Short Note
Basic Concepts in Seismology
Ali O. Oncel
Earth Sciences Department, King Fahd University of Petroleum and Minerals
31261 Dhahran, P.O Box 1946, Saudi Arabia
oncel@kfupm.edu.sa http://faculty.kfupm.edu.sa/es/oncel/
Abstract:. In this short paper, a general outline for Earth Sciences is provided, and then basic concepts of seismology
are introduced. Types of earthquake monitoring and earthquakes are briefly explained, and changes in the periodic
pattern of earthquake damage are discussed by using significant data.
Key-Words: Earthquake, Seismometer, Plates, and Monitoring Seismicity.
1 Introduction: What is Earth Science?
Understanding the Earth has been a great
challenge for mankind since the beginning of history.
The first important step towards understanding the
Earth's interior was the invention of the first modern
seismometer by John Milne in 1880. Based on the use
of this instrument, the first large earthquake in the San
Andreas Fault was successfully recorded and accurate
information for the structure in the layering models of
Earth, i.e. Crust, Mantle and Core, was obtained. Most
of the important data for understanding the Earth are
provided by earthquakes.
Earth Sciences can be classified mainly into
two divisions: Geology and Geophysics. Geology is a
science which uses field observations to explore
subsurface geology. Subsurface geology can involve
both economic materials and tectonic layers. Some
examples of economic materials are gold, coal, water
and oil. As a result of geological variety or complexity,
geologists have to select certain subjects as follows:
Hydrogeology, Mining Geology, Petroleum Geology and
Structural Geology. Geologists gain an expertise in
certain fields of geology. According to their expertise,
they can be called Hydrogeologists, Mining Geologists,
Petroleum Geologists and Structural Geologists.
Geology provides important hints through the Earth’s
surface, but it can not explain the accurate geometry of
buried materials. Thus, geophysics extends geology to
resolve the subsurface geometry of geological materials
which are exposed through the surface.
Geophysics is a science which significantly
extends surface geology by methods of applied physics.
Some examples of the measured physical constants of
subsurface geology are resistivity, gravity, magnetic
susceptibility and wave velocity. Geophysics extends
also eye-witnessed geological data on the surface to
resolve how those materials are layered undersurface.
Thus, geophysics uses geological data as a preliminary
means to explore the position of exposed geology under
the surface. Different specialized studies enable
geophysicists to obtain expertise in certain fields of
geophysics as follows: Petroleum Geophysicist, Mining
Geophysicist,
Earthquake
Geophysicist
and
Hydrogeophysicist.
2 Earthquake Monitoring
In this part of the paper, we will discuss
fundamental approaches of earthquake monitoring,
because this is an important variable for the purpose of
study.
In general, earthquake monitoring can be
classified into two types: global earthquake monitoring
and regional earthquake monitoring. Global earthquake
monitoring is monitoring which is based on larger
earthquakes, i.e. more than 2000 km distance. It
provides travel-time data of earthquake which comes
from deeper part of earth, e.g. mantle and crust. As a
result of continuous occurrences of larger earthquakes,
their data can be used freely to probe the Earth. It means
that one can estimate accurate thicknesses of crust,
which range from 7 km to 35 km through the Earth.
Regional monitoring is monitoring which is
based on sources and receivers within the study area. It
monitors activity of nearby possible faults or fields of oil
and gas. Examples are monitoring seismicity caused by
tectonic faults in the western province of Saudi Arabia
by the Saudi Geological Survey. Quality of monitoring
earthquake data depends significantly on the number of
seismographs and network design. Fault-specific or
oilfield-specific seismicity monitoring requires a
network design at first and then stations are deployed
according to the agreed design.
As a result of
deployed seismometers, data is acquired and transmitted
to the seismic laboratory by satellite based network
connection.
Monitoring Seismicity provides data for
seismologists to study stress changes along the fault or
oil-fields.
Figure 1
3. Earthquake Types: Intraplate or
Interplate?
We generally hear about the earthquakes in
Japan and appreciate the Japanese authorities' efforts to
mitigate damage from earthquakes. For example, an M8
earthquake occurred in Japan but did not cause damage.
Because most frequent earthquakes are deep-seated
earthquakes along the plate boundaries, which are called
intra-plate earthquakes. It means that earthquakes in
Japan occur frequently deeper and the damage due to
them is observed to be lighter. Let’s discuss the types of
earthquakes in order to understand significant factors
affecting the damage and periodicity of an earthquake.
