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Slip Earthquakes: a Diversity of Earthquake-Cycle

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Slip Earthquakes: a Diversity of Earthquake-Cycle PUBLICATIONS Geochemistry, Geophysics, Geosystems RESEARCH ARTICLE A Comparison of Geodetic and Geologic Rates Prior to Large 10.1002/2017GC007014 Strike-Slip Earthquakes: A Diversity of Earthquake-Cycle Key Points: Behaviors? Ratios of geodetic to geologic rates prior to large earthquakes are diverse James F. Dolan1 and Brendan J. Meade2 Observations show fault-system behavior is not yet fully explained by 1Department of Earth Sciences, University of Southern California, Los Angeles, CA, USA, 2Department of Earth and extant models Planetary Sciences, Harvard University, Cambridge, MA, USA Multiple intrinsic and extrinsic processes may interact to control fault behavior Abstract Comparison of preevent geodetic and geologic rates in three large-magnitude (Mw 5 7.6–7.9) strike-slip earthquakes reveals a wide range of behaviors. Specifically, geodetic rates of 26–28 mm/yr for the Correspondence to: J. F. Dolan, North Anatolian fault along the 1999 MW 5 7.6 Izmit rupture are 40% faster than Holocene geologic rates. [email protected] In contrast, geodetic rates of 6–8 mm/yr along the Denali fault prior to the 2002 MW 5 7.9 Denali earth- quake are only approximately half as fast as the latest Pleistocene-Holocene geologic rate of 12 mm/yr. In Citation: the third example where a sufficiently long pre-earthquake geodetic time series exists, the geodetic and Dolan, J. F., & Meade, B. J. (2017). geologic rates along the 2001 MW 5 7.8 Kokoxili rupture on the Kunlun fault are approximately equal at A comparison of geodetic and geologic rates prior to large strike-slip 11 mm/yr. These results are not readily explicable with extant earthquake-cycle modeling, suggesting that earthquakes: A diversity of earthquake- they may instead be due to some combination of regional kinematic fault interactions, temporal variations cycle behaviors? Geochemistry, in the strength of lithospheric-scale shear zones, and/or variations in local relative plate motion rate. What- Geophysics, Geosystems, 18. https://doi. org/10.1002/2017GC007014 ever the exact causes of these variable behaviors, these observations indicate that either the ratio of geo- detic to geologic rates before an earthquake may not be diagnostic of the time to the next earthquake, as Received 9 MAY 2017 predicted by many rheologically based geodynamic models of earthquake-cycle behavior, or different Accepted 16 NOV 2017 behaviors characterize different fault systems in a manner that is not yet understood or predictable. Accepted article online 22 NOV 2017 1. Introduction Geodetic observations of time-dependent surface deformation throughout an earthquake cycle can con- strain physical models of fault loading and rates of elastic strain accumulation. Unfortunately, these data are not yet generally extant along most faults that are capable of generating MW > 7 earthquakes due to long (>100 year) interevent times that exceed the duration of the observational epoch. However, geodetic obser- vations immediately prior to large (MW 7.6) earthquakes are now available along the North Anatolian, Denali, and Kunlun strike-slip faults. While short of spanning a complete earthquake cycle, these decadal pre-earthquake measurements do provide constraints late in the earthquake cycle where differences between long-term geologic fault slip rates and geodetically inferred slip-deficit rates are predicted to be greatest (Hetland & Hager, 2006). Earthquake-cycle models of periodic earthquakes within an elastic upper crust coupled to a linear viscoelastic mantle (Cohen, 1982; Hetland & Hager, 2006; Savage & Prescott, 1978) predict relatively low near-fault velocity gradients in fault-parallel surface velocities prior to periodic main shock ruptures. This is the case when either lower crustal viscosities are low or earthquake recurrence times are long, and where viscoelastic processes are responsible for postseismic relaxation. Interpreted in the con- text of classical elastic dislocation models (Savage & Burford, 1973), these velocities would yield relatively slow slip-deficit rate estimates as compared with longer-term (104 to 105 years) geologic slip rate estimates. It is for this reason that GPS observations of the surface velocity field late in the earthquake cycle provide strong constraints on viable earthquake-cycle models. In this paper, we summarize available geodetic and geologic rate data from the only three large-magnitude (MW 5 7.6–7.9) strike-slip earthquakes for which both sufficiently long preevent geodetic time series and well-constrained geologic fault slip rates are available. We discuss these results in terms of their implications for our understanding of fault mechanics, earthquake-cycle modeling, and the use of geologic and geodetic VC 2017. American Geophysical Union. rate data in probabilistic seismic hazard assessment, and conclude by outlining some potential mechanisms All Rights Reserved. that may explain the observed behaviors. DOLAN AND MEADE DIVERSE EARTHQUAKE-CYCLE BEHAVIORS? 