June 24, 2026 Venezuela M7.2 and 7.5 earthquake doublet

Yesterday at 6 pm local time (3 pm in Arizona) pair of large earthquakes occurred in rapid succession along the northern coast of Venezuela. The first, an M7.2, ruptured at a depth of about 20 km and about 180 km west of Caracas. 40 seconds later (while shaking from the first was still propagating), the second, an M7.5, initiated at a depth of about 10 km and ruptured from the eastern end of the M7.2 towards the east (towards Caracas). This pair of events, a foreshock followed rapidly by an adjacent mainshock, is being called a "doublet". The first ruptured into the second like one domino following into the next.

This is a devastating pair of earthquakes for Venezuela; they are shallow and occurred near to populated regions. The US Geological Survey anticipates significant loss of life and economic impacts.

The foreshock occurred along the large east-west Yaracuy valley that connects to the northern Venezuelan coast. Cities with hundreds of thousands of residents (San Felipe, Puerto Cabello, Los Guayos) experienced very strong to severe shaking (a total of ~1M people).

The second event with its ground motions propagating along with those of the first, ruptured eastward along the northern Venezuelan coast nearly to Caracas (a distance of ~150 km). This "directivity"--rupture moving in the same direction as the waves it is producing--can significantly enhance ground motions and that is likely what happened for places like Caracas. For this event, more than 2.5M people experienced very strong to severe shaking for at least tens of seconds. Eyewitness reports show many buildings collapsed. Many slopes may have failed in the nearby mountains, enhancing damage via cascading hazard.

The earthquakes occurred along a well known active fault zone variably named the Boconó-San Sebastián-El Pilar Fault zone. It represents the plate boundary between northern South America and the Carribean plates. The Carribean plate (north side of the fault) moves eastward about 20 mm/yr relative to the South American Plate. This dextral motion was thus accommodated in part by these two mostly right lateral earthquakes. This side to side rather than up-down sense of motion--even though the rupture was along the coast--is consistent with the lack of observation of any tsunamis.

These are the largest earthquakes to occur in the region in at least the past century. M6 events have occurred within 250 km in the last century, but nothing this large. An ~M7.4 event occurred in 1812 in the same area as yesterday's M7.2, while an ~M7.6 occurred possibly along these faults just offshore Caracas in 1900.

Aftershocks are occurring and will continue to make rescues more challenging. I expect that today the response will be vigorous and we will know more about this catastrophe.

Authoritative Links:

• Fundación Venezolana de Investigaciones Sismológica
• USGS M7.5
• USGS M7.2

Additional useful links:

• https://earthquakeinsights.substack.com/p/catastrophic-m75-earthquake-strikes
• ASU recordings: M7.5 and M7.2

Some papers I found useful:

• Audemard, F. A., 2007, Revised seismic history of the El Pilar fault, Northeastern Venezuela, from the Cariaco 1997 earthquake and recent preliminary paleoseismic results, J Seismol, 11, no. 3, 311–326, doi: 10.1007/s10950-007-9054-2.
• Pousse‐Beltran, L., R. Vassallo, F. Audemard, F. Jouanne, J. Carcaillet, E. Pathier, and M. Volat, 2017, Pleistocene slip rates on the Boconó fault along the North Andean Block plate boundary, Venezuela, Tectonics, 36, no. 7, 1207–1231, doi: 10.1002/2016TC004305.
• Higgins, M., P. C. La Femina, J. C. Weber, H. Geirsson, G. A. Ryan, and C. Wauthier, 2021, Strain Partitioning and Interseismic Fault Behavior Along the Caribbean‐South American Transform Plate Boundary, Tectonics, 40, no. 8, e2021TC006740, doi: 10.1029/2021TC006740.

Presentation for Harnessing Massive Data Across Geophysical Domains and Applications: Committee on Solid Earth Geophysics Spring Meeting 2026

I enjoyed the opportunity to present in the spring meeting of the National Academy of Sciences Committee on Solid Earth Geophysics Spring Meeting 2026. The meeting was entitled: Harnessing Massive Data Across Geophysical Domains and Applications.

--Photo by Wendy Bohon, PhD

PRESENTATION

I tried to combine some ideas building from our OpenTopography project with nascent ML collaborations with Dr. Zhiang Chen. Thanks for their contributions.

Opportunities associated with AI/ML are really exciting, but there is a lot to think about for infrastructure, research, and education.

San Andreas Fault in the Carrizo Plain Field trips

Recently, I pulled together some notes for the SCEC UNREST Field trip in the Carrizo Plain. It was a great conversation to share with the esteemed colleagues.

So I can find it, here are the notes that produced as a handout: LINK

Here are a few other items/guides:

• Field trip along the San Andreas Fault in the Carrizo Plain--Guide I wrote some time ago
• Akçiz, S.O., and Arrowsmith, J.R., 2013, New views on the evolution of the San Andreas fault zone in central California and the Carrizo Plain, in Geologic Excursions from Fresno, California, and the Central Valley: A Tour of California’s Iconic Geology, Geological Society of America, p. 1–12, doi:10.1130/2013.0032(01).
• Poster of B4 lidar data derived DEM of the Wallace Creek-Phelan Creeks area

Remembering Thomas C. Hanks

I attended the Tom Hanks, a Remembrance symposium at the USGS in Moffett Field Oct. 17, 2024. It was a great chance to remember Tom, reflect on his vast scientific and personal impacts, and to catch up with old friends and colleagues. It was also nice to meet Tom's daughters. I was fortunate to be able to make a presentation. Here is my talk: LINK.

