2008 AGU Fall Meeting
San Francisco, California Dec.15-19, 2008
Fault plane Determination for the Niigataken Chuetsu-oki Earthquake Using Tsunami Simulations
Yui, Y. and Mori, J. J.
On July 16th 2007, the Niigataken chuetsu-oki earthquake occurred along the Japan Sea coast of central Japan. This strike of the fault was estimated to be approximately northeast-southwest but from the preliminary aftershock distribution and crustal deformations, it was difficult to determine if the fault dipped toward the northwest or toward the southeast. A tsunami occurred due to the sea bottom deformation caused by this earthquake. This tsunami was not so large, so it did not cause significant damage along the Japan Sea coast, however, it was clearly recorded by a number of tide-gauge stations along the Japan Sea coast. In this study, we use the focal mechanisms from F-net and estimate the location of the fault plane from arrival times of the tsunami waves at Kashiwazaki station in the southern direction and Sado station in the northern direction. We calculated the sea floor deformation assuming both northwest and southeast dipping fault planes, and then calculated the resultant tsunami waveforms for tide-gauge stations that clearly recorded the tsunami. Using comparisons of the model and observed tsunami waveforms at 6 stations, we try to estimate whether the major slip was on a westward or eastward dipping fault plane. The results show that a fault that dips to the south-east fits both the arrival times and the shape of the tsunami waveforms, better than for a fault that dips toward the northwest.
Aftershock Rates and Spatial Complexity for Recent Moderate Earthquakes in Japan
We looked at the spatial and temporal distributions for 6 recent moderate earthquakes that occurred at shallow depth onshore of Japan and were well recorded by the regional networks. These events are the 2000 Western Tottori (Mw 6.7), 2004 Niigata Chuetsu (Mw 6.6), 2005 Fukuoka (Mw 6.6), 2007 Noto Peninsula (Mw 6.7), 2007 Niigata Chuetsu-oki (Mw 6.8), and 2008 Iwate-Miyagi-ken (Mw 6.8). All of these earthquakes are approximately of similar size, however, the rates of aftershock activity are quite different. The 2004 Niigata and 2008 Iwate-Miyagi earthquakes have significantly more aftershocks than the other 4 events. In the spatial locations of the aftershocks, these two earthquakes have more complex spatial distributions with more aftershocks occurring away from the mainshock fault plane. There appears to be a correlation between the rate of aftershock activity and the spatial complexity of the locations. The sequences with higher rates of aftershock occurrence may be associated with aftershocks triggered in a volume around the mainshock. In contrast, for the other sequences, aftershocks occur mainly in a planar pattern close to the mainshock fault plane. We also looked at the early time sequence of the aftershocks for these events. Using continuously recorded seismograms from nearby borehole stations of Hi-net, aftershocks were identified and counted. From about one minute following the mainshock origin time, we estimate that we can identify aftershocks with magnitudes down to Mj 3.5. For the first few minutes the rate of aftershocks is quite similar for all of the mainshocks. The higher rate of aftershocks for the 2004 Niigata and 2008 Iwate-Miyagi earthquakes appears to begin about 10 minutes after the mainshock. This suggests that the enhanced triggering of aftershock for these 2 earthquakes is caused by some changes in the aftershock region several minutes after the mainshock.
Fault Plane Determination And Possible Triggering Of The 2007 Kuril Islands Earthquake (Mw 8.1)
Norimatsu, K. and Mori, J. J.
