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An earthquake is usually caused by the rupture of a geologic fault, or the seam between two large blocks of land that suddenly move in different directions. The two predominant types of faults are thrust faults and strike-slip faults. A strike slip fault is the most common in the United States; it is where two geologic plates move in opposite directions relative to each other, such as the San Andreas in California. A thrust earthquake occurs when one plate moves under another. In 2004, a great earthquake off the Indonesian island of Sumatra was caused by a thrust fault; the rupture along the fault was greater than 93 miles (150 kilometers). The earthquake and the massive tsunami that was generated by the tsunami killed about ...

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