ntained at several points about San
Francisco bay, and the records have been sufficiently studied to
afford data for comprehension of the varied earth waves which have
made themselves felt either to the perception of the citizens of the
Golden Gate or to the sensitive instruments. Such observations have
been conducted by Prof. George Davidson, for many years in charge of
the Coast and Geodetic Survey upon the Pacific Coast; by Prof. Charles
Burckhalter, of the Chabot Observatory, in Oakland, and by the staff
of the Lick Observatory on Mount Hamilton.

Careful inspection of these records shows that two systems of
earthquake disturbances act upon San Francisco. Those of the lighter
series show a wave movement beginning in one of the easterly quadrants
and more commonly in the southeastern. This series of light shocks is
attributed to the slip along the line of the San Joaquin fault. While
they may occur at any season of the year, they are more frequently
observed when the San Joaquin river is running bank high under the
influence of the melting snows in the foothills of the Sierra. That
such a condition has recently existed is made clear by the report
within less than a month of floods in the interior valleys of the
State. Assuming, as the geologists do, that the fault in the valley
lies near the roots of the Monte Diablo range, on the western edge of
the alluvial plain, it will be seen that the physical factors
involving the slip are very simple. There is a wide, flat plain
bounded on the west by a line of weakness in the rock supports. When
this plain is carrying an abnormal weight of water the tendency is to
break downward at the line of the fault. This tendency will produce a
jar in the mountain mass which will be rapidly communicated to its
farthest extremity.

The earthquakes which have their origin in the disturbances to which
the oceanic basin is subject always approach San Francisco from the
direction of the southwest quadrant. These have been uniformly more
violent than those whose origin is attributed to the San Joaquin
fault. While the records of San Francisco earthquakes up to the
present have exhibited a mild type, the damage to property having
hitherto been slight, it would appear from the extent and violence of
the present temblor that both causes had for once united.

The possibility of such simultaneous action of the two known seismic
factors of the greater peninsula had been foreseen by Prof. Le Cont