Engineering Seismology(contd.):Prepared by:Dareen Angelica Castro

Engineering SeismologyMechanics of Earthquake EngineeringSeismic WavesEarthquake Magnitude & Seismic IntensitySeismometers & Seismic ObservationTsunamis

Seismometers & Seismic Observation

HYPOCENTER OR FOCUS: The point of origin of elastic disturbance constituting an earthquake. This point seems to represent the position of the initial rupture.Terminology

Terminology

EPICENTER: The point on the surface of the earth vertically above hypocenter is called an epicenter. : The destruction caused by the earthquake at this place will always be maximum and with an increasing distance from this point, the intensity of destruction also decreases.

Terminology

EPICENTRAL DISTANCE (): It is the distance from recording station to the epicenter.

HYPOCENTRAL DISTANCE: The distance (d+) from hypocenter to the recording station.

Terminology

ANTI-CENTER: The point on earth's surface diametrically opposite to the epicenter

Recording Earthquakes

The vibratory motion produced during an earthquake could be measured in terms of displacement, velocity or acceleration.

A seismologist is interested in even small amplitude ground motions (in terms of displacement) - Provides insight into the wave propagation characteristics and enables him to estimate the associated earthquake parameters.

Engineers are more concerned with the earthquake causing structural damage.- As accelerations are the causative phenomena for forces that damage structures. Force = mass x acceleration

- Hence, engineers are more interested in acceleration measurement.

SEISMOGRAPH: instruments measure the ground displacements. consists of a ground motion detection sensor, called a SEISMOMETER, coupled with a recording system.SEISMOGRAM: time-wise record of the ground motion obtained from a seismograph.

ACCELEROGRAPH: An instrument that records the ground motion in terms of acceleration in the epicentral region of strong shaking. It produces the time wise record of ground acceleration at a specific locality.

ACCELEROGRAM: The ground motion record produced by an accelerograph.Terminology

Principle of Seismometersdirection of ground shaking

Principle of Seismometersdirection of ground shaking

SeismometersModern research seismometers are electronic, and instead of using a pen and drum, the relative motion between the weight and the frame generates an electrical voltage that is recorded by a computer.

By modifying the arrangement of the spring, weight and frame, seismometers can record motions in all directions.

Seismogram

Measure of Earthquakes

Magnitude of EarthquakeA measure of the absolute size of the earthquake, without reference to distance from the epicenter. Over the years, a number of approaches for measurement of magnitude of an earthquake have come into existence.

Magnitude of EarthquakeA measure of the absolute size of the earthquake, without reference to distance from the epicenter. Over the years, a number of approaches for measurement of magnitude of an earthquake have come into existence.

Magnitude of EarthquakeFirst proposed by C.F. Richter, based on the data from Californian earthquakes.Defined the earthquake magnitude as the logarithm to the base 10 of the largest displacement of a standard seismograph situated 100 km from the focus.M=log10 A

Where, A = amplitude in micron (10-6m) recorded by the instrument located at an epicentral distance of 100 kmM = is the magnitude of the earthquake at a distance from the epicenter.

Magnitude of Earthquake

Intensity of Earthquakesa measure of severity of the shaking of ground and its damage. destruction or damage that takes place to a construction at agiven place depends on many factors.(i) distance from the epicenter,(ii) compactness of the underlying ground(iii) type of construction (iv) magnitude of the earthquake (v) duration of the earthquake and (vi) depth of the focus

The most popular intensity scale is the Modified Mercalli Intensity (MMI) Scale.This scale,composed of 12 increasing levelsIt does not have a mathematical basis; instead it isan arbitrary ranking based on observed effects. The lower numbers of the intensityscale generally deal with the manner in which the earthquake is felt by people.

Another intensity scale is Mendvedev-Spoonheuer-Karnik scale (MSK 64). Indian seismic zones were categorized on the basis of MSK 64 scale.Some of the other intensity scales used are Rossi-Forel (RF) scale, JapaneseMeteorological Agency (JMA) intensity scale

Correlation of Magnitude and Intensity(by Esteva and Rosenblueth)MM = 8.16 + 1.45 M - 2.46 ln R

where

MM = Modified Mercalli scale intensity grade.M = Earthquake magnitude (Richter scale).R = Distance from epicenter (km).

Comparison of Various Seismic Intensity Scales

Measure of Earthquakes

ISOSEISMALS: Curves connecting localities where equal intensity is observed in an earthquake. They are now more commonly mapped as boundaries between regions of successive intensity ratings.

MICROEARTHQUAKES: Very small earthquakes with magnitude less than three on Richter scale are called Microearthquakes.

Terminology

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