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The U.S. Geological Survey (USGS) has developed a national capability for aftershock forecasting after significant earthquakes. Use of this capability began in August 2018, and the 30 November 2018 Mw 7.1 Anchorage, Alaska, earthquake provided the first opportunity to apply this capability to a damaging earthquake in an urban area of the United States of America and observe how the forecast was discussed in the media. During this sequence, the forecasts were issued by a seismologist using interactive software that implements the Reasenberg and Jones (1989) model as updated in Page et al. (2016). The forecasts are communicated with a tiered template that provides basic information first before providing a more detailed numerical forecast and are posted on the mainshock’s event page on the USGS earthquake program. Experience from the Anchorage sequence showed that the process worked well, and the first forecast was issued only 54 min after the mainshock occurred. Updates over the coming days, weeks, and months adapted the forecast model from the initial generic parameters for the seismotectonic region to Bayesian and sequence‐specific models. Media reports accurately reported the forecast, demonstrating that the forecast template was successful except for a few reports that incorrectly merged the probability of one or more events in a given time–magnitude window with the likely range of the number of events. Changes to the template have been made to prevent that confusion in the future. We also released a special report on the possible duration of the sequence to assist in the federal disaster declaration and assistance process. Both our standard forecasts and this special report would benefit from more rapid determination of a sequence‐specific decay rate. more...

It has recently become practical to use triangular fault elements in earthquake simulators instead of the usual rectangular fault elements. A simulator spends most of its time computing how slip on some fault elements affects the stresses on other fault elements. We explore whether rectangles or triangles yield more accurate stress values on curved fault surfaces. One might expect triangles to perform better, because triangles can follow the shape of a curved surface, whereas rectangles leave gaps and overlaps between adjacent fault elements. Our test results show that, contrary to expectations, rectangles overall perform as well as or better than triangles when computing stresses on curved fault surfaces. We also find that one triangulation may perform significantly better than another triangulation. more...

The article of record as published may be found at http://dx.doi.org/10.1785/0220170222 We describe a set of benchmark exercises that are designed to test if computer codes that simulate dynamic earthquake rupture are working as intended. These types of computer codes are often used to understand how earthquakes operate, and they produce simulation results that include earthquake size, amounts of fault slip, and the patterns of ground shaking and crustal deformation. The benchmark exercises examine a range of features that scientists incorporate in their dynamic earthquake rupture simulations. These include implementations of simple or complex fault geometry, off-fault rock response to an earthquake, stress conditions, and a variety of formulations for fault friction. Many of the benchmarks were designed to investigate scientific problems at the forefronts of earthquake physics and strong ground motions research. The exercises are freely available on our website for use by the scientific community. Southern California Earthquake Center (SCEC) more...

Online Material: Explanation of the concept of a dynamic rupture simulation. Earthquakes are complex events that involve a myriad of interactions among multiple geologic features and processes. One of the tools that is available to assist with their study is computer simulation, particularly dynamic rupture simulation. A dynamic rupture simulation is a numerical model of the physical processes that occur during an earthquake. Starting with the fault geometry, friction constitutive law, initial stress conditions, and assumptions about the condition and response of the near‐fault rocks, a dynamic earthquake rupture simulation calculates the evolution of fault slip and stress over time as part of the elastodynamic numerical solution (![Graphic][1] see the simulation description in the electronic supplement to this article). The complexity of the computations in a dynamic rupture simulation make it challenging to verify that the computer code is operating as intended, because there are no exact analytic solutions against which these codes’ results can be directly compared. One approach for checking if dynamic rupture computer codes are working satisfactorily is to compare each code’s results with the results of other dynamic rupture codes running the same earthquake simulation benchmark. To perform such a comparison consistently, it is necessary to have quantitative metrics. In this paper, we present a new method for quantitatively comparing the results of dynamic earthquake rupture computer simulation codes. Over the past decade, the Southern California Earthquake Center–U.S. Geological Survey (SCEC‐USGS) Spontaneous Rupture Code Verification Project has published a series of computational dynamic rupture benchmark exercises (Harris and Archuleta, 2004; Harris et al. , 2009, 2011). The benchmarks, a subset of which of are shown in Figure 1, are listed on the project’s website (http://scecdata.usc.edu/cvws, last accessed November 2014) and cover many earthquake settings, including a range of fault geometries (strike‐slip faults, normal faults, … [1]: /embed/inline-graphic-1.gif more...

