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# Observation_seismology

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```									                Introduction to
Observational Seismology
D. Di Giacomo (domenico@isc.ac.uk),
A. Strollo (strollo@gfz-potsdam.de)
Outline

 Seismic Waves

 Earth interior and Global Earth models

 Seismograms     at   different   distance
ranges
Introduction

Tectonic earthquakes are
caused by slippage along
fractures (faults).
As slip along the faults
occurs, energy is released
which travels in the Earth
interior in the form of seismic
waves…
Harmonic wave parameters and their mutual relationship
NMSOP, Ch. 2                   Body waves
For homogeneous media and small
deformations the equation of the
motion for seismic waves is:

Volume change          Change of
(compression        shape without
and dilatation)     volume change

The equation of the motion has
two independent solutions:
For a Poisson solid λ = μ,
and Vp/Vs = √3.
P waves
The P-waves travel
faster and arrive earlier
S waves                 in the seismic record
than S-waves
Body waves – P waves
P waves, also known as primary or compressional waves are the first
arrivals, but generally not destructive.

Particle motion consists of alternating compression and dilation parallel to
the direction of propagation (longitudinal polarisation). The material
returns to its original shape after the wave passes.
Body waves – S waves
S waves, also known as secondary or shear waves arrive latter, but can
potentially be destructive.

Particle motion consists of alternating transverse motion perpendicular to the
direction of propagation.
Inelastic attenuation of the seismic waves

Waves travelling in the Earth medium are affected by a
loss of energy due to the anelastic properties of the
medium itself.

The amount of energy loss per oscillation is defined by the
Q factor:
Q = 2π E/ΔE, with ΔE being the dissipated energy per
cycle.
Low values of Q mean higher attenuation, and vice versa.
Q is different for P and S waves, with Qα systematically
larger than Qb.

Moreover, the Q factor depends on the frequency: it is
nearly constant between 0.001 and 1 Hz, but for higher
frequencies depends on frequency and, in general,
increases with frequency.
NMSOP, Ch. 2             Surface waves

According to theory, in the presence of a free surface and/or of a layered half-
space, other solutions are possible.

In the real Earth, due to the presence of the Earth free surface and of the
layered Earth’s crust, the body waves radiated by shallow earthquakes
generate also the surface waves:

Love waves (LQ), V(LQ) ≈ Vs in the near-     Rayleigh waves (LR), V(LR) ≈ 0.92 Vs
surface layers
Rayleigh waves
Rayleigh waves are a combination of P and SV waves.

Particle motion consists of elliptical motions (usually retrograde elliptical)
in the vertical plane and parallel to the direction of propagation.
Amplitude decreases with depth.
Material returns to its original shape after wave passes.
Love waves
Love waves result from SH waves trapped near the surface
within the upper layers.

Particle motion consists of alternating transverse motions, horizontal
and perpendicular to the direction of propagation.
Amplitude decreases with depth.
Material returns to its original shape after wave passes.
Surface waves
Surface waves decay more slowly with distance from the epicentre, r-1,
compared to body waves that decay as r-2. Hence, are more prominent
on distant seismograms.

RAYLEIGH
Example of a Mw 7.7
event in Vanuatu
12,250 km away.
RADIAL COMP.
Note the Love wave
LOVE                     on the transverse
component, and the
Rayleigh on the
TRANSVERSE COMP.                                     radial and vertical.
RAYLEIGH

VERTICAL COMP.

(Stein & Wysession, 2003)
Surface waves
Surface waves are dispersive, i.e. the propagation velocity depends on
frequency.
NMSOP, Ch. 2          Surface waves
Dispersive waves are characterized by two velocities:
• Group velocity U(T), with which the energy of a wave group travels;
• Phase velocity c(T), with which the wave peaks and troughs travel.
c(T) > U(T)
The Mantle Surface waves
Love and Rayleigh may also circle the Earth several times along great circle
paths.
These global surface waves have periods T > 50 s, are characterized by low
attenuation and are also used to determine the magnitude of very large
earthquakes.

