Segmentation of the northern San Jacinto Fault Zone, southern California

Abstract

Abstract. Accurate forecasting of large earthquakes in the San Jacinto fault zone depends on the correct determination of fault segmentation. We have searched for discontinuities in the fault zone using as data geological maps of the fault traces, relocated earthquake hypocenters, focal mechanisms, slip rates, and historic large earthquake rapture zones. We identify nine principal discontinuities in the northern San Jacinto fault zone. These are characterized as structural discontinuities (bends, steps, branches, intersections), geodetic discontinuities (slip rate), and lithologic discontinuities. Most of the discontinuities have more than one defining characteristic. We also analyzed the local strain field orientation along the fault zone. Significant lateral discontinuities in the strain-field orientation are coincident with many of the structural and geodetic discontinuities we identify. Segment lengths defined from the data vary from about 7 to 35 km. This range of segment lengths is consistent with the lengths of large historical earthquake ruptures. However, evidence from small earthquake hypocenter patterns suggests the possibility that in some areas large earthquakes may rapture multiple adjacent segments. In these cases, maximum magnitudes may be larger than those observed historically. Because of the short historic record and the sparse paleoseismic record, probabilistic estimates of earthquake recurrence for this fault zone have depended heavily on assumed characteristic lengths of individual rupture segments. Ours and previous delineations of segment lengths have varied considerably, indicating that assumed segment length is a major source of uncertainty in the recurrence calculations. This source of uncertainty should be taken into account in calculations of the reliability of the probabilistic hazard estimates.

Quaternary landscape evolution in the San Jacinto fault zone, Peninsular Ranges of Southern California: Transient response to strike-slip fault initiation

“…5); numbers indicate number of measurements at each site. Asterisks mark correlation points used to estimate distance and rate of knickpoint migration along the fault zone (see Discussion Sharp, 1967;Sanders, 1989;Sanders and Magistrale, 1997). The geometric complexity of the fault zone is characteristic of young, disorganized strike-slip faults with limited total displacement (Wesnousky, 1986(Wesnousky, , 1988.…”

Internal structure of the San Jacinto fault zone at Blackburn Saddle from seismic data of a linear array

“…Variations of lithological units and geometrical complexities (e.g. Sharp 1967) produce non-uniform distribution of strain and seismicity along the length of the fault (Sanders & Kanamori 1984;Sanders & Magistrale 1997, Hauksson et al 2012. Recent tomographic studies (Allam & Ben-Zion 2012; Allam et al 2014a; Zigone et al 2015) imaged with nominal resolution of 1-2 km large-scale variations of seismic velocities across the fault and significant damage zones at different locations.…”

Geologic and geophysical maps of the El Casco 7.5′ quadrangle, Riverside County, southern California, with accompanying geologic-map database

“…Frasier's geologic map implies that his Claremont Fault continues northwest from its junction with the more easterly Hot Springs Fault. We conclude that Frasier viewed the Claremont Fault as a major strand of the San Jacinto Fault flanking the northeast side of the San Jacinto Valley (also see Sharp, 1972;Sanders and Kanamori, 1984;Park and others, 1995;Lee and others, 1996;Sanders and Magistrale, 1997). We project the Claremont strand along the margin of the San Timoteo Badlands, continuing the fault northwestward from where Frasier (1931) mapped it along the east margin of San Jacinto Valley.…”

“…The two faults have an en echelon geometry (for example, Morton, 1972Morton, , 1977, and many workers propose that a right step from the Casa Loma Fault to the Claremont Fault has led to extensional subsidence of the San Jacinto Valley (Morton, 1977;others, 1992a, Park andothers, 1995). Seismicity associated with the San Jacinto Fault in the San Jacinto Valley region is discussed by Sanders (1993), Sanders and Kanamori (1984), Sanders and Magistrale (1997), and Magistrale and others (2000).…”

“…Seismic-imaging investigations were conducted in the El Casco quadrangle (Park and others, 1995) and in the southern San Jacinto Valley (Lee and others, 1996) in order to clarify the distribution of strands of the San Jacinto Fault zone. Sanders and Magistrale (1997) examined seismic evidence for segmentation of the northern San Jacinto Fault zone. Regional seismicity has been evaluated by Hauksson (2000) and by Magistrale and others (2000)).…”