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Applying time-reverse-imaging techniques to locate ...crowe/EQprocesses/Horst... · We demonstrate...
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1 Geophysical Institute, Karlsruhe Institute of Technology, Hertzstrasse 16, D-76187 Karlsruhe 3 US Geological Survey, 525 S. Wilson Ave, Pasadena, CA 91106 [email protected] Applying time-reverse-imaging techniques to locate individual low- frequency earthquakes on the San Andreas fault near Cholame, CA Tobias Horstmann 1 , Rebecca M. Harrington 2 , Elizabeth S. Cochran 3 , David R. Shelly 4 [email protected] [email protected] 2 Earth & Planetary Sciences, Mcgill University, 3450 University St. Montreal, QC, Canada H3A 2A7 [email protected] 4 US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025-3561 Location of a M 1.4 earthquake Location of an individual LFE 1 Introduction In spite of the abundance of LFE observations in recent years, locating arrivals. Here we use time-reverse-imaging techniques that do not require identifying phase arrivals to locate individual low-frequency- earthquakes (LFEs) within tremor episodes on the San Andreas fault near Cholame, California. We propagate a time reversed seismic signal back through the subsur- energy is a maximum. The grid point and time window occupying the spatial median of cross-correlation values within 10% of the maximum value indicates the source location and origin time. Location errors are based on the spatial extent of all cross-correlation and vertical errors are on the order of 4 km and 3 km respecitively. A test using earthquake data shows that the method produces an identi- cal hypocentral location (within errors) to that obtained by standard ray-tracing methods. We check the locations determined here with an LFE catalog determined by stacking hundreds of LFEs [Shelly and Har- debeck, 2010]. The LFE catalog uses stacks of at least several hundred templates to identify phase arrivals used to estimate the location. We - verse-imaging technique are within ~4 km of the catalog LFE locations [Shelly and Hardebeck, 2010]. LFEs locate at depths between 15-25 km, similar focal depths to previously published locations of LFEs or tremor in the region. Overall, the method can provide robust locations of individual LFEs without identifying and stacking hundreds of LFE templates, and is more accurate than envelope location methods with errors on the order of tens of km [Horstmann et al., 2013]. Location errors may be further reduced with increased velocity model resolution. 2 Data & Methods Figure 1: Station distribution We use continuous broadband waveforms from the PERMIT array (KIT stations), HRSN and PBO borehole sta- tions, and stations from the BK and NC networks. Cholame Shandon PAGB PHOB B072 B076 B079 ° ° ° ° ° ° ° ° ° ° road SAF surface station borehole station Cholame Parkfield Shandon MiniArray 1 MiniArray 2 MiniArray 3 KIT10 KIT11 KIT12 KIT13 GHIB EADB FROB MMNB JCSB VCAB VARB PKD ° ° ° ° ° ° ° ° ° m k 5 4 3 2 1 0 1 KIT05 KIT06 KIT04 KIT02 KIT01 KIT03 KIT08 KIT09 KIT07 ° ° ° Bakersfield Santa Barbara Santa Maria Parkfield Paso Robles KIT Array 35.9° N 35.8° N 35.7° N 35.6° N 120.6° W 120.4° W 120.2° W 50 25 0 y in km x in km 0 4 0 5 10 m k n i h t p e d 0 12 seconds seconds 12 0 I II III 0 50 100 for each receiver 0 100 time step of back propagation cross- correlation with moving time window Figure 2: Schematic of the method We use the 3D velocity model of Thurber et al. [2006] interpolated to a grid spacing of 100m. Back-propagations are calculated using a 1. back-propagate time-reversed waveforms recorded at each station through the velocity model. 3. time-dependent curl functions are then cross-correlated in a moving time window. 4. highest cross-correlation value in space and time provides the tremor source location. We demonstrate the ability of time-reverse-ima- ging to locate a M 1.4 earthquake to within com- parable accuracy as determined by ray-tracing methods at station distances of up to 26 km. Further, we demonstrate the ability to locate an in- dividual LFE to within 4.3 km uncertainty. For comparison, the same individual LFEs were located using other methods and result in location errors of ~20 km. based on the TRI technique are within 4 km rela- tive to the LFE catalog of Shelly and Hardebeck [2010] which determines locations by stacking events. Hypocentral source locations of individual LFEs in a single LFE family show scattering around the stacked family location within error. Therefore, scattering in hypocentral distribution within the single LFE family located here is not resolvable using the TRI method and the current velocity model. Improved velocity model resolution or a higher signal-to-noise ratio would reduce error and improve location accuracy. 