that seismographs 2 + 3 both record somewhat similar earthquake occurrence times and seismographs 1 and 4 also record occurrences times that are close to each other. All calculations (in theory) should have resulted in only 1 Specific Earthquake Occurrence Time (b/c there was only 1 earthquake that happened at a specific time). Besides human measurement errors, why were the earthquake occurrence times for each seismograph slightly different? Hint: Remember that an average P=wave travel time of 6km/sec was used for the calculation. What to you know about the ground material through which the P-waves have to travel that might "alter" the average P-wave speed slightly. Be specific in your answer, please, and use Fig. 10.9 (p.2 224 in EARTH lab manual) as part of your explanation