Data Reduction And Error Analysis For The Physical Sciences

• Stability of steady frictional sliding, inertia and the quasi-static limit • work on ps 4; see course web site

The Mechanics Of Earthquakes And Faulting

Seismic nowcasting uses counts of small earthquakes as proxy data to estimate the current dynamical state of an earthquake fault system. The result is an earthquake potential score that characterizes the current state of progress of a defined geographic region through its nominal earthquake "cycle." The count of small earthquakes since the last large earthquake is the natural time that has elapsed since the last large earthquake (Varotsos et al., 2006, https://doi.org/10.1103/PhysRevE.74.021123). In addition to natural time, earthquake sequences can also be analyzed using Shannon information entropy ("information"), an idea that was pioneered by Shannon (1948, https://doi.org/10.1002/j.1538-7305.1948.tb01338.x). As a first step to add seismic information entropy into the nowcasting method, we incorporate magnitude information into the natural time counts by using event self-information. We find in this first application of seismic information entropy that the earthquake potential score values are similar to the values using only natural time. However, other characteristics of earthquake sequences, including the interevent time intervals, or the departure of higher magnitude events from the magnitude-frequency scaling line, may contain additional information.

/pdf/global-seismic-nowcasting-with-shannon-information-entropy-52f3k4dgnd.pdf

Global Seismic Nowcasting With Shannon Information Entropy.

Nowcasting refers to the use of proxy data to estimate the current dynamic state of driven complex systems such as earthquakes, neural networks, or the financial markets. In previous papers, methods to nowcast earthquakes have been presented based on the natural time count of small earthquakes after the last large earthquake in a defined, seismically active geographic region. In this method, a large geographic region is identified in which a local region of interest is embedded. The primary assumption in the method is that the frequency-magnitude statistics of the local region are the same as the frequency-magnitude relation of the large region. The nowcasting technique relies on seismic catalogs that are complete in the sense that all events whose magnitude is larger than a completeness threshold have been detected. For this reason, the previous papers have been limited to nowcasting large earthquakes with magnitudes of approximately M7.5. In this article, we extend the nowcasting method to great global earthquakes of magnitudes as large as M9 by defining the surrounding large region as the entire earth. We then analyze the current hazard of a number of selected “local” regions and rank the regions in terms of current risk of great earthquakes. These great events also present significant hazards for generating mega-tsunamis as well as local damage due to intense shaking. We also perform a sensitivity analysis to establish the reliability of the nowcasts. One of our main results is that the eastern Aleutian Islands, site of the M8.6 earthquake of 1 April 1946, is currently the region most at current risk of a great earthquake larger than M8.0. We finish by presenting comments on the applicability of our methods for anticipating the occurrence of great destructive earthquakes and tsunamis.

Nowcasting Great Global Earthquake and Tsunami Sources

Monitoring the Earth's stress state plays a role in our understanding of an earthquake's mechanism and in the distribution of hazards Crustal deformation due to the July 2019 earthquake sequence in Ridgecrest (California) that culminated in a preceding quake of magnitude (M) 64 and a subsequent M71 quake caused stress perturbation in a nearby region, but implications of future seismicity are still uncertain Here, the occurrence of small earthquakes is compared to larger ones, using b-values, showing that the rupture initiation from an area of low b-values, indicative of high stress, was common to both M64 and M71 quakes The post-M71-quake sequence reveals that another low-b-value zone, which avoided its ruptured area, fell into an area near the Garlock fault that hosted past large earthquakes If this area were more stressed, there would be a high-likelihood of further activation of seismicity that might influence the Garlock fault

/pdf/were-changes-in-stress-state-responsible-for-the-2019-1i96tlekc8.pdf

Were changes in stress state responsible for the 2019 Ridgecrest, California, earthquakes?

This paper describes the use of the idea of natural time to propose a new method for characterizing the seismic risk to the world's major cities at risk of earthquakes. Rather than focus on forecasting, which is the computation of probabilities of future events, we define the term seismic nowcasting, which is the computation of the current state of seismic hazard in a defined geographic region.

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

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. In this paper, we discuss ideas and applications arising from the use of NT to understand earthquake dynamics, in particular by use of the idea of nowcasting. Nowcasting differs from forecasting, in that the goal of nowcasting is to estimate the current state of the system, rather than the probability of a future event. Rather than focus on an individual earthquake faults, we 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. 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. Among other results, we find, for example, that the current nowcast ranking of the Los Angeles region is comparable to its ranking just prior to the January 17, 1994 Northridge earthquake.

Alexis Giguere

Papers

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

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

Global Seismic Nowcasting With Shannon Information Entropy.