Dual-System Tectonics of the San Luis Range and Vicinity, Coastal Central California

Douglas H. Hamilton, Consulting Geologist 2 Bassett Lane, Atherton, California 94027

Text and illustrations from poster displayed at the Wednesday, December 15 Session of the 2010 American Geophysical Union Fall Meeting San Francisco, California

Dual – System Tectonics of the San Luis Range and Vicinity, Coastal Central California

Douglas H Hamilton Consulting Geologist, Atherton, CA, United States

ABSTRACT The M 6.5 "San Simeon" earthquake of December 22, 2003, occurred beneath the San Lucia Range in coastal central California, and resulted in around $250,000,000 property damage and two deaths from collapse of an historic building in the town of Paso Robles, located 40 km from the epicenter. The earthquake and more than 10,000 aftershocks were well recorded by nearby seismographs, which permitted detailed analysis of the event (eg: McLaren et al., 2008). This analysis facilitated evaluation of the hazard of the occurrence of a similar event in the nearby San Luis Range, located along the coast west of the City of San Luis Obispo some 55 km south of the San Simeon epicenter.

The future occurrence of earthquakes analogous to the 2003 event in this area had been proposed in the late 1960's (eg: Benioff and Smith, 1967; Richter, 1969) but the apparent hazard of such occurrences came to be overshadowed by the discovery of the "Hosgri" strike slip fault passing close to the area in the offshore. However data accumulated since the early 1970's clearly demonstrate the hazard as being partitioned between nearby earthquakes of strike slip origin, and underlying earthquakes of thrust origin analogous to that of the 2003 San Simeon earthquake. And for the onshore San Luis Range area, an underlying actively seismogenic thrust wedge appears to provide the maximum potential seismic ground motion; exceeding that potentially resulting from large events on nearby strike slip faults of the San Simeon-Hosgri system, for onshore sites.

Understanding and documentation of the geology, geomorphology, tectonics and seismogenesis of the San Luis Range and vicinity has recently experienced a quantum improvement as both new and accumulated data have been analysed. An integrated interpretation of all available data now clearly shows that a dual "side by side" system of active tectonics exists in the region.

1 Essentially the most obvious evidence for this is seen simply in the topography; the rapidly uplifting San Luis Range represents the field of NE-SW compression driving a thrust – backthrust thrust-fault wedge "popup" while the adjacent shear-strike slip faulting associated with the plate boundary San Gregorio-Hosgri splay of the San Andreas fault system results in only minor surface deformation of the sea floor surface of late Quaternary marine planation. Interaction between the two tectonic systems occurs mainly along the SE shoreline of Estero Bay where NNW aligned strike slip faults intersect the uplifting San Luis Range thrust fault "popup" wedge, and along the recently identified Shoreline fault, against which the SSW-vergent leading edge of the San Luis Range thrust impinges at depths of 1-5 km. The latter structural relationship gives rise to locally pronounced west facing sea floor surface scarps along a fault with mostly or entirely horizontal strike slip motion.

Overall the San Luis Range and vicinity constitutes an excellent full scale laboratory for observation of evidence of a variety of tectonic processes in action. The opportunity for studies of tectonism here arises not only from the geologically and topographically clearly exhibited effects of the two interacting tectonic fields (NNW shear; NE-SW compression) but also from the extensive baseline studies of the area conducted during the past 40 years.

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Source of Regional Right Shear in Coastal Central California (Figure 1)

At the north the San Gregorio fault splays south from the San Andreas fault and extends south as the only west-side first order branch within the San Andreas fault system. This splay diverts approximately 7 mm/yr right slip from the San Andreas fault onto the San Gregorio fault system.

- Continuing south, right slip is progressively diverted from the major segments of the San Gregorio-Hosgri fault system along SE – branching splays into Monterey Bay, the northern Santa Lucia Mountains and the Estero Bay offshore in the San Luis Range region.

- Southward from San Luis Obispo Bay, right slip along the Hosgri fault has largely dissipated with the southernmost eastward splay branch, the "Shoreline" fault probably accommodating much of that still occurring along the Hosgri at that latitude.

- The southernmost Hosgri evolves to a west-vergent thrust complex and appears to merge into the NNW-SSE aligned fold and thrust belt present in the southern part of the offshore Santa Maria Basin.

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Source of Sub-Regional NE-SW Compression and Resultant Crustal Shortening (Figure 1)

The region of the broad zone of the southern Coast Ranges bounded generally by the Salinas Valley on the NE and the Pacific Ocean coastline on the SW, is characterized by mountain chains that rise abruptly from intervening lowlands. The three principal lowlands of this region are from north to south, the Los Osos, Santa Maria, and Santa Ynez River valleys. These valleys are separated by the San Luis Range and the Casmalia – Solomon - Purisima Hills. Although the present shoreline of these valleys is deeply scalloped by marine erosion, the onshore structural trends continue offshore to their termination at the Hosgri basin boundary fault. The ranges in this SW sector of the Southern Coast Ranges appear to all be developed by Quaternary compressional deformation of long-lived Tertiary synclinal troughs in which thousands of meters of clastic sediments accumulated. The compressional deformation is active at the present. It is manifest most obviously by the terrain aspect, but also by geologically young reverse and thrust faults along the range margins and by compressional mechanism earthquakes.

