Magnitude vs. Intensity and Seismic Waves

Earthquake measurements involve two key concepts: magnitude and intensity.

Magnitude refers to the size of the earthquake and is a single value for each event. The Richter scale is a common magnitude scale.

Intensity, on the other hand, measures the shaking experienced from an earthquake and varies depending on location. The Modified Mercalli scale is used to measure intensity.

Definition: Magnitude is the size of the earthquake, while intensity is the level of shaking experienced at different locations.

Seismic waves, the radiated energy from earthquakes, come in two main types:

1. P-waves (Primary waves):

   • Compression waves
   • Fastest of the earthquake waves, arriving first at distant points

2. S-waves (Secondary waves):

   • Travel through solid rock
   • Slower than P-waves

These waves transmit energy through the Earth in all directions from the earthquake's focus. Some waves can reach the opposite side of the Earth in just 20 minutes, though they weaken as they travel farther from the focus.

Highlight: Seismic waves can travel vast distances, with some reaching the opposite side of the Earth in just 20 minutes.

Scientists locate earthquakes by comparing P-waves and S-waves, similar to how we determine the distance of lightning by timing the gap between the flash and the thunder. The time difference between P-wave and S-wave arrivals on a seismogram helps determine the distance of the earthquake from that location.

To locate the epicenter, scientists draw circles around three different seismograph stations. The radius of each circle equals the distance from that station to the earthquake. The intersection of these three circles marks the epicenter.

Example: To locate an earthquake's epicenter, scientists use data from at least three seismograph stations, creating intersecting circles to pinpoint the exact location.

What is an Earthquake?

An earthquake occurs when two blocks of the Earth suddenly slip past one another along a fault line. This movement causes ground shaking and radiates seismic energy in the form of waves. The focus, also known as the focal point or hypocenter, is the point within the Earth where an earthquake rupture starts. The epicenter is the point directly above the focus at the Earth's surface.

Definition: The focus is the point within the Earth where an earthquake rupture starts, while the epicenter is the point directly above the focus at the Earth's surface.

There are several types of faults associated with earthquakes:

1. Strike-slip fault: A fault where two blocks slide past one another horizontally.

   Example: The San Andreas Fault is a well-known strike-slip fault.

2. Dip-slip fault: This type includes normal and reverse faults.

   • Normal fault: The block above the fault moves downward relative to the block below.
   • Reverse fault: The block below the fault moves upward relative to the block above.

3. Oblique fault: A fault that moves parallel to both the strike and the dip of the fault plane.

Earthquakes are recorded using a seismograph, an instrument designed to detect and record seismic activity. The size of an earthquake is measured using magnitude, with each earthquake having a single magnitude value. The Richter scale is a well-known magnitude scale.

Vocabulary: A seismograph is an instrument used to detect and record earthquakes.