Geo736: Seismicity and California s Active Faults Introduction

Transcription

1 Geo736: Seismicity and California s Active Faults Course Notes: S. G. Wesnousky Spring 2018 Introduction California sits on the boundary of the Pacific - North American plate boundary (Figure 1). Relative motion of north America with respect to the Pacific plate is right lateral. As a result, one of the most populated states of the U.S. sits astride numerous active faults. Because of the financial and personal risk associated with potential earthquakes, and the presence of numerous academic and governmental scientific groups, the San Andreas system of faults is now one of the most studied in the world. It provides a convenient starting point to examine the characteristics of seismicity as they relate to the physical and tectonic environment in which they occur. Figure 1. The San Andreas transform fault system accommodates right-lateral slip between the Pacific and North-American plates in California (Figure taken from R. E. Wallace, 1990a). The San Andreas is not a single fault but more aptly described as a fault system. The major strands that compose this anastomosing subparallel system of faults are sufficiently large to have earned their own name (Figure 2). Faults of both reverse and Spring

2 strike-slip motion occur within the system. We'll first focus on the major strike-slip faults (Figure 2 right): the Newport-Inglewood, Elsinore, San Jacinto, Garlock, Calaveras- Green Valley, Hayward-Maacama, and San Andreas faults. Figure 02. (left) Active faults of California (taken from p.3 of Robert E. Wallace, 1990b). (right) Simplified fault map of California showing major strike-slip faults of San Andreas System (taken from Wesnousky, 1988) San Andreas Fault It is generally accepted that right-slip of 150 km or more has been accommodated by the San Andreas fault since early Miocene (Crowell, 1962; Grantz and Dickinson, 1968; Hill, 1981). The total length of the fault where exposed onshore is about 1000 km (Figure 3). Though profoundly bent, the trace of of the San Andreas is virtually continuous except for a complex, ill-defined restraining step near the San Gorgonio Pass fault zone and a 1-km releasing step at Parkfield. Surface ruptures during the Spring

3 1857 (M=7.9) Ft. Tejon earthquake extended southward from near the 1-km step near Parkfield for a distance of km to a point near Cajon Pass (Sieh, 1978). As well, the 1906 San Francisco earthquake produced surface ruptures extending northward to offshore Pt. Arena from a point near San Juan Bautista (Lawson, 1908). Figure 3. Strip map of the San Andreas fault, California. Strips overlap and progress northwest to southeast from top to bottom of figure, respectively. Extent along strike of surface ruptures occurring during the large ~M7.9 earthquakes of 1857 and 1906 are stippled (Figure from Wesnousky, 1988). Sisters of the San Andreas: the Garlock, Whittier-Elsinore, Calaveras-Green Valley, Hayward-Maacama and Newport-Inglewood fault zones. Garlock Fault The left-lateral Garlock fault trace strikes easterly from it s junction with the San Andreas a distance of ~240 km (Figures 2 and 4a). Like the San Andreas the fault trace is relatively smooth, broken only by a 3-4 km step in the fault trace at Fremont Valley and a lesser ~1/2 km step near it s eastern end (Figure 4a). The Garlock fault has not been the site of any large historical earthquakes. Whittier-Elsinore The Whittier-Elsinore fault zone (Figure 4b), including the Laguna-Salada strand, strikes northwest for about 330 km from south of the U.S.-Mexico border to the north of Lake Elsinore. Lamar and Rockwell (1986) summarize studies concerning the age and offset of the Whittier-Elsinore fault zone. Apparent offset of Paleocene facies across the Whittier-Elsinore fault suggests km of right-slip near Lake Elsinore (Lamar, 1961; Yerkes and Campbell, 1971; Sage, 1973). Weber (1977) in contrast estimates that the total right-slip along the Elsinore fault in the same area is about 9 to 11 km, based on the distribution of older basement rocks. Woodford et. al (1972) similarly believe that large post-cretaceous strike-slip on the Whittier-elsinore fault zone is precluded by the distribution of distinctive Upper Cretaceous and Paleocene strata. Farther to the southeast, Mueller and Rockwell (1984) constrains total slip to no more than 35 to 40 km based on the size and geometry of pullapart basins developed along the Laguna Spring

4 Salada fault zone. Maximum total offset along the Elsinore fault thus appears limited to between 10 km and 40 km, and may vary along fault strike. Figure 4. Mapped traces of the a) Garlock, b) Whittier-Elsinore, c)calaveras-green Valley and Hayward- Rogers Creek-Maacama, d) San Jacinto, and e) Newport-Inglewood fault zones. The extent of rupture during the 1911, 1979, 1984, and 1868 earthquakes along the Calaveras and Hayward fault systems are bracketed. Hachured segments of the San Jacinto and Imperial faults produced surface ruptures in 1980, 1968, 1979, and The Laguna Salada fault system (L.S.) is considered to have ruptured in a large earthquakes in 1892 and most definitely in Star marks the epicenter of the 1933 Long Beach earthquake. on opposite sides of the fault. The 330 km length of the fault zone is interrupted by 3 steps, each with an effective step width of about 3 km. Geological evidence in conjunction with reported isoseismal data have been used to strongly suggest that offsets of 5 m or more observed along a stretch of the Laguna Salada fault were produced by a magnitude 7 or larger Spring