Figure 2
Arabian
Plate
Figure 3
Tectonic earthquakes can be mainly classified as
follows: interplate earthquakes and intraplate
earthquakes. These earthquakes are similar in three
ways. Both are recorded by seismographs, and caused by
changes of tectonic stresses. Neither occurs through the
passive margins of plate zones.
However, these earthquakes are different in
some ways. The depths of interplate earthquakes
approach 20 to 40 km through the upper crust, but
intraplate earthquakes about 660 km.
Intraplate
earthquakes occur through convergent plate boundary
zones, while interplate earthquakes occur through the
off-boundary areas. The period of larger interplate
earthquakes (M>8) is about one hundred years, whereas
the period of intraplate earthquakes is about one
thousand years. Interplate earthquakes that are shallower
are always devastating, but intraplate earthquakes cause
less damage since they are generally deeper.
3 Function/Structure of Seismometer
We have discussed so far both types of
earthquakes and monitoring seismicity. Now, it is time
to introduce a basic fundamental device of earthquake
seismology, which is the Seismograph.
The seismograph comprises a recorder and
sensor which is called a seismometer. A seismometer is
a device which simply measures vertical ground motion.
It is made up of a weight hanging on a spring and of a
frame where the spring is attached.
The weight moves vertically, causing
differences in some relative motion between the weight
and the Earth when the earthquake happens. If a
recording system is completely installed, this ground
motion can be recorded over the rotating drum.
.4. Relationship between Tectonic Forces:
Example from Anatolia
Earthquakes are mainly caused by changes in
tectonic stresses. Tectonic stresses are driven due to
plate motion as a result of Dynamic Earth. In order to
make clear how the periodicity of damaging earthquakes
is related to plate interactions, a case region is selected.
The data covers the period AD 100-1990. The data are
taken from two graphs. The first graph shows spatial
changes of seismicity through the transform faults, i.e.
East Anatolian Fault Zone (EAFZ) and North Anatolian
Fault Zone (NAFZ).
The second graph shows the
periodic changes of earthquake damage for adjacent
larger tectonic faults. After a brief summary of some
kinds of tectonic faults, driven by different tectonic
forces, tectonic stresses causing earthquake activity in
Anatolia will be discussed.
Earth’s crust comprises mainly seven moving
larger plates and numerous smaller plates. The Pacific
plate is the largest, while the Nazca plate is an example
of the smallest.
Tectonic forces through plate
boundaries are different in different types of plate
interaction. Convergent plate boundaries, where plates
approach each other, are associated to compressional
stresses to create and form thrust folds. However,
extensional stresses drive divergent plate boundaries,
where plates move away from each other, and they form
normal faults.
Lateral stresses (shear) drives transforms fault
where plates move relatively to each other.
Compressional stresses, for example, are caused by
continental collision between the Arabian and Eurasian
Plates in the eastern part of Turkey, and the Anatolian
plate is extruded toward western Turkey.
Transform
Faults, i.e., North Anatolian Fault and East Anatolian
Fault, cause the extrusion of the Anatolian plate.
Long-term deformation of transform faults
forms storing underlined stresses are associated to both
number of larger earthquakes and their occurrence times.
The graphs show that changes in the number of
earthquakes for each transform fault increase in the early
years and decrease in later years or vice versa. Tectonic
stresses of inactive periods are built up, but stresses in
active periods are released by subsequence larger
earthquakes.
The following data are taken from the graphs.
The number of earthquakes for the EAFZ (East
Anatolian Fault Zone) increased slightly from one in 50
to two by 460 at first and went up sharply to 11 by 1250.
However, it rose slightly again until the end of period.
The number of earthquakes for the NAFZ
(North Anatolian Fault Zone) rose at first from one in
100 to five by the year 460. It remained constant until
1100, but it went up until the end of period.
In conclusion, the graphs can be interpreted as
follows. The changes in the number of earthquakes
show an anomalous inactive period that may probably be
caused by an active period on another transform fault.
However, the increase in the number of earthquakes for a
short period 1100-1250 was perhaps due to external
factors.
5 Conclusion
Understanding earthquake instruments might help to
show challenges for studying tectonic earthquakes.
Earthquakes are different depending on where they
occurred, and have different periodicity.
References:
[1] Hubert-Ferrai et al., 2003, Long-term elasticity in the
continental lithosphere: modeling the Aden Ridge
propagation and the Anatolian extrusion process,
Journal of Geophysical Research, Vol.153, pp. 111132.
[2]http://www.iris.edu/edu/onepagers/Hies/OnePager7.pdf