They’re more than
just scribbles
• P-waves (primary) travel faster than S-waves
(secondary) when produced by an
earthquake
• They arrive on a seismograph first because
they are faster
• A seismograph is an instrument that detects
and records waves produced by earthquakes
• They are the first squiggles on a seismogram
• The S-waves are slower and appear second
• As the P-waves and S-waves travel through the
ground, the slower S-waves lag behind
• The further a seismograph station is from the
epicenter, the larger the lag time
• Increased distance from quake = bigger lag
time for P-waves and S-waves
• Notice the S-waves appear at different times
based on distance to the epicenter
Which is closest to the epicenter or where the Earthquake started?
• The S-P time interval is the difference from
when the first P-wave and the S-wave arrived
Seconds
• Scientists use a travel-time graph
that shows the relationship between
P-wave and S-wave arrival times
• Given the difference between the Pwave and the S-wave, the distance to
the epicenter can be figured out
• What does each tick-mark mean?
Each tick is how many
seconds?
• What does each tick-mark mean?
Each tick is how many kilometers?
• We know the difference between the P and S
waves. Example: 1 min 40 sec
Time (minutes)
Distance to epicenter =
_____________ km
Distance (kilometers)
Time (minutes)
• The S-P time interval is 3 minutes
• What is the distance to the epicenter?
__________ km
Distance (kilometers)
• A distance of 3000 kilometers means that the
Earthquake originated 3000 kilometers away
from that specific seismic station
3000 km
• If you know the epicenter distance, you can
figure out P-wave and S-wave travel times
• Which are faster?
Primary waves or Secondary waves
• What is the travel time at a distance of:
P-wave:
1.) 6000 km: ______ minutes _______ seconds
2.) 2000 km: ______ minutes _______ seconds
S-wave:
3.) 7000 km: ______ minutes _______ seconds
4.) 8000 km: ______ minutes _______ seconds
If a P-wave arrived in 4 minutes and 30 seconds
…and an S-wave arrived in 8 minutes 40 seconds
… what is the lag time (S-P time) between them?
08:04:30
HOURS
If you subtract:
MINUTES
SECONDS
08:04:30
- 00:02:50
08:01:40
• Origin time is the time when an Earthquake
first triggers seismic waves or when it
orginates
• Example: The Earthquake that caused the
tsunami that hit Sumatra in December 2004
originated at 06:58:50 local time
• If P-waves are first recorded at a seismic
station at 10:00:10 a.m and it takes 5 minutes
40 seconds (travel time) for the P-waves to get
there, what is the origin time?
10:10:10
- 00:05:40
(P-wave arrival time)
(P-wave travel time)
• If P-waves are first recorded at 09:30:35 p.m
and it takes 3 minutes 40 seconds for the Pwaves to get there, what is the origin time?
• One seismic station isn’t enough to help you
locate the epicenter
?
Why or why
not???
4000 km
TO THE LAB!!!