Yoko Ota

Fault mechanism and recurrence time of major earthquakes in southern Kanto district, Japan, as deduced from coastal terrace data

The southern Kanto region has had two shocks of magnitude 8 or greater during the past 1,000 years. They were the 1703 and 1923 earthquakes, which occurred along the Sagami trough, a northeastern boundary of the Philippine Sea crustal plate in contact with the Asian plate. Although they occurred in nearly the same region, the 1703 earthquake was significantly different from the 1923 earthquake in the distribution of coastal uplift and tsunami height. The 1703 earthquake deformation is described on the basis of the height of the marine terraces along the coast of the southern Kanto region. The 1703 earthquake is interpreted, as is the 1923 earthquake, as the result of low-angle right-lateral faulting with a thrust component at the plate boundary. However, the fault surface in 1703 was longer (about 200 km) and was located farther east than that of the 1923 earthquake. On the basis of the pattern of coastal uplift and the trend of the Sagami trough, the fault surface of the 1703 earthquake can be divided into three planes, which involve the eastern part of the source region of the 1923 earthquake to the west (plane A), the Kamogawa submarine cliff in the middle (plane B), and a segment near the source region of the 1953 Boso-Oki earthquake (M equals 8.0) to the east (plane C). The Boso and Miura Peninsulas in the sourthern Kanto region have been uplifted during at least the last 6,000 years, and major uplifts have been accompained by earthquakes like those of 1703 and 1923 many times. The recurrence time of similar uplifts is estimated at 800 to 1,500 years on the basis of the numbers of the uplifted Holocene terraces in the Boso Peninsula, the rate of upheaval during the last 6,000 years, and the present geodetic data. Thus, it is unlikely that major earthquakes such as the 1703 and 1923 earthquakes will occur in the same segments in the near future. The Oiso area, however, which is located west of the western end of the 1703 faulting, seems higher in seismic risk than the other parts of the Sagami trough fault, because the sum of the recent uplift in the 1703 and 1923 earthquakes in that area is significantly less than the average rate of uplift there during the past 6,000 years. /Author/

Holocene paleoseismicity in the fold and thrust belt of the Hikurangi subduction zone, eastern North Island, New Zealand

Abstract Along the 500 km long eastern coastline of the North Island, New Zealand there are, at most, seven distinct Holocene marine terraces aged 7 ka B.P. or less. The highest of the terraces is 27 m above present-day mean sea level. The coastal region is subdivided into fourteen distinct subregions based on radiocarbon ages of marine deposits overlying wave-cut shore platforms and geographic distribution of similarly-aged terraces. Holocene marine terraces are the result of uplift associated with large earthquakes (co-seismic deformation). This conclusion is based on characteristic stepped terrace morphology, clustering of ages of terrace deposits within subrogions, and the occurrence of co-seismic uplift in historic time. Differential uplift across structures and distinct age variations at subregion boundaries are also characteristic. Recurrence intervals of uplift in any one terrace sequence vary from ∼ 0.4 ka to ∼ 2.0 ka, and individual amounts of uplift vary from ∼ 1.0 m to ∼ 4.0 m. In the past ∼ 2.5 ka ages from all terraces within subrogions indicate at least 21 paleoseismic events affecting coastal areas of eastern North Island in that period. These events cluster in time, and in separate parts of eastern North Island several earthquakes occurred ∼ 0.3, 0.6,1.0,1.5, 2.1 and 2.3 ka B.P. Our data strongly support the concept of segmentation of deformation along the subduction margin. The likely cause of coastal uplift is movement on steep reverse faults (local structures rather than the subduction thrust) that propagate from or near the subduction thrust some 20–25 km below the region. Earthquakes of moment magnitude 7.3–8.0 are estimated to be associated with these fault movements.

Tectonic and paleoclimatic significance of Quaternary river terraces of the Waipaoa River, east coast, North Island, New Zealand

Abstract Remnants of four aggradational terraces in the lower 45 km of the main branch of the Waipaoa River have been correlated with cold/cool climate episodes of the Otiran glaciation. The youngest of the aggradation levels—the Waipaoa‐1 terrace—has the c. 14.7 kaRerewhakaaituTephra as the oldest part of the coverbed sequence, indicating cessation of aggradation about 16 ka BP. This terrace is broadly correlated with Ohakean‐aged terraces in other parts of the North Island. The second most recent episode of aggradation—the Waipaoa‐2 terrace—is slightly older than the c. 28 ka Mangaone Tephra, and is broadly correlated with the Rata terrace. The third most recent aggradation episode— the Waipaoa‐3 terrace—is slightly older than the c. 55–57 ka Rotoehu Tephra (age estimate based on stratigraphic relationships in this study), indicating cessation of aggradation at c. 65 ka BP, and correlative with the Porewa terrace. The fourth, and oldest, aggradation episode we identify in the present landscape—the Waipaoa‐4 terrace—has poor age constraints, but is probably related to the cool period of late oxygen isotope stage 5 at c. 90 ka BP or the glacial period of oxygen isotope stage 6 at c. 140 ka BP. Tectonic deformation in the middle reaches of the Waipaoa catchment is deduced from the elevation difference of pairs of aggradation terraces, and takes the form of broad regional uplift in the range of 0.5–1.1 mm/yr. Uplift is probably driven by subduction processes in the middle part of the catchment and by a combination of deep‐seated subduction processes and local deformation associated with active faults and folds in the lower valley area. Downcutting rates of up to 7 mm/yr occur in upper reaches of the river. In the middle reaches of the valley, where there are both uplift and downcutting data, we find that downcutting is about four times faster than tectonic uplift. Thus, climate fluctuations are interpreted to be the primary control on formation of fluvial terrace landscapes in the region.

