# Earthquake intensity and magnitude scales of Phivolcs

by Quirino Sugon Jr.

I am working on earthquake risk measurements.  To measure risks, we have to determine earthquake energies: how they are measured and how they die down as a function of distance.  Here are some reference notes:

In the Philvocs website, the earthquake values are obviously magnitude not intensity scales, since we cannot have fractional intensity scales.  Intensity scales are subjective, as seen from the Phivolcs Earthquake Intensity Scale.  The Phivolcs scale is a Richter scale.

As stated above, the Richter scale itself is a logarithmic mathematical formula which is calibrated so that a ten fold increase in amplitude relates to a single whole number increase on the scale (e.g. an earthquake with a Richter magnitude of 5 has seismic waves with a maximum amplitude 10 times larger than those for a magnitude 4). It has a number of practical limitations, in that it is poor at recording earthquakes with magnitudes greater than 7 and at distances greater than 650 km from a seismometer.

The equation for calculating the Richter magnitude (MR) is shown below:

MR = (Log10A) – (Log10A0)

Where
A = maximum zero to peak amplitude of seismic wave (mm) recorded.
A0 = Empirical function derived from the distance from seismometer station to earthquake epicentre

Log10A0 From 0 to 200 km distance:
Log10A0 = 0.15 – 1.6 log(distance in km)

Between 200 and 600 km distance by:
Log10A0 = 3.38 – 3.0 log(distance in km)

Due to the limitations described above (distance and maximum size of measurable earthquake), it has since been replaced by the Moment Magnitude Scale in the measurement of large earthquakes – for information on this, please see the related question.

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Does anybody know what would be the empirical function for each point in the Philppines?