Macroseismic Survey Research Articles

Preliminary results of a macroseismic survey of the Colfiorito sequence (Central Italy)

Umbria-Marche region (Central Italy) has been interested by a seismic sequence, which caused a large amount of damage within an extensive area (around 5000 km2). The sequence produced eight shocks with magnitude higher than 5.0, the largest of them occurring on September 26, UTC 09:40, M1 = 5.8. The incidence of many shocks with a magnitude higher than 4.0 contributed to the creation of a damage pattern that was in evolution for more than one month. Such seismic behavior motivated field operators to perform a real-time macroseismic survey to update the data set. One of the major objectives was to differentiate the effects pertaining to each of the largest shocks. In this paper we present the macroseismic survey performed during the sequence, along with some preliminary results inferred. Intensity points of seven of the main shocks are shown, together with the filtered macroseismic fields produced following the application of specific statistical methods. Finally, consideration is given to the comparison of these study results with corresponding tectonic and geological data.

Distribution of effects in the urban area of Rome During the October 14, 1997 Umbria Marche earthquake

In the last years many studies were carried out in order to assess the correlation between nearsurface geology and seismic response of downtown Rome during local and regional earthquakes. Nevertheless, only historical macroseismic data and theoretical modelling could be used so far to estimate the ground motion in the historical part of the city. The occurrence of the October 14, 1997 Umbria Marche event (Mw = 5.7), located about 115 km far from Rome but largely felt in the city, allowed us to verify our knowledge of the seismic response on the basis of the present urban setting of Rome.A prompt macroseismic survey in the urban area within the ring-road (G.R.A.) was performed. 949 questionnaires were compiled by means of 1842 interviews: such a large amount of macroseismic information, for the first time available in Rome, was related to 669 observation points providing a picture of local intensity variations. The areal distribution of collected intensity points is quite homogeneous.The data analysis points out a preferential distribution of larger effects (65%) in correspondence of Holocene alluvium outcrops. Furthermore, a tight correspondence between remarkable intensities and alluvial sediments is pointed out in suburban areas characterized by the minor hydrographic network of the Tiber River. Macroseismic anomalies were outlined in sectors of the urban area where local hydrogeological conditions can be responsible for a significant deterioration of geomechanical properties.

From questionnaires to intensities — Assessing free-form macroseismic data in the UK

Questionnaires for macroseismic surveys can generally be categorised into two groups: structured questionnaires, where respondents are given a number of options and have to tick one, and free-form questionnaires, where open-ended questions are answered by the respondents in their own words. Both have advantages and disadvantages. The structured approach is easier for the seismologist to process and is more focussed on the answers he wants to receive. The free-form approach is better at preserving nuances and qualifications to the respondents' answers that might otherwise be lost. Free-form questionnaires can be a lot

Macroseismic evidence and site effects for the Lunigiana (Italy) 1995 Earthquake

On October 10, 1995, an ML= 4.8 (ING) earthquake occurred in the region of Lunigiana (northwestern Italy). The shock was felt over a large area and produced significant damage. We performed a macroseismic survey and damage zonation and assessed a maximum intensity VII MCS in the epicentral area. The damage pattern, that we investigated in detail for some of the villages in the most heavily damaged area, emphasises the role of surface geology in amplifying the effects. Topographic effects and near-surface geology are largely responsible for broadening the damage area. Given the moderate size of the earthquake, many of the macroseismic observations, including rotations of objects and the propagation of visible waves in the ground, are suggestive of amplification phenomena.

The Malawi earthquake of March 10, 1989: A report of the macroseismic survey

The Malawi earthquake of March 10, 1989: A report of the macroseismic survey

The Bishop's Castle (UK) Earthquake of 2 April 1990

ABSTRACTA large earthquake, by British standards, occurred near Bishop's Castle in the Welsh Borders on 2 April 1990 at 13:46 GMT. This magnitude 5.1 ML event was felt over a wide area of Britain, from Ayrshire in the north to Cornwall in the south, Kent in the east and Dublin in the west. The epicentre was near the village of Clun, 7 km SSW of Bishop's Castle. Damage was minor and limited to the epicentral area, north to Wrexham and in particular Shrewsbury, which suffered most. Results from a macroseismic survey by BGS revealed that the maximum intensity in the epicentral area was 6 MSK.The mainshock had a focal depth of 14.3±4.7 km; however, better located aftershocks further constrained the mid‐crustal seismicity to 15±0.2 km in the best cases. The marked lack of aftershocks contrasts with some previous similar magnitude events for intraplate earthquakes in Britain and throughout the world and may represent a large stress drop due to almost total relief of strain energy by the mainshock. The aftershock epicentral distribution shows a preference for an approximately N‐S orientation which is consistent with one of the focal planes of the mainshock focal mechanism and suggests that this is the fault plane. Movement on this plane was predominantly strike‐slip with a component of thrust and was consistent with a maximum compressive stress axis orientated NW‐SE. The NE striking Welsh Borderland Fault System dominates the epicentral area; however, there is no surface fault which can clearly be related to the seismicity.

BOOK REVIEWS

Book reviewed in this article: Drought and Hunger in Africa. Denying Famine a Future, edited by M.H. Glantz. Mallet's macroseismic survey of the Neapolitan earthquake of 16th December 1857, edited by E. Guidoboni and G. Ferrari. Behaviour and Poverty in Bangladesh, by Clarence Maloney.

The October 4th, 1983 — Magnitude 4 earthquake in Phlegraean Fields: Macroseismic survey

On Oct. 4th, 1983 the area of Phlegraean Fields, near Naples (Southern Italy) was shaked by an earthquake of magnitude (M L) 4.0 that caused some damage in the town of Pozzuoli and its surroundings. This seismic event was the largest one recorded during the recent (1982–84) inflation episode occurred in the Phlegraean volcanic area, and a detailed macroseismic reconstruction of the event was carried out. Failing macroseismic data on other earthquakes occurred in Phlegraean Fields, the attenuation law of the intensity as a function of the distance as obtained for the Oct. 4th earthquake was compared with those obtained for other volcanic areas in central Italy —i.e., Tolfa, Monte Amiata — in order to check the reliability of the results obtained for Phlegraean Fields. The Blake's model of the earthquake of Oct. 4th, 1983 does not agree with the experimental data because isoseismals contain areas larger than those shown by the model. This result has been interpreted as an effect of energy focusing due to a reflecting layer 6–8 km deep.