When the fault rings twice: repeated ruptures on the same fault stretch

The recent publication of a paper on the Weitin Thrust (Papua New Guinea) by Chen, Milliner and Avouac (Fig. 1) gave me the opportunity to dig out and look back to some notes I wrote few months ago. Chen et al. use optical image correlation to document coseismic surface ruptures along the Weitin Thrust occurred in a Mw 8.0 event in 2000 and in a Mw 7.7 event in 2019. The ruptures overlap along a 20-km long portion, with 3-4 m of slip (Fig. 2).

I’m particularly interested in re-rupturing: I’m Italian and the first ground-rupturing event I studied in the field was the 2016 Central Italy seismic sequence, when the same fault stretch ruptured twice on August 24^th and October 30^th, 2016 (Fig. 3). If I have to recall a single thing that surprised the scientific community, including researchers much more experienced than me, that is re-rupture.

I don’t want to get lost in defining how close in time ruptures should be, let’s simplify things saying “somehow below the resolution of dating techniques, so that they cannot be recognized by paleoseismic investigations” – after all, I’m writing on paleoseismicity.org 😊 

So, I started to look for other case histories and here’s what I found (some are well-constrained, other less):

• Ridgecrest (USA): Mw 6.4 on July 4^th and Mw 7.1 on July 5^th, 2019;
• Weitin Thrust (PNG): Mw 8.0 in 2000 and Mw 7.7 in 2019;
• Lake Miur (Australia): Mw 5.3 on September 16th and Mw 5.2 on November 8th, 2018;
• 2017 Hojdek triplet (Iran);
• Mw 6.5 and Mw 7.3 Kumamoto (Japan) earthquakes in 2016;
• Central Italy: Mw 6.0 and Mw 6.5 in 2016, three months apart;
• Mw 7.4 Izmit and Mw 7.1 Duzce (Turkey) earthquakes in 1999, 3 months apart;
• 1981 Sirch and 1998 Fandoqa (Iran) earthquakes;
• Mw 7.1 Dasht-e-Bayaz 1968 and Mw 7.1 Khuli-Buniabad 1979 (Iran);
• Ms 7.0 and 7.1 in 1957 and 1967 at the western end of the North Anatolian Fault (Turkey);
• 1936, 1979 and Mw 7.2 1997 Zirkuh (Iran);
• Mw 7.0 1940 El Centro earthquake and Imperial Valley 1979;
• 1944 and 1970 earthquakes in Gediz (Turkey).

It seems re-rupture is the new black…

I can’t see a common pattern in the above-listed case histories: they are different in terms of faulting style, plate motion rates, slip distribution, range of magnitudes and evolutive pattern (sometime the strongest shock came first, sometime not). The list is obviously far from complete and I would love to collect other case histories!

Re-rupture is challenging because it forces to rethink – or even abandon? – some assumptions widely-adopted in earthquake geology:

• Paleoseismology: in most cases, dating techniques will never be able to disentangle events occurred few weeks-years apart. The slip we see in a trench is due to a single event or to multiple events?
• To make things even more complex, distributed faulting lies at the opposite end of the spectrum of surface faulting behaviors: slip may occur on faults other than the seismogenic source. How to recognize in a trench if slip is due to distributed faulting?
• Recurrence models and hazard assessment: re-rupture totally alters the estimation of recurrence intervals and slip per event. To which extent it changes the output of hazard codes or earthquake forecasting?
• Coulomb stress transfer: it is usually assumed that all the accumulated stress is released during an earthquake and the stress goes to zero; is this assumption still valid?

Sometimes I have the impression that my research is guided by the drunkard’s search principle, the observational bias that occurs when people only search for something where it is easiest to look. Our observations are still too limited and shed a faint light on the earthquake process; analyzing re-rupturing events may improve our understanding and ultimately help in coping with seismic risk.

References:

Chen, K., Milliner, C., & Avouac, J.‐P. (2019). The Weitin Fault, Papua New Guinea, Ruptured Twice by Mw 8.0 and Mw 7.7 Earthquakes in 2000 and 2019. Geophysical Research Letters, 46. https://doi.org/10.1029/2019GL084645