Faulting and Folding of the Transgressive Surface Offshore Ventura Records Deformational Events in the Holocene

Identifying the offshore thrust faults of the Western Transverse Ranges that could produce large earthquakes and seafloor uplift is essential to assess potential geohazards for the region. The Western Transverse Ranges in southern California are an E-W trending fold-and-thrust system that extends offshore west of Ventura. Using a high-resolution seismic CHIRP dataset, we have identified the Last Glacial Transgressive Surface (LGTS) and two Holocene seismostratigraphic units. Deformation of the LGTS, together with onlapping packages that exhibit divergence and rotation across the active structures, provide evidence for three to four deformational events with vertical uplifts ranging from 1 to 10 m. Based on the depth of the LGTS and the Holocene sediment thickness, age estimates for the deformational events reveal a good correlation with the onshore paleoseismological results for the Ventura-Pitas Point fault and the Ventura-Avenue anticline. The observed deformation along the offshore segments of the Ventura-Pitas Point fault and Ventura-Avenue anticline trend diminishes toward the west. Farther north, the deformation along the offshore Red Mountain anticline also diminishes to the west with the shortening stepping north onto the Mesa-Rincon Creek fault system. These observations suggest that offshore deformation along the fault-fold structures moving westward is systematically stepping to the north toward the hinterland. The decrease in the amount of deformation along the frontal structures towards the west corresponds to an increase in deformation along the hinterland fold systems, which could result from a connection of the fault strands at depth. A connection at depth of the northward dipping thrusts to a regional master detachment may explain the apparent jump of the deformation moving west, from the Ventura-Pitas Point fault and the Ventura-Avenue anticline to the Red Mountain anticline, and then, from the Red Mountain anticline to the Mesa-Rincon Creek fold system. Finally, considering the maximum vertical uplift estimated for events on these structures (max ∼10 m), along with the potential of a common master detachment that may rupture in concert, this system could generate a large magnitude earthquake (>Mw 7.0) and a consequent tsunami.Depto. de Geodinámica, Estratigrafía y PaleontologíaFac. de Ciencias GeológicasTRUEUnión Europea. Horizonte 2020Comunidad de MadridSCECpu

Quaternary active tectonic deformation of the transgressive surface offshore Ventura, CA, constrained by new geophysical data

Seismological Society of America Annual Meeting, 18-20 April 2017, Denver, ColoradoThe Transverse Ranges are a thrust-and-fold belt that accommodates the contraction resulting from a regional restraining bend in the San Andreas fault. The E-W trending Ventura basin, which is filled by more than 5 km of Pleistocene sediment, is shortening at about 10 mm/yr as inferred from geodetic data. Although the geological structure is fairly well known in the onshore areas of the basin, there is still discussion about how the different onshore thrust and folds continue in the offshore, and the deep relationship between the main faults. New high-resolution seismic data (chirp) was acquired in the area, that when combined with existing geophysical information, allows for a better understanding of the current activity of geological structures in the offshore. The dense seismic dataset allows us to identify different latest Quaternary seismostratigraphic units and horizons, with the most regionally recognized being a transgressive surface (LGTS) associated to the Last Glacial maximum and subsequent sea level rise. This surface is cut across and deformed by a series of E-W regional folds that produce elongated and parallel highs and depressions, and some emergent faults. Below the LGTS there is Early to Late Pleistocene units that are deformed by high amplitude regional folds and some local faulting. Above the LGTS we have identified progradational and agradational units that are related to global sea level rise, and which show less deformation (folding and faulting) than the lower units and horizons. Based on our analysis of the entire dataset, we have mapped the late Quaternary active structures in the offshore Ventura basin, which leads us to propose a new deep structural geological model of the basin. In addition, a preliminary interpretation of some specific fold growth sequences has allowed us to identify different tectonic deformation events (e.g. earthquakes) and, thus, their deformation history may be determinedThis research was supported by PALEOSISQUAKE MSC-EU Project (H2020-MSCA-IF-2014 657769). Hector Perea was a fellow researcher under the Marie Sklodowska-Curie Actions (H2020-MSCA-IF-2014 657769)Peer Reviewe

