfer
Earthquake interaction is a fundamental feature of seismicity, leading to earthquake sequences, clustering, and aftershocks. One interaction criterion that promises a deeper understanding of earthquake occurrence, and a better description of probabilistic hazard, is Coulomb stress transfer.
In a series of computer animations and examples from great and small quakes around the world, Ross explains how static stress triggering and Coulomb stress transfer works for isolated earthquakes, earthquake couplets, and in progressive falling-domino sequences.
On the road to more accurate earthquake forecasts
At heart, we interpret our results to mean that earthquake stress changes do not simply turn on or off seismicity; rather, the background seismicity rate is enhanced by stress increases and suppressed by stress decreases. This best explains:
- why seismicity in stress trigger zones is often patchy or discontinuous
- why seismicity rate declines in stress shadows are often subtle or absent
- why some aftershock zones expand, migrate or densify
While static stress triggering is a far cry from an earthquake prediction, it is, perhaps, on the road to the more useful and accurate earthquake forecasts that we all seek.
WRN Seismic Seminar in London February 23rd
If you want to learn more about this in the context of the Re/Insurance industry, join us for an afternoon of cutting edge earthquake science discussions at our upcoming WRN Seminar on Seismic Risk, on February 23rd, where Ross will discuss the state of art in (Coulomb) stress triggering methodologies and real-time aftershock forecasting.
Ross Stein, CEO and cofounder, Temblor, Inc., Adjunct Professor of Geophysics, Stanford University, studies how earthquakes interact by the transfer of stress. He has largely contributed to science in the field of quantification of static stress transfer for probabilistic seismic hazard assessment (the principal tool of the engineering, insurance, financial, and emergency-response communities) that reflects or can reproduce such observations.
Stein is also a USGS Scientist Emeritus, and President-elect of the Tectonophysics section of the American Geophysical Union (AGU). In 2003, the Science Citation Index reported that Stein was the second most-cited author in earthquake science during the preceding decade. In 2009 he cofounded the Global Earthquake Model Foundation, a public-private partnership building a seismic risk model for the world, and chaired GEM’s Science Board until 2014. Ross has appeared in many documentary films, including the Emmy-nominated documentary, ‘Killer Quake’ (NOVA, 1995), the four-part ‘Great Quakes’ series (Discovery, 1997-2001), and the multiple award-winning 2004 National Geographic IMAX movie ‘Forces of Nature,’ which he helped to write and animate.