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Fault Dynamics of theApril 6, 2009 L'Aquila, Italy

Earthquake Sequence

Robert B. HerrmannSaint Louis University

Luca MalagniniINGV, Roma

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Overview• Determined moment tensor solutions

for 102 of 160 earthquakes with ML ≥ 3

• Developed regional crustal model that fits ground velocities well in the 0.02 – 0.20 Hz frequency band

• Evaluated INGV ML and source depth

• Noted stations that are difficult to fit• Make recommendations on digital

network

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Moment Tensor Solution

• Initially used WUS model• Developed regional velocity model

from–DSS study– Love and Rayleigh wave dispersion from

L'Aquila aftershocks–Receiver functions

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Group Velocity Dispersion

Love and Rayleigh group velocity observations and fits by models nnCIA (based only on dispersion) and ACI (based on joint inversion of dispersion and receiver function)

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Receiver function fits at AQU using ACI model

• Used iterative deconvolution

• Used filter α = 0.5 and 1.0

• Reverberations due to surface low-velocity and crustal velocity inversions

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• Group velocities require low velocities near the surface.

• Receiver functions provide more detail on crust.

• Both models fit waveforms very well in the 0.02 – 0.20 Hz band in for the 0 – 150 km epicentral distance range.

• At these frequencies, discontinuities are averaged

• We used the nnCIA model for all inversions

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Waveform fitsComparison of observed and model predicted waveforms for earthquake of 20090423151408

STK=345, DIP-30RAKE=-50, MW=3.84

•Each trace pair is plotted to the same scale: observed is red, blue is predicted

•All traces represent filtered velocity in the 0.02 – 0.10 Hz band

•Waveform shapes are excellent

•Some stations have problems and were not used for moment tensor

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Moment Tensor Solutions

• M < 3 (black for seismicity – no inversion)

• 3 < M < 4 (blue)

• 4 < M < 5 (blue-green)

• 5 < M < 5.5 (yellow)

• 5.5 < M (red) (IDIDE locations and magnitudes)

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Moment Tensorscolored by depth

• Main Mw=6.25 plotted at INGV epicenter not at centroid

• Northern group shows a pattern of down-dip to SE

• Southern group also shows same pattern

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Moment Tensorscoded P-T axis

• P-axis (open)• T-axis (solid)• Southeastern group

has more variability in orientation of P- axis. Also P-axis is not very vertical, thus nodal planes have dip ≠ 45°

• T-axes are quite uniform

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Comparison of Moment Tensor and INGV Depths and Magnitudes

Comparison of Depths Comparison of magnitudes

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Source Complexity• We were able to get the moment

tensor solution for the main event by using the frequency band 0.01 – 0.025 Hz

• This was strange since we used 0.01 – 0.05 Hz for the Mw=5.91 2008/02/21 Wells, Nevada earthquake

• Using a 0.01 – 0.05 band gave an unrealistic depth of 29 km for L'Aquila

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• The deep depth was forced by the lack of high frequency in the regional signal, and since this was dominated by surface-waves, this can be caused by a greater depth,

• Or by something in the source that removes high frequencies, such as a double event: if two identical events are separated by X seconds, there will be a spectral hole at 2X seconds

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Source deconvolution using empirical Green's function

• Use small event with same mechanism and location as main event

• Use iterative time-domain deconvolution

• Plot versus azimuth

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Comparison

• Wells, Nevada– 20080221141605 (Mw = 5.88)– 20080228151039 (Mw = 3.98)

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Comparison

• Laquila– 20090406013239 (Mw = 6.25)– 20090406035645 (Mw = 4.26)

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Wells

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L'Aquila

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• L'Aquila earthquake is more complex than Wells

• Azimuthal coverage is not very good though

• Speculation – is source complexity reason for low ML?

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Network Performance

• Excellent data through ISIDe• World class broadband network• Data drops–No problem for moment tensor inversion

because there are many other good stations

• Station responses–Never used CAMP since gains seem to

be afactor of 2-3 too large–Never used TRTR because of gain and

waveform shape – local site effect?

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– VVLD low frequency sensor noise/instability

–CERT – station gain too low?– LNSS – low frequency noise–RNI2 ?

To process the moment tensors rapidly, we did not keep a complete list of problem stations. CAMP was often the nearest station and would have been very useful for smaller earthquakes

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Observations

• Moment tensors can be done in near-real time by an analyst. All codes and Green's functions are set up and easy to use.

• The main event seems may be a multiple source. The lack of on-scale data at distances < 50 km, and for many < 100, makes study of main event difficult

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• Rapid finite fault inversion may be useful for estimating where significant aftershocks may occur. –Most moment tensor solutions are on

edge of region of the initial rupture.

• Real-time finite fault inversion requires on-scale data. Perhaps install continuous low-gain seismic channels (e.g., Episensor) at all or every second or third broadband station.

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Summary

• We compiled a very complete catalog of moment tensors down to M=3

• The pre-computed Green's functions can be used for other earthquakes in the country