Long-term dynamics of earthquake swarms in the Yellowstone caldera

Abstract

The factors controlling the spatial distribution and temporal evolution of earthquake swarms in volcanic systems remain unclear. We leverage leading-edge deep learning algorithms and a detailed three-dimensional velocity model to construct a 15-year high-resolution earthquake catalog of the Yellowstone caldera region. More than half of the region's earthquakes are clustered into swarm-like families characterized by episodes of hypocenter expansion and migration. Adjacent earthquake swarms, separated by long quiescent periods, are found to be a dominant feature. We suggest that these swarms are controlled by the interplay between slowly diffusing aqueous fluids and rapid episodic fluid injections, which may result from the breaking of permeability seals. Our analyses also indicate that clustered seismicity beneath the caldera occurs on relatively immature, rougher fault structures, compared to more planar faults outside. Our results provide additional context for understanding seismicity in hydrothermal systems, highlighting the key role played by long-term fluid diffusion processes in driving the occurrence of earthquake swarms.

Fig. 1.. Map and cross sections of relocated seismicity.

(A) Depth cross section A-A′. (B) Depth cross section B-B′. (C) Depth cross section C-C′. (D) Map view of the relocated catalog, colored by depth. Blue lines indicate depth cross sections, white line indicates the caldera boundary, gray light lines highlight Hebgen and Yellowstone lakes, and black thin ellipses outline areas where major swarms have been documented. Depths are referenced to sea level. All cross sections use a 5-km projection distance to display seismicity.

Fig. 2.. Statistics of clustered seismicity.

(A) Map view of clusters with $\overline{d}>5$ (green); open circles correspond to root earthquake locations and are colored by migration metric $\mathrm{\Delta }Z$ . C1, C2, and C3 refer to families in Fig. 3 (B) Map view of clustered seismicity (event pairs with ${\mathrm{\eta }}_{\mathit{ij}}\le -3.9$ ); the color represents the fractal dimension calculated for a corresponding rectangular grid cell. CNW, CNE, CBN, and CLC label areas with statistical differences in the $\mathrm{\Delta }Z$ metric; each area is bounded by a rectangle with a black thin outline.

Fig. 3.. Diversity of swarm migration patterns.

(A) Cluster C1: $\overline{d}=21.1$ , n = 657 and $\mathrm{\Delta }Z=5.9$ km. White open circle indicates the location of root earthquake, with seismicity colored by time in days relative to root event. Top: Map view in projected UTM Zone 12 coordinates. Middle: East-west depth cross section. Bottom: Topology of the time-oriented tree graph. Origin corresponds to root event. Arrows are drawn for linked events, pointing in the direction of the children, with circle sizes scaled by earthquake local magnitude. (B) Similar to (A) but for cluster C2: $\overline{d}=6.7$ , n = 75, and $\mathrm{\Delta }Z=1.1$ km. Note the clear vertical migration pattern. (C) Similar to (A) but for cluster C3: $\overline{d}=22.5$ , n = 329, and $\mathrm{\Delta }Z=-0.8$ km. Refer to Fig. 2 for the location of clusters relative to the study region.

Fig. 4.. Spatiotemporal patterns of swarm occurrence for the period 2008 to 2022.

(A) Map view of clustered seismicity (event pairs with ${\mathrm{\eta }}_c\le -3.9$ ), colored by time since 1 January 2008. Black lines indicate depth cross sections, white line indicates the caldera boundary, and gray light lines show lake boundaries. (B) Depth cross section A-A′ (6-km projection distance). (C) Depth cross section C-C′ (4-km projection distance). (D) Depth cross section B-B′ (5-km projection distance). Panels (B), (C), and (D) display families with $\overline{d}>5.0$ , where each family is labeled with a color corresponding to the root earthquake origin time. Note the spatial proximity between clusters.

Fig. 5.. Schematic model of possible mechanisms driving earthquake swarms.

Cartoon of Yellowstone’s upper crustal magmatic system from northwest (NW) to southeast (SE). The interplay between slowly diffusing aqueous fluids and rapid episodic fluid injections into active faults control the occurrence of earthquake swarms. Faults are represented by different colored lines to indicate that they become active years after an adjacent fault has experienced a major episode of swarm-like activity, a pattern widely observed throughout the caldera region.