Time-varying interseismic strain rates and similar seismic ruptures on the Nias–Simeulue patch of the Sunda megathrust

1 Time-varying interseismic strain rates and similar seismic ruptures on the Nias–Simeulue patch of the Sunda megathrust Aron J Meltzner,1,2,* Kerry Sieh,1,2 Hong-Wei Chiang,1,3 Chung-Che Wu,3 Louisa L H Tsang,1 Chuan-Chou Shen,3 Emma M Hill,1 Bambang W Suwargadi,4 Danny H Natawidjaja,4 Belle Philibosian,2,5 Richard W Briggs6 101 Earth Observatory of Singapore, Nanyang Technological University, 639798 Singapore 112 Tectonics Observatory, California Institute of Technology, Pasadena, CA 91125, USA 123 High-precision Mass Spectrometry and Environment Change Laboratory (HISPEC), 13 Department of Geosciences, National Taiwan University, Taipei 10617, Taiwan, ROC 144 Research Center for Geotechnology, Indonesian Institute of Sciences (LIPI), Bandung 15 40135, Indonesia 165 Équipe de Tectonique et Mécanique de la Lithosphère, Institut de Physique du Globe de 17 Paris, 75238 Paris, France 186 Geologic Hazards Science Center, U.S Geological Survey, Denver, CO 80225, USA 19 20* To whom correspondence should be addressed E-mail: meltzner@ntu.edu.sg -1- 21Abstract 22 Fossil coral microatolls from fringing reefs above the great (MW 8.6) megathrust 23rupture of 2005 record uplift during the historically reported great earthquake of 1861 24Such evidence spans nearly the entire 400-km strike length of the 2005 rupture, which 25was previously shown to be bounded by two persistent barriers to seismic rupture 26Moreover, at sites where we have constrained the 1861 uplift amplitude, it is comparable 27to uplift in 2005 Thus the 1861 and 2005 ruptures appear to be similar in both extent and 28magnitude At one site an uplift around AD 1422 also appears to mimic the amount of 29uplift in 2005 The high degree of similarity among certain ruptures of this Nias– 30Simeulue section of the Sunda megathrust contrasts with the substantial disparities 31amongst ruptures along other sections of the Sumatran portion of the Sunda megathrust 32At a site on the northwestern tip of Nias, reefs also rose during an earthquake in AD 331843, known historically for its damaging tsunami along the eastern coast of the island 34 The coral microatolls also record interseismic vertical deformation, at annual to 35decadal resolution, spanning decades to more than a century before each earthquake The 36corals demonstrate significant changes over time in the rates of interseismic deformation 37On southern Simeulue, interseismic subsidence rates were low between 1740 and 1820 38but abruptly increased by a factor of 4–10, two to four decades before the 1861 rupture 39This may indicate that full coupling or deep locking of the megathrust began only a few 40decades before the great earthquake In the Banyak Islands, near the pivot line separating 41coseismic uplift from subsidence in 2005, ongoing interseismic subsidence switched to 42steady uplift from 1966 until 1981, suggesting a 15-year-long slow slip event, with slip 43velocities at more than 120% of the plate convergence rate -2- 441 Introduction 45 Assessing future earthquake hazard relies upon an appreciation for the range of 46earthquake scenarios that are plausible for a particular fault and an understanding of the 47strain accumulation history along that fault The better we can characterize the 48earthquake recurrence in a region, the more that region can prepare for the hazards it 49faces And the more complete we can make our picture of strain accumulation, and how 50strain accumulation varies over time, the better our chances for accurately identifying 51faults that are likely to rupture in the near future 52 There have been limited efforts to apply earthquake recurrence models to 53subduction megathrusts Few long paleoseismic records exist for subduction zones with 54which to rigorously test these models, and the inaccessibility of megathrusts hinders 55attempts to compare displacements at a point along the fault from one event to the next 56In Sumatra, prior studies identified two persistent barriers to rupture, under the Batu 57Islands and under central Simeulue (Figure 1) These two barriers, which align with 58fracture zones in the subducting slab, divide the Sumatran portion of the Sunda 59megathrust into at least three segments with independent rupture histories North of the 60northern barrier (on the Aceh segment) and south of the southern barrier (on the 61Mentawai segment), paleoseismic evidence suggests that ruptures vary considerably: no 62two ruptures in the available paleoseismic, historical, or modern records