Download Presentation #18

Effect of Velocity Models on
the Accuracy of Earthquake
Hypocenters
Sudipta Sarkar, Youshun Sun, M. Nafi Toksöz
Earth Resources Laboratory
Massachusetts Institute of Technology
USA
Big Picture
Earthquake hypocenters could be in
error by tens of kilometers
Several factors contribute to errors
– Poor station coverage
– Phase reading errors
– Poor knowledge of velocity
– Lateral variations of velocity
Outline
Earthquake location errors due to errors in
velocity models: Examples I.
Local scale
II. Regional scale
Improving location accuracy by using a 3D
velocity model
China case
A practical approach to improving location
accuracy
Source Specific Station correction
Applicability to the Gulf region
Conclusions
Velocity Model and Depth of Events
Vp (m/s)
0
1000
2000
3000
4000
5000
6000
7000
0
500
1000
Depth (m)
1500
2000
Vel1
2500
3000
3500
4000
Velocity Model and Depth of Events
Vp (m/s)
0
1000
2000
3000
4000
5000
6000
7000
0
500
1000
Depth (m)
1500
2000
Vel1
Vel2
2500
3000
3500
4000
Velocity Model and Depth of Events
Vp (m/s)
0
1000
2000
3000
4000
5000
6000
7000
0
500
1000
Depth (m)
1500
2000
Vel1
Vel2
Vel3
2500
3000
3500
4000
Depth Error in Local Events Due to
Incorrect Velocity Model
Vel #1
Vel #2
Vel #3
Location Errors Due to Lateral
Variation of Velocity
Our experience in CHINA
1D
(km)
AK135
(km)
11 Events
(Explosions)
10
20
Earthquake
12
20
Location Errors Reduced by
Introducing a 3D Model
Our experience in CHINA
3D 1D
(km) (km)
AK135
(km)
11 Events
(Explosions)
1
10
20
Earthquake
3.5
12
20
Event Relocation Using 3D Travel
Times: A Regional Study in China
Method
Determine 3-D, regional velocity
structure in crust and upper mantle
(tomography)
Use 3-D velocity / travel time grids to
locate events
A Regional Study in China
A Regional Study in China
A Regional Study in China
Location Errors Reduced by
Introducing a 3D Model
Our experience in CHINA
3D 1D
(km) (km)
AK135
(km)
11 Events
(Explosions)
1
10
20
Earthquake
3.5
12
20
Station Distribution and Events in
the Gulf
Prospects of 3D Tomography and
Event Location in the Gulf
A Practical Approach
Doing true 3D locations is
computationally expensive and most
often not possible!
A practical solution to this problem –
use Source Specific Station Corrections.
SCij  tt  tt
3D
ij
ref
ij
Procedure
Use 3-D velocity models to calculate
travel times from all potential source
regions to all stations
Determine source specific station
corrections relative to reference model
used for locations
Determine an initial location
Apply the station correction to times at
each station specific for the source
region
Relocate the event
Conclusions
Local and regional seismic networks are
improving rapidly
Accurate event locations with these networks
require accurate 3D velocity models
Such velocity models can be derived through
regional seismic tomography
3D travel times can be implemented through
source specific travel time corrections
New networks in the Gulf region provides an
opportunity for 3D velocity models and more
accurate locations