Testing stress shadowing effects at the South American subduction zone

Abstract

S U M M A R YThe seismic gap hypothesis assumes that a characteristic earthquake is followed by a long period with a reduced occurrence probability for the next large event on the same fault segment, as a consequence of the induced stress shadow. The gap model is commonly accepted by geologists and is often used for time-dependent seismic hazard estimations. However, systematic and rigorous tests to verify the seismic gap model have often failed so far, which might be partially related to limited data and too tight model assumptions. In this study, we relax the assumption of a characteristic size and location of repeating earthquakes and analyse one of the best available data sets, namely the historical record of major earthquakes along a 3000 km long linear segment of the South American subduction zone. To test whether a stress shadow effect is observable, we compiled a comprehensive catalogue of mega-thrust earthquakes along this plate boundary from 1520 to 2015 containing 174 earthquakes with M w > 6.5. In our new testing approach, we analyse the time span between an earthquake and the last event that ruptured the epicentre location, where we consider the impact of the uncertainties of epicentres and rupture extensions. Assuming uniform boundary conditions along the trench, we compare the distribution of these recurrence times with simple recurrence models. We find that the distribution is in all cases almost exponentially distributed corresponding to a random (Poissonian) process; despite some tendencies for clustering of the M w ≥ 7 events and a weak quasi-periodicity of the M w ≥ 8 earthquakes, respectively. To verify whether the absence of a clear stress shadow signal is related to physical assumptions or data uncertainties, we perform simulations of a physics-based stochastic earthquake model considering rate and state-dependent earthquake nucleation, which are adapted to the observations with regard to the number of events, spatial extend, size distribution and involved uncertainties. Our simulations show that the catalogue uncertainties lead to a significant blurring of the theoretically peaked distribution, but the distribution would be still distinguishable from the observed one for M w ≥ 7 events. However, considering the stress transfer to adjacent fault segments and heterogeneous instead of constant stress drop within the rupture zone can explain the observed recurrence time distribution. We conclude that simplified recurrence models, ignoring the complexity of the underlying physical process, cannot be applied for forecasting the M w ≥ 7 earthquake occurrence at this plate boundary.

Testing of the Seismic Gap Hypothesis in a Model With Realistic Earthquake Statistics

“…This scenario is avoided if one restricts the study to magnitude m ≥ 8 earthquakes, which are sufficiently large to rupture the full seismogenic zone. Interestingly, in this case Roth et al (2017) find a weak quasi-periodic temporal organization of events, consistent with the hypothesis of alternation. The problem in this case is that the statistical sample is so small, only 20 recurrences, that a definite conclusion cannot be drawn.…”

“…This has been recently done in a study (Roth et al, 2017) conducted along the South American subduction zone for the last 500 years, which shows that recurrence times of magnitude m ≥ 7 earthquakes present some tendencies toward short-time clustering. This result, which is apparently in opposition to the hypothesis of alternation, can still be consistent with it if one takes into account that in the data analysis of Roth et al (2017) only the overlap between hypocentral coordinates is taken into account and that most of the m ≳ 7 earthquakes only break a portion of the seismogenic width. Consistently with the hypothesis of alternation, indeed, the partial rupture can cause the stress increase along the unbroken part of the fault width which, in turn, can raise the probability of subsequent earthquakes with similar hypocentral coordinates in the near future.…”

Seismicity clusters in Central Chile: investigating the role of repeating earthquakes and swarms in a subduction region

“…were reported in the years 1575, 1647, 1730, 1822, 1906, 1965, 1971, and 1985; of which the 1730 is the largest one, with a magnitude around 9.0 ( Fig. 1; see also, Comte et al 1986;Udías et al 2012;Roth et al 2017;Ruiz & Madariaga 2018). The last large earthquakes to rupture in this segment were the 1985 M W 8 Valparaíso earthquake and the 2010 M W 8.8 Maule earthquake, followed by the 2010 M W 6.9 and M W 7.0 Pichilemu earthquake doublet, see Fig.…”

The ISC-GEM Earthquake Catalogue (1904–2014): status after the Extension Project

“…Before the ISS was put in production starting with earthquakes that occurred in 1918, other seismic bulletins were compiled by different authors/agencies (e.g. Schweitzer and Lee, 2002;Storchak et al, 2015, and references therein). For this work we gathered station data from the following sources.…”

“…The ISA, SHIDE and RUS bulletins are available from the supplementary material of Schweitzer and Lee (2002), whereas scanned images of GUTE notepads were kindly provided by Katsuyuki Abe. The ISA, BAAS and ISS bulletins list arrival times from most of the stations operating at that time, whereas SHIDE mostly includes data from Milne stations and the GUTE notepads only a subset of global stations.…”