The 6 May 1947 Milwaukee, Wisconsin, Earthquake

The State of Wisconsin is not known for earthquake activity. The authoritative public-facing U.S. Geological Survey Comprehensive Catalog of earthquakes includes only three small (magnitude < 2) earthquakes in the state, all instrumentally recorded. Although other catalogs include more events in Wisconsin, experience has shown that many types of events, such as explosions and cryoseisms, have made their way into earthquake catalogs in this region. In this short report, I summarize available information about an earthquake that was felt in eastern Wisconsin at 15:27 local time on 6 May 1947. As what appears to be the largest historical earthquake in the State of Wisconsin, it is of public interest, its modest size notwithstanding. It appears that no useful instrumental records exist, due in part to a teleseismic event that occurred approximately 3 min later, generating surface waves that were recorded on early long-period instruments in the region. Instrumental data may exist for this event but have not been found. Comparing the felt area with information from recent earthquakes in the region, I estimate an intensity magnitude of 3.8 for the event, with a subjectively estimated uncertainty range 3.5–4.1. Relatively strong effects, including reports of broken dishes in Milwaukee, and shaking described as short but especially sharp, suggest that the event may have been among the sprinkling of shallow earthquakes now known to occur in the upper Great Lakes region.

Review of Historical Seismicity of West-Central Brazil: Newly Discovered Events and Implications for Seismic Hazard

ABSTRACT. The Pantanal basin seismic zone has experienced earthquakes with magnitudes up to 5.4 mb. New information on historical events was searched for in regional newspapers, especially the "Correio do Estado", Campo Grande, Mato Grosso do Sul State, Brazil. Several small new events, not previously reported in the Catalog of Berrocal et al. (1984), were discovered. Five events had new macroseismic information from different localities allowing magnitudes to be estimated from the felt area. Ten new events were also discovered, each one felt in a single locality. The event of 1906-Oct-24, with previous magnitude 4.2, was felt in a larger area indicating magnitude 4.9 mb. The large event of 1919-June-01, reported in the Catalog with 4.9 mb, was false, and had resulted from joining a local tremor probably due to a meteorite fall in Santa Luzia (GO) with wrong interpretation of records of the RDJ station, Rio de Janeiro, as if they were from the same event in Mato Grosso. The revised catalog for the West-Central Brazil, compared with geophysical maps, shows that earthquakes occur in areas of low velocities in the upper mantle, as well as in areas of positive gravity free-air anomalies. Stress concentration in the upper crust can be explained as due to both lithospheric thinning and flexure.Keywords: historical seismicity, Pantanal, macroseismic information. Revisão da Sismicidade História do Centro-Oeste do Brasil: Novos Sismos Recém Descobertos e Implicações para Ameaça SísmicaRESUMO. A zona sísmica da bacia do Pantanal tem sismos até 5,4 mb. Informações mais detalhadas sobre eventos históricos foram pesquisadas em jornais regionais, especialmente o "Correio do Estado", Campo Grande, Mato Grosso do Sul. Vários pequenos eventos novos foram descobertos que não constavam do Catálogo anterior de Berrocal et al. (1984). Cinco eventos tiveram novas informações macrossísmicas de diferentes localidades, permitindo estimar magnitudes pela área afetada. Foram descobertos dez novos eventos, cada um com informações de uma única localidade. O evento de 24-Out-1906, com magnitude anterior 4,2, foi sentido em uma área maior, indicando magnitude de 4,9 mb. Já o grande evento de 01-Jun-1919, relatado no Catálogo com magnitude 4,9 mb, era falso, sendo resultado da junção de um tremor local, possivelmente devido à queda de um meteorito em Santa Luzia (GO), com interpretação errada dos registros da estação RDJ (Rio de Janeiro), como se fossem provenientes de um único evento em Mato Grosso. O catálogo revisado para a região Centro-Oeste do Brasil, comparado com mapas geofísicos, mostra que os sismos ocorrem em áreas de baixas velocidades no manto superior, bem como em áreas de anomalias ar-livre positivas. A concentração de tensões na crosta superior pode ser explicada tanto por afinamento litosférico, como por flexura.Palavras-chave: sismicidade histórica, Pantanal, informação macrossísmica.

