Peninsula Earthquake Research Articles

A tectonic interpretation of NW-SE strike-slip faulting during the 2004 off the Kii peninsula earthquakes, Japan: Probable tear of the Philippine Sea plate

Abstract The 2004 off the Kii peninsula earthquakes (M w 7.5 for the main shock) occurred within the subducting Philippine Sea (PHS) plate near its boundary, the Nankai trough, southwest Japan. The rupture mode of the foreshock-main shock-aftershock sequence was complicated, a combination of ENE-WSW striking (almost trough parallel) reverse faulting beneath the trough and NW-SE trending (almost trough normal) strike-slip faulting mostly on the landward side of the former. In this paper, we discuss the tectonic meaning of this NW-SE running strike-slip fault. We examined hypocenter distribution and focal mechanisms of slab earthquakes from October 1997 through September 2004 and confirmed a NW-SE striking tear of the PHS slab beneath the middle part of the Kii Peninsula pointed out by Miyoshi and Ishibashi (2004). According to the Earthquake Research Committee (2004) there is a NW-SE trending structural discontinuity in the PHS crust to the southeast of the main shock epicenter. Putting all features together, we interpret that there is a NW-SE striking fracture within the PHS plate continuously from the Nankai trough region to the slab beneath the Kii Peninsula, and that a partial rupture of this fracture occurred during the off the Kii peninsula earthquakes together with trough-parallel reverse faulting. It should be noted that two disastrous M 7-class slab earthquakes seem to have occurred along this tear beneath the peninsula in 1899 and 1952.

Very low frequency earthquakes excited by the 2004 off the Kii peninsula earthquakes: A dynamic deformation process in the large accretionary prism

Abstract Anomalous seismic events were observed after the occurrence of the foreshock (Mw=7.2) and the main shock (Mw=7.5) of the 2004 off the Kii peninsula earthquakes. These anomalous events are characterized by very low-frequency energy of around 10 seconds with almost no higher-frequency energy and are considered the same as the very low-frequency (VLF) earthquakes discovered by Ishihara (2003) in some places along the Nankai trough, southwest Japan. The VLF seismic activity is mainly coincident with the aftershock area of the 2004 off the Kii peninsula earthquakes; however a minor activity was also excited in the southern Kii channel area. The VLF seismograms sometimes include higher-frequency wave trains with amplitudes much smaller than that of regular aftershocks. This indicates that VLF earthquakes have different source properties from the regular earthquakes. The centroid moment tensor analysis for one of the larger events suggests that the source depth is very shallow and the focal mechanism is the reverse faulting. These features suggest that the event occurs on the well-developed reverse fault system in the large accretionary prism near the Nankai trough. The swarm activity of VLF earthquakes might be considered as a chain-like occurrence of slips on the reverse fault system and thus the signature of a dynamic deformation process in the accretionary prism.

The seismological and geodetical roles of strain seismogram suggested from the 2004 off the Kii peninsula earthquakes

Abstract Dynamic strain seismograms obtained by high-speed sampling include not only long-period changes, such as ‘strain-step’, step change in strain, but also short period variations, such as body waves. We studied dynamic strain variations caused by the 2004 off the Kii peninsula earthquakes. Using such strain seismograms, we clarified the relation between dynamic strain and velocity. Our results will contribute greatly to seismology. We also clarified the strain-step formation process. This result and the concept of dynamic strain will bring more information to geodetics.

Spatial distribution of centroid moment tensor solutions for the 2004 off Kii peninsula earthquakes

Abstract Two large intraplate earthquakes (MW 7.0 and 7.3) occurred off the southeastern Kii Peninsula on September 5, 2004. We determined the centroid moment tensors of these earthquakes and their aftershocks. Earthquakes can be divided into two categories: (1) the reverse-fault type with a range in focal depth from 5 km to 20 km; and (2) earthquakes with a near-vertical nodal plane and a focal depth shallower than 5 km. Earthquakes of type (1) are distributed inside the Philippine Sea plate beneath the Nankai trough, while normal-fault events predominate in the other regions along the trough. The distribution of type (2) events shows two clear lineaments with strikes of NNW-SSE to the north and NE-SW to the west of the focal area. These lineament strikes coincide approximately with strikes at the nodal planes of such events.