Age and height distribution of holocene transgressive deposits in eastern North Island, New Zealand

Abstract Holocene transgressive deposits are frequently exposed near the present-day coastline of the study area along eastern North Island, New Zealand. They occur in sites of former estuaries that were filled during the postglacial rise in sea level. We present one hundred radiocarbon dates of Holocene transgressive deposits from the study area, ranging in age from ca. 10,000 to 5500 yr B.P. Relative sea level curves up to ca. 6000 yr B.P. were reconstructed for six locations. The curves have similar slopes prior to about 7000 yr B.P., indicating that sea level rise was much more rapid than any tectonic uplift at that time. The postglacial rise in sea level in New Zealand is considered, in general, to have culminated at about 6500 yr B.P. but the upper limit ages of transgressive deposits in our study area vary from ca. 5500 to 7000 yr B.P. At sites where the uplift rate is high the postglacial transgression culminated rather earlier than ca. 6500 yr B.P., and at sites where there is subsidence or there is very low uplift the culmination is later than ca. 6500 yr B.P. Nine of fourteen dates from fossil trees in growth position, that grew in and were buried by estuarine silt, cluster in the age range ca. 8000–8400 yr B.P. These data support the view that there was a minor regression or stillstand in the eustatic sea level rise at that time. Eleven tectonic subregions are recognized in the study area on the basis of average uplift rate. Most of these subregions coincide with those established from the number and ages of younger Holocene marine terraces of probable coseismic origin.

Contrasting styles and rates of tectonic uplift of coral reef terraces in the Ryukyu and Daito Islands, southwestern Japan

Abstract Coral reef terraces on Kikai, Hateruma and Yonaguni in the Ryukyu Islands on the Eurasian lithospheric Plate and those of Kita-Daito and Minami-Daito Islands on the Philippine Sea Plate indicate three distinct patterns of coral reef terrace formation and preservation, primarily reflecting different uplift rates. The Kikai-type, which includes many terraces from Oxygen Isotope Stages 5e to 1, is a result of high uplift rate of ca. 1.8 m/kyr on the fore-arc rise of a Chilean-type subduction zone. Arcward (westward) tilting and faulting are also characteristic of the Kikai-type. The Hateruma-type is characterized by two coral reef terraces younger than the Stage 5e terrace and they are arranged in a concentric annular pattern. Slow uplift rate of ca. 0.3 m/kyr, adjacent to a Mariana-type subduction margin is typical of the Hateruma-type. The emergence of Kikai and Hateruma Islands is recent, younger than Isotope Stage 5e and 7, respectively, reflecting the active deformations of the leading edge at the overriding Eurasian Plate. In the Daito-type, atolls surrounded by fringing reefs have been uplifted very slowly (less than 0.05 m/kyr), but over a time period of at least several million years. Slow uplift and long history of Daito-type islands is the result of up-bulging of the Philippine Sea Plate as it migrates toward the Ryukyu Trench.

Coseismic uplift of holocene marine terraces in the pakarae river area, Eastern North Island, New Zealand

Abstract Holocene marine terraces along 15 km of the northeastern coast of North Island record episodic tectonic uplift. A maximum of seven terraces are arranged in staircase fashion and lie about 20 km above the subduction interface between the Pacific and Australian plates. The highest (T1) corresponds with the maximum of the Holocene marine transgression about 6700 14 C yr B.P. Younger terraces are marine abrasion platforms overlain by thin beach deposits. Radiocarbon ages of marine shells from beach deposits indicate that uplift above marine conditions occurred ca. 6700(T1), 5500(T2), 3900(T3), 2500(4), 1600(T5), 1000(T6), and slightly younger than 600(T7) yr B.P. Uplift probably occurred coseismically. The maximum late Holocene uplift rate in the study area is 8 mm/yr. Altitudinal distribution of terraces suggests deformation exists as a ca. 20-km elongate dome, broken at the southern end by the Pakarae fault, which trends across the dome. Rupture on this fault has accompanied the growth of the dome but is not responsible for it. Bathymetric profiling suggests that an active fault, parallel to and about 5 km offshore, is probably responsible for the episodic coastal uplift.

