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The 2009 L’Aquila Earthquake
Richard Walters- Oxford University -
John Elliott, Marcus Bell, Nicola D’Agostino, Philip England, Ingrid Hunstad, James Jackson, Barry Parsons, Richard Phillips, Gerald Roberts
Ken McCaffrey, Max Wilkinson, Tim Wright
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(LAK-KWILL-A) Name, PhD Student at Oxford Talk to you today about the recent earthquake in central Italy. First of all thank you to collaborators at COMET, and Gerald (UCL), Richard (Edin), Nicola and Ingrid (INGV)
L’Aquila Earthquake
L’Aquila
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Peninsular Italy currently undergoing NE-SW extension. Expressed as normal faulting in the Apennines, mountain belt runs down the Italian peninsula Mediaeval city, capital of Abruzzo region History of earthquakes M 6.3 Normal Faulting Earthquake April 06, 2009 ~300 Killed, lots of damage
L’Aquila Region & Faults
Campo Imperatore FaultAssergi Fault
Paganica FaultL’AQUILA
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Assortment of faults Early seismology solutions for the earthquake suggest normal fault striking either NW or SE. 3 focal mechanism determinations for main shock AFTERSHOCKS bottom to top 5.5, 5.4, 5.2 on 9th 7th 7th. L’Aquila and Campo Imperatore faults bound the Gran Sasso range- highest topo in peninsular Italy. Largest Fault and initially thought to be potential source. Soon after the earthquake it became clear from InSAR that neither of these faults were to blame.
SeismologyStrike = 126
Dip = 52Rake = -104
Centroid = 4 kmMw = 6.25
Length = 12 kmTop depth = 0 km
Bottom depth = 9 kmSlip = 0.6 m
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Long period body wave modelling (James Jackson) produces a similar solution. If we assume a SW dipping fault plane then we get following parameters Small right lateral component Using scaling relationships can estimate…
IncoherenceIncoherence
DownDown
UpUp
Strike = 144, Dip = 54, Rake = -105, Slip = 0.7 m, Length = 12 km, Top = 3 km, Bottom = 12 km
Mw = 6.23
10 km RMS = 1.18 cm
RMS = 1.04 cm
i = 23°
along track
i = 23°
along track
090201-090412
090311-090415
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We use InSAR to model the earthquake. Data (left) for desc/asc from ENVISAT. Top Track 79, dates 090201-090412. Bottom 129 090311-090415 All processed with roi_pac Fringe =2.8cm, up 8 down 25 etc. Incoherence = vegetation and snow Modelled as rectangular dislocation in an elastic halfspace (i.e. Okada) for uniform slip, residuals smaller than 1 fringe Results: right lateral, doesn’t rupture surface, largely agrees with seismology
Slip distribution
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To give slightly better solution, fix fault geometry to results of uniform model and invert for variable slip (fixed rake) Fault does reach surface Errors.
10 km RMS = 0.98 cm
RMS = 0.88 cm
i = 23°
along track
i = 23°
along track
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Results from distributed slip model
Surface ruptures
Gerald Roberts
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Field Observations – only several cm offsets Typical surface ruptures, not particularly impressive
Surface ruptures
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The surface ruptures coincide well with the surface projection of our uniform slip fault
Geomorphology
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River Incision, sharp change in slope- overall subtle features in comparison to L’Aquila fault. Although Paganica fault was known, had been neglected in the literature and implied to be less recently active than the other faults. This earthquake highlights that we shouldn’t be working on the assumption that the faults associated with the largest topographic features are necessarily those with the largest seismogenic potential
Stress on Other Faults
Lago di Campotosto
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Model surrounding faults as planes that fail according to a coulomb failure criterion Surrounding Faults along strike loaded, whereas those sub-parallel and offset perpendicular to the fault are brought further Reservoir & Towns north; historic towns Campotosto and Amatrice Fault planes dip away from black lines. Describe equations Aftershocks on Campotosto fault
Seismic strain deficit
Deficit of
~2.3×1019 Nm
2.3×1019 Nm – 0.4×1019 Nm= 1.9×1019 Nm
Hunstad et al. 2003, GRL
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Geodetic strain rates for 126 years calculated from GPS reoccupation of first-order triangulation network. Calculate the amount of earthquakes we would expect for this 126 year strain and compare to the historical record of earthquakes. They found a deficit- particularly large in L’Aquila region; if it was all released seismically equivalent to a M6.8 earthquake. EXPENDABLE
Postseismic Deformation
T129 090415-090520
Slip distribution (m)
2 sigma errors (m)
0.1
0
0.04
0.02
0.08
0.06
Marcus Bell
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Work done by Marcus Bell at Oxford Here is the data. 3 signals that could be related to afterslip on the fault. Problem is that these signals are of similar magnitude to the atmospheric signals. If we compare the location and coverage of these signals to topography can see that one correlates with an intermontane basin, whilst two others show no clear correlation with topography. Model these data as afterslip on the same fault plane as for coseismic We find 3 slip patches that correspond to the 3 patches. 1 sigma formal errors found same as for coseismic; essentially as large as the signals, except for 1? Note the change in scale. Formally it would be difficult to say that these signals are definite. BUT spatial correlation with the fault, and elongate along the fault AND very sharp phase gradient AND not correlated with topographic features IN ADDITION a similar area of postseismic subsidence has been observed with both GPS and laser scanning data. Also persistent in 3 independent interferograms. It is likely that this is REAL