I have read the paper with interest. The article is well written and reads well. It is novel and presents a new approach towards man-made load on natural landforms of cultural and esthetic value.  The research methodology is sound and clearly presented.

My main critique is towards the unneeded extrapolations and conjectures.  The results speak for themselves, even if their practical significance is of second order.

For example, lines 233-237, “Thus the range of landforms susceptible to high-amplitude resonance from helicopter-sourced infrasound is likely narrow, i.e., the conditions such as frequency and modal vector alignment limit the number of landforms that might experience large vibration amplitudes during helicopter flight. However, a single past study reported peak vibration velocities of 4.1 mm s-1 for a ~13 m tall rock pinnacle (with similar values for two neighboring pinnacles), demonstrating the viability of these elevated amplitudes (King, 2001). Followed by lines 238 – 239, “We additionally note that heavier military helicopters not studied here likely generate higher power infrasound than lighter civilian models (Hanson et al., 1991), and further study is needed to test the effects of military helicopter overflights on the vibration response of rock landforms.

What can we learn from a single pass? For example, are military helicopters allowed to fly and hover over National Parks and Monuments?

Lines 246 – 248: “While this scenario is limited, there are thousands of arches, towers, and other culturally significant valuable rock landforms in Utah alone (e.g., Stevens and McCarrick, 1988), and a multitude of others other nationally significant geologic features around the world”.

This is stating the obvious. Instead, please provide a range of height, span, stiffness, or any other mechanical attribute of landforms that would be affected by helicopter-generated infrasound pressure.

My suggestion is to avoid generalized statements where and when possible.  

Also, there are some minor remarks:

1. Table 1 and Figure 2. There seems to be a mismatch with the frequencies of Arsenic and Squint arches. In the table, there are three frequencies for the Squint arch and a single frequency for Arsenic. The figure shows opposite.
2. Table 1. Young’s modulus of what? Structure, rock mass, rock?
3. Line 78, please provide the models of broadband and geophones.
4. Table 2. I am not convinced it is necessary.
5. Figure 2e is in mm/sec units; it is hard to see the “…increased 100–1000 times during helicopter flight ….” Please provide numbers for median background noise.
6. Line 223. It would be of value to provide a correlation between landform(s) vibration values and the expected change of stresses.

The authors present an interesting study associated with the vibration of rock arches and towers caused by helicopter-sourced infrasound. Both seismic and infrasound observations are carefully analyzed to evaluate the response of natural landforms to anthropogenic sources. The pieces of evidence and analyses are well organized and the paper is well written. However, there are a few aspects that need a further demonstration.

First, the authors mentioned 3D models of landforms were built to estimate the eigenfrequencies of the arches and towers using finite-element simulations. This is an important component of the analysis and can provide insightful discussion on this topic. Please consider, at least, adding a dedicated paragraph describing the details.

In addition, in L80-84, a couple of procedures and methods provided by the previous study have been cited without sufficient details. It is difficult for the audience to understand what has been done. Please add a sentence or two to briefly summarize these methods and procedures.

Further, in section 5.3, the author compared the maximum vibration velocities measured on landforms with the empirical levels of previous studies. Using vibration velocity is only a practical way to evaluate the damage. The damage should be determined by the stress perturbations and the structural strength. As the 3D finite-element model are available, it will be great to estimate the maximum stress/strain perturbations inside the landforms caused by the infrasound loading of helicopters. What’s the order of the stress perturbation when loading on a landform of ~10 Pa at its natural frequencies? The stress perturbation is a more direct parameter to compare with the rock strength. This should be the highlight of this paper and will cause an impact.

Last but not the least, when introducing new parameters, e.g., admittance, it is always good to demonstrate what does it mean when the value is high and low. 

Here are the detailed comments:

Figure 1. Can the authors also show the spectra of the infrasound records for comparison?

Table 2. Are the overtones always the multiples of blade pass frequency?

L80-81: Please briefly summarize the method performed to estimate the natural frequencies.

L82-85: Please consider adding a supplemental figure comparing the observed and modeled nature frequencies of the 4 arches and 3 towers analyzed in this study.

L97-99: Did the authors perform lowpass filtering before downsampling the records? Otherwise, there will be artifacts associated with aliasing.

L105: The authors mentioned the orientation of maximum vibration for arches is in the vertical. Can this be confirmed by the seismic observation? Please add a sentence or two to discuss.

L106: Are the dominant polarization azimuth associated with the geometry of hoodoos?

L108: “We plotted …” Is this referring to Figure 2 or Figure 3?