Holocene marine terraces on the northeast coast of North Island, New Zealand, and their tectonic significance

Abstract As many as seven Holocene marine terraces are preserved between Raukokore River and Gisborne on the northeast coast, North Island, New Zealand. Six terraces up to 20 m above present mean sea level (a.m.s.l.) are dated at c. 300, 600–700, 900–1200, 1600–2000, 4500, and 6000 radiocarbon yr B.P. to the west of East Cape. Seven terraces are preserved up to 27 m a.m.s.l. near Pakarae River mouth, and the higher six terraces have radiocarbon ages of c. 1000, 1600, 2500, 3900, 5500, and 7000 yr B.P. The coastal region from Waiapu River to Tolaga Bay has only two to three marine terraces, the highest attaining a maximum height of c.8m. Sponge Bay Terrace is generally the highest preserved marine terrace, and it is underlain by more than 10 m of estuarine deposits that record the rapid rise of postglacial sea level. The terrace surface records the culmination of this sea‐level rise at 5500 yr B.P. or slightly younger in areas of low average uplift rate (<1.5 m/1000 yr) and c. 7000 yr B.P. in areas of high...

Late Quaternary coseismic uplift events on the Huon Peninsula, Papua New Guinea, deduced from coral terrace data

Up to six regressive terraces occur on the Holocene raised reef tract and up to 15 occur on late Pleistocene raised reef tracts along 40 km of coastline at Huon Peninsula, Papua New Guinea. We suggest that the regressive terraces represent repeated episodic uplift caused by great earthquakes. Ages of Holocene coseismic uplift events are established by radiocarbon dating; the late Pleistocene events are bracketed by U series ages of the raised reef tracts on which they occur. The mean recurrence interval of great earthquakes that caused the uplift events is 970 to 1165 years in the Holocene and probably the same in the late Pleistocene; the interval ranged from about 200 to 1900 years. The uplift rate increases parallel to the coast from northwest to southeast, and the amplitude of coseismic uplifts generally increases similarly, although some events produced uplift with little shore-parallel tilting. The mean amplitude of coseismic uplifts throughout the study area is ∼3 m in for both Holocene and late Pleistocene sequences. Large, late Quaternary landslides are numerous, and some probably were triggered by the great earthquakes that caused coseismic uplift. There appears to be no continuum between historical large earthquakes at Huon Peninsula of magnitudes >7 that produced no or only minor uplift, and the great earthquakes represented by meter-scale coseismic uplifts and very large landslides. Two tectonic subregions are recognized, which were uplifted together by some Holocene events but not by others. There is no surface trace of Holocene faulting between the subregions, and a buried fault is thought to separate them.

Vertical tectonic movement in northeastern Marlborough: Stratigraphic, radiocarbon, and paleoecological data from Holocene estuaries

Abstract Height and age information from Holocene estuarine deposits along the northeastern Marlborough coast provide a database to evaluate coastal vertical tectonics. These data are related to the postglacial marine transgression and coastal geomorphic features formed since the culmination of sea‐level rise. Four tectonic domains are recognised. The Wairau domain is characterised by subsidence at rates over 4 mm/yr. About 60% of this subsidence is tectonic and may be related to Marlborough Sounds subsidence, and 40% is a result of compaction. The Vernon Fault at the south side of the lower Wairau plain separates the Wairau domain from the high‐standing Vernon domain. The Awatere Fault marks the southern boundary between the Vernon domain and the Grassmere domain, which extends from the Awatere River valley to Mussel Point. Slight uplift (c. 1 m in 6500 yr) characterises the Grassmere domain, based on data obtained from Blind River, Lake Grassmere, and, to a lesser extent, from Awatere River fluvial terr...

Late Holocene Paleoseismicity of the Southern Part of the Chelungpu Fault in Central Taiwan: Evidence from the Chushan Excavation Site

The geomorphic expression of the frontal Western Foothills in central Taiwan is usually defined by a late Holocene scarp that ranges from tens to hundreds of meters in height. This scarp is the product of displacement on a near-surface 20- 30 east-dipping thrust fault, the Chelungpu fault, which ruptured during the Chichi earthquake. The large scarp height may correspond directly to the accumulation of successive surface ruptures. The Chushan site is located on the southern part of this fault where the Chichi earthquake produced a scarp with a 1.7-m vertical offset for the total vertical separation. Based on core-boring estimates, the vertical displace- ment on both sides is 7 m along a 24 east-dipping thrust fault. The results from our paleoseismic analysis indicate that five large paleoearthquake events have caused the large offsets during the past 2 ka. The radiocarbon age constraints of the paleoearth- quakes suggest a clustering of 540-790 cal yr B.P. (E2), 710-950 cal yr B.P. (E3), 1380-1700 cal yr B.P. (E4), 1710-1930 cal yr B.P. (E5), and the 1999 Chichi earth- quake. Events E3 and E4 have not been reported in previous studies and we did not observe event E1 (300-430 cal yr B.P.) at the site. Based on displacement and fault segmentation from the geologic features, we argue that the two new events may have occurred along the northern part of the Chelungpu fault. The vertical slip rate is estimated to be at least 3.9 0.2 mm/yr over the past 2 ka, which is similar to the long-term estimation through a calculation of late Pleistocene-Holocene terrace el- evations on the hanging wall.