Directions and Motivation: This midterm is open book, open web search, open talk to your peers and professors. However if you are heavily relying on someone else or a web resource (very likely) for your information then I would like you to cite them following the model employed herein (see reference list at end of the document and in text citations throughout). The answers to some of these questions are very open ended and are more about showing critical thinking related to earthquake geology and hazards concepts. So there is no reason to copy anyone, nor will it be tolerated. I hope you have fun with this midterm and realize that you are capable of interrogating this type of data and producing knowledge.
Resources: I have placed several journal readings on CANVAS in a section marked midterm related to the questions on this midterm. Another useful link is the Holocene to Latest Pleistocene fault maps of the USGS. Here is a link to Google Earth fault map files: http://earthquake.usgs.gov/hazards/qfaults/google.php
The Magnitude of Earthquakes on the Wasatch Fault
1 – The Wasatch Fault has been divided into fault segments based upon fault geometries and paleoseismic records that suggest certain parts slip in earthquakes independently of other sections (segments). Use the Utah Geological Survey’s web resources, the Machette et al., 1991 paper, and other relevant sources (e.g., your Paleoseismology Book: McCalpin, 2009) to construct a data table showing segment names from north to south, segment lengths, the timing of the last few earthquakes for each segment, and if possible the estimated slip in individual past earthquakes. If you cannot determine the average slip in an earthquake on each segment please estimate this value and describe your reasoning (i.e., use the empirical relationships of Wells and Coppersmith, 1994 or another publication).
2 – Append the depth to which earthquakes rupture along the Wasatch Fault to your table. We have not seen a large historical earthquake on the Wasatch so we don’t know this information directly, but if you can find a map showing the depth of microseismicity along the Wasatch, this would allow you to infer the depth of seismicity. Otherwise, do an internet search to asking about either the depth of seismicity in the Basin and Range, on the Wasatch or along normal faults in general. Additionally, you might just simply ask the question, in what layer of the Earth do earthquakes occur and how thick or deep is this layer here in Utah? Justify your inferred depth of seismicity (tell me how you deduced it and cite your references).
3 – You now have the maximum depth of fault rupture, the total lengths of each segment, and the slip per earthquake….
a) Using the seismic moment equation directly calculate the moment magnitude for a full rupture of each segment of the Wasatch Fault.
The equation for moment magnitude is in your Paleoseismology textbook and can be found online, but here are the basics:
Moment = (shear modulus [i.e., rigidity] of crust) X (area that ruptured) X (mean displacement in the earthquake)
Mo = µ *A*D
The Shear modulus can be approximated through the relationship to s-wave velocities, because the velocity of an S wave (Vs) = [shear modulus (µ) / density (ρ)]1/2
Therefore µ = Vs2 x ρ
An S wave travels in continental crust at ~ 3.5 km/s and crustal densities are ~ 2.7 g/cm3
So if you can calculate a shear modulus µ (units should be in g/cm*s2) for the crust you can then calculate the moment for individual earthquakes on each segment of the Wasatch.
Please do this and then use the following logarithmic relationship to change your moment (energy) calculation to a moment magnitude (Mw) for earthquakes on each segment of the fault.
Mw = 2/3 Log (Mo) – 10.7
b) In addition to directly calculating the magnitude, use the relationships of Wells and Coppersmith, 1994 to determine the magnitudes of these potential fault ruptures. Please show or describe your work/assumptions and append both of these two earthquake magnitude estimates to your data table.
4 – Describe the range of maximum magnitudes that you calculated for the different segments of the Wasatch. These earthquake magnitudes are based upon long held segmentation models for the fault.
Fault Segmentation and Earthquake Rupture Boundaries
5 – Calculate the magnitude of an earthquake that ruptures the whole Wasatch at once. You will need to refer to the relationships of Wells and Coppersmith, 1994 to estimate the slip for an earthquake with this long of a rupture length so you can do the calculation appropriately. How does your entire Wasatch magnitude compare with the magnitudes of the individual segment bounded earthquakes?
6 – Consider our field work near Alpine, UT. We observed colluvial wedge deposits within that excavation which was incised by the recent debris flow. Based upon your collective field summaries we observed that there were at least two colluvial wedges in this exposure. At least one wedge was 1-2 meters in height. Using the Wells and Coppersmith, 1994 paper, figure 12; discuss what this might mean for the likelihood of multi-segment rupture between the Salt Lake and Provo segments. Do you believe that the geologic record along the Northern Provo segment and Southern Salt Lake City segment indicates that ruptures cross the segment boundary?