Here is the program from the event: LINK

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I heard that Thomas C. Hanks passed away recently. He was a mentor to me. He worked for his career with the US Geological Survey. The memorials of him from his colleagues will be many and deep. I wanted to capture some of my memories of him. Tom was very supportive of young scientists and very broad in his scientific thinking. While he was most well known as a seismologist, his work in geomorphology and fault scarps and fragile geologic features was transformative.

This was a sticky on a manuscript draft he once gave me after a discussion on uncertainties in morphologic datting. Look at the nice handwriting (usually from a well sharpened #2 pencil). And the signature THanks.

Tom was on my Ph.D. supervisory committee. I was at Stanford and Tom was in Menlo Park at the USGS. Like many of his colleagues there, he was very generous with his time with the Stanford students. We talked a lot about fault scarps and diffusion, but also about the San Andreas Fault and I was able to drive for him on a few field trips to the SAF in the southern Bay Area into the Creeping Section. With Professor Gordon Brown's support (chair of our department at the time), Tom helped to lead an active tectonics seminar one quarter.

Tom's work on the age of scarplike landforms from diffusion-equation analysis (title of one of his latter papers on the subject) was very influential. He teamed up with Robert Wallace and others to take something simple about how fault scarps apparently change shape over time and quantify it in a realistic way. There are numerous important papers on the topic with Tom as an author but two seminal ones are:
Hanks, T. C., Bucknam, R. C., Lajoie, K. R., & Wallace, R. E. (1984). Modification of wave-cut and faulting-controlled landforms. Journal of Geophysical Research. https://doi.org/10.1029/JB089iB07p05771
and
Hanks, T. C. (2000). The Age of Scarplike Landforms From Diffusion-Equation Analysis. https://doi.org/10.1029/rf004p0313 in Quaternary Geochronology: Methods and Applications. In AGU Reference Shelf 4 (Vol. 4).

Among many other contributions on the age of scarplike landforms, Tom introduced a simple morphological dating approach: reduced slope-offset. He argued for a measure of the scarp midpoint slope (reduced by the far field slope) versus the vertical offset and he developed a nice calibration along with his colleagues for the rate constant k. He favored analytical solutions (tolerating my numerical approach).
One small anecdote that I always appreciated on the geomorphology side was his desire to name a unit for GK Gilbert (1m²/kyr = 1GKG). See the seminal 1984 Hanks et al JGR paper. It did not catch on but was a fun idea.

In 2007, David Haddad and I went with Tom to Northern Arizona University to see the collection of his father's photographs that he had endowed: Repeat Photography Site for The James J. Hanks Photographs, 1927-1928. Tom, like always, was deeply engaged/obsessed with the topic at hand. He worked hard to relocate and repeat his father's photographs, as well as to tell their story.

Whilst on the trip to Flagstaff, Tom, David, and I stopped to see and discuss the Granite Dells (near Prescott, AZ). Tom had been leading parts of the seismic hazard analysis for the Yucca Mountain possible nuclear repository. The problem they were coming up with was the age of the landscape was great (million year old landforms) and there were fragile geologic features and precarious rocks that may have been there fragile for a large fraction of that time. However, the extrapolation of the ground motion predictions would be to extreme, possibly unrealistic levels. Tom was interested in these million-year-old landscapes of fragile geologic features and recognized their value as an observational constraint for seismic hazard analysis. This is an impressive product of their work:
Hanks, T. C., Abrahamson, N. A., Baker, J. W., Boore, D. M., Board, M., Brune, J. N., Cornell, C. A., & Whitney, J. W. (2012). Extreme Ground Motions And Yucca Mountain. Extreme Ground Motions and Yucca Mountain Open-File Report 2013–1245, US Geological Survey.

Tom was interested in precariously balanced rocks given their use as a part of seismic hazard analysis. He thought it might be helpful for new people to get involved. So, he pulled David and I into it. He was supportive and helped generate some funds for us. That lead to a couple of nice papers lead by David. I regret that we did not have Tom as a coauthor:
Haddad, D. E., Akciz, S. O., Arrowsmith, J. R., Rhodes, D. D., Oldow, J. S., Zielke, O., Toke, N. A., Haddad, A. G., Mauer, J., & Shilpakar, P. (2012). Applications of airborne and terrestrial laser scanning to paleoseismology. Geosphere, 8(4). https://doi.org/10.1130/GES00701.1
Haddad, D. E., Zielke, O., Arrowsmith, J. R., Purvance, M. D., Haddad, A. G., & Landgraf, A. (2012). Estimating two-dimensional static stabilities and geomorphic settings of precariously balanced rocks from unconstrained digital photographs. Geosphere, 8(5). https://doi.org/10.1130/GES00788.1

A final lesson from Tom is that senior scientists should be generous and use their privilege to do good. Tom was a widely appreciated mentor of younger scientists--men and women. He was also a leader who did not shy away from trying to do the right thing. Just one example relates to another senior scientist who recently passed away: Paul Tapponier. Professor Tapponier led a transformation of our understanding of continental tectonics. He favored results with relatively high slip rates and thus the inference that the deformation even in plate interiors was more plate-like. Tom supported his colleague Wayne Thatcher who had come up with a result based on geodesy for the deformation of the Asian continental interior (Thatcher W. 2007. Microplate model for the present-day deformation of Tibet. J. Geophys. Res. 112:B01401) (and that did not sit well with Paul). Zack Washburn and I had written a paper based on paleoseismology in which we could not support enough earthquakes to support a high slip rate). Tom stepped in to mediate between Wayne and Paul and consulted me as part of his preparations. Tom had the stature, the intelligence, maturity and deserved respect so that he was able to set the tone for what I gather was a productive meeting.