1 Geochemistry, Geophysics, Geosystems 10.1002/2017GC007014 2. Observations of Geodetic and Geologic Rates Prior to Large-Magnitude Strike-Slip Earthquakes 2.1. 1999 Izmit, Turkey, Earthquake (North Anatolian Fault, Northern Strand) The 1999 MW 5 7.6 Izmit earthquake was generated by rupture of 145 km of the main, northern strand of the North Anatolian fault (NAF-N; Barka, 1999; Barka et al., 2002; Reilinger et al. 2000), which accommodates most of the slip along the NAF system in northwestern Turkey (e.g., Barka, 1992; Hubert-Ferrari et al., 2002). Although fault slip along the NAF-N within the central Marmara Sea region to the west of the 1999 rupture may involve significant distributed deformation (e.g., Hergert & Heidbach, 2010), several well-constrained offsets from both the east and west of the central Marmara zone of structural complexity all yield NAF-N slip rates of 15–20 mm/yr, over a wide range of time scales (<1,000 years to 500 ka; Dolan, 2009; Grall et al., 2013; Kurt et al., 2013; Meghraoui et al., 2012; Pucci et al., 2007). These geologic rates are slower than geodetically constrained slip-deficit rates based on pre-1999 earth- quake GPS data (Figure 1). For example, modeling of the 1988–1997 geodetic data suggests slip-deficit rates for the NAF-N strand of 24–30 mm/yr (e.g., Meade et al., 2002; Nyst & Thatcher, 2004; Reilinger et al., 2006; DeVries et al., 2016). Although the earlier block models were based on the assumption of internally rigid blocks, which could result in artificially rapid estimates of slip-deficit rates at block boundaries (i.e., the mod- eled faults), and some used block boundaries that did not exactly match the active strands of the NAF sys- tem in northwestern Turkey (as noted by Floyd et al., 2010), DeVries et al. (2016) used a geologically accurate, three-dimensional earthquake-cycle model that incorporates realistic block boundaries corre- sponding to the main, northern NAF-N strand, as well as the central NAF and southern NAF strands. More- over, their model allows internal block strain and accounts for interseismic strain accumulation and viscoelastic deformation generated from 500 years of MW > 6.5 earthquakes. The DeVries et al. (2016) model yields a pre-1999 Izmit earthquake geodetic slip-deficit rate for the NAF-N strand rate in northwest- ern Turkey of 26–28 mm/yr, similar to the earlier-determined rates, and 40% faster than the geologic slip rate (Figure 1). This geologic-geodetic rate discrepancy could potentially be explained by distributed geologic deformation that is not accounted for in the fault offset measurements used to constrain geologic slip rates (e.g., Hergert & Heidbach, 2010). This hypothesis, however, is inconsistent with late Pleistocene to Holocene (104 to 105 years) geologic slip rate estimates (Dolan, 2009; Grall et al., 2013; Kozacı et al., 2009; Kurt et al., 2013; Megh- raoui et al., 2012; Pucci et al., 2007; Rockwell et al., 2009) that come from both to the east and west of the central Marmara region zone of structural complexity at locations where the NAF-N is single stranded. For example, to the west of the 17.3 central Marmara, studies by Rockwell et al. (2009; 15.8 /–3.8 mm/yr), Meghraoui et al. (2012; 17 6 5 mm/yr), and Grall et al. (2013; 17.4 6 2.3 mm/yr) yield rates that are similar to NAF-N rates measured to the east by Pucci et al. (2007; 15.0 6 3.2 mm/yr), Dolan (2009; 16 6 110.9 2 mm/yr), and Kurt et al. (2013; 18.5 /–5.9 mm/yr). Moreover, the similar, longer-term (100–500 ka) geologic slip rate estimates by Grall et al. (2013) and Kurt et al. (2013) are based on offsets of km-scale fea- tures and therefore likely incorporate any off-fault distributed defor- mation. Thus, the apparent geologic-geodetic rate discrepancy along the northern strand of the NAF-N is real, with the pre-Izmit earthquake Figure 1. Comparison of geologic and geodetic rate data for the three earth- slip-deficit rate being 40% faster than the longer-term geologic fault 4 5 quakes we discuss. Dots show ratios of long-term (10 to 10 years) average slip rate (Figure 1). fault slip rates with geodetic rates immediately prior to the earthquakes. Denisty of blue and brown vertical lines show probability distribution functions 2.2. 2001 Kokoxili, Tibet, Earthquake (Kunlun Fault) of geologic-geodetic ratios from Meade et al. (2013) and Thatcher (2009), respectively. Note that rate of elastic strain accumulation along the Denali fault The 2002 MW 5 7.8 Kokoxili rupture extended for more than 400 km was only about half as fast as the long-term rate prior to the 2002 earthquake, along the central and western Kunlun fault in Tibet (Klinger et al., whereas the rate of elastic strain accumulation on the NAF-N prior to the 2005; Lasserre et al., 2005; Van der Woerd et al., 2002a; Xu et al., 2002).
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