REVIEW OUTLINE:  EQ LOCATION & MAGNITUDE DETERMINATION

EQ Source Parameters

• Epicentral Location (latitude, longitude)
• Focal depth
• Time of earthquake (‘Origin Time’)
• Magnitude

EQ Location:

• Arrival times of P,S waves
• Convert ‘S-P’ time to distance
• Use of travel-time curves
• Distance from > 3 stations to determine location
• Multiple station readings used to improve precision
• Sources of error (travel time curves, timing errors, reading errors, etc.)

EQ Size determination

Magnitude	Intensity
Quantitative	Qualitative
Single-value	Multiple-value (depends on location)
Open-ended, logarithmic	12-point scale, ‘linear’
‘Intrinsic size’ of EQ	Variable, depends on site conditions

Intensity at a site depends on:

• Size of EQ (magnitude)
• Distance from EQ epicenter
• EQ focal depth
• Site conditions (soil, bedrock)

Magnitude determination

• Amplitude measurements at one or more seismograph stations
• Information on distance (from S-P times)
• Information on seismograph magnification
• Information on wave propagation (built into magnitude chart)