scholarly journals A slow earthquake in the Santa Maria basin, California

1992 ◽  
Vol 82 (5) ◽  
pp. 2087-2096
Author(s):  
Hiroo Kanamori ◽  
Egill Hauksson

Abstract An ML = 3.5 earthquake near Santa Maria, California, was recorded by the Southern California Seismic Network and a TERRAscope station at Santa Barbara (SBC) on 31 January 1991. The waveform of this event is dominated by 2- to 5-sec waves, and is different from that of ordinary events with similar size. Inquiries into operations in several oil fields in the area revealed that hydro-fracturing at a pressure of about 80 bars was being done at a depth of 100 to 300 m in the Orcutt oil field in the Santa Maria basin from about 9 to 11 a.m. on 31 January and the earthquake occurred in the afternoon. Field evidence of 30-cm displacement to a depth of 300 m was reported. The field evidence as well as the first-motion data indicates that the event had a thrust mechanism with the P axis in the NNE - SSW direction, which is in agreement with the regional stress field. From the analysis of the SBC record and the field evidence, we conclude that the source must be shallower than 1 km and the ratio of the radiated energy to the seismic moment is about 6.2 × 10−7, one to two orders of magnitude smaller than that of ordinary earthquakes. The occurrence of this earthquake demonstrates that release of regional tectonic stress in shallow sediments can yield significant seismic radiation at periods of a few seconds, the period range of engineering importance for large structures, and has important implications for excitation of long-period ground motions from large earthquakes in sedimentary basins.

Author(s):  
Lars Stemmerik ◽  
Gregers Dam ◽  
Nanna Noe-Nygaard ◽  
Stefan Piasecki ◽  
Finn Surlyk

NOTE: This article was published in a former series of GEUS Bulletin. Please use the original series name when citing this article, for example: Stemmerik, L., Dam, G., Noe-Nygaard, N., Piasecki, S., & Surlyk, F. (1998). Sequence stratigraphy of source and reservoir rocks in the Upper Permian and Jurassic of Jameson Land, East Greenland. Geology of Greenland Survey Bulletin, 180, 43-54. https://doi.org/10.34194/ggub.v180.5085 _______________ Approximately half of the hydrocarbons discovered in the North Atlantic petroleum provinces are found in sandstones of latest Triassic – Jurassic age with the Middle Jurassic Brent Group, and its correlatives, being the economically most important reservoir unit accounting for approximately 25% of the reserves. Hydrocarbons in these reservoirs are generated mainly from the Upper Jurassic Kimmeridge Clay and its correlatives with additional contributions from Middle Jurassic coal, Lower Jurassic marine shales and Devonian lacustrine shales. Equivalents to these deeply buried rocks crop out in the well-exposed sedimentary basins of East Greenland where more detailed studies are possible and these basins are frequently used for analogue studies (Fig. 1). Investigations in East Greenland have documented four major organic-rich shale units which are potential source rocks for hydrocarbons. They include marine shales of the Upper Permian Ravnefjeld Formation (Fig. 2), the Middle Jurassic Sortehat Formation and the Upper Jurassic Hareelv Formation (Fig. 4) and lacustrine shales of the uppermost Triassic – lowermost Jurassic Kap Stewart Group (Fig. 3; Surlyk et al. 1986b; Dam & Christiansen 1990; Christiansen et al. 1992, 1993; Dam et al. 1995; Krabbe 1996). Potential reservoir units include Upper Permian shallow marine platform and build-up carbonates of the Wegener Halvø Formation, lacustrine sandstones of the Rhaetian–Sinemurian Kap Stewart Group and marine sandstones of the Pliensbachian–Aalenian Neill Klinter Group, the Upper Bajocian – Callovian Pelion Formation and Upper Oxfordian – Kimmeridgian Hareelv Formation (Figs 2–4; Christiansen et al. 1992). The Jurassic sandstones of Jameson Land are well known as excellent analogues for hydrocarbon reservoirs in the northern North Sea and offshore mid-Norway. The best documented examples are the turbidite sands of the Hareelv Formation as an analogue for the Magnus oil field and the many Paleogene oil and gas fields, the shallow marine Pelion Formation as an analogue for the Brent Group in the Viking Graben and correlative Garn Group of the Norwegian Shelf, the Neill Klinter Group as an analogue for the Tilje, Ror, Ile and Not Formations and the Kap Stewart Group for the Åre Formation (Surlyk 1987, 1991; Dam & Surlyk 1995; Dam et al. 1995; Surlyk & Noe-Nygaard 1995; Engkilde & Surlyk in press). The presence of pre-Late Jurassic source rocks in Jameson Land suggests the presence of correlative source rocks offshore mid-Norway where the Upper Jurassic source rocks are not sufficiently deeply buried to generate hydrocarbons. The Upper Permian Ravnefjeld Formation in particular provides a useful source rock analogue both there and in more distant areas such as the Barents Sea. The present paper is a summary of a research project supported by the Danish Ministry of Environment and Energy (Piasecki et al. 1994). The aim of the project is to improve our understanding of the distribution of source and reservoir rocks by the application of sequence stratigraphy to the basin analysis. We have focused on the Upper Permian and uppermost Triassic– Jurassic successions where the presence of source and reservoir rocks are well documented from previous studies. Field work during the summer of 1993 included biostratigraphic, sedimentological and sequence stratigraphic studies of selected time slices and was supplemented by drilling of 11 shallow cores (Piasecki et al. 1994). The results so far arising from this work are collected in Piasecki et al. (1997), and the present summary highlights the petroleum-related implications.