Two great earthquakes occurred in the Kuril Islands area in 2006 and 2007. The 2006 earthquake (Mw 8.3) occurred on15 November and was located on the subduction boundary between the Pacific and Okhotsk plates. About two month later, the 2007 earthquake (Mw 8.1) occurred on 13 January and was located on the outer-rise portion of the Pacific Plate. It is relatively-uncommon that, great earthquakes (Mw >= 8.0) happen in the same region within a short span of time, and also great outer-rise earthquakes have occurred only three times since the early 20th century. The aim of this study is to estimate the orientations of fault planes for the 2007 outer-rise earthquake and investigate the possible of triggering for the 2007 earthquake. For the 2007 earthquake, there are currently not clear indications of the geometry so it is difficult to understand the relation between the 2006 and 2007 earthquakes. To relocate the associated seismicity, we used a master event technique. The depths of the master events were determined by teleseismic waveform modelling and other earthquakes were located relative to the master events. For the relocations of seismicity, we used P wave arrival times compiled by the United States Geological survey (USGS) during the period from 20 September 2006 to March 2007. The results of fault plane determinations show that, the fault plane of the 2006 earthquake has a shallowly northwest dipping plane and the 2007 earthquake has more steeply dipping plane to the southeast. To investigate the possible triggering, we estimate the static stress changes with the Coulomb Failure Function Change (¢ CFF). Using the slip distribution for the 2006 earthquake, we calculated the static stress changes in the hypocenter area of 2007 earthquake. The results of the determination of ¢ CFF show positive values (conductive to failure) for the hypocenter area of the 2007 earthquake and surrounding region.
Real-time Application of Earthquake Early Warning Considering Effects of Near-field Terms
Yamada, M. and Mori, J.
This research analyzes strong motion records of 24 large earthquakes to investigate the relationship between Ñc (the average period of the first motion) and moment magnitude. The records of large earthquakes very close to the source include large long-period near-field terms. Therefore, if we do not consider the effect of the near-field term, we may overestimate the final size of the magnitude. We processed the records which do not have strong near-field terms, and compute the Ñc for each event. Our analysis shows the value of Ñc takes between the corner frequency and the period determined by the record duration to compute Ñc. If the magnitude is less than 6, Ñc becomes close to the period corresponding to the corner frequency, whereas Ñc for larger earthquakes depends on the rupture process and location of area of large slip. Ñc for large earthquakes regulates the lower bound of the magnitude estimation, and it approaches the period corresponding to the corner frequency, if a longer record duration is used. We also propose a method to classify the records with and without large near-field terms. If the Pd3 (peak displacement of the first 3 seconds) exceeds 1cm and Ñc > 2 seconds, the record is more likely to contain a large near-field term. For the purpose of quick onsite warnings, stations observing large near-field terms provide valuable information. Large near-field terms can be observed only if the magnitude is large and the epicentral distance is small. Therefore, if the displacement exceeds a threshold (e.g. Pd = 0.5cm), the ground motion at the site would likely become very large. This criterion will help to issue warnings to the blind zone that is close to the source. We incorporated this algorithm into the Seismic Automatic Triggering and Recording Network (SATARN) system operated by the Research Center for Earthquake Prediction, Kyoto University. We constructed a prototype system using the data recorded by stations around Kyoto city. Event triggers are picked by using ratios of the short-term and long-term averages, and then the minimum AIC is computed to search for the onset of the P-arrival. Currently, near- field terms and values of tauc are monitored, which may provide quicker warnings for large nearby earthquakes.
Variations in Seismic Anisotropy with time on Volcanoes in Kyushu Island, Southern Japan
Savage, M. K., T. Ohkura, K. Umakoshi, H. Shimizu, Y. Kohno, M. Iguchi, A. Wessel, J. Mori
Using a newly developed automatic processing technique, we have calculated shear wave splitting on and near three active volcanoes in Kyushu, southern Japan (Aso, Unzen and Sakurajima). Shear wave splitting is considered to be caused by aligned cracks and microcracks. The polarisation of the first arriving phase, Ó, gives a measure of the crack orientation, which is expected to align with the maximum principal stress. The delay time dt between the two phases depends upon the crack density and the path length. High quality measurements include the following: a) over 1700 from local events recorded and located near Aso Volcano between 2001 and 2008; b) over 2000 from local events recorded and located near Unzen volcano between 1988 and 1997 (spanning the most active period of seismic activity related to the large eruption in 1991); c) over 600 from regional events originating in the subducting Phillipine Sea plate recorded near Sakurajima volcano between 2003 and 2005, (during which time numerous small eruptions have occcurred, and GPS measurements have been modeled as caused by inflation of a Mogi source and a near-vertical crack). Most of the stations were located in boreholes or tunnels, providing excellent signals. Common features at all three volcanoes are that stations closest to the craters yield the fewest good measurements, and even those tend to give varying results at closely spaced stations. Scattering from the volcanic edifice may be making the S waves difficult to pick, and the local stresses may be varied. Stations on the volcanic flanks give many good measurements. Some stations yield variations in Ó and dt that depend upon the earthquake location. But at each volcano, some stations show changes that are better explained by variations in time than in space. Where GPS measurements are available, the variations sometimes but not always correlate with previously-modeled inflation or deflation events. The temporal variations in Ó are large, ranging from 30 at some stations to 90 at other stations. These results will allow us to test models of stress changes with time on the volcanoes.
