S41D-01 08:00h
Offshore Double-Planed Shallow Seismicity of the NE Japan Forearc Region Revealed by Hypocenter Determination Using the sP Depth Phase
Precise locations of earthquakes in subduction zones are required to determine the geometry and structure of the plate interface and slab structure. High-resolution relocation for offshore earthquakes is especially difficult using conventional land-based seismic networks. In this study, we use a special depth phase often detected in shallow seismic events in the northeastern Japan forearc region and use it to redetermine focal depths. This distinct later phase is often observed in vertical component seismograms between direct P and S wave arrivals at epicentral distances of about 150 km or more for shallow earthquakes. This sP phase is an upgoing S wave from the focus which is then reflected and converted to P wave at the Earth_fs surface, diving into the Earth again, and finally arriving observation stations (Umino et al., 1995). The arrival time difference between this phase and the first P wave is very sensitive to the focal depth of the event, and hence we used this phase for the depth determination. We also determined focal mechanisms of the relocated shallow events by using P wave first motion data. In the relocation procedure, we use Japanese University Network data, Hi-net and JMA data for the period of 2000 to 2004. The distribution of relocated earthquakes clearly shows the configuration of the plate boundary at shallow depths beneath the Pacific Ocean. The Pacific plate in this region initially subducts at an extremely low dip angle and then gradually becomes a much steeper dip angle at a depth of approximately 30 km. We were also able to detect some events whose focal depths are significantly deeper than those of the interplate events. These events are apparently the near-trench counterpart of the lower plane of the double seismic zone beneath the land. Thus a double seismic zone is also formed near the Trench. For both northern and southern part of the NE Japan forearc region, most of the relocated upper plane events have low angle thrust fault type mechanisms, especially near the coastline. Although P wave initial motion data are not ideally distributed on the focal sphere, preliminary work indicates that upper-plane events occurring near the trench of the northern part of the subduction zone are normal fault type. However, due to the lack of mechanism data, we are unable to recognize such a pattern in the southern part of the study area. We also show that seismicity is high and with normal faulting mechanisms near the rupture area of 1933 Sanriku tsunami earthquake (M8.3). Preliminary results indicate that most of the upper plane events under the offshore of NE Japan are thrust faulting type and near the trench are normal faulting type. Bending and unbending of the Pacific plate appears to be the most natural explanation of the distribution and focal mechanism of these offshore events.
http://aob-new.aob.geophys.tohoku.ac.jp/~sng/
S41D-02 08:15h
Heterogeneity And Spatial Correlation Of Clusters Between The Upper And Lower Plane Seismicity Of The Double Seismic Zone Beneath NE Japan Revealed By Double-Difference Relocations
Seismicity within the subducted Pacific plate is very active beneath NE Japan, and this activity is expressed as a double seismic zone (Hasegawa et al., 1978). We relocated interplate and intraslab earthquakes in the entire Tohoku district of NE Japan using the Double-Difference method (Waldhauser and Ellsworth, 2000) where relative earthquake arrival times were determined by waveform cross-spectrum analysis. The result showed that both the lower-plane and upper-plane seismicity are distributed unevenly in space. Moreover, there is a spatial correlation between clusters of upper- and lower-plane seismicity. If intermediate-depth earthquakes are caused by dehydration and/or CO$_{2}$-bearing devolatilization of hydrated minerals (Kirby, 1995; Kirby et al., 1996; Seno and Yamanaka, 1996; Peacock, 2001; Yamasaki and Seno, 2003), the present result suggests that hydrated and/or carbonate minerals are distributed unevenly but in common in both the crust and deeper mantle of the slab. This correlated heterogeneity of upper- and lower-plane seismicity may be associated with the subduction of structures produced by intraplate volcanism (Seno and Yamanaka, 1996; Kirby et al., 1996). Our relocations indicate that at depths between 50 and 150km, most of the background upper-plane seismicity occurs in the slab crust. Okada and Hasegawa (2003) and Sakoda et al. (2004) relocated intraslab earthquakes occurring around off the northern part of NE Japan, including the source region of the 2003 off Miyagi earthquake (M7.1), which occurred on May 26, 2003 in the upper plane. Their results show that many earthquakes occurred near the hypocenter of the main shock before the 2003 event. These events are distributed near the Moho and/or within the mantle of the slab. Anomalous 'deep' upper-plane earthquakes taking place within the slab mantle as well as the slab crust such as the source area of the 2003 Miyagi earthquake could also be caused by faulting in the thicker crust of a subducted seamount.