The source of the NE-SW compression that drives this deformation has been attributed variously either to active or residual effects of clockwise rotation of the western Transverse Ranges (eg:, Luyenduyk, 1980, McLaren and Savage, 2001) or to "escape tectonics" acting along the north margin of the Western Transverse Ranges (eg: Wells et. al. 1998, Hardebeck, 2010). We see no compelling argument for preferring either the WTR clockwise rotation or the "escape tectonics" hypotheses but note that geometrically the "clockwise rotation" hypothesis appears to work well for the observed onshore deformation while the "escape tectonics" hypothesis could explain the E-W compressional deformation in the southernmost offshore Santa Maria Basin, west of the Hosgri fault.

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Seismicity of the South Central Coastal Region

The seismicity of the south central coastal region of California as related to terrain is shown on the two maps of earthquake hypocenters, with depth indicated by color code plotted on a digital terrain base (Figures 2, 3). The focal mechanisms of sufficiently well recorded events from this region are shown on separate plots (Figure 4, 5). Seismicity data is shown for the period prior to the occurrence of the San Simeon earthquake in 2003 (Figure 4) and for the period extending through November 2010 (Figure 5). This allows observation of the general pattern of seismicity (the pre 2003 plot) compared with that overprinted by the dense cloud of aftershocks of the San Simeon event, which obscure the general pattern of events that characterize the seismicity in the Santa Lucia Range. Focal mechanisms of both the pre 2003 seismicity in this region and of the main shock and aftershocks of the San Simeon earthquake show the dominant role of compressional seismic faulting in the San Lucia Range.

The inset to the post 2003 seismicity map is a cross section through the hypocenters of the main shock and aftershocks of the 2003 San Simeon earthquake (Figure 6). Note that the east-dipping principal source fault projects toward the surface near the offshore trace of the San Simeon fault, passing c. 3 km below the surface trace of the Oceanic fault. Compare this cross section with the cross section plotted at the same scale, of the seismicity beneath the San Luis Range (Figure 9).

The pattern of seismicity in the south central coastal onshore and near shore region is noteworthy for the close association of seismic activity with compressionally uplifted ranges of hills and mountains (Santa Lucia, San Luis, Casmalia-Solomon-Purisima) north of the Western Transverse Ranges and east of the San Simeon-Hosgri fault system. Linear patterns of seismicity are generally discernable only along the Hosgri and Shoreline faults opposite Estero Bay and the Irish Hills and less distinctly, within Estero Bay. As shown by the focal mechanism plots, these are all areas with predominantly strike slip faulting.

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Right-Slip Faulting and Seismicity in the San Simeon - Estero Bay – Offshore Irish Hills Region

The uplift–exposed landward part of the compressional Piedras Blancas antiform is transected by the active right slip San Simeon fault, as well as by older related faults. These faults have prominent erosionally produced geomorphic expression, and less obvious tectonic expression marked by linear low scarps. The more active-appearing traces have been explored by trenching (eg: Asquith, 1977, PG&E, 1988). Both the visible landform and the trenching results demonstrate beyond doubt that the San Simeon and related faults represent major loci of right slip faulting. This zone of faulting clearly extends offshore both north and south of the Piedras Blancas onshore exposure (Figure 1).

South of San Simeon Bay this zone of faulting continues along strike, paralleling and lying directly offshore from the shoreline between Cambria and Point Estero. In addition to the linear shoreline this reach of the San Simeon fault is marked by a few clusters of epicenters of small earthquakes recorded since 1987. Continuing southward into Estero Bay the faulting that was concentrated along the shoreline north of Point Estero divides into a broad (8 km wide) zone of at least 3 individual traces of right slip faulting, each marked by linear zones of hypocenters with predominately right lateral focal mechanisms (Figures 4, 5, 8). These faults appear to represent the southward termination of seismogenic right slip directly related to the San Simeon fault. Right slip instead has been transferred west to the basin-boundary Hosgri fault, which apparently originates within or even somewhat northward from, the Piedras Blancas antiform (Figure 1).

