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Density perturbations accompanying seismic waves are expected to generate prompt gravity perturbations preceding the arrival of P-waves. Vallée et al. (Science 358:1164–1168, 2017, https://doi.org/10.1126/science.aao0746) reported the detection of such pre-P-wave signals in broadband seismograms during the 2011 Tohoku-oki earthquake. Kimura et al. (Earth Planets Space 71:27, 2019, https://doi.org/10.1186/s40623-019-1006-x) considered that their detection involved some uncertain points, including a concern regarding their signal processing procedure. Specifically, to remove the instrumental response, Vallée et al. (2017) applied acausal deconvolution to the seismograms truncated at the P-wave arrivals. Generally, acausal deconvolution produces artifacts at the edge of the time window. However, they did not present quantitative assessment whether the detected signals were artifacts due to the signal processing. To avoid this concern, Kimura et al. (2019) employed another procedure that eliminated acausal processes, resulting in the detection of a pre-P-wave signal with a statistical significance of 7σ in stacked broadband seismograms. Subsequently, Vallée et al. (Earth Planets Space 71:51, 2019, https://doi.org/10.1186/s40623-019-1030-x) commented that the procedure employed by Kimura et al. (2019) for the signal detection was inappropriate because it dismissed the low-frequency components of data. Although we admit the loss of low-frequency components in the data in Kimura et al. (2019), Vallée et al. (2019) have not yet provided a full account of the validity of their own procedure. Here, we assessed the validity of the procedure employed by Vallée et al. (2017) by quantitatively evaluating the magnitude of the acausal artifacts. First, we investigated how the input acceleration waveform, having an ideal signal-like shape, was distorted by their procedure. Their acausal deconvolution indeed generated a large-amplitude terminal artifact; however, it was removed by the causal band-pass filtering performed after the deconvolution and consequently became negligible. Next, we constrained the maximum amplitude of the artifact due to the noise in a seismogram and showed that it was sufficiently small compared to the reported signal amplitudes. These results suggest that the signal waveforms seen after their procedure were not artifacts but were representing the input acceleration with sufficient accuracy. Namely, their procedure well functions as a detection method for pre-P-wave signals. In the context of this validation, we replied to the comments of Vallée et al. (2019). [ABSTRACT FROM AUTHOR]

This article describes improvement of the rapid earthquake analysis system, accurate and quick analysis system for source parameters (AQUA), which has automatically determined the location, magnitude, and fault type of M < ∼7 earthquakes with good accuracy by analyzing long-period waveform data, but could not process the 2011 Off Tohoku giant earthquake (M9.0) due to instrumental and program limitations. Here, we address these problems by incorporating strong-motion seismic data and changing processing parameters accordingly, which extends the upper limit of applicable magnitudes to M9. In a simulated process, this earthquake is properly determined as thrust faulting with M8.6 offshore northeast Japan within 431 s of the first phase arrival. We also evaluate its performance. After improvements, four recent large earthquakes (M7.0–7.9) were properly processed in real time. The successful determination ratio is 100% for events with Japanese seismic intensity ≥6 Lower, and 100% for events with M>7 except nearly simultaneous events. Source parameters with and without fixed horizontal locations are provided within 100–250 and 100–600 s of the first P arrival, respectively. This system is valuable for disaster mitigation because it rapidly provides information for use in tsunami warning and early damage assessment. [ABSTRACT FROM AUTHOR]

In earthquake hazard assessment studies, the focus is usually on horizontal ground motion. However, records from the 14 June 2008 Iwate-Miyagi earthquake in Japan, a crustal event with a moment magnitude of 6.9, revealed an unprecedented vertical surface acceleration of nearly four times gravity, more than twice its horizontal counterpart. The vertical acceleration was distinctly asymmetric; the waveform envelope was about 1.6 times as large in the upward direction us in the downward direction, which is not explained by existing models of the soil response. We present a simple model of a mass bounding on a trampoline to account for this asymmetry and the large vertical amplitude. The finding of a hitherto-unknown mode of strong ground motion may prompt major progress in near-source shaking assessments. [ABSTRACT FROM AUTHOR]

We stabilized a frequency-doubled Nd:YAG laser (λ=532 nm) with reference to an iodine absorption line, and applied it to a long-baseline interferometer for earth strain observations. To obtain unmodulated light, saturated absorption signals of an external iodine cell were detected by the modulation transfer technique using an acousto-optic modulator working as both amplitude and frequency modulators. Two feedback loops, which could control the laser frequency by a piezo-electric actuator and a thermal actuator, realized fast and wide-range frequency stabilization, and ensured long-term stable operation. From a beat-note measurement between two identical systems, we obtained a frequency stability (in Allan variance) of ...2 x 10[sup -13] for time intervals of 10 to 1000 s. By applying the stabilized Nd:YAG laser to a light source of a 10 m interferometer, we successfully observed earth tides and earthquakes in strain variations. Other geophysical signals, detectable by this strainmeter, are also discussed. [ABSTRACT FROM AUTHOR]