S41D-03 08:30h
Characteristics and Distribution of Small Asperities on the Plate Boundary in the NE Japan Subduction Zone Estimated From Repeating Earthquakes
A diversity of seismic events including large interplate earthquakes, tsunami earthquakes and intraplate earthquakes, have occurred in the northeastern Japan subduction zone. However the precise activity and nature of small events under the Pacific Ocean has not been clear except in special cases (e.g. Hino et al., 2000; Gamage et al., present meeting). Repeating earthquakes, which have been found in California and NE Japan subduction zone, are significant because they indicate recurrent rupture of small asperities on the plate boundary. We investigated the distribution and characteristics of these small asperities. We selected repeating earthquakes based on the waveform similarity, near by location and size (M$>$=2.5). The criterion for similarity was such that the cross-correlation coefficients must be larger than 0.95 for a 40s window for 1-4 Hz band-pass filtered waveform. This strict criterion for similarity ensures the events share the same asperity or are adjacent to each other for M3-4 events. The frequency distribution of the lifetime of the repeaters had two peaks. The burst type (duration $<$ 3years) repeating earthquakes have a relatively low cross correlation coefficients compared to the continual type (duration $>$= 3 years). It should be noted that successive rupture of nearby asperities sometimes involves burst-type repeating groups. The focal mechanisms of most repeating earthquakes involve low-angle thrust faulting. This suggests the repeating earthquakes occur on the plate boundary. The spatial distribution of repeating earthquakes are complementary with large asperities (Yamanaka and Kikuchi, 2004). These asperities occur in area of high coupling coefficient as determined from GPS data analysis (Suwa et al., 2003). Repeating earthquakes near the Japan Trench tend to have relatively low stress drop (Yamashita et al., 2004). These features of repeating earthquakes probably not only show the characteristics of small asperities but also the nature of the active plate boundary.
S41D-04 08:45h
Roles of Oceanic Intraplate Volcanism and Mantle CO$_{2}$ in the Seismicity of the Double Wadati-Benioff Seismic Zone of NE Japan: A New Proposed Mechanism for Lower-zone Seismicity
Research in recent years has given insights into the occurrence of intraslab seismicity at intermediate depths as expressions of dehydration embrittlement in faults previously formed in oceanic lithosphere (Kirby et al., 1996). A special challenge exists to explain the lower zone of the double seismic zone by this process because the mechanism by which hydration occurs at depths of 30-40 km into the Pacific Plate is unclear. Based on recent seismological advances and theoretical developments at Tohoku University, we propose that CO2 released into the Pacific Plate by ascending mafic magmas associated with Cretaceous intraplate volcanism later facilitates embrittlement of the mantle when such structures are subducted. Wilshire and Kirby (Tectonophysics, 1989) earlier proposed that CO$_{2}$ facilitates mantle fracture and seismogenic faulting beneath Hawaii by such a mechanism. Extensive seismological and marine geological and geophysical evidence suggest that that widespread Cretaceous plume volcanism, perhaps associated with the mid-Cretaceous Superplume, led to widespread mid-plate magmatic activity and a broad region of CO$_{2}$-induced fracture and faulting in the Pacific Plate. After subsequent cooling and initial subduction, such supercritical CO$_{2}$ should become unstable, producing magnesite by reaction with depleted harzburgite (depleted peridotite). Magnesite has the same structure as calcite and is, by analogy, expected to be dramatically weaker than peridotite and we posit that thermal runaway by localized viscous dissipation can lead to CO$_{2}$ embrittlement in the slab by release of CO$_{2}$ or by shear melting. This model for lower-zone seismicity supports the Superplume hypothesis of Seno and Yamanaka (1996), but employs a different physical mechanism for faulting. This proposed structure of intraplate volcanics in subducting lithosphere is major source of pre-existing fault and mineralogical heterogeneity that can affect the distribution and focal mechanisms of upper-zone seismicity in NE Japan and elsewhere.