even vaguely 63resemble one another The 28 March 2005 MW 8.6 rupture spanned the full distance 64between these two barriers, and since both rupture endpoints appear to have been 65structurally controlled, we speculate that earthquakes like 2005 may be a common feature 66of this portion of the megathrust -3- 67 As for fault behavior between earthquakes, researchers generally believed until 68recently that interseismic motions are roughly linear over time, punctuated only by 69sudden earthquakes and postseismic deformation that follows the earthquakes Although 70postseismic transients in deformation have been widely documented and result from a 71variety of processes during the post-earthquake deformation phase of the earthquake 72cycle , they are commonly observed to decay, over a period of years to decades, to a 73“background” interseismic rate The belief was that, subsequently, this “background” 74interseismic strain rate (or pattern of interseismic deformation) remained steady over 75most of the seismic cycle More recently, researchers discovered processes and 76phenomena previously unappreciated along subduction zones Numerous studies have 77explored slow slip events (SSEs) at a range of timescales, in a number of settings 78Multiple large SSEs, with durations of 2–4 years, and a series of abrupt changes in the 79width of the locked region, have now been documented in southern Alaska In the Tokai 80region of Japan, a 5-year long SSE occurred between 2000 and 2005, and longer-term 81changes in plate coupling have been observed Changes in plate coupling over time have 82also been proposed elsewhere 83 What if a fault system can appear for decades to be uncoupled and then suddenly 84start accumulating strain that could lead to seismic rupture? If this could happen, it 85would have profound implications for hazards along subduction zones and other faults 86that are not currently considered highly seismogenic Interseismic deformation rates, 87long assumed to be steady over time, may instead be a function of time Most modern 88geodetic networks have not been in operation for sufficiently long durations to address 89this question The geological record may provide unique insight -4- 90 In this paper, we explore the recent paleoseismic (earthquake) histories of sites on 91Nias, Bangkaru, and southern (eastern) Simeulue islands, which lie above the 28 March 922005 MW 8.6 rupture patch (Figures 1–2) We combine historical records with geological 93observations from in situ preserved coral colonies—namely coral microatolls—to 94determine details of the timing, extent, and magnitude of past coseismic deformation 95These data elucidate similarities and differences between various past earthquakes, 96including notable similarities between earthquakes in 1861 and 2005 We also explore 97the recent paleogeodetic (interseismic deformation) histories of these sites The coral 98microatolls provide information on gradual relative sea-level (RSL) change (hence land99level change) between earthquakes, which we can use to infer rates of interseismic 100deformation and patterns of strain accumulation These corals reveal that rates of 101interseismic vertical deformation are not constant over time 102 1032 Historical Accounts of Earthquakes since 1843 104 Limited historical information is available for three large earthquakes in the Nias– 105Simeulue region prior to 2005 (see Appendix) The earliest historical event, in January 1061843, caused severe shaking on Nias and a substantial tsunami that inundated (at 107minimum) the northeast coast of Nias and reached the adjacent mainland coast Few 108additional details are known historically about this earthquake; the lack of more widely 109reported effects, particularly along the west coast of Nias, does not necessarily indicate 110that other areas were unaffected Even today, the island’s west coast is rugged and 111sparsely inhabited, and a large tsunami there in 1843 would not necessarily have left a -5- 112historical record Prior to this study, no land-level changes were attributed to the 1843 113earthquake 114 In February 1861, a strong and widely felt earthquake affected Nias and northern 115Sumatra Tsunami inundation was reported along the southwest and east coasts of Nias, 116in the Batu Islands, and in numerous places along the coast of mainland northern Sumatra 117 Reports from the time unequivocally describe coseismic uplift of some parts of the west 118coast of Nias and permanent flooding (subsidence, slumping, or sediment compaction) 119along other portions of the west coast of Nias and in Singkil, on the