A Seasonally Modulated Earthquake Swarm near Maupin, Oregon

From December 2006 to November 2011, the Pacific Northwest Seismic Network (PNSN) reported 467 earthquakes in a swarm 60 km east of Mt Hood near the town of Maupin, Oregon. The swarm included 20 MD ≥ 3.0 events, which account for over 80 per cent of the cumulative seismic moment release of the sequence. Relocation of 45 MD≥ 2.5 earthquakes and moment tensor analysis of nine 3.3 ≤ Mw ≤ 3.9 earthquakes reveals right-lateral strike-slip motion on a north-northwest trending, 70° west dipping, 1 km2 active fault patch at about 17 km depth. The swarm started at the southern end of the patch and migrated to the northwest at an average rate of 1–2 m d−1 during the first 18 months. Event migration was interrupted briefly in late 2007 when the swarm encountered a 10° fault bend acting as geometrical barrier. The slow migration rate suggests a pore pressure diffusion process. We speculate that the swarm was triggered by flow into the fault zone from upwards-migrating, subduction-derived fluids. Superimposed on the swarm is seasonal modulation of seismicity, with the highest rates in spring, which coincides with the maximum snow load in the nearby Cascade Mountains. The resulting surface load variation of about 4 × 1011 N km−1 arc length causes 1 cm annual vertical displacements at GPS sites in the Cascades and appears sufficient to modulate seismicity by varying normal stresses at the fault and fluid flow rates into the fault zone