Recent earthquakes have demonstrated that rupture may propagate through geometrically complex networks of faults. Ancient exhumed faults have the potential to reveal the details of complex rupture at seismogenic depths. We present a new set of field observational criteria for determining which of a population of pseudotachylyte fault veins formed in the same earthquake and apply it to map rupture networks representing single earthquakes. An exceptional exposure of an exhumed ancient strand of the Norumbega Shear Zone preserves evidence of multistranded earthquake rupture in the deep seismogenic zone of a continental transform fault. Individual fault strands slipped at least 2–18 cm, so significant slip is represented by each rupture network. Our data show that synchronously slipped faults intersect at angles of 0 to ∼55°, with the opening angles of fault intersections directed toward the dilational quadrants for dextral slip. Multistranded rupture on a fault network instead of rupture of a single fault may result in greater and/or more variable slip and cause slip rake to vary spatially and temporally. Slip on intersecting faults unequivocally means that there will be motion perpendicular to the average fault plane. Modern earthquakes displaying non‐double‐couple components to focal mechanism solutions and spatially varying rake, slip, and anomalous stress drop may be explained by rupture across fault networks that are too close (spatially and temporally) to be resolved seismically as separate events.

Outburst floods from ice‐marginal lakes are poised to become more prevalent in a warming climate. As glaciers thin and tributaries detach, water can be impounded in these unstable lakes at valley confluences. To characterize the role of the little‐studied englacial hydrological system during an outburst flood cycle, we deployed a variety of geophysical and hydrometeorological instruments in and around an ice‐marginal lake dammed by the Kaskawulsh Glacier, Yukon, Canada, to capture its 2017 filling and drainage. The subaerial lake reached a maximum volume of 9.9 ±0.5×106 m 3 on 17 August before draining over the course of ∼19 days. During lake filling, abrupt changes in ice shelf uplift rates are associated with the formation of faults and fractures. These hydromechanical interactions are closely linked to a redistribution of englacial water as evidenced by fluctuations in shallow borehole water pressures, and changes in radar internal reflection power. Water balance calculations reveal that the subaerial, subglacial, and englacial reservoirs respectively store 20 (17–23)%, 40 (25–50)%, and 40 (30–60)% of water in the catchment at peak lake level. The calculated englacial storage volume implies saturated porosities up to 4–10% locally or 2–4% catchment‐wide. The englacial system is therefore both volumetrically important and influenced by the hydromechanical interactions between the lake and glacier. Its spatial extent and storage capacity may play a role in lake drainage initiation, modulation of the flood hydrograph, and lake refilling.

The Kaskawulsh Glacier is an iconic outlet draining the icefields of the St. Elias Mountains in Yukon, Canada. We determine and attempt to interpret its catchment-wide mass budget since 2007. Using SPOT5/6/7 data we estimate a 2007–18 geodetic balance of −0.46 ± 0.17 m w.e. a−1. We then compute balance fluxes and observed ice fluxes at nine flux gates to examine the discrepancy between the climatic mass balance and internal mass redistribution by glacier flow. Balance fluxes are computed using a fully distributed mass-balance model driven by downscaled and bias-corrected climate-reanalysis data. Observed fluxes are calculated using NASA ITS_LIVE surface velocities and glacier cross-sectional areas derived from ice-penetrating radar data. We find the glacier is still in the early stages of dynamic adjustment to its mass imbalance. We estimate a committed terminus retreat of ~23 km under the 2007–18 climate and a lower bound of 46 km3 of committed ice loss, equivalent to ~15% of the total glacier volume.

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