I ended up with a copy of Tom's USGS bio and I note the following which is a nice example of his writing and matter-of-fact approach:

A simple evolutionary model for fragile geologic features

The fragility of geologic features, such as precariously balanced rocks (PBRs), can be measured by a simple parameter like α. For the case of a PBR is the smallest of the angles between the vertical from the center of mass and its rocking points. In a landscape, each object will have a fragility and so the ensemble will be a fragility distribution. The controls on the initial distribution and its long term evolution will be from the environment and its history (material properties, landscape evolution (lowering), weathering, shaking, etc.). The distribution can be disturbed by an earthquake (or other loading like windstorm, human impacts, etc.) which abruptly removes features with fragility below a threshold α.

Example of PBRs in Granite Dells, Arizona

I started to think an analogy with a fruit tree. I am not sure this is an original idea maybe I heard it somewhere. The progressive ripening of the fruit can be interrupted by a shake which will remove completely a subset of the most ripe fruit. For the case of the rocks, the ripening is a gradual decrease in fragility over time and then a fragility reset of a subset rather than removal after they are toppled in a shaking event. Ripening continues and the processes repeat over time. This simple model does not account for changing ripening rates or much variation in shaking effects other than threshold α. The basic idea then for seismic hazard is that the fragility distribution at a location reflects the history of the long term ripening and episodic shaking and reset. Therefore, if we can produce fragility distributions for landscapes that otherwise comparible (ripening and threshold α), we might be able to say which has seen more recent shaking and of what severity. This does not directly address the age control for the history and this remains a significant problem.

I wrote a simple code to explore this problem with the hope that it helps us explain and isolate the basic controls on fragility distributions. See the figure below which presents the fragility distributions through the experiment.The algorithm is simple:

• Set up: Specify number of objects, the initial α distribution (assume normal), the ripening rate distribution (also normal; this is the loss of fragility per year), the timing of the earthquake, the threshold α, and the max time
• Interseismic period 1: ripen α until the earthquake (begin and end are the upper two plots below)
• Earthquake: remove α < threshold α (third plot below) and reset those α drawing from the same initial distribution (fourth plot below)
• Interseismic period 2: ripen α until the end of the model time (final plot below)

Fragility distributions through the experiment. Upper plot is initial α. 2nd plot is α and the end of interseismic period 1. 3rd plot shows removed objects with α < threshold α. 4th plot is reset distribution of < threshold α Note that some α results below threshold α. Lowest plot shows continued ripening until the end of the model.

The additional two figures show the evolution of fragility with time of a subset of the objects and a spatial view of the randomly positioned features, their fragility at the time of the earthquake, and the circled objects that failed. The evolutionary diagram helps illustrate the interruption of the more fragile features by the event while the others do not notice. The map provides an idea of the search challenge that this scenario presents.

Evolution of fragility of a subset of the objects. The red star indicates the time of the earthquake and the threshold α.

Map view randomly positioned features, their fragility at the time of the earthquake, and the circled objects that failed.

Maybe more realistic with a more sparse set of features and a lower threshold α (0.3 as opposed to 0.4 above).

This is just a sketch of the problem, but it is a toy model in which we can explore the importance of the distribution widths, timing of earthquake, etc.

The MATLAB code is in this repository: https://github.com/jrarrowsmith/MATLAB-Geomorphology; make sure to get the script PBRevolution.m and functions ripenPBR.m and shakePBRs.

Eminent earthquake scientists: Clarence Allen and Robert Wallace oral histories

For some "light reading" over the winter break, I have enjoyed a read of oral histories of Clarence Allen (Caltech) and Robert Wallace (USGS).
EARTHQUAKES, MINERALS AND ME: WITH THE USGS, 1942-1995 by Robert E. Wallace; Oral History Interviews With Stanley Scott; USGS Open-File Report 96-260

Connections EERI Oal History Series: Clarence Allen with interviewer Stanley Scott
CLARENCE R. ALLEN (1925-2021) INTERVIEWED BY DAVID A. VALONE Caltech archives. This latter one has a bit more about Caltech and is slightly less polished than the first.

Maybe I at times too sentimental, but I found these personal and scientific histories throught provoking and inspiring, not only for their tellings of important steps in the history of earthquake science and service, but also for their modest, laconic, and matter of fact story telling. I am also fortunate to have substantial memories of interacting with both of them personally and also of the transition at the end of their careers and the beginning of mine (I entered graduate school at Stanford University in Fall 1989). I also appreciate the effort of EERI to accumulate those and other oral histories.

One thing that comes to mind is that it would be nice to include some histories from women who have contributed in these areas. I will work on that for a future blog post.

