Introduction
Volcano‑tectonic earthquakes are seismic events generated by the emplacement and movement of magma within the crust. As magma intrudes and migrates it perturbs the local stress field and modifies pore pressure, provoking brittle failure or slip on adjacent rock; these ruptures produce measurable seismic waves that record the subsurface magmatic processes. Such events therefore represent a mechanical response of the host rock to magmatic forcing rather than the release of long‑accumulated plate‑boundary strain alone.
Seismically, volcano‑tectonic activity is characterized by relatively high frequency content and by temporal patterns that differ from typical fault‑rupture sequences. Rather than a single large mainshock followed by a decaying aftershock sequence, magma‑driven earthquakes commonly occur as swarms—clusters of events of similar size—often associated with dike propagation and other intrusive behavior as tabular magma bodies open fractures and redistribute stress in their path.
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Monitoring volcano‑tectonic seismicity is central to volcanic surveillance because changes in event rates, dominant frequencies, spatial clustering and depths provide direct constraints on magma movement, pressurization, and the evolving likelihood of eruption. Routine seismological observation—using networks of seismometers, magnitude and intensity measures, and operational forecasting frameworks—translates these seismic signals into real‑time assessments used by eruption forecasting committees and civil authorities.
Understanding volcano‑tectonic earthquakes sits within the broader seismological framework. Basic source and propagation concepts (epicenter and epicentral distance, hypocenter or focus depth, seismic shadow zones, and the principal P‑ and S‑wave phases) underpin event location and interpretation. Complementary techniques and disciplines—such as shear‑wave splitting to infer stress and flow anisotropy, density–elasticity relations for subsurface modeling (e.g., Adams–Williamson formulations), regional reporting schemes, earthquake engineering, and palaeoseismic indicators like seismites—augment seismic observations and improve interpretation of magma‑related and tectonic seismicity.
Cause of volcano tectonic earthquakes
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Volcano-tectonic seismicity arises primarily from the mechanical effects of migrating magmas within the crust rather than from slip on regional tectonic faults. In compressive subduction settings, plate convergence hampers the direct ascent of magma, favoring accumulation in shallow to mid-crustal storage zones beneath the overriding plate. Many arc volcanoes—including much of the circum‑Pacific Ring of Fire—are fed by these repeatedly recharged and drained magma reservoirs, and their episodic filling and evacuation is a major driver of associated seismicity.
Magma movement between reservoirs and into the surrounding rock produces deformation and failure distinct from classical fault‑slip earthquakes. Intrusive bodies such as dikes and sills propagate laterally and vertically, imposing stress changes on host rock, opening fractures, and locally weakening the crust; these processes generate measurable earthquakes that reflect rock failure, collapse, or slip on preexisting structures adjacent to the intrusion. Comparable processes occur at divergent boundaries and spreading centers, where shallow magma emplacement and intrusion induce fracturing and seismicity independent of plate‑boundary faulting.
Magmatically induced events commonly occur as swarms—clusters of many small to moderate earthquakes over short intervals—whose frequency, magnitude distribution, and duration are controlled by the rate and style of magma ascent, the geometry of propagation, and the mechanical interaction among magma, chamber walls, and surrounding lithology. Dike propagation in particular produces characteristic stress perturbations (dike‑induced stress) that govern the pattern of fracturing, collapse, and slip responsible for the observed seismic signals.
Although the mechanisms differ, volcanic and tectonic earthquakes can interact: eruptions or rapid magmatic intrusion may trigger regional faulting, and temporal or spatial overlap between magmatic and tectonic events can complicate classification. Consequently, analysis of seismic waveforms and temporal patterns is central to understanding subsurface magmatic behavior; distinct seismic signatures—swarms, rock‑failure events associated with intrusions, and stress‑change sequences—provide diagnostic evidence of magma movement even in the absence of visible surface deformation or eruptive activity.
Volcano-tectonic (VT) seismicity—brittle-failure earthquakes driven by stress changes from magma movement—is a primary geophysical indicator of magmatic unrest because it commonly precedes large eruptions and can be observed in near real time. Monitored VT activity has proven operationally valuable for short-term forecasting: the 1985 Nevado del Ruiz crisis is a canonical example in which elevated seismicity signaled an imminent eruption, and similar precursory VT sequences were evident before Unzen (1990), Pinatubo (1991) and Cotopaxi (2002).
VT earthquake swarms provide dynamic information about the onset, migration and intensity of intrusions, so patterns of event location, rate, frequency content and magnitude distribution are routinely used to infer the size, geometry, volume and pressurization of subsurface magma bodies. These signals also have predictive utility at long-dormant systems, where renewed VT activity can indicate reawakening. At the same time, volcanic systems are complex: interactions within the conduit and surrounding rock can produce counterintuitive precursors, including apparent quiescence or a settling of seismicity immediately before eruptive activity, which complicates interpretation and emphasizes the need to integrate VT observations with other monitoring data.
Seismicity is a primary tool in volcano surveillance because almost every documented eruption is preceded by earthquake activity beneath or adjacent to the edifice; such seismic precursors therefore serve as key indicators of magma movement and potential eruptive behaviour, though they do not invariably provide precise or sufficient warning. Continuous seismic monitoring—now applied to roughly 200 volcanoes—relies on multi‑year background records to categorize volcanic event types and to establish baseline patterns used in hazard assessment. Volcanic earthquakes typically occur as swarms rather than classic mainshock–aftershock sequences: event rates often rise prior to eruptions, swarms cluster spatially near or beneath future vents, and individual events within swarms tend to exhibit similar waveform characteristics that distinguish them from regional tectonic earthquakes. Because volcanic events generally attain smaller maximum magnitudes than tectonic earthquakes, their detectability and waveform interpretation require dense local networks and careful signal discrimination. Volcano‑tectonic (VT) earthquakes commonly nucleate laterally to eventual eruption sites—frequently on fault structures a few kilometres from the vent—and reflect brittle fracturing and stress changes driven by magmatic intrusion rather than simple rupture of the eruptive conduit. Many VT focal solutions display substantial non‑double‑couple components, implying complex source processes (volumetric changes, tensile cracking, or shear combined with opening) in contrast to pure shear faulting. Long‑period (LP) events are interpreted as records of sudden, transient movement of magma or magmatic fluids that were temporarily impeded (e.g., decompression pulses or slug flow), whereas harmonic tremor manifests as a continuous, quasi‑periodic signal attributed to sustained magma or fluid flow within the conduit or plumbing system; together these signal types provide complementary insights into transient versus steady-state fluid dynamics during eruptive unrest.