“Measuring Earthquakes” Earthquake Scales “Measuring Earthquakes” 3d - Students know why and how earthquakes occur and the scales used to measure their intensity and magnitude.

More than 1 million earthquakes occur each year More than 90% of earthquakes are not felt and cause little to no damage Magnitude Average Annually 8 and higher 1 ¹ 7 - 7.9 15 ¹ 6 - 6.9 134 ² 5 - 5.9 1319 ² 4 - 4.9 13,000 (estimated) 3 - 3.9 130,000 (estimated) 2 - 2.9 1,300,000 (estimated)

The Richter Scale Charles Richter (1935) Measures Magnitude (energy released) Scale ranges from 1 – 10 Each increase in magnitude is 10 times stronger than the previous number (shaking strength) Each increase in magnitude is 32 times the amount of seismic energy than the previous number A wave one millimeter (1000 microns) high on a seismogram would have a magnitude of 3 because 1000 is ten raised to the third power. In contrast, a wave ten millimeters high would have a magnitude of 4. For reasons that we won't go into, a factor of 10 change in the wave height corresponds to a factor of 32 change in the amount of energy released during the earthquake. In other words, a magnitude 7 earthquake would produce seismogram waves 10 x 10 = 100 times as high and release energy 32 x 32 = 1024 times as great as a magnitude 5 earthquake.

327 = 34,359,738,368 326 = 1,073,741,824 325 = 33,554,432 324 = 1,048,576 323 = 32,768 322 = 1024 321

Size of Amplitude depends on: Distance from epicenter Magnitude of the earthquake Based on the amplitude of the largest seismic wave recorded on a seismogram Amplitude is measured in millimeters (mm) from the center of the seismic record to the maximum on the trace. In this case the amplitude is Amplitude is measured in millimeters (mm) from the center of the seismic record to the maximum on the trace. In this case the amplitude is 180 mm (you can also measure the maximum width of the trace and divide by 2. 180 mm

Large Amplitudes imply: Earthquake epicenter is very close Earthquake is far away but very large Richter scale is logarithmic

The Mercalli Scale I on end for Intensity! Measures Intensity (damage) Uses roman numerals from I to XII (1 = low intensity and 12 = high intensity)

What does Intensity depend on? Amplitude of Surface Waves Most damage typically occurs in the region nearest the epicenter Depth of Focus Shallow – closer to surface Deep – further from surface A deep-focus earthquake produces smaller vibrations at the epicenter than a shallow-focus quake because it occurs further from the surface.

Solid rock – least damage Types of Soil Solid rock – least damage Moderately consolidated soil (clay, sand) – intermediate damage Unconsolidated soil (loose, not compact) – most damage Left seismogram = soft soils Right seismogram = harder rock

Mercalli scale data is used to make seismic intensity maps