1. The Pinnacles and Neenach volcanics are 23.5 million year old andesite to rhy
ID: 111400 • Letter: 1
Question
1. The Pinnacles and Neenach volcanics are 23.5 million year old andesite to rhyolite outcrops, on either side of the San Andreas Fault, today separated by ~320 kilometers. If they were originally products of the same eruptive center, they have been offset along the San Andreas Fault by 320 kilometers in 23.5 million years. What is the rate of offset in centimeters per year?
1.362 cm/yr
2. The maximum horizontal offset recorded anywhere along the San Andreas Fault after the 1906 earthquake was about 6 meters. If there has beencumulative offset along the fault of 320 kilometers, estimate how many earthquakes have occurred during that period? If 6 meters is taken as the maximum offset, would your answer represent a maximum or minimum estimate?
53,333; Maximum estimate (my answer, not sure if it is correct)
3. What would the recurrence interval be if your estimated number of earthquakes were accurate? Give your answer in years between earthquakes, oryrs/eq.
12,533 yrs/eq
4. Darwin recorded uplift of 3 meters after the 1835 Chilean earthquake. That was a subduction zone megathrust quake, with a primarily vertical component to the elastic rebound. Along the San Andreas Fault movement is primarily horizontal. At a restraining bend such as that forming the San Gabriel Mountains in the Transverse Ranges (a left step along the right-lateral San Andreas Fault), assume a maximum uplift of 0.5 meters per earthquake. Given the number of earthquakes you calculated above, what would be the resulting maximum uplift of the San Gabriel Mountains? (Don't freak out! Expect a huge number. Yuge!)
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5. The highest elevation in the San Gabriel Mountains is 3,069 meters. Given your answer to (4) above, does this seem at least possible as the result of cumulative small uplifts due to recurring earthquakes along the San Andreas Fault? What might account for the difference between the maximum uplift you calculated and the actual elevation of these mountains?
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Explanation / Answer
1 and 2 answers are correct
3. Ans you have given is 12,533 years/earthquake
But as per my calculations, Recurrence Interval = Number of years/ No of events
= 23.5 X 106 years / 53333
= 440.6 years/Earthquake aprroximately
4. Given maximum uplift due to earthquake as 0.5 metres and we have calculated the number of earthquakes as 53333.
To calculate resulting maximum uplift due to all earthquakes = 53333 X 0.5 = 26666.5 metres approximately
5. our estimate of maximum uplift is a 26666.5 metres which is fairly a maximum estimate. But in practical case all earthquakes is not of same magnitude and hence the uplift due to the earthquakes will be different for different earthquakes. We have assumed maximum uplift of 0.5 meters per earthquake. But it may not be the same in practical case. There is a possibility that maximum number of earthquakes leads to a very small uplift which may lead to an uplift of around 3069 metres. The maximum uplift calculated and actual elevation of the mountains is different due to the difference in the magnitude of earthquake thereby decreasing the uplift due to earthquake. Among the earthquakes calculated in a period, maximum number of earthquakes belongs to small magnitude and so there is much less possibility of uplift around 5 metres per earthquake also because this belongs to transform fault.
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