7 – Read the paper by Wesnousky, 2006 on fault segmentation, based upon bends and step overs. Then Read the papers by Schwartz and Coppersmith, 1984 and Machette et al., 1991 which describes the justification for segmenting the Wasatch the way it has been done. Use the Google Earth Fault map (see introduction) to test if the Wasatch segment boundary between the Salt Lake City and Provo segments fit with Wesnousky’s findings about bend and step over widths and how they control fault rupture termination? Use the measurement tool in google earth, how big are the segmentation boundary geometries? Based upon Wesnousky’s findings, is there reason to believe that these segments could rupture together or should they be rupture barriers? You will want to draw the geometries of these segment boundaries or produce some kind of google earth map showing your measurements.
UGS Setback Regulations
8 – On the PDF Map showing a Rock Canyon slope-shade underlain by aerial photography map any recently active surface traces of the Wasatch Fault. Also map prominent geomorphic surfaces (e.g., fans, lake terraces) and features (channels).
9 – On the PDF Map zooming into the prominent fault scarp just southwest of Rock Canyon position the fault through the fault scarp (use the three profiles to help) and determine if the four homes marked with an # meet the setback requirements according to the UGS guidelines. Please state any assumptions and refer to the Rock Canyon Paleoseismic Report (Lund and Black, 1998) for slip/earthquake information.
Distance to the Fault, Peak Ground Shaking, and Earthquake Felt Effects
10 – If I told you that the Wasatch Fault dips west beneath Utah Valley at 65 degrees and the hypocenter of the next Provo segment Earthquake was 10km deep at the latitude of Sherwood Hills Landslide on the Provo segment of the fault, please do the trigonometry to determine how far from the mountain front the epicenter would be. Where in the valley does this coincide? Seismic energy attenuates away from faults during the rupture, but the highest shaking is usually within 10 km of the fault rupture. Assuming a full Provo segment rupture, what places in the Salt Lake City metropolitan area are far enough from the fault, say 40km, that they would have relatively modest shaking with relatively little chance for major damage?
11 – Check out this webpage http://geology.utah.gov/utahgeo/hazards/eqfault/ibcshake/index.htm from the Utah Geological Survey and the associated map of expected strong ground motions with a 2% chance of occurrence in the next 50 years. Find where you live on these maps. At your home, what are the expected levels of ground shaking (levels are given in units of acceleration in G’s (units of gravity: e.g., 1 = 100% gravity) for both short and longer period shaking. Shorter period shaking is more important to low-rise buildings. So use the short period (high frequency) acceleration maps if you don’t live in a skyscraper. Given this information compare shaking intensities to modified Mercalli Intensities (they are correlated). What is the equivalent Modified Mercalli Intensity level for your ground acceleration?
http://quake.abag.ca.gov/shaking/mmi/ * Refer to this Association of Bay Area Governments website for modified Mercalli intensity (MMI) explanation after finding the correlation between % g and MMI in the chart above.
References:
Association of Bay Area Governments Resilience Program. The Modified Mercalli Intensity Scale (from Richter, C.F., 1958. Elementary Seismology. W.H. Freeman and Company, San Francisco, pp. 135-149; 650-653): http://quake.abag.ca.gov/shaking/mmi/ (last accessed September 30, 2015).
Lund, W.R. and B.D. Black (1998). Paleoseismic Investigation at Rock Canyon, Provo Segment, Wasatch Fault Zone, Utah County, Utah. Paleoseismology of Utah, vol. 8, Special Study 93.
Machette M.N., S.F. Personius, and A.R. Nelson (1991). The Wasatch Fault Zone, Utah – Segmentation and history of Holocene earthquakes. Journal of Structural Geology, vol. 13, no. 2, 137-149.
McCalpin, J.P. (ed.), 2009, Paleoseismology, 2nd Edition: International Geophysics Series, Vol. 95, Elsevier Publishing, 647 p.
Schwartz, D.P. and K.J. Coppersmith (1984). Fault Behavior and Characteristic Earthquakes: Examples from the Wasatch and San Andreas Fault Zones, Journal of Geophysical Research, vol. 89, no. B7, 5681-5698.
United States Geological Survey. Quaternary Faults in Google Earth: http://earthquake.usgs.gov/hazards/qfaults/google.php (last accessed September 30th, 2015).
Utah Geological Survey. Earthquake Ground Shaking Levels for the Wasatch Front: http://geology.utah.gov/utahgeo/hazards/eqfault/ibcshake/index.htm (last accessed September 30, 2015).
Wells, D. L., and K. J. Coppersmith (1994). New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement, Bulletin of the Seismological Society of America, vol. 84, no. 4, 974–1002.
Wesnousky, S.G. (2006). Predicting the endpoints of earthquake ruptures, Nature, vol. 444, 358-360.
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