Tsunami due to the 2004 September 5th off the Kii peninsula earthquake, Japan, recorded by a new GPS buoy

Abstract A tsunami due to the 2004 M7.4 September 5th earthquake off of the Kii peninsula, Japan, was recorded at the GPS buoy newly designed and established in April 2004, about 13 km off of the Muroto Promontory, southwestern Japan. The tsunami arrived at the buoy about 8 minutes before its arrival to the nearest tide gauge station at the tip of the Muroto Promontory. The predicted tsunami record based on preliminary source model shows excellent agreement (rms 2.7 cm) with the observed GPS record. This demonstrates that GPS buoy observations can be used not only for a tsunami warning system, but also for precise determination of the tsunami source.

Difference in the initial slopes of the P-wave envelope waveforms of the main shock M7.4 and foreshock M7.1 of the 2004 off the Kii peninsula earthquakes

Abstract We made a close investigation about the initial rising slopes of the envelope waveforms of the P-wave ground motion by using data for recent earthquakes. We have found that the P-wave initial slopes decrease almost linearly with increasing epicentral distance over the wide range of distance. This linear relation looks to be independent of the earthquake magnitudes and focal depths though the dispersion of data is somewhat large. We have found that the P-wave initial slopes of the foreshock M7.1 of the 2004 off the Kii peninsula earthquakes are very sharp. To the contrary, the slope of the main shock M7.4 is quite gentle. The main shock was preceded by a small event that occurred about 6.7 seconds before the main shock. This pre-event was generated near the initiation point of the main fracture. We are speculating that the difference in the initial slope between the foreshock and main shock may reflect the difference in the time variation of the stress drop at the early stage of the fracture and that the small pre-shock may be responsible for the slow rising of the initial P-wave motion of the main shock.

Co-seismic strain-steps associated with the 2004 off the Kii peninsula earthquakes-Observed with Ishii-type borehole strainmeters and quartz-tube extensometers

AbstractThe 2004 off the Kii peninsula earthquakes (Mj7.1 and Mj7.4) occurred at the Nankai trough, on 5 September 2004. Clear strain-steps associated with these earthquakes were observed with Ishii-type borehole strainmeters and quartz-tube extensometers in the Tokai and Kinki districts. We investigated the spatial and depth distribution of the observed principal strain changes and compared the observed strain-steps and theoretical calculations at all observatories. The following results were obtained: the observed strain-steps at all observatories are generally consistent with the polarities of the theoretical values, and the observed strain-step increases with depth at the same place. As for the Togari site, the following relationships are obtained: the strain-step and the tidal strains increase with depth and increasing of the modulus of elasticity, namely, hardness of rock. We consider that the geological structure around the observatory may cause a modification of the strain field.

The September 5, 2004 off the Kii Peninsula earthquakes as a composition of bending and collision

Abstract The 2004 off the Kii Peninsula earthquakes ruptured the subducting Philippine Sea plate near the axis of the Nankai Trough. A series of major events (the fore- and mainshocks with Mw>7) showed very peculiar rupture characteristics. The depth extent of the foreshock is around 12–23 km below the sea surface; in contrast, that of the mainshock is around 7–17 km, shallower and closer to the upper surface of the plate than the foreshock. The focal mechanism of the foreshock is almost pure reverse faulting, with a P-axis directing roughly N-S, that is rotated clockwise by 20–25° from the trough normal. In contrast, the mainshock involved a significant component of strike-slip faulting, with a similar P-axis. The OBS survey revealed a double layer of the aftershock activity and the upper NW trending layer contains aftershocks with strike-slip focal mechanisms. These features indicate that the compressional and tensional stress domains due to bending are significantly modified by tectonic stress disturbance and produced the fore- and mainshock ruptures deviated from simple reverse and normal faulting. I show that a supposed N-S compressional stress, which might be produced by the collision south of the Izu Peninsula, can rotate the principal stress axes and change the stress type, which are consistent with the P-axes of these events and the strike-slip component for the mainshock. The complex rupture mode of the mainshock might have been produced by superposition of the depth-dependent tensional bending stress in the upper half of the lithosphere and the collisional stress. This collisional stress should be comparable to the bending stresses in magnitude, which is much larger than the stress perturbation caused by nearby interplate locking.

