Peak Ground Displacement Saturates Exactly When Expected: Implications for Earthquake Early Warning

Abstract

The scaling of rupture properties with magnitude is of critical importance to earthquake early warning systems that rely on source characterization using limited snapshots of waveform data. ShakeAlert, a prototype earthquake early warning system that is being developed for the western United States, provides real-time estimates of earthquake magnitude based on P wave peak ground displacements measured at stations triggered by the event. The algorithms used in ShakeAlert assume that the displacement measurements at each station are statistically independent and that there exists a linear and time-independent relation between log peak ground displacement and earthquake magnitude. Here we challenge this basic assumption using the largest data set assembled for this purpose to date: a comprehensive database of more than 140,000 vertical-component waveforms from M4.5 to M9 earthquakes occurring near Japan from 1997 through 2018 and recorded by the K-NET and KiK-net strong-motion networks. By analyzing the time evolution of P wave peak ground displacements for these earthquakes, we show that there is a break, or saturation, in the magnitude-displacement scaling that depends on the length of the measurement time window. We demonstrate that the magnitude at which this saturation occurs is well-explained by a simple and nondeterministic model of earthquake rupture growth. We then use the predictions of this saturation model to develop a Bayesian framework for estimating posterior uncertainties in real-time magnitude estimates.Key Points: • We analyze P wave peak displacements (Pd) of magnitude M4.5-9 earthquakes in Japan from 1997 to 2018 • Time-dependent saturation in the linear scaling between log10 Pd and magnitude is consistent with nondeterministic rupture • We develop a Bayesian framework for rapid calculations of time-dependent uncertainties in real-time magnitude estimates Supporting Information:• Supporting Information S1 • Movie S1 • Movie S2 • Data Set S1

Seismogeodetic P‐wave Amplitude: No Evidence for Strong Determinism

“…In the present study, the larger events (>M7.5) appear indistinguishable this early (i.e., 3–5 s) in the rupture process, which is consistent with the magnitude saturation typical of seismic‐only observations and previous work describing the expected behavior of P‐wave amplitudes at these observation windows (Trugman et al, ). While the P‐wave amplitude measurements are generally larger in the unfiltered dataset, the evidence for magnitude saturation remains.…”

Source Parameters of Moderate-To-Large Chinese Earthquakes From the Time Evolution of P-Wave Peak Displacement on Strong Motion Recordings

“…Moreover, the rupture process of an earthquake is a chaotic process, and the simple exponential LPDT curve may hardly represent the complex nature of this process, especially at the very beginning. However, the now massive and well documented observation of LPDT empirical curves analysed in many worldwide seismic regions and in a wide magnitude and distance range confirm that the general ramp-like behavior of the curves is a universal evidence (Colombelli et al, 2012;Colombelli et al, 2015;Colombelli et al, 2020;Melgar et al, 2015;Trugman et al, 2019) which is well matched by the chosen exponential model (Colombelli et al, 2020). When the curve has reached its peak, the schematic representation with two parameters (plateau time and amplitude), may properly catch the essential features of the process we are interested in.…”

Evaluation of the Grillo sensor, a low-cost accelerometer for IoT-based Real-time seismology

“…Earthquake magnitude in EEW is commonly estimated via the peak ground displacement (Pd) of the initial portion of the earthquake's P-wave (Li et al, 2017;Trugman et al, 2019). The decadic logarithm of Pd increases linearly with earthquake magnitude up to a magnitude of saturation.…”

“…The decadic logarithm of Pd increases linearly with earthquake magnitude up to a magnitude of saturation. The magnitude of saturation depends on the length of the P-wave segment used for the calculation and can reach up to M 7.5 for roughly 10 s of initial P-wave (Trugman et al, 2019). We calculated the Pd for 722 earthquakes (Fig.…”

“…1) using records filtered by the 4-pole Butterworth bandpass filter between 0.075 and 3 Hz (as used e.g. Li et al, 2017;Trugman et al, 2019). We use 1, 3, and 5 s long segments of the initial earthquake P-wave and correct the Pd to the common epicentral distance of 10 km.…”