[email protected] SEISMIC SITE EFFECTS IN NORCIA BASIN (CENTRAL ITALY) Istituto Nazionale di...

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SEISMIC SITE EFFECTS IN NORCIA BASIN SEISMIC SITE EFFECTS IN NORCIA BASIN

(CENTRAL ITALY)(CENTRAL ITALY)

Istituto Nazionale di Geofisica e Vulcanologia, sezione Milano - PaviaIstituto Nazionale di Geofisica e Vulcanologia, sezione Milano - Pavia

Istituto Nazionale di Geofisica e Vulcanologia

30° convegno nazionale GNGTS, Trieste, 14-17 novembre 2011

Luzi L., Luzi L., Massa M.Massa M., Puglia R., Pacor F., Puglia R., Pacor F.

[email protected] 30° convegno nazionale GNGTS, Trieste, 14-17 novembre 2011

SEISMIC SITE EFFECTS IN NORCIA BASIN (CENTRAL ITALY)SEISMIC SITE EFFECTS IN NORCIA BASIN (CENTRAL ITALY)

Earthquakes data set from January to April 2009

Spectral analyses on noise and seismic events

Detection of surface waves and polarization phenomena

DPC-S4 Project 2007-2009, coordinated by INGV-MIAnomalous response of ITACA sites

Active geophysical surveys

Passive seismic investigations (in collaboration to GFZ, Potsdam, Germany)


Seismic hazard for Norcia (OPCM 3519, 28/04/2006) 10% of probability (T 475 yr) to exceede PGA (on rock site) in the next 50 yr between 0.250 and 0.275 g (http://esse1-gis.mi.ingv.it/)

Historical seismicity (Io >= 6)



Several RAN stations were installed in the basin. On the basis of EC8 provisions the installation sites were classified as B,C or D

NRC (since 1979) -> EC8 B category 360-800 m/s (Vs30) Di Alessandro et al., 2008 CL-VII Luzi et al., 2011 F-1 (HVSR amplitude 1.1 (+/- 0.5)

between station error/between station sigma evaluated introducing different site classification schemes in ad-hoc developped GMPE (from Bindi et al., BEE 2011)



Passive seismic investigation

• 1 microtremor array close to station 12 (15 Mark-1s, inter-distance from 4.6 to 83 m, sampling 200 Hz, 2h)

• Temporary velocimetric network (15 Mark-1s coupled to 24 bits Earth Data Logger, sampling 100 Hz)

Active geophysical survey

• 2 seismic reflection surveys

• 1 down-hole test close to station 14 (depth of 60 m)



Down-hole (Vs30 = 664 m/s)

Reflection profile

Microtremor array(Vs30 = 557 m/s)



Temporary seismic network (January-April 2009)15 velocimeters 15 Mark-1s coupled to 24 bits Earth Data Logger

Data setAbout 200 selected (S2N > 2.5) seismic events with 2 < ML < 5.9 and epicentral distance up to 100 km



Data processingLocation and ML were obtained from http://iside.rm.ingv.it/Automatic procedure to select the events based on STA/LTADeconvolution of signal using the sensor responseFiltering using a 2nd order a-causal Butterworth filter, 0.3-20 HzManual picking to select P and S arrivalsSelection of S-coda (twice the S wave travel time)FFT were calculated and smoothed using Konno and Omachi (1998) with b=40


http://iside.rm.ingv.it/


Signals : N = noise ; C = S-coda ; S = S phase

Stations: 04 = plain ; 01 = plain ; 11 = reference



SSR on S-phase for the stations installed in the basin (0.8 Hz, direction N°140)



SSR on S-phase for the stations installed on alluvial fan (2 Hz, direction N°140)


Vertical SSR for station installed in the basin

Vertical SSR for station installed on alluvial fan




0

5

10

15

20

0 5 10 15 20

Amplitude coda

Am

plitu

de S

-phase

HVSR

SSR

0

5

10

15

20

0 5 10 15 20

Amplitude HVSR

Am

plitu

de S

SR

Coda

S-phase

5

2

10 3

1

0.1

1

10

100

0.1 1 10 100f0

f0 S

-

HVSR

SSR

1

10

1 10f0

f0 S

-

SSR Vertical

SSR vs. HVSR and S-phase vs. codaS

-pah

se A

mp

litu

de

Coda amplitude HVSR Amplitude

Am

pli

tud

e S

SR

(H

)

f0 Coda

f 0 S

-ph

ase

f 0 S

-ph

ase

f0 Coda



Example of S-Transform for stations 1 and 5, installed in the basin 2009 04 06 (02:37:04)



Example of S-Transform for stations 6 and 11, installed on alluvial fan and limestone respectively - 2009 04 06 (02:37:04)



Esempio di Lag Cross Correlation da definire



SSRs for the stations installed in the basin (1,4,5,10,12) evidence an amplification peak around 0.8 Hz, with preferential direction of amplification N140°

SSRs for the station installed on alluvial fan (3,6,13,14,15) evidence a first amplification peak at 2 Hz (N140°) and a second peak around 6 Hz (N30°)

Due to vertical amplifications HVSRs and are not able to well highlight the amplification peak at low frequancy

Polarization effects appear more scattered if we consider results on coda

Also vertical SSRs highlight amplification for stations installed in the basin, even if at frequency slightly higher (1.5 Hz vs. 0.8 and 3.5 Hz vs. 2)

Selecting a sub set not including L’Aquila events the low frequancy peak (0.8 Hz) is presenr again but it is characterized by low amplitude values

The ananlyses computed by S-Transform highlight that the surface waves have a large contribution of Love wave (non torna con quello che dice GFZ)


Thank you for the attention !

[email protected]

30° convegno nazionale GNGTS, Trieste, 14-17 novembre 2011

Extra slides

HVSR for the reference stations (on S-phase)

[email protected] SEISMIC SITE EFFECTS IN NORCIA BASIN (CENTRAL ITALY) Istituto Nazionale di...

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