Bob Wallace was inspiring as an earthquake geologist. I followed some of his work quite closely as I shared an obsession with geomorphic indicators of faulting, although certainly with less of an impact...
Just a couple of examples from his papers:

1. 1949 - Wallace, R. E., Structure of a portion of the San Andreas rift in southern California: Bulletin of the Geological Society of America, v. 60, n. 4, p. 781-806.. This is cited as one of the earliest focused mapping efforts along the San Andreas Fault. He mapped substantial offset along the San Andreas Fault and also worried about the fault zone core and interactions of the drainage network with the fault zone. At one point, I colored the detailed map to appreciate it better.
2. 1968 - Wallace, R. E., Notes on stream channels offset by the San Andreas fault, southern Coast Ranges, California, in Dickinson, W. R., and Grantz, Arthur, eds., Proceedings of conference on geologic problems of San Andreas fault system: Stanford University Publications in Geological Science, v. 11, p. 6-21.. This was a landmark in my mind as he noted offsets along the San Andreas Fault in the Carrizo Plain as indicators of short and longer term indicators of recurrent fault slip. It was in a somewhat difficult to find publication. But, being around Stanford University (and having to move out of the Geology Corner after the Loma Prieta Earthquake), there were numerous copies to be found. Figures 6 and 7 of the histograms of numbers of channels with certain offset sizes was something we followed up on a fair bit.
3. 1990 - Wallace, R. E., (editor) The San Andreas Fault System, California: U. S. Geological Survey Prof. Paper 1515, 283 p.. This was his magnum opus. I was lucky once to meet him in his office at USGS Menlo Park and he asked if I had a copy yet. I did not even though I had stared at it. He reached into his filing cabinet and gave me his copy! I should have asked him to sign it or something but I certainly treasure that copy. It has now been rebuilt a few times...
4. 1992 - Wallace, R. E., Ground-squirrel mounds and patterned ground along the San Andreas fault in central California: U. S. Geological Survey, Open-file report n.91-149, p. 1-21.. This was a modest contribution, but I had a couple of conversations with him about it as we shared our enthusiasm for the Carrizo Plain. In his related GSA presentation, he even mentioned me as someone who might pick it up! Scared the crap out of me. We never really did a systematic effort on this but it remains a fascinating problem. I talk about it with people every time I get to the Carrizo Plain.

THere is a lot more to say and remember about Bob Wallace. My memories of him are also tied to Kerry Sieh who worked with Bob and honored him by naming the offset channel he explored in the 1968 paper "Wallace Creek". How many times have I read the Sieh and Jahns 1984 paper: Sieh, K. E., and Jahns, R. H. (1984). Holocene activity of the San Andreas fault at Wallace Creek, California. Geological Society of America Bulletin, 95, 883–896.. We even revisited this in our 2019 paper.

One of my San ANdreas Fault tour videos over the Carrizo Plain. Hillshades produced by me from the B4 project at OpenTopography.

I did not interact with Clarence Allen as much as I did with Bob Wallace, but I did have a few nice conversations with him. I think he was more serious in the conversations than I was. They main ones were when I was thinking about going to Caltech to work with Kerry Sieh. Clarence and I talked about science, but also about trout fishing. I regret never taking him up on an invite to fish in the San Bernardino Mountains. I think my father had fished some of the same places there around and below Lake Arrowhead or Big Bear Lake.
Two papers among many I would like to highlight from Clarence:

1. Allen, C. R. (1968). The tectonic environments of seismically active and inactive areas along the San Andreas fault system. Proceedings of Conference of Geologic Problems of San Andreas Fault System, 5(1496), 70–80.. This stands as an important first order characterization of the San Andreas Fault system and the recognition that the geology was an important control on the current behavior of the system. I have used Figure 1 here and there over the years when I kick of talks on the San Andreas Fault.
2. Geological Criteria for Evaluating Seismicity: Address as Retiring President of The Geological Society of America, Miami Beach, Florida, November 1974 CLARENCE R. ALLEN GSA Bulletin (1975) 86 (8): 1041–1057. This is a classic that helps to introduce the concepts of earthquake geology and the value of the geologic record in the study of recently active faults.

My recent attempt to follow Prof. Allen's ideas about the San Andreas Fault (some edits on the caption from Mike Oskin; we made this as a prototype for a SCEC request). The geology of the plate boundary shows the SAFS progressively dismembering the former subduction system (as indicated by the paired Mesozoic metamorphic--green and granitic--red rocks) (https://ngmdb.usgs.gov/gmna/; upper panel). This framework is a first order control on the behavior of the system (lower panel): active faults (USGS and CGS 2021) and M>4 seismicity (https://earthquake.usgs.gov/earthquakes/search/) overlain on the GMRT (Ryan, et al., 2009).

Simple computations of scalar seismic moment and moment magnitude

In my classes and for research, sometimes it is useful to calculate the scalar seismic moment (M₀; basically a geometric measure of the total static energy release at a 0th order). It is a function of the area of a fault that slipped times the average slip times the shear modulus of the volume. The latter is usually assumed to be 30GPa. The main challenge (after determining the parameters) is to get the units all to be the same (dimensions of Newtons and meters):

M₀= mu*Length*Width*U_bar.

And, once we have that scalar moment in Nm, then we usually want to convert it to moment magnitude (M_w):

M_w = 2/3 log10(M₀) – 6.

Here is a simple spreadsheet to do this calculation: LINK
Here is a simple and older lecture I have used in introductory level geoscience courses: LINK

Accumulating some links for M6.4 Tonopah area Nevada earthquake May 15, 2020

An M6.4 earthquake occurred this morning in SW Nevada 56 km west of Tonopah and 202 km ESE of South Lake Tahoe. It was widely but lightly felt across southern Nevada and the central California from the Sierra Nevada to the coast. It occurred in an area of NW-oriented dextral shearing called the Walker Lane. The size of the event would suggest that there is surface rupture. Fortunately, it is an area of low population density so hopefully no one was hurt and the damage is low. Aftershocks are continuing.

