Hidden fault puzzle revealed by whisper-quiet quakes off Northern California

Scientists using a dense seismometer network tracked ultra-small, low-frequency earthquakes offshore to map a five-piece fault system where the San Andreas fault meets the Cascadia subduction zone, including a deep Pioneer fragment from the Farallon plate, which helps explain the unusually shallow 1992 quake and challenges prior boundary models; the work, published in Science, improves understanding of seismic hazards in this complex region.