$10.$ The USGS computed that there is a $21\%$ chance of and earthquake with magnitude $>6\mathrm{.}7$ striking the Bay Area in the next $30$ years $($see slide in the lecture$).$ What is the return period for such earthquake? Pe$=21,$ n$-30$

$11.$ Obtain PGA for $2\%$ in $50$ years earthquake for the following sites. Treasure Island $(37\mathrm{.}823,-122,369)$ and Cal Memorial Stadium $(37\mathrm{.}871,-122\mathrm{.}251).$ Use the Coterminous US $2014$ update edition and assume site class D$/$E for Treasure Island and site class B$/$C for the stadium site.

$12.$ Determine the spectral accelerations for periods corresponding to the response of a$3-$story and a $10-$story building on each site in Question $10.$ Copy the spectral acceleration plots and explain what they show