NMSOP, Ch. 2
Free oscillations (Normal modes)
The waves reflected back from the
surface into the Earth and the
surface waves along great circles
will interfere destructively and
constructively. Constructive
interferences can occur only at
certain resonant frequencies, that
are called the normal modes of the
Earth (in practice, the Earth rings
like a bell).
The normal modes can be
generated especially by great
earthquakes and are important to
study the deep Earth interior
(however, they are generally not
considered in routine observatory
practice).
NMSOP, Ch. 2
Broadband Seismology
Seismic record

The waveform that we record at
the seismic station is the result of:

Therefore, the signal recorded at
the seismic station has been also
“distorted” by the transfer
function of the seismometer…
Seismic record

Records of a deep earthquake at one station in Germany. The different traces have been
obtained by filtering the original record with the response curves of some traditional
seismometers shown on the left of each trace.
Transfer function of some modern seismometer
Broadband
seismometer
Guralp 60T: natural
period of 60 s.

Broadband
seismometer
Streckeisen
STS-2: natural
period of 120 s.

(In the DS 5.1 of the NMSOP are described
the characteristics of some widely used
seismic sensors)
Broadband Seismology

No unique seismic sensor can cover
the whole frequency band required to
cover the Earth signals…
Seismic sensors
Frequency range of Earth signals and
typical seismometers.

Broadband
seismometer
Streckeisen
STS-2:
natural period
of 120 s.

Force balance accelerometer
Clinton, 2004.                                                Episensor ES-T.
Seismology and Earth interior

Frequency f (Hz)
The seismic
waves
travelling in the
0.001 Hz                                  Earth volume
from the
0.01 Hz                                seismic source
to the
0.1 Hz                                receivers bring
information
1 Hz                                    about the
properties of
10 Hz
the Earth
structure.
100 Hz
Seismic wave phases

Each arrival or phase is the
result of a given seismic ray
that has followed a certain path
through the Earth.
When combined they provide
multiple pieces of information
about the physical properties of
the Earth, in particular the
seismic velocity distribution.

Left: An example of some of
the wave
paths followed by various
phases
and their associated form on a
seismogram.

(Stein & Wysession, 2003).
Global Earth Models: AK135Q and PREM
Global Earth
models can be
retrieved by
inverting
phase travel
data (as well
as other
parametric
data).
Comparison
between the
reference Earth
models AK135Q
(solid lines,
Kennett et al.,
1995; Montagner
and Kennett,
1996) and PREM
(dashed lines,
Dziewonski and
Anderson, 1981).
Important discontinuities of the Earth: Crust

• Conrad discontinuity: Seismic boundary between the upper and middle
crust that is usually defined by an increase in seismic velocity from 6.2-
6.4 km/sec to about 6.6-6.8 km/sec. The term has fallen into disuse in
recent years due to the lack of universality of such a discontinuity.

• Mohorovicic discontinuity (Moho): the seismic boundary between the
crust and mantle named after A. Mohorovicic who discovered it from the
travel time data in Europe (Mohorovicic, 1909). The velocity contrast
across the boundary is such that the lower crust typically has a
compressional-wave velocity of 6.5-7.4 km/sec, while the uppermost
mantle a velocity greater than 7.6 km/sec with an average value of 8.1
km/sec. The boundary is between 20 and 60 km deep beneath the
continents and between 5 and 10 km deep beneath the ocean floor.
Important discontinuities of the Earth: Mantle

• Transition         zone:
delimited between the
410 km discontinuity
(which represents the
boundary between UM
and TZ) and the 660 km
discontinuity       (which
represents the boundary
between TZ and LM).
The     410     and    660
discontinuities        are
generated by solid state
phase transitions that
produce strong velocity
gradients     and    sharp
seismic discontinuities.
Important discontinuities of the Earth: Core
• Gutenberg discontinuity (CMB): it marks the discontinuity between mantle and
core at which the P waves drops from ~13.7 to ~8.0 km/s and that of S waves
from ~7.3 to 0 km/s. Indeed, the CMB reflects the change from the solid mantle
to the fluid outer core.

