Global occurrence and impact of small-to-medium magnitude earthquakes: a statistical analysis

Abstract

Despite their much smaller individual contribution to the global counts of casualties and damage than their larger counterparts, earthquakes with moment magnitudes Mw in the range 4.0–5.5 may dominate seismic hazard and risk in areas of low overall seismicity, a statement that is particularly true for regions where anthropogenically-induced earthquakes are predominant. With the risk posed by these earthquakes causing increasing alarm in certain areas of the globe, it is of interest to determine what proportion of earthquakes in this magnitude range that occur sufficiently close to population or the built environment do actually result in damage and/or casualties. For this purpose, a global catalogue of potentially damaging events—that is, earthquakes deemed as potentially capable of causing damage or casualties based on a series of pre-defined criteria—has been generated and contrasted against a database of reportedly damaging small-to-medium earthquakes compiled in parallel to this work. This paper discusses the criteria and methodology followed to define such a set of potentially damaging events, from the issues inherent to earthquake catalogue compilation to the definition of criteria to establish how much potential exposure is sufficient to consider each earthquake a threat. The resulting statistics show that, on average, around 2% of all potentially-damaging shocks were actually reported as damaging, though the proportion varies significantly in time as a consequence of the impact of accessibility to data on damage and seismicity in general. Inspection of the years believed to be more complete suggests that a value of around 4–5% might be a more realistic figure.

Green, yellow, red, or out of the blue? An assessment of Traffic Light Schemes to mitigate the impact of hydraulic fracturing-induced seismicity

“…This being the case, global observations of earthquake damage (e.g. Nievas et al, 2020a) can provide a basis for calibrating magnitude thresholds, provided certain assumptions are made regarding the completeness of reporting for damaging events (Nievas et al 2020b) and that the exposed building stock is analogous to that affected by the earthquakes for which damage was reported. However, a critical question is whether the red-light threshold of a TLS should be fixed at the target risk level, or whether it is necessary to set the red light at a lower threshold if the risk target is to be achieved.…”

A database of damaging small-to-medium magnitude earthquakes

“…The latter would facilitate future improvements and extensions of the database. Moreover, it has been a key component of a statistical study on the frequency with which upper-crustal earthquakes with magnitudes in the range of interest cause damage and/or casualties, which is explored in a separate paper (Nievas et al 2019a).…”

“…If the conclusion from step 5 is that the event needed to be added, export all potential earthquakes as output. For each potential earthquake, select one set of hypocentral coordinates and one magnitude estimate according to the criteria defined in Nievas et al (2019a), the top priority being for moment magnitudes. Additionally, retrieve body-wave and surface-wave magnitude estimates authored by the ISC or the USGS.…”

“…With depth being the most difficult parameter to constrain within this context-a fact that is particularly true in the case of small-to-medium magnitude earthquakes (e.g., Letort et al 2014)-the database contains several cases of supposedly deep yet reportedly damaging earthquakes. A small discussion on the potential causes of this and an analysis on a subset of 213 of these events can be found in Nievas et al (2019a).…”

“…An alternative approach, which can provide complementary insights, is to investigate the relative frequency with which such small-tomoderate earthquakes cause damage, and to explore general patterns in these cases. The database summarised in this paper is focused on this second class of study and has indeed been used in statistical analyses of the numbers of earthquakes in the defined magnitude range that have coincided with human settlements (Nievas et al 2019a). From the outset, however, it is important to have realistic expectations of such a database, which in aiming for breadth of coverage-by including as many reportedly damaging events as possible-inevitably has to compromise in terms of depth for the simple reason that the available information regarding many of the events is rather limited.…”