Natural Time, Nowcasting and the Physics of Earthquakes: Estimation of Seismic Risk to Global Megacities

Abstract

Natural Time ("NT") refers to the concept of using small earthquake counts, for example of M>3 events, to mark the intervals between large earthquakes, for example M>6 events. The term was first used by (Varotsos et al., 2005) and later by (Holliday et al., 2006) in their studies of earthquakes. As we discuss in this paper, it is particularly useful in describing complex stochastic nonlinear systems characterized by fat-tail statistics rather than Gaussian normal statistics. In this paper we discuss ideas and applications arising from the use of NT to understand earthquake dynamics. The usual end-user applications of fault-based studies are often applied to risk of a particular geographic location, so it seems best to start the analysis with that geographic region. Rather than focus on an individual earthquake faults, we have found it more productive to focus on a defined local geographic region surrounding a particular location. This local region is considered to be embedded in a larger regional setting from which we accumulate the relevant statistics. From this different philosophical point of view, we first discuss methods to use NT, counts of small earthquakes, to evaluate the current state of a regional collection of faults. We then use these concepts to first discuss the nucleation physics of large earthquakes. We introduce the idea of nowcasting, a term originating from economics and finance. The goal of nowcasting is to determine the current state of the fault system, or put another way, the current state of progress through the earthquake cycle. This is in contrast to forecasting, which is the calculation of probabilities of future large earthquakes. Finally, we apply the nowcasting idea to the practical development of methods to estimate the current state of risk for dozens of the world's seismically exposed megacities, defined as cities having populations of over 1 million persons. We compute a ranking of these cities based on their current nowcast value, and discuss the advantages and limitations of this approach. We note explicitly that the nowcast method is not a model, in that there are no free parameters to be fit to data. Rather, the method is simply a presentation of statistical data, which the user can interpret.

Natural Time Analysis: Results Related to Two Earthquakes in Greece during 2019

“…Moreover, the analysis in natural time of the seismicity subsequent to the SES activity in the area revealed [2] that critical conditions were obeyed early in the morning of 18 October 2018, i.e., almost a week before the strong earthquake occurrence, in agreement with earlier findings [4]. The application [2] of the recent method of nowcasting earthquakes [9][10][11][12][13], which is based on natural time, has revealed that an earthquake potential score of around 80% was observed just before the occurrence of this M w 6.8 earthquake. Here, we focus on the recording [14] of additional SES activities after the occurrence of the latter earthquake in the beginning of January 2019 (see below) that preceded the following two earthquakes in Greece during 2019: First, a M w 5.4 earthquake [15] close to Preveza city in Western Greece on 5 February 2019 and a M w 5.3 earthquake [16] on 30 March 2019 a few tens of km East of Patras SES measuring station (labeled PAT in Figure 1).…”

Global Seismic Nowcasting With Shannon Information Entropy

“…We have shown in companion papers (Rundle et al, 2018) that the nowcasting method is generally not very sensitive to the choice of large spatial region defining the histogram, leading to a standard error of approximately ±10%. However, the method is strongly sensitive to issues of catalog completeness (Rundle et al, 2018).…”

“…Nowcasting refers to the use of proxy data to estimate the current dynamical state of a driven complex system such as earthquakes, neural networks, or the financial markets . In previous papers Rundle et al, 2018), a method to nowcast earthquakes has been presented based on RUNDLE ET AL. 191 Earth and Space Science TECHNICAL…”

Fracture Advancing Step Tectonics Observed in the Yuha Desert and Ocotillo, CA, Following the 2010 Mw7.2 El Mayor‐Cucapah Earthquake

“…To better understand the earthquake potential, we compute nowcasts for the area around the Ocotillo site within circles of radius 25 and 100 km (Figure ). Nowcasting is a method for evaluating the current state of the fault system in a seismically active area, which is reset following a moderate to large earthquake (Rundle et al, ; Rundle et al, ). The method uses counts of small earthquakes to evaluate the potential for a large earthquake.…”

“…The method uses counts of small earthquakes to evaluate the potential for a large earthquake. Nowcasting is not a prediction, but it can be applied to estimate seismic risk in a region (Rundle et al, ). In the present calculation, which provides a snapshot of the fault system in October 2017, a small earthquake is an event having magnitude 3.99 < = M w < 5, whereas a large earthquake is an event having magnitude M w > 5.…”