I am collecting some links in this blog entry:

• USGS page on the event
• Nevada Seismological Laboratory
• UC Berkeley Seismological Laboratory
• IRIS backprojection--favors the E-W plane
• Here is the event recorded on our ASU seismometer
• Jay Patton report--lots of good content
• A few references:
• Delano, et al., 2019 Geosphere article on the tectonic setting
• Active Faulting in the Walker Lane; by Steve Wesnousky (2005)
• Pliocene sinistral slip across the Adobe Hills, eastern California–western Nevada: Kinematics of fault slip transfer across the Mina deflection; by Sarah Nagorsen-Rinke et al. (2013)
• Recent Wired article on the Walker Lane

Anniversary of Great Tohoku Japan earthquake and tsunami (20110311)

Today is the anniversary of the catastrophic great Tohoku Japan earthquake and tsunami of March 11, 2011. While I am not an expert of subduction systems nor tsunamigenesis, I was of course interested in the event and prepared some lectures about it. While there are many better and newer illustrations, I wanted to share the materials.

The first presentation was at the Arizona Science Center in 2011. Here is the folder of the materials.

I prepared a lot of content for a series of lectures at IT Bandung in Java that I presented in 2013 with the help of my former student Dr. Gayatri Marliyani. There is much high quality material at IRIS (some of which I have included). The main materials are in these three folders:

• Subduction I--has the most on the 3-11 Japanese earthquake.
• Subduction II--mostly about Sumatran research. This has youtube lecture recording.
• Tsunami--includes 2004 and Cascadia material. This has youtube lecture recording

Idea for an earthquake intensity exercise based on 1857 Ft. Tejon earthquake data

I was cleaning some files yesterday and I found an old exercise I had deployed when I was first teaching Introductory Geology. It was intended to help students understand earthquake intensity (vs.) magnitude. I took the felt intensities as reported by D. C. Agnew and K. Sieh (1978), A documentary study of the felt effects of the great California earthquake of 1857, Bull. Seismol. Soc. Amer., vol. 68, pp 1717-1729 and compiled some of the more easily interpreted ones into a table and then provided a simple map of California for the students to map the intensities. Here is a link to the compiled data from Agnew and Sieh. THe work of Kerry Sieh on the 1857 earthquake is seminal. I think it was an ok exercise, but there are probably more interesting and more recent datasets. For example, I like the twitter-based work that is coming from the USGS colleagues. It should be possible to take some sample tweets and do an intensity mapping.

The 1857 earthquake and its foreshocks and aftershocks are fascinating an a sobering reminder of what will happen one day in California.

This figure from Toké and Arrowsmith, 2006 shows the 1857 foreshocks and the mainshock distribution (the latter is what the exercise mentioned in the last paragraph is supposed to look like) and compares it with the historic Parkfield earthquakes.

Some new San Andreas Fault tour videos of 1 m bare earth hillshades

I have been preparing a lecture and I built some new simple videos flying along the San Andreas Fault. The videos are made by me flying along in Google Earth with 1 m hillshades produced from lidar topography data collected along the San Andreas Fault. The videos are on youtube in this play list: https://www.youtube.com/playlist?list=PLFfZSFyNZ_jZm86F1TsnYfuuYNef_Uh21. I also put the MP4s in this folder--they are numbered 1-8 from NW-SE.

The flights follow generally along the San Andreas Fault from Point Arena to the southern Carrizo Plain (Dragon's Back and Northern Elkhorn Hills):

The data were processed at www.opentopography.org and come from 3 really cool datasets:

• EarthScope Northern California Lidar Project. Data collected by National Center for Airborne Laser Mapping.
• B4 project: southern San Andreas Fault laser scan. Data collected by National Center for Airborne Laser Mapping in collaboration with USGS, UNAVCO, and Ohio State University.
• Dragons Back Ridge: San Andreas Fault, California acquisition. Data collected by National Center for Airborne Laser Mapping in collaboration with George Hilley.

One dimensional morphological modeling of transport and production- limited fault scarps

Over the years, I have maintained a steady obsession with fault scarps. For my Ph.D., I worked on a few aspects of fault-scarp development: Arrowsmith, J R., Pollard, D. D., and Rhodes, D. D., Hillslope development in areas of active tectonics, Journal of Geophysical Research, 101, B3, 6,255--6,275, 1996. Correction: Journal of Geophysical Research, 104, B1, 805, 1999. Since then, I have kept the work going along, mostly with teaching applications. In this blog post, I wanted to share some of the presentations and tools that are available to explore, learn about, perform one dimensional morphological modeling of transport and production-limited fault scarps.

A few definitions:

• One dimensional--means elevation (H) as a function of distance along a profile (x).
• Transport-limited--there is enough transportable material available for any erosion that comes from the application of the mass continuity equation. In this case, the transport capacity is equal to the sediment supply.
• Production-limited--there is insufficient transportable material (regolith=material between topographic surface and top of bedrock) for erosion. In this case, the transport capacity exceeds the sediment supply locally.
• Diffusion erosion--transport capacity is scaled by local slope and a constant k. The consequence of this transport rate choice and the application of continuity for transport-limited conditions yields a diffusion-like or heat-conduction-like behavior.

