EARTHQUAKE MAGNITUDE, INTENSITY, ENERGY, POWER LAW RELATIONS AND SOURCE MECHANISMWalter D. MooneyU.S. Geological SurveyCalifornia, USAe-mail: mooney @ usgs.gov

EARTHQUAKE CLASSIFICATION

MAGNITUDECLASSIFICATION M 8.0 Great Earthquake 7.0 M < 8.0 Major / Large Earthquake 5.0 M < 7.0 Moderate Earthquake 3.0 M < 5.0 Small Earthquake 1.0 M < 3.0 Microearthquake M < 1.0 Ultra MicroearthquakeHagiwara, 1964

NATURE OF EARTHQUAKES

Foreshocks

Main shock

Aftershocks

Earthquake Swarm

Normal Seismic activity

CLASSIFICATIONDISTANCE1) Teleseismic Earthquake> 1000 km2) Regional Earthquake> 500 km3) Local Earthquake< 500 kmTYPES OF EARTHQUAKES

Tectonic EarthquakeVolcanic EarthquakeCollapse EarthquakeExplosion Earthquake

EARTHQUAKE MAGNITUDERichter Magnitude ML (Local Magnitude)ML = log A - log Ao ( ) Body-wave Magnitude (mb) mb = log (A/T) - f (,h) Surface-wave Magnitude (Ms)Ms= log AHmax - log Ao (o) MS = log (A/T)max + 1.66 log + 3.3 Moment Magnitude (Mw)Mw = 2/3 log Mo - 10.7 Mo = A u Duration Magnitude (MD)MD = - 0.87 + 2.00 log + 0.0035 Macroseismic Magnitude (Mms)

Mms = 0.5Io + log h + 0.35

Richter Magnitude ScaleDistance S P Magnitude Amplitude km sec ML mm

EARTHQUAKE INTENSITY Rossi-Forel Intensity Scale (I X) Modified Mercalli (MM) Intensity Scale (1956 version), (I XII) Medvedev-Sponheuer-Karnik (MSK) Intensity Scale (1992 Version), (I XII)IsoseismalsIsoseismals are the curved lines joining the localities of same intensity.

EARTHQUAKE ENERGYlog E = 12 + 1.8 MLlog E = 5.8 + 2.4 mb log E = 11.4 + 1.5 Ms 1.010.0 timesabout 32 times0.5 3.2 timesabout 5.5 times0.3 2.0 timesabout 3 times0.1 1.3 timesabout 1.4 timesMagnitude Ground Motion Energy Magnitude versus ground motion and energy

POWER LAW RELATIONSFrequency- magnitude RelationLog10N = a bM Aftershock Attenuation (p-value)N(t) t -pFractal Dimension

b - VALUE ESTIMATION

The Least-Square Fit Method:

The log values of the cumulative number of earthquakes (N) are plotted against magnitude (M).

The Maximum Likelihood Method :

The maximum likelihood estimate of b-value is given by Aki ( 1965) : b =log10e/M-M0

b = 0.77MagnitudeLog NAn example showing frequency-magnitude relation in NE Indiab-value:

SOURCE MECHANISM(fault-plane solution) Classification of Faults Thrust Fault Normal Fault Strike-slip Fault Dynamics of FaultingElastic Rebound Theory Single Couple Double Couple

Normal fault RegimeThrust fault RegimeStrike-slip fault Regime

Fault Dimension

Different Types of Faulting

Graben & Horst in Fault System

Dynamics of Faulting

Elastic Rebound TheoryReid, 1910

Plotting of P-wave First-motion(Equal Area Projection)

Equal Area Plot of a Plane and its Pole

P-wave first-motion plot and fault plane solution Kayal, 1984

Source mechanisms of earthquakes at spreading centre

Source mechanisms of earthquakes at the subduction zone, Indo-Burma ranges Rao & Kalpana, 2005

Earthquakes have different magnitude and are given names based on their magnitude. These names are used for talking to the media and the public. But sometimes even moderate earthquakes can cause great damage.If there is a swarm of earthquakes on the same faults then the largest fault is called the main shock any earlier small earthquakes are called foreshocks and the later earthquakes are called aftershocks.

The largest aftershock is normally smaller in magnitude by 1.5 - e.g. an eq M7 will have aftershock of at most M5.5

Main shocks are NOT always preceded by foreshocks. Some large events have no warningMost earthquakes are tectonic due to movement on faults.

Also have many volcanic earthquakes, but these are normally smaller in magnitude.

Explosion earthquakes are related to making stream under volcanoes.

Earthquakes are also given names depending on how far the source (epicentre) is from the recording station.Many ways of describing Magnitude.

Duration Mag = amount of time the earthquake is recorded for large eqs cause the ground to shake for longer than small eqs.Richter Mag = the frist magnitude scale, but relates to a specific seismograph, but this instrument is no longer used (wood anderson seismograph)Surface wave Mag = used for distant earthquakesRichter and Body-wave Mag = used for small local earthquakesMoment Mag = is related to the amount of energy released by the fault (used for distant earthquakes). Therefore liked by seismolgists

How to calculate Richter Magnitude.The measure of damage from an earthquake is given by the intensity (not the magnitude).

We still use Intensity (even though there are now many instruments) because this is all we have for old earthquakes, so we can use modern recordings to estimate the magnitude of ancient earthquakes.

We can map the damage from historic earthquakes and use areas of high intensity to locate roughly where the earthquakes occurred.We are also interested in the total energy released from the earthquake. The amount energy is related to the magnitude, but because the magnitude scale is logarithmic the relationship not linear.Only talk about the first equation. The number earthquakes is related to the magnitude the b value is normally about 1. Therefore there are about 10 times as many M6 earthquakes as there M7 (there are about 17 M7 earthquakes each year in the world, there are about 1300 M6 earthquakes, and about 1 or 2 M8)

There are only 1 or 2 earthquakes of M9 in 10 yearsAn example of local variation in b value (usually about b= approximately 1)The names of bits of faults these names come from mining (minerals are sometimes along fault planes so the foot wall iPairs of normal faultsThis is about how to tell a normal fault from a thrust fault or a strike-slip fault using seismic data.

The circle represents the volume of the earth around the fault. The shaded area is where the first motion of the ground as towards the station (away from the fault), the white areas are where the ground moved away from the station (towards the fault)Example of a fault plane solution this has some normal component and some strike slip.