Kyle Bradley

The frequency of explosive volcanic eruptions in Southeast Asia.

There are ~750 active and potentially active volcanoes in Southeast Asia. Ash from eruptions of volcanic explosivity index 3 (VEI 3) and smaller pose mostly local hazards while eruptions of VEI ≥ 4 could disrupt trade, travel, and daily life in large parts of the region. We classify Southeast Asian volcanoes into five groups, using their morphology and, where known, their eruptive history and degassing style. Because the eruptive histories of most volcanoes in Southeast Asia are poorly constrained, we assume that volcanoes with similar morphologies have had similar eruption histories. Eruption histories of well-studied examples of each morphologic class serve as proxy histories for understudied volcanoes in the class. From known and proxy eruptive histories, we estimate that decadal probabilities of VEI 4–8 eruptions in Southeast Asia are nearly 1.0, ~0.6, ~0.15, ~0.012, and ~0.001, respectively.

New insights into Kawah Ijen's volcanic system from the wet volcano workshop experiment

Abstract Volcanoes with crater lakes and/or extensive hydrothermal systems pose significant challenges with respect to monitoring and forecasting eruptions, but they also provide new opportunities to enhance our understanding of magmatic–hydrothermal processes. Their lakes and hydrothermal systems serve as reservoirs for magmatic heat and fluid emissions, filtering and delaying the surface expressions of magmatic unrest and eruption, yet they also enable sampling and monitoring of geochemical tracers. Here, we describe the outcomes of a highly focused international experimental campaign and workshop carried out at Kawah Ijen volcano, Indonesia, in September 2014, designed to answer fundamental questions about how to improve monitoring and eruption forecasting at wet volcanoes.

Implications of the diffuse deformation of the Indian Ocean lithosphere for slip partitioning of oblique plate convergence in Sumatra

Oblique plate convergence between Indian Ocean lithosphere and continental crust of the Sunda plate is distributed between subduction on the Sunda megathrust and upper plate strike-slip faulting on the Sumatran Fault Zone, in a classic example of slip partitioning. Over the last decade, a destructive series of great earthquakes has brought renewed attention to the mechanical properties of these faults and the intervening forearc crustal block. While observations of forearc deformation over the earthquake cycle indicate that the forearc crust is fundamentally elastic, the spatial pattern of slip vector azimuths for earthquakes sourced by rupture of the Sunda megathrust is strongly inconsistent with relative motion of two rigid plates. Permanent and distributed deformation therefore occurs in either the downgoing lithospheric slab or the overriding forearc crust. Previous studies have inferred from geodetic velocities and geological slip rates of the Sumatran Fault that the forearc crust is undergoing rapid trench-parallel stretching. Using new geological slip rates for the Sumatran Fault and an updated decadal GPS velocity field of Sumatra and the forearc islands, we instead show that permanent deformation within the forearc sliver is minor and that the Sumatran Fault is a plate boundary strike-slip fault. The kinematic data are best explained by diffuse deformation within the oceanic lithosphere of the Wharton Basin, which accommodates convergence between the Indian and Australian plates and has recently produced several large earthquakes well offshore of Sumatra. The slip partitioning system in Sumatra is fundamentally linked with the mechanical properties of the subducting oceanic lithosphere.

Can the Updip Limit of Frictional Locking on Megathrusts Be Detected Geodetically? Quantifying the Effect of Stress Shadows on Near‐Trench Coupling