adjacent mainland 120coast (Figure 2) 121 In January 1907, a tsunami earthquake, with an estimated magnitude of MS 7.5 to 1228.0, appears to have involved rupture of the shallow, updip portion of the Nias–Southern 123Simeulue segment of the megathrust This event produced strong shaking on Simeulue 124and Nias and a tsunami that devastated Simeulue, Nias, and the Batu Islands and 125extended 950 km along the mainland Sumatra coast News accounts, although prone to 126exaggeration, stated that the southern coast of Simeulue was destroyed, and that the 127island “nearly disappeared” underwater The Malacca Strait Pilot , a nautical guidebook 128concerned with navigation, anchorage, and bathymetric depths, indicates that the 129southern coast of Simeulue “was partially submerged by an earthquake” in 1907 130 The March 2005 earthquake involved uplift of southern (eastern) Simeulue, 131Bangkaru, and most of Nias, and subsidence of the eastern Banyak Islands and 132easternmost Nias Uplift in 2005 peaked at 290 cm at Lahewa on the northwestern tip of 133Nias; uplift exceeding 50 cm extended from the southernmost west coast of Nias to the 134island’s northern tip and northward to the eastern half of Simeulue; and the southwestern -6- 10 135half of Bangkaru also uplifted >50 cm (Figure 2) At Simuk Island, just south of the 136Equator in the Batu Islands (Figure 1), 25 cm of uplift was recorded The endpoints of 137the 400-km-long 2005 rupture coincide with the persistent rupture barriers of Meltzner et 138al (Figure 1) 139 1403 Coral Microatoll Background and Methodology 141 1423.1 Limits on Coral Upward Growth: Diedowns 143 We extracted records of RSL change from coral microatolls In the absence of 144reef ponding (which in general has not been observed on the typically narrow reef flats of 145Simeulue or Nias), coral microatolls grow upward to a limit near mean low water springs 146(MLWS), and their upper surfaces record a history of RSL Microatoll shapes form 147because prolonged subaerial exposure at times of extreme low water limits the highest 148level to which the coral colonies can grow (Figures 3–4) A diedown to a uniform 149elevation around the perimeter of the coral is a clear indication that the diedown resulted 150from low water, and the elevation above which all coral died is termed the highest level 151of survival (HLS) A related term, the highest level of growth (HLG), reflects the 152highest elevation up to which a coral grew in a given year Although both HLS and HLG 153refer to the highest living coral at a particular time of interest, HLG is limited by a coral’s 154upward growth rate Hence, in years during which there is no diedown, HLG provides 155only a minimum estimate of the HLS that would theoretically be possible, given water 156levels 11 -7- 12 157 Any coral diedown at sites off the west coast of Sumatra may be related to 158tectonic uplift, the Indian Ocean Dipole (IOD), or both Positive IOD events result in the 159development of persistent surface easterly winds over the equatorial Indian Ocean, and 160lower sea surface height (SSH) in the tropical eastern Indian Ocean , whereas negative 161IOD events have the opposite effect If a diedown is sufficiently large, it is unlikely to 162result solely from IOD effects and is more likely to be related to tectonics Other criteria 163for distinguishing uplift from IOD-related diedowns include the spatial variability of the 164amplitude of the diedown in coeval corals at nearby sites (the amplitude of tectonic 165uplifts tends to vary markedly over short distances, whereas IOD-related diedowns 166should be similar over distances of tens to hundreds of kilometers) and the duration of the 167RSL change (the IOD causes fluctuations in RSL lasting weeks to months, whereas 168tectonic changes are more enduring, lasting decades to centuries) Meltzner et al 169suggested that a moderate non-tectonic diedown on a coral should be followed by 170unrestricted upward growth without additional diedowns until the coral grows back up to 171its former elevation Meltzner and Woodroffe provide further discussion of techniques to 172differentiate uplift from transient oceanographically induced diedowns 173 1743.2 Microatoll Records of Gradual RSL Change (Paleogeodesy) 175 A microatoll’s basic morphology reveals important information about RSL during 176the coral’s lifetime Flat-topped microatolls record RSL stability; colonies with diedowns 177(HLS unconformities) that rise radially outward toward their perimeter reflect rising sea 178level during their decades of growth As reefs subside or rise in the course of