Earthquake Measurement PDF

Measurement of Earthquakes Earthquakes are complex phenomena that require scientific measurement. Magnitude quantifies the energy release, while intensity measures the shaking at a specific location. Earthquake magnitude and intensity are analogous to a lightbulb and the light it emits. Magnitude...

Measurement of Earthquakes Earthquakes are complex phenomena that require scientific measurement. Magnitude quantifies the energy release, while intensity measures the shaking at a specific location. Earthquake magnitude and intensity are analogous to a lightbulb and the light it emits. Magnitude Lightbulb’s Wattage Intensity Light Intensity measured in footcandles Earthquake Magnitude a measure of the “size,” or amplitude, of the seismic waves generated by an earthquake source and recorded by seismographs frequently described using the Richter scale To calculate magnitude, the amplitude of waves on a seismogram is measured, correcting for the distance between the recording instrument and the earthquake epicenter Local Magnitude, 𝑴𝑳 Introduced by Charles F. Richter 𝑴𝑳 =local Magnitude defined for Southern California 𝑨= peak amplitude in µm, measured from a determined from the logarithm of seismogram the amplitude of waves recorded by 𝑨𝒐 = peak amplitude of a zero-magnitude seismographs earthquake in µm, which is used to adjust the variation of ground motion amplitude for 𝑀𝐿 = 𝑙𝑜𝑔𝐴 − 𝐿𝑜𝑔𝐴𝑜 epicentral distances other than 100 km Surface Wave Magnitude, 𝑴𝒔 used for observations near the 𝑴𝒔 =Surface wave magnitude earthquake epicenter where the 𝑬𝒔 =total energy in ergs surface wave is larger than the body wave. log10 𝐸𝑠 = 11.8 + 1.5𝑀𝑠 Body Wave Magnitude, 𝒎𝒃 calculated from the body waves and are 𝒎𝒃 =body wave magnitude usually used at larger distance from the 𝑴𝒔 =Surface wave magnitude earthquake epicenter Since body wave travels the fastest, the body wave magnitude is normally the earliest available earthquake magnitude 𝑚𝑏 = 2.5 + 0.63𝑀𝑠 Moment 𝑴𝒘 =moment magnitude Magnitude, 𝑴𝒘 𝑴𝒐 =Seismic Moment (dyne cm) for very large earthquakes, moment magnitude gives the most reliable estimate of earthquake size Moment is a physical quantity proportional to the slip on the fault multiplied by the area of the fault surface that 𝜇=material shear modulus slips; it is related to the total energy released in the earthquake A=area of fault plane rupture D=mean relative displacement between log 𝑀𝑜 = 1.5𝑀𝑤 + 16.0 the two sides of the fault (the averaged 𝑀𝑜 = 𝜇𝐴𝐷 fault slip)  Earthquake Intensity The intensity of an earthquake is a subjective, nonempirical approach for estimating the size of an earthquake based on the subjective assessment of human observations of the effects of earthquake shaking on buildings (amount of damage sustained by structures and land surface) and on people. Earthquake Intensity Modified Mercalli Intensity (MMI) is a subjective scale defining the level of shaking at specific sites on a scale of I to XII  Modified Mercalli Intensity Scale, and Comparison to Magnitude and PGA ACTIVITY: 1. In 3-4 sentences, differentiate magnitude from intensity 2. Relate earthquake magnitude and intensity to the lightbulb and the light it emits 3. Explain when to use local magnitude, surface wave magnitude, body wave magnitude, and moment magnitude