Structure: Sensitivity to crustal models
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The seismic modeling for the source depends to second order on the seismic velocity
structure used to calculate the Greens functions. In order to test the
sensitivity of the solution to the chosen crustal model we picked out two earthquakes
(events 01-138b and 02-117) with epicentral locations in the Eastern and Western
Sierras Pampeanas, respectively. For each seismic event, we performed a grid-search for
the crustal thickness, P-wave crust velocity, and crustal Vp/Vs ratio using the SMTI
method described previously. First, we constructed the Greens functions for event 01-138b with magnitude Mw=4.3 located near Córdoba for a set of simple average models consisted of a crustal layer over a halfspace. The models had a crustal layer of thickness varying from 25 through 65 km in steps of 5 km, P-wave crustal velocity varying from 5.8 to 6.6 km/s in steps of 0.2 km/s and Vp/Vs ratio fixed at 1.70. We maintained the P-wave velocity at 8.15 km/s for the halfspace with a Vp/Vs ratio of 1.80. We ran an inversion matching synthetic and observed seismograms using a series of source depths for each crustal model. We used a band-pass filter between 15-30 seconds. Accepting misfit-errors rms < 0.25 constrains the crustal thickness to be less than 45 km and P-wave crustal velocities around 6.0 – 6.2 km/s. The misfit increases rapidly (> 75 %) using a high crustal Vp and/or a thickened crust. The second step consisted of fixing Vp at 6.0 km/s and initiating a grid-search around crustal thickness and Vp/Vs ratio using the SMTI for the same earthquake. This means that only the S-wave velocity and crustal thickness varied. We ran an inversion for each model of crustal thickness varying between 25 and 65 km in steps of 5 km and Vp/Vs ratio varying from 1.60 to 1.85 in steps of 0.05. Again, changes in Vp/Vs ratio correspond to S-wave crustal velocity variations only. No modifications were made in the halfspace described above. |
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Therefore, models using a low crustal Vp of 6.0 – 6.2 km/s, low Vp/Vs ratio of 1.65 – 1.70,
and a crustal thickness corresponding to a normal crust (30 km) or at least no thicker
than 45 km predict the best results. Nevertheless, the seismic raypaths for the earthquake
considered (event 01-138b) recorded at the CHARGE seismic stations used in the inversion
test traveled mostly through the Eastern Sierras Pampeanas. The focal
mechanism solutions did not change significantly around the best depth and
remained stable for the entire range of parameters tested. The same grid-search process was carried out for event 02-117 with magnitude Mw=5.1 and epicenter location in the Western Sierras Pampeanas. For this test, we only considered stations located in the western part of the CHARGE network region. We started testing crustal models of P-wave crustal velocity varying from 5.80 to 6.60 km/s in steps of 0.2 km/s, Vp/Vs=1.80 and thickness varying from 25 to 65 km in steps of 5 km. We maintained the mantle parameters Vp=8.15 km/s and Vp/Vs=1.80. We ran an inversion for each model and a set of fixed source depths using a band-pass filter of 15-50 seconds. Figure 7a shows a map of the SMTI minimum amplitude misfit-errors results for each crustal model. The best fit (> 75%) occurs around crustal Vp=6.40km/s and thickness of 45-52 km. Figure 7b shows SMTI results for a grid-search around Vp/Vs and crustal thickness for the same event. We tested models fixing crustal P-wave velocity at 6.20 km/s and varying crustal S-wave velocity to obtain Vp/Vs ratios in the range 1.60 - 1.85 in steps of 0.05. The crustal thickness varied from 25 to 65 km in steps of 5 km. The mantle parameters were the same as before. In contrast to the test results for the event in the Eastern Sierras Pampeanas, the best results for event 02-117 occur when considering a crustal structure with Vp/Vs greater than 1.80 and thickness between 40 km and 57 km. Figure 7d shows examples of the amplitude misfit errors and CLVD versus source depth for a set of 45-km thick crust models. We conclude that the best results are observed using a higher Vp/Vs (1.80-1.85) if the raypaths travel in the western Sierras Pampeanas geologic terranes. |
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Latest revision: January 9, 2005 |