Evidence for fluid‐triggered slip in the 2009 Mount Rainier, Washington earthquake swarm

Abstract

[1] A vigorous swarm of over 1000 small, shallow earthquakes occurred 20-22 September 2009 beneath Mount Rainier, Washington, including the largest number of events ever recorded in a single day at Rainier since seismic stations were installed on the edifice in 1989. Many events were only clearly recorded on one or two stations on the edifice, or they overlapped in time with other events, and thus only~200 were locatable by manual phase picking. To partially overcome this limitation, we applied waveform-based event detection integrated with precise double-difference relative relocation. With this procedure, detection and location goals are accomplished in tandem, using cross-correlation with continuous seismic data and waveform templates constructed from cataloged events. As a result, we obtained precise locations for 726 events, an improvement of almost a factor of 4. These event locations define a~850 m long nearly vertical structure striking NNE, with episodic migration outward from the initial hypocenters. The activity front propagates in a manner consistent with a diffusional process. Double-couple-constrained focal mechanisms suggest dominantly near-vertical strike-slip motion on either NNW or ENE striking faults, more than 30 different than the strike of the event locations. This suggests the possibility of en echelon faulting, perhaps with a component of fault opening in a fracture-mesh-type geometry. We hypothesize that the swarm was initiated by a sudden release of high-pressure fluid into preexisting fractures, with subsequent activity triggered by diffusing fluid pressure in combination with stress transfer from the preceding events.

A new strategy for earthquake focal mechanisms using waveform‐correlation‐derived relative polarities and cluster analysis: Application to the 2014 Long Valley Caldera earthquake swarm

“…A similar process might accompany the 2014 swarm, but given the much sparser permanent network (Figure ) that recorded it, any non‐double‐couple components are not readily resolved. Outside of Long Valley, a positive isotropic component was also suggested for a similar 2010 swarm in Yellowstone [ Shelly et al ., ], which could have contributed to an excess of compressional first motions seen both in that swarm and in a 2009 swarm at Mount Rainier [ Shelly et al ., ].…”

A fluid‐driven earthquake swarm on the margin of the Yellowstone caldera

“…In this swarm, we find that an assumed diffusivity of ~1.5 m 2 /s fits the activity front well (Figure ), using the location of the first event as a proxy for the point of injection. This estimate is somewhat higher than diffusivities of 0.2–0.8 m 2 /s estimated for the 1989 earthquake swarm at Mammoth Mountain, on the rim of Long Valley caldera [ Hill and Prejean , ], as well as the ~1 m 2 /s estimated for the 2009 swarm at Mount Rainier [ Shelly et al ., ], and it is several times greater than diffusivities of ~0.3 m 2 /s from the 2000 and 2008 Vogtland/NW Bohemia swarms in central Europe [ Hainzl and Ogata , ; Hainzl et al ., ]. On the other hand, Parotidis et al .…”

“…While this could simply be an artifact of relatively uniform focal mechanisms of swarm earthquakes and an inhomogeneous station distribution, it opens the possibility of a volumetric component of the source. Similar observations of dominantly compressional first motions for the 1985 swarm at Yellowstone [ Waite and Smith , ] and a 2009 swarm at Mount Rainier [ Shelly et al ., ], both of which are hypothesized to be fluid triggered, lend support to the idea that this may be more than an observational artifact. Despite the overall dominance of compressional first motions, the 22 polarity observations of the largest earthquake of the 2010 swarm ( M l 3.9) can be well fit by an assumed double‐couple source (Figure a).…”

“…In particular, numerical modeling has suggested that the strong spatial spreading of hypocenters is indicative of fluid triggering rather than stress triggering [ Hainzl , ]. We observe migration of the front of the swarm activity in a manner consistent with fluid diffusion (Figure ), similar to the pattern seen with other swarms hypothesized to be fluid triggered [ Parotidis et al ., ; Hill and Prejean , ; Hainzl and Ogata , ; Chen et al ., ; Shelly et al ., ], as well as those triggered by controlled injection of fluids at depth [e.g., Shapiro et al , ]. Often, small earthquakes lead the propagation front, with larger events following only after weaker activity has become established in an area (Figure ).…”

Complex spatiotemporal evolution of the 2008 Mw 4.9 Mogul earthquake swarm (Reno, Nevada): Interplay of fluid and faulting

“…Earthquake swarms are broadly defined as sequences of earthquakes that cluster in space and time and do not fit a typical main shock‐aftershock pattern [ Mogi , ; Hill , ; Vidale and Shearer , ]. Swarms are common in areas of volcanic [e.g., Hill , ] and geothermal activity [e.g., Waite and Smith , ; Shelly et al ., ] and have been associated with driving mechanisms such as aseismic creep [e.g., Lohman and McGuire , ], magma injection [e.g., Hill , ], and fluid diffusion [e.g., Shapiro et al ., ; Spicak and Horalek , ; Antonioli et al ., ]. They are most commonly attributed to fluid circulation reducing normal stress via increased pore pressure on preexisting structures, particularly in extensional and transform fault environments [ Vidale and Shearer , ; Chen et al ., ].…”