Search

A network of 10 satellite-telemetered broadband stations was established under a cooperative project between Japan and the Philippines, and a source analysis system based on waveform inversion of regional seismograms was adapted to operationalize a regional moment tensor analysis of Philippine earthquakes. This study presents the source information generated by the system for recent damaging earthquakes: theMw6.7 Negros andMw7.6 offshore Samar in 2012, and theMw7.2 in Bohol in 2013. Results show that the Negros event was generated by shallow NE–SW thrust faulting with a small strike-slip component, and that the centroid was located slightly offshore. The Samar event occurred in relation to an outer-trench thrust fault within the Philippine Sea Plate, adjacent to a part of the Philippine Trench that has relatively low seismicity. Our centroid moment tensor (CMT) solutions show that the Samar event triggered distinct clusters of outer-rise normal and thrust aftershocks, which we explain as being consistent with a Coulomb stress change in the area. Finally, we infer that the previously unidentified fault zone that generated the Bohol earthquake has a length of ∼ 100 km, is oriented ENE–WSW, transects parts of Bohol, and extends offshore towards Cebu. These examples show how recent improvements in Philippine earthquake monitoring could contribute to the characterization of earthquake sources and in the understanding of the seismotectonics of the area.

During a phreatic explosion at Mayon Volcano, Philippines, on 7 May 2013, a very long period seismic event with a peak frequency of 0.4 Hz was observed. Our frequency-domain waveform inversion solution of the event in the frequency range 0.1–0.6 Hz is consistent with a subhorizontal tensile crack and a vertical single force at a shallow location beneath the summit crater. The source time functions obtained by the waveform inversion are band-passed forms. We estimated the deconvolved forms of the source time functions (DSTFs), which are source time functions corrected for the effects of the band-pass filter. The DSTF of the crack can be approximated by an impulse-type function composed of inflation followed by deflation, whereas the DSTF of the single force can be approximated by a downward impulse. The inflation of the crack may be attributed to boiling of underground water and its deflation can be attributed to discharge of water vapor, whereas the downward force may be understood as the counterforce of the explosion. Our results suggest that only a portion of the crack wall was destroyed by the explosion. We could not find clear precursors in seismic, thermography, geothermal, geodetic, and meteorological data. We present a model of repeated explosions in which an explosion can occur once the fragmented portion of the crack is sealed by precipitation of clay minerals or hydrothermal secondary deposits. This model may explain the absence of clear precursory signals before the 2013 explosion.

Taal Volcano, Philippines, is one of the world's most dangerous volcanoes given its history of explosive eruptions and its close proximity to populated areas. A real-time broadband seismic network was recently deployed and has detected volcano-tectonic events beneath Taal. Our source location analysis of these volcano-tectonic events, using onset arrival times and high-frequency seismic amplitudes, points to the existence of a region of strong attenuation near the ground surface beneath the east flank of Volcano Island in Taal Lake. This region is beneath the active fumarolic area and above sources of pressure contributing inflation and deflation, and it coincides with a region of high electrical conductivity. The high-attenuation region matches that inferred from an active-seismic survey conducted at Taal in 1993. These features strongly suggest that the high-attenuation region represents an actively degassing magma body near the surface that has existed for more than 20 years.

We developed a centroid moment tensor (CMT) catalogue of earthquakes in and around Indonesia (InaCMT) using data from the nationwide broadband seismograph network in Indonesia. We obtained CMT solutions for about 500 earthquakes that occurred in and around Indonesia between 2006 and 2009. The moment magnitudes ranged from 4.5 to 8.3. We examined the accuracy of the CMT solutions of the InaCMT catalogue by comparing them with those obtained by the Global CMT (GCMT) Project. The seismic moments and focal mechanisms of these catalogues were highly consistent with each other, but we found systematic differences between the catalogues in the source centroid locations of earthquakes off Sumatra. The InaCMT source centroid locations were closer to the hypocenter locations in the Preliminary Determination of Earthquakes (PDE) of the U.S. Geological Survey compared to those of GCMT. The systematic deviations in the GCMT source centroid locations may mainly reflect insufficient azimuthal coverage of the stations used for the inversions as well as uncertainties in the Earth model. Using the InaCMT catalogue, we investigated seismic activity related to the off Bengkulu seismic sequence on 12 September 2007 (Mw = 8.3, 7.9, and 6.8), southwest of Sumatra, and the earthquakes northwest of the island of New Guinea on 3 January 2009 (Mw = 7.7 and 7.4). In the aftershock activity of the 2007 off Bengkulu seismic sequence, we found that shallow earthquakes were aligned along the eastern coast of Siberut Island, located between the Sunda trench and Sumatra. These earthquakes may have occurred along the Mentawai fault or another unknown fault. The focal mechanisms of the earthquakes were dominantly reverse slip, although the Mentawai fault has been considered to be a strike-slip fault. Shallow large earthquakes along this fault may cause damage above the source region and generate large tsunamis. We found that the 2009 earthquakes northwest of New Guinea occurred along the Manokwari trough and that the aftershocks were aligned along a plane dipping about 30° to the south, indicating that the Pacific and/or Philippine Sea plate subducts below the Australian plate at the Manokwari trough. The InaCMT catalogue enables us to perform detailed analyses of seismic activity in and around Indonesia, which contributes to better understanding of tectonic features and potential seismic and tsunami hazards in Indonesia as well as the coastal regions of the Pacific and Indian Oceans.