5 earthquake on Feb 24, 1892 (Strand, 1980; Toppozada et al., 1981; Mueller, 1984; Mueller and Rockwell, 1984; Anderson et al., 1988). The same section of the fault zone produced a ~75 km zone of surface ruptures along the Laguna Salada fault system on April 04, 2010 M 7.2 Sierra Mayor earthquake (it is not annotated on the figure but coincides at the scale of the map with stippling for the proposed 1892 event). Displacement during the earthquake was mostly right-lateral strike-slip with a component of down to the west normal motion. Calaveras-Green Valley The Calaveras-Green Valley fault zone strikes northward and approximately parallel to the San Andreas fault starting from a point east of Monterey (Figure 4c). I am aware of only one study regarding the total offset of this fault zone. Kintzer et al. (1977) report that a Middle Miocene shoreline exposed near Calaveras Reservoir may be offset in a right-lateral sense for a distance of 24 km across the Calaveras fault zone. The 220 km length of the mapped fault zone is broken by 4 distinct steps of width greater than about 1 km. The largest recent earthquakes to occur on the fault were the 1979 Coyote Lake (M=5.9) and l984 Morgan Hill (M =6.2) events (Reasenberg and Ellsworth, 1982; Bakun et al., 1984). Although significant surface ruptures were not observed for either event, aftershock studies show rupture lengths for each were on order of 25 km. The 1984 earthquake was apparently preceded by a larger magnitude 6.6 earthquake in 1911, which is the largest historical earthquake attributed to slip along this fault zone (Bakun et al., 1984). Hayward-Maacama The Hayward-Maacama fault branches from the Calaveras a few kilometers north of Calaveras Reservoir (Figure 4d). I'm not aware of data bearing directly on the total offset of the Hayward-Maacama fault zone, though the discontinuous trace and weak geomorphic expression of the fault have been used as evidence to suggest a relatively recent initiation for the fault zone (Herd, 1978; 1979; Pampeyan et al., 1981) [[Herd, 1978; Herd, 1979; Pampeyan et al., 1981] and Herd (pers. Comm.). The 200 km length of the fault characterized by strike-slip offset is interrupted by 3 steps with width greater than 1 km. An approximately 60 km segment of the fault zone ruptured in a magnitude 6.8 earthquake in 1868 (Toppozada et al., 1981). There are also reports that a similar sized event ruptured the same segment of the fault in 1836, though evidence for the actual location of the 1836 event is scant (Toppozada et al., 1981). (I believe there are also some more recent works that have addressed this issue). Newport-Inglewood The Newport-Inglewood fault zone strikes southward from near the Baldwin Hills and is expressed topographically by an aligned series of low hills that rise as much as 120 m above the adjacent plains (Figure 4e). Structurally, the fault zone near the surface is a series of discontinuous north- to northwest striking faults and northwest- to west-trending folds. Evaluations of total right-lateral displacement along the fault zone range from 200 meters near the Baldwin Hills to a maximum of 10 km near Huntington Beach. The fault zone is about 60 km long where evident onshore and broken by at least 4 prominent steps. The 1933 Long Beach earthquake (M=6.3) epicenter is located off Newport Beach (Richter, 1958). Aftershocks for the 1933 event apparently concentrated along a 30 km stretch of the fault zone between Signal Hill and the Spring

6 epicenter (Benioff, 1938; Richter, 1958). An analysis of the focal mechanism shows essentially pure right-lateral strike-slip motion on a nodal plane parallel to the surface expression of the fault (Hauksson, 1987). Seismicity in California Detection and Recording The earliest seismographs capable of systematically detecting earthquakes in California (and Nevada) were installed at sites around the World beginning in Reports of local effects in newspapers and other written media continued to be a main source of information until the development and deployment of Wood-Anderson seismographs throughout most of California beginning in 1926, at which time instrumental recordings begin to supplant non-instrumental sources as the primary recorder of earthquake history. With the implementation of seismographs, the historical record of earthquakes in California (at least southern California) is reportedly complete down to ~M6 events. Installation of a dense seismograph array was commenced in southern California in 1932, from which time, the record of earthquakes is complete for M>=4 events. The distribution of stations as of 1986 is shown in Figure 5. The distribution of stations would ideally be an equally spaced array across the state. But due to geologic and logistical considerations, it is not. Figure 5. Seismograph network stations operating in California in Dot, single (vertical)-component; star, multicomponent station. (Figure 5.2 fromhill et al., 1990) Spring

7 Spatial and Depth Distribution The locations 64,000 M>1.5 earthquake in California and western Nevada recorded during the period of 1980 to 1986 are shown on a map of known active faults in Figure 6. The relationship of the location of seismicity to the location of known active faults that cut the surface of the earth is a question of some importance to understanding the mechanical behavior of faults as well as seismic hazard. Extended alignments of epicenters indicate a northwest trending structural fabric similar to the orientation of major mapped faults. Upon closer examination, there is not always a one to one relationship of the location of seismicity to faulting. Some faults are clearly marked by alignments of epicenters whereas others are not. Questions one may ask as we survey the character of seismicity along the faults include Do small earthquake actually represent slip on the major fault, or small breaks in the volume surrounding the faults? Why are some faults quiescent and others marked by abundant seismicity? What is the depth of the seismicity and how does it relate to the surrounding physical and tectonic environment. Figure 6. Seismicity of M>1.5 recorded during 1980 to (Figure 5.4 of Hill et al., 1990) Spring