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Abstract
SUMMARYWe recently found the original Omori seismograms recorded at Hongo, Tokyo, of the 1922 Atacama, Chile, earthquake (MS = 8.3) in the historical seismogram archive of the Earthquake Research Institute (ERI) of the University of Tokyo. These recordings enable a quantitative investigation of long-period seismic radiation from the 1922 earthquake. We document and provide interpretation of these seismograms together with a few other seismograms from Mizusawa, Japan, Uppsala, Sweden, Strasbourg, France, Zi-ka-wei, China and De Bilt, Netherlands. The 1922 event is of significant historical interest concerning the cause of tsunami, discovery of G wave, and study of various seismic phase and first-motion data. Also, because of its spatial proximity to the 1943, 1995 and 2015 great earthquakes in Chile, the 1922 event provides useful information on similarity and variability of great earthquakes on a subduction-zone boundary. The 1922 source region, having previously ruptured in 1796 and 1819, is considered to have significant seismic hazard. The focus of this paper is to document the 1922 seismograms so that they can be used for further seismological studies on global subduction zones. Since the instrument constants of the Omori seismographs were only incompletely documented, we estimate them using the waveforms of the observed records, a calibration pulse recorded on the seismogram and the waveforms of better calibrated Uppsala Wiechert seismograms. Comparison of the Hongo Omori seismograms with those of the 1995 Antofagasta, Chile, earthquake (Mw = 8.0) and the 2015 Illapel, Chile, earthquake (Mw = 8.3) suggests that the 1922 event is similar to the 1995 and 2015 events in mechanism (i.e. on the plate boundary megathrust) and rupture characteristics (i.e. not a tsunami earthquake) with Mw = 8.6 ± 0.25. However, the initial fine scale rupture process varies significantly from event to event. The G1 and G2, and R1 and R2 of the 1922 event are comparable in amplitude, suggesting a bilateral rupture, which is uncommon for large megathrust earthquakes.
Highlights
GPS measurements indicate that the plate boundary is accumulating slip deficit along the entire 1922 source region and there is potential for future large earthquakes in this region (e.g. Nishenko 1985; Metois et al 2016, Fig. 1)
(3) By the time the 1922 event occurred, the basic structure of the earth’s interior had been established (e.g. Oldham 1906; Gutenberg 1914), and the many seismic phases Gutenberg (1924) identified on the seismograms of the 1922 event provided the basis for further significant refinement
The Hongo Omori seismograms are the best long-period records we have found so far for the 1922 earthquake, these estimates are from a single station with some assumptions on the instrument constants, and considerable uncertainty is inevitable
Summary
We found the original Omori seismograms of the 1922 earthquake in the historical seismogram archive of the Earthquake Research Institute (ERI) of the University of Tokyo This record allows us to make a quantitative investigation of long-period seismic radiation from the 1922 earthquake. We could find the original Omori long-period smoked paper seismograms recorded at Hongo, Tokyo, Japan (35.717◦, 139.767◦) along with some instrumental details in the ERI historical seismogram archive. The Hongo Omori seismograms are the best long-period records we have found so far for the 1922 earthquake, these estimates are from a single station with some assumptions on the instrument constants, and considerable uncertainty is inevitable. We will discuss this later after we investigate the Uppsala Wiechert records
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