T32C-01 INVITED 10:20h
Crustal Deformation Around the Central-Northern Itoigawa-Shizuoka Tectonic Line Fault System, Central Japan, and its Tectonic Implications
The Itoigawa-Shizuoka Tectonic Line (ISTL) is a major geologic boundary dividing the Japanese mainland into the northeastern and the southwestern parts. Under the present compressive stress regime of the Japanese islands, active deformation is ongoing along ISTL. The ISTL fault system is considered to be one of the most active faults in the Japan Islands. In particular, the Gofukuji Fault has a slip rate of 8-9 mm/yr, which is the largest value for active faults in the Japanese inland. We investigated contemporary crustal deformation by deploying an array of permanent GPS (Global Positioning System) stations. The GPS observation demonstrates conspicuous shortening (about 0.3ppm/yr) around the East Matsumoto Basin Fault at Omachi city. This contraction is concentrated in a narrow zone of about 30km, indicating that a responsible deformation source is located at a shallow depth. On the other hand, the geodetic strain rate around the Gofukuji Fault is consistent with a geologically inferred left-lateral strike slip motion although the strain rate is much smaller (0.05 ppm/yr) compared with that around the East Matsumoto Basin Fault. The deformation is widely distributed around the Gofukuji Fault, and the relative displacement rate over 100km-long baseline across the fault is comparable to its geological slip rate. These observation data indicate that the strain accumulation is quite heterogeneous along ISTL. One explanation for this heterogeneity is a difference in the crustal structure. Seismological as well as electromagnetic structural exploration revealed a complicated upper crustal structure and a possible lower crustal heterogeneity beneath the East Matsumoto Basin fault. Such a heterogeneous structure reflects a geologic history of tectonic inversion in this area after the opening of the Japan Sea. Another factor to be considered is a transient deformation component in the interseismic period. There is no historical record of a large earthquake around the Gofukuji Fault, while the 1918 Omachi Earthquake (M6.5) occurred around Omachi city. Taking these factors into account, the present seismic potential is not proportional to the geodetic strain rate. Rather, the probability of earthquake occurrence is much larger at the Gofukuji Fault, as deduced from the geological exploration.
T32C-02 INVITED 10:40h
Characterization of Late Quaternary Deformation of the Mt. Diablo Fold-And-Thrust Belt, Eastern San Francisco Bay Area, California
Geomorphic and structural geologic studies provide evidence for the loci, style, and rates of late Quaternary uplift, and possibly insights into the paleoseismic behavior of the Mt. Diablo fold-and-thrust belt (MDFTB). The MDFTB is comprised of map-scale fold-thrust fault structures oriented obliquely to and lying largely within a restraining left-stepover between the dextral Greenville and Concord faults of the eastern San Andreas fault system. Reverse slip on the blind, southwest-verging Mt. Diablo thrust fault, the largest structure in the belt, has uplifted and folded the Great Valley Sequence about the Franciscan-cored Mt. Diablo anticline, which culminates in the 1,173 m peak of Mt. Diablo. Detailed longitudinal profiles of well-preserved stream terraces that cross the structure at a high angle place the loci of active surface deformation above the tip of the thrust fault, which underlies the Tassajara anticline fault-propagation fold. Profiles along Alamo and Tassajara creeks, separated by 4 km strike distance, similarly record ~4 m of localized uplift of terraces radiocarbon dated between 3 and 1 ka. These data suggest tectonic quiescence between 3 and 1 ka followed by considerable tectonic uplift. Estimated source dimensions of the Mt. Diablo thrust fault and historical earthquake data suggest uplift resulted from multiple earthquakes on the fault in the past 1 ka. Fold-thrust structures within the deep Neogene sedimentary basin beneath Livermore Valley south of Mt. Diablo have a more westerly vergence direction and exhibit lesser structural and geomorphic relief. Based on construction of cross sections through the basin, we interpret a NE-dipping imbricate system of thrust faults, the "Vallecitos thrust fault system", that intersects the Greenville fault at seismogenic depths to the NE, and impinges on the Calaveras fault at shallow crustal levels to the SW. Detailed surficial geologic mapping and geomorphic profiles reveal `hanging' geomorphic surfaces, wind gaps, and abandoned stream channels, all of which are consistent with late Quaternary activity of the fold-thrust structures. The distribution and elevation of these features constrain late Quaternary geomorphic growth of the "Springtown anticlines", "Livermore anticline", Verona homocline, and "Vallecitos hills anticline". Subsurface data show late Quaternary growth of a southeast-plunging syncline flanked on the east by the Livermore anticline and on the west by the Verona homocline, the depocenter for reportedly the tenth largest gravel resource in the U.S.