tectonic 179elastic strain accumulation and release, microatoll morphologies record changes in RSL 13 -8- 14 180Because these corals’ skeletons have annual growth bands, we can precisely calculate 181rates of change in elevation, when those changes are gradual 182 In order to determine gradual (interseismic) land-level changes, we first estimate 183rates of RSL change (and associated errors) from the coral growth histories, following 184Meltzner et al Unless a compelling argument can be made otherwise, a “worst-case 185scenario” is considered in which there is an 8-cm error in the apparent elevation gain 186recorded by a microatoll slab due to differential erosion of one part of the coral compared 187to another part, or due to deficient upward growth The error in the rate of RSL change, 188then, is cm divided by the length of the record 189 If sea level itself was steady as the coral grew, then the land-level change is 190simply the opposite of RSL change Alternatively, if the rate of sea-level rise or fall was 191not negligible but is known (within error), it can be subtracted from the overall rate of 192RSL change before the negative of that rate is used to calculate land-level change To the 193extent that rates of regional sea-level change are unknown at various times in the past, 194this adds uncertainty to our estimates of land-level change However, we also consider 195the spatial scales over which SSH trends vary: analyses of satellite altimetry data since 1961993 suggest that sea-level trends vary fairly smoothly at low latitudes In particular, 197average sea-level rise calculated over the period 1993–2009 varied only from ~2.0 mm/yr 198just northwest of Simeulue, to ~2.5 mm/yr just southeast of Simeulue, to ~3.0 mm/yr near 199Nias and the Batu Islands, a distance of 600 km If this spatial variability in SSH trends 200since 1993 is characteristic of the spatial variability in SSH trends in the same region at 201earlier times, it puts a limit on how much variability among coeval RSL records from 202corals at nearby sites can be explained by spatial variability in SSH trends In other 15 -9- 16 203words, if an abrupt and sustained change in RSL trends of >1 mm/yr occurs at one site 204but not contemporaneously at another site 10–100 km away, it is unlikely that this is the 205result of changes in SSH trends 206 2073.3 Coseismic Uplift Inferred from Sudden RSL Fall (Paleoseismology) 208 For sudden uplifts inferred from diedowns, we attempt to estimate formal errors 209Aside from any uncertainty in the amplitude of the diedown that may result from erosion 210of the microatoll, there are two primary sources of uncertainty in estimating the uplift 211The first is the inherent variability in the corals’ HLG or HLS In the Mentawai Islands, 212the variation of HLS on a single Porites microatoll is usually about ±2.6 cm (2σ) , which 213is consistent with our observations farther north on Simeulue and Nias Hence, for 214diedowns measured on a single microatoll, where neither the pre-diedown HLG nor the 215post-diedown HLS are significantly eroded, the uncertainty should be roughly [ (2.6 cm)2 216+ (2.6 cm)2 ] 1/2 , or less than about ±4 cm 217 The amplitude of the diedown is generally treated as a proxy for the amount of 218uplift, but there is an additional source of uncertainty that is incurred in this conversion: 219inherent variability in SSH associated with phenomena such as the IOD (Figure 3) 220Diedowns unrelated to tectonic uplift during 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http://dx.doi.org/10.1785/0119980016 1200Zurcher, F., Margollé, É., 1866 Volcans et Tremblements de Terre, 372 pp., Hachette et 1201 Cie, Paris Republished by Adamant Media Corporation, 2006 Newer edition 1202 available at http://books.google.com/books?id=iQcKAAAAIAAJ&pg=PP9 or 1203 https://archive.org/details/volcansettrembl01marggoog 1204Zurcher, F., Margollé, É., 1868 Volcanoes and Earthquakes, translated by Lockyer, W., 1205 105 261 pp., Richard Bentley, London Republished by Cambridge University Press, New - 54 - 106 1206 York, 2012, http://dx.doi.org/10.1017/CBO9781139226806 Available at 1207 https://archive.org/details/volcanoesearthqu00zurcrich 1208Zweck, C., Freymueller, J.T., Cohen, S.C., 2002 The 1964 great Alaska earthquake: 1209 present day and cumulative postseismic deformation in the western Kenai Peninsula 1210 Phys Earth Planet Inter 132, 5-20, http://dx.doi.org/10.1016/S0031-9201(02)00041- 1211 1212 107 - 55 - 108 1213Figure Captions 1214 1215Figure (a) Regional map of the Sunda megathrust and large megathrust ruptures since 1216AD 1865 Rupture