Site-Dependent Amplification on Topography during the 2016 Amatrice Seismic Sequence, Central Italy
Marta PischiuttaRodolfo PugliaPaola BordoniSara LovatiGiovanna CultreraAlessia MercuriAntonio FodarellaMarco MassaEzio D’Alema
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ABSTRACT Following the Mw 6.0 Amatrice earthquake on 24 August 2016 in central Italy, the Emersito task force of the Istituto Nazionale di Geofisica e Vulcanologia installed a temporary seismic network focusing on the investigation of amplification effects at municipalities located on topographic reliefs. Fourteen stations were installed at three sites: Amandola, Civitella del Tronto, and Montereale. During the operational period, stations recorded about 150 earthquakes with Mw up to 4.7. Recorded signals were analyzed calculating the horizontal-to-vertical spectral ratios at single station, using both ambient noise and earthquake waveforms, as well as standard spectral ratios (SSRs) to a reference site. To robustly estimate site amplification at each station of the site amplification effect at each station, the influence of backazimuth and epicentral distance is investigated. With the aim of reproducing the observed amplification pattern, 2D numerical simulations were performed on a section orthogonal to the topography major axis, constrained through in situ geological investigations and geophysical surveys. Although at Montereale site no clear amplification effects were observed, at Amandola site, all stations on the relief consistently detected significant peaks at about 4 Hz and along N120–150 azimuth. At Civitella del Tronto, a proper reference station is missing, implying a misleading of site response evaluation in terms of SSRs. Moreover, even if all stations show amplification in the frequency band 1–3 Hz, the direction of the maximum amplification varies from northeast to northwest. At the three sites, observations were successfully reproduced by 2D numerical models, the latter suggesting that topography alone cannot reproduce data, and the interplay with subsoil velocity structure is needed to produce a clear amplification effect. We conclude that according to the previous articles, rather than the sole topography convex shape, the geophysical structure has often a predominant role in controlling the observed amplification pattern on topography.Keywords:
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The March 31, 2002, Hualien offshore earthquake (M L 6.8, Mw7.0) caused minor damage near its epicenter but significant damage in Taipei, about 110 km from the epicenter. The recorded PGA values around the Taipei basin vary in a factor of 5. The high‐PGA values in the southeastern part of the basin correspond to the most significant damage area during the earthquake. Strong ground‐motion recordings from this event recorded at stations around the Taipei basin exhibit several distinct large‐amplitude seismic arrivals that can be interpreted as the PmP, SmP and SmS phases. The largest amplitude of the critically reflected S‐wave from the Moho (SmS) and the amplification of soft shallow sediments unique for each site are responsible for the maximum peak ground motion and the significant damage in the Taipei basin during the Hualien offshore earthquake.
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Unprecedented hydroacoustic observations of the megathrust earthquake of 26 Dec, 2004 were afforded by a network of 5 small hydroacoustic arrays located in the Indian Ocean, at distances of 2800 to 7000 km from the epicenter. Each array recorded acoustic waves, called T waves, generated by this event. Analysis of a series of short time windows within the T wave coda shows that the receiver to source azimuth varies smoothly as a function of time, indicating that the apparent T wave source is not stationary. The apparent T wave source moves northward along the Sunda trench at an average velocity of 2 km/s, closely tracking event rupture. The hydroacoustic data suggest that the rupture proceeded in two distinct phases; initially it progressed northwest along the Sunda trench with a velocity of approximately 2.4 km/s. At 600 km from the epicenter the rupture slowed to approximately 1.5 km/s, as it continued to propagate to the northwest.
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Abstract An M 7.0 earthquake struck Lushan, Sichuan, on 20 April 2013. We attempt to reveal its rupture details around 1 Hz using back‐projecting teleseismic P waves. The results show that this event was spawned by a 20‐km‐long and 26‐s‐duration rupturing of the source, which occurred bilaterally during the first 4 s, and unilaterally afterwards. The biggest energy release area is located north of the epicenter. We compared the rupture details of the Wenchuan and Lushan earthquakes, which occurred on the same fault, and found three common characters in rupture direction, location of the biggest energy burst and multi sub‐event. Two sources ruptured along the northeast trending Longmenshan fault zone. The biggest energy release regions are both off the epicenter. The source included more than one sub‐event, with the second one having the biggest energy release. The total rupture length of the Wenchuan and Lushan earthquakes accounts for about two thirds of the entire Longmenshan fault zone.
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The rapid and precise determination of the azimuth information, purely based on the satellite positioning technology of global positioning system (GPS), is tested and evaluated by this paper. The main consideration of using GPS for the azimuth determination is expected to replace the traditional astronomical azimuth measurement, which is believed to be time-consuming and weather-dependent. A GPS approach is simply based on setting up the GPS receivers at the two ends of a baseline, recording and processing the phase observable, and converting its coordinate solution into an azimuth with the inverse geodetic formulas. This type of azimuth obtained by the GPS static solution has been assessed to be well consistent with the astronomical azimuth by a level of better than ±1″. The GPS kinematic mode of azimuth value, however, is biased from the standard value for a RMS error of ±9″. The correction of the deflection of the vertical, theoretically required by the geodetic and astronomical azimuth conversion, is also implemented and found to be only effective at the observation site where the ηtanφ value is higher than the estimation accuracy of the η value provided by a gravimetric geoid model.
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We use the back-projection method, with data recorded on the dense USArray network, to estimate the rupture propagation for the Mw 9.0 earthquake that occurred offshore of the Tohoku region, Japan. The results show a variable rupture propagation ranging from about 1.0 to 3.0 km/s for the high-frequency radiation. The rupture propagates over about 450 km in approximately 150 s. Based on the rupture speed and direction, the high-frequency source process can be divided into two parts. The first part has a relatively slow rupture speed of 1.0 to 1.5 km/s and propagates northwestward. In the second part, the rupture progresses southwestward starting with a slow speed of about 1.5 km/s and accelerating to about 3.0 km/s. We see three large pulses at 30 s, 80 s and 130 s. The first two, including the largest second pulse, were located 50 to 70 km northwest of the epicenter. The third occurred about 250 km southwest of the epicenter. The variability of rupture velocity may be associated with significant changes of physical properties along the fault plane. Areas of low/high rupture speed are associated with large/small energy releases on the fault plane. These variations may reflect the strength properties along the fault. Also, locations of the high-frequency radiation derived from the back-projection analysis are significantly different from the areas of very large slip for this earthquake.
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The epicenter of the Numadate Earthquake on March 28, 1914 (JMT) has been reported to be at 140°.9E, 39°.8N in the "Rika-Nenpyo", which is a standard reference on big earthquakes in Japan. However, this location is inconsistent with the distribution of damage or with other evidences. The epicenter is relocated in this paper based on the newly collected reports about damage as well as the S-P times at several stations. Both the macroseismic and instrumental results give the epicenter at 140°.4E, 39°.2E.
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