This electronic supplement contains unit descriptions from the trenches and the step-wise reconstructions from trench 1. It also includes extra annotated photographs of the La Laja region to aid with visualizing the local tectonic framework and environment. Refer to the main text for a discussion on the timing of interpreted past ruptures and for the displacements inferred from the reconstructions.
Unit CW1, the uppermost colluvial wedge, is a poorly sorted, wedge-shaped gravel deposit composed of pebbles and cobbles located immediately below the main fault plane. A thick layer of limestone clasts that is several centimeters thick covers the slope of the cumulative colluvial wedges, as well as the hanging wall deposits. CW1 likely formed as the result of the 1944 earthquake, as is evident by the presence of a small scarp (Fig. 7 of the main text).
Unit CW2 is a 35–40 cm thick, poorly stratified, matrix-supported gravel composed of coarse, subangular limestone pebbles and cobbles within a brownish sandy matrix. A weak brown soil is present in the top 15 cm of the CW2 deposit, although the presence of gypsum and secondary clay appears to be very minor. We interpret this wedge as resulting from the penultimate coseismic rupture of the La Laja fault.
Unit CW3 is predominantly composed of reddish sand with scattered limestone clasts overprinted by soil development. The base of CW3 is probably an erosional contact developed over a secondary thrust splay and the CW4 deposits. CW3 becomes sandier westward and then becomes indistinguishable from the CW4 distal sand deposits.
Both CW4 and CW5 are matrix-supported fine gravel deposits, with intercalated clast-supported pebble layers. Units CW4 and CW5 are differentiated on the basis of textural differences below and above a discontinuous gypsiferous horizon. The average clast size of unit CW4 is smaller than that of unit CW5, and unit CW5 also contains subrounded limestone clasts within a reddish sand matrix. A weakly developed gypsiferous horizon caps CW5, which represents a partially preserved soil. Consequently, we interpret CW4 and CW5 as discrete colluvial wedge deposits. The discontinuous and incomplete intervening soil likely reflects disturbances of sediments as a consequence of fault motion. Although this is our preferred interpretation, it is also possible that units CW4 and CW5 comprise a single wedge that has been dismembered by displacement on a subhorizontal thrust splay during a subsequent earthquake. If correct, this interpretation requires an exceptionally large event to account for the large slip this model would require, and it discounts the presence of the gypsiferous soil.
Units CW9 to CW6 share similar characteristics. Triangular-shaped basal gravel deposits are truncated by the fault and are overlain by a wedge-shaped, fine- to medium-grained sand deposit with scattered pebbles (Figs. 7 and 8 in the main text). Each of these wedge-shaped units has a gypsiferous soil developed at its upper surface, with most of the gypsum concentrated near the upper part of the colluvial wedge, consistent with our interpretation that it represents a soil. A thin horizon of gravel (stone line) that covers the sandy deposits caps each colluvial wedge.
Trench T2 exposed about a meter of bedded fine- to medium-grained sand and silty fine sand with scarce gravel stringers overlying a 30–50 cm-thick gravel bed that pinches out southeast of the fault, suggesting that the gravel was deposited across a low eroded scarp. The gravel overlies the Neogene Tapias formation strata with an angular unconformity.
Trench 4 exposed a sequence of bedded fluvial deposits, fine-grained sandy silt and silt interpreted to be ponded alluvium east of the scarp, several colluvial wedge units, and Tertiary Las Tapias formation strata. The units are numbered from 1 (youngest) to 13 (oldest), with the colluvial units interfingering with specific strata.
Units 1–3 are interpreted to postdate the 1944 earthquake scarp, with unit 3 comprising colluvium and alluvium, whereas units 1 and 2 are predominantly alluvium. These strata are generally massive, with abundant gravel, sand, and silt, and unit 3 thickens into the fault scarp and is interpreted as a colluvial wedge. Note that the very base of unit 3 appears faulted, which we interpret as being the result of the initial collapse of the thrust snout as it was bulldozed over the ground surface and subsequently faulted.
Unit 4 is composed of silty sand, locally overlain by a 10 cm-thick gravel stratum, and this unit is the uppermost stratum deformed by the 1944 earthquake, whereas unit 5 is a boulder sandy gravel layer. Units 4 and 5 are interpreted as fluvial in origin.
