Northern San Andreas Rupture History from the Turbidite Record

Synchroneity

Following the discovery of the first buried marsh sequences on land along hte Cascadia coast, Adams (1990) used existing cores to test the possibility that the Cascadia cores contained a record of Holocene great earthquakes of the Cascadia margin.  Fortunately, Oregon and Washington cores all contain a unique datable event, the ash layer from the eruption of Mount Mazama, at 7627 ± 150 cal yr BP (Zdanowicz et al., 1999).  The ash was distributed to the channel system via the drainage basins of major rivers, similar to the distribution of Mt St. Helens ash following the 1980 eruption (Nelson et al., 1968).  Only channel cores contain the ash, indicating that airfall offshore was not significant. 

Adams (1990) examined core logs for Cascadia Basin cores, and determined that nearly all of them had 13 turbidites overlying the Mazama ash. Following the discovery of the first buried marsh sequences on land Adams, (1990) assessed the possibility that turbidites in channels of Cascadia Basin contained a record of great earthquakes along the Cascadia margin. He found that cores along the length of Cascadia channel contain 13 turbidites and argued that these 13 turbidites correlate along the channel.  Adams observed that cores from Juan de Fuca Canyon, and below the confluence of Willapa, Grays, and Quinault Canyons, contain 14-16 turbidites above the Mazama ash.  The correlative turbidites in Cascadia channel lie downstream of the confluence of these channels.  If these events had been independently triggered events with more than a few hours separation in time, the channels below the confluence should contain the sum of the tributaries, from 26-31 turbidites, not 13 as observed (Figure 1).  The importance of this simple observation is that it demonstrates synchronous triggering of turbidite events in tributaries, the headwaters of which are separated by 50-150 km.  Similar inferences about regionally triggered synchronous turbidites in separate channels are reported in Pilkey (1988).  This elegant relative dating technique is used extensively in our Cascadia and SAF work.   

Using 51 new cores along the Northern San Andreas, we have confirmed and extended the event record temporally and spatially from our original three cores in Noyo Canyon collected in 1999. 

Triggering mechanisms: Are theyEarthquakes?     

Are these events all triggered by earthquakes?  Common sense suggests that such a scenario is absurdly simplistic, yet our Cascadia work has led us to the unlikely conclusion that Adams (1990) was correct.  We now discuss the methods used to test the hypothesis and why it seems to work.  Adams (1990) suggested four plausible mechanisms for turbid flow triggering: 1) storm wave loading; 2) great earthquakes; 3) tsunamis; and 4) sediment loading. To these we add 5) crustal earthquakes, 6) slab earthquakes, 7)  hyperpycnal flow, and 8) gas hydrate destabilization. 

All of these mechanisms could trigger a turbid event, but how often do they actually occur, and can earthquake-triggered events be distinguished from other events?  Two basic techniques can be used to distinguish seismic from non seismic events:

1)  Sedimentological determination of individual event origin.

2)  Regional correlations that require synchronous triggering.  

Individual event determination can in some cases distinguish seismic turbidites from storm, tsunami, and other deposits using several methods.  Nakajima and Kanai (2000) and Shiki et al. (2000) report that seismo-turbidites can in some cases be distinguished sedimentologically.  They observe that historically known seismically derived turbidites in the Japan Sea and Lake Biwa are distinguished by wide areal extent, multiple coarse fraction pulses, variable provenance, and greater depositional volume than storm-generated events. These investigators traced known seismo-turbidites to multiple slump events in many parts of a canyon system, generating multiple pulses in an amalgamated turbidity current, some of which sampled different lithologies that are separable in the turbidite deposit.  These turbidites are complex, with reverse grading, cutouts, and multiple pulses.  Gorsline et al. (2000) make similar observations regarding areal extent and volume of seismoturbidites.  In general, these investigators observe that known storm triggered events are thinner, finer grained and have simple normally graded Bouma sequences, although complexity is also a function of proximity to the source, and some reports reach different conclusions (Mulder and Syvitski, 1996).  While there may yet be applicable global, regional or local criteria to make such distinctions, these are at present poorly developed and somewhat contradictory.

Thus far in Cascadia and the San Andreas, we have not attempted to distinguish between triggering mechanisms directly because the physiography, numerous tephra layers, and long historical records favorable to this method in Japan are not present on the US west coast.  Favorable factors that are present favor regional correlation and determination of synchronous triggering.  Determination of synchronous triggering can eliminate non-earthquake triggers with the possible exception of storm wave loading or multiple hyperpycnal flows for very large storms, and perhaps triggering by a tele-tsunami.  West Coast physiography favors filtering of non-seismic events from the record because a wide shelf separates river sources from canyon heads.  Hyperpycnal flow, or direct turbid injection from rivers, can produce turbid flows, and can even mimic earthquakes in that they may affect several rivers over a span of days.  We have found that while this certainly occurred during the Pleistocene when lowered sea-level resulted in direct river-canyon connections resulting in a major increase in turbidite frequency at the Holocene Pleistocene boundary.  Once sea level-rise in the Holocene isolated most west coast canyons from the rivers, sediment was spread across the shelf and not directly injected in the canyons (e.g. Sternberg, 1986).  Some exceptions are the Eel river record, which probably contains storm events, and the Viscaino channel along the northern San Andreas.  Both of these occur where the shelf is very narrow, and river injection is possible.  Deep canyon heads also prevent triggering by storm wave loading and distant tsunami, the last two non-earthquake sources.  For example, storm wave loading is an unlikely trigger in Cascadia, where, although deep water storm waves are large, the canyon heads where sediment accumulation occurs are at water depths of 150-400 m, too deep for disturbance by maximal storm waves of ~20 meters.   Tsunamis may also conceivably act as a regional trigger, however the tsunami from the 1964 Alaska Mw 9.0 event did not trigger a turbidite observed in any of the cores, although it did serious damage along the Pacific coast (Adams, 1990).  Crustal or slab earthquakes could also trigger slumps and turbid flows, though not regionally.  To test for this, we resampled the location of a 1986 box core in Mendocino channel, where the uppermost event is suspected to be the 1906 San Andreas event.  Since 1986, the Mw7.1 Petrolia earthquake occurred in 1992, with an epicentral distance of only a few km from the canyon head.  We found no turbidite in the 1999 box core, suggesting that triggering at that site may require earthquakes larger than Mw 7.2. Conversely, the Loma Prieta earthquake apparently did trigger a turbid flow event in Monterey Canyon at a greater epicentral distance (Garfield et al., 1994), though it is not known whether a discernable turbidite record exists from this event.  Japanese investigators have suggested a minimum magnitude of ~ Mw=7.2 for turbidite triggering, though we suspect that this minimum value is site and event specific.

Synchronous Triggering, Relative Dating, and Regional Correlations

Taking advantage of favorable physiography, we have used spatial and temporal patterns of event correlations that are unlikely be the result of triggers other than earthquakes.  We use multiple techniques to test for linkage between sites and thus test for synchroneity.  Typically, paleoseismologic investigations use radiocarbon constraints to establish these linkages, but often are unable to determine synchronous event chronology due to the inherent limits in dating techniques.  Relative dating techniques, if available, and if of sufficient resolution, are strongly preferred to test for synchroneity.  The “confluence test” of Adams (1990) is powerful in that it requires synchronous triggering within a few hours. Comparisons of the number of events between time markers is a somewhat less powerful technique that can be applied in some cases.  Recently we have begun to use direct physical property correlations, which are proving to be a powerful new method of testing for linkages (if present) between sites. 

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