1857 Rupture 300 km Ray Weldon (with material from colleagues and students) How to do Paleoseismology for SOSAFE

Conclusions of the “Wrightwood” Paleoseismology Meeting Ray Weldon University of Oregon [>5 years ago!]

#1) Focus more effort of the San Andreas and San Jacinto faults There was a perception that the San Andreas zone had received inadequate attention while SCEC focused on LA Basin and Mojave

#1) Focus more effort of the San Andreas and San Jacinto faults #2) Attack sites with larger multi-PI teams This will allow sites to be completed more rapidly. Results will be more robust (more experienced eyes interpreting the same features). Will force prioritization of sites (with finite money and PIs fewer sites can be pursued at a time).

#1) Focus more effort of the San Andreas and San Jacinto faults #2) Attack sites with larger multi-PI teams #3) Run a lot more C-14 dates and standardize methodology To assure that the results from different sites are truly comparable, we need to run enough samples with the same methodology (and perhaps even the same lab) to fully understand the variability of different carbon sources and how it affects the age of events.

#1) Focus more effort of the San Andreas and San Jacinto faults #2) Attack sites with larger multi-PI teams #3) Run a lot more C-14 dates and standardize methodology #4) Place more emphasis on getting slip per event data Fewer than ¼ of the ~60 dated paleoseismic events on the SSAF have displacements and most high quality paleo-offsets cannot be uniquely attributed to a single event.

#1) Focus more effort of the San Andreas and San Jacinto faults #2) Attack sites with larger multi-PI teams #3) Run a lot more C-14 dates and standardize methodology #4) Place more emphasis on getting slip per event data #5) Increase quantification and reproducibility in the characterization and analysis of paleoseismic data Including field procedures like photo-logging, multiple trenches or 3d excavations, better documentation of samples, and quantification of event indicators - as simple as tabulation of all positive and negative evidence or, better, the development of a community-wide “paleoseismic index” to allow comparison of the quality of evidence for different “events.”

#1) Focus more effort of the San Andreas and San Jacinto faults #2) Attack sites with larger multi-PI teams #3) Run a lot more C-14 dates and standardize methodology #4) Place more emphasis on getting slip per event data #5) Increase quantification and reproducibility in the characterization and analysis of paleoseismic data #6) Develop a few keystone sites in regions with large data gaps We especially need to find a good site in the Big Bend region (or nearby on the Northern Mojave or Southern Carrizo) of the SAF, the Northern San Jacinto fault, and extend the 5-6 event records on the northern and southern ends of the SSAF.

#1) Focus more effort of the San Andreas and San Jacinto faults #2) Attack sites with larger multi-PI teams #3) Run a lot more C-14 dates and standardize methodology #4) Place more emphasis on getting slip per event data #5) Increase quantification and reproducibility in the characterization and analysis of paleoseismic data #6) Develop a few keystone sites in regions with large data gaps Most important we need more qualified people working on the problem and adequate money to support them

Many site have a relative resolving power that we are not communicating. These sites could help answer many questions about fault behavior that might be more important than absolute dates of earthquakes.

An earthquake occurred while peat 590 was growing and it began growing again before the capping c591 was deposited.

C-14 dates suggest peat grows continuously at rates that can be used to say how much time passes at an event horizon. Also, abrupt changes in peat accumulation rate might be a regionally significant climate indicator that could be used to correlate between sutes cm/yr 23.5 yr/cm Wrightwood Deep 0.06 cm/yr 15 yr/cm

“Map” on prints at ~1:7

We can show that events are relatively fast, much larger than our threshold of resolution and that there are relatively long period of time when nothing happens. VolumeTime

Lake records offer the potential to study earthquakes occurring in a continuous, complete record. Here is another example of a small normal fault, Ana River, in Oregon

While we can only crudely date the earthquakes using the ages of the ashes, due to the continuous sedimentation in a lake environment, we can say that major deformation occurred in periods of time that cannot be much more than decades (and based on the nature of deformation can be argued to be seconds to minutes, like in an earthquake) whereas for periods of time 10,000 to 20,000 years long, nothing happened. This is not a section being chewed up by a GR type range of big, little and medium sized earthquakes, but a section that experiences rare events much larger than the threshold of detection.

Event 2 Folding can also be shown to be rare discrete events

3. Calibration and trimming1. Physical pretreatment2. Chemical Pretreatment Dating methodology has improved, but we are still not there. Multiple dates of the same layer yields multiple “ages” that must be interpreted

Detrital Charcoal

We need to better document what we are seeing in our trenches and interpreting as earthquakes (or not).

