Homework 1. Earthquakes and focal mechanisms Apr. 9, 2015 (due Apr. 16) Name: 1) An earthquake is recorded by a seismograph located on the other side of the Earth (see a diagram below). The seismograph can detect P and S wave arrivals. Estimate travel times and sketch how the seismograms would look like if (a) the Earth was completely liquid; (b) the Earth was completely solid; (c) the Earth was solid, but with a liquid core starting halfway to the center. Choose average values of P and S waves that are appropriate for the Earth, and make sure to include numbers on your time axis. You may assume that the seismic velocities are constant (independent of depth). 2) Location of earthquake epicenters. A network of seismographs shown on a map below has recorded seismograms from two earthquakes that were felt in Washington D.C. P and S wave arrivals from these earthquakes are listed in Table 1 (in decimal minutes). Using these arrival times, and the attached chart for travel times for P and S waves, (a) determine the distance to the earthquake from each station for which the data are available; (b) locate the earthquake on the map. More data than you need is given, but at a minimum you will need three stations (note that using data from more stations will reduce the location error). You need to locate only one earthquake; use data from earthquake 1 if your last name starts with A-M, and earthquake 2 if your last name starts with N-Z. Was the earthquake you located a distant “big one”, or a moderate local event? 2 3 3) Sketch focal mechanisms for the following faults. Next to each of the sketches, draw geologic cross sections illustrating the earthquake fault and the auxiliary nodal plane that would produce the same focal mechanism. (a) Thrust fault, dip 15◦ W, strike N30◦ W. (b) Normal fault, dip 50◦ W, strike N-S. (c) Reverse fault, dip 75◦ E, strike N30◦ W. (d) Left-lateral strike-slip fault, dip 90◦ , strike N20◦ E. You may use the Lambert equal area net (Fig. 2.6 in KK&V textbook) to draw the fault lines in the lower hemisphere projection. 4