Double-difference relocations of the 2004 off the Kii peninsula earthquakes

Abstract Two large earthquakes (Mw7.3 and Mw7.5) occurred off-shore of the Kii peninsula (Japan) on 5th September 2004. The insufficient station coverage in the area where the large events and their aftershocks occurred led to significant uncertainty regarding their depth location. By using a double-difference approach, we were able to determine more accurate hypocenter locations and better define the main characteristics of the aftershock distribution. The data we used in this study comprise of about 700 earthquakes recorded by the Japanese Meteorological Agency (JMA) from 5 to 12 September 2004. We assume several depth ranges for the initial double difference earthquake locations and select the solution with the smallest rms residual. The resulting hypocenter distribution has two distinctive features: 1) on average the depths are about 20 km shallower than the initial JMA location and 2) the hypocenters are divided into two groups: one shallow (around 12 km depth) and another deep (around 27 km depth). The epicenters of the relocated events define a finer, more clustered structure compared with the epicentral distribution of JMA. The splitting of the earthquake locations into two depth groups is still observed when using a significantly different velocity structure. Our results of the two depth groups for the hypocenters resemble the independently determined hypocentral distribution from the OBS survey.

Urgent aftershock observation of the 2004 off the Kii Peninsula earthquake using ocean bottom seismometers

Abstract The 2004 off the Kii Peninsula earthquake occurred on September 5, 2004. Knowing the precise aftershock distribution is important for understanding the mechanism of this earthquake. However, the hypocenter of the main shock was located more than 100 km offshore from the nearest station of the land observation network. In the three days after the main shock, we started ocean bottom seismometer (OBS) observation in order to determine the precise distribution of the aftershocks. We assumed a seismic velocity structure for the hypocenter calculation, based on the results of previous seismic refraction study. The station corrections were incorporated to locate the hypocenter precisely. The hypocenters located within an area covered by five OBSs show relatively small errors. It is found that the OBS-located hypocenters are located about 5.5 km east-southeast from those by JMA and the depth range of the aftershocks is about 5–25 km just beneath the Nankai trough axis. The aftershock hypocenters can be grouped into two clusters at different depths of about 10 km and about 20 km. It is inferred that the main shock also has a depth of 5–25 km. Since this extent of the main shock was larger than one of the oceanic crust of the Philippine Sea plate, the fault plane of the main shock extended at the upper most mantle of the Philippine Sea plate. Although we cannot assign the actual fault plane of the main shock form our observation results, it is clarified that intra-plate earthquakes occurred near the trench region. Our OBS result supports that the main shock was the earthquake not at the plate boundary but within the bending Philippine Sea plate near the trough axis.

Estimation of the source model for the foreshock of the 2004 off the Kii peninsula earthquakes and strong ground motion simulation of the hypothetical Tonankai earthquake using the empirical Green’s function method

AbstractWe estimated the source model for the foreshock of the 2004 off the Kii peninsula earthquakes by empirical Green’s function modeling. The size and the rise time of the strong motion generation area (SMGA) were estimated to be 30 × 15 km, and 0.9 sec, respectively. The stress drop of the SMGA was calculated to be 8.3 MPa. This model could reproduce long-period ground motions following the direct S-wave observed in the Osaka basin well. Using the derived source parameters, we simulated strong motions of the hypothetical Tonankai earthquake. Distribution of the seismic intensity derived here is similar to that obtained by the previous report. We could predict long-period ground motions which last for a long duration at the basin sites.

