A question we commonly get following an earthquake large enough to be reported in the news is “What fault did this earthquake occur on?” Earthquakes are, after all, the release of elastic strain energy following brittle failure of the crust. Since tectonic earthquakes are a response to the stress applied to a dynamic crust experiencing pressure from ongoing sources, it makes sense that we try to identify the zones of weakness on which we expect those stresses to be manifested. Unfortunately the answer to this question is rarely straightforward and earthquakes in Arizona are certainly no exception.

From Seismicity and active faulting of the Kanab-Fredonia area of the southern Colorado Plateau (2008) Brumbaugh, D.S.

Fault plane solutions

An analysis of the fault associated with a given earthquake starts with a focal mechanism generated by observations taken from the seismic record. Modeling and analysis of waveforms from events give us meaningful information about source dynamics. For small to moderate earthquakes in regions with sparse network coverage phase data is often too poor to yield unambiguous moment tensor solutions. For our record of 1216 cataloged events in Arizona, only a small handful have associated fault plane solutions. Availability of data and relevant scientific interest will always be legitimate barriers to a complete understanding of crustal structure, but they are certainly not the only limitations.

Going from source mechanisms to known fault:

Once the orientation and direction of movement that define the fault plane are identified, pinpointing the responsible fault isn’t always as easy as looking at a map. An understanding of historic faults is heterogeneously created from the research of scientists interested in the crustal structure of a certain area. Generally this interest stems from the need for hazard mitigation or known historical events. The USGS for example compiled a map of Quaternary age faults for the western US thought to be associated with events greater than magnitude 6 from thousands of journal articles, maps, theses and other research (http://earthquake.usgs.gov/hazards/qfaults/google.php).

GoogleEarth KMZ of quaternary faults by USGS

This is a good community resource for understanding the location and names of known faults, however there is likely a level of incompleteness for certain types of faults as well as poorly monitored regions or unpopulated places. Often, as in the case of Christchurch, NZ, little is known about the existence or extent of a fault until large events focus attention on them (NZ article on extent of Greendale fault from last years 7.0 quake: http://www.stuff.co.nz/the-press/news/4586170/Whose-fault-is-it-anyway-The-scientists-remain-divided). Likewise after the earthquake in Haiti last January, scientist blamed the Enriquillo fault. After months of dedicated research and investigation we now know that the earthquake was a result of rupture on a previously unknown parallel fault named the Léogâne fault. These are earthquakes of considerable size. The amount of data collected for them is enormous. The USGS reported 428 phase picks for the Haitian quake. By comparison, an earthquake in the magnitude 2.5 range in California generates on average probably 20 phase picks, in some areas of Arizona or Alaska the number of stations which register that magnitude might be as few as 6.

Blind thrust animation from the Southern California Earthquake Center

A lot of our understanding of fault recurrence intervals is built upon observations from the geologic record. Mike Conway’s Lidar post on January 21st is a good example of new tools being used to identify and understand surface faults. Or the surface trenching that was done to help understand the Chino Fault covered in yesterdays anniversary post. These are often our best tools for understanding fault dynamics over the long term. But surface rupture is not a guaranteed part of the earthquake process. Whether an earthquake breaks the crust at the surface or anywhere near it is a function of many variables: the amount of energy released during the event, the depth of the focus, the composition of the overlying material, etc. The deeper the earthquake focus, the larger the earthquake has to be to rupture the surface. It is often the case that we are unable to use ground based observations to verify movement or location for a fault surface and it is often these faults which small to moderate seismicity is related to. A large fraction of global seismicity is not associated with correlative surface rupture because there isn’t any. The 1994 Northridge earthquake (Mw 6.7) occurred on a blind thrust and produced the strongest ground motions ever instrumentally recorded in an urban setting in North America. The Southern California Earthquake Data Center provides an animation for understanding this type of fault (http://www.data.scec.org/chrono_index/pop_blind.html). The amount of data we have for the Northridge fault is a function of the location of the fault and size of the events it tends to generate. It is often the case that these faults go undetected unless they pose a specific threat or generate obvious seismicity.

For each earthquake, realistically what we can do is use the data we collect from an event to characterize the style and orientation of failure in a certain zone, and then use what we know about current and historical structure to tell the best story we can. This is harder for some events than others. For some earthquakes like the February 4th Chino Valley event, there is a likely culprit on hand. This is also true for a good bit of seismicity in the northeastern part of Arizona. Faults are often less discrete than the lines on our maps indicate. When an earthquake is located within the region of a fault but not on top of it- its likely that the fault is better characterized as a zone of weakness than a single, clean line of failure. It is also sometimes the case that earthquakes occur in places that are unexpected. New faults like the Léogâne are being found all the time. The closer we look, the more we find, and the more complete our stories become.

Lisa Linville, 4 February 2011