Defining the Coseismic Phase of the Crustal Deformation Cycle With Seismogeodesy

Abstract

Earth's crustal deformation cycle is traditionally divided into coseismic, postseismic, and interseismic phases upon which transient motions from various sources may be superimposed. Here we present a new seismogeodetic methodology to define and identify the transition from the coseismic to the early postseismic phase. While this early period of postseismic deformation has not been well observed, it plays an important role in better understanding fault processes and crustal rheology because it is where the fastest evolution of fault slip occurs. Current methods often choose to rely on geodetic displacements with several hour to daily resolution to estimate static coseismic offsets. The choice of data span is often arbitrary and introduces some fraction of postseismic motion. Instead, we apply a more physics‐based approach that is applicable to interleaved regional networks of high‐rate Global Navigation Satellite System (GNSS) and seismic sensors. The start time of the coseismic phase is based on P wave arrivals and its end time on the total release of energy derived from seismic velocities integrated from strong‐motion accelerations. In the absence of physical collocations, we interpolate the coseismic time window to the GNSS stations and estimate the static offsets from the high‐rate displacements. We demonstrate our methodology by applying it to 10 earthquakes over a range of magnitudes and fault mechanisms. We observe that the presence of early postseismic motions within the widely used estimates of daily coseismic offsets can lead to an overprediction of coseismic moment and fault slip, up to several meters depending on the magnitude and mechanism of the event.

Very Early Postseismic Deformation Following the 2015 Mw 8.3 Illapel Earthquake, Chile Revealed From Kinematic GPS

“…To further identify the exact transition from the coseismic to the early postseismic phase, one can refer to the seismogeodetic methodology by Golriz et al. (2021).…”

“…Here, we are just presuming an initiation time for the postseismic period, rather than presuming that the coseismic and postseismic phases are separated clearly over time in the physical sense. To further identify the exact transition from the coseismic to the early postseismic phase, one can refer to the seismogeodetic methodology by Golriz et al (2021).…”

Real‐Time Seismogeodetic Earthquake Magnitude Estimates for Local Tsunami Warnings

“…For the other parameters in Equation 4, we choose representative values for the uppermost mantle of 𝐴𝐴 𝐴𝐴= 3400 kg∕m 3 and 𝐴𝐴 𝐴𝐴= 7900 m∕s (Tsuboi et al, 1995). As indicated earlier, we determine the coseismic time window for the integration in Equation 4 at each station individually using the seismogeodetic velocity (Golriz et al, 2021). We then compute the seismogeodetic moment magnitude at each station, following Kanamori (1977)…”

“…We continue to test 𝐴𝐴 𝐴𝐴wg estimation with a seismogeodetic approach extended for interleaved high-rate GNSS and seismic instruments that may only have a subset of collocated stations. This scenario requires spatial interpolation of the coseismic time window from regional seismic stations to the GNSS stations, as demonstrated by Golriz et al (2021). Since most local tsunamis are caused by thrust events, we have not considered strike-slip fault mechanisms in this study although this is certainly of interest for rapid magnitude estimation for earthquake early warning systems such as ShakeAlert (Given et al, 2014;Kohler et al, 2020) in the U.S. West Coast, which includes major strike-slip fault regimes.…”

“…We designed a Kalman filter to optimally combine the GNSS displacements and strong motion accelerations (Bock et al., 2011; Smyth & Wu, 2007) to estimate seismogeodetic velocities and displacements. The seismogeodetic velocities are used to detect P ‐wave onset times and estimate the coseismic time window (Golriz et al., 2021) over a network of near‐ to far‐field collocated GNSS and accelerometer stations. Using the vertical component of seismogeodetic displacement as an approximate source time function (Kikuchi & Ishida, 1993; Tsuboi et al., 1995), we rapidly estimate the seismic moment and $${M}_{w}$$.…”