Holocene deformation events in the offshore Transverse Ranges (California, USA) constrained by new high-resolution geophysical data

8th International INQUA Meeting on Paleoseismology, Active Tectonics and Archeoseismology (PATA), 13-16 November 2017, Blenheim, New ZealandThis research has received funding from the SCEC (award #12026) and from the European Union's Horizon 2020 research and innovation programme under grant agreement No H2020-MSCA-IF-2014 657769Peer Reviewe

Faulting and Folding of the Transgressive Surface Offshore Ventura Records Deformational Events in the Holocene

25 pages, 15 figures, supplementary material https://www.frontiersin.org/articles/10.3389/feart.2021.655339/full#supplementary-material.-- Data Availability Statement: The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authorsIdentifying the offshore thrust faults of the Western Transverse Ranges that could produce large earthquakes and seafloor uplift is essential to assess potential geohazards for the region. The Western Transverse Ranges in southern California are an E-W trending fold-and-thrust system that extends offshore west of Ventura. Using a high-resolution seismic CHIRP dataset, we have identified the Last Glacial Transgressive Surface (LGTS) and two Holocene seismostratigraphic units. Deformation of the LGTS, together with onlapping packages that exhibit divergence and rotation across the active structures, provide evidence for three to four deformational events with vertical uplifts ranging from 1 to 10 m. Based on the depth of the LGTS and the Holocene sediment thickness, age estimates for the deformational events reveal a good correlation with the onshore paleoseismological results for the Ventura-Pitas Point fault and the Ventura-Avenue anticline. The observed deformation along the offshore segments of the Ventura-Pitas Point fault and Ventura-Avenue anticline trend diminishes toward the west. Farther north, the deformation along the offshore Red Mountain anticline also diminishes to the west with the shortening stepping north onto the Mesa-Rincon Creek fault system. These observations suggest that offshore deformation along the fault-fold structures moving westward is systematically stepping to the north toward the hinterland. The decrease in the amount of deformation along the frontal structures towards the west corresponds to an increase in deformation along the hinterland fold systems, which could result from a connection of the fault strands at depth. A connection at depth of the northward dipping thrusts to a regional master detachment may explain the apparent jump of the deformation moving west, from the Ventura-Pitas Point fault and the Ventura-Avenue anticline to the Red Mountain anticline, and then, from the Red Mountain anticline to the Mesa-Rincon Creek fold system. Finally, considering the maximum vertical uplift estimated for events on these structures (max ∼10 m), along with the potential of a common master detachment that may rupture in concert, this system could generate a large magnitude earthquake (>Mw 7.0) and a consequent tsunamiFunding for this research was provided by SCEC award #12026 (TR, ND, and GK). HP was supported by the European Union’s Horizon 2020 research and innovation programme under grant agreement No H2020-MSCA-IF-2014 657769 (PALEOSEISQUAKE) and by the Madrid Community’s “Atracción de Talento Investigador” call 2018 programme under the grant 2018-T1/AMB-11039 (UNrIDDLE)With the institutional support of the ‘Severo OchoaCentre of Excellence’ accreditation (CEX2019-000928-S)Peer reviewe

Holocene deformation offshore Ventura basin, CA, constrained by new high-resolution geophysical data