Inversion analysis of the 2004 off the Kii peninsula earthquakes using seismic intensity data

Abstract An inversion analysis has been developed to evaluate short-period radiation zones on an earthquake fault plane using seismic intensity data. It is robust and widely applicable even for historical earthquakes without ground motion, tsunami and geodetic data observed by instruments. It was applied to the 2004 off the Kii peninsula earthquakes that occurred in the Philippine Sea plate along the Nankai Trough. The short-period seismic wave radiation zone of each event during the 2004 off the Kii peninsula earthquakes from the inversion analysis was included in epicenter area of the corresponding aftershocks and slipped fault zone derived from the waveform inversion analysis as well. It may be also found that there were slip areas where little short period energy radiated. It also applied to the 1899/3/7 earthquake whose felt area was similar to that of the 2004 off the Kii peninsula earthquakes. However, it was indicated that the short-period radiation zones of the 1899 earthquake was located near the coast of the Kii peninsula and departed from that of the 2004 earthquake. The 1899 event might be an intermediate-depth earthquake in the Philippine Sea plate or a relative small interplate event at the west margin of the Tonankai earthquake source region.

Crustal deformations in Kii peninsula associated with the SE off the Kii peninsula earthquake sequence of September 5, 2004 derived from dense GPS observations

AbstractWe present crustal deformations in southern Kii peninsula, southwest Japan, before and after the SE off the Kii peninsula earthquake sequence of September 5, 2004, which were obtained from repeated surveys of dense GPS network. Disaster Prevention Research Institute (hereafter DPRI), Kyoto University made a reoccupation of their dense GPS network network that was occupied in February-March, 2004, right after the event to obtain crustal deformations associated with this event. Since we do not have the data just before the event, we apply an inversion of displacements of GEONET sites during the period from February-March to August, and interpolate displacements at DPRI’s sites using the inverted fault model. Southward movements of about 2 cm are obtained in the middle of Kii peninsula, while displacements are rotated to southwest on the west side. These characteristics of horizontal displacement field cannot be explained only by a pure thrust faulting, but additional sources such as right-lateral faulting on a NW-SE trending fault are required.

Tsunami source of the 2004 off the Kii Peninsula earthquakes inferred from offshore tsunami and coastal tide gauges

Abstract Tsunamis from the 2004 off the Kii Peninsula earthquakes (M 7.1 and 7.4) were recorded on offshore tsunami gauges, a GPS tsunami gauge and eight bottom-pressure gauges, as well as coastal tide gauges located south of Honshu and Shikoku. The maximum amplitudes on the GPS and bottom-pressure gauges were several to ten cm, while those on tide gauges were up to 0.9 m. We first computed tsunami waveforms from the earthquake source models proposed Yamanaka (2004) and Yagi (2004) from seismic waveform analysis, and compared them with the observed waveforms. For the first event (foreshock), both models produce similar waveforms with the observations. For the second event (mainshock), the waveforms computed from the Yamanaka model is closer to the observed waveforms, but there are still discrepancies between the observed and computed waveforms. We then performed tsunami waveform inversions to estimate the water height distributions in the source area. The foreshock source is ≈1600 km2 with the maximum water height of 0.2 m. The estimated tsunami source area for the mainshock, ≈3600 km2 with the maximum of 0.6 m, extends ≈60 km toward northwest and ≈40 km southwest from the epicenter along the aftershock distribution, suggesting that multiple faulting was involved in the mainshock.

Long-period ground motion simulation in the Kinki area during the MJ 7.1 foreshock of the 2004 off the Kii peninsula earthquakes

AbstractLong-period (5–20 s) ground motion simulations in the Kinki area during theMJ7.1 foreshock of the 2004 off the Kii peninsula earthquakes are performed. We construct a three-dimensional underground structure model that includes the source region and Osaka basin area to confirm propagation-path effects of crustal structure on long-period ground motions. Finite difference (FD) simulations by our three-dimensional underground structure model agree well with long-period observed ground motions at several rock sites in the Kinki area. We found that a sedimentary wedge structure along the Nankai trough amplifies and elongates long-period ground motions from this event. We also examine the effect of source depth for this event by comparing the observed with synthetic waveforms. These results suggest that precise modeling of the shallower parts of the underground structure, such as the sedimentary wedge, are quite important for long-period ground motion simulation of the hypothetical Nankai and Tonankai earthquakes.