The 1885 M 6.9 Earthquake in the French Guiana–Brazil Border: The Largest Midplate Event in the Nineteenth Century in South America

In 4 August 1885, 06:30 local time, a strong earthquake (reported intensities up to VI–VII modified Mercalli intensity [MMI]) was felt in the French Guiana, causing slight damage. Recently discovered newspaper records show that this event was also felt as far as Georgetown (British Guyana), Belém, and several other localities along the Amazon River toward Manaus (Brazil). The distribution of intensities and the radius of the felt area indicate a magnitude around Mw 6.9, which makes it the largest known earthquake in the stable continental region of South America, since the nineteenth century. The epicenter, determined with four different attenuation relations, lies onshore near the border between the French Guiana and Brazil, although an epicenter offshore in the continental slope cannot be ruled out with 95% confidence. The epicenter (03.4° N, 52.9°W±100 km) likely lies in the Transamazonian (2.2–2.0 Ga) geochronological province in the Guyana shield of the Amazon craton. No nearby failed rift is known onshore near the epicenter, which would place this event in the ∼30% class of nonextended stable continental crust. Other nearby smaller earthquakes (both historical and instrumental) with magnitudes up to mb 5.2, indicate a cluster of seismicity in the region of the 1885 earthquake, possibly delineating an onshore seismic zone separate from the sparse seismicity along the continental shelf. This large midplate earthquake will likely affect future reevaluations of seismic hazard in midplate South America.

The Međimurje (Croatia) Earthquake of 1738

The 30 March 1738 earthquake with an epicenter near Čakovec in Međimurje (Croatia) is the largest known earthquake in the low-seismicity area that includes northernmost Croatia, northeastern Slovenia, southeastern Austria, and southwestern Hungary. So far, it has attracted very little attention in the seismological communities of those countries. It is missing or has wrong source parameters in all of the relevant earthquake catalogs (including the Seismic Hazard Harmonization in Europe (SHARE) catalog, Stucchi et al., 2013), which may influence seismic hazard assessment in this part of Europe, most critically in the Međimurje region itself. We present contemporary historical data shedding some light on the effects that the earthquake had on settlements mostly in Međimurje, but also elsewhere in Croatia, Slovenia, and Hungary. We were able to assign intensities to 12 localities surrounding the epicenter and to resolve the confusion about its date of occurrence. The intensity points were inverted for the location of the macroseismic hypocenter and epicentral intensity (I0=7.9 MSK [Medvedev–Sponheuer–Karnik]). The epicenter is found to lie on the hanging wall of the reverse Čakovec fault, about 6 km from its surface trace, and 8 km north-northwest of the town of Čakovec. The rather small felt area for an earthquake of this maximum intensity implies a shallow macroseismic focal depth of 6 km. These values of intensity and depth correspond to a macroseismic magnitude of MLm 5.1.

Sismicidade do Nordeste do Brasil

ABSTRACTA revised catalogue of earthquakes, which occurred in the Northeastern region of Brazilup to 1980, is presented. Most of the data come from historical macroseismic informationcollected in old publications and newspapers. Recent instrumental data is used to deriveempirical relations between magnitude and felt area in order to estimate mb magnitudesfor the events not recorded by seismographic stations. Macroseismic data for two largeevents (Pereiro 23.02.68, and Pacajus 20.11.80) are presented in detail. Epicentres of fourrecent events recorded at NAT were determined by combining the macroseismic andinstrumental data and used to define a preliminary value of the ratio K=Vp/Vs = 1.77 forthe region under study. A swarm of activity at Parazinho, Rio Grande do Norte, wasstudied in more detail to estimate the parameter b of the Gutenberg and Richter frequency-relation.

On the footprints of a major Brazilian Amazon earthquake

Combining historical accounts and seismological studies, three hundred years of dormant information emerged as a source of the largest known seismic event that rocked Brazil since the beginning of our colonization. The probable epicenter location of the 1690 tremor lies on the left bank of the Amazon River, about 45 km downstream from the modern day Manaus. A year later, while passing this area, a missionary met witnesses of the tremor and observed remarkable changes in the topography and vegetation along the margin of the river. By 1692 another priest confirmed this event and the occurrence of large waves in the river, which led to the flooding of the Native Indians' terrains. The tremor spread seismic waves throughout the forest and shook indigenous constructions as far as one thousand kilometers away. A calculation of the seismic parameters shows an estimated magnitude of 7, a maximum intensity of IX MM and a felt area of about 2 million km2. Due to the long recurrence period for this type of tremor, the discovery of one of these events is valuable for seismic global intraplate studies. As for Brazil, it unravels the myth that the country was never hit by severe earthquakes.