locations and magnitudes are from Briggs et al , Konca et al , 1217Meltzner et al , Hill et al , and references therein; the 1907 location is speculative 1218Relative plate motions from Shearer and Bürgmann Black lines are faults; gray lines are 1219fracture zones (b) The 2005 rupture spanned the Nias–Southern Simeulue segment of 1220the megathrust (NSS), which is bounded by persistent rupture barriers (gray bars) 1221Ruptures to the north on the Aceh segment (A) and to the south on the Mentawai segment 1222(M) have been highly variable, but there may be a higher degree of similarity among the 1223largest ruptures along the Nias–Southern Simeulue segment Bdg, Badgugu; Smk, Simuk 1224Island 1225 1226Figure Map of sites with coral data published in this and earlier papers Of the sites 1227over the 2005 rupture patch, some show evidence for deformation in 1843 and others 1228have evidence for uplift in 1861 No site has direct evidence for uplift in both 1229earthquakes, although sites with 1843 deformation are also inferred to have risen in 1861 1230Data from sites to the northwest, over the 2004 patch and the boundary region between 1231the two ruptures, were published in earlier papers Contours show uplift and subsidence 1232in the 2005 earthquake, updated from Briggs et al and Meltzner et al Sites published 1233by Meltzner et al : USL (Ujung Salang), LDL (Lhok Dalam), LNG (Langi), LKP (Lhok 1234Pauh), LWK (Lewak), USG (Ujung Sanggiran), PST (Pulau Salaut Besar) Sites 1235published by Meltzner et al : ULB (Ujung Lambajo), BUN (Bunon), PPY (Pulau Penyu) 109 - 56 - 110 1236Sites with microatolls analyzed in this paper: SLR (Silinggar), SMB (Sambay), UTG 1237(Ujung Tinggi), LBJ (Labuhan Bajau), LAT (Latiung), PBK (Pulau Bangkaru), PWG 1238(Pulau Wunga), AFL (Afulu), PSN (Pulau Senau), MZL (Muzoi Ilir), BWL 1239(Bawelowalani), LAG (Lagundri) Additional sites with data listed in the supplementary 1240tables of this paper: SBG (Sinabang), GSG (Teluk Gosong, or Busung), SBA (Siaba) 1241 1242Figure (Top) Microatoll development under different relative sea-level history 1243scenarios: under stable sea level conditions (Case 1), under gradually falling sea level 1244(Case 2), and under gradually rising sea level (Case 3) In all three cases, we 1245superimpose on the long-term trend a realistic interannual variability: at 11.1 and 14.1 yr, 1246we simulate temporary local sea level lowerings as might occur during positive Indian 1247Ocean Dipole (IOD) events Concentric annuli form simply from year-to-year 1248fluctuations in low water level (Bottom) Microatoll development affected by sudden 1249changes in land level Case illustrates the microatoll from Case 3, followed by 1250coseismic uplift at 17.1 yr; Case illustrates the microatoll from Case 1, followed by 1251coseismic subsidence at 17.1 yr In each case, the long-term trend is superimposed on a 1252typical IOD cycle, with an additional period of lower local sea level at 20.1 yr In Case 4, 1253the uplift must have been sudden at 17.1 yr, but if we found the microatoll in Case 5, we 1254could not distinguish between sudden coseismic subsidence at 17.1 yr and rapid 1255interseismic subsidence (at an average rate exceeding the coral’s growth rate) beginning 1256at 17.1 yr 1257 111 - 57 - 112 1258Figure Photos of microatolls that died in 1861 on southeastern Simeulue Both 1259exhibit a wide, low-relief interior that is surrounded by concentric “stair step” rings rising 1260outward (a) LBJ-2, Labuhan Bajau site, diameter ~5.0 m (b) LAT-1, Latiung site, 1261diameter ~3.5 m 1262 1263Figure Cross sections through parts of (a) slab UTG-5 and (b) slab UTG-6, both from 1264site UTG The U-Th dates suggest both died in the historical 1861 earthquake, but UTG12655 has been moderately eroded and is missing ~2.5 bands; UTG-6 sustained less erosion 1266and is missing only ~0.5 band Both corals experienced diedowns in early 1833 and 1267~13.5 years later in mid-1846 that are inferred to have resulted from regional sea-level 1268lowerings related to the IOD An earlier diedown on UTG-6 may correlate with a mid12691817 diedown on UTG-5, but ambiguities in band counting on UTG-6 (where the 1270banding is not shown in the figure) preclude a definitive correlation Both cross sections 1271are plotted at the same scale; note the finer banding in UTG-6, a Goniastrea sp coral, 1272than in UTG-5, a Porites sp coral For the full slabs, see Figures S16 and S17 1273 1274Figure Fossil corals from the PBK site provide information not only on the timing of 1275the predecessor to the 2005 earthquake, but also on the amount of uplift resulting