Effects of source model variations on Coulomb stress analyses of a multi-fault intraplate earthquake sequence
Introduction
Multi-fault earthquakes and earthquake sequences are common in many tectonic settings on Earth (e.g., Sieh et al., 1993; Fletcher et al., 2016; Wei et al., 2011; Beavan et al., 2012; Elliott et al., 2012; Hamling et al., 2017; Quigley et al., 2019; Clark et al., 2012). Distinct fault ruptures may be triggered as part of a quasi-continuous cascading seismic moment release, or be triggered seconds, minutes, hours, days, or even decades following preceding earthquakes (Belardinelli et al., 2003a, Belardinelli et al., 2003b; Freed, 2005; Nissen et al., 2016; Stein, 1999). Stress changes due to moderate-to-large earthquakes may affect the location of subsequent events by processes including static stress transfer, dynamic (i.e., coseismic) stress changes and viscoelastic stress change (e.g., King et al., 1994; Stein, 1999; Nostro et al., 1997; Reasenberg and Simpson, 1992; Stein et al., 1997; Lin and Stein, 2004; Steacy et al., 2013). The relative importance of these different types of stress changes in earthquake sequences may be challenging to evaluate as they all may provide scientifically valid explanations for aspects of the same sequence. Static stress changes estimated from Coulomb stress calculations (e.g., Harris and Simpson, 1998) have been shown to provide an important explanation for some instances of earthquake triggering and clustering (e.g., King et al., 1994; Harris, 1998; Stein, 1999; Harris et al., 1995; Freed and Lin, 2001; Freed, 2005; Steacy et al., 2005; Mohammadi and Bayrak, 2015; Mohammadi et al., 2017) by suppressing or encouraging rupture on receiver faults. Coulomb stress change analysis may be a potentially powerful forecasting tool if it can be reliably applied to rapidly-developed fault models (Steacy et al., 2014).
Although the calculation of Coulomb stress changes is well-established, the early stages of many earthquake sequences see the emergence of multiple diverse models for fault ruptures that vary in fault geometries and co-seismic slip due to different modelling approaches and utility of different datasets (e.g., seismological data, geologic data, optical data, InSAR, geodetic data). Model variations impart significant epistemic uncertainty to interpretation of the relationships between stress transfer and earthquake sequencing (Wang et al., 2014; Zhan et al., 2011). Changing parameters of the Coulomb stress change model, including the stress change tensor (related to changing geometries and slip distributions of the source fault), receiver fault geometry, and the friction coefficient and Skempton's coefficient, may impact on how well static stress change analyses explain observation of spatiotemporal patterns of earthquake sequences (Lin and Stein, 2004; Zhan et al., 2011; Wang et al., 2014; Mildon et al., 2016). Fault model uncertainties are rarely applied to stress-triggering studies; although some studies have examined uncertainties in receiver fault geometries (e.g., Harris and Simpson, 2002; Steacy et al., 2005; Lasocki et al., 2009). Woessner et al. (2012) conducted a thorough analysis of the effects of fault model uncertainties on Coulomb stress models for the moment magnitude (Mw) = 5.9 June 2000 Kleifarvatn earthquake in southwest Iceland.
Another limitation of many Coulomb stress change studies is the ambiguity with which estimated stress changes on receiver faults are interpreted to have been enough to trigger rupture. A Coulomb stress increase of 0.01 MPa (0.1 bar) is commonly proposed to be the threshold for potential earthquake triggering (Harris, 1998; Reasenberg and Simpson, 1992; Freed, 2005; King et al., 1994; Stein, 1999). However, the stress threshold to trigger instantaneous rupture on receiver faults concurrent with the hypocentral source fault rupture may be significantly higher. For example, Coulomb stress changes of >0.1 MPa (Zhan et al., 2011) and 1 to 1.5 MPa (Walters et al., 2018) were insufficient to generate spontaneous rupture during the 2010–2011 Canterbury earthquake sequence and 2016 Central Italy seismic sequence, respectively. Instead subsequent receiver fault ruptures occurred days (Walters et al., 2018) to months (Zhan et al., 2011) after initial stress loading from prior mainshocks.