Units 6–8 are finer-grained strata, with unit 6 containing appreciable clay and silt, unit 7 being mostly well-sorted fine sand, and unit 8 being predominantly silt with fine sand. Unit 6 exhibits some oxidation and contains secondary calcium carbonate and gypsum (CaSO4), and possibly some secondary clay, whereas unit 8 contains secondary salt (NaCl) (Fig. S1). We interpret these observations as evidence that these accumulations represent soil development due to a period of surface exposure. The gypsum is more soluble than calcium carbonate, if carbonate is present at all, so gypsum is typically found deeper than the carbonate in arid soils. In this profile, the carbonate was at the top, and most of the white filaments in unit 6 are gypsum. The salt, which is even more soluble than gypsum, is interpreted to have hung up at the top of unit 8 because of the increased moisture retention of the silt. We interpret most of the clay of unit 6 to be primary or depositional in origin because the secondary calcium carbonate is superposed on the clayey layer and because the lower contact is abrupt; typically, argillic horizons occur above calcic and gypsic horizons unless there has been a dramatic change in climate, which is unlikely for this region during the Holocene.
Unit 9 is a massive, matrix-supported gravelly layer that thickens into the fault and merges with a wedge-shaped deposit interpreted as a colluvial wedge (CW2). Unit 9 is essentially the more distal part of this colluvial wedge.
Unit 10 is a fine silt sand that interfingers with or terminates at the foot of CW3, whereas units 11 and 12 are coarse gravel strata interpreted as fluvial in origin. Unit 12 thickens into the fault, and a sliver of unit 12 / CW4 is caught within the fault zone. Finally, unit 13 is a clayey silty fine sand that is localized adjacent to the fault and probably represents ponded fine-grained sediment after a scarp-forming event—probably the same event that formed CW4 and unit 12.
Fine-grained sandy silt was found ponded along the uphill-facing fault scarp intermittently along the 1944 rupture (Fig. S2). In some trench exposures, this deposit was as much as 10–20 cm thick (trench T4N exposure; Fig. S3), whereas in others, it was absent, as in trench T4S exposure. This probably reflects variations in surface topography, with the silt accumulating is shallow depressions.
We conducted stepwise reconstruction of the stratigraphy and deformation exposed in the south wall of trench T1 (Fig. S4a–r). Each panel shows the reconstruction required to remove the deformation from each interpreted earthquake, until the Qt3 surface is restored (Fig. S4r). Displacements for each reconstruction are presented in Table 3 and used in Figure 10 in the main text. We arbitrarily assign an uncertainty of ±10 cm to the reconstruction of each event, even though we have measured each to the centimeter.
We include here an annotated photograph of the La Laja fold front and surrounding area at the request of the reviewers to provide a visual context to the field area. View is to the south in Figure S5, with the Andes rising in the very faint distance to the west. The region is arid, with scattered vegetation. Terrace formation indicates that structural relief is up to the west, implying that the deep structure that produced the 1944 earthquake verges to the east and that the hypocenter is likely to the west of the fold front.
Figure S1. Soil developed into the ponded fine-grained alluvium of units 6–8. The abrupt lower contact of unit 6 indicates that much or all of the clay is depositional, not pedogenic, and the reddening may also be primary, in part, although we do observe reddening in most of the colluvial wedge soils. Nevertheless, the secondary carbonates, gypsum, and salt are clearly secondary and, considering their ordering of increasing solubility with depth, almost certainly are associated with the same period of surface exposure.
Figure S2. As the 1944 rupture produced an uphill facing scarp for its entire 7 km length, smaller drainages that cross the fault experienced ponding of coarse- and fine-grained sediments behind (west of) the scarp. The modern channel was found to be breached in every drainage that we walked in which ponded sediment was evident, and sediment now bypasses the scarp.
Figure S3. Ponded silt and alluvium on the north face of trench Tr4.
Figure S4: a–e, f–k, l–o, and p–r. Stepwise reconstructions of events E1 (1944) through E9.
Figure S5. Photograph of the La Laja area showing the active axial surface at the monoclonal front. The uplifted terraces to the west that show that topography and structural relief are being built to the west of the monoclonal, indicating that the causative thrust dips to the west
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