American Geophysical Union Fall Meeting, 11-15 December 2017, New OrleansThe Transverse Ranges (Southern California, USA) accommodate the contraction resulting from a regional restraining bend in the San Andreas Fault to form a thrust-and-fold belt system. The southern boundary of this system corresponds to the E-W trending Ventura basin, which is filled by more than 5 km of Pleistocene sediment and is shortening at about 10 mm/yr as inferred from geodetic data. Although the different thrust and folds are fairly well known in the onshore areas of the basin, there is still uncertainty about their continuation in the offshore. The analysis of new high-resolution (SIO CHIRP) and existing (USGS sparker and chirp) seismic data has allowed us to characterize better the active geological structures in the offshore. In the dataset, we have identified different latest Quaternary seismostratigraphic units and horizons, with the most regionally recognized being a transgressive surface (LGTS) associated to the Last Glacial maximum and subsequent sea level rise. A series of E-W regional folds related to thrust faults have deformed the LGTS producing highs and depressions. The correlation of these structures between profiles shows that they are elongated and parallel between them and continue to the coastline. In addition, considering their trend and kinematics, we have been able to tie them with the main onshore active thrusts and folds. Above the LGTS we have identified progradational and agradational units that are related to global sea level rise, which exhibit less deformation (folding and faulting) than the lower units and horizons. However, we have recognized some specific fold growth sequences above LGTS associated with the activity of different thrust-related anticlines. Accordingly, we have identified between 3 and 5 tectonic deformation events (e.g., earthquakes) associated to thrust fault activity. These results may help us to determine the deformation history for the offshore Ventura basin and the potentiality of the thrust faults that may be tsunamigenic, and compare our observations to the onshore resultsThis research has received funding from the SCEC (award #12026) and from the European Union's Horizon 2020 research and innovation programme under grant agreement No H2020-MSCA-IF-2014 657769 (PALEOSEISQUAKE project; www.researchgate.net/project/PALEOSEISQUAKE)Peer Reviewe

Palaeoseismology of the North Anatolian Fault near the Marmara Sea: implications for fault segmentation and seismic hazard

We conducted palaeoseismic studies along the North Anatolian fault both east and west of the Marmara Sea to evaluate its recent surface rupture history in relation to the well-documented historical record of earthquakes in the region, and to assess the hazard of this major fault to the city of Istanbul, one of the largest cities in the Middle East. Across the 1912 rupture of the Ganos strand of the North Anatolian fault west of the Marmara Sea, we excavated 26 trenches to resolve slip and constrain the earthquake history on a channel–fan complex that crosses the fault at a high angle. A distinctive, well-sorted fine sand channel that served as a marker unit was exposed in 21 trenches totaling over 300 m in length. Isopach mapping shows that the sand is channelized north of the fault, and flowed as an overflow fan complex across a broad fault scarp to the south. Realignment of the feeder channel thalweg to the fan apex required about 9±1 m of reconstruction. Study of the rupture history in several exposures demonstrates that this displacement occurred as two large events. Analysis of radiocarbon dates places the age of the sand channel as post ad 1655, so we attribute the two surface ruptures to the large regional earthquakes of 1766 and 1912. If each was similar in size, then about 4–5 m of slip can be attributed to each event, consistent with that reported for 1912 farther east. We also found evidence for two additional surface ruptures after about ad 900, which probably correspond to the large regional earthquakes of 1063 and 1344 (or 1354). These observations suggest fairly periodic occurrence of large earthquakes (RI=c. 283±113 years) for the past millennium, and a rate of c. 16 mm/a if all events experienced similar slip. We excavated six trenches at two sites along the 1999 Izmit rupture to study the past earthquake history along that segment of the North Anatolian fault. One site, located in the township of Köseköy east of Izmit, revealed evidence for three surface ruptures (including 1999) during the past 400 years. The other trench was sited in an Ottoman canal that was excavated (but never completed) in 1591. There is evidence for three large surface rupturing events in the upper 2 m of alluvial fill within the canal at that site, located only a few kilometres from the Köseköy site. One of the past events is almost certainly the large earthquake of 1719, for which historical descriptions of damage are nearly identical to that of 1999. Other earthquakes that could plausibly be attributed to the other recognized rupture of the Izmit segment are the 1754, 1878 or 1894 events, all of which produced damage in the region and for which the source faults are poorly known. Our palaeoseismic observations suggest that the Izmit segment of the North Anatolia fault ruptures every one and a half centuries or so, consistent with the historical record for the region, although the time between ruptures may be as short as 35 years if 1754 broke the Izmit segment. Release of about 4 m of seismic slip both west and east of the Marmara Sea this past century (1912, 1999) support the contention that Istanbul is at high risk from a pending large earthquake. In that historical records suggest that the last large central Marmara Sea event occurred in 1766, there may be a similar 4 m of accumulated strain across the Marmara basin segment of the North Anatolian fault