Precise geodetic measurements made over broad swaths of tectonically active regions record patterns of interseismic strain accumulation, providing key insights into the locus and timing of pending earthquakes. Recent studies of geodetic position time series, including that of Melnick et al. (2017), illustrate temporal variation in the pattern of interseismic deformation. These authors propose that the 2010 Mw = 8.8 Maule, Chile, earthquake enhanced coupling on the Andean subduction zone adjacent to the rupture, including on the portion of the megathrust that broke 5 years later in the Mw = 8.3 Illapel event

Loveless, John P.

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Masthead Logo Smith ScholarWorksGeosciences: Faculty Publications Geosciences2-4-2017Super-Interseismic Periods: Redefining EarthquakeRecurrenceJohn P. LovelessSmith College, jloveles@smith.eduFollow this and additional works at: https://scholarworks.smith.edu/geo_facpubsPart of the Geology CommonsThis Article has been accepted for inclusion in Geosciences: Faculty Publications by an authorized administrator of Smith ScholarWorks. For moreinformation, please contact scholarworks@smith.eduRecommended CitationLoveless, John P., "Super-Interseismic Periods: Redefining Earthquake Recurrence" (2017). Geosciences: Faculty Publications, SmithCollege, Northampton, MA.https://scholarworks.smith.edu/geo_facpubs/7Confidential manuscript submitted to Geophysical Research Letters  Super-interseismic periods: Redefining earthquake recurrence 1 John P. Loveless1  2 1Department of Geosciences, Smith College, Northampton, MA 01063, USA. 3 Corresponding author: John P. Loveless (jloveless@smith.edu)  4 Key Points: 5  Recent studies show temporal variation in interseismic strain patterns. 6  Great earthquakes modify coupling on adjacent parts of fault. 7  Earthquake clusters may arise from such fluctuations in coupling. 8   9 Confidential manuscript submitted to Geophysical Research Letters  Abstract 10 Precise geodetic measurements made over broad swaths of tectonically active regions 11 record patterns of interseismic strain accumulation, providing key insights into the locus 12 and timing of pending earthquakes. Recent studies of geodetic position time series, 13 including that of Melnick et al. [2017] in this issue, illustrate temporal variation in the 14 pattern of interseismic deformation. These authors propose that the 2010 Mw=8.8 Maule, 15 Chile earthquake enhanced coupling on the Andean subduction zone adjacent to the 16 rupture, including on the portion of the megathrust that broke five years later in the 17 Mw=8.3 Illapel event.  18 1 Introduction 19 The classical concept of an earthquake recurrence interval [Shimazaki and 20 Nakata, 1980] is rooted in an assumption that spatial patterns of interseismic strain 21 accumulation that precede seismic rupture are consistent through time. However, a 22 growing body of evidence based on geodetic and geologic observations suggests 23 fundamental deviations from this simple model, including spatially and temporally 24 clustered earthquakes and short-term fluctuation in patterns of interseismic fault coupling 25 preceding large earthquakes. Paleoseismic data provide long records of earthquake 26 deformation, which are essential for providing insight into patterns of fault slip over 27 multiple seismic cycles. However, these data can be complicated by imprecise temporal 28 resolution and ambiguity in correlating multiple observations along a single seismogenic 29 fault [Biasi and Weldon, 2009]. As a result, distinguishing, for example, a single large 30 magnitude rupture along a great length of the fault from a temporally clustered sequence 31 of smaller earthquakes can be difficult [Gold et al., 2017]. On the other hand, modern 32 satellite geodesy observations provide a more spatially complete picture of earthquake 33 cycle deformation but thus far have only been capable of imaging single earthquakes and 34 small fractions of the preceding and subsequent interseismic periods. In several instances, 35 major subduction zone earthquakes have occurred in regions inferred from geodetic 36 observations to be pre-seismically coupled [Loveless and Meade, 2011; Moreno et al., 37 2010; Protti et al., 2014], but the extent to which coupling patterns vary within an 38 interseismic period or between sequential earthquake cycles is unclear owing to the short 39 time duration of available observations. To improve our understanding of earthquake 40 occurrence and recurrence, it is important to synthesize insights gained about fault system 41 behavior throughout the earthquake cycle from both geologic and geodetic data. 42 The proliferation of geodetic data along subduction zone fore arcs has led to 43 unprecedented insight into megathrust earthquake cycle processes. Specifically, geodetic 44 observations can be used to constrain the distribution of interseismic coupling and 45 coseismic slip, as well as aseismic slip and postseismic processes [Wang et al., 2012]. 46 Geodetic velocities near seismogenic faults are often taken to represent steady, secular 47 motion in response to spatially variable but temporally constant interseismic fault 48 coupling, even for data spanning major earthquakes and hence including portions of two 49 sequential interseismic periods [Ergintav et al., 2009]. This assumption may reflect, at 50 least in part, the relatively short duration of geodetic observations compared to the length 51 of a decades- to centuries-long earthquake cycle, which inhibits exploration of temporal 52 variation in interseismic behavior beyond that attributed to postseismic processes. 