What can we Learn From Dynamic Triggering of Low-Frequency Earthquakes?
Miyazawa, M., E.E. Brodsky, J. Mori
Remote triggering of small low-frequency seismic events near the seismic-aseismic transition zone of subduction zones, by surface waves from large distant earthquakes, has been reported in southwest Japan and the Cascadia region. Recent observed triggering in southwest Japan from three large earthquakes (2003 Tokachi-oki (Mw 8.1), 2007 Solomon (Mw 8.1), and 2008 Wenchuan (Mw 7.9)) covering wide azimuthal information that is necessary to distinguish the triggering processes, shows significant triggering from Rayleigh waves rather than Love waves. This observation provides strong evidence for the influence of fluids in the source area because fluid can be affected by normal stress changes and not by shear stress changes. In the Cascadia region, it has been reported that seismic tremor associated with episodic slip is triggered by shear stress changes from Love waves. The low-frequency events both in southwest Japan and the Cascade region are thought to be fluid related events. The Coulomb failure stress analyses suggest the effective friction coefficient is large for southwest Japan and small for the Cascadia region, which could be related to the amount of fluid in the source regions of the low-frequency events.
7th General Assembly of Asian Seismological Commission
2008 Seismological Society of Japan Fall Meeting
Tsukuba November 24-27, 2008
Back-Projection Analysis of the 2008 Wenchuan, China Earthquake using Hi-net Data
P waveforms from about 700 Hi-net stations in Japan were used in a back projection analysis of the rupture for the May 12, 2008 Wenchuan, China earthquake (Mw7.9). The stations in Japan are at a distance of about 25 to 30 degrees from the earthquake. The P waveforms have durations of about 100 sec and these data were used to estimate the rupture propagation for the earthquake.
The back-projection method tests a grid of points to determine the best location for the source of seismic radiation for a designated time window of the P wave. The initial arrival of the first time window was assumed to come from the grid point corresponding to the earthquake hypocenter. For each subsequent time window, the data were stacked assuming a source at each grid point. The grid point that corresponds to the stack with the highest amplitude is determined to be the source of the P-wave energy. Relative time shifts for each time series were calculated using the theoretical travel times from the station to the grid point, using the IASPEI91 model. The grid of the 290 tested source locations over a fault length of 400 km was used for each time window. Time windows of 10 to 30 seconds were used and produced generally similar results.
There may be some significant complications in the waveforms from the depth phases for a shallow strike-slip earthquake. Part of this problem may be overcome if large enough time windows are used that include the direct arrivals and depth phases, and the whole wavetrain is used.
The results shown in the figure below show the rupture progresses from the epicenter in the southwest toward the northeast for a distance of over 300 km. The rupture velocity appears to be variable with an average speed of about 3 km/second.