This presentation (PPT and PDF) provides a bit of a review of fault scarp research as I saw it mostly about 5-10 years ago. This PPT has two embedded movies which illustrate this basic behavior: PPT. Transport-limited scarp movie; Production-limited scarp movie

Transport-limited models:

This web page from my Computers in Earth and Space Exploration class lays out the main derivation and numerical implementation: Lecture 8: Exploring diffusion using Excel. This older page has some Matlab and Excel implementations of 1D transport-limited linear diffusion: Scarp diffusion exercise. Finally, here is a 2D version of transport-limited non-linear diffusion in a paper by Mattia de Michieli Vitturi and me: de Michieli Vitturi, M. and Arrowsmith, J R., Two dimensional nonlinear diffusive numerical simulation of geomorphic modifications to cinder cones, Earth Surface Processes and Landforms, doi:10.1002/esp.3423, 2013.

Production-limited models:

George Hilley significantly updated my original code and produced the Penck1D imlpementation in MATLAB: zip file. Here is an older version of the MATLAB (no gui): zip file.
The software is delicate in some ways so you may have to try it a few times! If it crashes, just start over. One important thing is that it works best if downhill is to the right.

Note in particular the user's manual we wrote in 2006: Hilley, G. E., and Arrowsmith, J R., Penck1d: Transport- and production-limited fault scarp simulation software, user's manual for software used at 2001 Geological Society of America Short-course: Tectonics and Topography: Crustal Deformation, Surficial Processes, and Landforms Cosponsored by GSA Structural Geology and Tectonics Division and taught by Dorothy Merritts and Roland Bürgmann.

Landers earthquake fault scarp Structure from motion

I made a movie of structure from motion high resolution view of 1992 Landers California earthquake fault scarp. Video starts with 2012 hillshade (Johnson, et al., 2014; available from OpenTopography here: link) in Google Earth to show location and then to a ground based set of photographs (see blue rectangles as focal planes) visualized in Agisoft Photoscan.

I am pretty pleased that the ground-based model worked so well. Now we can move forward with fine scale alignment with earlier topographic point clouds and compute differences over the 25 years since the earthquake--a project I have worked on with Dallas Rhodes for many years (see Arrowsmith and Rhodes, 1994 and also Haddad, et al., 2012).

See also these posts:

2015 Anniversary of 1992 Landers California earthquake

SfM mapping--also has an orthophoto kmz of the Johnson, et al., 2014 data

Structure from Motion micro documentary from Merri Lisa Trigilio

References:

• Arrowsmith, J. R., & Rhodes, D. D. (1994). Original forms and initial modifications of the Galway Lake Road scarp formed along the Emerson Fault during the 28 June 1992 Landers, California, earthquake. Bulletin - Seismological Society of America, 84.
• Haddad, D. E., Akciz, S. O., Arrowsmith, J. R., Rhodes, D. D., Oldow, J. S., Zielke, O., … Shilpakar, P. (2012). Applications of airborne and terrestrial laser scanning to paleoseismology. Geosphere, 8(4). https://doi.org/10.1130/GES00701.1
• Johnson, K., Nissen, E., Saripalli, S., Arrowsmith, J. R., McGarey, P., Scharer, K., … Blisniuk, K. (2014). Rapid mapping of ultrafine fault zone topography with structure from motion. Geosphere, 10(5). https://doi.org/10.1130/GES01017.1

Goodbye to SoSAFE (Southern San Andreas Fault Evaluation): review and a recent workshop

The SoSAFE activity was a very successful data gathering and interdisciplinary science rallying activity in the Southern California Earthquake Center (SCEC) for 10 years. Its early leader was Dr. Ken Hudnut from the USGS. The original aspiration was to develop understanding of the last 2000 years of activity along the Southern San Andreas Fault. I was fortunate to be invited to help co lead SoSAFE in 2011. Dr. Kate Scharer (USGS) was the other co-leader. Her recent research has been largely focused on understanding the southern San Andreas Fault paleoseismic history. I very much appreciated the chance to work with Kate to help coordinate the research activity of SoSAFE withing SCEC as part of its Planning Committee. Our main efforts included evaluating proposals and helping coordinate and promote the research of our colleagues. We did a few additional activities including the "FieldShop" in which we made a group field trip to discuss and assess small offset landforms along the San Andreas Fault Mojave segment near Pearblossom. The outcome of the FieldShop was a paper lead by Kate: Scharer, K. M., Salisbury, J. B., Arrowsmith, J R., Rockwell, T. K., Southern San Andreas Fault Evaluation field activity: Approaches to measuring small geomorphic offsets and challenges and recommendations for active fault studies, Seismological Research Letters, v. 85, no. 1, ppl 68 - 76, 2014. In addition, we also organized (along with Prof. Mike Oskin from UC Davis) the SoSAFE and Earthquake Geology Geochronology workshop in 2014. The discussions of novel applications of geochronology for earthquake geology were really interesting and helpful for the community. SoSAFE has been sunsetted as an activity within SCEC as it moves into its 5th iteration (SCEC5). Much of the important work of SoSAFE will be subsumed (and hopefully continued) in the San Andreas Fault System Working Group to be led by Kate and Prof. Michele Cooke (UMass). It makes some sense I suppose as a refresh on SCEC structure, but I and others are concerned about the loss of emphasis on paleoseismic data gathering.

Nice cover image produced by Barrett Salisbury (ASU).

In our last SoSAFE activity, Kate and I organized a workshop for Sept. 10, 2016: SCEC SoSAFE Workshop: Recent Successes and Future Challenges. The workshop had 3 main themes:

1. Earthquake recurrence and slip over short and long term: how does it all add up?
2. Integrating earthquake and paleoclimate/paleoenvironmental chronologies on the SoSAFE System
3. Outside looking in: Broad applications of Behavior of high slip rate faults

The workshop was well attended (we capped it at 40 people, but many more wanted to join).

