NEAR-SURFACE SEISMIC IMAGING ACROSS THE PITAYCACHI FAULT, NORTHEASTERN SONORA, MEXICO
Frank H. Wagner, III (Trey) and Roy A. Johnson
Department of Geosciences, University of Arizona, Tucson, 85721,AZ
Email: fwagner@geo.arizona.edu; johnson@geo.arizona.edu
West
Data Processing
Abstract
The Pitaycachi normal fault of northeastern Sonora, Mexico is the source of the
3 May 1887 Bavispe earthquake (Mw ~7.4). This fault bounds the eastern
margin of the San Bernardino Valley, extending roughly north-south for over
100 km with fault-scarp exposures from the 1887 earthquake of up to 4
meters. In the fall of 2001, a near-surface seismic survey across this fault was
undertaken to attempt to image shallow structures associated with the fault in
order to better constrain recent fault activity. Ray-trace modeling and
tomographic inversion of first arrivals show that unconsolidated sediments in
the footwall are thin (~1 meter) and overlie more consolidated course
alluvial-fan sediments. Unconsolidated sediments near the surface in the
hanging wall are greater than 8 meters thick and show significant lateral and
vertical velocity variations. Total throw on the fault is estimated at over 4000
meters since fault initiation at about 23 Ma, giving a total slip rate of 0.17
mm/yr. Although previous estimates of Quaternary slip rates on this fault are
believed to be significantly less, fault activity remains vigorous. The nature of
the faulting in the San Bernardino Valley is similar to Quaternary fault scarps
flanking numerous basins in southern Arizona, and provides evidence of
continued extensional deformation in the southern Basin and Range Province.
East
Figure 5. Depth/velocity model across Pitaycachi fault
zone. East is towards the right. Velocities increase
dramatically with depth on the eastern side,
corresponding to the footwall block. Thick low velocity
layers on the west side indicate looser alluvium shed from
the fault scarp and suggest multiple rupture events over
time.
The data were acquired with the intention of processing for seismic reflections, but
due to local conditions, loose, dry alluvium and the extreme groundroll generated by
the sledge hammer source, clear reflections were not obvious in the data. Also,
because the ground roll and any possible near-surface reflections would be
expected to have very similar velocities, little could be done in processing to
enhance one while removing the other. Instead, changes in first-break slopes were
picked, indicating subsurface velocity changes that could be inverted for a
depth/velocity model.
First-Break Picking
Surface geometry was applied to the data and first-breaks were automatically
picked using Promax seismic processing software. First-breaks were picked on
unfiltered data (Figure 3), as filter effects can change the true timing of first-breaks.
As such, close attention was paid to ensure that the automatic picks were accurately
following first breaks and not triggering on noise. Bad picks were manually edited.
Some areas required manual picking, as extremely low-velocity first-breaks were
superimposed within groundroll and airwave. This is a common occurrence in the
loose alluvium that characterizes many southwestern desert seismic surveys.
Tomography
The first-break picks were inverted using Promax Turning Ray Tomography software.
This tomographic model works by tracing turning rays through an approximate
starting model, in this case a 1-dimensional, linear increase in velocity with depth.
The predicted travel times were subtracted from the actual travel times to produce
travel time residuals. The ray paths and the travel time residuals were then matrix
inverted to produce a 2-dimensional velocity field that best reduced the travel-time
residuals . This velocity model solution could then be used as a new starting model
and new ray paths and travel time residuals could be computed to converge on a
final depth/velocity model.
Introduction
The Pitaycachi normal fault is a 100 km long basin bounding fault, on the
eastern side of the San Bernardino Valley in the western fringe of the
Chihuahuan Desert, Northern Sonora, Mexico (Figure 1). This fault was the
source of the Great Sonoran earthquake of 1887 (Mw ~7.4) (Pearthree et
al.,1990), the largest historical normal fault earthquake of the southern Basin
and Range (de Polo et al., 1991). A 1-4 m fault scarp resulting from the 1887
rupture dominates the landscape and testifies to the major nature of this
event. On the basis of correlations of the Sierra Madre Occidental basalt
flows, total throw on the fault was estimated previously at over 4000 meters
since fault initiation at about 23 Ma, giving a total slip rate of 0.17 mm/yr;
Quaternary estimates of slip are much lower at only 0.015 mm/yr (Bull and
Pearthree, 1988).
Ray Trace Modeling
A more detailed ray trace inversion was accomplished using the program RAYINVR
(Figure 4) (Zelt, 1992). This step was undertaken for several reasons: 1) to confirm
the results obtained with tomographic inversion 2) to incorporate topography, which
the tomographic model didn’t handle adequately, and 3) to allow more user
interaction/intervention to produce a more geologically feasible model.
The
general tomographic velocity-model was used as a starting point. First-break pick
times were imported from Promax. Forward modeling was guided through trail and
error. The velocity-model was kept as simple as possible to avoid anomalous
complexities. Consequently, not every observed first break is accurately predicted.
In the fall of 2001 seismic reflection/refraciton data were acquired across the
Pitaycachi normal fault. In addition to its educational value as a field
exercise for members of the Society of Earth Science Students (SESS) at the
University of Arizona, the goal of this seismic survey was to image shallow
structures associated with the recently active fault in order to build a
velocity/depth model of the fault zone to better understand sedimentation
characteristics in the hanging wall and
perhaps extrapolate fault
movement history. Identification of multiple depositional events in the
hanging wall could provide proxy information on past earthquake
magnitude and recurrence intervals.
Issues and concerns
Figure 1. Above, areal view of the 1887 fault
scarp (from DuBois and Smith, 1980). Below,
generalized geologic map of the San Bernardino
Valley area of northern Sonora, Mexico (from
Pearthree et al., 1990).
