FinDer v.2: Improved real-time ground-motion predictions for M2–M9 with seismic finite-source characterization

Abstract

S U M M A R YRecent studies suggest that small and large earthquakes nucleate similarly, and that they often have indistinguishable seismic waveform onsets. The characterization of earthquakes in real time, such as for earthquake early warning, therefore requires a flexible modeling approach that allows a small earthquake to become large as fault rupture evolves over time. Here, we present a modeling approach that generates a set of output parameters and uncertainty estimates that are consistent with both small/moderate (≤M6.5) and large earthquakes (>M6.5) as is required for a robust parameter interpretation and shaking forecast. Our approach treats earthquakes over the entire range of magnitudes (>M2) as finite line-source ruptures, with the dimensions of small earthquakes being very small (<100 m) and those of large earthquakes exceeding several tens to hundreds of kilometres in length. The extent of the assumed line source is estimated from the level and distribution of high-frequency peak acceleration amplitudes observed in a local seismic network. High-frequency motions are well suited for this approach, because they are mainly controlled by the distance to the rupturing fault. Observed ground-motion patterns are compared with theoretical templates modeled from empirical ground-motion prediction equations to determine the best line source and uncertainties. Our algorithm extends earlier work by Böse et al. for large finite-fault ruptures. This paper gives a detailed summary of the new algorithm and its offline performance for the 2016 M7.0 Kumamoto, Japan and 2014 M6.0 South Napa, California earthquakes, as well as its performance for about 100 real-time detected local earthquakes (2.2 ≤ M ≤ 5.1) in California. For most events, both the rupture length and the strike are well constrained within a few seconds (<10 s) of the event origin. In large earthquakes, this could allow for providing warnings of up to several tens of seconds. The algorithm could also be useful for resolving fault plane ambiguities of focal mechanisms and identification of rupturing faults for earthquakes as small as M2.5.

How Often Can Earthquake Early Warning Systems Alert Sites With High‐Intensity Ground Motion?

“…The FinDer algorithm (Böse et al, 2012;Böse et al, 2015;Böse et al, 2018) rapidly determines line source models of small (M3) to large (M9) earthquakes by matching the spatial distribution of observed ground motion amplitudes in a seismic network with theoretical template maps. These templates are precalculated from empirical ground-motion prediction equations (GMPEs) for line sources of different lengths and magnitudes, and they are rotated to constrain the strike of the earthquake fault rupture.…”

“…These templates are precalculated from empirical ground-motion prediction equations (GMPEs) for line sources of different lengths and magnitudes, and they are rotated to constrain the strike of the earthquake fault rupture. The template with the highest correlation with the observed ground motion pattern is found from a combined grid-search and divide-and-conquer approach (Böse et al, 2018). The resulting finite-source model, characterized by the line source centroid, length, strike, and corresponding likelihood functions, is updated every second until peak shaking is reached.…”

“…The answer to this question is complex, because whether or not timely and correct alerts can be provided depends on a variety of factors, including the source/site geometry, the temporal evolution of the fault rupture, site conditions, and end user sensitivities (Meier, 2017;Minson et al, 2018). Experience in operational EEW systems during large earthquakes has revealed that providing timely warnings for sites with strong ground motion is possible but challenging and that near-epicentral sites may not always receive alerts in time (Böse et al, 2018;Cochran et al, 2018;Hoshiba et al, 2011;Kodera et al, 2016).…”

“…Here we evaluate and compare the performance of three operational EEW algorithms on a large set of strong-motion data: (1) The Earthquake Point-Source Integrated Code (EPIC) algorithm (Chung et al, 2019) estimates earthquake point-source parameters in real time from empirical early ground motion scaling relations; it is the point source algorithm in the operational ShakeAlert EEW system for the U.S. West Coast (Given et al, 2018). (2) The Finite-Fault Rupture Detector (FinDer) algorithm (Böse et al, 2018) uses precomputed binary ground motion templates to identify and track the line source that best explains the observed high frequency ground motion; it is the seismic finite-source algorithm in ShakeAlert. (3) The Propagation of Local Undamped Motion (PLUM) algorithm (Kodera et al, 2018) uses ground motion amplitude observations at nearby sites to predict impending ground motion at target sites without characterizing the earthquake source; it is one of the two operational algorithms in the Japanese public EEW system operated by the Japan Meteorological Agency (JMA, Doi, 2011;Kodera et al, 2018).…”

Near‐Real‐Time Estimates on Earthquake Rupture Directivity Using Near‐Field Ground Motion Data From a Dense Low‐Cost Seismic Network

“…Recent development of low cost sensors (Cochran et al, 2009;Wu et al, 2013), fiber sensors (Lindsey et al, 2017), and even smartphones (Kong et al, 2016) make the buildup of such dense networks and noninterrupted transmission more feasible nowadays. That would pave the way for the development of real-time-based methods such as ours and others (Bose et al, 2018;Yang et al, 2018) to provide information of rupture directivity and possible amplified ground shaking areas as fast as possible for hazard mitigation and emergency management.…”