Macroseismology

Macroseismology: the study of felt earthquakes

Nowadays seismology mainly focuses on digital data acquisition gathered by seismometers installed in a seismic network and the detection of earthquakes. Earthquakes, however, happened long before we were able to measure them in the instrumental era. By gathering and mapping witness’ reports on felt earthquakes, the impact of an earthquake can be reliably mapped and the magnitude and depth of historical earthquakes can be estimated based on the distribution of the felt reports. Usually the German mathematician P.N.G. Egen is credited for the development of the first modern intensity scale based on the perception of 28 February 1828 Ms ~4.6 earthquake near Jauche in Belgium. Later pioneering work of Nicholas Ambraseys in the 1970ies showed the enormous value of studying historical earthquakes.

Since 1932, after the ML 4.7 Uden earthquake, when a felt earthquake occurs, the Royal Observatory of Belgium sends out national earthquake inquiries to each Belgian municipality to construct an intensity map of the felt area. Based on the responses, an earthquake intensity value is assigned to each municipality resulting in an Communal Intensity Map. The first success of macroseismology in Belgium was shown by Somville (1939) who gathered the felt reports of the largest instrumentally recorded earthquake on the Belgian territory: i.e. the Zulzeke-Nukerke (near Oudenaarde) 11 June 1938 Ms 5.0 earthquake.

Since the arrival of the internet, the paper inquiry has been complemented by an online internet "Did You Feel It?" (DYFI) macroseismic inquiry. Online DYFI inquiries allow rapid and automatic construction of community internet intensity macroseismic maps with which the authorities and the population can be quickly informed about the damage distribution and felt area of an earthquake, either officially or through social media. This information is complementary to the recordings by the seismic stations to study earthquakes and their impact.

Europe: a problem in gathering macroseismic data?

In the US, the “Did You Feel It?” (DYFI?) system of the USGS asks questions about nationally and internationally felt earthquakes. The advantage of DYFI? is that macroseismic data are collected systematically over the whole North American continent and worldwide by ONE SINGLE INSTITUTE (USGS) through a well-calibrated algorithm (Dengler and Dewey, 1998; Worden et al., 2000).

In Europe, however, the situation is more complex: at least 34 seismological institutes in 24 countries and 2 international institutes (EMSC and USGS) manage and maintain their own web-based macroseismic questionnaire (see table). Germany wins the award of having the most inquiries (8 !) in one country. The benefit of having different European inquiries is that every institute has made the inquiry available in its own national language(s) and hence can deal with specifics in their language. However, disadvantages are that

(1) people can answer multiple national or international inquiries when they felt an earthquake

(2) the data on the perception of internationally-felt seismic events is strongly fragmented across different institutes and countries.

Merging databases carries the risk of duplicate entries in the database but, more importantly, also has the risk of merging intensities that result from different questionnaires, countries and intensity calculation algorithms, which will smooth the mean of the merged intensities. Because of this European fragmentation and intensity variability, performing a proper macroseismic assessment of transfrontier-felt earthquakes remains a complex and not straightforward task.

A solution?

The current challenge in Europe is to generate denser and more accurate shaking maps by merging all available intensity datasets from national and international seismological institutes. For example, the ROB cooperates with the Cologne University on a trans-frontier online macroseismic data acquisition. Small- to moderate-magnitude (M4 to M5) earthquakes affecting the Belgian and neighbouring territories cause (several thousand) respondents to fill this online questionnaire. To apply this methodology on an European scale, a consensus on a common questionnaire and data gathering is required. Something that will take time and effort.

To provide a simple solution how data can be shared, I have investigated the possibility to store intensity data from different institutes in equally-sized grid cells. First a python software was developed to geocode individual addresses into their geographical coordinates. Afterwards, instead of using classic DYFI representations (average internet intensities per community), I assign average intensities to single 100 km² grid cells. The resulting macroseismic grid cell distribution of the affected area shows a less subjective and more homogeneous intensity distribution than the classical irregular community. This method was extensively explored in my open access 2017 Solid Earth paper.

Averaging intensities in grid cells is however not ideal. All earthquake effect need to be taken into account for a proper intensity assessment. Determining the intensity of a community, area or grid cell thus requires a more complex approach than just averaging. HOWEVER, because different countries have different questionnaires and are not transparent in their intensity algorithm, averaging intensities is the currently the best we can do. Before questionnaires are homogenised in Europe, this grid cell methodology provides a simple yet rough solution.