Takeo Ishibe

Analysis of the Completeness Magnitude and Seismic Network Coverage of Japan

Abstract A reliable estimate of completeness magnitude, M c , above which all earthquakes are considered to be detected by a seismic network, is vital for seismicity-related studies. We show a comprehensive analysis of M c in Japan. We use the catalog maintained by the Japan Meteorological Agency (JMA) and also available information on seismic stations that report to JMA. For computing M c , we adopt a commonly used method based on the Gutenberg–Richter frequency-magnitude law. Presently, M c =1.0 might be typical in the mainland, but to have a complete catalog, one needs to use earthquakes with magnitudes of 1.9 or larger. Comparison with the Southern California Seismic Network (SCSN) suggests that the recent event detectability in the mainland generally shows similar completeness levels to that in the authoritative region of SCSN. We argue that the current M c of Japan is due to the success of network modernization over time. Particularly, we show that the spatiotemporal change of M c closely matches the addition of the Hi-net borehole stations to enhancing seismic-station density; it started in October 1997 in southwestern Japan, continuing to northeastern Japan until 2002. As suggested from this matching, we confirm that M c inversely correlates with station density. Further, we find that irrespective of the network change after 1997, this correlation is unchanged in time, demonstrating that the influence on M c from factors beyond station density does not vary in time. Contrary to Alaska and California (Wiemer and Wyss, 2000), our results do not attribute such factors simply to anthropogenic noise. Because this is due to the borehole stations that reduce ambient noise, we conclude that in Japan the anthropogenic noise has an insignificant effect on M c .

Comparative study of two tsunamigenic earthquakes in the Solomon Islands: 2015 Mw 7.0 normal‐fault and 2013 Santa Cruz Mw 8.0 megathrust earthquakes

The July 2015 Mw 7.0 Solomon Islands tsunamigenic earthquake occurred ~40 km north of the February 2013 Mw 8.0 Santa Cruz earthquake. The proximity of the two epicenters provided unique opportunities for a comparative study of their source mechanisms and tsunami generation. The 2013 earthquake was an interplate event having a thrust focal mechanism at a depth of 30 km while the 2015 event was a normal-fault earthquake occurring at a shallow depth of 10 km in the overriding Pacific Plate. A combined use of tsunami and teleseismic data from the 2015 event revealed the north dipping fault plane and a rupture velocity of 3.6 km/s. Stress transfer analysis revealed that the 2015 earthquake occurred in a region with increased Coulomb stress following the 2013 earthquake. Spectral deconvolution, assuming the 2015 tsunami as empirical Greens function, indicated the source periods of the 2013 Santa Cruz tsunami as 10 and 22 min.

Characteristic Earthquake Model and Seismicity around Late Quaternary Active Faults in Japan

A total of 172 late Quaternary active fault zones in Japan are examined to determine whether the Gutenberg–Richter relationship or the characteristic earthquake model more adequately describes the magnitude–frequency distribution during one seismic cycle. By combining seismicity data for more than 100 active fault zones at various stages in their seismic cycles, we reduced the short instrumental observation period compared to the average recurrence interval. In only 5% of the active fault zones were the number of observed events equal to or larger than the number of events expected by the Gutenberg–Richter relationship. The average and median frequency ratios of the number of observed events to the number of expected events from the Gutenberg–Richter relationship are only 0.33 and 0.06, respectively, suggesting that the characteristic earthquake model more appropriately describes the magnitude–frequency distribution along the late Quaternary active faults during one seismic cycle. Moreover, the larger an average slip rate is or the shorter an average recurrence interval, the larger the gap in magnitude tends to be between the characteristic earthquake and the largest among other events. A fault zone with a shorter average recurrence interval and/or a larger average slip rate has generally produced more earthquakes in the past or is likely to be at a more mature or developed stage. Thus, these tendencies may reflect a change in the magnitude–frequency distribution related to the maturity or development of fault zones.

