Relationship Between Dike Injection and b-Value for Volcanic Earthquake Swarms

Dike swarms are the fossil remains of regions of the crust that have undergone repeated magma injections. Volcanic earthquake swarms and geodetic measurements are, at least in part, a record of active injection of fluids (water, gas, or magma) into fractures. Here, we link these two ways of observing magmatic systems by noting that dike thicknesses and earthquake magnitudes share similar scaling parameters. In the Jurassic Independence dike swarm of eastern California median dike thickness is similar to 1 m, similar to other swarms worldwide, but glacially polished exposures reveal that a typical dike comprises a number of dikelets that are lognormally distributed in thickness with a mean of similar to 0.1 m. Assuming that dikes fill penny-shaped cracks of a given aspect ratio, the geodetic moment and earthquake magnitude of a diking event can be estimated. A Monte Carlo simulation of dike-induced earthquakes based on observed dike thickness variations yields a frequency-magnitude distribution remarkably like observed volcanic earthquake swarms, with a b-value of similar to 1.7. We suggest that swarms of dikes composed of dikelets, as well as plutons built incrementally by sheet intrusions, are physical complements to volcanic seismic swarms, and that at least some earthquake swarms are a palpable expression of incremental magma emplacement. Plain Language Summary Dike swarms are the geologically preserved expressions of magmatic intrusion. The dikes have different thicknesses, with many more small ones than large ones. We model the size distribution using Monte Carlo simulations and a variety of inputs. All yield similar numerical results with a value of the frequency-magnitude distribution of b similar to 1.7. This value is very close to observed seismic b-values for contemporary observations of earthquakes at active volcanoes. There are many more small earthquakes than larger ones, similar to the dike distributions. We suggest that the similar size distributions indicate that seismic swarms are the geophysical expression of the same processes that occur in dike formation.