Catalogue of deformation EQTrenchMeterTypeUpper Unit Lower Unit QComment W610T21SE46-51gs Older units folded into syncline, units above debris flow undeformed. Significant coarsening and thickening of debris flow. Excellent example of single event growth strata. W610T37SE gs Moderate thickening, no coarsening. W592T41ANE10 -16gs Unit thickens by 5x in fold, also coarsens, lower boundary more arcuate. W590T21SE62fis Fissure has relative normal separation, nice clean break of 590 and excellent fill genetics. W592T31NE4-5fis Very large fissure oriented sub parallel to trench. Edges of fissure have been subsequently faulted making correlation of thin upper units across feature difficult. Timing constraint is poor. Has very similar internal structure as fissure in T37, possibly continuation of same feature. E3T31SW2, 3fis592/ Large fissure with abstract shape. Filled with silt, not cg. Fis is very thin in cross-section (seen in T37SE) and has poorly defined upper horizon. Note bedding parallel shape under 520, suggests liquefaction feature. W594T35NE14fis Possible hanging wall accommodation feature E3T31SW2ut592/ Normal separation, consistent offset below W520T35SW6ut

Catalogue of deformation EQTrenchMeterTypeUpper Unit Lower Unit QComment W610T21SE46-51gs Older units folded into syncline, units above debris flow undeformed. Significant coarsening and thickening of debris flow. Excellent example of single event growth strata. W610T37SE gs Moderate thickening, no coarsening. W592T41ANE10 -16gs Unit thickens by 5x in fold, also coarsens, lower boundary more arcuate. W590T21SE62fis Fissure has relative normal separation, nice clean break of 590 and excellent fill genetics. W592T31NE4-5fis Very large fissure oriented sub parallel to trench. Edges of fissure have been subsequently faulted making correlation of thin upper units across feature difficult. Timing constraint is poor. Has very similar internal structure as fissure in T37, possibly continuation of same feature. E3T31SW2, 3fis592/ Large fissure with abstract shape. Filled with silt, not cg. Fis is very thin in cross-section (seen in T37SE) and has poorly defined upper horizon. Note bedding parallel shape under 520, suggests liquefaction feature. W594T35NE14fis Possible hanging wall accommodation feature E3T31SW2ut592/ Normal separation, consistent offset below W520T35SW6ut

We need to better document what we are seeing in our trenches and interpreting as earthquakes (or not). And we need to quantitatively (or at least semi-quantitatively) assess their relative merits.

11 probable, 3 possible, 15 very unlikely Histograms of Event Quality Total Observations Sum Q < 1 Avg Q Sum Q

sum of rank # observations Scharer et al., BSSA, preferred, 3 maybe, 15 very unlikely paleoearthquakes Only about 10% of the event indicators are associated with nonevent horizons BUT about 50% of all horizons with > 1 piece of evidence are not earthquakes, SO you need a thorough study to get a useable record, and there will always be some events that are not certain. rank sum - #obs

We need to develop correlation tools beyond C-14 dating.

Traditionally, correlation has been based on the overlap in age ranges between sites, such as shown here. While improvements in dating will allow more precise determination of overlap, overlap alone cannot distinguish ruptures separated by months, years, or even decades.

See Weldon et al., Science, 2005, for details and references to data sources. If we plot all of the pdfs for all earthquakes on the southern San Andreas we find that most overlap at relatively narrow intervals of time. [Each color is a different site.] Does this mean that large portions of the fault rupture together or that a series of smaller events propagate along the fault over a period of decades? The “megaQuake” or “Decades of Terror” ?

To get past age and event recognition inconsistencies we need an independent way to determine whether adjacent sites are likely to share earthquakes or not. One promising tool is displacement

NS Slip per event Fault Zone Thickness changes across the fault will allow us to determine the amount of slip associated with each earthquake

This technique requires that we know a lot more about displacement per event, how displacement varies along strike and through time.

Solid: p(ground rupture|M)*p(M) Dashed: p(M) Uniform: equally likely on an interval GR: exponential increase in frequency with decreasing M Constrained Mean: Accepts paleoseismic event count and total slip.

If we know (or can provide reasonable estimates for) these parameters we can start to develop earthquake ensembles that can be tested in other ways, like consistency with longer term slip rates.

What to do? Focus on relative age, not absolute Careful documentation, esp event evidence but also section resolution etc. multiPI teams with their noses at the wall day after day Trench review? Works or not? Are we being held to higher standard? Should we offer incentives for documenting data/trench review? Meter by meter documentation, so can evaluate quality and number of features, and potentially re-excavate efficiently Develop measures of quality, between events at a single site and btw sites (the latter might be where trench review could be useful, to give community idea of a site’s quality. Field review in middle of project (not end) to get input about how to proceed (also easier to schedule and more useful than grading or evaluating final results.