from 1276that earlier rupture (a) Cross section through part of slab PBK-7, from subsite PBK-B 1277For the full slab, see Figure S37 (b) Relative sea-level history (coral growth history) for 1278subsite PBK-B, derived from slabs PBK-5, PBK-7, and PBK-8 Different colors 1279represent data from different corals PBK-7 had almost reached HLS just prior to the 12801861 earthquake (determined from its elevation relative to coeval microatolls at the site), 113 - 58 - 114 1281and it survived the 1861 diedown We estimate the 1861 uplift at this site to be the 1282amplitude of the diedown on PBK-7, 30 cm, plus up to cm to account for erosion of the 1283top of the central hemisphere Between 1751 and 1861, the corals record an average rate 1284of RSL rise of 2.2 ± 0.7 mm/yr That RSL rise appears to have been fairly steady from 12851812 onward (2.5 ± 1.6 mm/yr if only data from 1812 to 1861 are considered), but there 1286is ambiguity in the RSL history before 1812 because of erosion of a portion of PBK-5: 1287we can neither preclude nor confirm decadal-scale fluctuations in the rate of RSL change 1288prior to 1812 1289 1290Figure Relative sea-level history (coral growth history) for site LAG, derived from 1291slabs LAG-1, LAG-2, LAG-3B, and LAG-4 Elevations are plotted relative to the HLG 1292just before the 16 February 1861 earthquake Different colors represent data from 1293different corals As at site PBK, these fossil corals provide information on both the 1294timing of the predecessor to the 2005 earthquake and the amount of uplift resulting 1295therefrom We estimate the 1861 uplift at LAG from the difference between the pre1296diedown HLG on LAG-1 and the post-diedown HLS on LAG-3B, which is 28 cm, and 1297then add cm to account for the estimated erosion of the outer rim of LAG-1; this yields 1298an estimate of ~34 cm For the slab LAG-1 cross section, see Figure S75 1299 1300Figure Maps showing uplift derived from coral microatolls, for earthquakes in AD 13011422, 1843, 1861, and 2005 The 2005, 1861, and possibly 1422 earthquakes were 1302similar to one another with regard to both extent and amount of uplift In 1843, the uplift 1303distribution is clearly different, and site PSN likely subsided Sites where microatolls 115 - 59 - 116 1304preclude statistically significant land-level change are indicated For 2005, contours 1305show uplift and subsidence in cm, updated from Briggs et al and Meltzner et al The 1306circle color for each site is as in Figure 2: yellow denotes sites with evidence for 1861 1307uplift, whereas red denotes sites with evidence for 1843 deformation 1308 1309Figure Cross sections of (a) slab AFL-3 and (b) chiseled hand sample AFL-4, both 1310from site AFL The U-Th dates suggest both died in the historical 1843 earthquake, and 1311both appear to have less than a few millimeters (less than 0.5 band) of erosion AFL-3 1312experienced diedowns in mid-1817 and early 1833, among others, but AFL-4 did not 1313because it was lower in the water Both cross sections are plotted at the same scale; note 1314the finer banding in AFL-4, a Goniastrea sp coral, than in AFL-3, a Porites sp coral 1315 1316Figure 10 Cross section through part of slab PSN-2 (slice a), from site PSN Unlike at 1317nearby site AFL, no coral mortality occurred at PSN in 1843, and unlike all other sites on 1318Nias, no diedown occurred here in 1846 The lack of a diedown in mid-1846 suggests 1319that subsidence occurred at the PSN site, either during the 1843 earthquake or as a 1320postseismic response soon thereafter, thereby lowering the coral relative to sea level and 1321inhibiting subsequent diedowns For the full slab, see Figure S62 1322 1323Figure 11 Relative sea-level histories for the 18th–19th centuries for (a) southern 1324Simeulue, (b) northern Nias, and (c) northern Bangkaru Different colors represent data 1325from different sites On southern Simeulue and possibly northern Nias, rates of relative 1326sea-level rise were slow prior to 1819–1839 but much faster from then until 1861 Rates 117 - 60 - 118 1327on Bangkaru were fairly constant from 1812 or earlier until 1861; relative sea level 1328dropped as land rose during the 1861 earthquake, but much of that change was recovered 1329by 1875, by which time steady relative sea-level rise had resumed at the site In (b), the 1330elevation of all data from site AFL has been systematically shifted by 50 cm to account 1331for an inferred surveying error, discussed in the supplement, Text S9.2 For the 1332uncorrected elevations, see Figure S83 1333 1334Figure 12 Histories of interseismic subsidence (and where