On September 1, 2004, Mt. Asama in central Japan erupted for the first time in 21 years. Between this moderate eruption and mid-November of the same year, 4 additional moderate eruptions occurred. We installed 8 broadband seismic stations in addition to the short period seismic network around the volcano and succeeded in recording the near-field seismic signals associated with the summit eruptions. The results of the waveform inversions clearly show that the force system exerted at the source region is dominated by vertical single force components. The source depths of the single force are shallower than 200 m from the bottom of the summit crater, and the order of magnitude of the single force is 1010–1011N. The source time history of each vertical single force component consists of two downward forces and one upward force. The initial downward force probably corresponds to the sudden removal of a lid capping the pressurized conduit. The drag force due to viscous magma moving upward in the conduit can explain the upward force. The correlation between the single force amplitudes and the amounts of volcanic deposits emitted from the summit crater are not necessarily positive, suggesting that the amount of deposits remaining within the summit crater may have played an important role in the excitation of the single force.

[1] We performed numerical simulations of seismic waveforms with frequencies up to 10 Hz in heterogeneous media with topography to investigate the effects of topography and structural heterogeneity on seismic scattering. We used the simulated waveforms to test the source location method assuming isotropic radiation of S waves for long-period events and tremor at volcanoes. The assumption of isotropic radiation has been shown previously to be valid in a high-frequency band because of the path effect caused by the scattering of seismic waves. Our simulation results showed that distortion of the wavefields increased as the correlation distance of structural heterogeneity (a) decreased, as the frequency increased, and as the travel distance increased. Topography alone considerably distorted the wavefields. However, we found that strong scattering due to topography was suppressed if the correlation distance of structural heterogeneity was longer than the S wave wavelength. Isotropic radiation of S waves by scattering due to topography was not achieved in our simulations. Our results indicated that scattering due to structural heterogeneity becomes stronger than that due to topography at ka ≅ 1 and a = 50 m, where k is the wave number of the S waves. This suggests that strong short-scale structural heterogeneity is required to achieve isotropic radiation of S waves. Although we could not reproduce isotropic radiation, our results support the validity of the assumption of isotropic radiation in the source location method.

SUMMARY On 2007 March 6, an earthquake doublet occurred along the Sumatran fault, Indonesia. The epicentres were located near Padang Panjang, central Sumatra, Indonesia. The first earthquake, with a moment magnitude (M w) of 6.4, occurred at 03:49 UTC and was followed two hours later (05:49 UTC) by an earthquake of similar size (M w = 6.3). We studied the earthquake doublet by a waveform inversion analysis using data from a broadband seismograph network in Indonesia (JISNET). The focal mechanisms of the two earthquakes indicate almost identical rightlateral strike-slip faults, consistent with the geometry of the Sumatran fault. Both earthquakes nucleated below the northern end of Lake Singkarak, which is in a pull-apart basin between the Sumani and Sianok segments of the Sumatran fault system, but the earthquakes ruptured different fault segments. The first earthquake occurred along the southern Sumani segment and its rupture propagated southeastward, whereas the second one ruptured the northern Sianok segment northwestward. Along these fault segments, earthquake doublets, in which the two adjacent fault segments rupture one after the other, have occurred repeatedly. We investigated the state of stress at a segment boundary of a fault system based on the Coulomb stress changes. The stress on faults increases during interseismic periods and is released by faulting. At a segment boundary, on the other hand, the stress increases both interseismically and coseismically, and may not be released unless new fractures are created. Accordingly, ruptures may tend to initiate at a pull-apart basin. When an earthquake occurs on one of the fault segments, the stress increases coseismically around the basin. The stress changes caused by that earthquake may trigger a rupture on the other segment after a short time interval. We also examined the mechanism of the delayed rupture based on a theory of a fluidsaturated poroelastic medium and dynamic rupture simulations incorporating a rheological velocity hardening effect. These models of the delayed rupture can qualitatively explain the observations, but further studies, especially based on the rheological effect, are required for quantitative studies.

[1] The Kuril arc collides with the northeast Japan arc in the southern part of Hokkaido, Japan. 3-D tomographic inversion of data from a dense network of sensitive ocean-bottom seismographs and land stations has allowed imaging of previously unseen details of the arc-arc collision structure. A low velocity body dips gently southwestward, at depths of 35 to 45 km, from east of the Hidaka Mountains to the source area of the 1982 Urakawa-oki destructive earthquake (Ms 6.8). The low velocity body is the lower half of the lower crust of the Kuril arc, which must have been delaminated by the collision. We believe that the continuing collision of the delaminated lower crust with the northeast Japan arc resulted as an episode of aseismic slow slip prior to the 1982 Urakawa-oki earthquake as well being the reason for the high seismic activity in this region.

In the early morning of 27 May 2006, local time, central Java was jolted by strong seismic ground motion. In spite of the moderate size of the earthquake (Mw = 6.4), it caused severe damage nearYogyakarta city. According to a report from the Social Affairs Ministry of Indonesia, more than 5700 people were killed, 38,000 injured, and 423,000 evacuated. As a result of the shaking, more than 126,000 buildings collapsed, and more than 392,000—including those of the famous Prambanan temple complex, located about 17 kilometers east of Yogyakarta—were severely damaged (Figure 1).