By Ross Stein and David Jacobson, Temblor
For several decades, geologists assumed that faults more than 5 km (3 mi) apart could not rupture together. This inference was based on examination the surface ruptures of all M≥7 shocks around the world by geologists such as Steve Wesnousky (Univ. Nevada Reno), as well as by numerical modeling of rupture propagation by Ruth Harris (USGS) and Steve Day (San Diego State Univ.). If you asked earthquake researchers how to interpret this work, they would say that for faults separated by less than a kilometer, through-going ruptures are possible; if separated by more than a few kilometers, rupture is unlikely; and if more than about 5 km, jumping from one fault to another becomes extremely unlikely. Further, the width of rupture along the fault can be as narrow as several centimeters (an inch), and rarely exceeds several kilometers. The caveat is that faults are three-dimensional, and that we are condemned to live on the surface of the Earth, and so faults widely separated at the Earth's surface could be connected at depth.
Kaikoura changed everything
But in one fell swoop, the Kaikoura earthquake chucked this hallowed view out the window—or at least showed that there are glaring exceptions.
In the earth sciences, good data always trumps good theory. This means that seismology is an experimental science in which you cannot plan your own experiments: Mother Nature plans them for you. Each large, well recorded event furnishes not only a chance to learn, but also the prospect to falsify the current dogma or consensus. What a gift this is for young scientists, because evidence, not eminence, rules.
In the New Zealand event, faults as much as 20 km (12 km) apart ruptured together, and the width of the faulting along parallel faults was as much as 40 km (25 mi). Was this unprecedented? Probably not. Rather, thanks to New Zealand geologists and geophysicists, the fault slip was very well recorded and extensively documented. Publications have just begun to emerge about this quake, and they make two things clear:
• Mapped active faults did only a fair job of foretelling where the faults would rupture
• Mapped faults 25 km apart can indeed rupture together
The faults unzipped at 3300 mph (5300 kph), pausing briefly before the jumps
The story at the Earth's surface is also fairly consistent with the slip inferred at depth by Kaiser et al. (2017) and independently by Hollingsorth et al. (2017): They both show fault ruptures separated by ~20 km. Here, below, the energy recorded at seismometers is projected backwards to the source, a technique used to watch ruptures propagate. The earthquake nucleates at the star and unzips northeast for 20 seconds (purple to blue colored dots), then it jumps ~15 km (9 mi) to the north and ruptures for another 20 seconds (blue to turquoise colored dots). Then it jumps north for the third time and ruptures for another 40 seconds (green to yellow dots). The entire leaping rupture process lasted an astonishing 2 minutes, the same amount of time that it took the 2011 Tohoku M=9.0 earthquake, which was 40 times larger.
What does this mean for the seismic hazards in LA and SF?
Through the prism of Kaikoura, two fault systems jump out that deserve another look. In southern California, the Newport-Inglewood Fault has always been an oddity, because it is straight but highly discontinuous, with as much as 10 km between mapped active faults. But jumping from one to another now looks much more possible than it did before.
In the San Francisco Bay Area, the northern tip of the Calaveras Fault is separated by 10 km from the southern tip of the Concord Fault, and 15 km from the parallel Greenville Fault. Two, or perhaps all of these could rupture together, creating a much larger earthquake.
The Newport-Inglewood Fault, with a 1 mm/yr slip rate, has 1.0-1.5% per year chance of rupturing in a M≥6.7 shock in the next 30 years. If that sounds unlikely, think again: It means there is 2 chances out of 3 that it will strike in a typical lifetime, and the consequences of such a quake would be immense, tearing through the densely populated south central Los Angeles corridor. Such a rupture was included in current USGS model (Field et al., 2014), and that judgement now seems validated.
The Calaveras and Concord Faults have slip rates of 3.5-6.0 mm/yr, and about a 1% per year chance of rupturing. But the combined ruptures were not considered in the 2014 USGS hazard model, and so much larger, longer duration ruptures now seem possible.
What can you do about it?
If you live in an older (pre-1977) home, retrofit it. A $5,000 retrofit can likely save $100,000 in damage in a M~7 earthquake. Second, consider earthquake insurance. Because the insurance companies are using the older, 2008 USGS model, they are not considering any ruptures that can jump from one fault to another, regardless of distance. The possibilities of the long jumps seen in Kaikoura mean that companies are probably selling insurance for less than its true risk-based price, so you will be effectively getting a discount.
Edward H. Field, Glenn P. Biasi, Peter Bird, Timothy E. Dawson, Karen R. Felzer, David D. Jackson, Kaj M. Johnson, Thomas H. Jordan, Christopher Madden, Andrew J. Michael, Kevin R. Milner, Morgan T. Page, Tom Parsons, Peter M. Powers, Bruce E. Shaw, Wayne R. Thatcher, Ray J. Weldon II, and Yuehua Zeng (2015), Long-Term Time-Dependent Probabilities for the Third Uniform California Earthquake Rupture Forecast (UCERF3), Bull. Seismol. Soc. Amer., 105, 511–543, doi: 10.1785/0120140093
Ruth A. Harris and Steven M. Day (1993). Dynamics of fault interaction–parallel strike-slip faults, J. Geophys. Res. 98, 4461–4472.
James Hollinsworth, Lingling Yec, Jean-Philippe Avouac, Dynamically triggered slip on a splay fault in the Mw 7.8, 2016 Kaikoura (New Zealand) earthquake (2017), Geophys. Res. Letts., doi: 10.1002/2016GL072228.
A. Kaiser, N. Balfour, B. Fry, C. Holden, N. Litchfield, M. Gerstenberger, E. D'Anastasio, N. Horspool, G. McVerry, J. Ristau, S. Bannister, A. Christophersen, K. Clark, W. Power, D. Rhoades, C. Massey, I. Hamling, L. Wallace, J. Mountjoy, Y. Kaneko, R. Benites, C. Van Houtte, S. Dellow, L. Wotherspoon, K. Elwood, and K. Gledhill (2017), The 2016 Kaikoura, New Zealand, Earthquake: Preliminary Seismological Report, Seismol. Res. Letts., 88, doi: 10.1785/0220170018.
Steven G. Wesnousky, (2006). Predicting the endpoints of earthquake ruptures, Nature 444, 358–360, doi: 10.1038/nature05275.
Litchfield NJ, Benson A, Bischoff A, Hatem A, Barrier A, Nicol A, Wandres A, Lukovic B, Hall B, Gasston C, Asher C, Grimshaw C, Madugo C, Fenton C, Hale D, Barell DJA, Heron DW, Strong DT, Townsend DB, Noble D, Howarth JD, Pettinga J, Kearse J, Williams J, Manousakis J, Mountjoy J, Rowland J, Clark KJ, Pedley K, Sauer K, Berryman KR, Hemphill-Haley M, Stirling MW, Villeneuve M, Cockcroft M, Khajavi N, Barnes P, Villamor P, Carne R, Langridge RM, Zinke R, Van Dissen RJ, McColl S, Cox SC, Lawson S, Little T, Stahl T, Cochran UA, Toy V, Ries WF, Juniper Z. 2017. 14th November 2016 M7.8 Kaikoura Earthquake. Summary surface fault rupture traces and displacement measurements. GNS Science. http://dx.doi.org/10.21420/G2