The Loma Prieta earthquake differs from most of the observed seismicity along the San Andreas in northern California in its depth and style of rupture. The combination of crustal and thermal structure in the Loma Prieta region allowed strain accumulation to 18--20 km in the crust, leading to a rupture at that depth during the event. A discrepancy in displacement direction of crust and mantle components of the Pacific plate near Loma Prieta led to a decoupling of the crust from the mantle within the ductile lower crust. This played a key part in the rupture initiation at the base of the seismogenic layer. ¿ American Geophysical Union 1990