The temperature and pressure in the asthenosphere generally keep the rocks there below the melting point.
Hawaiian eruptions are extremely violent and produce large pyroclastic flows.
Mafic magma has a high amount of dissolved gases and is very explosive.
The population density of San Francisco is extremely dense, making it a high seismic risk compared to other areas of the United States. Population does not make a high seismic risk; it makes a high environmental risk for humans.