constrained, coseismic uplift) 1335through the 18th–19th centuries at sites on southern Simeulue, northern Nias, and 1336Bangkaru Islands Subsidence rates abruptly increased on southern Simeulue and 1337possibly northern Nias in the decades prior to the 1861 earthquake, whereas rates were 1338fairly uniform on Bangkaru before 1861 and after 1875 Shown in a geodetic reference 1339frame, these rates and elevations have been inverted from the corresponding relative sea1340level histories (Figure 11), and the time series have been shifted vertically by 16 cm to 1341account for eustatic sea-level rise since the 20th century, following Meltzner et al Data 1342constrain solid parts of the curves well; dashed portions are inferred or less reliable; 1343queried portions are questionable Interseismic subsidence rates (all negative, in mm/yr) 1344are shown Vertical dotted white lines mark the 1843 and 1861 earthquakes The zero 1345elevation at each site is defined as the site’s elevation immediately prior to the 2004– 13462005 uplift 1347 1348Figure 13 The modern microatoll from the PBK site has an unusual growth history 1349Relative sea level at the site was rising gradually as the coral began to grow in the 1950s, 119 - 61 - 120 1350but, after ~1966, successive diedowns were lower and lower, indicating relative sea level 1351was gradually falling Around 1981, the trend reversed again, and relative sea level 1352gradually rose until the 2005 earthquake We infer these changes to result from changes 1353in rates of vertical tectonic deformation (a) Cross section of slab PBK-4 (slice a); for 1354cross sections of two parallel slices, see Figure S34 (b) Relative sea-level history (coral 1355growth history) for site PBK, derived from and plotted at the same vertical scale as slab 1356PBK-4 The 6-cm diedown in late 1971 is a little larger than most and may reflect a pulse 1357of more rapid uplift within the 15-year period of gradual uplift; if so, it would be at the 1358limit of our resolution 1359 1360Figure 14 Relative sea-level history for the 20th century for (a) southern Simeulue, (b) 1361northern Nias, and (c) northern Bangkaru Different colors represent data from different 1362sites Abrupt changes in the rates of relative sea-level change were seen on Bangkaru, 1363but not elsewhere 1364 1365Figure 15 Histories of interseismic vertical deformation preceding the 2005 earthquake 1366at sites on southern Simeulue, northern Nias, and Bangkaru Islands Although 1367uncertainties on some rates are large, the pre-2005 rates on Simeulue and Nias generally 1368differ from the pre-1861 rates shown in Figure 12 On Bangkaru, interseismic subsidence 1369before 1966 switched to gradual uplift until 1981, and then reverted to subsidence 1370through 2005 Shown in a geodetic reference frame, these rates and elevations have been 1371inverted from the corresponding relative sea-level histories (Figure 14), and the time 1372series have been adjusted by mm/yr to account for 20th-century eustatic sea-level rise 121 - 62 - 122 1373Interseismic uplift (positive) or subsidence (negative) rates are given in mm/yr Vertical 1374dotted white line marks the 2005 earthquake The zero elevation at each site is taken as 1375the site’s elevation immediately prior to the 2004–2005 uplift, as defined by the HLG in 1376late 2004 Some pre-1997 rates project to 2004 elevations that are lower than zero either 1377because the site rose in an earthquake in 2002, or because the coral had not yet grown 1378back up to its HLS following the late 1997 IOD diedown 1379 1380Figure 16 Rates of interseismic land-level change during various time periods on 1381southern Simeulue The error ellipses have a fixed width for visualization purposes only 1382 1383Figure 17 Back-slip (elastic dislocation) models suggest that most of the spatiotemporal 1384variations in interseismic subsidence rates on southern Simeulue can be explained by 1385along-strike variations in the locking depth along the megathrust The upper panels show 1386profiles of predicted uplift (or subsidence) rates along a hypothetical row of surface 1387points located 110 km from the trench (the approximate distance from the trench of our 1388sites), as a consequence of along-strike variations in the downdip limit of locking In 1389each of the “Pre-1819,” “1839–1861,” and “Pre-2005” upper panels, several models are 1390shown: on one side of the boundary (the side that happens to have deeper locking) the 1391locking depth is fixed, but on the other side, along-strike uplift-rate profiles are shown for 1392a selection of locking depths to illustrate the effect