The updip limit of the seismogenic zone of megathrusts is poorly understood. The relative absence of observed microseismicity in such regions, together with laboratory studies of friction, suggests that the shallow fault is mostly velocity strengthening, and likely to creep. Inversions of geodetic data commonly show low to zero coupling at the trench, reinforcing this view. We show that the locked, downdip portion of the megathrust creates an updip stress shadow that prevents the shallow portion of the fault from creeping at a significant rate, regardless of its frictional behavior. Our models demonstrate that even if the shallowest 40% of the fault is frictionally unlocked, the expected creep at the fault tip is at most 30% of the plate rate, often within the uncertainties of surface geodetic measurements, and below current resolution of seafloor measurements. We conclude that many geodetic models significantly underestimate the degree of shallow coupling on megathrusts, and thus seismic and tsunami hazard. Plain Language Summary When one tectonic plate dives beneath another, the fault between them is called a megathrust. The shallow part of these faults is not well understood. Generally, it is thought that if this area is pushed, it will freely slip (and will not store energy that would be released as earthquakes). Researchers make models of megathrusts using GPS measurements to determine which parts of it are slipping and which are not (which means they are storing energy that will be released as earthquakes). These models are not well constrained far from the GPS measurements, and in many areas it is difficult to make measurements near the shallow megathrust because they are under the ocean. We use a simple model that considers the forces acting on the fault to show that if the deeper megathrust is not slipping, then it will act as a buffer to prevent the shallow part from moving. We compare our model to megathrusts with many measurements in Japan and Nepal and show that the GPS data cannot tell us if the shallow fault is stuck together by friction or not. This is important because the behavior of the shallow fault affects potential earthquake size and tsunami risk.

The 2015 Mw 6.0 Mt. Kinabalu earthquake: an infrequent fault rupture within the Crocker fault system of East Malaysia

The Mw 6.0 Mt. Kinabalu earthquake of 2015 was a complete (and deadly) surprise, because it occurred well away from the nearest plate boundary in a region of very low historical seismicity. Our seismological, space geodetic, geomorphological, and field investigations show that the earthquake resulted from rupture of a northwest-dipping normal fault that did not reach the surface. Its unilateral rupture was almost directly beneath 4000-m-high Mt. Kinabalu and triggered widespread slope failures on steep mountainous slopes, which included rockfalls that killed 18 hikers. Our seismological and morphotectonic analyses suggest that the rupture occurred on a normal fault that splays upwards off of the previously identified normal Marakau fault. Our mapping of tectonic landforms reveals that these faults are part of a 200-km-long system of normal faults that traverse the eastern side of the Crocker Range, parallel to Sabah’s northwestern coastline. Although the tectonic reason for this active normal fault system remains unclear, the lengths of the longest fault segments suggest that they are capable of generating magnitude 7 earthquakes. Such large earthquakes must occur very rarely, though, given the hitherto undetectable geodetic rates of active tectonic deformation across the region.

Structural Control on Downdip Locking Extent of the Himalayan Megathrust

Author(s): Lindsey, EO; Almeida, R; Mallick, R; Hubbard, J; Bradley, K; Tsang, LLH; Liu, Y; Burgmann, R; Hill, EM | Abstract: ©2018. The Authors. Geologic reconstructions of the Main Himalayan Thrust in Nepal show a laterally extensive midcrustal ramp, hypothesized to form the downdip boundary of interseismic locking. Using a recent compilation of interseismic GPS velocities and a simplified model of fault coupling, we estimate the width of coupling across Nepal using a series of two-dimensional transects. We find that the downdip width of fault coupling increases smoothly from 70 to 90 km in eastern Nepal to 100–110 km in central Nepal, then narrows again in western Nepal. The inferred coupling transition is closely aligned with geologic reconstructions of the base of the midcrustal ramp in central and eastern Nepal, but in western Nepal, the data suggest that the location is intermediate between two proposed ramp locations. The result for western Nepal implies either an anomalous coupling transition that occurs along a shallowly dipping portion of the fault or that both ramps may be partially coupled and that a proposed crustal-scale duplexing process may be active during the interseismic period. We also find that the models require a convergence rate of 15.5 ± 2 mm/year throughout Nepal, reducing the geodetic moment accumulation rate by up to 30% compared with earlier models, partially resolving an inferred discrepancy between geodetic and paleoseismic estimates of moment release across the Himalaya.