53 Confidential manuscript submitted to Geophysical Research Letters  However, in order to fully exploit the information that fault coupling maps provide about 54 the potential for future damaging earthquakes, we must acknowledge the possibility that 55 the distribution of coupling is heterogeneous in both space and time.  56 2 Temporal variation in earthquake cycle processes 57 In this issue, Melnick et al. [2017] underscore the importance of temporal 58 variation in fault coupling patterns by suggesting that the 2010 MW=8.8 Maule 59 megathrust earthquake offshore south-central Chile resulted in a “super-interseismic 60 phase” of the earthquake cycle to the north and south of the rupture area, enhancing 61 coupling on these along-strike adjacent regions of the subduction interface. Five years 62 later, the MW=8.3 Illapel earthquake struck central Chile, ~200 km to the north of the 63 Maule rupture in a region of interplate coupling increased by the Maule event. On 64 December 25, 2016, while the Melnick et al. [2017] manuscript was under revision, a 65 MW=7.6 earthquake occurred south of the Maule rupture, also in a region that likely also 66 showed increased coupling owing to the 2010 event. The authors note that the close 67 temporal and spatial spacing of these large to great earthquakes is similar to five past 68 great earthquake doublets along the central Chilean subduction zone dating back to 1570, 69 with doublet spacing ranging 5–16 years separated by remarkably consistent interseismic 70 periods of 71–88 years.  71 Melnick et al. [2017] suggest that the Maule earthquake induced bending in the 72 upper and possibly lower plates of the subduction zone, geodetically imaged by 73 examining patterns of vertical axis rotation extracted from spatial gradients in station 74 velocities (Figure 1a). The distribution of rotation is symmetric along the strike of the 75 subduction zone about the center of the Maule earthquake rupture area, with clockwise 76 rotation to the north and counterclockwise to the south (Figure 1b). The decoupling of the 77 rupture area during the earthquake, as well as following the event as a result of 78 postseismic afterslip, can increase the degree of subduction coupling adjacent to the 79 slipping region by bending the upper plate and “dragging” it against the interface (Figure 80 1c), thereby increasing shear stress at the periphery of slipping zones, which may trigger 81 failure on these along-strike sections.   82 That the occurrence of one earthquake may influence the timing of nearby 83 earthquakes is a commonly employed model in studies of earthquake sequences. Changes 84 in static stress within the crust owing to finite fault slip [e.g., King et al., 1994] and 85 dynamic stress arising from the passage of seismic waves through the crust [e.g., Kilb et 86 al., 2000] have been invoked to explain the spatial and temporal relationships between a 87 mainshock and the spatial and temporal patterns of its aftershock sequence. However, 88 several recent studies, including Melnick et al. [2017], suggest that coseismically induced 89 changes in earthquake cycle behavior can occur over spatial and temporal scales beyond 90 that of a triggered event or aftershock sequence. Based on analysis of GPS position time 91 series, Heki and Mitsui [2013] suggest accelerated subduction of the Pacific Plate 92 following two major earthquakes offshore Japan — the 2003 MW=8.1 Tokachi-oki and 93 2011 MW=9.1 Tohoku-oki events. Loveless and Meade [2016] propose that, despite 94 accounting for postseismic deformation from major earthquakes and assuming that 95 nominally interseismic geodetic velocities vary minimally through time, spatial patterns 96 Confidential manuscript submitted to Geophysical Research Letters  of coupling on the Japanese subduction interfaces shift on time scales as short as a few 97 years, influenced primarily but not exclusively by megathrust earthquakes. Such abrupt 98 fluctuation in interseismic deformation has substantial implication for earthquake hazard 99 assessment, as it indicates that identifying likely sites of future seismicity is not as simple 100 as integrating a static image of fault coupling over time. Rather, continued geodetic 101 observations throughout interseismic phases of global subduction zone earthquake cycles 102 will shed light on the degree to which temporal changes in the patterns of coupling 103 influence the location, timing, and recurrence of great earthquakes.  104 Ongoing geodetic observation will complement several recent studies that have 105 highlighted temporal variations in earthquake recurrence over historical (~100-yr) to 106 paleoseismic (~10-kyr) time scales on continental strike slip faults [Dolan et al., 2007; 107 Dolan et al., 2016; Gold et al., 2017] and subduction zones [Nocquet et al., 2016; 108 Philibosian et al., 2016; Sieh et al., 2008]. Combining the satellite geodetic record with a 109 paleogeodetic record from uplifted fossil coral microatolls, studies has enabled 110 documentation of spatiotemporal variation in earthquake cycle processes on the Sumatra 111 subduction zone over hundred- to thousand-year timescales [Meltzner et al., 2015; 112 Philibosian et al., 2016; Sieh et al., 2008]. Most recently, Philibosian et al. [2016] used 113 fossil corals recording vertical deformation since the year 1500 to estimate great 114 earthquake rupture patterns, as well as coupling