Improve gallery example "meca.py" (#2199)

Author: yvonnefroehlich

examples/gallery/seismology/meca.py

@@ -2,18 +2,18 @@
 Focal mechanisms
 ----------------
 
-The :meth:`pygmt.Figure.meca` method can plot focal mechanisms, or beachballs.
+The :meth:`pygmt.Figure.meca` method can plot focal mechanisms or beachballs.
 We can specify the focal mechanism nodal planes or moment tensor components as
-a dict using the ``spec`` parameter (or they can be specified as a 1d or 2d
-array, or within a specified file). The size of plotted beachballs can be
+a dictionary using the ``spec`` parameter (or they can be specified as a 1d or
+2d array, or within a specified file). The size of plotted beachballs can be
 specified using the ``scale`` parameter.
 """
 
 import pygmt
 
 fig = pygmt.Figure()
 
-# generate a basemap near Washington state showing coastlines, land, and water
+# generate a map near Washington State showing land, water, and shorelines
 fig.coast(
     region=[-125, -122, 47, 49],
     projection="M6c",
@@ -23,11 +23,18 @@
     frame="a",
 )
 
-# store focal mechanisms parameters in a dict
+# store focal mechanism parameters in a dictionary based on the Aki & Richards
+# convention
 focal_mechanism = dict(strike=330, dip=30, rake=90, magnitude=3)
 
-# pass the focal mechanism data to meca in addition to the scale and event
-# location
-fig.meca(focal_mechanism, scale="1c", longitude=-124.3, latitude=48.1, depth=12.0)
+# pass the focal mechanism data through the spec parameter. In addition provide
+# scale, event location, and event depth
+fig.meca(
+    spec=focal_mechanism,
+    scale="1c",  # in centimeters
+    longitude=-124.3,
+    latitude=48.1,
+    depth=12.0,
+)
 
 fig.show()