Small-Magnitude Felt Earthquakes Near the Pajarito Fault System, New Mexico: Updated Locations and Faulting Implications Using Improved Tools
Abstract
On July 25, 2017 a magnitude 1.5 earthquake was recorded by The Los Alamos Seismic Network (LASN) in Northern New Mexico. Surprisingly, this small event was felt nearby by at least 30 Los Alamos residents, who (all while indoors) reported a range of disturbances from a sudden jolt or impact to their house, to 3-5 seconds of rumbling or floor vibrations, to objects rattling on tables and walls. These reports correspond to Modified Mercalli Intensity Scale values of at least II to III and possibly as high as IV. LASN seismic data for this event were recorded on 11 broadband digital stations and six older short-period analog stations. The event was located about 3-5 km northwest of Los Alamos town site at a calculated depth of less than 5 km. Five similar (small, shallow, felt) events were recorded in 1991, 1998 and 2003, when LASN consisted only of six or seven short-period stations. As a result, the 2017 earthquake location is better constrained and can be used to improve the locations of the previous events through relative-location techniques. All six events were located on or near the Pajarito fault system in an area where the faulting geometry (especially at depth) and tectonics are complex and not well constrained with previously available information. The apparent shallow focal depths are consistent with the observations that all six events had small magnitudes and were nonetheless felt. This in turn has implications on possible shallow intersections of the main Pajarito fault with several known antithetic faults located to the east (whose dips are not well constrained), all of which lie within the epicentral area of the felt events. In addition to recent LASN station improvements, intensity data and maps for the earlier events have been updated, geologic mapping of fault structures for the upper Rio Grande Rift region has been improved, and high-resolution LIDAR data for the region are now available. We will report results for relocations of all six felt earthquakes and for combining this information with geologic and LIDAR data to obtain an updated understanding of the complex structure and tectonic behavior of the northern region of the Pajarito fault system.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFM.S53B0701G
- Keywords:
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- 7209 Earthquake dynamics;
- SEISMOLOGY;
- 7215 Earthquake source observations;
- SEISMOLOGY;
- 7221 Paleoseismology;
- SEISMOLOGY;
- 7230 Seismicity and tectonics;
- SEISMOLOGY