4 Conclusions 3 Results 5 10 15 KIT01 KIT11 Z X KIT12 Y 5 10 15 GHIB EADB MMNB Figure 3: Back-propagated seismograms Seismograms of a cataloged M 1.4 earthquake at selected stations with origin time 04:27:00, 06/24/2010, and epicenter at 35.85 N, 120.42 W. The window highlighted in red indicates the back-propagated waveform. Figure 4: M 1.4 hypocentral location (A) Map view and (B) fault plane view of the hypocentral location (red X) using time-reverse-imaging (TRI). Red lines show error range determined by the spatial mean values. Stations used shown by black triangles, and the surface trace of the San Andreas fault shown in yellow. 5 20 time in seconds 5 20 time in seconds S41B-2441 depth in km X in km Y in km 5 25 50 15 5 5 15 10 20 N 0.2 0.25 A B 0 5 km Figure 6: Individual LFE hypocentral location the red square indicates the location of a LFE family from Shelly and Hardebeck [2010] for comparison. Figure 5: LFE seismograms Seismograms of a LFE at selected stations recor- ded at 19:27:40, on 09/02/2010. The window highlighted in red indicates the back-propagated waveform. 0 10 20 KIT10 KIT11 Z X KIT01 Y 0 10 20 KIT13 EADB B901 0 25 time in seconds 0 25 time in seconds depth in km X in km Y in km 5 25 50 15 25 5 15 10 20 N 0.25 0.3 0.35 A B 0 5 km Location of multiple LFEs Figure 7: Individual LFE hypocentral locations Figure 8: Hypocentral locations of a single LFE family X in km depth in km Y in km 5 10 15 10 20 30 40 50 5 10 15 20 25 A B N 0 5 km X in km depth in km Y in km 5 10 15 10 20 30 40 50 5 10 15 20 25 A B N 0 5 km (A) Map view and (B) fault plane view of hypocentral locations for 35 individual LFEs (Figure 7) and 12 LFEs in a single LFE family (Figure 8). Red stars indicate source position, black lines show the estimated error range. Small blue dots mark the stacked LFE family solution of Shelly and Hardebeck [2010]. Black triangles denote all possible station positions and the yellow lines indicate the San Andreas Fault surface trace.
Transcript of Applying time-reverse-imaging techniques to locate ...crowe/EQprocesses/Horst... · We demonstrate...
1Geophysical Institute, Karlsruhe Institute of Technology, Hertzstrasse 16, D-76187 Karlsruhe
3US Geological Survey, 525 S. Wilson Ave, Pasadena, CA 91106
Applying time-reverse-imaging techniques to locate individual low-frequency earthquakes on the San Andreas fault near Cholame, CA
Tobias Horstmann1, Rebecca M. Harrington2, Elizabeth S. Cochran3, David R. Shelly4
[email protected]@usgs.gov
2Earth & Planetary Sciences, Mcgill University, 3450 University St. Montreal, QC, Canada H3A 2A7
[email protected] Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025-3561
Location of a M 1.4 earthquake
Location of an individual LFE
1 IntroductionIn spite of the abundance of LFE observations in recent years, locating
arrivals. Here we use time-reverse-imaging techniques that do not require identifying phase arrivals to locate individual low-frequency-earthquakes (LFEs) within tremor episodes on the San Andreas fault near Cholame, California.
We propagate a time reversed seismic signal back through the subsur-
energy is a maximum. The grid point and time window occupying the spatial median of cross-correlation values within 10% of the maximum value indicates the source location and origin time.
Location errors are based on the spatial extent of all cross-correlation
and vertical errors are on the order of 4 km and 3 km respecitively. A test using earthquake data shows that the method produces an identi-cal hypocentral location (within errors) to that obtained by standard ray-tracing methods. We check the locations determined here with an LFE catalog determined by stacking hundreds of LFEs [Shelly and Har-debeck, 2010]. The LFE catalog uses stacks of at least several hundred templates to identify phase arrivals used to estimate the location. We
-verse-imaging technique are within ~4 km of the catalog LFE locations [Shelly and Hardebeck, 2010]. LFEs locate at depths between 15-25 km, similar focal depths to previously published locations of LFEs or tremor in the region. Overall, the method can provide robust locations of individual LFEs without identifying and stacking hundreds of LFE templates, and is more accurate than envelope location methods with errors on the order of tens of km [Horstmann et al., 2013]. Location errors may be further reduced with increased velocity model resolution.
2 Data & Methods
Figure 1: Station distribution We use continuous broadband waveforms from the PERMIT array (KIT stations), HRSN and PBO borehole sta-tions, and stations from the BK and NC networks.
