Historical Seismograms - Evidence from the AD 2000 Izu Islands ...

2 14 D. Mun"oz and A. Udiasgiven by Herrman et al. (1978) for earthquakes in California and Central UnitedStates. Although the attenuation of Spanish earthquakes is more similar to thatof California than of Central United States, a mean value of both places has beentaken. For the radius of intensity VI we have taken 135 km, a mean between themaximum 175 km and the minimum 100 km. Using this value, the area obtainedis 1.2 x lo5 km2 and resulting seismic momentM, = 1.25 x 101'Nm. (4)A comparison of this earthquake with that occurred between Granada and MBlagathe 25th of December of 1884 with maximum intensity IX (Udias and Muiioz, 1979)is of interest. Although the maximum intensity seems to have been higher in 1884,the area of intensity VI is one order of magnitude smaller. Also, the 1680 earthquakewas felt with greater intensity in Madrid, even though its epicenter was more distant.The extent of the felt area and the fact that no aftershocks are mentioned anywhereseem to substantiate the fact that this was a deeper shock.Evaluation of damage in the city of Mdlaga has been made from all existingdocuments, but in first place we base our analysis on the Act of the Notary ofthe Episcopal Audience of MBlaga, the 15th of October of 1680, where the numberof houses, churches and convents that suffered damage is specified, as well as thekind of damage affecting them. This information is presented on a contemporarymap in Figure 4. Location of churches and convents has been identified in the mapand the extent of damage is shown by the black sections of the circles. Numbersin the map situating the buildings correspond to those of Table 2. Four conventswere destroyed and two had to be abandoned after the earthquake. The city ofMdlaga was divided into four parishes. The Act gives the total number of houses ineach parish, and the number houses that suffered total destruction, heavy damage,etc. This permits a calculation for each parish of the percentages of the damagesuffered by the buildings. These percentages are represented in the large circles ofFigure 4 for each individual parish; complete destruction is shown in black, heavydamage in narrow lines, appreciable damage in wide lines and light damage in dots.Heaviest damage is located in the parishes of San Juan and Santos Mjrtires with39 persons dead, 87 injured, and total destruction of nearly 25% of all buildingsand appreciable damage of between 50% and 65%. In the parish of San Juan, thedistrict of Percheles, located on the alluvial deposit of the Guadalmedina River,was almost totally ruined. In Table 2, a summary of the damage suffered by eachof the four parishes of the city of Mdlaga is presented. Of a total of 4320 houses,795 were destroyed, 355 suffered serious damage, 2385 considerable damage and 635moderate damage. This means that practically all houses were somewhat affectedby the earthquake. Personal casualties are given as 42 dead and 110 injured. Allthese numbers are based on the Notarial Act mentioned above.In Figure 5, a present day map of the city of MQlaga is presented, where theoutline of the limits of the 1600 city has been drawn. It can be seen that majordevelopment has taken place to the west, precisely where damage from this earthquakewas most intense. As has been mentioned before this zone is built on recentsedimentary terrain of the Guadalmedina and Guadalhorce Rivers. This part of thecity can be expected to suffer heaviest damage in the case of a future earthquakesimilar to that of 1680. In that event it is not sure what will be the behaviour ofthe modern structures, most of them reinforced concrete.