An earthquake is what happens when two blocks of the earth suddenly slip past one another. The surface where they slip is called the fault or fault plane. The location below the earth’s surface where the earthquake starts is called the hypocenter, and the location directly above it on the surface of the earth is called the epicenter.

Question

2 While the edges of faults are stuck together, and the rest of the block is moving, the energy that would normally cause the blocks to slide past one another is being stored up. When the force of the moving blocks finally overcomes the friction of the jagged edges of the fault and it unsticks, all that stored up energy is released. The energy radiates outward from the fault in all directions in the form of seismic waves like ripples on a pond. The seismic waves shake the earth as they move through it, and when the waves reach the earth’s surface, they shake the ground and anything on it, like our houses and us!

3 The size of an earthquake depends on the size of the fault and the amount of slip on the fault, but that’s not something scientists can simply measure with a measuring tape since faults are many kilometers deep beneath the earth’s surface. So how do they measure an earthquake? They use the seismogram recordings made on the seismographs at the surface of the earth to determine how large the earthquake was. A short wiggly line that doesn’t wiggle very much means a small earthquake, and a long wiggly line that wiggles a lot means a large earthquake. The length of the wiggle depends on the size of the fault, and the size of the wiggle depends on the amount of slip.

4 The size of the earthquake is called its magnitude. There is one magnitude for each earthquake. Scientists also talk about the intensity of shaking from an earthquake, and this varies depending on where you are during the earthquake.

5 Sometimes an earthquake has foreshocks. These are smaller earthquakes that happen in the same place as the larger earthquake that follows. Scientists can’t tell that an earthquake is a foreshock until the larger earthquake happens. The largest, main earthquake is called the mainshock. Mainshocks always have aftershocks that follow. These are smaller earthquakes that occur afterwards in the same place as the mainshock. Depending on the size of the mainshock, aftershocks can continue for weeks, months, and even years after the mainshock!

Answer 1

This passage provides a clear overview of the science behind earthquakes, explaining the key concepts of faults, hypocenters, and epicenters, as well as the mechanisms that lead to the release of energy and the resulting seismic waves. Here’s a summary of the key points:

1. Earthquake Basics:

   • An earthquake occurs when two blocks of the earth slip past each other along a fault.
   • The fault is the surface where this slipping occurs.
   • The hypocenter is the point below the surface where the earthquake originates, while the epicenter is directly above it on the surface.

2. Energy Release:

   • Energy builds up as the edges of the fault stick together due to friction while the blocks move.
   • When this friction is overcome, the accumulated energy is released, generating seismic waves that shake the ground.

3. Measuring Earthquakes:

   • The size of an earthquake is determined by the size of the fault and the degree of slip, but these cannot be directly measured.
   • Seismographs record the seismic waves, producing seismograms. Wiggliness on these recordings indicates the earthquake's size—a short line means a small quake, while a long, wiggly line indicates a larger quake.

4. Magnitude and Intensity:

   • The magnitude of an earthquake refers to its size and is a singular value for each event.
   • Intensity describes the varying shaking experienced at different locations during an earthquake.

5. Foreshocks, Mainshocks, and Aftershocks:

   • Foreshocks are smaller earthquakes that precede the larger main earthquake, or mainshock, but cannot be identified as foreshocks until after the mainshock occurs.
   • Aftershocks are smaller quakes that follow the mainshock and can persist for extended periods, depending on the mainshock's size.

This summary captures the mechanics, measurement, and categorization of earthquakes, illustrating the complexity and dynamism of seismic activity.