patterns during the intervening 115 interseismic periods. In general, they find spatial anti-correlation between the locus of 116 coseismic slip and concentrations of coupling in the subsequent interseismic phase, with 117 earthquake slip apparently enhancing interplate coupling on adjacent along-strike 118 segments of the subduction interface. These zones of enhanced coupling are then often 119 the sites of the next great earthquakes on the subduction zone. This pattern is consistent 120 with the interpretations of Melnick et al. [2017] but extends their plate bending 121 hypothesis beyond a portion of a single earthquake cycle to a repeatable pattern that may 122 be pervasive over many seismic cycles along global subduction zones. Notably, the 123 concept of a super-interseismic phase has been inferred from two very different datasets: 124 horizontal GPS observations recording variations in coupling over 5 years [Melnick et al., 125 2017], and vertical fossil coral data that suggest coupling fluctuations over 10–50 year 126 intervals [Philibosian et al., 2016].  127 Over longer time scales, and in a continental strike-slip setting, Dolan et al. 128 [2007] noted that several-thousand-year periods of faster-than-average seismic activity in 129 the Los Angeles basin region of southern California have coincided with relative 130 quiescence on faults of the Eastern California Shear Zone and vice versa. Furthermore, 131 temporal variations in slip rate on the Garlock fault apparently coincide with those on the 132 Mojave segment of the San Andreas fault [Dolan et al., 2016], suggesting that when the 133 Garlock and San Andreas faults show faster than average slip rates, they suppress activity 134 on Eastern California Shear Zone faults, potentially by ejecting a Mojave Desert crustal 135 block eastward, which clamps faults to the east [Dolan et al., 2007]. To achieve these 136 coordinated changes in fault slip rates across hundreds of kilometers of a fault system on 137 millennial time scales, Dolan et al. [2016] propose that mechanical changes in the deep 138 roots of active faults alternate between strain hardening during bursts of seismic activity 139 and annealing during periods of comparative quiescence, and whichever portions of the 140 Confidential manuscript submitted to Geophysical Research Letters  fault system are annealed at a given time are most likely to slip at faster-than-average 141 rates in order to accommodate relative plate motion.  142 The above-mentioned temporal variations in slip rates and coupling patterns 143 necessarily complicate use of these metrics in seismic hazard assessment. Estimating 144 earthquake recurrence intervals in the face of time-varying rates of interseismic strain 145 accumulation presents a challenge in conveying the very concept of earthquake 146 recurrence to the public. Brian Tucker, director of GeoHazards International, a nonprofit 147 organization that helps communities prepare for natural hazards, relayed in the 148 Washington Post an anecdote following the 2015 Gorkha, Nepal earthquake in which he 149 was told by a government official in the late 1990s that, because of the occurrence of a 150 major earthquake in 1934, Nepal need not worry about future seismic hazard [Achenbach, 151 2015]. Recognition and understanding of the concept of earthquake recurrence is 152 necessary for building seismic resilience. Continued collection and analysis of geodetic 153 data will ideally clarify this concept, but as studies such as that of Melnick et al. [2017] 154 show, these data will also reveal additional complexities in earthquake cycle deformation. 155 Acknowledgments 156 I thank editor Andrew Newman for his invitation to write this commentary as well as 157 suggestions to clarify the presentation.  158 References 159 Achenbach, J. 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J. Meltzner, C.-C. Shen, H. Cheng, K.-S. Li, B. W. Suwargadi, J. Galetzka, 202 B. Philibosian, and R. L. Edwards (2008), Earthquake Supercycles Inferred from Sea-Level Changes 203 Recorded in the Corals of West Sumatra, Science, 322(5908), 1674–1678, doi: 10.1126/science.1163589. 204 Wang, K., Y. Hu, and J. He (2012), Deformation cycles of subduction earthquakes in a viscoelastic Earth, 205 Nature, 484(7394), 327–332. 206  207 Figure 1. Schematic model of post-earthquake changes in a) trench-normal fore-arc 208 velocities, b) fore-arc vertical axis rotation, and c) coupling on the subduction interface. 209 Decoupling occurs in the earthquake rupture zone (solid contour in c.) and continues with 210 afterslip. This induces clockwise fore-arc rotation north of the rupture area and 211 counterclockwise rotation to the south; the red and blue color scheme follows that of 212 Melnick et al. [2017]. Rotation of the upper plate increases landward velocity adjacent to 213 the rupture zone, consistent with enhanced coupling on the interface beneath these 214 regions. Loci of enhanced coupling (shaded red in c.) may be the site of subsequent 215 earthquakes (dotted contours in c.), facilitated by the “super-interseismic” coupling.  216  217  218 Figure 1.  a. Trench-normal velocities b. Fore-arc rotationMorelandwardMorelandward MoreseawardNo changeNo changec. Interface couplingEnhancedEnhancedReduced/decoupledEarthquake/afterslip

Super-Interseismic Periods: Redefining Earthquake Recurrence

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Super-Interseismic Periods: Redefining Earthquake Recurrence

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