In this paper, we apply Coulomb stress modelling to investigate the rupture behaviour of the 1987 to 1988 Tennant Creek earthquake sequence in Australia. We aim to investigate (1) whether static stress changes on receiver faults induced by preceding earthquakes provide an explanation for the observed spatiotemporal patterns of this sequence, including the hypocentral locations and inter-event timing, and (2) whether differences in fault geometry and rupture kinematics (associated with different rupture models), influence static stress changes significantly enough to cast uncertainty over whether Coulomb stress models adequately explain this sequence. We also consider maximum calculated stress change increases on receiver faults in the context of stress triggering thresholds and time lags between source and receiver fault ruptures for this and other earthquake sequences globally.
Section snippets
Seismotectonics of the 1987–1988 Tennant Creek earthquake sequence
The Australian continent moves north relative to the ITRF 2014 NNR model (Altamimi et al., 2017) at 6.7 cm/yr but accommodates little internal strain (Tregoning, 2002). Australia has a complex intraplate stress field that reflects interactions amongst plate tectonic forces exerted from distant plate boundaries (Hillis et al., 2008) with local stress perturbations associated with faults, lithological density and strength contrasts and intraplate volcanism (Rajabi et al., 2017 and references
Coulomb stress change calculations: theory
One of the most straightforward and powerful physics-based methods to forecast the distribution of triggered seismicity is Coulomb stress change modelling (Rybicki, 1973; Smith and Van de Lindt, 1969; Stein, 1999; Toda and Stein, 2003; Toda et al., 2005). This method is successful in forecasting locations of aftershocks, with accuracy that improves as seismic network density increases and slip inversions become more accurate. In this study we focus on static stress changes, although other
Method and assumptions
To address a source of epistemic uncertainty in the applicability of Coulomb stress models to earthquake sequences, where source models vary significantly, we model five scenarios for ΔCFS triggering based on four different sets of fault models (described as ‘sequence models’) which primarily reflect variable interpretations of hypocentre and kinematics. The models are variably derived from (1) P wave polarity fault plane solutions, (2) teleseismic centroid body wave inversions, (3) teleseismic
Defining the fault model for the largest foreshock
For the largest recorded foreshock (LFS), we use the slip kinematics and seismic moment magnitude from the USGS database (https://earthquake.usgs.gov/earthquakes/eventpage/usp00031hu/executive) and assume that Bowman and Dewey's (1991) relocated hypocentre occurs in the centre of the fault. As we do not know which fault plane (nodal vs. auxiliary) is the actual fault plane, we construct two fault models using Leonard's (2014) scaling relation and consider both fault orientations in each
Coulomb triggering and the Tennant Creek sequence
We explored the possible stress triggering relationship of the Tennant Creek earthquakes with different fault source (sequence) models. We do not account for other sources of stress such as dynamic or visco-elastic stress changes (Van Der Elst and Brodsky, 2010; Kilb et al., 2000; Felzer and Brodsky, 2005, Felzer and Brodsky, 2006) or secondary aftershock triggering (e.g., Meier et al., 2014) although these processes may be relevant. Dynamic stress triggering is caused by the passage of seismic
Conclusions
The effects of Coulomb stress changes on the Tennant Creek earthquake sequence have been evaluated using five different planar fault source sequence models developed from different input data. Our major conclusions are:
- 1)
In most fault models, for most of the major earthquakes in the Tennant Creek sequence, the hypocentral area of the forthcoming receiver fault rupture is located in an area of positive Coulomb stress loading (ΔCFS) from preceding ruptures. This suggests that ΔCFS provides a
Acknowledgements
This work was supported by the Australian Research Council (Discovery Grant #DP170103350). We wish to thank Kelin Wang (Editor in chief) and two anonymous reviewers for their comment and suggestions that improved the manuscript. Hiwa Mohammadi received a Baragwanath Travel Scholarship from the University of Melbourne to assist in research development.
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