How DO they actually locate earthquakes? Alternatives to S-P Triangulation Hubenthal, M. Taber, M.

An Inverse Problem We know the end product; arrival times We have a velocity model for Earth and we want to calculate the original event – Location – Origin time Forward Problem = Model parameters → Data Inverse Problem = Data → Model parameters

The bisector method P arrivals are much easier to pick Assumes an extremely simple velocity model Which station of this record section was closest?

Bisector Steps 1.Mark and measure path between TATO/WAKE 2.Find midpoint of path 3.Draw bisector perpendicular to path 4.Label Bisector with station names 5.Determine which side of the bisector was close to the event. 6.Repeat with TATO/YAK 7.Repeat with WAKE/YAK 8.Repeat with additional stations of your choice

Path Bisector TATO WAKE X

Bisector Steps 1.Mark and measure path between TATO/WAKE 2.Find midpoint of path 3.Draw bisector perpendicular to path 4.Label Bisector with station names 5.Determine which side of the bisector was close to the event. 6.Repeat with TATO/YAK 7.Repeat with WAKE/YAK 8.Repeat with additional stations of your choice

Solution

Compare to regional seismicity

Earth is Complex PREM Model

Earth is Complex PREM Model

Solution to complexity Iterative approach – Solution through the model provides result – Result is run back through the model to see if it matches the observations (forward problem) – Differences used to refine the model to compute a new solution. – Process continues until some condition is met

In reality More data More complex models Iterative approach