Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

sparse vegetation. One such area is the steep slopes along lower Green Creek. Other fans
vulnerable to debris flows are the fans along the south side of the South Arkansas River,
both west of Maysville (draining Proterozoic rock) and east of Maysville (draining Dry
Union Formation).
SEISMICITY
The Maysville quadrangle lies in the Rio Grande rift, an active zone of crustal
extension. The level of historic seismicity is low in the Colorado portion of this rift. A
search of the USGS/NEIC Internet catalog of earthquakes "Preliminary Determination of
Epicenters" (1973-2003 A.D.) reveals only one measured earthquake within a 6 mi radius
of the center of the quadrangle (table 6).
Table 6. Summary of measured earthquakes within a 6-mi radius of the center of the
Maysville quadrangle. Source: USGS website,
http://neic.usgs.gov/neis/epic/epic_circ.html
Catalog= PDE
Circle Search, centered on UTM-E 407472, UTM-N 4268501
Radius: 6.0 mi
Data Selection: Historical & Preliminary Data
YEAR MO DA ORIG TIME UTM-E UTM-N DEP(mi) MAG DIST(mi)
1994 08 04 164941.35 412932 4273767 3.1 2.50 ML 4.3
However, in 1921 a swarm of earthquakes occurred a short distance west of the
Maysville quadrangle, at roughly 386800E and 4273000N (near the ghost town of
Shavano in the North Fork). This swarm was felt at St. Elmo (northwest of the Maysville
quadrangle) and Garfield (west of the Maysville quadrangle) and included 14 small
events ranging from intensity II to IV from February and July of 1921 (table 7;
Humphreys, 1921). These are the only historic earthquakes known to have occurred near
the southern end of the Sawatch Range. Unfortunately, very little is known about this
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