The August,24 01:36:32 (UTC) Mw 6.2 Earthquake at Rieti, Central Italy

Informative Notes

RISSC Lab

University of Naples Federico II

Earthquake Location

Origin Time: 24-08-2016

01:36:32 (UTC)

Magnitude: Mw 6.0 (moment

magnitude, source INGV); Mw

6.2 (moment magnitude, source

EMSC)

Geographical Coordinates:

42.7, 13.24

Depth: 4 km (source INGV). ; 8

km (source IPGP, Paris)

Recent & Historical Seismicity

Epicenters of eqs with magnitude >= 2.5 since 1985 (source INGV). White star indicate the Mw 6.2, 2016/08/24 mainshock

Mw 6.2

Historical earthquakes (red squares) in the area (1000-2006) . The largest event in the proximity of the 2016 mainshock occurred in 639 with estimated magnitude M 6.2 (source CPTI, INGV

Geological Setting 1/2

Localisation du choc principal et de la forte réplique de 02h33 par rapport au réseau de failles des Apennins de l'Italie Centrale, avec les crises sismiques proches vers le Nord (1979 Norcia; 1993 Umbria-Marche, Colfiorito; 1984 Perugia) (Lucilla Benedetti, CNRS, 24/08/2016)

Source: https://sites.google.com/site/cellulepostsismique/central-italy-24-august-2016-01h36utc-mw-6-2

The EQK epicenter & potential Faults

The Rieti eqk epicenter (white star) is plotted on the map of known seismogenic sources (yellow squares and lines) and active faults of the area. The earthquake nucleated nearby the SE end of the seismogenic source ITSI016 (Norcia Basin) and NW of the active fault ITCS028_67 Amatrice Accumoli 1. Source : DISS (Database of italian seismogenic sources – INGV)

Mw 6.0-6.2

Aftershocks 11 hours after the Mainshock

This figure shows map of aftershocks during the 11 hours following the RIETI event of 24/08/2016 01:32 (UTC). The events delineate a presumable fault zone with a NNW-SSE direction extending for a total length of about 28 km (source: ISIDE and DISS databases, INGV).

Aftershocks : 36 hours after the mainshock

This figure shows map of aftershocks during the 36 hours following the RIETI event of 24/08/2016 01:32 (UTC). The events delineate a presumable fault zone with a NNW-SSE direction extending for a total length of about 28 km . The blue zones indicate debated active faults. source: ISIDE and DISS databases, INGV)

EQK Impact: Shake- Maps

The Intensity shake map (MMI Mercalli Modified Intensity) is computed from observed and interpolated peak ground acceleration. The colors show the areal distribution of perceived shaking/potential damages produced by the main event.

The peak ground acceleration map is computed from the measured and interpolated PGA values recorded at INGV stations. The largest peak acceleration value (0.45g=4.5 m/s^2) has been recorded at the station of Arquata del Tronto, fron the National Accelerometric Network of the Department of Civil Protection, located at 11 km from the epicenter (source INGV)

Seismic Hazard Map

The largest magnitude events of the Rieti Earthquake sequence occurred within the highest hazard zone of the Central Apennines chain. The map shows with the darkest color the zone where the Peak Ground Acceleration of 0.25-0.275 g had a probability of 10% to be exceeded in 50 yrs.

Impact: Did you feel it?

The map of perceived earthquake shaking (according to the MCS-Mercalli Cancani Sieberg scale) as reconstructed from the online questionnaires (7544) compiled by the citizens and sent to the INGV server, updated at 24/08/2016 16:32

Source parameters 1: Magnitude & Focal Mechanism

- Compilation of fault plane solutions & Moment magnitudes determined by different agencies. Using regional seismic waveforms.

- All solutions indicate a normal-fault mechanism (dominant extensional stress regime), with plane strikes oriented along the Apenninic direction and SW-NE horizontal tensional axis .

- Magnitude estimation varies between Mw 6.1 and 6.7, with an average value of Mw 6.3.

- Centroid Depth estimations varies between 5 and 15 km, with an average of 10 km.

Source parameters 2: Rupture duration & length

The source time function illustrates the release of average seismic moment as a function of the time during the earthquake rupture.

The figure shows the solution obtained by Martin Vallee (IPGP, Paris) using the code SCARDEC applied to regional and teleseismic waveforms.

The source time function of the Rieti eqk shows a total rupture duration of 12 sec with an interval of dominant moment realease of about 8 sec. Assuming a rupture velocity of 2.8-3 km/sec , this implies a minimum rupture length of 22-24 km. This is consistent with the preliminary spatial extent of the aftershock area (slide 7).

Source parameters 2: Average fault-slip and stress release

The average fault-slip can be determined by using the Wells & Coppersmith(1994)’ empirical relations (see figures) either assuming the moment-magnitude or the rupture length as estimated by the source time function. The static stress drop is derived from the theoretical scaling relations for rectangular fault models (e.g. Madariaga, 1977). The table below summarizes these preliminary estimations:

Mw Length(km) Width (km) Average Slip (m) Stress drop (Mpa)

Known Mw 6.2-6.3 17-19 9-10 0.7-0.8 2.3-2.4

Known rupture length

6.4 22-24 11 0.8-0.9 2.3

References & Credits

• Donald L. Wells and Kevin J. Coppersmith, 1994. New Empirical Relationships among Magnitude, Rupture Length, Rupture Width, Rupture Area, and Surface Displacement. Bull. Seismol. Soc. Am. 84, 974–1002.

• R. Madariaga, 1977. Implications of Stress-Drop Models of Earthquakes for the Inversion of Stress Drop from Seismic Observations. PAGEOPH 115, 301–316.

• Vallée, M., J. Charléty, A.M.G. Ferreira, B. Delouis, and J. Vergoz, SCARDEC : a new technique for the rapid determination of seismic moment magnitude, focal mechanism and source time functions for large earthquakes using body wave deconvolution, Geophys. J. Int., vol.184, pp.338-358, 2011.

• Database of Individual Seismogenic Sources (DISS) http://diss.rm.ingv.it/diss/ • Italian Seismological Instrumental and Parametric Database (ISIDE)

http://iside.rm.ingv.it/iside/standard/index.jsp • CSEM-EMSC http://www.emsc-csem.org/#2 • Hai sentito il terremoto ? http://www.haisentitoilterremoto.it/xml-

server.php?item=quest&action=compile&event_id=7075961&output=int • INGV – Shake Map http://shakemap.rm.ingv.it/shake/index.html • Benedetti (1999)