Effect of the Near-field Term on Earthquake Early Warning
Yamada, M. and J. Mori
Test of Seismic Hazard Maps from 500-year Recorded Intensity Data of Japan
Miyazawa, M. and J. Mori
Attenuation and Thermal Structures under Shikoku and the Bungo Channel of Japan
Nugraha, A.D., J. Mori, S. Ohmi
Vp and Vp/Vs Imaging of the Subduction Zone Beneath Kyushu, Japan
Nugran, A.D., J. Mori, S. Ohmi
Seismic Structure Associated with the Subducting Philippine Sea Plate beneath Southwestern Japan
Hayashida, T., F. Tajima, J. Mori
Rupture Process of the 1975 Solomon Earthquake (Ms7.8) Estimated using WWSSN Pdiff Waveforms
Park, S.-C., J. Mori, Y, Park, J.H. Lee
2008 Association of Pacific Rim Universities Symposium
Multi-Hazards around the Pacific Rim
University of California Davis, August 21-22, 2008
Estimates of Rupture Propagation for the July 17, 2006 West Java Tsunami Earthquake
A tsunami earthquake (Mw = 7.7) occurred south of Java on July 17, 2006. The event produced relatively low levels of high-frequency radiation and local felt reports indicated only weak shaking in Java. There was no ground motion damage from the earthquake but there was extensive damage and loss of life from the tsunami along 250 km of the south coasts of Western and Central Java. An inspection of the area a few days after the earthquake showed extensive damage to wooden and unreinforced masonry buildings that were located within several hundred meters of the coast. Since there was no tsunami warning system in place, efforts to escape the large waves depended on the reaction of people to the earthquake shaking, which was only weakly felt in the coastal areas. This experience emphasizes the need for adequate tsunami warning systems for regions around the Indian Ocean.
An important aspect of identifying tsunami earthquakes, is the slow rupture velocity. We used 717 short-period vertical components of the Hi-Net array in Japan (National Institute for Earth Science and Disaster Prevention) for a back projection analysis of the July 17, 2006 West Java earthquake. The earthquake had a long duration of over 150 sec, so that there was sufficient time resolution to see the rupture propagation. The array is located at distances of 52 to 70 degrees from the earthquake and clearly records the direct P wave. Data were high-passed filter at 0.2 hz and aligned on the first arrival using waveform cross correlations. The initial arrival of the first time window was assumed to come from the grid point corresponding to the earthquake hypocenter. For subsequent time windows, the data were stacked assuming a source at each grid point. The stack with the highest correlation was interpreted to be the source of the seismic radiation for that time window. We tried several values from 10 to 30 sec for the time windows and found the results to be fairly stable. Our results show an overall low rupture speed of about 1 km/sec for the earthquake, but the progression is irregular with areas of faster propagation. This suggests that the overall low rupture speed may be due to delayed multiple events and not a continuously slow rupture.
Japan Geoscience Union Meeting 2008
Makuhari, Chiba May, 25-30, 2008
Fault plane Determination for the Niigataken Chuetsu-oki Earthquake in 2007 Using Tsunami Simulations
Yui, Y. and J. Mori
On July 16th 2007, the Niigataken chuetsu-oki earthquake in 2007 occurred along the Japan Sea coast of central Japan. This strike of the fault was estimated to be approximately northeast-southwest but from the preliminary aftershock distribution and crustal deformations, it was difficult to determine if the fault dipped toward the west or toward the east. A tsunami occurred due to the sea bottom deformation caused by this earthquake. This tsunami was not so large, so it did not cause significant damage along the Japan coast, however, it was clearly recorded by a number of tide-gauge stations along the Japan Sea coast.
In this study, we use various models for the slip on the fault plane, as estimated by teleseismic body-wave inversions and local GPS data. We calculated the sea floor deformation from these models and then calculated the resultant tsunami waveforms for tide gauge stations that clearly recorded the tsunami. We pay particular attention to stations east and west of the fault at Kashiwazaki and Sado Island, respectively.
Using comparisons of the model and observed tsunami waveforms, we try to estimate whether the major part of the slip was on a westward or eastward dipping fault plane. In general it is difficult to distinguish between the two fault planes, because both give similar patterns of sea floor deformations. There are some differences due to the shallow slip on the fault that cause different timing of the waves to the east and west.