Fault geometry of the 2004 off the Kii peninsula earthquake inferred from offshore pressure waveforms

We could successfully obtain water pressure fluctuations after a series of two off the Kii peninsula earthquakes that took place on September 5 in 2004, using two pressure gauges deployed at deep ocean-bottom off Muroto. The earthquakes caused two tsunamis in the southern coast of the southwestern Japan with maximum wave heights of 0.9 m, and we could identify the tsunami signals from the acquired dataset ca. 20 min before the arrivals of the tsunamis to the coastline nearest to the sensors. Two earthquakes are assumed as one for a fore-shock and the other for a main-shock. Although seismological studies, without aftershock distributions, showed a lack of constraints to distinguish if the main-shock was caused by either southwest dipping fault or by the other, i.e., northeast dipping fault, we found that the difference in the pressure fluctuations for the tsunamis could attribute to the determination of the fault plane in the fault models. In this paper, we evaluate the fault geometry of the main shock by using tsunami waveforms obtained by those unique instruments in following procedure: (1) We first simulate pressure waveforms caused by earthquakes of the two fault geometries, one for southwest dipping place and the other for the northeast, at the location of the pressure gauges, and (2) we then compare the simulated results with observed waveforms. Our results demonstrate that the main-shock should be caused by a fault whose strike lies in northwest-to-southeast with a dip towards southwest direction.

Change of the source mechanism of the main shock of the 2004 off the Kii peninsula earthquakes inferred from long period body wave data

Abstract Waveform inversion of long period body wave data was performed to determine the temporal distribution of moment release of the main shock of the 2004 off the Kii peninsula earthquakes. Our result suggests that the source mechanism varied during the rupture, with the first 20 sec dominated by the strike slip component, while a thrust mechanism was predominant between 30–40 sec. The employed dataset has enough resolving power to detect temporal change on such a time scale, and thus our results suggest that this earthquake is a compound event consisting of the two different source mechanisms.

Tsunami run-up heights of the 2004 off the Kii peninsula earthquakes

Abstract A tsunami height survey was conducted immediately after the 2004 off the Kii peninsula earthquakes. Results of the survey show that the largest tsunami height was about 4.6 m locally at Kiho-cho, Mie prefecture. Numerical simulation of the tsunami due to the earthquake was carried out using the model parameters estimated by NIED. The distribution pattern of the observed tsunami heights along the coast cannot be explained by the computed heights, because the model equation is linear long-wave theory and the run-up computations with a finer grid system are not included in this simulation. In order to explain tsunami run-up heights, it is necessary that the non-linear and run-up computation model should be used with a finer grid system.

Compound fault rupture during the 2004 off the Kii Peninsula earthquake (M 7.4) inferred from highly resolved coseismic sea-surface deformation

For a tsunami inversion analysis, we incorporated a new technique that uses many Green’s functions to improve model spatial resolution. Since this method precisely reproduces observed tsunami waveforms, we can obtain a better model of the tsunami source process. We applied the proposed method to the 2004 off the Kii Peninsula earthquake to determine what fault or faults ruptured to cause the earthquake. The estimated coseismic sea-surface deformation extended to two directions, and it was consistent with the distributions of the two aftershock swarms. Hence, we concluded that the earthquake resulted from the rupture of two faults.

Long-period ground motions from a large offshore earthquake: The case of the 2004 off the Kii peninsula earthquake, Japan

AbstractThe 2004 off the Kii Peninsula earthquake excited long-period ground motions over a wide area of Honshu Island of Japan. This remarkable excitation was observed in the Osaka, Nobi, and Kanto basins as well as in the Omaezaki region. The record section indicates two types of developed long-period motions by the basin surface waves, either by the source or passage effect of the shallow and large offshore earthquake. Their combination resulted in the well-developed long-period ground motions observed within the distant basins as those during the 1985 Michoacan and 2003 Tokachi-oki earthquakes did. The distributions of pseudo-velocity response spectra confirmed this development at periods of 5–7 s in the Osaka and Nobi basins and of 7–10 s in the Kanto basin. The comparison of the distributions with the thicknesses of the sediments and the S-wave velocities of the surface layers shows that these characteristics of the long-period ground motions are closely related to the structures of the basins. The earthquake provided a timely warning of damaging long-period ground motions from future megathrust events in the Tonankai, Nankai, and Tokai regions.