T32C-03 11:00h
Estimating Slip Rate and Last Event Timing for the Blind Fukaya Fault System in Greater Metropolitan Tokyo Through Precise Analyses of Continuously Cored Boreholes
The NW-trending Fukaya fault system is located along the northwestern margin of the Kanto Plain where greater metropolitan Tokyo is expanding. The fault system comprises an 80-km-long array of left-stepping west-side-up reverse faults. The faults appear as flexures or fault-related folds at the ground surface and are widely concealed beneath fluvial deposits. Our boring and trenching surveys in 2000 and 2001 revealed a 2-to-6-ka earthquake event on the fault system. However, paleoseismological data such as the last event timing, recurrence interval and rupture extent are still not constrained enough for reliable probabilistic evaluation of future earthquakes from the fault system. In addition, historical and archeological data suggest a large earthquake in AD 818 in the nearby area, but no evidence is known for an earthquake in the period between 2 ka and 6 ka. Under the circumstances, we have conducted a complementary study for the Fukaya fault, an about 38-km-long main segment of the fault system. We extracted a 173-m-long continuous core from the downthrown side of the southernmost part of the fault concealed beneath the alluvial plain in the Fukiage area. The core is subdivided into five stratigraphic units 20 to 50 m thick. The upper part of each unit includes fine-grained deposits, which are correlative to Marine Isotope Stage (MIS) 1, 5, 7, 9 and 11, respectively, in descending order. The average subsidence rate in the Fukiage area is estimated to be around 0.35 to 0.4 m/ky on the assumption that the base of the core (153 m below the sea level) is ca. 430 ka (base of MIS 11). The upthrown side seems to be absolutely uplifted because middle or late Pleistocene fluvial terraces are distributed at the level about 20 m high from the present alluvial surface. Consequently, the vertical slip rate of the southernmost part of the Fukaya fault is estimated to be more than 0.35 m/ky. We also conducted a precise stratigraphic correlation of Holocene deposits for a boring array comprising 12 existing and 3 newly extracted cores, and extending over 600 m across the flexure zone in the Fukiage area. Each newly extracted core records a liquefaction trace at the same stratigraphic horizon. Two of the three cores preserve liquefied sands jetting out to the ground surface at that time. We identify the liquefaction event, which is dated to be 2,700 to 3,000 years BP, as the latest faulting event on the Fukaya fault in the Fukiage area. The event timing on the southernmost Fukaya fault is consistent with a 2-to-6-ka event on the northern and middle parts of the Fukaya fault system. The result suggests a possibility that a 2.7-to-3-ka earthquake ruptured the whole extent of the Fukaya fault system.
T32C-04 11:15h
Structural architecture of the active thrust systems in Japan, revealed by deep seismic reflection profiling
In the central to northern Japan, active thrusts were developed by the subduction of the Pacific and Philippine Sea plates and the arc-arc collisions. Since the Kobe earthquake of 1995, the evaluation of earthquake risk has become an important problem. To reveal the geometry of active thrust fault systems, including deep geometry of source fault and connectivity of faults in a fault system, is important for the estimation of strong ground motions and the risk evaluations. Deep to shallow seismic reflection profiling were performed across the several active thrusts in the central to northern Japan, such as the Hidaka thrust system in the central Hokkaido (A), the Senya thrust system in the central northern Honshu and the Kozu-Matsuda active fault system associated with the subduction mega-thrust in the southern part of Kanto (Greater Tokyo) area (B). The geometry of active thrust systems is well presented by these seismic sections down to 15 km in depth. In this paper, we present the structural architecture of these active thrust systems based on the recent results of seismic reflection profiling. The deeper part of the above-mentioned thrust-systems shows relatively simple geometry. In the shallower part within the sedimentary wedge, thrust system shows flat-and-ramp geometry associated with fault-related folds and spray faults. Basin-ward migration of thrusting is common feature within the sedimentary layer. It is well demonstrated by geomorphological and geologic evidences. The structural architecture in the shallow part of thrust system is marked by wedge-thrust, emergent thrust and fault-related folds. Large scale wedge-thrust was found by deep seismic profiling in the deeper active thrust systems in the arc-arc collision zones (A and B). Arc block is collided into the mid-crust forming a "crocodile structure" associated with lower crustal delamination in the Izu collision zone (B) and the axial part of Hokkaido (A). Due to complex geometry and existence of spray faults in the shallow part of thrust system, the evaluation of seismic risk and slip-rate have to be performed based on the knowledge of the structural architecture of a whole thrust system.
T32C-05 11:30h
Structure of the Seattle Fault Zone, Washington State, from vibroseis seismic reflection profiles
We acquired high-resolution vibroseis seismic reflection profiles east of Lake Washington in June 2004 to characterize the geology in the upper one km across a 15 km-wide swath of the Seattle fault zone. We collected more than 20 km of north-trending profiles to image the southern part of the Seattle Basin, the northern edge of the Seattle uplift, and the intervening Seattle fault zone. Between Lakes Washington and Sammamish, the profiles show flat-lying and hummocky glacial strata in the southern part of the Seattle Basin, overlying north dipping ($>$25 degrees), faulted reflectors. Disrupted reflectors in the southern end of the Seattle Basin suggest that active fault strands are present north of the topographic scarp in the Seattle fault near Vasa Park and Interstate 90, where shortening from a M7 earthquake uplifted strata A.D. 900-930. A nearly identical pattern of dipping reflectors appears along strike on a profile 5 km east of Lake Sammamish, suggesting that the entire length of the Seattle fault east of Lake Washington has a similar fault history and is unsegmented. Deformation in the fault zone east of Lake Sammamish implies a documented fault length of at least 50 km, and the eastern extent of the fault still undetermined. In the Seattle uplift south of the Seattle fault, steeply-dipping reflections ($>$50 degrees) within Eocene and older sediments match known attitudes in outcrops and can be used to constrain structural models of the fault motion. Further processing of these data should reveal the history, character, and continuity of faulting and help identify likely trench sites for paleoseismic studies.
