1. The Pinnacles and Neenach volcanics are 23.5 million year old andesite to rhy

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? (It's easiest to convert everything first; see NOTE above)

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?

3. What would the recurrence interval be if your estimated number of earthquakes were accurate? Give your answer in years between earthquakes, oryrs/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!)

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?

Explanation / Answer

1. Given the seperation between the outcrops as 320 km and time as 23.5 X106 years.

So, the offest per year = 320 X 103 / 23.5 X 106 m/year = 13.6 X 10-3 m/year = 1.36 cm/year

2. Given the offset per earthquake as 6m and total cumulative offset of 320 km

Then the total number of earthquakes = 320 X 103 / 6 = 53330 earthquakes.

Our answer represent a maximun estimate because 6metres is the maximum offest recorded anywhere on San Andreas Fault

3. Recurrence Interval = Number of years / number of events

= 23.5 X106 / 53330 = 440 years/earthquake

4. Given maximum uplift due to earthquake as 0.5 metres

So the total uplift due to 53330 earthquakes = 53330 X 0.5 metres = 26665 metres

5. The highest elevation in San Gabriel Mountains is given as 3069 meters but we got maximum uplift as 26665 metres. The difference may be because of the magnitude and number of earthquakes occured during teh years. We have calculated the number of earthquakes by taking the maximum offset of 6metres for all earhquakes. But practically all the earthquakes are different in magnitude and size and hence the offset for all teh earthquakes will not be same. Our number of earthquakes calculated is actually maximum estimate.

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