Overview
This lecture covers how to analyze an earthquake seismogram, focusing on determining wave arrival times, lag time, distance to the epicenter, wave travel times, and the earthquake's origin time.
Identifying Seismic Waves on a Seismogram
- The first vibrations on a seismogram indicate the arrival of P-waves (primary waves), which travel fastest.
- The second, larger vibrations show the arrival of S-waves (secondary waves), which travel slower than P-waves.
Determining Arrival and Lag Times
- Record the arrival time of the P-wave using the time scale (e.g., 8:21).
- Record the arrival time of the S-wave (e.g., 8:30).
- Lag time is calculated by subtracting P-wave arrival time from S-wave arrival time (e.g., 8:30 − 8:21 = 9 minutes).
Calculating Distance to the Epicenter
- Use the lag time and the P- and S-wave travel time chart (from reference tables).
- Mark the lag time on scrap paper and align it between the P and S curves using the "wedge method".
- Trace down to the distance axis to determine the epicenter distance (e.g., 7,600 km).
Determining Wave Travel Times
- Find the epicenter distance on the travel time chart.
- Read the P-wave travel time at this distance (e.g., 11 minutes for 7,600 km).
- Read the S-wave travel time at this distance (e.g., 20 minutes for 7,600 km).
Calculating Origin Time of the Earthquake
- Origin time = wave arrival time − wave travel time.
- For P-waves: 8:21 (arrival) − 11 min (P travel) = 8:10.
- For S-waves: 8:30 (arrival) − 20 min (S travel) = 8:10.
- Both methods should yield the earthquake's origin time.
Key Terms & Definitions
- Seismogram — a recorded graph of ground vibration during an earthquake.
- P-wave (Primary wave) — the fastest seismic wave, arrives first.
- S-wave (Secondary wave) — a slower seismic wave, arrives after the P-wave.
- Lag time — time difference between the arrivals of the S- and P-waves.
- Epicenter — point on Earth's surface above where the earthquake originates.
- Origin time — the actual time the earthquake occurred.
Action Items / Next Steps
- Practice using a seismogram to find P and S-wave arrivals and calculate lag time.
- Use Earth Science reference tables to determine epicenter distance and wave travel times.
- Review the steps to calculate earthquake origin time.