Lecture Notes
Course: CE 407/021 – (Principles of) Earthquake Engineering
MODULE 03: Measurement of Earthquakes
Intended Learning Outcome/s:
At the end of the lecture, the students will be able to:
Define and differentiate intensity and magnitude;
Be familiar with PHIVOLCS Earthquake Intensity Scale & Richter Magnitude Scale;
Describe what a seismograph and a seismogram are;
Estimate the location of the epicenter of an earthquake from seismograms;
Estimate the magnitude of an earthquake from seismograms; and
Describe what an accelerographs and an accelerograms are.
Intensity
Measure of the extent of damage caused by a certain earthquake event
Based on a qualitative description of the damage caused by an earthquake to the natural and built
environment at a particular location and the associated human reaction
PHIVOLCS Earthquake Intensity Scale (see page 2)
Isoseismal Maps – contour maps that indicate plotted locations with same intensity due to a certain
earthquake event
Magnitude
Measure of size or strength of an earthquake
Widely used measure of earthquake strength by scientists and engineers nowadays
Seismograph
Instrument used to detect earthquakes and measure its magnitude
Data coming from seismographs are called seismograms
Basic Components of a Seismograph
Illustration of a Seismogram
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Lecture Notes
Course: CE 407/021 – (Principles of) Earthquake Engineering
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Lecture Notes
Course: CE 407/021 – (Principles of) Earthquake Engineering
Locating the Arrival of P-waves and S-waves using a Seismogram
Determining the Location of the Epicenter of an Earthquake Event
At least three seismograms from seismic stations are necessary for determining the estimated
location of the epicenter of an earthquake.
Determine the time lag between the arrival of P-waves and S-waves for each seismogram.
Determine the average P-wave and S-wave velocities depending on the local site conditions for
each seismic station.
- On rock, average P-wave and S-wave velocities are 6.0 and 3.5 km/s, respectively.
Determine the hypocentral distance from each seismic station using the given equation:
𝑡𝑠𝑝
∆= 1 1
−𝑉
𝑉𝑠 𝑝
where tsp is the time lag, Vs is the average S-wave velocity, and Vp is the average P-wave velocity.
Plot three circles from each seismic station with a map (in scale) with the determined hypocentral
distance as theie corresponding radii. The point of concurrency of the circles is the estimated
location of epicenter of the earthquake.
Locating Epicenter of an Earthquake Event
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Lecture Notes
Course: CE 407/021 – (Principles of) Earthquake Engineering
Determining the Magnitude of an Earthquake Event
Richter or Local Magnitude (ML)
- For shallow earthquakes recorded at distances of < 600 km
- Determine the peak amplitude of a given seismogram considering no earthquake event
(A0).
- Determine the peak amplitude of the same seismogram considering the earthquake event
(A).
- Determine the magnitude using the given equation:
𝐴
𝑀𝐿 =
𝐴0
- Apply corrections when necessary or available (depending on site conditions and
availability of historical data).
Surface Wave Magnitude (MS)
- For moderate to large shallow earthquakes (epicentral distance of > 1000 km and focal
depth of < 70 km)
- MS is determined using the given equation (Gutenberg & Richter, 1936):
𝑀𝑆 = log 𝑎 + 1.66 log ∆ + 2.0
where a is the peak ground displacement (in micrometers), and Δ is the epicentral distance
(in degrees; 360° corresponds to circumference of the Earth).
Body Wave Magnitude (MB)
- For deep focus earthquakes
- MB is determined using the given equation (Gutenberg, 1945):
𝑎
𝑀𝐵 = log + 𝑄(ℎ, ∆)
𝑇
where a is the actual ground motion amplitude (in micrometers), T is the measured wave
period (in Hertz), and Q is an empirical function dependent on focal depth h and epicentral
distance Δ.
Accelerograph
Instrument used to record ground motions; may also be called accelerometers
Data from accelerographs are called accelerograms; in civil engineering context, accelerograms
may also be called ground motion data or earthquake time-history data
Nowadays, smartphones are also equipped with accelerometers for motion sensing; accelerograph
applications may also be installed on smartphones.
Accelerograms are data of greater interest for civil engineers since these may be used to analyze
and design structures considering a certain earthquake event.
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Lecture Notes
Course: CE 407/021 – (Principles of) Earthquake Engineering
Schematic Representation of an Accelerograph
Illustrations of Accelerograms from Various Earthquake Events
Reference/s:
Villaverde, R. (2009). Fundamental Concepts of Earthquake Engineering. Florida, USA: CRC
Press.
