Microquakes

Microquakes, or microearthquakes, are small seismic events produced by slip on faults or crack propagation in rocks. They are typically defined as events with local magnitude below 3.0, though definitions vary. Most microquakes are not felt on the surface, but can be detected by sensitive instruments and dense seismic networks. In crustal studies they are used to image fault zones because their high frequency content and abundance provide detailed information about conditions at depth.

Detection relies on high-sensitivity seismometers, borehole sensors, and dense arrays to record and locate events. Waveforms are analyzed to estimate arrival times of P- and S-waves, magnitudes, and sometimes source mechanism through inversion. This yields localizations with uncertainties that improve with network coverage and velocity models.

Causes include natural tectonic loading and release of stress, aftershock sequences, volcanic or magmatic activity, and fluid-induced seismicity from reservoirs, geothermal operations, mining, or hydraulic fracturing. Microquakes can occur in swarms, clusters of events with little or no single dominant event, often linked to changing pore pressure or fault friction.

Significance lies in their ability to reveal fault structure, rock properties, and stress state at depths inaccessible by other means. Catalogs of microquakes underpin studies of fault slip behavior, help monitor induced seismicity in energy and engineering projects, and support seismic hazard assessments by providing high-resolution images of subsurface processes.