• Inner core boundary (ICB): discontinuity between the fluid OC and the solid IC
Refraction, reflection, and conversion of a wave of a boundary
In the example below is shown a P wave that hits a boundary separating
two layers having different seismic velocities. Part of the energy of the
incoming P wave is transmitted, part is reflected, part is converted and
transmitted as SV wave, and part is converted and reflected as SV wave.
Phase nomenclature: general rules
Body wave phase nomenclature
Because of refraction, reflection and conversion, the majority of phases
have a complex path history between the source and receiver, and are
described by combining letters representing each portion of a ray path.

K       P wave through the outer core
I       P wave through the inner core
J       S wave through the inner core
PP      P wave reflected at the surface
PPP     P wave reflected at the surface
twice
SP      S wave reflected at the surface
as a P wave
SS      S wave reflected at the surface
as a S wave
pP      P wave upward from the focus,
and reflected at the surface
sP      S wave upwards from the
surface, and converted at the
surface to a P wave.
c       Reflection at the core-mantle
boundary (e.g. ScS)
i       Wave reflected at the inner
core-outer core boundary
(e.g. PKiKP)
Earthquake distance
The distance to an earthquake is generally divided between:

Local:            D < 100 km, seismic recordings are strongly
affected by shallow crustal structures.

Regional:         100 < D < 1400 km (1° < D < 13°), recordings
dominated by seismic energy refracted along, or
reflected several times from the crust-mantle boundary.

Upper mantle:         paths 13° < D < 30°, dominated by seismic energy that
turns in the depth range 70 to 700 km, a complex
part of the Earth's internal structure.

Teleseismic:      D > 30°, direct P-and S-waves relatively simple, but
with complex arrivals from traversing the mantle,
as well as core and surface reflections.

(Lay & Wallace, 1995; Stein & Wysession, 2003)
Broadband Seismology
Crustal phases
NMSOP, IS 2.1, p. 7
Moho: Crust-Mantle Boundary

Pg

Here Pn
arrives at
Pn                  station 3
before Pg
Local - Regional Distance Range

The distance at which the
Pn phase arrives at the
same time of the direct Pg
is called cross-over
distance.
Local - Regional Distance Range

Records of
an event in
Germany
(NMSOP,
DS 11.1)
Local - Regional Distance Range

In the real Earth, the crust is
characterized by
heterogeneities and the crust
layering is generally not flat. In
the example on the right, the
recordings of a regional event in
Switzerland (NMSOP, Ch. 2, p.
46) are shown. Here the y-axis
is the reduced time.
The time phase arrivals here
are more complex due to the
dipping layers that characterize
the crust.

Records of a regional event in
Switzerland (NMSOP, Ch. 2, p. 46)
Local - Regional Distance Range (3)

NMSOP, Ch. 11, p. 66
Local - Regional Distance Range (4)

Lg is a complex
mixture of multiple S-
waves reverberations
between the surface
and the Moho of SV
to P and/or P to SV
conversions, as well
as of energy
scattered at lateral
heterogeneities.
Lg follows close after
Sg at D < 300 km, but
is well separated
beyond.

NMSOP, Ch. 2, p. 18
Broadband Seismology

NMSOP, Ch. 11, p. 3
Mantle phases
NMSOP, IS 2.1, p. 8
Mantle phases
NMSOP, IS 2.1, p. 8          PP phase (red), PPP
(green), and PcP (blue)
Core phases
NMSOP, IS 2.1, p. 9           PKP phase (green),
and PKPdf (red)
Teleseismic Distance Range

NMSOP, Ch. 2, p.
24
Teleseismic Distance Range
Travel Times
Combining these arrivals gives what
are termed travel time curves.
The dots represent the arrival
times of a given phase.
A travel time curve therefore
shows results for each phase.
(Global Earth models can be retrieved by
inverting phase travel data)
Travel Times

S   PP

PcP

P
Travel Times
Polarization
In the real Earth, the particle motion can be in any direction, and
the shape and orientation in space of the ground-motion particle trajectory
is called POLARIZATION. It differs for different types of seismic waves
such as P, S and surface waves and may be ± linear or elliptical, prograde
or retrograde. It is also influenced by heterogeneities and anisotropy of the
medium in which the seismic waves propagate and depends on their
frequency or wavelength, respectively. The polarization of ground motion
may be reconstructed by analyzing three-component seismic
recordings.

NMSOP, Ch. 2

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