Tremor-genic slow slip regions may be deeper and warmer and may slip slower than non-tremor-genic regions

Abstract

Slow slip events (SSEs) are observed worldwide and often coincide with tectonic tremor. Notable examples of SSEs lacking observed tectonic tremor, however, occur beneath Ki¯lauea Volcano, Hawaii, the Boso Peninsula, Japan, near San Juan Bautista on the San Andreas Fault, California, and recently in Central Ecuador. These SSEs are similar to other worldwide SSEs in many ways (e.g., size or duration), but lack the concurrent tectonic tremor observed elsewhere; instead, they trigger swarms of regular earthquakes. We investigate the physical conditions that may distinguish these non-tremor-genic SSEs from those associated with tectonic tremor, including slip velocity, pressure, temperature, fluids, and fault asperities, although we cannot eliminate the possibility that tectonic tremor may be obscured in highly attenuating regions. Slip velocities of SSEs at Ki¯lauea Volcano (∼10-6 m/s) and Boso Peninsula (∼10-7 m/s) are among the fastest SSEs worldwide. Ki¯lauea Volcano, the Boso Peninsula, and Central Ecuador are also among the shallowest SSEs worldwide, and thus have lower confining pressures and cooler temperatures in their respective slow slip zones. Fluids also likely contribute to tremor generation, and no corresponding zone of high vp/vs has been noted at Ki¯lauea or Boso. We suggest that the relatively faster slip velocities at Ki¯lauea Volcano and the Boso Peninsula result from specific physical conditions that may also be responsible for triggering swarms of regular earthquakes adjacent to the slow slip, while different conditions produce slower SSE velocities elsewhere and trigger tectonic tremor.