Geophysics ◽  
1986 ◽  
Vol 51 (5) ◽  
pp. 1127-1140 ◽  
Author(s):  
Paul M. Kieniewicz ◽  
Bruce P. Luyendyk

The Santa Maria Basin in southern California is a lowland bounded on the south by the Santa Ynez River fault and on the northeast by the Little Pine‐Foxen Canyon‐Santa Maria River faults. It contains Neogene sedimentary rocks which rest unconformably on a basement of Cretaceous and older clastic rocks. Analysis of over 4 000 gravity stations obtained from the Defense Mapping Agency suggests that the Bouguer anomaly contains a short‐wavelength component arising from a variable‐density contrast between the basin’s Neogene units and the Cretaceous basement. A three‐dimensional inversion of the short‐wavelength component (constrained by wells drilled to basement) yields a structure model of the basement and the average density of the overlying sediments, assuming that the basement does not contain large‐scale density variations. The density anomalies modeled in the Neogene sediments, showing higher densities in the basin troughs, can be related to diagenetic changes in the silica facies of the Monterey and Sisquoc formations. The basement structure model shows the basin as composed of parallel ridges and troughs, trending west‐northwest and bounded by steep slopes interpreted as fault scarps. The basin is bounded on the west by a north‐south trending slope which may also represent a fault scarp.


2012 ◽  
Vol 2012 ◽  
pp. 1-16
Author(s):  
Yudong Wu ◽  
Quanlin Hou ◽  
Yiwen Ju ◽  
Daiyong Cao ◽  
Junjia Fan ◽  
...  

29 oriented and 10 nonoriented coal samples are collected in the study from three different regions of the Huaibei coalfield, eastern China, and their vitrinite reflectance indicating surface (RIS) parameters are systematically calculated and analyzed. Using the available methods, Kilby’s transformations and RIS triaxial orientations are obtained. The magnitudes and orientations of the RIS axes of the three regions were respectively projected on the horizontal planes and vertical sections. The results show that the samples in high deformed region have significant anisotropy magnitudes (higher Bw/Rmax and Ram values) with a biaxial negative style, whereas the samples in the slightly deformed area have unimpressive anisotropy magnitudes with a biaxial negative style. Thermal metamorphism superposed might enhance the complication and variation of RIS style. RIS projection analysis deduced that the RIS orientation is mainly controlled by regional tectonic stress, and likely influenced by deformation mechanisms of coal.


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