The strike of the Hosgri fault diverges a few degrees westerly from that of the San Simeon fault, so that the Hosgri continues southward along a trace that remains seaward of the most westerly points of land between Piedras Blancas Point and Point Arguello (Figure 1). The Hosgri therefore has no onshore expression and because it is predominantly strike slip, only very subdued expression in the sea floor. It is however, well marked by a parallel, large gravity

6 gradient associated with the 3 km vertical contact between onshore basement uplifts and the depressed offshore Santa Mara Basin, also by the seaward termination of linear magnetic gradients along uplifted magnetic basement rocks that are truncated by the Hosgri.

Although there are several important southeastward branchings of the San Gregorio fault (Monterey Bay fault zone, Garrapata Creek-Palo Colorado - Church Creek faults, conveying some right slip into the northern Santa Lucia mountain block, Figure 1), the only such branching from the east side of the Hosgri is the recently recognized "Shoreline" fault which splays southeastward from the Hosgri in an area located approximately 6 km northwest of Point Buchon in the outer SW quarter of Estero Bay. The area of branching is marked by a cluster of small earthquakes recorded since 1987. Most of these events have right lateral strike slip mechanisms but at least one has a thrust mechanism (Figures 4, 5).

The "Shoreline" branch fault trends directly along the linear reach of the shoreline of the Irish Hills between Diablo Cove and Point San Luis, and extends some 8 km or farther to the southeast in San Luis Obispo Bay. The fault is marked by linear west-facing scarps in the near shore wave cut platform, by aligned magnetic and gravity gradient anomalies, and by earthquake hypocenters that define a vertical planar zone extending to c.15 km depth (Figure 9). Focal mechanisms of small earthquakes in this zone consistently indicate right lateral strike slip faulting (Figures 4, 5).

Despite its well marked trace the "Shoreline" fault apparently has only limited cumulative displacement since it transects the uplifted Mesozoic basement rocks of the Point San Luis structural high which is exposed both onshore NE of the "Shoreline" fault and offshore across and seaward from its trace. The contrast between a well marked trace and little cumulative movement suggests that the Shoreline" fault is of geologically recent origin. The existence of sharp recent-appearing east side up scarps of probable tectonic rather than marine erosion origin, present along the trace of a purely strike slip fault where it crosses an <18 ka wave-cut platform, can be explained by the structural relationship between the "Shoreline" and San Luis Range faults. In this scenario the 35 degree SE dipping San Luis Range thrust ramp along which the Irish Hills are being uplifted intersects the vertical shoreline fault and its SW-side block at depths

7 ranging from 1 to c. 5 km, as defined by the hypocenter seismicity pattern. This results in deflection to vertical of the thrust-driven movement of the leading edge of the thrust wedge. This movement then deforms the sea floor as block uplift along the NE side of the "Shoreline" fault surface trace, with resultant formation of the observed prominent sea floor scarps (Figures 11). It is mentioned above that the "Shoreline" fault appears to be a geologically young structure. We are not aware of data that would constrain its time of origin but would speculate that the fault splay may have originated around the time of the acceleration of uplift of the Irish Hills, c. 500 ka. The development of the "Shoreline" splay from the Hosgri could have been related to this acceleration of local tectonism either as a triggering or a resultant event.

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Active Compressional Deformation and Uplift of the Irish Hills of the NW San Luis Range

The Irish Hills is the name given to the northwesternmost part of the San Luis Range. This segment of the range forms a broad peninsula and so is exposed to marine shoreline erosion on three sides with only its NE side, bordering the Los Osos Valley, having been subject mostly or possibly entirely, to subaerial erosion (Figures 7, 8).

Evidence for active compressional deformation and uplift of the Irish Hills is of 3 types:

1. The steep high terrain, developed in relatively weak layered sedimentary rocks. The formation and current existence of such terrain suggests rapid, ongoing tectonic uplift.

2. Evidence, consisting of a flight of marine terraces, of incremental uplift during at least the last half million years, with a pronounced rejuvenation or acceleration of uplift c. 500 ka. This approximately 500 ka event is represented by steep-walled V-notch drainage course incision of older broad-valley terrain present in the interior valleys of the Irish Hills (Figure 10,11).

3. Presence of late Quaternary reverse and thrust faults along the margins of the Irish Hills. The thrust boundary along the inland Los Osos Valley margin of the hills is the Los Osos fault; the thrust boundary along the seaward side apparently impinges at depth on the east side of the adjacent "Shoreline" fault (Figure 12). Onshore, SE of Arroyo Grande, the SW margin thrust is known as the Wilmar Avenue fault. Smaller late Quaternary faults, present in the leading edge of the SW margin (San Luis Range) thrust include the San Luis Bay, Rattlesnake, Olson and Diablo Cove faults (Figure 8).

4. High quality seismicity data obtained since 1987, including

9 a. A pattern defined by hypocenters of small earthquakes recorded in the shallow crust beneath the Irish Hills and Los Osos Valley that appears to define a NE- dipping source structure that extends from an updip termination at the vertical "Shoreline" fault downdip beneath the Los Osos Valley (Figure 9). This pattern closely resembles that of the post earthquake hypocenter patterns of well-recorded earthquakes in California including the 1971 San Fernando and 1984 Northridge events. b. Focal mechanisms of numerous earthquakes showing compressional or oblique compression faulting (Figures 2, 3).

