Seismicity and Crustal Structure Studies of Southern California: Tectonic Implications from Improved Earthquake Locations

Citation

Corbett, Edward John (1984) Seismicity and Crustal Structure Studies of Southern California: Tectonic Implications from Improved Earthquake Locations. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/h8mm-4v50. https://resolver.caltech.edu/CaltechTHESIS:10082018-123143470

Abstract

This thesis consists of studies of: 1) the 1978 Santa Barbara earthquake and its aftershocks; 2) the depth distribution of seismicity in the Transverse Ranges; 3) crustal-velocity structure of the Continental Borderlands derived from explosion data; 4) the 1981 Santa Barbara Island earthquake and its aftershocks; and 5) earthquake location procedures, in particular the calibrated master-event technique.

The 5.1 M_L Santa Barbara earthquake of 13 August 1978 occurred at 22^h 54^m 52.8^s GMT. The epicenter was located 3 km southeast of Santa Barbara at 34° 23.9' N latitude and 119° 40.9' W longitude with a focal depth of 12.7 km. The mainshock was followed between 13 August and 30 September by 373 aftershocks that were located with the Caltech-USGS array. The aftershock zone extended 12 km west-northwest from the epicenter and was 6 km wide in the north-south direction, and it had a very clear temporal development. During the first 20 minutes of activity, all the aftershocks were located in a cluster 7 km west-northwest of the mainshock epicenter. During the next 24 hours the aftershock zone grew to 11 km in the west-northwest direction and 4 km in the north-south direction. During succeeding weeks, the zone extended to 12 by 6 km. This temporal-spatial development relative to the mainshock epicenter may indicate that the initial rupture propagated 7 km unilaterally to the west-northwest, and the initial rupture plane may have been considerably smaller than that of the eventual aftershock zone. This smaller area suggests that the stress drop may have been significantly greater than that derived from the area of the final aftershock zone.

In cross-section, the aftershock hypocenters outline a nearly horizontal plane (dipping 15° or less) at 13-km depth. The mainshock focal mechanism indicates north-northeast/south-southwest compression and vertical extension. The preferred fault plane strikes N 80° W and dips 26° NNE, indicating north-over-south thrusting with a component of left-lateral movement. Focal mechanisms for 40 aftershocks also indicate compression in the general north-south direction. For most of these events, the north-dipping nodal plane dips between 7° and 45°, with most dipping 25° or more, which is significantly steeper than the plane delineated by the hypocenters themselves. These observations are consistent with a tectonic model in which much of the slip during the Santa Barbara earthquake occurred on a nearly horizontal plane. The aftershocks then might represent movement on a complex series of imbricate thrust faults that flatten into the plane of primary slip. Hence, the Santa Barbara earthquake may be taken as evidence for mid-crustal horizontal shearing in the western Transverse Ranges.

To further test the decollement hypothesis, Caltech catalog locations were reviewed to determine the depth distribution of earthquakes in the Transverse Ranges. Only events with ERH < 1 km and ERZ < 2 km were utilized. These were scrutinized further with a numerical test of location procedures to test the reliability of the Caltech catalog quality assignments. These tests confirmed location qualities within 40 km of the east-west axis of the Transverse Ranges, but cast doubt on locations to the north and south.

The bottom of the seismogenic zone is clearly deepest along the southern front of the Transverse Ranges, with the deepest earthquakes occurring in the Pt. Mugu-Malibu area and under San Gorgonio Pass. Seismic activity is noticeably shallower north and east of the San Andreas fault than it is across the fault to the southwest. The seismogenic zone is thinnest in the southern Mojave Desert and at the east end of the Transverse Ranges. The seismicity of the western Transverse Ranges is typified by several north-dipping planar structures that correlate with the aftershock zones of recent earthquakes. The eastern Transverse Ranges are typified by ubiquitous seismicity extending from the surface down to the floor of the seismogenic zone. The San Bernardino Mountains are underlain by a well-defined bottom of the seismogenic zone that dips southward from 5-km depth under the Mojave Desert to 15-km depth where it intersects the San Andreas fault. South of the San Andreas fault, seismic activity deepens abruptly to as much as 22-km depth. The most intense seismicity is localized in the San Gorgonio Pass between the north and south branches of the San Andreas fault. This study falls short of the solving the decollement question, but it does add more intriguing evidence to the puzzle.

A large quarry explosion detonated on Catalina Island produced clear signals at stations throughout southern California. Data from near-shore and Island stations were utilized to derive velocity structure by the slope-intercept method. A 5.2-km/sec layer underlain by a 6.3-km/sec refractor was typically observed in most azimuths. A 7.8-km/sec Moho refraction was observed at ranges beyond 120 km. The interpretation is that the crustal refractor is at 5.5-km depth and the Moho is at 22-km depth. The upper crustal layer is significantly faster (5.5 km/sec) and thinner (2.5 km) under Catalina Island. An early P_n arrival and possible Moho reflections observed at San Nicolas Island may constrain the Moho to be an average of 2 km shallower in the direction west from Catalina. This velocity structure was successfully used to improve the locations of the 1981 Santa Barbara Island earthquakes.

The Santa Barbara Island earthquake occurred at 15:50:50 GMT on September 4, 1981, at 30° 40.9' N and 119° 3.6' W, and registered 5.3 M_L. Aftershocks exhibited a clear northwest-southeast alignment that coincides with the northeast-facing escarpment of the submarine Santa Cruz-Catalina ridge. This alignment also coincides with a mapped bedrock fault which is herein referred to as the Santa Cruz-Catalina fault. Focal mechanisms of the mainshock and the 3 largest aftershocks consistently show right-lateral strike slip on a northwest-trending plane, with possibly a component of dip slip. Aftershock depths show a near-vertical fault plane. The aftershock zone was initially 6 km long or less, and was concentrated southeast of the mainshock, suggesting unilateral rupture. The aftershock zone grew bilaterally to 15-km length after 24 hours to 21 km after 10 days, and to 35 km long after several months. This behavior may be Interpreted in tenns of an asperity model.

This seismic activity suggests strike-slip motion on the Santa Cruz-Catalina fault, with Santa Monica basin being displaced southeastward relative to points west. Structural complexities at the northwest and southeast ends of this fault suggest that the Santa Monica basin and Catalina Island are behaving as a coherent block pulling away from the Transverse Ranges, with extension at the northwest corner of the basin and compression to the south at the Catalina escarpment. Thus the Santa Monica basin may have formed as a triangular gap opening up between Peninsular Ranges blocks and the Transverse Ranges along the lines of the model of Luyendyk et al. (1980).

Nearly all earthquake location programs use Geiger’s (1912) method of least squares. This rigorous statistical method assumes that all the data are of equal quality and the only source of error is in measuring arrival times. This is not generally true of real earthquake data, which has led to a number of attempts at improvement. One of the most common modifications is data weighting of three types: quality weighting, distance weighting, and residual weighting. Programs that use all three must be used carefully to avoid feedback between weighting routines, with residual weighting being the worst cause of feedback. Station corrections are used to correct for systematic velocity variations and permit higher precision relative locations. The two most popular relative location methods are Joint Hypocenter Determination (JHD) and the master-event technique. The locations in Chapters 2 and 5 were performed with a modification termed the calibrated master event (CME) method. First, an intermediate-sized event is calibrated (preferably by explosion data) to achieve the best possible absolute location. Then, the residuals and hypocenter of this master event are used for establishing station delays and starting location, respectively, for relocating the seismicity of interest. Case histories of previous location attempts document the improvement attained with the CME method.

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