Mapping 3D fault geometry in earthquakes using high‐resolution topography: Examples from the 2010 El Mayor‐Cucapah (Mexico) and 2013 Balochistan (Pakistan) earthquakes

Abstract

Fault dips are usually measured from outcrops in the field or inferred through geodetic or seismological modeling. Here we apply the classic structural geology approach of calculating dip from a fault's 3‐D surface trace using recent, high‐resolution topography. A test study applied to the 2010 El Mayor‐Cucapah earthquake shows very good agreement between our results and those previously determined from field measurements. To obtain a reliable estimate, a fault segment ≥120 m long with a topographic variation ≥15 m is suggested. We then applied this method to the 2013 Balochistan earthquake, getting dips similar to previous estimates. Our dip estimates show a switch from north to south dipping at the southern end of the main trace, which appears to be a response to local extension within a stepover. We suggest that this previously unidentified geometrical complexity may act as the endpoint of earthquake ruptures for the southern end of the Hoshab fault.

Extent of Low‐Angle Normal Slip in the 2010 El Mayor‐Cucapah (Mexico) Earthquake From Differential Lidar

“…Finally, computed dip angles along the Pescadores and Laguna Salada faults in the southernmost Sierra Cucapah are highly variable, reflecting the predominantly right‐lateral sense of slip; in fact, all dip values along the southern half of this section are filtered from Figure e due to rake exceeding 160°. Nevertheless, along the northern Pescadores fault our average dip of ∼70° is in agreement with field measurements (Fletcher et al, ), geodetic models (Figures d–f), and estimates based on planar fitting of the fault trace using lidar topography (Zhou et al, ). However, our computed slip measurements differ from those of Fletcher et al ().…”

“…For an independent check on the dip along the reported lowest‐dip angle rupture section—the Paso Superior fault—we adopted a method for computing fault dip using only the high‐resolution postearthquake lidar topography (Zhou et al, ). By fitting planes through the 3‐D rupture trace, which is the intersection of the underlying fault plane with topography, we can estimate the fault dip representative of the shallow depths (tns to hundreds of meters) sampled by the local relief.…”

“…Each digitized rupture trace comprised roughly ∼100 vertices at ∼5‐m point spacing. Following Zhou et al (), we computed the dip, θ , of each segment by minimizing least squares residual distances d i between the fault trace vertices, x i , y i , and z i , and a plane defined by the plane equation coefficients, A , B , C , and D (equations ): …”

Characterizing Complex Surface Ruptures in the 2013 Mw 7.7 Balochistan Earthquake Using Three‐Dimensional Displacements

“…The rupture was bilateral but propagated much further to the south of the epicenter than to the north where fault creep (∼3.4 mm/year) has released a large fraction of the accumulated stress (Fattahi & Amelung, ). The southward propagation ended in a relay stepover (Zhou, Walker, Elliott, & Parsons, ). Previous studies have measured an average of ∼8‐m left‐lateral strike‐slip (Avouac et al, ) and ∼1.6‐m vertical (Zhou, Elliott, et al, ) motion during the earthquake.…”

The role of space-based observation in understanding and responding to active tectonics and earthquakes

“…Hillshade and slope maps aid with interpretation of surface features, and pick out where fault ruptures reach the surface. By tracing the lateral shift in a fault surface trace from an earthquake break and combining this with the derived DEM it is possible to determine the changing fault dip along strike of a rupture120. Fault perpendicular profiles, as well as displaced features such as offset streams and alluvial fans, provide estimates of fault displacement and can be compared with slip models.…”