Large, sustained well water level changes (>10 cm) in response to distant (more than hundreds of kilometers) earthquakes have proven enigmatic for over 30 years. Here we use high sampling rates at a well near Grants Pass, Oregon, to perform the first simultaneous analysis of both the dynamic response of water level and sustained changes, or steps. We observe a factor of 40 increase in the ratio of water level amplitude to seismic wave ground velocity during a sudden coseismic step. On the basis of this observation we propose a new model for coseismic pore pressure steps in which a temporary barrier deposited by groundwater flow is entrained and removed by the more rapid flow induced by the seismic waves. In hydrothermal areas, this mechanism could lead to 4 × 10⁻² MPa pressure changes and triggered seismicity.

The style and evolution of volcanic eruptions are dictated by the fluid mechanics governing magma ascent. Decompression during ascent causes dissolved volatile species, such as water and carbon dioxide, to exsolve from the melt to form bubbles, thus providing a driving force for the eruption. Ascent is influenced not only by the nucleation and growth of gas bubbles, but also magma rheology and brittle deformation (fragmentation). In fact, all processes and magma properties within the conduit interact and are coupled. Ultimately, it is the ability of gas trapped within growing bubbles to expand or to be lost by permeable gas flow, which determines whether ascending magmas can erupt nonexplosively. We review and integrate models of the primary conduit processes to show when each process or property dominates and how these interact within a conduit. In particular, we illustrate how and why ascent rate may control eruptive behavior: slowly ascending magmas erupt effusively and rapidly ascending magmas erupt explosively.

Approximately 0.4% of explosive volcanic eruptions occur within a few days of large, distant earthquakes. This many “triggered” eruptions is much greater than expected by chance. Several mechanisms have been proposed to explain triggering through changes in magma overpressure, including the growth of bubbles, the advection of large pressures by rising bubbles, and overturn of magma chambers. Alternatively, triggered eruptions may occur through failure of rocks surrounding stored magma. All these mechanisms require a process that enhances small static stress changes caused by earthquakes or that can convert (the larger) transient, dynamic strains into permanent changes in pressure. All proposed processes, in addition to viscoelastic relaxation of stresses, can result in delayed triggering of eruptions, although quantifying the connection between earthquakes and delayed, triggered eruptions is much more challenging. Mud volcanoes and geysers also respond to distant earthquakes. Mud volcanoes that discharge mud from depths greater than many hundreds of meters may be triggered by liquefaction caused by shaking, and may thus be similar to small mud volcanoes that originate within a few meters of the surface. Changes in permeability of the matrix surrounding main geyser conduits, by opening or creating new fractures, may explain the observed changes in their eruption frequency.

One of the biggest challenges in volcanic hazard assessment is to understand how and why eruptive style changes within the same eruptive period or even from one eruption to the next at a given volcano. This review evaluates the competing processes that lead to explosive and effusive eruptions of silicic magmas. Eruptive style depends on a set of feedback involving interrelated magmatic properties and processes. Foremost of these are magma viscosity, gas loss and external properties such as conduit geometry. Ultimately, these parameters control the speed at which magmas ascend, decompress and outgas en route to the surface, and thus determine eruptive style and evolution.