Online Material: Supplemental figures of space‐time and frequency‐magnitude relations, scaling plots, mean and covariance plots of interevent times, probability distribution functions of recurrence intervals, and earthquake density plots. In order to understand earthquake hazards we would ideally have a statistical description of earthquakes for tens of thousands of years. Unfortunately the ∼100‐year instrumental, several 100‐year historical, and few 1000‐year paleoseismological records are woefully inadequate to provide a statistically significant record. Physics‐based earthquake simulators can generate arbitrarily long histories of earthquakes; thus they can provide a statistically meaningful history of simulated earthquakes. The question is, how realistic are these simulated histories? This purpose of this paper is to begin to answer that question. We compare the results between different simulators and with information that is known from the limited instrumental, historic, and paleoseismological data. As expected, the results from all the simulators show that the observational record is too short to properly represent the system behavior; therefore, although tests of the simulators against the limited observations are necessary, they are not a sufficient test of the simulators’ realism. The simulators appear to pass this necessary test. In addition, the physics‐based simulators show similar behavior even though there are large differences in the methodology. This suggests that they represent realistic behavior. Different assumptions concerning the constitutive properties of the faults do result in enhanced capabilities of some simulators. However, it appears that the similar behavior of the different simulators may result from the fault‐system geometry, slip rates, and assumed strength drops, along with the shared physics of stress transfer. This paper describes the results of running four earthquake simulators that are described elsewhere in … more...

Numerical simulations of earthquake rupture dynamics are now common, yet it has been difficult to test the validity of thesesimulations because there have been few field observations and no analytic solutions with which to compare the results. This paper describes the Southern California Earthquake Center/U.S. Geological Surve(SCEC/USGS) Dynamic Earthquake Rupture Code Verification Exercise, where codes that simulate spontaneous rupture dynamics in three dimensions are evaluated and the results produced by these codes are compared using Web-based tools. This is the first time that a broad and rigorous examination of numerous spontaneous rupture codes has been performed—a significant advance in this science. The automated process developed to attain this achievement provides for a future where testing of codes is easily accomplished. Scientists who use computer simulations to understand earthquakes utilize a range of techniques. Most of these assume that earthquakes are caused by slip at depth on faults in the Earth, but hereafter the strategies vary. Among the methods used in earthquake mechanics studies are kinematic approaches and dynamic approaches. The kinematic approach uses a computer code that prescribes the spatial and temporal evolution of slip on the causative fault (or faults). These types of simulations are very helpful, especially since they can be used in seismic data inversions to relate the ground motions recorded in the field to slip on the fault(s) at depth. However, these kinematic solutions generally provide no insight into the physics driving the fault slip or information about why the involved fault(s) slipped that much (or that little). In other words, these kinematic solutions may lack information about the physical dynamics of earthquake rupture that will be most helpful in forecasting future events. To help address this issue, some researchers use computer codes to numerically simulate earthquakes and construct dynamic, spontaneous rupture (hereafter called “spontaneous rupture”) solutions. For these types of numerical simulations, rather than prescribing the slip function at each location on the fault(s), just the friction constitutive properties and initial stress conditions are prescribed. The subsequent stresses and fault slip spontaneously evolve over time as part of the elasto-dynamic solution. Therefore, spontaneous rupture computer simulations of earthquakes allow us to include everything that we know, or think that we know, about earthquake dynamics and to test these ideas against earthquake observations. more...

Online Material: Specifications for the standard earthquake simulator input and output files. An earthquake simulator is a computer program that simulates a long sequence of earthquakes, which may range from tens of thousands of years up to millions of years. Different simulator codes have different input requirements, but in general the inputs to a simulator program include the fault system geometry, fault slip rates, rake angles, friction parameters, initial stress state, and elastic parameters. The output from a simulator program is a list of events, each of which is characterized by time, location, magnitude, rupture area, mean slip, stress change, and, optionally, a map that shows the distribution of slip over the fault surface. It is challenging to demonstrate that an earthquake simulator program is reliable, in the sense that it produces a simulated history whose statistical properties are similar to the properties of real earthquake sequences. The Southern California Earthquake Center (SCEC) Earthquake Simulator Comparison Project addresses this challenge by comparing the results from five different earthquake simulators with each other and, to the extent possible, with what is known about real earthquakes (Tullis et al. , 2012a). The five simulators are independently developed, so agreement among the simulators can provide some confidence in their reliability. To carry out the comparison, it is necessary for each simulator code to receive precisely the same inputs. It is also necessary for each simulator code to generate output in the same format, so that output from all the codes can be analyzed by the same set of statistical tools. We have achieved this by designing standard file formats for transferring data into and out of simulator codes. Although the standard files are formatted as ASCII text and are human‐readable, they are intended primarily for transferring data from one computer program to another. … more...