T32C-06 11:45h
Active Thrusting in the Inner Fore arc of Middle America, Costa Rica
The Fila Costena is an active thrust belt within the forearc basin of the Middle America convergent margin in Costa Rica, with a rate of shortening that represents a significant proportion of the rapid convergence between the Cocos and Caribbean plates. New geologic mapping of this thrust belt between the Panama border on the east and the Terraba gorge on the west depict a duplex with 3-4 horses that incorporate Eocene limestones and clastics of the Oligocene-early Miocene Terraba Formation. A thrust sheet at the rear of the thrust belt displaces the entire basin sedimentary sequence, including the Pliocene Curre Formation. All the thrust faults are emergent and cut the synorogenic land surface in the mapping area. Cross sections were constructed along two NE-SW trending transects across the thrust belt at the Terraba gorge and near the town of Guaria on the Pan-American Highway. Estimates of fault slip based on cutoffs of Eocene limestones on these sections are 4.2 km, 5.4 km, 5.0 km and 6.6 km for the four thrust sheets in the Terraba gorge, where hanging wall cutoffs are exposed, to 4.5 km, 5.5 km, 6.3 km, 8.1 km, and 11.9 km for the five thrusts in the Guaria area, where only minimum estimates of shortening were possible. The Eocene limestones at the base of thrust sheets pinch out to the west due to decreasing slip on faults and/or a lateral ramp in the basal decollement. To the east, the duplex terminates abruptly near the Panama border at the on-land projection of the subducting Panama Fracture Zone, suggesting that shortening is propagating rapidly to the east with the migration of the triple junction and the onset of rapid shallow subduction of thickened Cocos plate. Total shortening is greatest in the vicinity of Guaria, where the restored cross section shows 36 km of slip, as compared to 21 km near the Terraba gorge. Minimum long-term shortening rates are constrained by the presence of faulted Pliocene marine sediments in the thrust belt. The time since the passage of the Cocos-Nazca-Caribbean triple junction, migrating southeast at approximately 50 mm/yr, provides a maximum shortening rate based on the assumption that lateral propagation of the thrust belt is smooth and not episodic. It is estimated that the rates of active thrusting at the front of the Fila Costena are between 0.34 mm/yr and 1.5 mm/yr, based upon marine terrace development (Fisher et al., Tectonics, 2004). These uplift rates, determined from uplifted marine terrace elevations, correspond to the dip on the underlying faults, as exhibited on structural maps. Where the thrust belt extends offshore in the east, there is a regionally extensive marine platform where erosion largely keeps pace with the thrust rate. Comparatively, inboard of the subducting Cocos ridge the thrust front lies inland and the total shortening is greater. The south sides of the prominent ridges within this emergent thrust front are sites of extensive landsliding, with deposits up to 39 sq. km. A minimum of 40% of the total Cocos-Caribbean convergence is taken up by shortening of the inner fore 80 km inland from the active trench. Absence of similar features in the Nicaraguan forearc where the subducting crust is older, subducts more steeply, and lacks incoming ridges and seamounts indicates that demise of the forearc basin in Costa Rica reflects the greater coupling inboard of the Cocos Ridge.
T32C-07 12:00h
The latest event and start period of the thrust system in the Kuromatsunai lowland, northernmost part of the back arc zone of Northeast Japan
The Kuromatsunai-teich fault zone is an N-S trending fold and thrust zone in southwestern Hokkaido. Kuromatsunai lowland is located in the northernmost area of the fold and thrust zone in the back arc of Northeast Japan. Only few data had been obtained for the active faults in Hokkaido. We made trenching surveys and seismic profiling in this fault zone and obtained new data on the timing of the latest faulting events and the subsurface structures. The results of trench and pit excavation surveys at two sites (Shirozumi and Warabitai) show the latest event occurred after the Last Glacial period. At the Warabitai site, the timing of the latest faulting was narrowed down around 5,500 yBP. The results of seismic profiling on two lines show westward tilting. The westward dip of the layers decrease upward and the thickness of each layers increases toward the axis of syncline. This indicates the growth of tilting through the Middle to the Late Quaternary period. Based on the tilting rates of the late Pleistocene terraces, the deformation in this fault zone was informed to have started Early to Middle Pleistocene period (ca. 0.8 Ma).