S41D-05 09:00h
NanTroSEIZE: The Nankai Trough Seismogenic Zone Drilling Project
The Integrated Ocean Drilling Program has recently accepted NanTroSEIZE as a multi-year, multi-phase drilling and observatory installation project of unprecedented scope. NanTroSEIZE is an integrated program of geophysical studies, drilling, and instrumentation designed to investigate the aseismic to seismic transition of the megathrust system and the processes of earthquake propagation and tsunami generation at the Nankai Trough subduction zone. The fundamental goal is the creation of a distributed observatory spanning the up-dip limit of seismogenic and tsunamigenic behavior. This will involve sampling and instrumenting key elements of the active plate boundary fault system at several locations off the Kii Peninsula, Japan. Here, the plate interface and active mega-splay faults are accessible to drilling within the region of coseismic rupture and tsunami source in the 1944 Tonankai M8 great earthquake. The ultimate objective is to access and instrument the Nankai plate interface to advance our knowledge of fundamental aseismic and seismic faulting processes and controls on the transition between them. NanTroSEIZE will test models for the frictional behavior of fault rocks across the aseismic - seismogenic transition, the composition of faults and fluids and associated pore pressure and state of stress, partitioning of strain spatially between basal interface and splays, temporally between coseismic and interseismic periods, and between infraseismic and aseismic events vs. seismic events. Long-term borehole observations potentially will ultimately test whether interseismic variations or detectable precursory phenomena exist prior to great subduction earthquakes. Drilling and instrumentation will span the zone from input of sediment, crust, and fluids at the trench through the mega-splay and decollement fault systems at up to 6 km below the sea floor. Eight distinct drilling sites are targeted, with a comprehensive program of coring, geophysical logging, downhole geophysical and hydrological experiments. Opportunities exist for many new researchers to become involved in the NanTroSEIZE effort.
http://ees.nmt.edu/nantroseize
S41D-06 09:15h
Distinct Geodetic and Seismic Up-dip Limits to the Northern Costa Rica Seismogenic Zone: Evidence for Two Mechanical Transitions
Results from CRSEIZE (Costa Rica Seismogenic Zone Experiment), a joint seismic and geodetic experiment designed to elucidate seismogenic processes at the southern end of the Middle America Trench, indicate distinct patterns of geodetic strain accumulation and interplate microseismicity in northern Costa Rica. Geodetic inversions for interseismic slip on the plate interface reveal locking equal to $\sim$75% of the plate velocity between 8-15 km depth below sea-level while shallow interplate microearthquakes begin deeper, between 15-18 km below sea-level, in a region that appears to be freely slipping. Interplate microseismicity terminates at $\sim$30 km depth along the up-dip edge of a second patch of partial geodetic locking that represents $\sim$30-40% of the plate velocity. The strike parallel alternation of geodetically locked regions with no microseismicity and freely slipping regions with abundant microseismicity suggests that down-dip increases in temperature and pressure may generate two distinct transitions in mechanical behavior along the plate interface. We interpret the up-dip limit of the shallow geodetically locked patch as the frictional stability transition from stable sliding to stick-slip behavior. The shallow transition correlates with models of the 100$^{o}$C isotherm. The up-dip limit of interplate seismicity and the freely slipping zone most likely correspond to a change in mechanical properties along the plate interface; increased pore fluid pressure resulting from basalt dehydration reactions and/or decreased permeability in the upper plate may lead to fault weakening. Modeled temperatures at the deeper transition to the onset of seismicity are estimated at 200-250$^{o}$C, in the range where low-grade metamorphic reactions potentially release 0.3-3 wt % water and experimental data reveal significant decreases in permeability for a variety of rock types.
