Most earthquakes originate from compressional or tensional stresses built up at the margins of the huge moving plates that make up the earth’s surface. The immediate cause of most shallow earthquakes like in Nepal now, occuring within 70 km of the surface, is the sudden release of stress along a fault, or fracture in the earth’s crust, resulting in movement of the opposing blocks of rock past one another. The focus of an earthquake is the point where it originates within the earth. The earthquake epicentre is the point on the earth’s surface directly above the focus. The size or magnitude of earthquakes is determined by measuring the amplitude of the seismic waves, which depends on many factors such as the magnitude, distance from the epicentre, depth of focus, topography, and local ground conditions.