GEOPHYSICS (08/430/0012)
PRACTICAL 7: SEISMIC PHASES THROUGH THE INTERIOR OF THE EARTH
Name the seismic phases associated with the ray paths ending at points a to j in the figure below. Solid lines represent P
waves and dotted lines S waves.
Why are the ray paths curved?
e
d
f
g
h
c
i
b
a
j
focus
inner
core
outer
core
mantle
GEOPHYSICS (08/430/0012)
PRACTICAL 7: TELESEISMIC PHASES AND THEIR TRAVEL TIMES
Teleseismic recordings are recordings at distance, such as the recordings from a largish earthquake at seismograph stations
around the world. The figure overleaf shows the LPZ seismograms recorded at four seismograph stations from a magnitude 7.0
earthquake recorded on 30 January 1987 at four seismograph stations. The four stations are Bangui (BCAO) in the Central
African Republic, Charters Towers (CTAO) in Queensland, Australia, Hobart (TAU) in Tasmania and Zongo (ZOBO) in La
Paz, Bolivia. The P and S-wave phases are marked on the figure. We shall use these seismograms to find the origin time and
locate the epicentre of this earthquake in practical 8.
Measure and draw up a table of their arrival times:
Station Arrival Time (minutes after 22h ) t(S) − t(P )
P
S
min.
BCAO
CTAO
TAU
ZOBO
t(P )
min.
Distance
∆◦
1. For each station, calculate the difference t(S) − t(P ) in travel time between the P and S waves. Then use the graph of
P and S-wave travel-times from a shallow earthquake on page 4 to estimate the P-wave travel time t(P ) to the station
and the epicentral distance ∆ from the earthquake to the station.
2. Use the travel-time graph to identify the phase arriving at CTAO with a peak at 22h 48m .
3. The phase just before 22h 52m at CTAO is probably SKiKP. Sketch the ray path of this phase.
4. What are the travel times of the PP and PPP phases to ZOBO? How long after the P phase do they arrive? Identify
the PP and PPP waveforms on the ZOBO LPZ seismogram.
GEOPHYSICS (08/430/0012)
.
6
1
LPZ recordings from an earthquake on 30 Jan 1987
x 10
ZOBO LPZ
0
P
−1
38 5
40
x 10
1
BCAO LPZ
S
42
44
46
48
50
52
54
56
58
52
54
56
58
54
56
58
56
58
0
P
−1
38 4
40
x 10
1
TAU LPZ
42
S
44
46
48
50
0
P
−1
38 4
40
42
x 10
4
CTAO LPZ
2
S
44
46
48
50
52
0
−2
−4
38
P
40
42
44
S
46
48
50
h
minutes after 22
52
54
GEOPHYSICS (08/430/0012)
IASPEI91 TRAVEL TIMES FOR A SURFACE FOCUS
This graph should plot with a time scale of 1 cm ≡ 2 minutes. The graph on the next page should plot with a time scale of
1 cm ≡ 1 minute, the same as the time scale of the seismograms on page 3.
Travel times from the 1991 Tables for P, PP, PPP and S (surface focus)
26
24
22
travel time (minutes)
20
S
18
PPP
16
14
PP
12
10
P
8
50
55
60
65
70
75
80
geocentric distance (degrees)
85
90
95
100
Travel times from the 1991 Tables for P, PP, PPP and S (surface focus)
26
.
24
22
travel time (minutes)
20
S
18
PPP
16
14
PP
12
10
P
8
50
55
60
65
70
75
80
geocentric distance (degrees)
85
90
95
100