Several synthesis points were evident from the presentations and discussions:

• The opportunity to and importance of documenting the full spectrum of slip behavior at a range of slip speeds (creep to seismic) and both on and off fault. Where in space and time along and adjacent to the fault surface is there deformation, and how might the behavior vary over time? How to capture the integrated effect of fine scale fracturing? (Toké, Milliner, Lindsey)
• Variable slip at a point in successive earthquakes is suggested by detailed evidence from many sites. In some paleoseismic records, we may be seeing both small and large events mixed in the same record. This is a good picture of the earthquake phenomena but a greater challenge for interpretation. (Dawson, Salisbury, Rockwell, Biasi)
• Paleoseismic event recognition and resolveability. Are we under or over counting paleo earthquakes? The consensus among the group (and as analyzed by Biasi) was that both happen and so suggestions of systematic overcounting (e.g., D. Jackson) were not supported by experience. But, there certainly is value in exploring ways to systematize paleoseismic data (evidence, age control, correlation, etc.). (Dawson, Rockwell, Biasi, Milner). Tim Dawson reminded us of the important work of Bonilla and Lienkamper, 1991. The time scale during which many of the comparisons are being made (~1000 yrs) may be too short to completely assess the question of moment rate fluctuation across the SoSAFE system.
• Slip rates vary in space and time probably as a function of evolving fault geometry (at multiple length scales) and mechanical interaction (Cooke, Onderdonk).
• Interpreting offset per event from the reconstruction of fine scale landforms remains challenging. 3D excavation and reconstruction is desired but time consuming (and itself can have subtleties and ambiguities). There is a "... tension between collection of observations at as many locations as possible (assuming there will be signal in the noise [Large N]) versus inclusion of only data that are clearly offsets (rather than deflections) and have good quality ranking." (Scharer, et al. 2014). Interpretation of small geomorphic offsets will not go away given its convenience, the availability of high resolution imagery and topography, and the potential to assess remote structures. But, we must continue to validate and push to understand what we are measuring, entertain alternatives, identify 3D anchor sites, etc. (Salisbury)
• Earthquake simulators are moving forward as the preferred integrative tool for forecasting earthquake behavior across the SAF system ("Paleoearthquake data and slip rates battle it out in UCERF3--RSQSIM to the rescue"--Kevin Milner, USC). Jaqui Gilchrist (USC) gave a nice presentation reviewing the simulator approach. Her faults are quite smooth from the perspective of the many geologists in the room. Importantly, we learned about the tuning that is done by adjusting fault normal stresses to match paleoearthquake rates across the system. She showed that there is a gap between the curated paleoearthquake datasets she has used and the data producers. There is a need for deeper access to paleoearthquake data and metadata. SCEC-VDO was rebuilt in 2016 and is a valuable tool for visualization and exploration.
• Field earthquake geology is time and resource consuming. Assuming that the site conditions are good enough to preserve a high quality record, it is important to recognize that it takes at lot to produce high quality field studies, both in the field, as well as in the geochronology laboratory.

The early afternoon featured thought provoking talks about paleoclimate and paleoenvironmental chronologies. Here are a few of my takeaways:

• Kate showed in her 2014 paper (Scharer, K. et al., 2014b. Paleoearthquakes at Frazier Mountain, California delimit extent and frequency of past San Andreas Fault ruptures along 1857 trace. Geophysical Research Letters, 41(13), pp.4527– 4534.) how using paleosol and sediment accumulation curves would illuminate similarly timed landscape variation in California. She demonstrated that it was likely that earthquakes with overlapping ages at separate paleoseismic sites were different because their evidence was above and below a period of slow sediment accumulation (a paleosol). This is quite exciting and a frontier for earthquake geology. The challenge is what is the best proxy? Paleoprecipitation indicators? or Pollen or fire? How to balance convenience and the ability to measure with environmental sensitivy and ability to date?
• Prof. Matt Kirby (CSU Fullerton) presented a nice review of paleoclimate and paleoenvironmental proxies (temperature, precipitation, circulation, flooding, fire, other geomorphic disturbance). He focused on the rare lake records of southern California and differentiated millenial, centennial, sub-centenial to decadal, and annual time scales and their different drivers (mostly interactions with the Pacific Ocean). The millenial scale offers an opportunity to look at (synchronous formation of large markers along the SoSAFE System--e.g., Wallace Creek at 3,700 years BP and other similarly aged offset landforms). The sub-centennial to decadal (or finer scales) offer the opportunity for further differentiation of paleoearthquakes (e.g., Scharer, et al., 2014b) as well as the formation of small scale markers for single or few event offsets.
• Matt called out the need for common protocols, the value of looking at sections together, and that there were lots of interesting possible sites out there. We can use existing sites as benchmarks and prospect for new ones. This work costs $$!
• Prof. Nick McKay (NAU) offered valuable perspective as well as a potential path forward for organizing paleoseismic and paleoclimate data. He talked about the PAGES2K effort as a distributed global collaboration on past climate. It relies on a cyberinfrastructure of linked paleodata (McKay, N., and Emile-Geay, J., 2015, Technical Note: The linked paleo data framework – a common tongue for paleoclimatology, Climate of the Past, 11, 4309-4327.). He ended with some commentary on the challenges of supporting such a data effort (1) Identify metadata, 2) Structure metadata hierachically, 3) Be thoughtful about Selection Criteria, 4) Iterative data and metadata assimilation, and 5) Flexible scientific control).