S41D-07 09:30h
A Creep Event on the Shallow Interface of the Nicoya Peninsula, Costa Rica Seismogenic Zone
A continuously recording, three-station GPS network on the Nicoya Peninsula, Costa Rica, recorded what we believe is the first slow slip event observed along the plate interface of the Costa Rica subduction zone. The Nicoya segment of the Middle America Trench has been recognized as a mature seismic gap with potential to generate an Mw$>$7.5 earthquake in the near future (it ruptured with large earthquakes in 1853, 1900 and 1950). The transient displacement is well resolved and nearly opposite to the direction of plate convergence and strain accumulation. Slow slip began in mid-September 2003 and lasted about 4 weeks. The motion is observed earliest, with the largest amplitude at the stations closest to the trench; it arrives with diminished amplitude, some 10-14 days later at the inland-most station. Simple dislocation modeling suggests this is consistent with several cm of slip initiating at shallow depth on the plate interface (~10-20 km) and propagating down-dip. Slip in this event is focused in the region we previously identified as partly or fully slipping, between two locked patches on the plate interface using campaign GPS data. Moreover, this region is characterized by frequent microseismcity, as observed in the 1999-2001 Costa Rica Seismogenic Zone (CRSEIZE) dense deployment of land and ocean bottom seismometers. Silent earthquakes or creep events have been recorded at other subduction zones instrumented with continuous GPS stations. At most of these, including the Cascadia margin which experiences periodic creep events, slip occurs far from the trench (>150 km), below the seismogenic zone in a transition regime between stick-slip and stable sliding frictional behavior. The slip event in Costa Rica clearly occurs within the seismogenic zone making it unique. Collaborative efforts between Costa Rica, Japan and the US are focused on expanding this GPS network to provide improved resolution of future creep events and enhanced understanding of the mechanical behavior of the Nicoya subduction segment of the Middle American Trench.
S41D-08 09:45h
Transient Fluid Pulsing and Seismic Tremor-like Seismic Noise: Episodic Creep and/or Fluid Expulsion at the Updip Edge of the Seismogenic Zone, Costa Rica
We propose that we have recorded coupled ultra-slow "nearly silent" ruptures and associated subsurface tremor related to fracture flow events originating from the stable/unstable slip transition zone near or at the up dip limit of the seismogenic zone in the Cost Rica subduction system. Long-term measurements of fluid flow were made across the Nicoya Peninsula, Costa Rica, convergent margin utilizing osmotically driven fluid flow meters that are designed to quantify both inflow and outflow rates on the order of $\sim$10$^{-5}$ cm/d to 3 cm/d. The instruments are capable of measuring volumetric strain rates as low as 10$^{-7} - 10$^{-8}$ d$^{-1}$. Significant transience in flow was observed through the surface of the forearc. Three periods of correlated flow signals are seen on three instruments located in the out-of-sequence-thrust region separated by $\sim$30 km. Seismic noise amplitude recorded on collocated ocean bottom seismometers (OBS) increases during the three correlated flow events. The seismic noise has a number of frequency characteristics similar to both volcanic and non-volcanic tremor (non-double coupled seismic sources) suggestive of flow though fracture systems and associated generation of oscillatory instabilities. We show one model where tremor is related to non-linear effects associated with changes in the fluid momentumat fracture constriction points similar to the "knocking of pipes" in old buildings. The momentum changes transmit pressure impulses to the elasticlly damped fracture walls. Elevated flow rates cause the induced oscillations to become unstable leading to sustained and elevated seismic noise. We propose either a series of somewhat complex rupture dislocations and/or pulses of high pressure fluids along the decollement that generate accelerated flow episodes through the fracture networks. These are recorded both as increased seismic noise, and by the fluid flow meters at the surface as direct complex strain-related pulses of outflow or indirectly by incipient liquefaction events caused by the action of the noise on shallow poorly consolidated sediments. The nearly silent strain and flow events may be coupled and migrate across the toe of the wedge at probably 100s m to several kms a day with along strike dimensions of $>$30km. The flow measurement systems appear more sensitive to these signals at the toe of the wedge than the OBSs are to the primary low amplitude seismic noise.