of varying the locking depth 1393Subsidence rates estimated from corals are overlain on these model-predicted profiles 1394For each time period, the preferred model profile is indicated with a thicker line Back1395slip models have more difficulty explaining the spatial distribution of subsidence rates 123 - 63 - 124 1396between 1819 and 1839, a transition period during which SMB had already experienced 1397an increase in the subsidence rate, but the other sites had not Even with a three-patch 1398model, it is difficult to simultaneously fit all three rates during the 1819–1839 transition 1399period (See text for further discussion.) For all time periods, the assumed subduction 1400(convergence) rate is 40 mm/yr The lower panels show the surface projections of the 1401partially (light blue) and fully (yellow) locked patches in the preferred model for each 1402time period; downdip limits of full locking (depths in km) are labeled Along-strike 1403boundaries between patches with different locking depths are arbitrarily located to 1404maximize the fit to the data The coupling ratio along the portion of the fault shallower 1405than 18 km (light blue patches) is fixed at 0.4 in all models Arrows indicate observed 1406subsidence rates; the actual years over which each rate is determined is indicated in 1407Figure 16 Isobaths along the megathrust from Hayes et al are shown at 20-km 1408intervals 1409 1410Figure 18 Back-slip models, incorporating a range of plausible locking depths, can 1411readily explain the subsidence rates at Bangkaru during the 18th–19th centuries, from 14121956 to 1966, and from 1981 to 2005, but no standard back-slip model does a good job of 1413fitting the 1966–1981 uplift rate; the observed rate is simply too fast (a) Rates of 1414interseismic land-level change during the 20th century on Bangkaru The error ellipses 1415have a fixed width for visualization purposes only (b) Model predictions of surface 1416uplift and subsidence are plotted as a function of distance from the trench, for various 1417choices of the downdip limit of locking (LD, locking depth) Superimposed on these 1418model predictions are observed vertical deformation rates at the PBK site on Bangkaru, 125 - 64 - 126 1419for the 18th–19th centuries (left) and for the 20th–21st centuries (right) The assumed 1420subduction (convergence) rate is 40 mm/yr, and for simplicity full coupling is assumed all 1421the way to the trench; nonetheless, the state of coupling near the trench has little effect at 1422the PBK site, 135 km from the trench 1423 1424Figure 19 Back-slip models, modified to incorporate a slow slip event (SSE; slip at 1425rates exceeding the plate convergence rate) on a portion of the otherwise locked fault 1426zone, can explain the steady but rapid uplift at PBK between 1966 and 1981 Trench1427normal model profiles of uplift and subsidence rates, under the specified conditions, are 1428shown in the upper panels (a) Steady deformation (no SSE): the fault is fully locked 1429above 43 km and freely slipping at the full subduction rate below 43 km Superimposed 1430on the model profile is the subsidence rate at the PBK site between 1981 and 2005, a 1431period during which no SSE is inferred (b) Deformation from slow slip at 49 mm/yr 1432between 30 and 43 km depth Ongoing steady deformation below 43 km is not 1433considered in (b), so this profile is hypothetical only and should not match real-world 1434observations (no data are overlain) (c) SSE between 30 and 43 km depth, with ongoing 1435steady slip below that; this is the sum at each point along the profile of curves (a) and (b) 1436This profile is intended to match real-world observations during the SSE; hence, we 1437superimpose the uplift rate at the PBK site between 1966 and 1981 for comparison 1438Lower panels depict a simplified geometry of the model fault: the fault is normally 1439locked above 43 km depth, but between 1966 and 1981 a SSE occurs between 30 and 43 1440km, with slip at ~49 mm/yr 1441 127 - 65 - ... 30Simeulue section of the Sunda megathrust contrasts with the substantial disparities 31amongst ruptures along other sections of the Sumatran portion of the Sunda megathrust 32At a site on the northwestern... of the megathrust, 785updip of the 2005 patch ; and in April 2010), any similarities among the largest ruptures 78 6of the Nias–Southern Simeulue portion of the megathrust are noteworthy 787 The. .. 2012 885 8868 Conclusions 75 - 39 - 76 887 Coral microatolls reveal histories of interseismic strain accumulation and 888coseismic strain release along the patch of the Sunda megathrust that