Many thanks to our colleagues for their great ideas and community spirit in support of SoSAFE. Adieu!!

SoSAFE and Earthquake Geology Geochronology workshop report (Southern California Earthquake Center)

Kate Scharer (USGS), Mike Oskin (UC Davis) and I organized a geochronology workshop for the Southern California Earthquake Center community this fall. We emphasized methodologies useful for investigating fault slip behavior over time scales from 10² to 10⁶ years. That included Terrestrial Cosmogenic Nuclides, Uranium Series, and Optically Stimulated Luminescence. We did not talk so much about ¹⁴C given its relative maturity, but we certainly recognize its continued value.

Kate lead the charge on the completion of the workshop report. It has a nice summary of the topics covered (here is the original agenda). I had a sense of a real acceleration in the number and quality of applications of the methods and the resulting rich depiction of deformation rates and their variations across time and space in Southern California. I was particularly moved by the climate modulation on the development of landforms (alluvial fans, channels, etc.) that serve as markers as well as the possible temporal coherence of wet and dry times across the region. I think that this modulation and coherence can be exploited with more intensive application of geochronology.

I learned a lot more geochronology, especially having just been at the EarthScope Geochronology Institute a week or so earlier--note that the talks and some of the videos are up (where we covered some similar topics and where Kate was a speaker on ¹⁴C). It was very nice to see many SCEC friends and colleagues.

Fall 2014: PATA-Days in Busan, Korea

I had a wonderful trip to Busan, Korea for the Paleoseismology, Active Tectonics, and Archeoseismology (PATA Days) meeting. I really enjoyed it. It was great to see old and new friends, catch up on the latest developments, and to travel in southern South Korea.
Here is a presentation/overview I made for my students and colleagues for our seminar:
PPT
Here is the paper I wrote for the meeting: pdf.

I spent some nice time with my friends Koji Okumura and Shmulik Marco:

It was also an interesting trip because my father was based at the Pusan East (K-9) air base in 1951. He was curious to hear about the trip and mentioned that he turned 21 there. He said they played a lot of cards and that a few times he got to go off base to help his friends who worked at a reservoir.

Southern California Earthquake Center 2014 meeting group picture

Many of my students and colleagues and I just returned from the 2014 Southern California Earthquake Center meeting. It was really great: so much energy, so many old and new friends and colleagues. It is a very strong community with a world-leading emphasis on earthquake system science. I am on the Planning Committee and was very engaged many aspects of the meeting.

Here is a fun picture from the meeting:

Back row: Kate Potter (ASU), Kendra Johnson (Colorado School of Mines), Ed Nissen ((Colorado School of Mines), Wei Zhanyu (Chinese Earthquake Administration). Front row: Emily Kleber (ASU), Barrett Salisbury (ASU), me, and Gayatri Marliyani (ASU). I am always proud of my students and associates!

I flew to Palm Springs for the meeting and on the flight home, I had a nice view of the San Andreas Fault zone in the Indio Hills just to the east of Palm Springs and the meeting location. Here are two pictures:

The main fault trace cuts diagonally across the middle of the upper view. Two large scale right lateral offsets are evident along the fault. On the right side of the lower image, secondary (normal?) faults cut the uplifted alluvial fan units.

Along the San Andreas Fault in this area the the B4 laser scan data were collected by the Ohio State University, the National Center for Airborne Laser Mapping, USGS, and UNAVCO. The data are available on line at OpenTopography. Here is a screen capture of the hillshade in google earth:

And, here is the kmz file for that area of the hillshade of the DEM I calculated at OpenTopo.

M5 Earthquake in Eastern Arizona last night (June 28, 2014)

This is quite interesting and unusual historically. There was an earthquake in SE Arizona near the New Mexico border at about 10 pm local time. Here are a few links:

1. USGS site on the event
2. USGS site Did You Feel It shows that it was widely and lightly felt across southeastern Arizona and southwestern New Mexico
3. TUC vertical long period seismogram from Tucson show the event and aftershocks
4. Arizona earthquake history from AEIC does not show a lot of activity historically there
5. AZ Oil and Gas information I wonder if associated with oil and gas activities? AZ Is Hydraulic Fracturing a Threat in Arizona? (from AZGS).

Normal fault focal mechanism with roughly N-S striking planes is generally consistent with the few active faults in the area (USGS QFaults; see also AZGS fault map). I will post more when I have some more time.

Short Course at LIPI : Techniques in Active Tectonic Study

In July 2013, Gayatri Marliyani and I taught a short course at LIPI in Bandung, Indonesia on Techniques in Active Tectonic study. I put together almost 30 lectures on various topics in active tectonics, neotectonics, tectonic geomorphology, paleoseismology, earthquake geology, and related topics. Special thanks to Mudrik Daryono (course coordinator for LIPI/ITB/GREAT), Irwan Meilano (ITB/GREAT), Danny Hilman Natawidjaja (LIPI/GREAT), Eko Yulianto (LIPI/GREAT), and the participants.

Gayatri built a nice website with all of the freely available lectures and other course content. We also recorded most of the lectures and they are on youtube.

The course web site is: http://activetectonics.la.asu.edu/lipi/. Look under the schedule link for the lectures and the link for some exercise content.

Group picture on field trip along the Lembang Fault.