A complete suite of closed analytical expressions is presented for the surface displacements, strains, and tilts due to inclined shear and tensile faults in a half-space for both point and finite rectangular sources. These expressions are particularly compact and free from field singular points which are inherent in the previously stated expressions of certain cases. The expressions derived here represent powerful tools not only for the analysis of static field changes associated with earthquake occurrence but also for the modeling of deformation fields arising from fluid-driven crack sources.

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Surface deformation due to shear and tensile faults in a half-space

We report the repeating occurrence of short- and long-term slow slip events (SSE) which are accompanied by deep tremor activity around the Bungo channel region, southwest Japan. Both of these activities are detected by NIED Hi-net, which is composed of densely distributed observatories equipped with a set of tiltmeter and a high-sensitivity seismograph. Since the short-term SSE is small in magnitude, GPS can detect only the long-term SSE. Some of these episodes have nearly the same surface deformation pattern. This shows the existence of ‘slow slip patches’ on a plate interface, where the episodic slow slip is the characteristic slip behavior. We observe a change in periodicity and size of the short-term episode after the onset of the long-term SSE. Moreover, the long-term slow slip accelerates when the short-term activity takes place. This suggests that there is an interaction between these two types of SSEs.

/pdf/repeating-short-and-long-term-slow-slip-events-with-deep-4b6qxvps0u.pdf

Repeating short- and long-term slow slip events with deep tremor activity around the Bungo channel region, southwest Japan

The techniques and instrument designs developed for continuously recording tiltmeters and strainmeters are reviewed. Consideration is given to short-base and long-base tiltmeters as well as to rod, wire, laser and borehole hydraulic strainmeters. Since a good design is difficult to achieve, it is necessary to select the goals an instrument must meet and be prepared to compromise on others. The importance of comparison tests is demonstrated, and it is concluded that good results cannot be gotten at low cost or without careful attention to details.

Strainmeters and tiltmeters

National Research Institute for Earth Science and Disaster Resilience (NIED) integrated the land observation networks established since the 1995 Kobe earthquake with the seafloor observation networks established since the 2011 Tohoku earthquake and tsunami as MOWLAS (Monitoring of Waves on Land and Seafloor) in November 2017. The purpose of MOWLAS is to provide comprehensive, accurate, and rapid observation and monitoring of earthquake, tsunami, and volcano events throughout Japan and its offshore areas. MOWLAS data are widely utilized for long-term earthquake forecasting, the monitoring of current seismic activity, seismic and tsunami hazard assessments, earthquake early warning, tsunami warning, and earthquake engineering, as well as earthquake science. Ocean bottom observations provide an extension of observations to areas where no people are living and have the advantage of increasing lead time of earthquake early warning and tsunami warning. The application of recent technology advancements to real-time observations as well as the processing of MOWLAS data has contributed to the direct disaster mitigation of ongoing earthquakes. These observations are fundamental for both science and disaster resilience, and thus it is necessary to continue ceaseless operation and maintenance.

/pdf/mowlas-nied-observation-network-for-earthquake-tsunami-and-17yntr8yp8.pdf

MOWLAS: NIED observation network for earthquake, tsunami and volcano

We observed very long period seismic events that are associated with the 1998 activity of Iwate Volcano, northeast Japan. The events show a dominant period of 10 s and duration of 30–60 s, often with accompanying short-period waves at the beginning and at the end of the long-period signals. By analyzing the broadband seismograms we find that the source elongates in the east-west direction for ∼4 km at a depth of 2 km beneath the western part of Iwate Volcano. Results of moment tensor inversions show a source mechanism of mutual deflation and inflation of two chambers located at the western and eastern edges of the source region. The source region coincides with the low seismic velocity zone detected by seismic tomography and is very close to the locations of pressure sources estimated from crustal deformation data. On the basis of these results we infer that the very long period seismic events are generated by transportation and movement of magmatic fluid (hot water and/or magma) in a shallow part of the volcano. We further present a simple source model of very long period seismic events based on one-dimensional flow dynamics and propose a new parameter to characterize the size of very long period event: the energy flow rate, which is obtained by dividing the seismic moment by the dominant period. The energy flow rate was estimated as 3.1×1012 J/s for the event on July 29, 1998.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JB900155

Source process of very long period seismic events associated with the 1998 activity of Iwate Volcano, northeastern Japan

Continuous monitoring of the crustal tilt is one of the most effective approaches for the purpose of earthquake prediction. Drilling a borehole is not so expensive as construction of an underground vault, which is necessary for installation of water-tube tiltmeters and horizontal pendulum tiltmeters, and a borehole station is possible to be made even in a plain area. The National Research Center for Disaster Prevention has not only developed the boreholetype tiltmeter itself but has improved borehole drilling techniques and installation methods for the tiltmeter. The force-balanced pendulum tiltmeter developed for down-hole observation is installed at the bottom of the borehole of which the depth is about 100m. The measuring range is +2 x 10-4 radian, the resolution is 6 x 10-9 radian, and the long term drift is smaller than a few micro-radian per year after half a year since the installation. Clear records of the earth tides demonstrate the high quality of the instrument. Tilt changes of the order of micro-radian caused by precipitation are remarkable in the data of some stations. But, the crustal tilt observation by using borehole-type tiltmeters is expected to be very effective for detecting anomalous tilt changes of the order of 5 x 10-8 radian per day with a time constant of several hours to several tens of days.

Eiji Yamamoto

Papers

Development of the Crustal Tilt Observation Method Using Borehole-type Tiltmeters