Low-angle normal fault earthquakes and triggering

Geologic evidence for low-angle normal faulting is overwhelming, and recent seismic, geodetic, and neotectonic studies have identified probable active examples. Thus, apparent low levels of historical seismicity on such fault s is enigmatic. However, one large mainshock was a low-angle normal event, and three others may have been. One earthquake sequence had significant mom ent release from a triggered low-angle normal fault, and three earthquakes may have had normal subevents on low-angle planes. Such (sub)events are har d to recognize, may be underrepresented in catalogs (helping to explain the apparent seismic enigma), and pose increased seismic hazard due to delayed strong motions. Triggered (sub)events may be along strike or down dip from their triggering shock. In cases where the triggered slip is downdip of th e steep fault and under its hanging wall,elevated pore fluid pressure or fa ult weakness may be required.