disasters10.12notes

disasters10.12notes - 1 4.3: Ch. 4 Earthquake Geology and...

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1 4.3: Ch. 4 – Earthquake Geology and Seismology Date:_________ Richter scale, M(small L) is quick and useful for moderate- sized EQs that occur nearby (i.e., local EQs). It does not work well on distant, large EQs. For example, a Richter scale of 8.3 was assigned to both the 1906 San Francisco EQ and the 1964 Alaska EQ, yet using other scales the San Fran EQ is a 7.8 and the Alaska EQ a 9.2. The Alaska EQ was at least 100 times bigger in terms of energy! ♦♦ Seismic Moment, M O : relies on the amount of movement along the fault that generated the EQ; in other words, M O equals the shear strength of the rocks times the fault rupture area times the average displacement (slip) on the fault ♦ (One most used today) Moment Magnitude Scale, M w : used for big EQs; can take weeks or months to determine, as time is required to assess all aftershocks to define the area of the rupture zone: M W = 2/3 log 10 (M O ) – 6 Largest M w to date: (1) 1960 Chile EQ: 9.5 (2) 2004 Sumatra EQ: 9.2 ; (3) 1964 Alaska EQ: 9.2. ● Large EQs increase stress on adjacent sections of a fault, resulting in large aftershocks, especially in the three days following the mainshock.
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2 If a big EQ is followed by an even bigger EQ in the same fault zone, then the first EQ is reclassified as a foreshock. ● Fault-rupture length influences EQ magnitudes. A rupture along a fault typically moves 2 to 4 km / sec. (The longer the rupture, the lengthier duration of movement ). Seismic waves die off with distance traveled.
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disasters10.12notes - 1 4.3: Ch. 4 Earthquake Geology and...

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