Characteristics of the stress and barometric seismograms produced by the 2011 Tohoku Earthquake (M9.0) and vertical movements derived from barometric seismograms
© Ishii and Asai. 2016
Received: 3 December 2015
Accepted: 4 April 2016
Published: 23 April 2016
High-quality data concerning the Tohoku Earthquake (M9.0) on March 11, 2011, were obtained from the deep borehole observation network (maximum depth of 1030 m; epicentral distance of approximately 600 km) of the Tono Research Institute of Earthquake Science. In addition to data acquired via seismometers, stress meters, and strain meters, barometric seismograms were recorded by several barometers that are usually used for weather observations. We examined the characteristics of barometric and stress seismograms and compared them to the data obtained using broadband seismometers, finding a shared feature: large amplitudes and long-period waveforms began with the arrival of surface waves. We also investigated the relationship between vertical movements observed with GPS and barometric variations and discovered that the barometric variations were related to the differential of vertical movements, while the vertical movements corresponded to the integral of barometric variations. All these results demonstrate that vertical movements at observation points can be computed from the barometric variations observed at those points.
The phrase “barometric seismogram” is used in the title and throughout the paper. We define “atmospheric pressure changes caused by an earthquake” as “barometric seismograms” hereafter. In research based on microbarograph data concerning pressure changes associated with the 2003 Tokachi-Oki Earthquake in Japan (Watada et al. 2006), the relationship between seismic velocities and barometric variations has been investigated using the records obtained with a velocity seismograph.
Most of the broadband seismic data were out of scale for the Tohoku Earthquake, which occurred on March 11, 2011, because the magnitude was over 8 (M9.0). However, the stress meters, strain meters, and barometers installed for the deep borehole observation network (maximum depth 1030 m; epicentral distance of approximately 600 km) at the Tono Research Institute of Earthquake Science (TRIES) could capture some high-quality data. Several barometers usually used for weather observation (F4711, Yokogawa Denshikiki. Co., Ltd.)—inexpensive microbarographs—were able to record the barometric seismograms caused by the earthquake. The GPS observations conducted near our observation sites by the Geospatial Information Authority of Japan (GSI) captured vertical movements attributable to the earthquake. We investigated the features of the recorded stress and barometric seismograms as well as the relationship between the barometric variations and vertical movements.
TRIES observation sites
The observations at STG200 were made in a borehole excavated 20 m below the horizontal shafts at a depth of 200 m. The data used in this study are as follows: barometer records (at SN3 and STG200), water pressure records (at STG200), stress records (at STG200), vertical movements observed with GPS (at TOKI by GSI), acceleration record (at TOS), and STS seismogram (at MIZ). STS is a name of broadband seismometer manufactured by Streckeisen Company.
Comparison of data obtained with stress meters with data obtained by STS seismometers
Barometric variations derived from vertical movements
In this present study, we differentiated the observed vertical movements and converted them into ground motion velocities, obtaining the black line shown in the lower portion of Fig. 4. The red and green lines show the barometric seismograms observed at SN-3 and STG200, respectively. The shapes of those lines are similar to the shape of the line derived from the differential of vertical movements, indicating that barometric variations are proportional to the velocities of vertical movements at the ground surface. A differentiated vertical movement of 0.11 m/s corresponds to barometric variations of 50 Pa. Watada et al. (2006) explained the observed pressure change by (air density) × (sound velocity in air) × (ground vertical velocity). The present case can be explained in the same way.
Vertical movements derived from barometric variations
Large variations began with the arrival of S waves (at around 14:49) for all observation factors.
Data from both the stress meter and the barometer show large amplitudes and long-period waveforms from 14:50:12 to 14:50:48 (marked with a horizontal arrow). Large amplitudes and long-period waveforms like this are observed by water pressure meters, barometers, stress meters, and vertical movements, but not by seismometers. These data will yield useful information about earthquake mechanisms in the future.
The vertical movements (6 and 7) computed from the integrals of the observed barometric variations (3 and 4) nearly match the observed vertical movements (8).
The barometric variations (5) computed through the differentiation of the observed vertical movements (8) nearly match the observed barometric variations (3 and 4).
The aforementioned 3 and 4 suggest that vertical movements can be estimated from barometric variation data.
HI designed this study, analyzed the data, and wrote the manuscript. YA and HI installed the instruments (stress meters, strain meters, water pressure meters, and barometers) and maintained them, and YA helped interpretation of the data. Both authors read and approved the final manuscript.
We would like to extend our gratitude to the Geospatial Information Authority of Japan, who provided us with the GPS observation data. The authors wish to thank two anonymous reviewers for providing valuable comments.
The authors declare that they have no competing interests.
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