Earthquakes Susan Bilek Associate Professor of Geophysics New Mexico Tech How to figure out the who, what, where, why… (or the location, size, type)

Topics for Today Earthquake location Earthquake magnitude Focal mechanisms and moment tensors Relationship to tectonics

Earthquake Location One of the first pieces of information determined for an earthquake Location estimated by measurements of P, S, other phase arrival times Time for arrival to reach station will depend on distance away from station, and the velocity of the wave With known(-ish) velocity structure, use time measurements to estimate distance from station Combine with many observations (many stations) to solve inverse problem for location

Simple Location Example IRIS

More details Equation solved: t s = time at station s t e = origin time at earthquake e v = velocity of seismic waves through given medium x e, y e, z e = x, y, z position of earthquake x s, y s, z s = x, y, z position of recording station Set up system of equations, use (many) arrival times and known station locations, specified velocity, solve for earthquake location

Sources of Error Figs from Shearer Velocity uncertainties: Earth is made of rocks!! so earth not constant velocity (or even layer cake velocities) Picking errors: Some data can be hard to pick phase arrivals consistently

Earthquake Magnitude

Estimating m b in the 1st 5 s of P wave Estimating P-wave magnitude

Seismic Moment and Moment Magnitude Better estimates of earthquake size – seismic moment M o Relates size to fault and earthquake parameters M o =  AD (A: area of fault that slipped (=LxW), D: amount of slip,  : rigidity of fault) Get numbers like 1x10 18 Nm or 1x10 25 dyn-cm Convert to moment magnitude, get similar form as m b, M s

2011 Japan earthquake contributed large amount of moment to the overall global totals Japan 2011 Moment release Magnitude 8+ events contribute the most in moment, with the 5 M 9 events being the largest components

Fault Geometry and Focal Mechanisms: Using seismology to describe faults Shearer

Basic Types of Faulting Stein and Wysession

Fault Geometry

Direction of motion on fault -- produces compressional or dilatational first arrivals at stations

LW 8.23 Lay and Wallace

Stereonet used for plotting earthquake first motions to determine focal mechanism Plot azimuth, takeoff angle, polarity at station Find planes that separate up (compressional) and down (dilatational) motions

Example focal mechanism and data Note small amplitude 1st motions near nodal planes

Build double couple forces - motion on fault plane or auxillary plane

Earthquake locations and focal mechanisms around the world – why this sort of pattern? Relates to Plate Tectonics!

Hayward Fault, CA Large strike-slip fault east of the San Andreas in San Francisco Bay area

km ~350 km ~700 km

Thrust mechanism earthquakes along Sumatra subduction zone

BirthQuake! What is your birthquake (what kind of fault? What sort of tectonics?)