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Potential Earthquake Hazards Including Strong Ground Motion, Ground Deformation, Ground Failure, and Local Tsunami Generation

The earthquake hazard potential from seismic source structures present in the south central coastal region derives from either, or a combination of, offshore earthquakes of predominantly strike slip mechanism, and nearshore/onshore earthquakes of predominantly reverse or thrust mechanism.

Seismic strong ground motion. This hazard exists throughout the region but would appear to be greatest, based on proximity to known active seismic source structures, along and within around 20 km inland from the coastline, at least as far south as Arroyo Grande. The principal seismic source structures that give rise to this hazard are the strike slip San Simeon, (including its splays in Estero Bay), Hosgri, and "Shoreline" faults, and the San Luis Range and Los Osos, and Oceanic – West Huasna thrust faults.

Ground Deformation. This hazard category includes seismic uplift, subsidence, and surface faulting; tsunami generation results from seismic uplift of the sea floor but is here listed as a separate category of hazard. Surface uplift has clearly affected the Irish Hills especially during the last 500 ka, and the southern Santa Lucia Range underwent measurable tectonic uplift during and following the 2003 San Simeon earthquake. The lagoon of Morro Bay owes its existence to tectonic subsidence and there is certainly potential for this to continue in recurrent episodes. Surface faulting can be expected to occur along the submerged traces of both the strike slip and reverse faults but only faulting involving vertical uplift of the sea floor is likely to produce directly damaging effects. However changes in shoreline elevation could impact sea water intake and waste water discharge hydraulics.

Onshore surface faulting, most probably related to "popup" fault wedge uplift of the Irish Hills, could occur along both their landward boundary Los Osos fault, and along smaller faults in the hanging wall leading edge of the underlying San Luis Range thrust along the seaward shoreline of the Irish Hills

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Ground Failure

This general category of earthquake-related effects includes soil liquefaction, landsliding, rockfalls, and cliff failures. With the exception of soil liquefaction, all of these phenomena occur in the south coastal region even in the absence of triggering earthquakes, but their occurrence is certain to be enhanced by earthquake strong motion. Liquefaction of water saturated sandy soils along the coastline occurred as far south as Arroyo Grande during the M6.5 San Simeon earthquake, which was centered 50 km NW of Morro Bay and 90 km NW of Arroyo Grande. Large historically dormant slump and earthflow landslides, present at various locations in the Irish Hills and elsewhere in the region, are susceptible to reactivation and could be damaging to roads, power transmission facilities and other infrastructure. Rockfalls from the fractured volcanic rock cores of the steep hills between Morro Rock and Islay Hill, have yielded marginal boulder fields with blocks as large as small houses. Rockfalls similar to those that have occurred at intervals throughout late Quaternary time could again be triggered by strong earthquake shaking.

Tsunami Generation

The issue of central California coast tsunami hazards has recently been the subject of extensive studies by the Pacific Gas and Electric Company (eg: Nishenko et. al., 2009) but we are not aware that their studies have included the potential occurrence and effects of tsunamis generated by local sea floor uplift in Estero and/or San Luis Obispo bays. Our interpretation presented here suggests that some tens of square km of shallow sea floor in Estero Bay and San Luis Obispo Bay, could be subject to seismic uplift ranging up to one meter or more in amount. The effects of local tsunamis generated by such uplift on shoreline developments could be severe, especially since unlike with tsunamis originating in distant locations, there would be no more advance warning than the immediately preceding occurrence of the causative local earthquake.

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Conclusions

1. Geologic and seismologic (including paleoseismologic and detailed high resolution recording of local earthquakes since 1987) studies in the south central California coastal region during the past c.100 years have yielded the basis for recognizing and formulating a realistic tectonic model for characterizing contemporary tectonism and earthquake hazard in the region.

2. Since 1967 several "earthquake hazard" tectonic models have been proposed for this region but all have been deficient in failing to recognize one or another critical aspect of the local tectonic setting (eg: not including nearby offshore faults, not considering local compressional seismic source faults, etc.)

3. Among the most important investigations since c. 1967 were a.) the offshore geophysical surveys by Shell Oil, the USGS, and PG&E, which resulted in discovery and partial characterization of the offshore Hosgri fault, and b.) the establishment and operation by PG&E of a local seismograph network.

4. The results of all geoscience research available to date for the south central coastal region of central California, should be adequate to permit formulation of a comprehensive, realistic estimate of the probable location and upper bound magnitude of future earthquakes that will eventually occur in this region. But to date, so far as we are aware, no such estimate has been brought forward.

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