Cholame
Shandon
PAGB
PHOB B072
B076
B079
° ° ° ° ° °°
°
°
°
roadSAFsurface stationborehole station
Cholame
Parkfield
Shandon
MiniArray 1
MiniArray 2
MiniArray 3
KIT10 KIT11
KIT12
KIT13
GHIB
EADB FROB
MMNB
JCSB VCAB VARB
PKD
° ° ° ° °°
°
°
°
mk 5432101
KIT05KIT06
KIT04
KIT02 KIT01
KIT03
KIT08KIT09
KIT07
°
°
°
Bakersfield
Santa Barbara
Santa Maria
Parkfield
Paso Robles KIT Array
35.9° N
35.8° N
35.7° N
35.6° N120.6° W 120.4° W 120.2° W
50
25
0
y in km
x in km 0 4
0
5
10
mk ni htped
012
seco
nds
seco
nds
120
I
II
III
0 50 100time step of back propagation
for eachreceiver
0 100time step of back propagation
cross-correlationwith movingtime window
Figure 2: Schematic of the method
We use the 3D velocity model of Thurber et al. [2006] interpolated to a grid spacing of 100m. Back-propagations are calculated using a
1. back-propagate time-reversed waveforms recorded at each station through the velocity model.
3. time-dependent curl functions are then cross-correlated in a moving time window.
4. highest cross-correlation value in space and time provides the tremor source location.
We demonstrate the ability of time-reverse-ima-ging to locate a M 1.4 earthquake to within com-parable accuracy as determined by ray-tracing methods at station distances of up to 26 km.
Further, we demonstrate the ability to locate an in-dividual LFE to within 4.3 km uncertainty. For comparison, the same individual LFEs were located using other methods and result in location errors of ~20 km.
based on the TRI technique are within 4 km rela-tive to the LFE catalog of Shelly and Hardebeck [2010] which determines locations by stacking events.
Hypocentral source locations of individual LFEs in a single LFE family show scattering around the stacked family location within error. Therefore, scattering in hypocentral distribution within the single LFE family located here is not resolvable using the TRI method and the current velocity model. Improved velocity model resolution or a higher signal-to-noise ratio would reduce error and improve location accuracy.
4 Conclusions
3 Results
5 10 15
KIT01
KIT11
Z
X KIT12
Y
time in seconds
M14_EQ_withT
5 10 15
GHIB
EADB
MMNB
time in seconds
Figure 3: Back-propagated seismograms
Seismograms of a cataloged M 1.4 earthquake at selected stations with origin time 04:27:00, 06/24/2010, and epicenter at 35.85 N, 120.42 W. The window highlighted in red indicates the back-propagated waveform.
Figure 4: M 1.4 hypocentral location
(A) Map view and (B) fault plane view of the hypocentral location (red X) using time-reverse-imaging (TRI). Red
lines show error range determined by the spatial mean
values. Stations used shown by black triangles, and the surface trace of the San Andreas fault shown in yellow.
5 20time in seconds
5 20time in seconds
S41B-2441
dept
h in
km
X in
km
Y in km 52550
15
5
5
15
10
20 N
0.20.25
A
B
0 5 km
Figure 6: Individual LFE hypocentral location
the red square indicates the location of a LFE family from Shelly and Hardebeck [2010] for comparison.
Figure 5: LFE seismograms
Seismograms of a LFE at selected stations recor-ded at 19:27:40, on 09/02/2010. The window highlighted in red indicates the back-propagated waveform.
0 10 20
KIT10
KIT11
Z
X KIT01
Y
time in seconds
TR_19Dt
0 10 20
KIT13
EADB
B901
time in seconds0 25
time in seconds0 25
time in seconds
dept
h in
km
X in
km
Y in km 52550
15
25
5
15
10
20 N
0.250.30.35
A
B
0 5 km
Location of multiple LFEs
Figure 7: Individual LFE hypocentral locations Figure 8: Hypocentral locations of a single LFE family
X in
km
dept
h in
km
Y in km
5
10
15
1020304050
5
10
15
20
25
A
B
N0 5 km
X in
km
dept
h in
km
Y in km
5
10
15
1020304050
5
10
15
20
25
A
B
N0 5 km
(A) Map view and (B) fault plane view of hypocentral locations for 35 individual LFEs (Figure 7) and 12 LFEs in a single LFE family (Figure 8). Red stars indicate source position, black lines show the estimated error range. Small blue dots mark the stacked LFE family solution of Shelly and Hardebeck [2010]. Black triangles denote all possible station positions and the yellow lines indicate the San Andreas Fault surface trace.