Seismicity in source regions of large interplate earthquakes around Japan and the characteristic earthquake model

The occurrence rate of the characteristic earthquake (CE) was compared with seismicity in nine source regions of interplate earthquake (i.e., the regions off Shikotan Island, off Nemuro, off Tokachi, off Northern Sanriku, off Miyagi, far off Miyagi, and the Tonankai, Nankai, and Kanto regions) by combining instrumental data and the recurrence interval of CEs evaluated by the Headquarters for Earthquake Research Promotion (HERP) in Japan. The recurrence interval of the Nankai earthquake was estimated on the basis of long historical records and found to be one of the least uncertain ones. We used the unified catalog of earthquakes obtained by the Japan Meteorological Agency (JMA) on the basis of a recently improved seismic network, together with the old JMA catalog. Seismicity of all the interplate source regions indicated that the number of observed events was much less than the number of events predicted from the Gutenberg-Richter (G-R) relation and the occurrence rate of the CE. In all regions except for far off Miyagi, the CEs occurred during the interval of the earthquake catalog. Thus, our data set included the highest seismicity period during an earthquake cycle. In the region off Tokachi where the 1952 and 2003 Tokachi-Oki earthquakes occurred, the magnitude frequency distribution (MFD) during one seismic cycle exhibited a magnitude gap of 1.1 between the CE and the other events. Therefore, our results favored the CE model. Moreover, we conversely estimated an average recurrence interval of the CEs in each region, based on the assumption that the G-R relation holds. Most estimated recurrence intervals were far longer than the evaluation given by HERP.

Testing the Coulomb stress triggering hypothesis for three recent megathrust earthquakes

We test the static Coulomb stress triggering hypothesis for three recent megathrust earthquakes (the 2004 Sumatra–Andaman earthquake, the 2010 Maule earthquake, and the 2011 Tohoku-Oki earthquake) using focal mechanism solutions for actual earthquakes as receiver faults to calculate Coulomb stress changes. For the 2004 Sumatra–Andaman and 2011 Tohoku-Oki earthquakes, the median values of the Coulomb stress changes for 100 consecutive earthquakes revealed temporal changes from approximately zero before the megathrust earthquake to significant positive values following the mainshock, followed by decay over time. Furthermore, the ratio of the number of positively to negatively stressed receiver faults increased after the megathrust. These results support the triggering hypothesis that the static stress changes imparted by megathrust earthquakes cause seismicity changes. This is in contrast to the results of a previous study that used optimally orientated receiver faults to calculate Coulomb stress changes, and this difference indicates the importance of considering the spatial and temporal heterogeneities of receiver fault distributions. For the 2010 Maule earthquake, however, the results are strongly dependent on fault-slip models. Since most receiver faults are concentrated in the mainshock source region, slip models significantly affect the computed Coulomb stress changes and sometimes cause anomalous stress concentrations along the edge of each sub-fault.

A Focal Mechanism Solution Catalog of Earthquakes (M≥2.0) in and around the Japanese Islands for 1985–1998

Abstract We determined focal mechanism solutions for 14,544 earthquakes that occurred in and around the Japanese Islands from July 1985 to December 1998 using first‐motion polarities reported by the Japan University Seismic Network and compiled by the Japan University Seismic Network Earthquake Catalog of First‐Motion Focal Mechanisms (JUNEC FM ∧ 2). This catalog covers small‐magnitude earthquakes ( M ≥2.0) prior to the recent development of seismic observation networks and automated waveform data processing systems. This catalog will prove helpful in understanding the spatial and temporal heterogeneities of stress fields by combining recent focal mechanism solutions and also will be useful for statistical analyses. Online Material: Figures of velocity structure, focal mechanisms, magnitude–frequency distributions, and temporal changes in station locations.

Source model of the 16 September 2015 Illapel, Chile,Mw8.4 earthquake based on teleseismic and tsunami data: ILLAPEL (CHILE) TSUNAMI OF 16 SEPTEMBER 2015

Teleseismic data were provided by the Incorporated Research Institutions for Seismology (http://www.iris.edu/wilber3/find_event). Tide gauge data can be found at the Intergovernmental Oceanographic Commission website (http://www.ioc-sealevelmonitoring.org/). DART records were provided by NOAA (http://nctr.pmel.noaa.gov/Dart/). Earthquake catalogs by the USGS National Earthquake Information Center (http://earthquake.usgs.gov/earthquakes/search/) and Global Instrumental Earthquake Catalogue (1900-2009) of International Seismological Centre Global Earthquake Model (http://www.globalquakemodel.org/what/seismic-hazard/instrumental-catalogue/) were used in this study. We used the GMT software for drawing the figures [Wessel and Smith, 1998]. This article benefited from constructive review comments by Costas E. Synolakis (University of Southern California, USA) and Yuichiro Tanioka (Hokkaido University, Japan) for which we are grateful. We acknowledge financial supports from the Japan Society for the Promotion of Science.