Scattering of high-frequency seismic waves caused by irregular surface topography and small-scale velocity inhomogeneity

We analyze data from two orthogonal seismic lines 336 m in length collected at Piñon Flat, California, over weathered granodiorite bedrock. Each line was made up of 10 reversed segments 84 m in length. We analyzed the first arrivals from these data and found dramatic variations in velocity along the profiles. An upper layer (approximately 2-m thick) known from trenching to be composed of soil and sandy grus had measured velocities ranging from 400 to 700 m/s. Velocities inferred from refraction analysis of first arrivals of the reversed lines revealed a heterogeneous lower layer below the soil with measured velocities of 1600–2700 m/s by a depth of 15 m. We interpret these data to be measuring velocities of a deeply weathered unit characterized by granodiorite corestones embedded in a matrix of saprolite. The most remarkable feature of these data emerged from attempting to process the same data as reflection data. Simple bandpass filtering in the 250–400 Hz band revealed a bright, impulsive arrival with three characteristic properties: (1) irregular velocity moveout that is inconsistent with that expected from a layered earth model, (2) the arrival is at a nearly constant time‐depth on all data, and (3) the arrival tends to be followed by a ringing coda whose frequency varies from trace to trace. This arrival ties exactly with a velocity discontinuity measured in a borehole located on one of the profiles that we interpret as the base of the weathered layer. We suggest this arrival is a specular reflection from a weathering front that occurs along horizontal sheeting joints at a fixed depth below the surface. This surface acts as an effective mirror for high‐frequency seismic waves which are then channeled upward through an intact, high-Q path of unaltered blocks of granodiorite to define the observed signals at the surface.

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Nonlinear Interaction of High‐Frequency Seismic Waves With Sliding Fault Planes

Journal of Geophysical Research Solid Earth ◽

10.1029/2019jb017579 ◽

2019 ◽

Vol 124 (11) ◽

pp. 11748-11770

Author(s):

Norman H. Sleep

Keyword(s):

Nonlinear Interaction ◽

High Frequency ◽

Seismic Waves ◽

High Frequency Seismic Waves

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Surface topography effects on seismic ground motion and correlation with earthquake-induced landslide: An example of the JiuJiu peaks in 1999 Chi-Chi Taiwan earthquake

10.5194/egusphere-egu2020-6510 ◽

2020 ◽

Author(s):

Chun-Te Chen ◽

Shiann-Jong Lee ◽

Yu-Chang Chan

Keyword(s):

Surface Topography ◽

Seismic Response ◽

Ground Motion ◽

Seismic Waves ◽

Velocity Structure ◽

Seismic Hazard Assessment ◽

Buffer Layers ◽

Small Scale ◽

Ground Shaking ◽

Mesh Model

<p>The topography effect has been thriving investigated based on numerical modeling. It impacts the seismic ground shaking, usually amplifying the amplitude of shaking at top hills or ridges and de-amplifying at valleys. However, the correlation between the earthquake-induced landslide and the topographic amplification is relatively unexplored. To investigate the amplification of seismic response on the surface topography and the role in the Chi-Chi earthquake-induced landslide in the JiuJiu peaks area, we perform a 3D ground motion simulation in the JiuJiu peaks area of Taiwan based on the spectral element method. The Lidar-derived 20m resolution Digital Elevation Model (DEM) data was applied to build a mesh model with realistic terrain relief. To this end, in a steep topography area like the JiuJiu peaks, the designed thin buffer layers are applied to dampen the mesh distortion. The three doubling mesh layers near the surface accommodate a more excellent mesh model. Our results show the higher amplification of PGA on the tops and ridges of JiuJiu peaks than surrounding mountains, while the de-amplification mostly occurs near the valley and hillside. The relief topography could have a &#177;50% variation in PGA amplification for compression wave, and have much more variety in PGA amplification for shear wave, which could be in the range between -50% and +100%. We also demonstrate that the high percentages of the landslide distribution right after the large earthquake are located in the topographic amplified zone. The source frequency content interacts with the topographic feature, in general, small-scale topography amplifies the higher-frequency seismic waves. It is worthy of further investigating the interaction between the realistic topography and the velocity structure on how to impact the seismic response in the different frequency bands. We suggest that the topographic seismic amplification should be taking into account in seismic hazard assessment and landslide evaluation.</p>

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Landslide Characteristics Revealed by High-Frequency Seismic Waves from the 2017 Landslide in Central Japan

Seismological Research Letters ◽

10.1785/0220200032 ◽

2020 ◽

Vol 91 (5) ◽

pp. 2719-2729

Author(s):

Issei Doi ◽

Takuto Maeda

Keyword(s):

High Frequency ◽

Seismic Waves ◽

Wave Packets ◽

Impulsive Phase ◽

Frequency Component ◽

Source Location ◽

High Frequency Component ◽

Central Japan ◽

Location Determination ◽

High Frequency Seismic Waves

Abstract The recent development of advanced seismograph networks offers us a chance to remotely detect landslide occurrences with high-frequency (>∼1 Hz) components. This study examined a landslide in central Japan that produced clearly detectable seismic signals at multiple seismic stations in a permanent network. Wave packets propagated with a group velocity of 3 km/s from the landslide area. Using a source location determination method with amplitude information from the high-frequency component, the source location of the wave packets was shown to be in the vicinity of the landslide with an error of 5 km. Moreover, seismograms specific to this landslide also contained a distinct impulsive phase with a source located in the vicinity of the landslide. The study demonstrated that seismic waves with a high-frequency component from landslides can be used to estimate their mechanisms as well as their locations when they are recognized by a routine seismic network.

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