Removing Some of the Mystery from the “Mystery Booms” in the Mother Lode Region Glen White, Geologist September 19, 2014

Background: Numerous people throughout the Mother Lode region have reported deep booming noises of unknown origin. Since the origins were not immediately evident, they have been referred to as “mystery booms”. A Facebook page was set up to help record observations from witnesses and to allow people to speculate on the source of the noises. Anecdotal information indicates that the booms have been heard for many years (some reporting up to 200 years), at all times of the day, and throughout the year. Information is available to help remove the mystery from some of the booms. Historical, anecdotal, and reports from outside the Mother Lode region are not addressed here. Only scientifically collected data are used in this paper to explain some (perhaps most) of the noises heard in this region. The term “brontides” has sometimes been used to refer to these booms. Brontides are simply defined as deep booming sounds, possibly of seismic origin. (http://www.merriam-webster.com/dictionary/brontide)

Summary of Observations: Numerous witnesses reported their observations on the Mother Lode Mystery Booms Facebook page. The predominant pattern of reports indicated that booms tended to occur on weekdays between the hours of 11:00 AM and 2:00 PM. Occasional booms were reported outside of those boundaries. Some booms were heard widely from Amador, Calaveras, and Tuolumne counties, some seemed much more localized, while some were heard in a patchwork of locations. Portions of Calaveras and Amador counties had more reports than Tuolumne County. Some booms were reported shortly after their observation, others were reported up to many hours later. Thus, the timing of individual observations cannot be accurately correlated with each other. The booms seem to follow a seasonal pattern. Most booms are reported during the summer months, with few booms reported the remainder of the year.


Speculation of Origins: Many hypotheses have been posted on the Facebook page including sonic booms from aircraft, tectonic shifting within the Earth’s crust, changes in groundwater and reservoir levels, stress in rock units being relieved, quarry blasting, ordnance disposal, air pressure changes in the atmosphere, the collision of warm and cold fronts in the atmosphere, among others.
Electronic Monitoring Systems: There are several seismic monitoring stations located within the area, the principal one located on the Columbia College campus. That seismic station is part of the Northern California Earthquake Data Center operated by the Berkeley Seismic Laboratory through UC Berkeley (http://quake.geo.berkeley.edu/). The University of Nevada, Reno also operates a seismic network (http://www.seismo.unr.edu/).
In August 2014, researchers from Southern Methodist University (SMU) in Texas completed the installation of an infra-sound array at Columbia College. The infra-sound array has been developed by SMU in conjunction with various US agencies to monitor nuclear and conventional bomb detonations. Infra-sound are acoustic waves that are below the frequency of normal human hearing. They are detected as pressure waves that move through the atmosphere.
Limitations: This report does not attempt to address “mystery booms” at locations outside of the Mother Lode region, nor is there any attempt to explain anecdotal reports of historical booms. Additionally, the source identified in this paper is not intended to be described as the source for all booms heard throughout the Mother Lode region. The evidence is clear for a particular set of booms, and that is the focus of this work.
The Big Reveal: The weekday booms heard during the summer months originate at the Hawthorne Army Depot’s “New Bomb” ordnance disposal facility located south of Hawthorne, Nevada. http://www.jmc.army.mil/Installations.aspx?id=Hawthorne
Evidence in Support of Hawthorne as the Source of Mystery Booms:
SMU researchers have been working with the Hawthorne facility for over a decade using their ordnance disposal operation to help calibrate their infra-sound sensors. Previous work used high speed cameras and seismic monitoring equipment to accurately time the blast and the arrival of the pressure waves.
Blasting occurs five days a week, between the hours of 11:00 AM and 2:00 PM, throughout the year.
Seismic data can be used to accurately record the time of the blasts at Hawthorne. The acoustic waves travel much slower than seismic waves and thus lag far behind the seismic waves. The travel times are well known and the correlation is accurate.
While completing the installation of the infra-sound array, the SMU researchers experienced the booms themselves. They were shocked not only that they could be heard, but that they were heard so loudly. They reported that the instrument trailer that they were in was noticeably rocked by the booms.
Questions:
1. If the blasts occur throughout the year, how come we only hear them during certain times of the year?
The atmospheric conditions that allow the audible sound waves to reach this far do not occur throughout the year. During the summer months, conditions are just right to allow the sounds to reach this far. The SMU and UNR scientists were equally surprised that any sounds at all were audible. They were not surprised that the infra-sound array was detecting the pressure waves.
2. What about booms that happen at times outside the known blasting times?
More work and much more accurate recording of those events needs to be conducted to see if those are local events such as quarrying or mining, or something else. Once the datasets for the various monitoring instruments can be collated, the source of those booms may also be tracked down.
3. How come some people hear them and others don’t, even when they’re in rather close proximity to one another?
The pressure waves that are generated by the ordnance demolition propagate outward radially. As they move they attenuate in strength, but they also interact with layers within the atmosphere, and the topography of the landscape. The waves bounce (reflect) and bend (refract) as they encounter different layers and surfaces. Some of the waves interact constructively, increasing wave amplitude, and some interfere destructively, decreasing wave amplitude. Some locations will be ideally suited to receiving waves that are audible, while others will not. The conditions in the atmosphere are in constant change so boo ms may be louder one day than another, while the size of the initiating blast in Hawthorne could be the same.
4. What about the booms that have been reported long before Hawthorne began ordnance disposal?
Tracking down the source of historical or anecdotal observations isn’t possible. They could be local events like quarrying, mining, shooting, timber falling, thunder, rockfalls, etc. Nothing can be said with certainty regarding events for which no reliable data exist.
5. Why don’t people much closer to Hawthorne report hearing them?
The pressure waves that are generated by the explosions do not travel in straight lines. Changes in temperature and pressure in the atmosphere cause waves to bend (refract). In this study done by SMU researchers, the “zone of silence” is illustrated. While it sounds strange, it’s just physics and is a phenomenon that is well-known to researchers.
http://www.geology.smu.edu/~seismic/infrasound/infra_petru/negraruSRL_ZOS2010.pdf

Going Forward: The SMU infra-sound array has just recently been installed and some sensor upgrades were completed. Therefore, this data does not extend back in time and only future data will be used to further research in this area. Data from the Northern California Earthquake Data Center can be retrieved for past events, but the infra-sound array will have no corresponding dataset. Future events will be able to be correlated using the UNR seismic network, the NCEDC network, and the SMU infra-sound array. A computer is being sent from SMU so that the infra-sound data can be accessed and shared. Seismic data is already available online. Graduate students from SMU may be interested in collaborating with Citizen Scientists to record observations that they can compare to their instrumental readings. Similarly, research involving UC Berkeley and/or UNR seismology students may be possible. Conclusion: Strong instrumental data, combined with researcher field experience and observations, indicate that ordnance disposal that is conducted at the Hawthorne Army Depot in Hawthorne, Nevada is the source of most of the “mystery booms”. The weekday occurrence during well constrained hours fits their blasting schedule very well. Atmospheric conditions provide optimal conditions for audible acoustic waves to be experienced during summer months, and prevent them from being experienced during the remaining months of the year. Booms that occur outside the normal weekday schedule may be attributed to other, as of yet unidentified, natural or manmade sources.