On April the 18th, 1906, it was a few minutes after five oclock in the morning. Most people in San Francisco were asleep, but the rattling of the milkmens carts and bottles meant that the city was waking to another busy day. At that moment the land suddenly moved. The vibration was so strong that great buildings fell down, including the new seven-million-dollar City Hall, which the community had good reason to be proud of. Main water pipes burst. Cooking stoves overturned and electric wires flashed. The fires which started caused damage in large areas of the city. What had happened? The rocks had broken apart along nearly three hundred miles of a crack in the earth of California, a feature of the physical map of that region known as a 'fault'. The damage was greatest in San Francisco which was near the center of the fault. Many buildings were destroyed by fire or by the earthquake itself, and hundreds of people were killed. Many people also died from diseases which broke out in the dirty camps later occupied by homeless people. The fires got out of control and, before they died out, four square miles of the city were burn out. The loss of property was serious. The loss from fire alone amounted to 400,000,000 dollars, more than nine-tenth of the total damage. In those days this was an enormous sum. The effects of the earthquake were widespread. Rivers and streams began to run in new directions and their flow patterns were changed. Trees six feet in diameter were uprooted within half a mile of the central break. An area of wet fields on the side of a hill actually moved half a mile downwards. A road which crossed the fault burst apart and a gap of 21 feet remained between the broken ends. The California Earthquake is remembered because it was so sudden and because it occurred in a city, where the damage and destruction were plainly visible and where many people were killed simultaneously. Actually, deaths on American roads from car accidents are now greater in almost any week of the year, but we are so accustomed to road accidents that we do not pay much attention to them. Scientists and engineers studied the effects of the San Francisco earthquake. The city was rebuilt, and new features were introduced to strengthen buildings and maintain a constant water supply in the event of another earthquake. The water mains(main pipes)were fitted with control valves which would enable water to travel by different routes round broken places. Large underground tanks were constructed to supply water if normal supplies could not be tapped. Special measures were taken to prevent fires, which often do more damage than earthquakes themselves. The San Francisco earthquake provided scientists with valuable information , since the effects of the break were visible and reports of the incident were an important contribution to the worlds store of knowledge about earthquakes. The main cause of the great loss of property after the San Francisco earthquake in 1906 is______.