Dike Injection, Incremental Emplacement, and Seismic b-Value

Abstract:

Active volcanic areas are typically characterized by intense, episodic seismicity. Although many of these earthquakes are double-couple events resulting from frictional slip, others have a dilational component that is interpreted to reflect injection of a fluid (e.g., water, gas, or magma) into fractures. The frozen plutonic roots of volcanic systems are commonly characterized by dike swarms or by plutons that were emplaced incrementally. Our studies of a Jurassic dike swarm in eastern California show that, as in many swarms, typical dike thickness is ~1 m. However, glacially polished exposures of the swarm reveal that a typical dike comprises a number of dikelets that are lognormally distributed in thickness with a mean of ~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 swarms. The b-value, defined by log₁₀N = a – bM, corresponds to the slope of a cumulative frequency-magnitude distribution. Worldwide b-values for tectonic seismicity are typically ~1. In contrast, seismic bursts in active volcanic systems typically have elevated b-values of 1.5 or more, and b therefore has been used to map out (by inference) the magmatically active parts of a volcanic system. b-values estimated in our simulations from dike thickness distributions are typically ~1.5, similar to active volcanic systems, for a wide variety of input parameters. Comparison of representative curves for volcanic seismicity at Long Valley caldera (Wiemer et al., GJI, 134:409, 1998) and two dike simulations with crack aspect ratios of 1000 and 100 illustrates this similarity (Fig. 1). We suggest that swarms of dikes composed of dikelets, as well as plutons built incrementally by sheet intrusions, are physical products of volcanic seismic swarms, and that at least some swarms are a palpable expression of incremental emplacement.