Both the tomographic modeling and the ray-trace modeling brought out several
issues to note in the final solution. The most important is the limitation in depthresolution that the extremely short (<15 meters) source / receiver combinations
allowed. The short offsets allowed reliable imaging of only 2 meters in the footwall
and only 8-10 meters in the hanging wall. Also, extreme lateral variations in velocities
in the hanging wall necessitated approximations in the final velocity-model.
Meters
West
Figure 4. Depth/Velocity model plotted with calculated ray paths. (Below) First breaks (x’s) plotted with
calculated travel time curves (solid lines). Extreme lateral velocity variations in the hanging wall (west)
were averaged across these zones, resulting in some predicted trave ltime mismatches.
Interpretation
The gross distinguishing characteristic of this velocity model is the thin (<2m) low-velocity zone
on the east side of the profile, a much thicker (>6m) low velocity zone to the west, and a
gradational zone connecting along the topographic slope between the two (Figure 5). The
thick low velocity zone to the west corresponds to the hanging wall on the Pitaycachi normal
fault. The low velocities (200-500 m/s) are consistent with dry, sandy, loosely-packed alluvium
(Figure 6). On the eastern side of the fault, the footwall velocities increase quickly with depth to
over 800 m/s. This is still consistent with velocities expected for alluvium, but represents a
higher degree of consolidation.
Bull and Pearthree (1988) identified an earlier surface rupture along the Pitaycachi fault with
similar offset of 2-3 meters and hypothesized others previous to that. Based on similar offsets
per event, the 10-12 meters thick zone of loose alluvium imaged in the hanging wall block
possibly indicate 3-6 separate faulting event. Based on average recurrence rates for
Holocene Basin and Range faulting of ~100 ky/ event (Pearthree et al, 1983; Bull and
Pearthree, 1988), this survey possibly imaged 500+ ky of fault movement history. This would
represent an average slip rate of about 0.01-0.02mm/yr. Long term movement on this fault
(since 23 Ma) is an order of magnitude more (Suter and Contreras, 2002).
Line orientation was perpendicular to the fault, roughly east-west, rolling
towards the west. Acquisition began on the footwall side and progressed
across the fault zone into the hanging wall. A total of 90 shots were acquired,
rolling and leapfrogging active receivers to maintain constant fold, resulting in
a total line length of about 100 meters (Figure 2).
Figure 2. Data acquisition across the
Pitaycachi fault in northern Sonora,
Mexico. Acquisition was well
documented in still and moving
pictures.
Figure 3. Sample field shot gathers with first break picks in red. Note high
amplitude ground roll obscuring any possible reflections. Because this
ground roll has similar frequency and velocities to expected reflections,
little can be done to remove it an d enhance reflections.
Acknowledgments
--The University of Arizona SESS (Society of Earth Science Students) is thanked for their assistance in acquiring this data.
--IRIS (Incorporated Research Institutions for Seismology) is thanked for providing the seismic recording equipment.
Conclusions
The implications from this survey are that active faulting (i.e. extension) is still ongoing in the
southern Basin and Range, although at rates that may be much less than earlier Basin and
Range deformation. Indeed, similar faults and fault scarps, with varying amounts of scarp
erosion since last movement, are known throughout the southern Basin and Range (Menges et
al, 1982; Pearthree et al, 1983; Bull and Pearthree, 1988; Suter, 2002). Similar recurrence rates,
approximately one event every 100,00 years (Bull and Pearthree, 1988), with several meters of
offset per event are also common. From a seismic hazard perspective, these faults could
represent a distinct danger to population centers in the southern Basin and Range. From a
tectonic perspective, they attest to ongoing and active extension in a region assumed by
many to be inactive.
Seismic data were acquired with an IRIS supplied 24 bit, 60-channel
Geometrics StrataVisor NZ seismic recorder. Record lengths were 500 ms
sampled at intervals of 0.25 ms, with acquisition filters out. A three-pound
sledge hammer stuck against a steel plate was selected as the source as this
configuration seemed to provide the highest frequency content and least
ground roll. 40-Hz geophones spaced at intervals 0.5 m were deployed in a
split-spread configuration about the source. Maximum source-receiver offset
was 15 m. The source interval was 1 m, resulting in a nominal 15-fold CMP line.
Loose alluvium, fault scarp
derived
Consolidated alluvium
--Landmark is thanked for providing the seismic processing software.
The near-surface refraction survey produced a 2-D velocity profile for the first two to sixteen
meters of the earth centered about the historically active Pitaycachi normal fault in Sonora,
Mexico, a fault responsible for a Mw ~7.4 earthquake in 1887. Interpreted in the model is a
sequence of low-velocity alluvial fill >10 meters thick in the hanging wall. These sediments
could correspond to several (3-6) separate faulting events over the last half million years.
Data Acquisition
Figure 6. Geologic interpretation of velocity
model. Upper sediments on both sides of the
fault are characterized by loose alluvial fill. On the
downthrown side fill is much thicker, attesting to
numerous faulting events. Fault zone is inferred,
with several individual faults shown as suggested
from direct observation in nearby arroyos by Bull
and Pearthree, 1988.
East
Future work is necessary in this area to better understand the structural and tectonic processes
taking place. Very little is known about the geometry of this fault at depth, or about other similar
faults in the region. A more detailed knowledge of the deeper structure of the Pitaycachi fault
zone could be combined with regional stress data to begin to understand and predict future
movement on this and similar faults.
References
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northeastern Sonora, Mexico. Bulletin of the Seismological Society of America, Vol. 78, Number 2, pp. 956-978.
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